Note: Genital development involves transition through an indifferent stage in which gonads,
genital ducts and external features appear the same in both sexes.
Most genital anomalies involve some vairation of intersex development & appearance.


Gonad indifferent stage — gonads arises bilaterally from gonadal ridges in the embryonic abdomen
• the gonadal ridge, located medial to the mesonephric kidney, consists of thickened intermediate
mesoderm coated by coelomic mesothelium; the ridge contains germ cells and supporting cells
• supporting cells are derived from invading coelomic mesothelial cells, augmented by cells fromdisintegrating mesonephric tubules
• the supporting cells form gonadal cords, cellular bands that radiate into gonadal ridge mesoderm
• primordial germ cells arise from yolk sac endoderm and must migrate along the gut wall and gut
mesentery to reach the gonadal ridge; germ cell arrival induces further gonadal development
• germ cells proliferate and enter gonadal cords where they can be surrounded by supporting cells;
germ cells that fail to occupy a cellular cord complete the meiosis cycle and then undergo
cell degeneration and death

Testis — secretes hormones that drive male phenotype development; in the absence of testicular
hormones, a female phenotype results by default
• germ cells within supporting-cell gonadal cords differentiate into spermatogonia and then become
dormant (inhibited by supporting cells until puberty when they initiate spermatogenesis)
• gonadal cords hypertrophy, forming seminiferous cords containing spermatogonia germ cells;
at puberty, the cords become hollow seminiferous tubules
• deeply positioned gonadal cords that lack germ cells become rete testis, a collection of
sperm conducting tubules located centrally in the adult testis

• supporting cells that comprise seminiferous tubule walls become sustentacular (Sertoli) cells, which
secrete an inhibitory hormone that suppress spermatogenesis and female duct development
• supporting cells outside seminiferous cords differentiate into two populations of interstitial cells:- one population produces androgens immediately to cause male genital development
- the other produces androgens at puberty, for masculine maturation
• coelomic mesothelium covering the testis becomes visceral peritoneum; mesenchyme deep to the
mesothelium proliferates and becomes tunica albuginea

Ovary — the adult ovary is a source of oocyte gametes and estrogen & progesterone hormones
• gonadal cords that contain germ cells undergo reorganization such that each germ cell is
surrounded by a sphere of flat supporting cells—forming primordial follicles
• germ cells (oogonia) differentiate into primary oocytes that commence meiosis, but remain in
prophase of Meiosis I until ovulation (following puberty)
• 90% of germ cells fail to become incorporated into follicles; they complete meiosis and degenerate
• follicle & germ cell production is completed in-utero; the full allotment of primary oocytes that is
available during a lifetime is already present in the neonatal ovary (as primary oocytes within
primordial follicles)


Indifferent stage — an embryo has ducts for both genders; thus, anomalies are intersexual in nature
• initially every embryo has a urogenital sinus and the potential genital ducts for both genders:
- the mesonephric (Wolffian) duct, which persists following mesonephros degeneration,
becomes ductus deferens in males
- a paramesonephric (Mullerian) duct, which originates beside the mesonephric duct, starts as a groove that forms a tube and ultimately becomes uterus in females
• testicular hormones determine which duct system develops and which atrophies in an embryo:
- the mesonephric duct (ductus deferens) requires testosterone in order to develop
- the paramesonephric duct (uterus) develops spontaneously, unless its grow is blocked by an
inhibitory hormone released by the testes (sustentacular cells)

Female development — uterine tube, uterus, vagina, and vestibule formation
• mesonephric ducts (male ducts) cease to develop in the absence of testosterone from testes
• each paramesonephric duct (growing uninhibited in the absence of testes) becomes:
uterine tube - cranial region of each paramesonephric duct; it remains open-ended
uterine horn- each paramesonephric duct (caudal to the level of the inguinal fold/gubernaculum)
uterine body & cervix - formed when caudal regions of bilateral paramesonephric ducts merge and
fuse into a single midline tube; the degree of fusion (length of the body) is species dependent
• the vagina has a dual origin:
- the cranial one-third comes from fused paramesonephric ducts
- the caudal two-thirds comes from the urogenital sinus (future vestibule), as follows:
a solid tubercle (vaginal plate) grows outward from the urogenital sinus at the site of contact
between the fused paramesonephric ducts and the urogenital sinus
cavitation of the center of the solid tubercle creates the vaginal lumen; a hymen may persist
where the vagina joins urogenital sinus
• the vestibule is derived from the caudal end of the urogenital sinus


Male development — efferent ductules, epididymis, and ductus deferens formation
• growth of paramesonephric ducts (female ducts) stops, a result of exposure to the inhibitory
hormone produced by testicular sustentacular cells in the walls of seminiferous tubules
• some tubules of the degenerating mesonephros are converted to efferent ductules of the testis; they
already communicate with the mesonephric but, but they must establish communication
with sperm conducting rete testis tubules in the center of the testis
• the cranial region of the mesonephric duct undergoes extensive elongation and coiling; it becomes
epididymis
• the remainder of the mesonephric duct enlarges and becomes ductus deferens
• the mesonephric duct (ductus deferens) empties into the embryonic urogenital sinus; the latter
becomes the adult pelvic & penile urethra
• prostate and bulbourethral glands develop as outgrowths of urogenital sinus endoderm (vestibular
glands are female homologues of male bulbourethral glands)
• vesicular glands (seminal vesicles) arise as mesoderm outgrowths from the caudal region of the
mesonephric duct; gland smooth muscle comes from surrounding mesenchyme

Ligaments — genital ligaments are formed when the mesonephros degenerates
• genital ducts run along the ventral edge of the mesonephros which is coated by coelomic mesothelium
• when the mesonephros degenerates, the genital ducts and gonad become suspended by a double layer
of coelomic mesothelium, called the genital fold
• in females, the genital fold becomes: suspensory ligament of the ovary , mesovarium, mesosalpinx, and
mesometrium
• in males, the genital fold becomes mesorchium and mesoductus deferens
• the caudal extension of the genital fold that runs along the body wall and into the inguinal region is
called inguinal fold; it becomes the gubernaculum of the male fetus
• in the adult, the inguinal fold (gubernaculum) becomes:
- female: the proper ligament of the ovary and round ligament of the uterus, and
- male: the proper ligament of the testis and ligament of the tail of the epididymis
• the gubernaculum prevents closure of the body wall and is responsible for formation of the
inguinal canal and vaginal process

Gonad Descent — gonads shift caudally to a greater or lesser extent in both genders
• the gonad shifts caudally because of retention by the inguinal fold (gubernaculum) during
elongation of the growing body
• in females, the ovary remains intra-abdominal and the extent of caudal shift is species dependent
(e.g., slight in the bitch vs. caudal descent into the pelvis in the cow)
• in males the testis descends into the inguinal region, as follows:
- the gubernaculum develops as a condensation of the mesenchyme within each inguinal fold
- under the influence of gonadotropin hormones and testicular androgens, the gubernaculum
accumulates fluid and become a gel as large in diameter as the testis.
- the swollen gubernaculum enlarges the future inguinal canal
- subsequent outgrowth of the scrotal wall and dehydration of the gubernaculum passively pulls the
tail of the epididymis and testis to the inguinal canal, where a subsequent sudden increase in
intra-abdominal pressure can pop the testis through the canal into the scrotum



Indifferent stage — three mesoderm swellings give rise to the external genitalia of both sexes
1] bilateral urogenital folds - the caudal walls of the urogenital sinus bordering the urogenital orifice;
the folds & the caudal end of the urogenital sinus elongate ventrally
2] genital tubercle - an enlargement located at the ventral commissure of the urogenital folds
3] bilateral genital (labioscrotal) swellings - located lateral to the urogenital folds (in domestic
mammals these develop only in males; in humans the swellings develop in both sexes,
forming major labia in women)

Female genitalia — includes: clitoris, labia, vulval cleft, and vestibule
• the genital tubercle gives rise to the adult clitoris
• urogenital folds enlarge and become become labia of the vulva, overgrowing the genital tubercle
• the caudal portion of the urogenital sinus becomes the the adult vestibule; its caudal opening,
the urogenital orifice, becomes the vulval cleft, which opens into the vestibule

Male genitalia — includes: penis, prepuce, and scrotum
• the genital tubercle grows from its base to form an elongate phallus; the original genital tubercle
becomes the adult glans of the penis
• urogenital folds, including the urogenital sinus (space) they enclose, elongate along the caudal surface
of the phallus forming a urogenital groove
• the penile urethra is created when the urogenital groove closes by medial merger of urogenital folds
in proximal to distal sequence
• communication between the penile urethra and the exterior is established when ectoderm on the
surface of the genital tubercle invades and canalizes genital tubercle mesenchyme
• mesenchyme of the genital tubercle phallus gives rise to penile erectile tissue, tunica albuginea,
smooth muscle, and bone (carnivores)
• the prepuce is formed when a ring of surface ectoderm grows deep into the mesenchyme of the free
end of the phallus, dividing the phallic tissue into a penis encircled by preputial skin;
except in cats, the prepuce of domestic mammals is formed differently, by the phallus
elongating subcutaneously along the body wall.
• bilateral genital (labioscrotal) swellings enlarge and merge at the midline to form a single scrotum
with two compartments; the scrotum initially develops over the gubernaculum in the
inguinal region, subsequently it shifts to its species-specific adult position


Mammary Glands — arise from bilateral lines of thickened ectoderm in both sexes
• bilateral mammary ridges of thickened ectoderm develop from the axillary to the inguinal region
• mammary buds develop periodically along the ridge (between buds, ridge ectoderm regresses)
• each bud develops into a mammary gland (2, sheep, goat, mare; 4, cow; 8, queen; 10, bitch; 14, sow)
• at each bud, the ectoderm induces proliferation of underlying mesoderm to produce the teat per gland
• teat mesoderm induces surface ectoderm to proliferate and form lactiferous duct(s), as follows:
- a cord of epithelial cells grow into underlying mesoderm and canalizes to form a lactiferous duct
- the number of cell cord invasions determines the number of lactiferous ducts per teat per species
(1 in sheep, goat, cow; 2 in mare & sow; 6 in queen; 12 in bitch)
- in some cases, multiple lactiferous ducts open into a pit (inverted nipple) that becomes a
nipple (teat) following proliferation of underlaying mesoderm
• commonly, extra buds develop and degenerate, failure to degenerate results in supernumerary teats