Oogenesis and Gametogenesis

• Study notes on gamete formation, focusing on oogenesis

Gametogenesis

• Gametogenesis is the conversion of germ cells into male and female gametes.
• Male gametes are sperm, female gametes are oocytes, and a zygote is the product of their union.
• Somatic cells contain 46 chromosomes which are 23 pairs, including 22 pairs of autosomes and one pair of sex chromosomes.
• Each gamete contains a haploid number of 23 chromosomes.

Phases of Gametogenesis

• Gametogenesis involves four phases:
• Origin and migration of germ cells from the endodermal layer of the yolk sac to the primordia of the gonads via the dorsal mesentery.
• Increase in the number of germ cells by mitosis.
• Reduction in chromosomal number by meiosis.
• Structural and functional maturation of eggs and sperm.

Mitosis and Meiosis

• Mitosis is the cell division process that results in two daughter cells genetically identical to the parent cell.
• Meiosis occurs in germ cells in both males and females producing gametes: sperm and egg cells.
• Meiosis requires two cell divisions: Meiosis I and Meiosis II.

Chromosomal Abnormalities

• These can be numerical or structural.
• Numerical abnormalities:
• Non-disjunction results in daughter cells with unequal chromosome numbers.
• Trisomy: one daughter cell with three copies of a chromosome.
• Monosomy: the other with only one chromosome.
• Structural abnormalities:
• Do not affect the total chromosome number but have serious consequences.
• Translocation: A part of a chromosome breaks off and attaches to another, potentially exchanging segments between chromosomes.
• Deletion: A segment of a chromosome is lost, which occurs at chromosome 5 to form cri-du-chat syndrome.

Early Development of Gametes

• In human embryos, undifferentiated primordial gonadal cells (PGC) can be seen in the epithelium of the yolk sac at 3 weeks.
• By 4 weeks PGCs migrate from the yolk sac to the genital ridges and proliferate by mitosis.
• PGCs induce the genital ridge to differentiate into a primitive germinal epithelium that becomes embedded in it, forming primary sex cords.
• The gonads then become histologically distinct and bipotent, potentially becoming testis or ovaries.
• By 8 weeks ovaries and testes are histologically distinct.
• In males, products directed by the SRY gene activation cause undifferentiated sex cords to enlarge, split, and form the primitive testis, beginning PGC differentiation into spermatogonia.
• In females, PGCs begin to differentiate into oogonia within follicles.

Gametogenesis: Spermatogenesis vs Oogenesis

• Spermatogenesis produces sperm by spermatogonium differentiating into spermatozoa.
• Oogenesis produces ova by oogonium differentiating into ovum.

Oogenesis

• Immature oocytes are distributed throughout the ovary in follicles
• Oocytes vary in size and composition depending on the differentiated state.
• Each human ovary has 200,000 to 400,000 follicles, each with a single oogonium.
• Few oogonia reach maturity, with typically fewer than 400 oocytes maturing and being released by ovulation in humans.
• The location of oocytes during oogenesis depends on its maturity.
• Primordial follicles are most prevalent in the stroma which is near the ovaries outside.
• Oocytes become more centrally located in the ovaries as they mature.

Events During Oogenesis

• Primordial germ cells migrate to the ovary during fetal and embryonic development.
• Differentiation of primordial germ cells into an unfertilized egg requires proliferation, growth in size, and increase in number.

Follicle Development

• Primordial Germ Cells are detectable at 4 weeks of development, migrating from the allantois to the genital ridge (Oogonia).
• In early Primary Follicle stage it develops at puberty with the oocyte completes growth, and includes the zona pellucida.
• Primordial Follicle:
• Consists of a single layer of follicle cells, contain a primary oocyte, is arrested in Prophase I, and is present at birth.
• Primary Follicle (late) has multiple layers of follicle cells and is FSH induced.

Hormonal Control

• Hormonal control of oogenesis involves the pituitary gland which releases LH and FSH.
• FSH promotes growth of follicles and the LH receptors on follicles.
• LH: causes estrogen production by follicles and oocyte maturation.
• Estrogen: causes proliferation of endometrium, thinning of cervical mucus, increased FSH follicle receptors, and release of inhibin.
• Corpus luteum causes progesterone secretion which effects the uterine wall and FSH inhibition.