Lec:10 Dr.Sajida
Spermatogenesis (sperms formation)
The seminiferous tubules contain large number of germ cells called
spermatogonia, located in 2-3 layers. In the 1
st
stage of spermatogenesis,
type A spermatogonia divided 4 times (mitosis) to form 16 more
differentiated cells called type B spermatogonia, at this stage spermatogonia
migrate centrally among sertoli cells.
For a period of 24 days, each spermatogonium that crosses the barrier into
sertoli cell layer becomes by mitosis a large primary spermatocyte. At the
end of 24 days, each primary spermatocyte divide to form 2 secondary
spermatocytes, this division is called the 1
st
meiotic division, in this process
each of the 46 chromosomes become 2 chromatids, so each secondary
spermatocyte have 23 chrom.
Within 2-3 days a 2
nd
meiotic division occurs in which the 2 chromatids in
each of the 23 chrom split apart forming 2 sets of 23 chrom, one set passing
into one daughter spermatid.
The importance of these meiotic divisions is that the eventual sperms that
fertilized the ovum provide one half of the genetic material to the fertilized
ovum and the ovum will provide the other half.
These stages from spermatogonia to spermatid are androgen dependent.
During the next few weeks after meiosis each spermatid is nursed and
reshaped by its sertoli cells and changed into a sperm by losing some of its
cytoplasm, formation of compact head and formation of a tail, maturation of
spermatid to spermatozoa depends on estrogen (does not depend on
testosterone) acting on sertoli cells. The entire period of spermatogenesis
takes 74 days.
Acrosome reaction
The acrosome enzyme hyaluronidase and proteolytic enzymes released to
open a pathway for passage of sperm head through zona pellucida within 30
minutes, so fertilization will occur.
Why does only one sperm enter the oocyte?
1- Only few sperms get as far as the zona pellucida so that it might be
10, 20 or 30 minutes before the 2
nd
sperm arrives.
2- Within few minutes after the 1
st
sperm penetrate the zona pellucida,
Ca
++
diffuse through the oocyte membrane and cause release of
cortical granules that prevent binding of additional sperms.
3- The oocyte membrane its fusion with sperm is believed to be
electrically depolarized.
Hormonal factors that stimulate spermatogenesis:
1. LH secreted by anterior pituitary stimulates leydig cells to secrete
testosterone.
2. Testosterone secreted by leydig cells is essential for growth and
division of germinal cells in forming sperms.
3. FSH also secreted by anterior pituitary stimulates secretion of sertoli
cells and without this stimulation spermatogenesis will not occur, in
addition it promotes production of ABP.
4. Estrogen formed by sertoli cells essential for spermatogenesis, ABP is
important for sperm maturation (capability of movement).
5. GH promotes early division of spermatogonia and its absence results
in deficient or absence spermatogenesis (in dwarf).
6. Prolactin it may potentiate the stimulatory effect of LH on lydig cells
and of testosterone on many of its target cells (in hyperprolactinemia
causes infertility).
Formation of sperms:-
The sperms is composed of head and tail , the head composed of condensed
nucleus with only a thin cytoplasmic cell membrane layer around its surface.
On the outside of anterior two third of the head is a thick cap called
acrosome, This contain a number of enzymes including Hyaluronidase
which has a powerful proteolitic enzymes ,these enzymes play important
role it allowing the sperm to fertilize the ovum.
The tail of the sperm called flagellum which responsible of movement. The
tail provides motility for the sperm, normal sperm moves in a velocity of 1-4
mm/min.
Maturation
of sperm in epididymis:
The sperm after its formation in the seminiferous tubules pass into the
epididymis, they remain for 18-24 hours and become capable of motility,
although in the epididymal fluid there are inhibitory proteins which prevent
sperm motility until ejaculation, after ejaculation the sperm become motile
and capable of fertilization of the ovum, this process called maturation, from
sertoli and epididymis there is a fluid ejaculated with the sperms contains
estrogen and testosterone, this is essential for maturation, the adult testis
form about 120 million sperms per day sperms, small quantity of them
stored in the epididymis, while large quantities stored in vas deferens.
Function of prostate gland:
The prostate gland secrete a thin milky fluid (30% of total vol.) contain
citrate ions, calcium ions, phosphate ions and clotting enzyme and
fibrinolysin, prostatic fluid is alkaline and helps to neutralized the acidity of
vaginal secretion and of the vas deferens.
[Sperm don’t live in acidic medium (if it lived-according to the PH-) it will
be not capable to fertilize the ovum]
Semen:
The fluid that ejaculated at the time of orgasm contains sperms and secretion
of seminal vesicle, prostate and cowpers gland.
Normal semen volume 2.5-5 ml, sperm count 20,000,000 per ml, activity
(motility) 50 %, progressive movement 60% .
Subfertile means decrease in volume count …. Etc..
Capacitation of spermatozoa:-
Several changes believe to occur that activate the sperm for final process of
fertilization, these changes are:-
1. The uterine and fallobian tube fluids washaway the inhibitory factors
that suppress sperm motility
2. The sperm deposited in female genital tract loses much cholesterol
from the head, so the membrane at the head of the sperm becomes
weak.
3. The membrane of sperm’s head become small permeable for Ca, and
Ca enter the sperm giving it powerful motion , without capacitation
the sperm cannot fertilize the ovum
Function of seminal vesicle:-
The secretion of seminal vesicle form 60% of total volume,its mucoid
contain fructose,citric acid and other nutrients,also prostaglandin and
fibrenogin, prostaglandin are belived to aid fertilization by :
1. Reacting with cervical mucous making it more receptive to sperm
movement
2. Causing reverse peristatitic,contraction of uterus and fallobian tubes
to move the sperm toward ovaries
Neuroendocrine Regulation of the Testis:-
In hypothalamus GnRH secreted in pulses one pulse every 3 hours, if
secreted continuously the anterior pituitary not responds to it, anterior
pituitary secrete FSH AND LH (LH follows the pulsatile pattern of GnRH
secretion while FSH increase or decrease according to demand.
FSH has specific receptors on steroli cells, after binding to it:
1. They stimulate spermatogenesis.
2. Stimulate sertoli cells secretion estrogen and inhibin which have –ve
feedback on anterior pituitary LH effect on leydig cells stimulate it to
secrete testosterone which acts locally on sertoli cells that effect
spermatogenesis and estrogen secretions also effect target tissues
outside the testis (sexual characteristics ).
Testosterone have negative feedback on anterior pituitary that inhibit FSH
and LH secretion which causes negative feedback on hypothalamus to
decrease GnRH secretions
The mature sperm is capable of movement in a straight line. Its activity is
increased with increasing temperature, also increase neutral & alkaline
medium & decreased in acidic medium. Average PH of semen is 7.5, the
sperm remain in genital tract for 1-2 days.
Effect of temperature on spermatogenesis:
Normal temperature to spermatogenesis is 32
o
C, increasing temperature as
hot bath of 48
o
C for 30 minutes reduces sperm count to 90%, sperms can
live in male genital tract for weeks at low temperature and can be preserved
for years at -100 C (Bank for AID).