GAMETOGENESIS

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GAMETOGENESIS OVARIAN UTERINE CYCLES
ANATOMY DEPARTMENT
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OBJECTIVES

• At the end of the lecture, the students should be
  able to
• 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.

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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.

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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).

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IMPORTANCE OF MEIOSIS

• (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).

Segregation of chromosomes
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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).

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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.

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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.

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SPERMATOGENESIS

• 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.

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The mature sperm

• 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.

Mature sperm
Secondary oocyte (Mature female gamete)
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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.

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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)

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OOGENESIS

• At birth all primary oocyte (46 ch) remain
  arrested and do not finish their 1st
  meiotic division until puberty.

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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).

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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
  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.

Graafian Follicle
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OOGENESIS

• 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.

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OOGENESIS

• At ovulation the nucleus of the secondary
  oocyte begins the second meiotic division,
  leading to
• The secondary oocyte (23)
• The 2nd polar body.

At puberty /Before ovulation
At ovulation
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OOGENESIS

• 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.

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MATUREOOCYTE

• Has a covering of a morphous material (Zona
  Pellucida) and a layer of follicular cells
  (Corona Radiata).

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Comparison of Gametes

• 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

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Oogenesis postnatal maturation of Oocytes

• 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.

At puberty
At ovulation
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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.

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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.

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Ovarian cycle

• FSH and LH produce cyclic changes in the
  ovaries-the ovarian cycle (development of
  follicles, ovulation, and corpus luteum
  formation).

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Ovarian cycle

• (FSH) and (LH) produce cyclic changes in the
  ovaries.
• 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).

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Ovarian cycle
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Ovarian cycle

• Monthly series of events associated with the
  maturation of an egg.
• Follicular phase period of follicle growth
  (days 114).
• Ovulation occurs midcycle.
• Luteal phase period of corpus luteum activity
  (days 1428).

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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.

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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.

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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.

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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.

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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.

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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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