FIRST YEAR EMBRYOLOGY: GAMETOGENESIS

BY MUNEEB HASAN KHAN

 

GAMETOGENESIS

“Biological process by which specialized, diploid cells undergo meiotic cell divisions to form mature, haploid gametes, which are specialized generative cells.”

·         Human gametes arise in embryonic life from Progenitor Germ Cells (PGCs) that:  Form in epiblast in 2^nd week.

Ø  Migrate through primitive streak and into endoderm of umbilical vesicle during gastrulation.

Ø  Migrate to gonadal regions starting from 4^th and arriving by end of 5^th week.

Ø  Increase in number by mitotic division

Ø  Remain dormant during Infancy

Ø  Undergo gametogenesis and cytodifferentiation to form gametes at puberty.

 

SPERMATOGENESIS:

·         Male gamete formation is a continuous process that begins at puberty and continues throughout life.

·         One cycle completes in 74 days and approximately 300 million sperms are produced daily.

·         In seminiferous tubules, PGCs in germinal epithelium give rise to spermatogonia that are dormant in fetal life and infancy.

By puberty, 3 types are seen:

1)      Type A, Dark – reserve cells that produce a clone of pale Type A cells.

2)      Type A, Pale – produce type B cells as well as increasing their own number. 3) Type B –divide mitotically into daughter cells.

• These daughter cells enlarge to form diploid primary spermatocytes, largest cells in the region – which enter a prolonged prophase (22 days)
• Then they rapidly undergo 1^st meiotic division to form 2 haploid secondary spermatocytes.
• These undergo 2^nd meiotic division to form 4, nonmotile haploid spermatids.
• Until this stage, all progeny of single type A cell clone connected by cytoplasmic bridges, due to incomplete cytokinesis.
• Spermatids differentiate into 4 sperms by spermiogenesis, involving:

Ø  Acrosome formation from Golgi apparatus

Ø  Condensation of nucleus

Ø  Shedding of most of cytoplasm as residual bodies, which are phagocytosed by Sertoli cells.  Flagellum/Tail formation from centrioles.

·         Mature Spermatozoa are haploid, motile cells with 22 autosomes and either X or Y sex chromosome. They have 3 parts:

1) Head – contains nucleus and acrosome (cap-like, saccular organelle that covers anterior 2/3^rd of head & contains enzymes that help in penetration of ovum) 2) Neck – junction point of head and tail.

3) Tail – confers mobility and assists in transport & penetration. Has 3 segments:

a. Middle piece (contains mitochondria in a sheath for energy supply). b. Principle piece

c. End piece

 

Supporting cells & Hormonal Control: Include

·         Leydig cells –produce testosterone in response to LH from pituitary gland.

·         Sertoli cells – line the seminiferous tubules.

Ø  Bind testosterone to promote spermatogenesis.

Ø  Bind follicle-stimulating hormone to stimulate testicular fluid production.

Ø  Support, protect, nourish, and assist in release of sperms.

 

OOGENESIS:

PRENATAL DEVELOPMENT:

·         PGCs in gonadal region differentiate into oogonia, that proliferate mitotically – reaching peak number of `7 million at about 5^th fetal month.

·         About 2 million of these enlarge and form primary oocytes before, which are:

o   Arranged near surface of ovary

o   Surrounded by a layer of squamous cells i.e. primordial follicle (derived from ovarian surface epithelium and connective tissue)  

o   Acquire zona pellucida (glycoproteinaceous layer secreted by both oocyte and follicle) o Begin first mitotic division that is arrested in diplotene stage of prophase I until adolescence due to production of oocyte maturation inhibitor by follicular cells.

o   Cannot perform mitosis (hence do not proliferate after birth)

·         Remaining 5 million oogonia degenerate by atresia and are neither present at nor formed after birth.

 

POSTNATAL MATURATION:

·         Majority of primary oocytes regress during infancy and only 40,00 remain by adolescence.

·         About 400 of these undergo further maturation & ovulation usually at rate of once per ovarian cycle.

·         A pool of 15-20 growing follicles is established and maintained in ovary; usually 1 matures while the rest die by atresia, forming scar tissue i.e. corpus atreticum.

·         Following events occur in maturing follicle:

1.      Primary oocyte completes 1^st meiotic division in response to an LH surge near ovulation to form 2 haploid cells:

a)      Secondary oocyte – receives most of the cytoplasm

b)      1^st Polar Body – a small cell destined for degeneration.

2.      Secondary oocyte undergoes second meiotic division that proceeds upto metaphase, at which point ovulation occurs. 2 things can happen to it:

a)      If unfertilized, it degenerates and menstrual phase occurs.

b)      If fertilized, meiosis completes and results in formation of mature oocyte and 2^nd polar body.

3.      Both polar bodies are extruded into perivitelline space.

4.      Primordial follicle matures into Graafian follicle, with its cells becoming first cuboidal then columnar, & formation of granulosa, fluid-filled antrum and external theca folliculi.

Supporting Cells & Hormonal Control:

Hypothalamus forms gonadotrophin-releasing hormone at puberty that passes to anterior pituitary and causes it to release 2 hormones that regulate the ovarian cycle:

1)      Follicle-stimulating hormone:

i.         Stimulates growth of ovarian follicles

ii.      Promotes estrogen production by follicular cells and interstitial glands of ovary.

2)      Luteinizing hormone:

i.       Triggers ovulation by an upsurge in its levels

ii.      Inhibits FSH secretion to prevent anymore follicles from growing iii. Stimulates progesterone secretion from corpus luteum.