Monday, June 25, 2007

Epiblast cells

The most characteristic event occurring during the third week is gastrulation,
which begins with the appearance of the primitive streak,
which has at its cephalic end the primitive node. In the region of the
node and streak, epiblast cells move inward (invaginate) to form new cell layers,
endoderm and mesoderm. Hence, epiblast gives rise to all three germ
layers in the embryo. Cells of the intraembryonic mesodermal germ layer
migrate between the two other germ layers until they establish contact with
the extraembryonic mesoderm covering the yolk sac and amnion (Figs. 4.3
and 4.4).
Prenotochordal cells invaginating in the primitive pit move forward until
they reach the prechordal plate. They intercalate in the endoderm as the notochordal
plate (Fig. 4.4). With further development, the plate detaches from the
endoderm, and a solid cord, the notochord, is formed. It forms a midline axis,
84 Part One: General Embryology
Figure 4.18 Stem villi (SV) extend from the chorionic plate (CP) to the basal plate (BP).
Terminal villi (arrows) are represented by fine branches from stem villi.
which will serve as the basis of the axial skeleton (Fig. 4.4). Cephalic and caudal
ends of the embryo are established before the primitive streak is formed.
Thus, cells in the hypoblast (endoderm) at the cephalic margin of the disc form
the anterior visceral endoderm that expresses head-forming genes, including
OTX2, LIM1, and HESX1 and the secreted factor cerberus. Nodal, a member
of the TGF-β family of genes, is then activated and initiates and maintains the
integrity of the node and streak. BMP-4, in the presence of FGF, ventralizes
mesoderm during gastrulation so that it forms intermediate and lateral plate
mesoderm. Chordin, noggin, and follistatin antagonize BMP-4 activity and
dorsalize mesoderm to form the notochord and somitomeres in the head region.
Formation of these structures in more caudal regions is regulated by the
Brachyury (T) gene. Left-right asymmetry is regulated by a cascade of genes;
first, FGF-8, secreted by cells in the node and streak, induces Nodal and Lefty-2
expression on the left side. These genes upregulate PITX2, a transcription factor
responsible for left sidedness.
Epiblast cells moving through the node and streak are predetermined by
their position to become specific types of mesoderm and endoderm. Thus, it is
possible to construct a fate map of the epiblast showing this pattern (Fig. 4.11).
Chapter 4: Third Week of Development: Trilaminar Germ Disc 85
By the end of the third week, three basic germ layers, consisting of ectoderm,
mesoderm, and endoderm, are established in the head region, and
the process continues to produce these germ layers for more caudal areas of
the embryo until the end of the 4th week. Tissue and organ differentiation has
begun, and it occurs in a cephalocaudal direction as gastrulation continues.
In the meantime, the trophoblast progresses rapidly. Primary villi obtain
a mesenchymal core in which small capillaries arise (Fig. 4.17). When these
villous capillaries make contact with capillaries in the chorionic plate and connecting
stalk, the villous syste

Teratogenesis Associated With Gastrulation

Teratogenesis Associated With Gastrulation
The beginning of the thirdweek of development, when gastrulation is initiated,
is a highly sensitive stage for teratogenic insult. At this time, fate maps can
be made for various organ systems, such as the eyes and brain anlage, and
these cell populations may be damaged by teratogens. For example, high
doses of alcohol at this stage kill cells in the anterior midline of the germ disc,
producing a deficiency of the midline in craniofacial structures and resulting
in holoprosencephaly. In such a child, the forebrain is small, the two lateral
ventricles often merge into a single ventricle, and the eyes are close together
(hypotelorism). Because this stage is reached 2 weeks after fertilization, it is
approximately 4 weeks from the last menses. Therefore, the woman may not
recognize she is pregnant, having assumed that menstruation is late and will
begin shortly. Consequently, she may not take precautions shewould normally
consider if she knew she was pregnant.
Gastrulation itself may be disrupted by genetic abnormalities and toxic
insults. In caudal dysgenesis (sirenomelia), insufficient mesoderm is formed
in the caudal-most region of the embryo. Because this mesoderm contributes
to formation of the lower limbs, urogenital system (intermediate mesoderm),
and lumbosacral vertebrae, abnormalities in these structures ensue. Affected
individuals exhibit a variable range of defects, including hypoplasia and fusion
of the lower limbs, vertebral abnormalities, renal agenesis, imperforate anus,
and anomalies of the genital organs (Fig. 4.13). In humans, the condition is
associated with maternal diabetes and other causes. In mice, abnormalities
of Brachyury (T), Wnt, and engrailed genes produce a similar phenotype.
Situs inversus is a condition in which transposition of the viscera in the
thorax and abdomen occurs. Despite this organ reversal, other structural abnormalities
occur only slightly more frequently in these individuals. Approximately
20% of patients with complete situs inversus also have bronchiectasis
and chronic sinusitis because of abnormal cilia (Kartagener syndrome). Interestingly,
cilia are normally present on the ventral surface of the primitive node
and may be involved in left-right patterning during gastrulation. Other conditions
of abnormal sidedness are known as laterality sequences. Patients with
these conditions do not have complete situs inversus but appear to be predominantly
bilaterally left sided or right sided. The spleen reflects the differences;
those with left-sided bilaterality have polysplenia, and those with right-sided
bilaterality have asplenia or hypoplastic spleen. Patients with laterality sequences
also are likely to have other malformations, especially heart defects.

Establishment of the Body Axes

Establishment of the Body Axes
Establishment of the body axes, anteroposterior, dorsoventral, and left-right,
takes place before and during the period of gastrulation. The anteroposterior
axis is signaled by cells at the anterior (cranial) margin of the embryonic disc.
This area, the anterior visceral endoderm (AVE), expresses genes essential for
head formation, including the transcription factors OTX2, LIM1, and HESX1
and the secreted factor cerberus. These genes establish the cranial end of the
embryo before gastrulation. The primitive streak itself is initiated and maintained
by expression of Nodal, a member of the transforming growth factor β
(TGF-β) family (Fig. 4.5). Once the streak is formed, a number of genes regulate
formation of dorsal and ventral mesoderm and head and tail structures.
Another member of the TGF-β family, bone morphogenetic protein-4 (BMP-
4) is secreted throughout the embryonic disc (Fig. 4.5). In the presence of this
protein and fibroblast growth factor (FGF), mesoderm will be ventralized to
contribute to kidneys (intermediate mesoderm), blood, and body wall mesoderm
(lateral plate mesoderm). In fact, all mesoderm would be ventralized if
the activity of BMP-4 were not blocked by other genes expressed in the node.
For this reason, the node is the organizer. It was given that designation byHans Spemann, who first described this activity in the dorsal lip of the blastopore,
a structure analogous to the node, in Xenopus embryos. Thus, chordin
(activated by the transcription factor Goosecoid ), noggin, and follistatin antagonize
the activity of BMP-4. As a result, cranial mesoderm is dorsalized into
notochord, somites, and somitomeres (Fig. 4.5). Later, these three genes are
expressed in the notochord and are important in neural induction in the cranial
As mentioned, Nodal is involved in initiating and maintaining the primitive
streak (Fig. 4.6). Similarly, HNF-3β maintains the node and later induces
regional specificity in the forebrain and midbrain areas. Without HNF-3β, embryos
fail to gastrulate properly and lack forebrain and midbrain structures. As
mentioned previously, Goosecoid activates inhibitors of BMP-4 and contributes
to regulation of head development. Overexpression or underexpression of this
gene results in severe malformations of the head region, including duplications
(Fig. 4.7).
Regulation of dorsal mesoderm formation in mid and caudal regions of the
embryo is controlled by the Brachyury (T) gene (Fig. 4.8). Thus, mesoderm
formation in these regions depends on this gene product, and its absence
results in shortening of the embryonic axis (caudal dysgenesis; see p. 80).
The degree of shortening depends upon the time at which the protein becomes
Left-right sidedness, also established early in development, is orchestrated
by a cascade of genes. When the primitive streak appears, fibroblast growth
factor 8 (FGF-8) is secreted by cells in the node and primitive streak andinduces expression of Nodal but only on the left side of the embryo (Fig. 4.9A).
Later, as the neural plate is induced, FGF-8 maintains Nodal expression in the
lateral plate mesoderm (Fig. 4.10), as well as Lefty-2, and both of these genes
upregulate PITX2, a transcription factor responsible for establishing left sidedness
(Fig. 4.9B). Simultaneously, Lefty-1 is expressed on the left side of the
floor plate of the neural tube and may act as a barrier to prevent left-sided signals
from crossing over. Sonic hedgehog (SHH ) may also function in this role
as well as serving as a repressor for left sided gene expression on the right. The
Brachyury(T) gene, another growth factor secreted by the notochord, is also
essential for expression of Nodal, Lefty-1, and Lefty-2 (Fig. 4.9B). Genes regulating
right-sided development are not as well defined, although expression of thetranscription factor NKX 3.2 is restricted to the right lateral plate mesoderm
and probably regulates effector genes responsible for establishing the right side.
Why the cascade is initiated on the left remains a mystery, but the reason may
involve cilia on cells in the node that beat to create a gradient of FGF-8 toward
the left. Indeed, abnormalities in cilia-related proteins result in laterality defects
in mice and some humans with these defects have abnormal ciliary function

Gastrulation: Formation of Embryonic

Gastrulation: Formation of Embryonic
Mesoderm and Endoderm

The most characteristic event occurring during
the thirdweek of gestation is gastrulation, the process
that establishes all three germ layers (ectoderm,
mesoderm, and endoderm) in the embryo. Gastrulation
begins with formation of the primitive streak on the
surface of the epiblast (Figs. 4.1–4.3A). Initially, the streak
is vaguely defined (Fig. 4.1), but in a 15- to 16-day embryo,
it is clearly visible as a narrow groove with slightly bulging
regions on either side (Fig. 4.2). The cephalic end of the streak,
the primitive node, consists of a slightly elevated area surrounding
the small primitive pit (Fig. 4.3). Cells of the epiblast migrate
toward the primitive streak (Fig. 4.3). Upon arrival in the region
of the streak, they become flask-shaped, detach from the epiblast,
and slip beneath it (Fig. 4.3, B–D). This inward movement is known
as invagination. Once the cells have invaginated, some displace the
hypoblast, creating the embryonic endoderm, and others come to lie
between the epiblast and newly created endoderm to form mesoderm.
Cells remaining in the epiblast then form ectoderm. Thus, the epiblast,
through the process of gastrulation, is the source of all of the germ
layers (Fig. 4.3B), and cells in these layers will give rise to all of the
tissues and organs in the embryo.
As more and more cells move between the epiblast and hypoblast
layers, they begin to spread laterally and cephalad (Fig. 4.3). Gradually,they migrate beyond the margin of the disc and establish contact with the extraembryonic
mesoderm covering the yolk sac and amnion. In the cephalic
direction, they pass on each side of the prechordal plate. The prechordal plate
itself forms between the tip of the notochord and the buccopharyngeal membrane
and is derived from some of the first cells that migrate through the
node in a cephalic direction. Later, the prechordal plate will be important forinduction of the forebrain (Figs. 4.3A and 4.4A). The buccopharyngeal membrane
at the cranial end of the disc consists of a small region of tightly adherent
ectoderm and endoderm cells that represents the future opening of the oral
Formation of the Notochord
Prenotochordal cells invaginating in the primitive pit move forward cephalad
until they reach the prechordal plate (Fig. 4.4). These prenotochordal cells
become intercalated in the hypoblast so that, for a short time, the midline of the
embryo consists of two cell layers that form the notochordal plate (Fig. 4.4, B
and C ). As the hypoblast is replaced by endoderm cells moving in at the streak,
cells of the notochordal plate proliferate and detach from the endoderm. They
then form a solid cord of cells, the definitive notochord (Fig. 4.4, D and E ),
which underlies the neural tube and serves as the basis for the axial skeleton.
Because elongation of the notochord is a dynamic process, the cranial end
forms first, and caudal regions are added as the primitive streak assumes a
more caudal position. The notochord and prenotochordal cells extend cranially
to the prechordal plate (an area just caudal to the buccopharyngeal membrane)
and caudally to the primitive pit. At the point where the pit forms an indentationin the epiblast, the neurenteric canal temporarily connects the amniotic and
yolk sac cavities.

Day 8
At the eighth day of development, the blastocyst is partially
embedded in the endometrial stroma. In the area over the embryoblast,
the trophoblast has differentiated into two layers:
(a) an inner layer of mononucleated cells, the cytotrophoblast,
and (b) an outer multinucleated zone without distinct cell boundaries,
the syncytiotrophoblast (Figs. 3.1 and 3.2). Mitotic figures are
found in the cytotrophoblast but not in the syncytiotrophoblast. Thus,
cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast,
where they fuse and lose their individual cell membranes.
Cells of the inner cell mass or embryoblast also differentiate into two
layers: (a) a layer of small cuboidal cells adjacent to the blastocyst cavity,
known as the hypoblast layer, and (b) a layer of high columnar cells
adjacent to the amniotic cavity, the epiblast layer (Figs. 3.1 and 3.2).
Together, the layers form a flat disc. At the same time, a small cavity
appears within the epiblast. This cavity enlarges to become theamniotic cavity. Epiblast cells adjacent to the cytotrophoblast are called amnioblasts;
together with the rest of the epiblast, they line the amniotic cavity
(Figs. 3.1 and 3.3). The endometrial stroma adjacent to the implantation site
is edematous and highly vascular. The large, tortuous glands secrete abundant
glycogen and mucus.

Day 9
The blastocyst is more deeply embedded in the endometrium, and the penetration
defect in the surface epithelium is closed by a fibrin coagulum (Fig. 3.3).
The trophoblast shows considerable progress in development, particularly at
the embryonic pole, where vacuoles appear in the syncytium. When these vacuoles
fuse, they form large lacunae, and this phase of trophoblast development
is thus known as the lacunar stage (Fig. 3.3).
At the abembryonic pole, meanwhile, flattened cells probably originating
from the hypoblast form a thin membrane, the exocoelomic (Heuser’s) membrane,
that lines the inner surface of the cytotrophoblast (Fig. 3.3). This membrane,
together with the hypoblast, forms the lining of the exocoelomic cavity,
or primitive yolk sac.

Days 11 and 12
By the 11th to 12th day of development, the blastocyst is completely embedded
in the endometrial stroma, and the surface epithelium almost entirely covers
the original defect in the uterine wall (Figs. 3.4 and 3.5). The blastocyst now
produces a slight protrusion into the lumen of the uterus. The trophoblast is
characterized by lacunar spaces in the syncytium that form an intercommunicating
network. This network is particularly evident at the embryonic pole; at
the abembryonic pole, the trophoblast still consists mainly of cytotrophoblastic
cells (Figs. 3.4 and 3.5).
Concurrently, cells of the syncytiotrophoblast penetrate deeper into the
stroma and erode the endothelial lining of the maternal capillaries. These capillaries,
which are congested and dilated, are known as sinusoids. The syncytial
lacunae become continuous with the sinusoids and maternal blood enters the
lacunar system (Fig. 3.4). As the trophoblast continues to erode more and more
sinusoids, maternal blood begins to flow through the trophoblastic system, establishing
the uteroplacental circulation.
In the meantime, a new population of cells appears between the inner
surface of the cytotrophoblast and the outer surface of the exocoelomiccavity. These cells, derived from yolk sac cells, form a fine, loose connective
tissue, the extraembryonic mesoderm, which eventually fills all of the
space between the trophoblast externally and the amnion and exocoelomic
membrane internally (Figs. 3.4 and 3.5). Soon, large cavities develop in the
extraembryonic mesoderm, and when these become confluent, they form
a new space known as the extraembryonic coelom, or chorionic cavity
(Fig. 3.4). This space surrounds the primitive yolk sac and amniotic cavity except
where the germ disc is connected to the trophoblast by the connecting stalk
(Fig. 3.6). The extraembryonic mesoderm lining the cytotrophoblast and amnion
is called the extraembryonic somatopleuric mesoderm; the lining covering
the yolk sac is known as the extraembryonic splanchnopleuric mesoderm
(Fig. 3.4).
Growth of the bilaminar disc is relatively slowcompared with that of the trophoblast;
consequently, the disc remains very small (0.1–0.2 mm). Cells of the
endometrium, meanwhile, become polyhedral and loaded with glycogen and
lipids; intercellular spaces are filled with extravasate, and the tissue is edematous.
These changes, known as the decidua reaction, at first are confined to the
area immediately surrounding the implantation site but soon occur throughout
the endometrium.

Day 13
By the 13th day of development, the surface defect in the endometrium has
usually healed. Occasionally, however, bleeding occurs at the implantation site
as a result of increased blood flow into the lacunar spaces. Because this bleeding
occurs near the 28th day of the menstrual cycle, it may be confused withnormal menstrual bleeding and, therefore, cause inaccuracy in determining
the expected delivery date.
The trophoblast is characterized by villous structures. Cells of the cytotrophoblast
proliferate locally and penetrate into the syncytiotrophoblast,
forming cellular columns surrounded by syncytium. Cellular columns with
the syncytial covering are known as primary villi (Figs. 3.6 and 3.7) (see
Chapter 4).
In the meantime, the hypoblast produces additional cells that migrate along
the inside of the exocoelomic membrane (Fig. 3.4). These cells proliferate and
gradually form a new cavity within the exocoelomic cavity. This new cavity is
known as the secondary yolk sac or definitive yolk sac (Figs. 3.6 and 3.7). This
yolk sac is much smaller than the original exocoelomic cavity, or primitive yolk
sac. During its formation, large portions of the exocoelomic cavity are pinched
off. These portions are represented by exocoelomic cysts, which are often
found in the extraembryonic coelom or chorionic cavity (Figs. 3.6 and 3.7).
Meanwhile, the extraembryonic coelom expands and forms a large cavity,
the chorionic cavity. The extraembryonic mesoderm lining the inside of the
cytotrophoblast is then known as the chorionic plate. The only place where
extraembryonic mesoderm traverses the chorionic cavity is in the connecting
stalk (Fig. 3.6). With development of blood vessels, the stalk becomes the
umbilical cord.

At the beginning of the second week, the blastocyst is partially embedded
in the endometrial stroma. The trophoblast differentiates into (a)
an inner, actively proliferating layer, the cytotrophoblast, and (b) an
outer layer, the syncytiotrophoblast, which erodes maternal tissues (Fig. 3.1).
By day 9, lacunae develop in the syncytiotrophoblast. Subsequently, maternal
sinusoids are eroded by the syncytiotrophoblast, maternal blood enters the
lacunar network, and by the end of the second week, a primitive uteroplacental
circulation begins (Fig. 3.6). The cytotrophoblast, meanwhile, forms
cellular columns penetrating into and surrounded by the syncytium. These
columns are primary villi. By the end of the second week, the blastocyst
is completely embedded, and the surface defect in the mucosa has healed
(Fig. 3.6).
The inner cell mass or embryoblast, meanwhile, differentiates into (a) the
epiblast and (b) the hypoblast, together forming a bilaminar disc (Fig. 3.1).
Epiblast cells give rise to amnioblasts that line the amniotic cavity superior
to the epiblast layer. Endoderm cells are continuous with the exocoelomic
membrane, and together they surround the primitive yolk sac (Fig. 3.4). By
the end of the second week, extraembryonic mesoderm fills the space between
the trophoblast and the amnion and exocoelomic membrane internally. When
vacuoles develop in this tissue, the extraembryonic coelom or chorionic cavity
forms (Fig. 3.6). Extraembryonic mesoderm lining the cytotrophoblast and
62 Part One: General Embryology
amnion is extraembryonic somatopleuric mesoderm; the lining surrounding
the yolk sac is extraembryonic splanchnopleuric mesoderm (Fig. 3.6).
The second week of development is known as the week of twos: The
trophoblast differentiates into two layers, the cytotrophoblast and syncytiotrophoblast.
The embryoblast forms two layers, the epiblast and hypoblast.
The extraembryonic mesoderm splits into two layers, the somatopleure and
splanchnopleure. And two cavities, the amniotic and yolk sac cavities, form.
Implantation occurs at the end of the first week. Trophoblast cells invade the
epithelium and underlying endometrial stroma with the help of proteolytic enzymes.
Implantationmay also occur outside the uterus, such as in the rectouterine
pouch, on the mesentery, in the uterine tube, or in the ovary (ectopic pregnancies).


Infertility is a problem for 15% to 30% of couples. Male infertility may be
a result of insufficient numbers of sperm and/or poor motility. Normally, the
ejaculate has a volume of 3 to 4 ml, with approximately 100 million sperm
per ml. Males with 20 million sperm per ml or 50 million sperm per total
ejaculate are usually fertile. Infertility in a woman may be due to a number of
causes, including occluded oviducts (most commonly caused by pelvic inflammatory
disease), hostile cervical mucus, immunity to spermatozoa, absence
of ovulation, and others.
In vitro fertilization (IVF) of human ova and embryo transfer is a frequent
practice conducted by laboratories throughout the world. Follicle growth in the
ovary is stimulated by administration of gonadotropins. Oocytes are recovered
by laparoscopy from ovarian follicles with an aspirator just before ovulation
when the oocyte is in the late stages of the first meiotic division. The egg is
placed in a simple culture medium and sperm are added immediately. Fertilized
eggs are monitored to the eight-cell stage and then placed in the uterus
to develop to term. Fortunately, because preimplantation-stage embryos are
resistant to teratogenic insult, the risk of producing malformed offspring by
in vitro procedures is low.
A disadvantage of IVF is its low success rate; only 20% of fertilized ova
implant and develop to term. Therefore, to increase chances of a successful
pregnancy, four or five ova are collected, fertilized, and placed in the uterus.
This approach sometimes leads to multiple births.
Another technique, gamete intrafallopian transfer (GIFT), introduces
oocytes and sperm into the ampulla of the fallopian (uterine) tube, where
42 Part One: General Embryology
fertilization takes place. Development then proceeds in a normal fashion. In a
similar approach, zygote intrafallopian transfer (ZIFT), fertilized oocytes are
placed in the ampullary region. Both of these methods require patent uterine
Severe male infertility, in which the ejaculate contains very few live sperm
(oligozoospermia) or even no live sperm (azoospermia), can be overcome
using intracytoplasmic sperm injection (ICSI). With this technique, a single
sperm, which may be obtained from any point in the male reproductive tract,
is injected into the cytoplasm of the egg to cause fertilization. This approach
offers couples an alternative to using donor sperm for IVF. The technique
carries an increased risk for fetuses to have Y chromosome deletions but no
other chromosomal abnormalities.


Fertilization, the process by which male and female gametes fuse, occurs in the
ampullary region of the uterine tube. This is the widest part of the tube and
38 Part One: General Embryology
is close to the ovary (Fig. 2.4). Spermatozoa may remain viable in the female
reproductive tract for several days.
Only 1% of sperm deposited in the vagina enter the cervix, where they
may survive for many hours. Movement of sperm from the cervix to the oviduct
is accomplished primarily by their own propulsion, although they may be assisted
by movements of fluids created by uterine cilia. The trip from cervix
to oviduct requires a minimum of 2 to 7 hours, and after reaching the isthmus,
sperm become less motile and cease their migration. At ovulation, sperm
again become motile, perhaps because of chemoattractants produced by cumulus
cells surrounding the egg, and swim to the ampulla where fertilization
usually occurs. Spermatozoa are not able to fertilize the oocyte immediately
upon arrival in the female genital tract but must undergo (a) capacitation and
(b) the acrosome reaction to acquire this capability.
Capacitation is a period of conditioning in the female reproductive tract
that in the human lasts approximately 7 hours. Much of this conditioning,
which occurs in the uterine tube, entails epithelial interactions between the
sperm and mucosal surface of the tube. During this time a glycoprotein coat
and seminal plasma proteins are removed from the plasma membrane that
overlies the acrosomal region of the spermatozoa. Only capacitated sperm can
pass through the corona cells and undergo the acrosome reaction.
The acrosome reaction, which occurs after binding to the zona pellucida,
is induced by zona proteins. This reaction culminates in the release of enzymes
needed to penetrate the zona pellucida, including acrosin and trypsin-like substances
(Fig. 2.5).
The phases of fertilization include phase 1, penetration of the corona radiata;
phase 2, penetration of the zona pellucida; and phase 3, fusion of the
oocyte and sperm cell membranes.
Of the 200 to 300 million spermatozoa deposited in the female genital tract,
only 300 to 500 reach the site of fertilization. Only one of these fertilizes the
egg. It is thought that the others aid the fertilizing sperm in penetrating the
barriers protecting the female gamete. Capacitated sperm pass freely through
corona cells (Fig. 2.5).
The zona is a glycoprotein shell surrounding the egg that facilitates and maintains
sperm binding and induces the acrosome reaction. Both binding and the
acrosome reaction are mediated by the ligand ZP3, a zona protein. Release
of acrosomal enzymes (acrosin) allows sperm to penetrate the zona, thereby
coming in contact with the plasma membrane of the oocyte (Fig. 2.5). Permeability
of the zona pellucida changes when the head of the sperm comes
in contact with the oocyte surface. This contact results in release of lysosomal
Chapter 2: First Week of Development: Ovulation to Implantation 39
enzymes from cortical granules lining the plasma membrane of the oocyte.
In turn, these enzymes alter properties of the zona pellucida (zona reaction)
to prevent sperm penetration and inactivate species-specific receptor sites for
spermatozoa on the zona surface. Other spermatozoa have been found embedded
in the zona pellucida, but only one seems to be able to penetrate the oocyte

The initial adhesion of sperm to the oocyte is mediated in part by the interaction
of integrins on the oocyte and their ligands, disintegrins, on sperm. After
adhesion, the plasma membranes of the sperm and egg fuse (Fig. 2.5). Because
the plasma membrane covering the acrosomal head cap disappears during the
acrosome reaction, actual fusion is accomplished between the oocyte membrane
and the membrane that covers the posterior region of the sperm head
(Fig. 2.5). In the human, both the head and tail of the spermatozoon enter the
cytoplasm of the oocyte, but the plasma membrane is left behind on the oocyte
surface. As soon as the spermatozoon has entered the oocyte, the egg responds
in three ways:
1. Cortical and zona reactions. As a result of the release of cortical oocyte
granules, which contain lysosomal enzymes, (a) the oocyte membrane
becomes impenetrable to other spermatozoa, and (b) the zona pellucida
alters its structure and composition to prevent sperm binding and
penetration. These reactions prevent polyspermy (penetration of more
than one spermatozoon into the oocyte).
2. Resumption of the second meiotic division. The oocyte finishes its second
meiotic division immediately after entry of the spermatozoon. One
of the daughter cells, which receives hardly any cytoplasm, is known as
the second polar body; the other daughter cell is the definitive oocyte.
Its chromosomes (22+X) arrange themselves in a vesicular nucleus
known as the female pronucleus (Figs. 2.6 and 2.7).
3. Metabolic activation of the egg. The activating factor is probably carried
by the spermatozoon. Postfusion activation may be considered to
encompass the initial cellular and molecular events associated with early
The spermatozoon, meanwhile, moves forward until it lies close to the
female pronucleus. Its nucleus becomes swollen and forms the male pronucleus
(Fig. 2.6); the tail detaches and degenerates. Morphologically, the male
and female pronuclei are indistinguishable, and eventually, they come into
close contact and lose their nuclear envelopes (Fig. 2.7A). During growth of
male and female pronuclei (both haploid), each pronucleus must replicate its
DNA. If it does not, each cell of the two-cell zygote has only half of the normal
amount of DNA. Immediately after DNA synthesis, chromosomes organize on
the spindle in preparation for a normal mitotic division. The 23 maternal and
23 paternal (double) chromosomes split longitudinally at the centromere, and
sister chromatids move to opposite poles, providing each cell of the zygote
with the normal diploid number of chromosomes and DNA (Fig. 2.6, D and
E ). As sister chromatids move to opposite poles, a deep furrow appears on the
surface of the cell, gradually dividing the cytoplasm into two parts (Figs. 2.6F
and 2.7B).
The main results of fertilization are as follows:
 Restoration of the diploid number of chromosomes, half from the father
and half from the mother. Hence, the zygote contains a new combination
of chromosomes different from both parents.
 Determination of the sex of the new individual. An X-carrying sperm
produces a female (XX) embryo, and a Y-carrying sperm produces a male
(XY) embryo. Hence, the chromosomal sex of the embryo is determined
at fertilization.
 Initiation of cleavage. Without fertilization, the oocyte usually degenerates
24 hours after ovulation.


Ovarian Cycle
At puberty, the female begins to undergo regular
monthly cycles. These sexual cycles are controlled
by the hypothalamus. Gonadotropin-releasing hormone
(GnRH) produced by the hypothalamus acts on
cells of the anterior pituitary gland, which in turn secrete
gonadotropins. These hormones, follicle-stimulating
hormone (FSH) and luteinizing hormone (LH), stimulate
and control cyclic changes in the ovary.
At the beginning of each ovarian cycle, 15 to 20 primary
(preantral) stage follicles are stimulated to grow under the
influence of FSH. (The hormone is not necessary to promote
development of primordial follicles to the primary follicle stage,
but without it, these primary follicles die and become atretic.) Thus,
FSH rescues 15 to 20 of these cells from a pool of continuously
forming primary follicles (Fig. 2.1). Under normal conditions, only
one of these follicles reaches full maturity, and only one oocyte is
discharged; the others degenerate and become atretic. In the next
cycle, another group of primary follicles is recruited, and again, only
one follicle reaches maturity. Consequently, most follicles degenerate
without ever reaching full maturity. When a follicle becomes atretic,
the oocyte and surrounding follicular cells degenerate and are replaced
by connective tissue, forming a corpus atreticum. FSH also stimulates
maturation of follicular (granulosa) cells surrounding the oocyte. In
turn, proliferation of these cells is mediated by growth differentiation
factor-9 (GDF-9), a member of the transforming growth factor-β (TGF-β) family.
In cooperation, granulosa and thecal cells produce estrogens that (a) cause the
uterine endometrium to enter the follicular or proliferative phase; (b) cause
thinning of the cervical mucus to allow passage of sperm; and (c) stimulate the
pituitary gland to secrete LH. At mid-cycle, there is an LH surge that (a) elevates
concentrations of maturation-promoting factor, causing oocytes to complete
meiosis I and initiate meiosis II; (b) stimulates production of progesterone
by follicular stromal cells (luteinization); and (c) causes follicular rupture and

In the days immediately preceding ovulation, under the influence of FSH and
LH, the secondary follicle grows rapidly to a diameter of 25 mm. Coincident
with final development of the secondary follicle, there is an abrupt increase in
LH that causes the primary oocyte to complete meiosis I and the follicle to enter
the preovulatory stage. Meiosis II is also initiated, but the oocyte is arrested in
metaphase approximately 3 hours before ovulation. In the meantime, the surface
of the ovary begins to bulge locally, and at the apex, an avascular spot, the
stigma, appears. The high concentration of LH increases collagenase activity,
resulting in digestion of collagen fibers surrounding the follicle. Prostaglandin
levels also increase in response to the LH surge and cause local muscular contractions
in the ovarian wall. Those contractions extrude the oocyte, which
together with its surrounding granulosa cells from the region of the cumulus
oophorus, breaks free (ovulation) and floats out of the ovary (Figs. 2.2 and
2.3). Some of the cumulus oophorus cells then rearrange themselves around
the zona pellucida to form the corona radiata (Figs. 2.4–2.6).

During ovulation, some women feel a slight pain, known as middle pain
because it normally occurs near the middle of the menstrual cycle. Ovulation
is also generally accompanied by a rise in basal temperature, which can be
monitored to aid in determining when release of the oocyte occurs. Some
women fail to ovulate because of a low concentration of gonadotropins. In
these cases, administration of an agent to stimulate gonadotropin release and
hence ovulation can be employed. Although such drugs are effective, they
often produce multiple ovulations, so that the risk of multiple pregnancies is
10 times higher in these women than in the general population.


Maturation of Sperm Begins at Puberty
Spermatogenesis, which begins at puberty, includes all of the events by which
spermatogonia are transformed into spermatozoa. At birth, germ cells in the
male can be recognized in the sex cords of the testis as large, pale cells surrounded
by supporting cells (Fig. 1.21A). Supporting cells, which are derived
from the surface epithelium of the gland in the same manner as follicular cells,
become sustentacular cells, or Sertoli cells (Fig. 1.21C ).
Shortly before puberty, the sex cords acquire a lumen and become the
seminiferous tubules. At about the same time, primordial germ cells give
rise to spermatogonial stem cells. At regular intervals, cells emerge from this
stem cell population to form type A spermatogonia, and their production
marks the initiation of spermatogenesis. Type A cells undergo a limited number
of mitotic divisions to form a clone of cells. The last cell division produces
type B spermatogonia, which then divide to form primary spermatocytes
(Figs. 1.21 and 1.22). Primary spermatocytes then enter a prolongedprophase (22 days) followed by rapid completion of meiosis I and formation
of secondary spermatocytes. During the second meiotic division, these cells
immediately begin to form haploid spermatids (Figs. 1.21–1.23). Throughout
this series of events, from the time type A cells leave the stem cell population
to formation of spermatids, cytokinesis is incomplete, so that successive
cell generations are joined by cytoplasmic bridges. Thus, the progeny of a single
type A spermatogonium form a clone of germ cells that maintain contact
throughout differentiation (Fig. 1.22). Furthermore, spermatogonia and spermatids
remain embedded in deep recesses of Sertoli cells throughout their
development (Fig. 1.24). In this manner, Sertoli cells support and protect the
germ cells, participate in their nutrition, and assist in the release of mature
Spermatogenesis is regulated by luteinizing hormone (LH) production by
the pituitary. LH binds to receptors on Leydig cells and stimulates testosterone
production, which in turn binds to Sertoli cells to promote spermatogenesis.
Follicle stimulating hormone (FSH) is also essential because its binding to
Sertoli cells stimulates testicular fluid production and synthesis of intracellular
androgen receptor proteins.
The series of changes resulting in the transformation of spermatids into spermatozoa
is spermiogenesis. These changes include (a) formation of the acrosome,
which covers half of the nuclear surface and contains enzymes to assist in penetration
of the egg and its surrounding layers during fertilization (Fig. 1.25);
(b) condensation of the nucleus; (c) formation of neck, middle piece, and tail;
and (d) shedding of most of the cytoplasm. In humans, the time required for
a spermatogonium to develop into a mature spermatozoon is approximately
64 days.
When fully formed, spermatozoa enter the lumen of seminiferous tubules.
From there, they are pushed toward the epididymis by contractile elements
in the wall of the seminiferous tubules. Although initially only slightly motile,
spermatozoa obtain full motility in the epididymis.

Abnormal Gametes
In humans and in most mammals, one ovarian follicle occasionally contains
two or three clearly distinguishable primary oocytes (Fig. 1.26A). Although
these oocytes may give rise to twins or triplets, they usually degenerate before
reaching maturity. In rare cases, one primary oocyte contains two or even
three nuclei (Fig. 1.26B). Such binucleated or trinucleated oocytes die before
reaching maturity.
In contrast to atypical oocytes, abnormal spermatozoa are seen frequently,
and up to 10% of all spermatozoa have observable defects. The
head or the tail may be abnormal; spermatozoa may be giants or dwarfs;
and sometimes they are joined (Fig. 1.26C ). Sperm with morphologic abnormalities
lack normal motility and probably do not fertilize oocytes.

Primordial germ cells appear in the wall of the yolk sac in the fourth
week and migrate to the indifferent gonad (Fig. 1.1), where they arrive
at the end of the fifth week. In preparation for fertilization, both
male and female germ cells undergo gametogenesis, which includes meiosis
and cytodifferentiation. During meiosis I, homologous chromosomes
pair and exchange genetic material; during meiosis II, cells fail to replicate
DNA, and each cell is thus provided with a haploid number of chromosomes
and half the amount of DNA of a normal somatic cell (Fig. 1.3). Hence, mature
male and female gametes have, respectively, 22 plus X or 22 plus Y
Birth defects may arise through abnormalities in chromosome number
or structure and from single gene mutations. Approximately 7% of major
Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes 29
birth defects are a result of chromosome abnormalities, and 8%, are a result
of gene mutations. Trisomies (an extra chromosome) and monosomies
(loss of a chromosome) arise during mitosis or meiosis. During meiosis, homologous
chromosomes normally pair and then separate. However, if separation
fails (nondisjunction), one cell receives too many chromosomes and
one receives too few (Fig. 1.5). The incidence of abnormalities of chromosome
number increases with age of the mother, particularly with mothers
aged 35 years and older. Structural abnormalities of chromosomes include
large deletions (cri-du-chat syndrome) and microdeletions. Microdeletions
involve contiguous genes that may result in defects such as Angelman syndrome
(maternal deletion, chromosome 15q11–15q13) or Prader-Willi syndrome
(paternal deletion, 15q11–15q13). Because these syndromes depend
on whether the affected genetic material is inherited from the mother or the
father, they also are an example of imprinting. Gene mutations may be dominant
(only one gene of an allelic pair has to be affected to produce an alteration)
or recessive (both allelic gene pairs must be mutated). Mutations responsible
for many birth defects affect genes involved in normal embryological
In the female, maturation fromprimitive germ cell to mature gamete, which
is called oogenesis, begins before birth; in the male, it is called spermatogenesis,
and it begins at puberty. In the female, primordial germ cells form
oogonia. After repeated mitotic divisions, some of these arrest in prophase of
meiosis I to form primary oocytes. By the seventh month, nearly all oogonia
have become atretic, and only primary oocytes remain surrounded by
a layer of follicular cells derived from the surface epithelium of the ovary
(Fig. 1.17). Together, they form the primordial follicle. At puberty, a pool of
growing follicles is recruited and maintained from the finite supply of primordial
follicles. Thus, everyday 15 to 20 follicles begin to grow, and as they mature,
they pass through three stages: 1) primary or preantral; 2) secondary
or antral (vesicular, Graafian); and 3) preovulatory. The primary oocyte remains
in prophase of the first meiotic division until the secondary follicle is
mature. At this point, a surge in luteinizing hormone (LH) stimulates preovulatory
growth: meiosis I is completed and a secondary oocyte and polar
body are formed. Then, the secondary oocyte is arrested in metaphase of
meiosis II approximately 3 hours before ovulation and will not complete this
cell division until fertilization. In the male, primordial cells remain dormant
until puberty, and only then do they differentiate into spermatogonia. These
stem cells give rise to primary spermatocytes, which through two successive
meiotic divisions produce four spermatids (Fig. 1.4). Spermatids go through
a series of changes (spermiogenesis) (Fig. 1.25) including (a) formation of
the acrosome, (b) condensation of the nucleus, (c) formation of neck, middle
piece, and tail, and (d) shedding of most of the cytoplasm. The time required
for a spermatogonium to become a mature spermatozoon is approximately
64 days.

Maturation of Oocytes Continues at Puberty

Near the time of birth, all primary oocytes have started prophase of meiosis I,
but instead of proceeding into metaphase, they enter the diplotene stage, a
resting stage during prophase that is characterized by a lacy network of chromatin
(Fig. 1.17C ). Primary oocytes remain in prophase and do not finish
their first meiotic division before puberty is reached, apparently because of
oocyte maturation inhibitor (OMI), a substance secreted by follicular cells. The
total number of primary oocytes at birth is estimated to vary from 700,000 to
2 million. During childhood most oocytes become atretic; only approximately
400,000 are present by the beginning of puberty, and fewer than 500 will be
ovulated. Some oocytes that reach maturity late in life have been dormant in
the diplotene stage of the first meiotic division for 40 years or more before
ovulation. Whether the diplotene stage is the most suitable phase to protect
the oocyte against environmental influences is unknown. The fact that the risk
of having children with chromosomal abnormalities increases with maternal
age indicates that primary oocytes are vulnerable to damage as they age.
At puberty, a pool of growing follicles is established and continuously maintained
from the supply of primordial follicles. Each month, 15 to 20 follicles
selected from this pool begin to mature, passing through three stages: 1) primary
or preantral; 2) secondary or antral (also called vesicular or Graafian);
and 3) preovulatory. The antral stage is the longest, whereas the preovulatory
stage encompasses approximately 37 hours before ovulation. As the primary
oocyte begins to grow, surrounding follicular cells change from flat to cuboidal
and proliferate to produce a stratified epithelium of granulosa cells, and the unit
is called a primary follicle (Fig. 1.18, B and C ). Granulosa cells rest on a basement
membrane separating them from surrounding stromal cells that form the
theca folliculi. Also, granulosa cells and the oocyte secrete a layer of glycoproteins
on the surface of the oocyte, forming the zona pellucida (Fig. 1.18C ). As
follicles continue to grow, cells of the theca folliculi organize into an inner layer
of secretory cells, the theca interna, and an outer fibrous capsule, the theca
externa. Also, small, finger-like processes of the follicular cells extend across
the zona pellucida and interdigitate with microvilli of the plasma membrane
of the oocyte. These processes are important for transport of materials from
follicular cells to the oocyte.
As development continues, fluid-filled spaces appear between granulosa
cells. Coalescence of these spaces forms the antrum, and the follicle is termed
a secondary (vesicular, Graafian) follicle. Initially, the antrum is crescent
shaped, but with time, it enlarges (Fig. 1.19). Granulosa cells surrounding the
oocyte remain intact and form the cumulus oophorus. At maturity, the secondary
follicle may be 25 mm or more in diameter. It is surrounded by the
theca interna, which is composed of cells having characteristics of steroid secretion,
rich in blood vessels, and the theca externa, which gradually merges
with the ovarian stroma (Fig. 1.19).
With each ovarian cycle, a number of follicles begin to develop, but usually
only one reaches full maturity. The others degenerate and become atretic
(Fig. 1.19C ). When the secondary follicle is mature, a surge in luteinizing
hormone (LH) induces the preovulatory growth phase. Meiosis I is completed,
resulting in formation of two daughter cells of unequal size, each with 23 doublestructured
chromosomes (Fig. 1.20, A and B). One cell, the secondary oocyte,
receives most of the cytoplasm; the other, the first polar body, receives practically
none. The first polar body lies between the zona pellucida and the cellmembrane of the secondary oocyte in the perivitelline space (Fig. 1.20B). The
cell then enters meiosis II but arrests in metaphase approximately 3 hours
before ovulation. Meiosis II is completed only if the oocyte is fertilized; otherwise,
the cell degenerates approximately 24 hours after ovulation. The first
polar body also undergoes a second division.

Gene Mutations

Gene Mutations
Many congenital formations in humans are inherited, and some show a clear
mendelian pattern of inheritance. Many birth defects are directly attributable
to a change in the structure or function of a single gene, hence the name single
gene mutation. This type of defect is estimated to account for approximately
8% of all human malformations.
18 Part One: General Embryology
With the exception of the X and Y chromosomes in the male, genes exist
as pairs, or alleles, so that there are two doses for each genetic determinant,
one from the mother and one from the father. If a mutant gene produces an
abnormality in a single dose, despite the presence of a normal allele, it is a
dominant mutation. If both alleles must be abnormal (double dose) or if the
mutation is X-linked in the male, it is a recessive mutation. Gradations in the
effects of mutant genes may be a result of modifying factors.
The application of molecular biological techniques has increased our
knowledge of genes responsible for normal development. In turn, genetic
analysis of human syndromes has shown that mutations in many of these
same genes are responsible for some congenital abnormalities and childhood
diseases. Thus, the link between key genes in development and their role in
clinical syndromes is becoming clearer.
In addition to causing congenital malformations, mutations can result in
inborn errors of metabolism. These diseases, among which phenylketonuria,
homocystinuria, and galactosemia are the best known, are frequently accompanied
by or cause various degrees of mental retardation.
Diagnostic Techniques for Identifying Genetic Abnormalities
Cytogenetic analysis is used to assess chromosome number and integrity.
The technique requires dividing cells, which usually means establishing cell
cultures that are arrested in metaphase by chemical treatment. Chromosomes
are stained with Giemsa stain to reveal light and dark banding patterns
(G-bands; Fig. 1.6) unique for each chromosome. Each band represents 5 to
10×106 base pairs of DNA, whichmay include a fewto several hundred genes.
Recently, high resolution metaphase banding techniques have been developed
that demonstrate greater numbers of bands representing even smaller
pieces of DNA, thereby facilitating diagnosis of small deletions.
New molecular techniques, such as fluorescence in situ hybridization
(FISH), use specific DNA probes to identify ploidy for a few selected chromosomes.
Fluorescent probes are hybridized to chromosomes or genetic
loci using cells on a slide, and the results are visualized with a fluorescence
microscope (Fig.1.15). Spectral karyotype analysis is a technique in which
every chromosome is hybridized to a unique fluorescent probe of a different
color. Results are then analyzed by a computer.

C L I N I C A L C O R R E L A T E S - Birth Defects and Spontaneous Abortions:

Birth Defects and Spontaneous Abortions:
Chromosomal and Genetic Factors
Chromosomal abnormalities, which may be numerical or structural, are
important causes of birth defects and spontaneous abortions. It is estimated
that 50% of conceptions end in spontaneous abortion and that 50% of these

Figure 1.4 Events occurring during the first and second maturation divisions. A. The
primitive female germ cell (primary oocyte) produces only one mature gamete, the mature
oocyte. B. The primitive male germ cell (primary spermatocyte) produces four spermatids,
all of which develop into spermatozoa.
abortuses have major chromosomal abnormalities. Thus approximately 25%
of conceptuses have a major chromosomal defect. The most common chromosomal
abnormalities in abortuses are 45,X (Turner syndrome), triploidy,
and trisomy 16. Chromosomal abnormalities account for 7% of major birth
defects, and gene mutations account for an additional 8%.
Numerical Abnormalities
The normal human somatic cell contains 46 chromosomes; the normal gamete
contains 23. Normal somatic cells are diploid, or 2n; normal gametes
are haploid, or n. Euploid refers to any exact multiple of n, e.g., diploid or
triploid. Aneuploid refers to any chromosome number that is not euploid; it is
usually applied when an extra chromosome is present (trisomy) or when one
is missing (monosomy). Abnormalities in chromosome number may originate
during meiotic or mitotic divisions. In meiosis, two members of a pair
of homologous chromosomes normally separate during the first meiotic division
so that each daughter cell receives one member of each pair (Fig. 1.5A).
Sometimes, however, separation does not occur (nondisjunction), and both
members of a pair move into one cell (Fig. 1.5, B and C ). As a result of
nondisjunction of the chromosomes, one cell receives 24 chromosomes,
and the other receives 22 instead of the normal 23. When, at fertilization,
a gamete having 23 chromosomes fuses with a gamete having 24 or 22 chromosomes, the result is an individual with either 47 chromosomes
(trisomy) or 45 chromosomes (monosomy). Nondisjunction, which occurs
during either the first or the second meiotic division of the germ cells, may
involve the autosomes or sex chromosomes. In women, the incidence of
chromosomal abnormalities, including nondisjunction, increases with age,
especially at 35 years and older.

The Chromosome Theory of Inheritance

The Chromosome Theory of Inheritance
Traits of a new individual are determined by specific genes on chromosomes
inherited from the father and the mother. Humans have approximately 35,000
genes on 46 chromosomes. Genes on the same chromosome tend to be inherited
together and so are known as linked genes. In somatic cells, chromosomes
appear as 23 homologous pairs to form the diploid number of 46. There are
22 pairs of matching chromosomes, the autosomes, and one pair of sex chromosomes.
If the sex pair is XX, the individual is genetically female; if the pair is
XY, the individual is genetically male. One chromosome of each pair is derived
from the maternal gamete, the oocyte, and one from the paternal gamete, the
Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes 5
sperm. Thus each gamete contains a haploid number of 23 chromosomes, and
the union of the gametes at fertilization restores the diploid number of 46.
Mitosis is the process whereby one cell divides, giving rise to two daughter
cells that are genetically identical to the parent cell (Fig. 1.2). Each daughter
cell receives the complete complement of 46 chromosomes. Before a cell enters
mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During
this replication phase the chromosomes are extremely long, they are spread
diffusely through the nucleus, and they cannot be recognized with the light microscope.
With the onset of mitosis the chromosomes begin to coil, contract,
and condense; these events mark the beginning of prophase. Each chromosome
now consists of two parallel subunits, chromatids, that are joined at a
narrow region common to both called the centromere. Throughout prophase
the chromosomes continue to condense, shorten, and thicken (Fig. 1.2A),
but only at prometaphase do the chromatids become distinguishable
(Fig. 1.2B). During metaphase the chromosomes line up in the equatorial plane,

Figure 1.2 Various stages of mitosis. In prophase, chromosomes are visible as slender
threads. Doubled chromatids become clearly visible as individual units during
metaphase. At no time during division do members of a chromosome pair unite. Blue,
paternal chromosomes; red, maternal chromosomes.
6 Part One: General Embryology
and their doubled structure is clearly visible (Fig. 1.2C ). Each is attached by
microtubules extending from the centromere to the centriole, forming the mitotic
spindle. Soon the centromere of each chromosome divides, marking the
beginning of anaphase, followed by migration of chromatids to opposite poles
of the spindle. Finally, during telophase, chromosomes uncoil and lengthen,
the nuclear envelope reforms, and the cytoplasm divides (Fig. 1.2, D and E ).
Each daughter cell receives half of all doubled chromosome material and thus
maintains the same number of chromosomes as the mother cell.
Meiosis is the cell division that takes place in the germ cells to generate male
and female gametes, sperm and egg cells, respectively. Meiosis requires two cell
divisions, meiosis I and meiosis II, to reduce the number of chromosomes to
the haploid number of 23 (Fig. 1.3). As in mitosis, male and female germ cells
(spermatocytes and primary oocytes) at the beginning of meiosis I replicate
their DNA so that each of the 46 chromosomes is duplicated into sister chromatids.
In contrast to mitosis, however, homologous chromosomes then align
themselves in pairs, a process called synapsis. The pairing is exact and point
for point except for the XY combination. Homologous pairs then separate into
two daughter cells. Shortly thereafter meiosis II separates sister chromatids.
Each gamete then contains 23 chromosomes.
Crossovers, critical events in meiosis I, are the interchange of chromatid segments
between paired homologous chromosomes (Fig. 1.3C ). Segments of
chromatids break and are exchanged as homologous chromosomes separate.
As separation occurs, points of interchange are temporarily united and form an
X-like structure, a chiasma (Fig. 1.3C ). The approximately 30 to 40 crossovers
(one or two per chromosome) with each meiotic I division are most frequent
between genes that are far apart on a chromosome.
As a result of meiotic divisions, (a) genetic variability is enhanced through
crossover, which redistributes genetic material, and through random distribution
of homologous chromosomes to the daughter cells; and (b) each germ cell
contains a haploid number of chromosomes, so that at fertilization the diploid
number of 46 is restored.
Polar Bodies
Also during meiosis one primary oocyte gives rise to four daughter cells, each
with 22 plus 1 X chromosomes (Fig. 1.4A). However, only one of these develops
into a mature gamete, the oocyte; the other three, the polar bodies, receive
little cytoplasm and degenerate during subsequent development. Similarly, one
primary spermatocyte gives rise to four daughter cells, two with 22 plus 1

Figure 1.3 First and second meiotic divisions. A. Homologous chromosomes approach
each other. B. Homologous chromosomes pair, and each member of the pair consists of
two chromatids. C. Intimately paired homologous chromosomes interchange chromatid
fragments (crossover). Note the chiasma. D. Double-structured chromosomes pull apart.
E. Anaphase of the first meiotic division. F and G. During the second meiotic division,
the double-structured chromosomes split at the centromere. At completion of division,
chromosomes in each of the four daughter cells are different from each other.
X chromosomes and two with 22 plus 1 Y chromosomes (Fig. 1.4B). However,
in contrast to oocyte formation, all four develop into mature gametes.

Primordial Germ Cells

Development begins with fertilization, the process
by which the male gamete, the sperm, and the
female gamete, the oocyte, unite to give rise to a zygote.
Gametes are derived from primordial germ cells (PGCs)
that are formed in the epiblast during the second week
and that move to the wall of the yolk sac (Fig. 1.1). During
the fourth week these cells begin to migrate from the yolk
sac toward the developing gonads, where they arrive by the
end of the fifth week. Mitotic divisions increase their number
during their migration and also when they arrive in the gonad.
In preparation for fertilization, germ cells undergo gametogenesis,
which includes meiosis, to reduce the number of chromosomes and
cytodifferentiation to complete their maturation.
Primordial Germ Cells (PGCs) and Teratomas
Teratomas are tumors of disputed origin that often contain a variety
of tissues, such as bone, hair, muscle, gut epithelia, and others. It is
thought that these tumors arise from a pluripotent stem cell that can
differentiate into any of the three germ layers or their derivatives.

Figure 1.1 An embryo at the end of the third week, showing the position of primordial
germ cells in the wall of the yolk sac, close to the attachment of the future umbilical
cord. From this location, these cells migrate to the developing gonad.
Some evidence suggests that PGCs that have strayed from their normal migratory
paths could be responsible for some of these tumors. Another source
is epiblast cells migrating through the primitive streak during gastrulation