Development of male reproductive system
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When fertilization occurs, the genetic makeup of the male
gamete (22X or 22Y) determines the sex of the developing embryo.
The process of sex differentiation is complicated and
multigenic.
The most significant one is the Y chromosome gene's sex-determining region
(SRY), whereas certain autosomal chromosomal genes are also involved in this
process.
On the short arm of Y chromosome Yp11, the SRY gene is
located. Male development is influenced by the SRY protein, whereas female
development is developed in its absence.
Indifferent
gonad
Up to 6 weeks of development, gonads are two longitudinal genital
ridges, at the dorsal body wall of embryo which develop from intermediate
mesoderm.
The genital ridge is formed by coelomic epithelium and
underlying mesoderm. The coelomic epithelium forms irregular sex cords in both
male and female embryos, and it is not possible to distinguish between male and
female gonads at this stage, so this stage of development is known as indifferent
gonads.
By the 12-week mark, the male and female structures of the
external genitalia can be differentiated.
By the 20th week, the phenotypical differentiation is
complete.
Source of
development of gonad
- Genital
ridge: it is formed by the elevation of the intermediate mesoderm, which
is covered by the coelomic epithelium.
- Primordial
germ cells
The STY gene produces a protein known as a
testis-determining factor.
TDF induces the testis to secrete FGF9, which stimulates the
mesonephric duct to penetrate the gonadal ridge.
The SRY gene also produces SF1, which stimulates the
differentiation of sertoli and leydig cells, which produce testosterone and
Mullarrian-inhibiting factor (MIF), respectively.
In the presence of TDF, testosterone, and MIF, the indifferent
gonad will produce a male phenotype.
In the absence of TDF, testosterone, and MIF, the
indifferent gonad will produce a female phenotype.
Sequence of
development of reproductive system
Gonads > then genital duct
formation > then formation of external genitalia
Development
and migration of primordial germ cells originate from the epiblast, then
migrate through the primitive streak to reach the endodermal cell of the yolk
sac close to allantois by the 3rd week. During the 4th
week of development, they pass through the dorsal mesentery of the hindgut, and
by the 5th week of development, they reach near the genital ridges
and start to penetrate the genital ridge by the 6th week.
If
primordial germ cells have an inductive influence on the development of the
gonad, if they fail to reach the genital ridges, the gonad will not develop.
Development of gonads
Development of testis
1.
Intermediate mesoderm
forms the urogenital ridge, a longitudinal elevation at the dorsal body wall of
the fetus.
The coelomic epithelium and underlying mesoderm of the urogenital ridge
protifelate to form the gonadal ridge.
The coelomic epithelium forms irregular primary sex cords and incorporative
primordial germ cells, which migrate from the wall of the yolk sac.
The primary sex
The development of the testis ord extends deep into the medulla to form
medullary cords, which lose their connection to the surface epithelium as the
thick tunica albuginea forms in between the medullary cord and coelomic
epithelium.
2. Near the
hilum medullary cord break up into network of tiny cell stands that form rete
testis
3. By the 4th
month of interuterine life medullary cord is continuous with rete testis
4. The
testis or medullary cord contain and primitive germ cells and sustentacular
cell of sertoli cell which derived from
the surface epithelium of genital ridge.
5. Sertoli
cell secrete mullarian inhibiting factor (MIF) which degenerate mullarian duct
6. Interstitial
cell of leydig derive from mesenchyme of genital ridge which derived from
intermediate mesoderm ,By the 8th week of gestation leydig cell
begin to produce testosterone which influence development of genital duct and
external genitalia
7. The
testis or medullary cord of testis remain solid cord until puberty , then it become
canalize and known seminiferous tubule, the canal of seminiferous tubules are
continuous with rete testis which inturn join with efferent ductile. The
efferent ductules and other excretory part of male genital system derived from
mesonephric or wolffian duct
Relative descent of the testis
The testis originally developed
within the abdomen but later descent to scrotum due to disproportionate growth
of upper abdomen region away from the pelvic region
The gubernaculum testis a fibrous
band extend from testis to scrotum pull the testis
The gubernaculum also pull part
of peritoneum with it the processus vaginalis which obliterate later except a
remnat of peritoneal sac the tunica vaginalis of the testis
12th week , testis
reach inguinal region
28th week, inguinal
canal
By 33rd week it reach the scrotum
Genital
duct in male
There are two genital ducts,
mesonephric and paramesonephric ducts
are present in both sex at the initial period of development
In case male mesonephric duct is
developed but paramesonephric duct mostly degenerate, while in case female para
mesonephric duct is developed but mesonephric duct mostly degenerate
Gonads influence the development
of genital duct and genitalia.
The Sertoli cell secrete
mullarian inhibiting factor (MIF) which inhibit the development of
paramesonephric or mullarian duct
Interstitial cell of leydig
derive from mesenchyme of genital ridge which derived from intermediate
mesoderm ,
By the 8th week of
gestation leydig cell begin to produce testosterone which stimulates the
mesonephric ducts to form genital ducts .
Structure derived from mesonephric duct : Efferent ductules, epididymis,
vas deferens, seminal vesicle and ejaculatory duct
The paramesonephric duct regress
due to presence of Mullarian inhibiting factor secreted from the sertoli cell
The remnant of paramesonephric
duct in male: Appendix testis , appendix epididymis,
, The remnant of mesonephric duct
in male: paradidymis,
Development
of external genitalia
Indifferent
stage
In the 3rd week of
development, mesenchymal cells origining in the region of the primitive streak
migrate aroung the cloacal membrane to form a pair of slightly elevated cloacal
folds , cranially these folds unit and form genital tubercles , caudally the
folds are subdivided into urethral folds anteriorly and anal folds posteriorly
. another pair of swellings lateral to
it known as genital swellings
A the end of the 6th
week of development it is impossible to distinguish between two sexes.
Development of male external
genitalia
Under influence of testosterone
genital tubercle is elongated known as phallus which pull urethral folds
forward
Phallus forms the (glans penis,
corpora cavernosa and corpus spongiosum of penis )
Urethral folds form the ventral
part of the penis
The genital swelling form the
scrotum
