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Thedevelopment of the urinary system begins duringprenatal development, and relates to the development of theurogenital system – both the organs of theurinary system and thesex organs of thereproductive system. The development continues as a part ofsexual differentiation.
The urinary and reproductive organs are developed from theintermediate mesoderm. The permanent organs of the adult are preceded by a set of structures which are purely embryonic, and which with the exception of the ducts disappear almost entirely before birth. These embryonic structures are on either side; thepronephros, themesonephros and themetanephros of thekidney, and theWolffian andMüllerian ducts of thesex organ. The pronephros disappears very early; the structural elements of the mesonephros mostly degenerate, but thegonad is developed in their place, with which the Wolffian duct remains as the duct in males, and the Müllerian as that of the female. Some of the tubules of the mesonephros form part of the permanent kidney.
In the outer part of theintermediate mesoderm, immediately under the ectoderm, in the region from the fifthcervical segment to the thirdthoracic segment, a series of shortevaginations from each segment growsdorsally and extendscaudally, fusing successively from before backward to form thepronephric duct. This continues to grow caudally until it opens into the ventral part of thecloaca; beyond thepronephros it is termed theWolffian duct. Thus, the Wolffian duct is what remains of the pronephric duct after the atrophy of the pronephros.
The original evaginations form a series of transverse tubules each of which communicates by means of a funnel-shaped ciliated opening with theabdominal cavity, and in the course of each duct a glomerulus also is developed. A secondary glomerulus is formed ventral to each of these, and the complete group constitutes the pronephros. In humans, the pronephros is just rudimentary, and undergoes rapid atrophy and disappears.
On the medial side of the Wolffian duct, from the sixth cervical to the third lumbar segments, a series of tubules, theWolffian tubules, develops. They increase in number by outgrowths from the original tubules. They change from solid masses of cells to instead become hollowed in the center. One end grows toward and finally opens into the Wolffian duct, the other dilates and is invaginated by a tuft of capillary bloodvessels to form a glomerulus. The tubules collectively constitute themesonephros.
The mesonephros persists and form the permanent kidneys in fish and amphibians, but in reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the permanent kidney (metanephros) develops beginning during the sixth or seventh week, so that by the beginning of the fifth month only the ducts and a few of the tubules of the mesonephros remain.
In the male, theWolffian duct persists, and forms for example theepididymis, theductus deferens, theejaculatory duct,seminal vesicle and efferent ducts.
In the female, on the other hand, the Wolffian bodies and ducts atrophy, leaving behind only remnants in the adult, involving e.g. thedevelopment of the suspensory ligament of the ovary.
Shortly after the formation of the Wolffian ducts, a second pair of ducts is developed; these are theMüllerian ducts. Each arises on the lateral aspect of the corresponding Wolffian duct as a tubular invagination of the cells lining the abdominal cavity. The orifice of the invagination remains open, and undergoes enlargement and modification to form the abdominalostium of the fallopian tube. The ducts pass backward lateral to the Wolffian ducts, but toward the posterior end of the embryo they cross to the medial side of these ducts, and thus come to lie side by side between and behind the latter—the four ducts forming what is termed thecommon genital cord, to distinguish it from thegenital cords of the germinal epithelium seen later in this article. The Müllerian ducts end in an epithelial elevation, theMüllerian eminence, on the ventral part of the cloaca between the orifices of the Wolffian ducts. At a later stage, the eminence opens in the middle, connecting the Müllerian ducts with the cloaca.
In the male, the Müllerian ducts atrophy, but traces of their anterior ends are represented by theappendices testis (hydatids of Morgagni of the male), while their terminal fused portions form theutriculus in the floor of theprostatic urethra.
In the female, the Müllerian ducts persist and undergo further development. The portions which lie in thegenital cord fuse to form theuterus andvagina. This fusion of the Müllerian ducts begins in the third month, and the septum formed by their fused medial walls disappears from below upward.
The parts outside this cord remain separate, and each forms the correspondingfallopian tube. Theostium of the fallopian tube remains from the anterior extremity of the original tubular invagination from the abdominal cavity.
Around the fifth month, a ring-like constriction marks the position of thecervix of the uterus, and after the sixth month, the walls of the uterus begin to thicken. For some time, the vagina is represented by a solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the futurevaginal fornix. At about the fifth or sixth month, thelumen of the vagina is produced by the breaking down of the central cells of the epithelium. Thehymen represents the remains of the Müllerian eminence.[1]
The metanephros is the definite, permanent, but yet immature kidney. It arises from two directions. On one hand, the precursor of the ureter buds from the Wolffian duct, while on the other hand, the precursor of the renal tubules develop from the metanephrogenic blastema. The ureteric bud subsequently grows into the latter mass, forming the parts of the nephron. Other changes include e.g. the translocation of the ureteric opening directly into the cloaca.
The rudiments of the permanentkidneys make their appearance about the end of the first or the beginning of the second month. Each kidney originates as aureteric bud from the caudal end of the Wolffian duct, which, in turn, originates fromintermediate mesoderm. The ureteric bud starts close to where the Wolffian duct opens into the cloaca, and grows dorsalward androstralward along the posterior abdominal wall, where its blind extremity expands and subsequently divides into several buds, which form the rudiments of therenal pelvis andrenal calyces; by continued growth and subdivision it gives rise to thecollecting duct system of the kidney. The other, more superficial, portion of the diverticulum, on the other hand, becomes the ureter.
Therenal corpuscles andrenal tubules, in contrast, are developed from the metanephrogenic blastema instead of from the ureteric bud. The metanephrogenic blastema is moulded over the growing end of the latter, and becomes a part of the metanephros in this way. The renal tubules of the metanephros, unlike those of the pronephros and mesonephros, do not open into the Wolffian duct. Instead, the tubules rapidly elongate to form the parts of thenephron: theproximal tubules, theloops of Henle and thedistal convoluted tubules. These last join and establish communications with the collecting duct system derived from the ultimate ramifications of the ureteric bud. In the other end, the renal tubules give rise toBowman's capsules andglomeruli.
The mesoderm around the tubules becomes condensed to form the connective tissue of the kidney. The ureter opens at first into the hind-end of the Wolffian duct; after the sixth week it separates from the Wolffian duct, and opens independently into the part of the cloaca which ultimately becomes theurinary bladder. The renal tubules become arranged intorenal pyramids, and the lobulated condition of the kidneys exists for some time after birth, while traces of it may be found even in the adult. The kidney of the ox and many other animals, on the other hand, remains lobulated throughout life.
Theurinary bladder is formed partly from the endodermalcloaca and partly from the ends of the Wolffian ducts. In other words, theallantois takes no share in its formation.
After the separation of the rectum from the dorsal part of the cloaca, the ventral part becomes the primaryurogenital sinus.[2] The urogenital sinus, in turn, divides into the superficial definitive urogenital sinus and the deeper anterior vesico-urethral portion.
The definitive urogenital sinus consists of a caudal phallic portion and an intermediate narrow channel, the pelvic portion.
The vesico-urethral portion is the deepest, continuous with the allantois. It absorbs the ends of the Wolffian ducts and the associated ends of the renal diverticula, and these give rise to thetrigone of urinary bladder and part of theprostatic urethra. The remainder of the vesico-urethral portion forms the body of the bladder and part of the prostatic urethra; its apex is prolonged to the umbilicus as a narrow canal, theurachus, which later is obliterated and becomes themedian umbilical ligament of the adult.
Until about the ninth week of gestational age,[3] thesex organs of males and females look the same, and follow a common development. This includes the development of a genital tubercle and a membrane dorsally to it, covering the developingurogenital opening, and the development of labioscrotal folds.
Even after differentiation can be seen between the sexes, some stages are common, e.g. the disappearing of the membrane. On the other hand, sex-dependent development include further protrusion of the genital tubercle in the male to form the penis. Furthermore, the labioscrotal folds evolve into the scrotum in males, while they evolve into labia in females.
A.—Diagram of the primitive urogenital organs in the embryo previous to sexual distinction.
B.—Diagram of the female type of sexual organs.
C.—Diagram of the male type of sexual organs.