Transcript 1-gametogenesis

GAMETOGENESIS
OVARIAN & UTERINE CYCLES
ANATOMY
DEPARTMENT
OBJECTIVES
• At the end of the lecture, the students should
be able to :
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Describe the process of gametogenesis.
List the importance of meiosis.
List the stages of spermatogenesis.
List stages of oogenesis.
Compare between the 2 gametes.
Describe the ovarian cycle.
Describe the uterine cycle.
PROCESS OF GAMETOGENESIS
• It is the process of
conversion of immature germ
cells (spermatogonium or
oogonium), into mature
gametes (sperm or oocyte).
• This maturation is called
spermatogenesis in males
and oogenesis in females.
• During gametogenesis, a
special type of cell division
(meiosis) occurs, in which the
number of chromosomes is
reduced from diploid to
haploid number.
• It prepares gametes for
fertilization.
MEIOSIS
• It is the cell division that
takes place in the germ
cells only to produce male
and female gametes.
• It consists of two cell
divisions :
• meiosis I during which the
chromosome number of the
germ cells is reduced to half
(23), the Haploid number
(with double chromatid
chromosoes).
• Meiosis II during which the
haploid number (23) is
retained (with single
chromatid chromosome).
Segregation of
chromosomes
• (1) Reduces the chromosome
number from the Diploid to
Haploid.
• (2)Allows random movement
of maternal and paternal
chromosomes between the
gametes (Segregation)
• (3)Allows Crossing Over of
chromosome segments.
• (4)It enhances genetic
variability through cross over
and segregation (separation or
disjunction of paired
homologous chromosomes).
Spermatogenesis
• It is a conversion of
primitive germ cell
spermatogonium into
mature germ cell sperm.
• It begins at puberty and
continues into old age.
• It occurs in the
seminiferous tubules of
testes.
• After several mitotic
divisions, spermatogonia
increase in number and
grow into primary
spermatocytes
(46 chromosomes).
Spermatogenesis
• Each primary spermatocyte
undergoes a reduction
division- 1st meiotic
division to form 2 haploid
secondary spermatocytes.
• Secondary spermatocytes
undergo 2nd meiotic
division to form 4 haploid
spermatids (half size).
• Spermatids are
transformed into 4 mature
sperms by a process called
spermiogenesis.
Spermiogenesis
• It is the last phase of
spermatogenesis.
• The rounded spermatid is
transformed into elongated
sperm.
• Note the loss of cytoplasm,
development of the tail,
and formation of
acrosome, which is derived
from Golgi region of
spermatid.
• Acrosome contains
enzymes that are released at
the biginning of fertilization to
help sperm in penetrating
corona radiata & zona
pellucida surrounding
secondary oocyte.
• Spermatogenesis
including
spermiogenesis requires
about 2 months for
completion.
• It takes place in the
Seminiferous Tubules.
• The sperms are stored
and become functionally
mature in the
Epiddidymis.
The mature sperm
Mature sperm
Secondary oocyte
(Mature female gamete)
• It is a free-swimming actively motile cell,
consisting of a head, neck and a tail.
• The head, composed mostly of haploid
nucleus.
• The nucleus is partly covered by a
caplike acrosome, an organelle
containing enzymes to help sperm in
penetrating corona radiata & zona
pellucida of secondary oocyte during
fertilization.
• The tail of sperm consists of
3 segments : middle, principal &
end pieces. it provides motility of sperm
to the site of fertilization.
• The middle piece of the tail contains
mitochondria, providing adenosine
triphosphate (ATP) necessary for
activity.
Oogenesis
• It is a maturation of the germ
cells (oogonia) into mature
secondary oocytes.
• It occurs in the ovary, which
contains a large number of
oogonia that differentiate
into primary oocytes.
• It begins before birth
(in early fetal life) and is
completed after puberty and
continues to menopause.
• No oogonia are found in
ovary after birth because
they differentiate into
primary oocytes before
birth.
Oogenesis : Prenatal maturation of
Oocytes
• During early fetal life :
Oogonia proliferate by
mitosis to form daughter
oogonia, they enlarge and
grow to form primary
oocytes before birth
(each contains 46 ch.).
• A Single layer of flattened
follicular epithelial cells
from the cotex of ovary
surrounds the primary
oocyte, forming primitive or
primordial ovarian follicles
containing primary oocytes
(A)
• At birth all primary oocyte
(46 ch) remain arrested
and do not finish their
1st meiotic division until
puberty.
Oogenesis
 During puberty, follicular
epithelial cells become
cuboidal then columnar
and the primary oocyte
becomes surrounded by
zona pellucida (acellular
glycoprotein material)
forming growing follicle
(B).
 Then, Primary oocyte
has several layers of
follicular cells +
Z.P.membrane, forming
the primary follicle (C).
Oogenesis
 At puberty : cavities appear between
the follicular cells, then join together
forming a single large cavity called
(Antrum) filled with a watery fluid
(liquor folliculi).
 The follicular cells differentiated into :
Stratum granulosum, forming the wall of
follicle.
Cumulus oophorus : cells surrounding
the ovum (primary oocyte).
 Theca folliculi differentiated into :
Graafian Follicle
theca externa : outer fibrous &
theca interna : inner vascular and cellular
layer.
 At this stage the growing primary
follicle changed into mature
secondary follicle or Graafian follicle.
• Shortly before ovulation :
Primary Oocyte completes
first meiotic division :
• the Secondary Oocyte
(23 ch) receives almost all
the cytoplasm.
• The First Polar Body
receives very little.
• It is small nonfunctional
cell and soon degenerates.
At puberty /Before
ovulation
At ovulation
• At ovulation : the
nucleus of the
secondary oocyte
begins the second
meiotic division,
leading to :
• The secondary oocyte (23)
• The 2nd polar body.
• If the secondary oocyte
is fertilized by a sperm,
the second meiotic
division is completed
otherwise it degenerates
24 hours after ovulation.
• Most of the cytoplasm is
retained by the Mature
Oocyte (Fertilized
Oocyte).
• The rest is in the 2nd
Polar Body which soon
degenerats.
• Has a covering of
a morphous
material (Zona
Pellucida) and a
layer of follicular
cells (Corona
Radiata).
Comparison of Gametes
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Sperm
It is highly motile.
It contains little cytoplasm.
It is not surrounded by Z.P & C.R.
It has 2 kinds of sex
chromosomes : 23,x and 23,y /
so the difference in sex
chromosome complement of
sperms forms the basis of
primary sex determination.
Secondary oocyte
It is immotile.
It has an abundance of cytoplasm.
It is surrounded by Z.P and a layer
of follicular cells-the C.R.
It has only one kind of sex
chromosome : 23,x
Oogenesis: postnatal maturation of Oocytes
At puberty
At ovulation
• No primary oocytes form after birth in
females, in contrast to continuous
production of primary spermatocytes
in males.
• Primary oocytes are formed in the
ovaries in early fetal life and remain
dormant in ovarian follicles until
puberty.
• At puberty,as a follicle matures, each
primary oocyte divides by 1st meiotic
division into one large secondary
oocyte (contains haploid chr.) and
one small non-functional cell- first
polar body (degenerates ,contains
haploid chr.).
• At ovulation, secondary oocyte
begins 2nd meiotic division, which is
completed after fertilization, giving
one ovum or fertilized oocyte + one
small nonfuctional cell- second polar
body , each one having haploid chr.
Female reproductive cycles
• Commencing at puberty and normally continuing
throughout the reproductive years.
• Involving activity of :
• Hypothalamus.
• Hypophysis.
• Ovaries.
• Uterus.
• Uterine tubes.
• Vagina.
• Mammary glands.
Female reproductive cycles
• Gonadotropine releasing hormone by hypothalamus
(GnRH) stimulates the release of :
• Follicle stimulating hormone (FSH);
• Stimulates development of ovarian follicles.
• Production of estrogen by follicular cells.
• Lutinizing hormone (LH);
• Trigger for ovulation (secondary oocyte)
• Stimulate production of progesterone by follicular cells
and corpus luteum.
• These hormones induce growth of endometrium.
Ovarian
cycle
•FSH and LH produce cyclic changes in the ovaries-the ovarian cycle
(development of follicles, ovulation, and corpus luteum formation).
Ovarian
cycle
changes in the ovaries.
• (FSH) and (LH) produce cyclic
• Follicular development, with the
resulting growth and proliferation of
follicular cells, formation of zona pellucida,
and development of theca folliculi.
• Subsequent estrogen production,
necessary for reproductive organs
development and function.
• Ovulation; occurs at mid menstrual cycle,
stimulated by increase of (LH) production,
and high estrogen level.
• Expulsion of the secondary oocyte,
surrounded by zona pellucida, and corona
radiata,
• Corpus luteum; the wall of ovarian follicle
after expulsion of the secondary oocyte
and develop under influence of (LH).
• It is of two types : corpus luteum of
pregnancy and corpus luteum of
menstruation (corpus albicans).
Ovarian cycle
Ovarian cycle
• Monthly series of events associated with the
maturation of an egg.
• Follicular phase – period of follicle growth (days
1–14).
• Ovulation occurs midcycle.
• Luteal phase – period of corpus luteum activity
(days 14–28).
Menstrual cycle
• It is the cyclic changes in the endometrium
(every 28 days) under fluctuating effect of
gonadotropic, ovarian follicle and corpus
luteum hormones (estrogen and
progesterone).
• It is the peroid during which the oocyte
matures, ovulated, and enters the
uterine tube.
Menstrual cycle
• Phases of the menstrual cycle :
• Menstrual phase; the functional layer of
the uterine wall is sloughed, discarded
with the menstrual flow, it lasts about 4-5
days.
• The endometrium is thin.
Menstrual cycle
• Proliferative phase :(follicular estrogenic),
coincides with growth of the ovarian follicle
and controlled by estrogen.
• It lasts about 9 days.
• Increase thickness of endometrium.
Menstrual cycle
• Luteal phase; (secretory progesterone),
coincides with formation, function, and growth of
corpus luteum,
• It lasts about 13 days.
• increase endometrial thickness under influence
of progesterone and estrogen of corpus luteum.
Menstrual cycle
• If fertilization occurs;
• corpus luteum under influence of hCG
continues to secrets estrogen and progesterone,
the luteal phase persists and
menstruation does not occur.
• The endometrium passes into the pregnancy
phase.
• If fertilization dose not occur; corpus
luteum degenerates, with fall of estrogen and
progesterone level, menstruation occurs.
Menstrual cycle
• Ischemic phase; due to hormonal
withdrawal as a result of non fertilization of
the oocyte,
• Shrinkage of the endometrium, with
patchy ischemic necrosis, resulting in
bleeding in the uterine cavity.
• The cycle is repeated.
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