Describe Fertilization and gametogenesis OBG

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Gametogenesis and Fertilization

Sources: The Developing Human: Clinically Oriented Embryology (Moore et al.) & Langman's Medical Embryology

GAMETOGENESIS

Gametogenesis is the process of formation and development of specialized sex cells (gametes — oocytes and spermatozoa) from bipotential primordial germ cells (PGCs). It prepares the gametes for fertilization by:

Reducing the chromosome number from diploid (46) to haploid (23) via meiosis
Altering cell morphology (cytodifferentiation)

Primordial Germ Cells (PGCs)

Formed in the epiblast during the 2nd week
Move through the primitive streak during gastrulation → migrate to the wall of the yolk sac
During the 4th week, migrate toward the developing gonads (arriving by end of the 5th week)
Mitotic divisions increase their number during migration
Misplaced PGCs may give rise to teratomas (tumors containing derivatives of all three germ layers)

— The Developing Human, p. 75

MEIOSIS — Key Points

Meiosis involves two successive cell divisions:

Division	What happens
Meiosis I (Reduction division)	Homologous chromosomes pair in prophase, then separate → two haploid (secondary) cells, each with 23 double-chromatid chromosomes
Meiosis II (Equational division)	Each double-chromatid chromosome splits; chromatids drawn to opposite poles → four haploid cells

Key functions of meiosis:

Maintains constant chromosome number across generations (46 → 23)
Random assortment of maternal/paternal chromosomes
Crossing over during prophase I → genetic recombination

SPERMATOGENESIS

Fig: Normal Spermatogenesis — The Developing Human

Location: Seminiferous tubules of the testes
Onset: Puberty; continuous throughout adult life

Stages:

Spermatogonium (46,XY) — stem cell; undergoes mitosis to replenish itself and produce primary spermatocytes
Primary spermatocyte (46,XY) — undergoes Meiosis I → 2 secondary spermatocytes (23,X and 23,Y)
Secondary spermatocytes (23,X or 23,Y) — undergo Meiosis II → 4 spermatids
Spermatids → mature spermatozoa via spermiogenesis (nuclear condensation, acrosome formation, flagellar development)

Net yield: 1 primary spermatocyte → 4 functional spermatozoa

Sperm structure: Head (contains acrosome with hydrolytic enzymes + nucleus), midpiece (mitochondria), and tail (flagellum for motility)

Two types of sperm: 23,X and 23,Y — determines genetic sex of the embryo
Up to 10% of ejaculated sperm are morphologically abnormal; those >20% abnormal may affect fertility

OOGENESIS

Fig: Normal Oogenesis — The Developing Human

Location: Ovary
Unique feature: Begins before birth; completed only after fertilization

Stages:

Stage	Timing	Chromosome status
Oogonia proliferate by mitosis	Fetal life (2nd–5th month)	46,XX
Primary oocytes (enter Meiosis I)	By birth (~2 million); arrested in prophase I	46,XX
Meiosis I resumes → secondary oocyte + 1st polar body	Puberty onwards, just before ovulation (LH surge)	23,XX
Secondary oocyte ovulated; arrested at metaphase II	Each menstrual cycle	23,X
Meiosis II completed + 2nd polar body expelled	Only if fertilized by a sperm	23,X
Mature oocyte (female pronucleus forms)	After fertilization	Haploid (23,X)

Net yield: 1 primary oocyte → 1 mature oocyte + 3 polar bodies (polar bodies are non-functional and degenerate)

The asymmetric cytoplasm division conserves cytoplasm in the large oocyte — essential for supporting early embryonic development.

Follicular Development (associated with oogenesis):

Primordial follicle: Primary oocyte + single layer of flat follicular cells
Primary follicle: Columnar follicular (granulosa) cells + zona pellucida formation
Secondary follicle: Multiple granulosa cell layers + theca interna/externa develop + antrum formation
Mature (Graafian) follicle: Large antrum; oocyte surrounded by cumulus oophorus + corona radiata
FSH drives growth of 5–12 primordial follicles; only one typically reaches maturity and ovulates

FERTILIZATION

Definition: The process by which sperm and secondary oocyte unite to form a zygote.

Site: Usually the ampulla of the uterine tube (the widest part)

Timing: Oocyte is viable for ~24 hours post-ovulation; sperm viable for ~48–72 hours in the female tract

Pre-Requisites Before Fertilization

Capacitation — occurs in the female reproductive tract (~7 hours):

Sperm surface glycoproteins and seminal plasma proteins are removed
Sperm plasma membrane altered → becomes capable of the acrosome reaction
Sperm motility changes to a hypermotile "whiplash" pattern

Sperm Transport:

Of 200–300 million sperms deposited in the vagina, only 300–500 reach the fertilization site
Transported by muscular contractions of the uterine wall and flagellar movement
Chemical signals (attractants) from the oocyte guide capacitated sperm (sperm chemotaxis)

Phases of Fertilization

Molecular events of fertilization showing acrosome reaction, sperm-oocyte fusion, cortical reaction with Ca²⁺ wave, and pronucleus formation

Fig: Molecular events in fertilization — The Developing Human

Phase 1 — Passage through the Corona Radiata

Hyaluronidase released from the acrosome disperses the follicular cells of the corona radiata
Tail movements of the sperm assist penetration

Phase 2 — Penetration of the Zona Pellucida

The sperm binds to ZP3 (zona pellucida glycoprotein) via SED1 protein
Acrosome reaction triggered: hydrolytic enzymes (esterase, acrosin, neuraminidase) are released
Acrosin (proteolytic) is the most important enzyme; lysed a path through the zona
Once one sperm penetrates → Zona reaction (block to polyspermy):
- Cortical granules release lysosomal enzymes into the perivitelline space
- These digest sperm receptors ZP2 and ZP3, making the zona impermeable to further sperm

Phase 3 — Fusion of Cell Membranes

Plasma membranes of sperm and oocyte fuse
Head and tail of sperm enter oocyte cytoplasm; sperm's plasma membrane and mitochondria remain behind (mitochondria are maternally inherited)
Phospholipase C-zeta from the sperm causes a wave of Ca²⁺ release, reactivating cell cycling in the oocyte

Phase 4 — Completion of Meiosis II of the Oocyte

Sperm penetration activates the secondary oocyte to complete Meiosis II
Results in a mature oocyte + 2nd polar body
Maternal chromosomes decondense → female pronucleus forms

Phase 5 — Formation of the Male Pronucleus

Sperm nucleus enlarges within oocyte cytoplasm → male pronucleus
Sperm tail degenerates
Both pronuclei replicate their DNA (1n, 2c)
The oocyte at this stage is called an ootid

Phase 6 — Syngamy (Fusion of Pronuclei) → Zygote

Male and female pronuclei fuse → diploid chromosome complement (46)
Chromosomes arrange on a cleavage spindle in preparation for first mitotic division
The cell is now a zygote — the first cell of the new individual

Results of Fertilization

Result	Significance
Restoration of diploid number (46)	Genetic continuity maintained
Determination of chromosomal sex	23,X sperm → female (46,XX); 23,Y sperm → male (46,XY)
Initiation of cleavage	First mitotic division of zygote begins
Genetic uniqueness	New combination of maternal + paternal chromosomes
Completion of meiosis II	Oocyte matures only upon fertilization

Abnormal Gametogenesis

Nondisjunction — failure of chromosome pairs to separate during meiosis:

Results in gametes with 24 chromosomes (trisomy if fertilized) or 22 (monosomy)
Trisomy 21 (Down syndrome) is the classic example; risk increases with maternal age
Ideal maternal age for reproduction: 20–35 years
Paternal age also matters — advanced age increases fresh gene mutations in offspring

— The Developing Human: Clinically Oriented Embryology, pp. 75–124; Langman's Medical Embryology, pp. 34–40

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