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.