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 5th week)
Differentiate into oogonia ( at5th
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 1st polar body receives
very little.
The secondary oocyte begins 2nd
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
- Menstrual phase
- Proliferative(follicular) phase
- 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
