1. Dr. asmaa A. al sanjary

2. Following fertilization the normal embryo contains 23 sets of chromosomes,including 22 autosomes and one sex chromosomes from each parent. 46XY embryo will develop as a male 46XX embryo will develop as a female It ’ s the presence or the absence of the Y chromosome which determines whether the undifferentiated gonad becomes a testis or an ovary.

3. The testis determining factor is on the Y chromosome on a gene termed the SRY (sex determining region of the Y chromosome). This gene triggers testis formation from the indifferent gonad.

4. The development of the differentiated gonad is fundamental for the development of the other genital organs. testes leads to male genital organs. absence of testis leads to female genital Organ.

5. The genital organs and those of the urinary tract arise in the intermediate mesoderm on either side of the root of the mesentery beneath the epithelium of the coelom. The mesonephric(wolffian) duct appear in association of the pronephros (transient tubules in the cervical) and extends caudally to open at the cloaca.

6. The mesonephros, the second primitive kidney develops as a swelling bulging into the dorsal wall of the coelom of the thoracic and upper lumber region. The mesonephros in the male persists in part as the excretory portion of the male genital system ; in the female only a small vestiges survive

7. The genital ridge in which the gonads of each sex develop is visible as a swelling on the medial aspect of the mesonephros. The paramesonephric duct which forms much of the female genital tract develops as in growth of the coelomic epithelium laterally, then become a groove then as a tube below the surface

8. The two paramesonephric ducts extend caudally until they reach the urogenital sinus (about nine weeks gestation). The blind end will project into the posterior wall of the sinus to form the Mullerian tubrcule. At the beginning of the third month both the Mullerian and the wolffian ducts are present and capable of development.

9. In the male there is development of the wolffian duct and degeneration of the paramesonephric duct as a result of the production of the Mullerian inhibitory factor (MIF) produced by the fetal testis. In the female there is degeneration of the wolffian system and marked growth of the mullerian system.(absent MIF)

10. The lower ends of the Mullerian ducts come together in the midline,fuse and develop into the uterus and cervix. The cephalic ends of the duct remain separate to form the fallopian tubes. Proliferation of the mesenchyme a round the fused portion of the ducts form the thick muscular wall of the uterus and the cervix.

11.  At the point of fusion of the para -mesonephric duct to the dorsal wall of the urogenital sinus ( the Mullerian tubercule) there is marked growth of the tissues from which the vagina will forms, known as the vaginal plate, extending between the cervix and the urogenital sinus.  canalization of the vagina occurs and completed around the 20-24 weeks gestation. Incomplete canalization at any level result into a vaginal septum and outflow tract obstruction.

12. The primitive cloaca becomes divided by a transverse septum into an anterior urogenital portion and posterior rectal portion. The urogenital portion of the cloacal membrane breaks down and it’s divided into three portions ; external expanded phallic part, a deeper narrow pelvic part (between it and the region of the Mullerian Tubercle ), and a vesico -urethral part connected superiorly into the allantois.

13.  Externally in this region the genital tubercle forms a conical projection around the anterior part of the cloacal membrane.  Two pairs of swelling on the medial part (genital folds),  Two lateral pairs (genital swelling) formed by proliferation of mesoderm around the end of the urogenital sinus.  Development till ten weeks is the same in the male and female and differentiation occur later.

14.  The vesicourethral portion of the urogenital sinus forms the bladder and urethra, the pelvic and the phallic portion forms the vestibule,  the genital tubercle enlarge slightly and forms the clitoris,  the genital fold form the labia minora  the genital swelling enlarges to form the labia majora.

16.  The primitive gonads will appear in the embryo at 5 wks. as a proliferation of the coelomic epithelium on the medial aspect of the urogenital ridge and grows into mesenchyme (primary sex cord) and the gonads now have outer cortex and inner medulla.  In embryos with XX complement, the cortex differentiate to form most of the ovary and the medulla regresses.  The primordial germ cells develop by the four week in the endodermal cells of the yolk sac and during the five week they migrate along the dorsal mesentery of the hind gut to the gonadal ridge and become incorporated into the mesenchyme and the primary sex cord by six week.

18.  In the ovary there is active growth of the sex cord forming the pregranulosa cells which surround the oocyte. There is active growth phase make the largest number of oocyte surrounded by pregranlosa cells there is about seven million at twenty weeks, at birth it’s about (two million).  Those oocyte who fails to surround themselves with the pregranulosa cells will disintegrate  Follicular development will become evident by twenty eight wks. of intrauterine life.

19.  In XY embryo the testicular differentiation is evident by seven week by disappearance of the germ cells from the cortex and the cells differentiate into fibroblasts which form the tunica albuginea. The deeper parts of the sex cord give rise to the rete testis and the seminiferous and straight tubules.