General embryology : PART 2 Gemetogenesis

 

Gametogenesis

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It is the process of formation and development of male and female gametes.

·         Gametes are derived from primordial germ cells.

·          It is formed in the epiblast during the 2^nd  week

·         Then  move to the wall of the yolk sac  during the 4^th week ,

·         these cells begin to migrate from the yolk sac to the developing gonad, where they arrive by the end of the 5^th week

Oogenesis:it is the sequence of events by which oogonia are transformed into mature

oocytes.

This maturation begins before birth and completed after puberty and continues to the menopause (the permanent cessation of the menses)

Maturation of oocytes:

Before birth

Primordial germ cells (arrived in the gonad by the end of the 5^th week)

Differentiate into oogonia ( at5^th month its number reaches maximum about 7 million)

Primary oocytes : oogonia enlarge to form primary oocytes.

Graafian follicle

It is the mature form of ovarian follicle containsingle cavity within the granulosa cells

Shortly before ovulationthe primary oocyte completes thefirst meiotic division and became secondary oocyte.

The division is unequal, the secondary oocyte receives almost all the cytoplasm but the 1^st polar body receives very little.

The secondary oocyte begins 2^nd meiotic division but arrest at metaphase.

Meiosis II is completed only if the oocyte is fertilized.

 

Ovarian cycle

It is a regular monthly cyclic change in the ovaries (which start at puberty) comprising the development of ovarian follicles, ovulation and formation of corpus luteum constitute the ovarian cycle.

It has three phases are

i) Follicular phase: follicular development

ii) LH surge (ovulation)

iii) Luteal phase

•       Correlation between ovarian and menstrual cycle

Ovarian cycle	Development / event occurred	Hormone	Duration(Normal duration 28 days )	Menstrual cycle (Normal duration 28 days)
Follicular phase	Growth of ovarian follicles by FSH Follicle stimulating hormone secreted from anterior pituitary	Growing ovarian follicle secreted estrogen	14-21 days before ovulation	Menstrual phase 1-4 days (Without fertilization degeneration of corpus luteum is occurred so less progesterone is produce which  causebreak down of endometrium.) Proliferative phase (5-14 days )proliferation of endometrium by estrogen secreted from growing ovarian follicles
LH surge	Ovulation occurred due to sudden increase secretion of LH (luteinizing hormone) from anterior pituitary		24 hrs
Luteal phase	Development of corpus luteum by LH	Luteinizing hormone, develop corpus luteum which secrete estrogen and progesterone.	14 days after ovulation (14-28 days)	Secretory phase (From 14-28 days) progesterone promotes growth of endometrium by increasing proliferation, cellular hypertrophy, and deposition of extracellular matrix and uterus ready for implantation. Corpus luteum produce progesterone, inhibitin and estradiol .

SN:  An LH surge is a rapid increase of the luteinizing hormone (LH) in a woman's bloodstream that occurs 24-48 hours prior to ovulation which is responsible for  release of a mature egg around the second week of each menstrual cycle

•       At luteal phase of ovarian cycle, estrogen level in the body become high. This is known as a "secondary estrogen surge.

Menstrual cycle :

•       The endometrium of uterus undergoes monthly cyclic changes for implantation of zygote during reproductive life of a woman called menstrual cycle.

The phases of menstrual cycle

 

1. Menstrual phase
2. Proliferative(follicular)  phase
3. Secretory (progestational) phase

 

How the menstrual cycle is controlled?

 

Both ovarian and menstrual cycle is controlled by the Hypothalamic-Pituitary-Gonadal (HPG) Axis.

Gonadotropin releasing hormone (GnRH) from the hypothalamus stimulates luteinizing hormone (LH) and follicular stimulating hormone (FSH) release from the anterior pituitary gland.

LH and FSH are gonadotropins that act primarily on the ovaries in the female reproductive tract:

·         FSH binds to granulosa cells to stimulate follicle growth, permit the conversion of androgens (from theca cells) to oestrogens and stimulate inhibin secretion

·         LH acts on theca cells, cause ovulation, growth of corpus luteum which produce progesterone, inhibin A and estradiol.

The menstrual cycle is controlled by both positive and negative feedback systems:

·         High oestrogen levels (in the absence of progesterone) positively feedback on the HPG axis.

·         Moderate oestrogen levels exert negative feedback on the HPG axis

·         Oestrogen in the presence of progesterone exerts negative feedback on the HPG axis

·         Inhibin selectively inhibits FSH at the anterior pituitary

1. Hypothalamus secrete GnRH > act on anterior pituitary > anterior pituitary secrete FSH > FSH act on ovary > FSH developed ovarian follicles > ovarian follicle secrete estrogen > estrogen cause proliferation of  endometrium of uterus, this phase known as  proliferative phase of menstrual cycle.

2. After ovulation corpus luteum start to grow and secrete inhibitin which selectively inhibit anterior pituitary to secrete FSH > so no further development of ovarian follicles

3. Well developedcorpus luteum also secrete progesterone promotes growth of endometrium by increasing proliferation, cellular hypertrophy, and deposition of extracellular matrix and uterus ready for implantation, this phase known as secretory phase of menstrual cycle.

Spermatogenesis

Definition: It is the sequence of events by which spermatogonia are transformed into mature sperms or spermatozoa.

This maturation process begins at puberty and continues into old age

In male the primordial germ cells remain dormant until puberty.

At puberty, after several mitotic divisions of the spermatogonia, primary spermatocytes (diploid)  are formed. They are the largest cells of seminiferous tubules and enters a prolonged prophase (22 days)

Each primary spermatocyte is completed meiosis Ito form two haploid secondary spermatocytes

Subsequently, two secondary spermatocytes complete the meiosis II to form four spermatids

Spermiogenesis

The spermatids are gradually transforms into mature sperm or spermatozoa by a process known as spermiogenesis.

These changes include:

1. Loss of cytoplasm.
2. Formation of acrosomes which covers half the nuclear surface, derived from the golgi region of spermatid contain enzymes that assist penetration of corona radiata and zona pellucida during fertilization
3. Condensation of nucleus
4. Formation of neck, middle piece, and tail
5. The mitochondria arranged in the middle piece as mitochordial sheath

The time required for a spermatogonium to become a mature spermatozoon is approximately74 days

 Difference between oogenesis and spermatogenesis

Location	Testes	Ovary
Number of gamete produce	Life long production (millions)	Fixed amount ( only mature 400)
Per day gemate produce	Millions sperm produce per day	One ovum  per 28 days cycle
Gamete per germ cells	Four	One ovum and 2-3 polar body
Beginning of the process	At puberty	At fetal life
Time of gamete formation	Continuous	Once in a month
End of process	Life long but reduces in later part of life	End at menopause
Time of gamete release	Anytime	Once in a month
Meiotic division	Uninterrupted	Arrested
Name of immature cell	Oogonia	Spermatogonia
Name of mature cell	Ovum	Spermatozoa
Release of germ cell	During ejaculation	During ovulation
Motility of mature germ cell	Motile	Not motile

 

Primordial follicle – a primary oocyte with its surrounding flattened follicular epithelium.

Near the birth, the primary oocyte remainarrested in prophase of meiosis I(diplotene stage) and do not finished meiosis I before puberty

At puberty

Primary follicle: follicular cell became cuboidal and zona pellucida begin to form

Growing follicle: follicular cells forming the stratified layer of granulose cells and well-defined zona pellucida

Secondary follicle: fluid filled spaces developed within the granulosacells