Download Sexual differentiation and genitals 2008

Embryology #13 Sexual differentiation and genitals
Discuss the determination of genetic sex.
 The genetic makeup of an individual determines their sex.
 XY = males, while XX = females
 For additional points about this objective, refer to previous notes in embryology on fertilization and
development.
Discuss the formation of germ cells and the features of the indifferent gonad.
 While the sex of a baby is determined at fertilization, the gonads do not acquire “noticeable” male or
female characteristics until week 7
 Primordial germ cells appear at an early stage of development in the wall of the yolk sac
 They travel up the dorsal mesentery of the hindgut, where they then enter into an area called the genital
ridge
 The epithelium of the genital ridge proliferates, and these cells begin to penetrate the underlying
mesenchyme
 In this layer of mesenchyme, the primordial germ cells are formed; at this time they are considered
“indifferent” because a contrast can’t be made between the name and female gonad
Describe the development of the testes and ovaries.
 Testes (pp. 240-241)
o Under the influence of the SRY gene on the Y chromosome, the primitive sex cords penetrate deep
in the medulla to form the medullary cords
o At the hilum of the gland, the cords break up into a network of strands which gives rise to the rete
testis
o The tunica albuginea (a dense layer of connective tissue) also forms, separating the medullary cords
from the surface epithelium
o During the fourth month, the testis cords become horseshoe shaped, and are composed of primitive
germ cells and Sertoli cells; Leydig cells lie between the testis cordis, and by the 8th week, begin the
production of testosterone
o At puberty, testis cords develop a lumen and form the seminiferous tubules
 Ovary (pp. 241-242)
o In the medullary part of the ovary are found many clusters, which contain groups of primitive
germ cells
o Unlike the surface epithelium of a male, that of a female continues to proliferate; in week 7, it gives
rise to the cortical cords, which penetrate the underlying mesenchyme
o In the fourth month, these cords split into cell clusters which surround the germ cells; these germ
cells then develop into oogonia, while the surrounding epithelium becomes follicular cells (** think
back to chapter 2)
Describe the development of the genital ducts.
 Pages 242-246
 Indifferent Stage
o Both female and male embryos start off with two pairs of genital ducts, the mesonephric (wolffian)
and paramesonephric (mullerian)
o In one’s inferior portion, the paramesonephric duct runs lateral to the mesonephric duct, only to
later cross it and become more medial (and closer to the paramesonephric duct of the opposite side)
o Although initially separated, the two paramesonephric ducts unite to form the uterine canal
o
o
In male: Mesonephros regresses; Epigenital tubules form the efferent ductules of testis, while
paragenital tubules have no contact with rete testis; Below entrance of efferent ductules, the
mesonephric ducts elongate and form the epididymis
In female: Paramesonephric ducts develop into the main genital ducts; With descent of an ovary,
the cranial vertical and horizontal portions become the uterine tube, while the caudal vertical
portion forms the uterine canal; Once the two paramesonephric ducts fuse at the midline, the broad
ligament of the uterus is formed; the uterus and broad ligaments divide the pelvic cavity into the
uterorectal pouch and the uterovesical pouch
Describe the formation of the vagina.
 Pages 246-247
 It all starts with two evaginations from the pelvic part of the urogenital sinus
 These evaginations, coined sinovaginal bulbs, form a solid vaginal plate
 As the vaginal plate proliferates, a greater distance exists between the uterus and urogenital sinus
 By month five, this vaginal outgrowth is entirely canalized, and is said to be of dual origin – its top part
evolved from the uterine canal, and its bottom from the urogenital sinus
 The female may retain some remnants of the cranial and caudal excretory tubules in the epoophoron and
paroophoron
Describe the development of the male and female external genitalia.
 Pages 248-251
 Indifferent Stage
o Week 3 of development, a pair of slightly elevated cloacal folds form
o At the top of the folds is found the genital tubercle, while the bottom is divided into anterior
urethral folds and posterior anal folds
o At the same time, genital swellings on the side of these folds will become noticeable. They later go
on to form the scrotal swellings in the male and the labia majora in the female.
o External Genitalia in male
 Androgen-influenced
 Characterized by quick elongation of the genital tubercle (now called phallus)
 Urethral folds pulled forward to form the lateral walls of urethral groove, whose lining
forms the urethral plate
 Penile urethra formed once the two urethral folds close
 Scrotal swellings make up the scrotum
o External Genitalia in female
 Estrogen-influenced
 Genital tubercle forms the clitoris
 Urethral folds don’t come together, but form the labia minora
 Genital swellings form the labia majora
 Urogenital groove remains open and becomes the vestibule
Describe the descent of the testes and ovaries.
 Pages 253-255
 Descent of Testes
o Testis originally begins in abdominal wall
o As the testis begins to descend toward inguinal ring, the gubernaculum grows from the inguinal
region toward the scrotal swellings
o Testis reaches inguinal region by week 12, and the scrotum by about week 33
o

An evagination known as the processus vaginalis follows the course of the gubernaculums into the
scrotal swellings
o Eventually, this processus vaginalis develops into the tunica vaginalis (which has both a visceral
and parietal component)
o Muscles in abdominal wall (except for transverus abdominis) contribute to the three-layered sheath
which also surrounds testis
Descent of Ovary
o Considerably less than that of the male
o Ovaries settle just below rim of true pelvis
o Cranial genital ligament forms the suspensory ligament whereas the caudal congenital ligament
forms the ligament of the ovary proper and the round ligament of the uterus
Discuss the common congenital malformations associated with development of the genital system
NOTE: I would know these in and out, since the last test consisted
fully of congenital defects.
1) Duplications of the uterus – Picture on page 247; Results from lack of fusion of paramesonephric ducts at their
normal area; many different forms exist, although one of the most common is the uterus bicornis, in which the
uterus has two horns entering a common vagina
2) Hypospadias – Page 250; fusion of the urethral folds is incomplete, and abnormal openings of the urethra occur
along the inferior aspect of penis
3) Epispadias – Page 250; Rare condition; Urethral meatus is found on the dorsum of the penis; MOST OFTEN
ASSOCIATED WITH EXSTROPHY OF THE BLADDER
4) Klinefelter Syndrome – Page 251, Patient presents with karyote of 47 XXY (or 48 XXXY); characterized by
infertility, gynecomastia, impaired sexual maturation; most common cause is nondisjunction of the XX
homologues
5) Turner’s Syndrome – Page 18; 45, X karyotype; only monosomy compatible with life; presents with a “female”
appearance, webbed neck, broad chest, and widely spaced nipples
6) Androgen-Insensitivity Syndrome – Page 252; occurs in patients with a 46, XY chromosome complement but
have the external appearance of normal females; spermatogenesis does not occur
7) Hermaphrodites
True: have both ovary and testis tissue present; very rare, although most raised as female
Pseudo male: Underdeveloped gonadal structures; genetically XY, but may not be able to produce testosterone
Pseudo female: genetic makeup of XX, excessive levels of androgens (because there is maybe a tumor on adrenal
gland or too much testosterone)