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 chro-matin (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

Antrum Fluid

Figure 1.19 A. Secondary (antral) stage follicle. The oocyte, surrounded by the zona pellucida, is off-center; the antrum has developed by fluid accumulation between intercellular spaces. Note the arrangement of cells of the theca interna and the theca externa. B. Mature secondary (graafian) follicle. The antrum has enlarged considerably, is filled with follicular fluid, and is surrounded by a stratified layer of granulosa cells. The oocyte is embedded in a mound of granulosa cells, the cumulus oophorus. C. Photomicrograph of a mature secondary follicle with an enlarged fluid-filled antrum (cavity, Cav) and a diameter of 20 mm (x65). CO, cumulus oophorus; MG, granulosa cells; AF, atretic follicle.

Figure 1.19 A. Secondary (antral) stage follicle. The oocyte, surrounded by the zona pellucida, is off-center; the antrum has developed by fluid accumulation between intercellular spaces. Note the arrangement of cells of the theca interna and the theca externa. B. Mature secondary (graafian) follicle. The antrum has enlarged considerably, is filled with follicular fluid, and is surrounded by a stratified layer of granulosa cells. The oocyte is embedded in a mound of granulosa cells, the cumulus oophorus. C. Photomicrograph of a mature secondary follicle with an enlarged fluid-filled antrum (cavity, Cav) and a diameter of 20 mm (x65). CO, cumulus oophorus; MG, granulosa cells; AF, atretic follicle.

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 double-structured 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 cell

Zona pellucida

Granulosa cells

Secondary oocyte in division

Zona pellucida

Granulosa cells

Secondary oocyte in division

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