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SCOPE
• Oogenesis
• Phases of the menstrual cycle
• Regulation of the menstrual cycle
Oogenesis
• Involves
– Differentiation of oogonia into mature oocytes
– Begins before birth and is completed after
puberty
– Continues to menopause
Oogenesis
Prenatal maturation of oocytes OOGENESIS
• Begins before birth - Primordial germ cells migrate from yolk sac to the developing in ovary
• Differentiation of primordial germ cells into oogonia begins shortly after primordial germ cells arrive in the ovary
• Early fetal life, Oogonia prolifrerate by mitosis
OOGENESIS CONTD…
• By about 5 months of gestation, ovaries
contain about 6 -7 million oogonia
• Production of oogonia stops & never
resumes, but cell death begins – atresia
• Some of the oogonia differentiate into
primary oocytes
• Primary oocytes is surrounded by
flattened follicular epithelial cells and
zona pellucida
OOGENESIS CONTD…
• A primary oocyte together with its surrounding flat epithelial cells is called primordial follicle
• Primary oocytes begin first meiotic division( prophase I of meiosis ) before birth
• By 7th month all surviving primary oocytes have entered 1st meiotic division
• Oogenesis is arrested by oocyte maturation inhibitor at prophase I (diplotene stage) until just be4 ovulation
Postnatal maturation of oocytes
• Completion of first meoitic
division occurs shortly after birth
and becomes a secondary oocyte
• At ovulation, Secondary oocyte
begins second meiotic division
and is arrested at metaphase II
until penetration of sperm to
become functional ovum
Postnatal maturation of oocytes
• At birth - about 700,000 - 2 million
primary oocytes
• At puberty - about 400,000
• Only < 500 become secondary
oocytes and get ovulated
SEXUAL CYCLES IN FEMALES
• Ovarian cycle
Follicular phase
Luteal phase
• Uterine (menstrual) cycle
Menstrual phase
Proliferative phase
Secretory phase
OVARIAN CYCLE - 28 DAYS
• It is the monthly series of
events associated with
maturation of an egg.
• It has 2 consecutive phases:
1. Follicular Phase - period of
follicular growth (day 1 - 14)
2. Luteal phase - period of corpus
luteum activity (days 14 - 28)
• Typical ovarian cycle repeats at
intervals of 28 days with
ovulation occuring midcycle
FOLLICULAR PHASE
• Days 1 - 14
• When primordial follicles are
activated, squamouslike cells
surrounding primary oocyte grow to
become cuboidal & stratified
granulosa
• Follicle is now called primary follicle
• Next, zona pellucida is formed by
granulosa cells & primary oocyte
• Clear liquid accumulates between
granulosa cells forming a cavity called
antrum - the follicle is now known as
secondary follicle
FOLLICULAR PHASE CONTD…
• At maturity, follicle is called Graafian,
vesticular or tertiary follicle (10 mm or
more in diameter)
• It bulges on the ovarian surface like an
“angry boil” (stigma) - on day 14
• Meiosis resumes 10 to 12 hrs be4
ovulation
• Ovulation of secondary oocyte follows,
stimulated by LH which increases
contraction of ovarian wall
• Ovulated oocyte is surrounded by zona
pellucida & granulosa cells that form
corona radiata
LUTEAL PHASE
Days 14 – 28
• After ovulation, ruptured follicle
becomes corpus luteum under
the influence of LH
• Corpus luteum begins to
produce progesterone & some
estrogen
• If pregnancy fails, corpus
luteum degenerates & what
remains is corpus albicans
(scar)
• If pregnancy succeeds, corpus
luteum persists until placenta is
ready to take hormonal duties
HORMONAL REGULATION OF
THE OVARIAN CYCLE • The Gonadotropin-releasing hormone
(GnRH), the pituitary gonadotropins
(FSH, LH) & ovarian estrogen and
progesterone interact to produce cyclic
events in ovaries
HORMONAL REGULATION OF THE
OVARIAN CYCLE
OVARIAN CHANGES & HORMONAL
SECRETION
UTERINE (MENSTRUAL) CYCLE – 28
DAYS
• It is a series of cyclic
changes that
endometrium goes
through monthly as it
responds to changing
ovarian hormonal
levels in blood
• It has 3 phases:
Menstrual
Proliferative
Secretory
MENSTRUAL PHASE
Days 1-5
• Uterus sheds its functional endometrium layer
• Due to decline in ovarian hormone levels, endometrium
enters ischemic phase (reduction in blood supply) -
experiences hypoxia.
• Functional layer of endometrium detaches from uterine
wall accompanied by bleeding for 3-5 days
• Detatched tissue & blood pass out as menstrual flow -
marking the beginning of a new cycle
UTERINE CYCLE
UTERINE CYCLE
PROLIFERATIVE PHASE Days 6 -14
• Also called estrogenic or follicular phase
• Endometrium rebuilds itself
• Under the influence of rising levels of estrogen; endometrium generates new functional layer
• Endometrium becomes thick & well vascularized
• Estrogen also induces synthesis of progesterone receptors in endometrial cells
PROLIFERATIVE PHASE
• Rising estrogen levels cause the thick
& sticky cervical mucus to become thin
to facilitate sperm passage
• Ovulation occurs at the end of this
stage (day 14) in response to sudden
release of LH
SECRETORY PHASE
Days 15 - 28
• Also called progestational or progravid phase
• Endometrium prepares for implantation of embryo
• Rising levels of progesterone converts functional layer
of endometrium to a secretory mucosa
• Uterine glands secrete nutritious glycoproteins into the
uterine cavity to sustain embryo until it has implanted
• Increasing progesterone levels also cause cervical
mucus to be thick again forming cervical plug, which
blocks sperm entry
SECRETORY PHASE CONTD…
• Increasing progesterone levels inhibits LH release by anterior pituitary
• If fertilization has not occurred, corpus luteum begins to degenerate toward the end of secretory phase as LH levels decline
• Progesterone levels fall, depriving endometrium hormonal support, hence, menstruation begins on day 28
• Menstrual cycle starts over again on this first day of menstrual flow
SECRETORY PHASE
CONTD… • If fertilized, human chorionic
gonadotropin from embryo prevents
corpus luteum degeneration until 4th
month when placenta takes over
progesterone production
MENSTRUAL CYCLE
• Thank you
