The number of segments occupied by the mesonephros is further reduced, extending approximately from T-11 to L-1. The capsules and tubules at the cranial end disappear. Only the more caudal tubules persist and come to lie close to the lateral side of the gonad. See Section IV, E below for their fate in each sex.
Evidence that the human mesonephros functions as an excretory organ during the embryonic period has not been demonstrated.
KIDNEY (METANEPHROS)
The collecting tubules continue to divide at the periphery of the metanephros until approximately 12 generations are formed by the end of the fifth month. The second, third and fourth generations enlarge and coalesce to form the minor calyces of the renal pelvis. The tubules of the fifth generation become the collecting tubules that empty into the minor calyces.
Cell groups in the renal capsule differentiate into small spheres called renal vesicles in the vicinity of the collecting tubules. Each vesicle will form a nephron, which is the functional unit of the kidney. Each renal vesicle first becomes S shaped. The lower part of the S is invaginated subsequently by blood vessels forming a renal capsule (glomerulus and glomerular capsule). The upper part of the S will become a secretory tubule that establishes a communication with one of the collecting tubules. The secretory tubule will elongate and form the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule.
The kidney is functional in the late fetal period.
BLADDER AND URETHRA (UROGENITAL SINUS)
The bladder portion of the urogenital sinus becomes the definitive bladder as the proximal segment of the ureter and mesonephric duct become incorporated into its lower wall to form the trigone. During the incorporation the orifice of the ureter moves cranial to the orifice of the mesonephric duct.
In the male the pelvic portion of the urogenital sinus will become the prostatic and membranous parts of the urethra. The phallic portion forms the penile urethra after the urogenital folds fuse on the underside of the phallus (penis). In the female the pelvic portion gives rise to part of the urethra and vagina. The phallic portion remains open thus forming most of the vaginal vestibule into which open the urethra and eventually the vagina.
By the end of the third month buds of the epithelial lining of the urethra extend into the surrounding mesenchyme. These buds will form the prostate gland in the male and the urethral and paraurethral glands in the female.
GONAD (DIFFERENT STAGE)
The gonad by its microscopic appearance, can be determined to be a testis in a genetically male embryo or an ovary in a genetically female embryo.
TESTIS
The gonadal cords elongate into discrete, solid testis cords that communicate with one another. The cords join a network of minute cords near the mesonephric remnant that will canalize at puberty and form the rete testis.
The testis cords are made up of germ cells and sustentacular cells (Sertoli). At puberty a lumen appears in the cords converting them into seminiferous tubules and the germ cells begin producing sperm. The seminiferous tubules empty into the rete testis, which in turn empties into the duct remnants of the mesonephros.
A tough capsule of dense connective tissue called the tunica albuginea begins to develop around the testis. Mesenchymal cells between the testis cords will differentiate into the interstitial cells (Leydig).
OVARY
The gonadal cords extend into the central part of the ovary where they break up into cell clusters containing groups of primordial germ cells. The cell clusters disappear and are replaced by a vascular stroma that forms the definitive ovarian medulla. No rete ovarii is formed.
The surface cells continue to proliferate and give rise to a second generation of cords called cortical cords. These cords remain close to the surface where they break up and surround the naked oogonia (germ cells). The surrounding cells will become the follicular cells.
GENITAL DUCTS
The mesonephric duct, mesonephric tubules and female duct develop differently in the two sexes. Differences in the pattern of development are apparent in the eighth week.
MALE
As the mesonephros regresses, five to 10 of the tubules adjacent to the testis make contact with the rete testis and become the efferent ducts. The efferent ducts drain into the mesonephric duct, which terminates in the upper (prostatic) part of the urethra and mainly becomes the ductus deferens (vas). The orifice of the mesonephric duct in the urethra moves closer to the midline. The segment of the mesonephric duct that receives the efferent ducts will become highly convoluted to form the ductus epididymis. Near its termination in the urethra an outpocketing will develop that produces the seminal gland (vesicle). Between the seminal gland and the urethra the mesonephric duct is called the ejaculatory duct. Caudal to the testis, vestiges of the mesonephric tubules will become the paradidymis.
The female duct regresses and most of it eventually disappears. Its cranial end will persist as the appendix testis. Its caudal end fuses with its counterpart on the opposite side. Some embryologists believe this portion is represented in the adult by a small diverticulum of the prostatic urethra called the prostatic utricle.
FEMALE
The mesonephric duct and tubules regress and will mainly disappear. Small remnants of the cranial portion may persist as the epoophoron and paroophoron in the mesentery of the ovary (mesovarium).
The female duct enlarges and canalizes. It can be divided into three segments: a) a cranial longitudinal segment that opens into the peritoneal cavity; b) an intermediate horizontal segment that passes toward the midline ventral to the mesonephric duct and c) a caudal longitudinal segment that fuses in the midline with its counterpart on the opposite side. The female ducts together assume a Y shape. The first two segments form the uterine tube, which opens into the peritoneal cavity at the abdominal ostium. As the intermediate segment passes medially, it brings with it a fold of tissue that will become the broad ligament of the uterus. The gonad lies dorsal to the fold or ligament. The fused caudal segment is the uterine primordium that will give rise to the uterus. Initially a septum passes through the middle of the primordium, which disappears thus forming the uterine cavity.
The caudal segment of the female duct makes contact with the dorsal wall of the pelvic portion of the urogenital sinus. The wall in the area of contact proliferates forming the sinus (Müllerian) tubercle. In the ninth week the tubercle gives rise to two solid evaginations called sinuvaginal bulbs. The bulbs fuse to form a solid vaginal plate with which the uterine primordium is continuous. A lumen begins to form in the plate by the end of the third month converting it into the vagina. By the end of the fifth month the vagina is entirely canalized. Its upper portion expands around the lower part of the uterus (cervix) to form fornices. A thin membrane called the hymen initially separates the vaginal canal from the vestibule of the vagina. It ruptures at a variable time after birth.
EXTERNAL GENITALIA
When the cloacal membrane disappears, the phallic part of the urogenital sinus opens to the exterior through the urethral groove and the rectum opens to the exterior through the anal canal and anus. Differences appear in the external genitalia (perineum), which permit sexual identification during the eighth week.
MALE
The genital tubercle elongates and enlarges into the phallus, which will become the penis. Mesenchymal tissue within the phallus organizes into two very vascular erectile masses called corpora cavernosa. Each corpus cavernosum attaches to the pubic bone by a crus. The urogenital groove and folds extend onto the underside of the phallus. The epithelial lining of the groove produces a urethral plate. By the end of the third month the urogenital folds fuse forming a raphe and converting the urethral plate into the penile part of the urethra. The penile part of the urethra is surrounded by erectile tissue called the corpus spongiosum that is continuous with the rounded glans penis at the distal tip of the phallus. During the fourth month an epithelial cord of cells at the tip of the glans penis grows deep to join the penile urethra. The cord soon canalizes whereby the penile urethra opens to the exterior through the meatus on the glans penis. The proximal part of the corpus spongiosum enlarges to form the bulb of the penis.
The labioscrotal swelling becomes the scrotal swelling. The swelling will fuse in the midline with its counterpart on the opposite side to form the scrotum into which the testes will later descend. A scrotal septum will develop in the fusion area and separate the two testes. The fusion area is indicated on the surface by a raphe that joins with the raphe above and together they course from the anus to the glans penis.
FEMALE
The genital tubercle elongates and enlarges to a lesser extent producing a smaller phallus that will become the clitoris. The glans clitoris develops as a small rounded area at the distal tip of the phallus. Both the clitoris and its glans are also composed of erectile tissue. The urogenital folds do not fuse but remain open to become the labia minora. A mass of erectile tissue called the bulb of the vestibule develops at the base of each labium minus. The urogenital groove and phallic portion of the urogenital sinus together form the vestibule of the vagina located between the labia minora.
The labioscrotal swellings enlarge and remain separated thereby forming the labia majora.
DESCENT OF THE GONADS
By the end of the second month the gonads in both sexes begin to shift caudally in relation to the expanding abdominal wall. Since the initial blood supply from the aorta in the upper abdominal region is essentially retained, both the testicular and ovarian arteries descend from their original level to the testis and ovary, respectively.
The once broad urogenital mesentery, which attaches the mesonephros and gonad to the dorsal body wall, becomes very thin. Its cranial portion disappears along with the mesonephros. Its caudal portion becomes the gubernacular fold and extends caudally to the inguinal region. A band of mesenchyme runs from the caudal pole of the gonad, through the base of the gubernacular fold and inguinal region, into the labiogenital swelling. This band is called the gubernaculum, which is the "governor" of gonadal descent.
The change in position of the testis is greater than the ovary. The cause of the descent is not clear but it has been shown to be under hormonal influence.
Testis—By the end of the third month the testis lies behind the peritoneum near the inguinal part of the abdominal wall. A diverticulum of the peritoneal cavity called the vaginal sac (process) invades the gubernacular mesenchyme in the inguinal wall and marks the position of the inguinal canal. During the seventh month the testis passes through the canal but remains behind the peritoneum of the vaginal sac. When the vaginal sac reaches the scrotal swelling, it converts the swelling into a sac that receives the testis usually by the end of the eighth month. The lower part of the vaginal sac lines the scrotal cavity around the testis with a serous membrane called the tunica vaginalis testis. The upper part disappears when its walls come together thereby isolating the scrotal cavity from the peritoneal cavity. After the testis descends completely, the only remnant of the gubernaculum is a short, broad ligament between the caudal pole of the testis and the scrotal sac. It is necessary that the testis descends completely if it is to actively produce sperm at puberty.
Ovary—The ovary descends only as far as the brim of the true pelvis. Its gubernaculum becomes the ligament of the ovary and the round ligament of the uterus. The two ligaments together form a continuous band that runs from the caudal pole of the ovary to the labium majus.
Source: Atlas of Human Embryos.