Sexual Organs,
Gametogenesis and
Fertilization
Lecture 1
Cellular Division: Mitosis
Cellular Division: Mitosis
• Interphase – active stage of cellular function
• DNA replication occurs here
• Prophase
• Centrosomes replicate and migrate to opposite poles, forms
spindle fibers and asters to join with the chromatids
• Nuclear envelope disintegrates
• Chromatin  chromosome  chromatids
• Metaphase
• Starts when sister chromatids start to migrate and ends when the
chromosomes are lined up at the metaphase plate.
Cell Division: Mitosis
• Anaphase
• Starts when sister chromatids splits into independent
chromosomes
• Ends when chromosomes just prior to reaching the centrosomes
• Telophase
• When chromosomes are at each pole and nuclear envelope
begins to form
• Appearance of cleavage furrow and cytokinesis begins
1st Meiotic Division
• Prophase I
• Leptotene stage: Chromosomes divide into identical
chromatids
• Zygotene stage: Sister chromosomes (1 chromosome has 2
chromatids each) join together through synapsis.
• Pachytene stage: sister chromosomes (bivalent pairs)
thicken thru coiling
• Diplotene stage: portions of bivalent chromosomes overlap
one another thru chiasmata, called crossing-over, and
separating of the sister chromosomes at the synaptinemal
complex.
• Diakinesis stage: moving of chiasmata to the terminal ends
of the chromosomes called terminalization.
1st Meiotic Division
• Metaphase I
• Tetrads line up along the metaphase plate
• Anaphase I
• Paired chromatids (1 chromosome) move to opposite
poles
• Chiasmata separate
• Telophase I
• Nuclear membranes form
• Cleavage furrow
2nd Meiotic Division
• Similar to mitosis, but without DNA replication
• Chromatids separate into each daughter cell
Trisomy 21
Trisomy 21
Reproductive Organs
Spermatogenesis
Histology
Spermatogonium
Primary
spermatocyte (1)
Secondary
spermatocytes (2)
Spermatids (4)
Spermatozoa
Meiotic Events
Chromosomal
Complement
DNA replication complete 2N, 4C
First meiotic division in
progress
Second meiotic division
2N, 4C
1N, 2C
Immature haploid
1N, 1C
gametes
Release of residual bodies 1N, 1C
Supportive cells
• Sertoli Cells
• Target cells for FSH
• Synthesizes androgen-binding proteins to maintain a high level of
testosterone in the seminiferous tubules
• Blood testis barrier
• Degrading residual bodies for recycling
• Interstitial cells/Leydig cells
• Produce testosterone
• Spermatogenesis in humans = 64-74 days
Passage of sperm
• Seminiferous tubules  tubuli recti  rete testis 
efferent ductules (ductuli efferentes)  epididymis 
ductus or vas deferens  ejaculatory duct  urethra
• Epididymis = area of storage and further maturation of
sperm cells
• Seminal vesicles: produce fructose and seminal fluid
• Prostate: alkaline secretions of seminal fluid
• Bulbourethral glands or Cowper’s gland: alkaline fluid
and lubrication
• Glands of Littre: mucus for lubrication
Semen analysis
Semen
Normal Values
Volume
1.5-5.5mL (Ave. 2.5mL)
Sperm concentration
20,000,000/mL or more
Motility
>50%
Forward progression
2 (scale of 1-4)
Morphology
>30% WHO normal forms
Liquefaction
Within 20-60min (30min)
Collection time
2-3 days of abstinence
Oogenesis
Age
Histology
Meiotic Event
Oogonium
Chromosomal
complement
Fetal
period
No follicle
Fetus
Primordial follicle Primary oocyte
2N, 4C
After
birth
Primary follicle
2N, 4C
Puberty
Secondary follicle Primary oocyte
2N, 4C
Tertiary follicle
Secondary oocyte
+ polar body
1N, 2C
Ovulated ovum
Secondary oocyte
(arrested in
metaphase II)
1N, 2C
Fertilized ovum
Ovum + polar
body
1N, 1C +
sperm
Primary oocyte,
arrested in
prophase I
(diplotene stage)
2N, 2C
Oogenesis
• First Phase
• Primary follicle surrounded by squamous follicular
cells
• Follicular cells form several layers of cuboidal cells
which are now called granulosa cells with a membrane
between the oocyte called the zona pellucida
Primary follicle
Oogenesis
• Second Phase
• Growth of oocyte and surrounding granulosa cells
• Formation of theca folliculi (interna and externa)
outside the granulosa cells, with a basement
membrane separating the two cell types.
• Progresses to secondary follicle with formation of
antrum
• Theca cells develop LH receptors and produce
testosterone, and the granulosa cells convert
testosterone to estrogen via aromatase enzyme
Secondary follicle
Oogenesis
• Third Phase
• Further growth of follicle and selection of the
dominant follicle
• Dominant follicle is stimulated by FSH to mature to the
Graafian follicle
• The egg cell covered by granulosa cells now become
the cumulus oophorus cells
• The single layer of granulosa cells that is immediately
surrounding the secondary oocyte is the corona
radiata = produces hormones and makes the egg cell
larger to facilitate transport along the fallopian tube
• Egg cell is at metaphase II stage
Ovulation
• Egg cell ruptures from ovary
• Stigma = area where Graafian follicle meets wall of
ovary until it ruptures to the fallopian tube.
• Formation of corpus luteum
• Granulosa and theca cells
• Secretes both estrogen and progesterone to prepare
zygote for implantation in the uterus
• If fertilization occurs, the embryo secretes beta-HCG to
maintain the corpus luteum of pregnancy
• If no fertilization occurs, the corpus luteum
degenerates and becomes the corpus albicans
Sexual or estrous cycle
• 4 phases
• 1. Period of preparedness:
• only a short time for reproduction, ovulation and
increased sexual desire
• 2. Period of regression
• Postestrum or metestrum
• 3. Period of rest
• Diestrum
• 4. Period of active preparation
• Proestrum
Sexual or estrous cycle
• Monoestrous – one breeding season a year
• Polyestrous – multiple breeding seasons in a year
• Anestrous – period of dormancy between estrous cycles
Hormonal Regulation
• Hypothalamus = GnRH
• Pituitary = FSH and LH
• Gonadal axis
• Male
• Female
Fertilization
1.
2.
3.
4.
5.
6.
Initial membrane contact between egg and sperm
Entry of sperm
Prevention of polyspermy
Metabolic activation of egg
Completion of meiosis by the egg
Fusion of male and female pronuclei
Fertilization
• Site of insemination = upper 1/3 of fallopian tube
• Barriers to insemination: acidity of vaginal canal, cervix and
isthmus of fallopian tube
• Contractions of vagina and uterus to propel sperm to the
fallopian tubes
• Cilia in the fallopian tube propel the egg cell towards the
uterus, and the sperm are guided by swimming against a
current (egg cell also contains sperm attracting properties)
• Capacitation = process of making sperm cell able to fertilize
egg cells
Fertilization
• Sperm enters egg cell through enzymes in the acrosome
cap
• Hyaluronidase = penetration through corona radiata
• Acrosin = penetration through zona pellucida
• Prevention of Polyspermy
• Fast block = electrical barrier
• Depolarization of egg’s plasma membrane by Na+ influx
• Slow block = water barrier
• Cortical granules fuse with plasma membrane to create
a vitelline membrane (filled with water and hyaline)
• Produce substances which kills other sperm attached
Viability of gametes
• Egg cell = 24 hours
• Sperm cells = 1-2 days able to fertilize (3-5 days motile)
Contraceptive Methods
• Natural
• Calendar method
• Billing’s method
• Withdrawal
• Artificial
• Hormonal
• Barrier methods + spermicides
• Surgical procedures