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SPERMATOGENESIS
Spermatogenic cycle
The following is an example of how the number of spermatozoa is increased by repetitive mitotic divisions of
spermatogonial cells followed by the two meiotic divisions. There are actually more than 4 types of spermatogonia,
so the actual number of mature spermatozoa originating from the initial division of a type A 1 spermatogonium is
actually greater than 96.
A1
24 primary spermatocytes
Mitosis
Meiosis I
2A1
48 secondary spermatocytes
Mitosis
A1
3A2
Reductional division
Meiosis II
Mitosis
Equational division
96 spermatids
Mitosis
6B1
Mitosis
Spermiogenesis
= Spermateleosis
= Spermatozoan metamorphosis
12B2
The maturing spermatids remain
attached by cytoplasmic bridges
as they mature
=> syncytium
96 mature spermatozoa
First meiotic division lasts
several weeks in humans
Second meiotic division takes
about 8 hours in humans
An entire spermatogenic cycle in
humans takes about 64 days.
Let’s think about meiosis:
1. Where did the chromosomes come from?
2. Why are there pairs of chromosomes?
3. How must the chromosomes segregate if
you’re going to have sexual reproduction?
What do you have to end up with in he
gametes?
4. How would you accomplish this in two
divisions?
CHROMOSOME TERMINOLOGY
I
Single Chromosome
Monad
Homologous
Gene Loci
M
X
Replication
Replicated Chromosome
Dyad - Two Chromatids
P
M
II
XX
Replication
Homologous Pair
Replicated
Homologous Pair
Synapsis
M
X
P
P
X
Tetrad = Two Synapsed replicated
Homologues
Replicated
Homologous
Gene Loci
Genetic content vs Chromosome number
M
Diploid genetic content - 2n - the 2
represents the fact that each gene locus
can contain a maximum of 2 different gene
alleles (e.g., one dominant and one
recessive). “n” is the total number of
homologous gene loci in the genome (a
very big number).
P
Diploid chromosome number - 2c - the 2
represents the fact that there are 2
chromosomes in each homologous pair.
“c” is the total number of homologous
pairs of chromosomes.
M
P
M
M
P
P
Meiosis
Genetic Content (n) and Chromosome Number (c)
Turn off pic-in-pic
First meiotic prophase:
Leptotene
Chromosomes start to condense, dyads (replicated chromosomes) of homologous
pairs first become visible as linear strings of DNA
Zygotene
Chromosomes condense further. Dyads of homologous pairs of chromosomes pairup and their chromatids start to undergo synapsis.
Pachytene
Synapsis is completed with synapsed chromosomes forming tetrads. Cross-over
takes place. Synapsed chromosomes thicken up (pachy - thick).
Diplotene
Chromosomes condense further, Desynapsis begins. Chromatids of homologous
pairs of chromosomes remain connected at chiasmata where cross-over may have
occurred.
Diakinesis
Chromosomes condense even further. Separating tetrads form strange shapes like
crosses, fish, infinity signs as chiasmata move toward the ends of the paired
chromatids of the homologous chromosomes (called terminalization). Eventually
desynapsis is completed and the homologues separate from one and other
completely and metaphase begins.
Spermatogenesis in an
Invertebrate - grasshopper
Spermatogenesis in Vertebrates
Figure on Page 89 of your text
Spermatogenesis in Vertebrates
Figure of rat testis in digital lab
manual
Spermatogenesis in Vertebrates
Figure of human testis in digital
lab manual
Spermiogenesis
1.
Nucleus condenses (chromosomes
condense and nuclear sap is removed)
2.
Flagellum develops
3.
Spermatocyte elongates
4.
Acrosome formed from golgi body
5.
Mitochondria aggregate around base of
forming flagellum
6.
Mitochondria fuse to form
supermitochondrion (in humans)
7.
Most of cytoplasm is shed and phagocytosed
by sertoli cell (tubulobulbar processes)
1.
Nucleus condenses (chromosomes
condense and nuclear sap is removed)
2.
Flagellum develops
3.
Spermatocyte elongates
4.
Acrosome formed from golgi body
5.
Mitochondria aggregate around base of
forming flagellum
6.
Mitochondria fuse to form
supermitochondrion (in humans)
7.
Most of cytoplasm is shed and phagocytosed
by sertoli cell (tubulobulbar processes)
Sertoli cell function
1.
Remove excess cytoplasm from developing
spermatid - tubulobulbar processes
2.
Move spermatids toward the lumen of the
seminiferous tubules - ectoplasmic specializations
3.
Nurture and mediate maturation of spermatids
4.
Segregate groups of developing gametes
5.
Secrete fluid to transport sperm in reproductive
tract
6.
Secrete hormones and other factors
a.
Embryonic - anti-mullarian hormone
b.
Adult
(1) inhibin (inhibits FSH production)
(2) estrogen - may act to inhibit GnRH production by
basal hypothalamus
(3) Other factor (not a hormone) - androgen binding
protein (helps transport androgens from interstitial
fluid into seminiferous tubule - promotes
spermatogenesis)
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab27/EXAMPLES/EXSERTOL.HTM