Gametogenesis and Oogenesis Overview

Gametogenesis

• Gametes are derived from primordial germ cells (PGCs) formed in the epiblast during the second week of development
• PGCs migrate to the wall of the yolk sac
• PGCs migrate from the yolk sac to the developing gonads during the fourth week
• During migration, PGCs undergo mitosis to increase their numbers

Oogenesis Overview

• Oogenesis begins with primordial germ cells differentiating into oogonia
• Oogonia undergo mitotic divisions surrounded by flat epithelial cells (follicular cells)
• By the end of the third month of pregnancy, each ovary contains 7 million oogonia
• Some oogonia enter prophase I of meiosis I and become primary oocytes
• By the end of the fifth month, primary oocytes arrest their cell division in prophase of meiosis I and form primary oocytes
• By the seventh month, most oogonia degenerate, except for some near the ovarian surface

Oogenesis - Postnatal Maturation

• Primary oocytes remain in prophase I until puberty due to an oocyte maturation inhibitor (OMI) secreted by the follicular cells
• At birth, there are approximately 700,000 - 2 million primary oocytes
• During childhood, the number decreases to around 400,000
• Only a few hundred oocytes (less than 500) will ovulate during a woman’s lifetime
• At puberty, 15-20 primordial follicles mature each ovarian cycle
• One follicle continues to develop into a growing primary, secondary, and Graafian follicle
• The rupture of the Graafian follicle releases the ovum, which is captured by the fallopian tube fimbria
• The primary oocyte completes meiosis I and forms two daughter cells: a secondary oocyte and a first polar body
• The secondary oocyte receives most of the cytoplasm while the first polar body receives very little
• Both the secondary oocyte and first polar body have 23 chromosomes (1n) and double the amount of DNA (2N)
• The secondary oocyte enters meiosis II but arrests in metaphase II about 3 hours before ovulation
• Meiosis II is completed only if fertilization occurs; otherwise, the oocyte degenerates within 24 hours
• The first polar body also enters meiosis II without DNA replication

Spermatogenesis Overview

• Begins with spermatogonia in the sex cords of the testis, which differentiate into spermatogonia Type A and Type B
• Spermatogonia Type A divide and give rise to Type B spermatogonia
• Type B spermatogonia undergo mitosis to produce primary spermatocytes
• Primary spermatocytes enter a prolonged prophase of meiosis I (22 days)

Spermatogenesis - Meiosis I & II

• Meiosis I is completed rapidly, resulting in the formation of secondary spermatocytes with 23 chromosomes (1n) and double the amount of DNA (2N)
• Secondary spermatocytes immediately enter meiosis II to form spermatids

Spermatogenesis - Spermiogenesis

• Cytokinesis is not completed during the progression from type A spermatogonia to spermatids, forming a cluster of cells connected during differentiation
• Spermatids remain in contact with Sertoli cells for support and protection
• Spermiogenesis transforms rounded spermatids into elongated, motile sperm cells

Spermiogenesis Changes

• Formation of an acrosome, which helps penetrate the ovum
• Condensation of the nucleus
• Development of a neck, middle piece, and tail
• Shedding of most of the cytoplasm

Learn about the intricate processes of gametogenesis and oogenesis, including the migration of primordial germ cells and the stages of oocyte development. This quiz covers the timeline from the formation of primordial germ cells to the maturation of oocytes postnatally. Test your knowledge on these critical concepts in reproductive biology.