General embryology : Oogenesis and ovarian cycle

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

oocytes.

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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.

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

.

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