Development of the Urinary Organs
The intermediate mesoderm reflects in muted fashion the segmentation that is so evident in the adjoining somites. It soon forms in its caudal domain a continuous solid longitudinal (nephrogenic) thickening from which arise, in craniocaudal and temporal sequence, three attempts at the formation of an excretory organ.
The first attempt, which is transient and nonfunctional, constitutes the pronephros and forms in the presumptive neck region. The second attempt, the mesonephros, forms in the thoracic and lumbar regions and is functional through a large part of embryonic life. The third attempt, the metanephros, forms in the lumbar region and becomes the adult kidney (Fig. 5.3).All three structures have a series of excretory tubules as their essential histologic feature. In the pronephros one end of each tubule turns caudally to meet its neighbor to form a continuous pronephric duct (Fig. 5.3/4), which grows at its caudal end to open into the cloaca. The duct survives the regression of the pronephric tubules, which are nonfunctional, and becomes the drainage of the mesonephric tubules that now appear.
Each of the many mesonephric tubules resembles a rather simple version of the nephron of the adult kidney (see Fig. 5.27). The blind end is invaginated by a capillary tuft to form a filtration mechanism and the connection of the other end with the pronephric duct, now more appropriately termed the mesonephric duct, provides an outlet for the urine. The mesonephros may be a very prominent organ at its apogee, when it projects from the roof of the abdomen (Fig. 5.4). Its size varies among species and is in inverse proportion to the permeability (and thus the excretory efficiency) of the placenta. The mesonephros is supplanted by the metanephros when it begins its craniocaudal regression. Parts, however, survive to be given fresh use by the male reproductive system (Fig.
5.5).The metanephros has two primordia. One is provided by an outgrowth, the ureteric bud, from the lower end of the mesonephric duct close to its opening into the cloaca. This bud grows cranially into the metanephric blastema constituted by the caudal part of the nephrogenic cord (Fig. 5.3/5). The extremity of the bud undergoes a dozen or so dichotomous divisions. Branches of the later orders become the collecting tubules of the kidney, whereas those of the first few orders are later reabsorbed into the terminal expansion of the duct in a variable fashion that accounts for the specific forms of the renal pelvis and calices. The outer part of the metanephric mass forms the capsule and interstitium of the kidney, and cellular condensation in the inner part creates the cell cords that are transformed into nephrons. One end of each cell cord makes contact with a connecting duct, and once canalization has occurred, a continuous passage is established (Fig. 5.6). The other extremity of the nephron becomes invaginated by a vascular tuft supplied from a local branch of the aorta; this structure forms the glomerulus (see also Fig. 5.27).
FIG. 5.1 The male urinary and reproductive organs (dog). 1, Right kidney; 2, ureter; 3, bladder; 4, testis;
5, epididymis; 6, spermatic cord; 7, vaginal ring; 8, deferent duct; 9, prostate; 10, corpus spongiosum (spongy body); 11, retractor penis; 12, corpus cavernosum (cavernous body); 13, glans penis; 13, bulb of glans; 14, os penis; 15, preputial cavity; 16, prepuce; 17, rectum.
FIG. 5.2 The female urinary and reproductive organs (bitch). 1, Right kidney; 2, ureter; 3, bladder; 4, ovary; 5, uterine tube; 6, uterine horn; 7, cervix; 8, vagina; 9, urethra; 10, vestibule; 11, clitoris; 12, vulva; 13, vaginal process; 14, rectum.
FIG.
5.3 Differentiation of the intermediate mesoderm. 1, Pronephros; 2, mesonephros, segmented cranially but continuous caudally; 3, metanephros; 4, pronephric (later mesonephric) duct; 5, ureteric bud; 6, urachus.The horizontal division of the cloacal region of the hindgut forms the lower urinary passages. The division itself is due to the caudal growth of a wedge of mesoderm, the urorectal septum, present within the angle between the hindgut and the allantoic bud. The septum eventually reaches the cloacal membrane, which is thus divided into dorsal (anal) and ventral (urogenital) parts (Fig. 5.5/9). The fusion site corresponds to the perineal body. The breakdown of the anal membrane transforms the dorsal passage into a continuous rectoanal canal. A similar process in the urogenital membrane opens the ventral passage to the surface of the body. This urogenital passage differentiates into a cranial part, the future bladder and allantois, and a caudal part, from which the urethra is formed.
FIG. 5.4 (A) Ventral and (B) lateral views of the abdominal roof in a pig embryo of 2.5 cm. The pronephric duct drains the mesonephros and is now more aptly termed the mesonephric duct. 1, Developing gonad; 2, mesonephros; 3, mesonephric duct; 4, paramesonephric duct; 5, metanephros; 6, ureter.
FIG. 5.5 The development of the metanephros from two primordia (metanephric cord and ureteric bud). Note the gradual regression of the mesonephros. 1, Metanephros; 2, ureteric bud (future ureter); 3, mesonephric (deferent) duct; 4, rectum; 5, cloaca; 5', cloacal membrane; 6, urogenital sinus; 7, gonad; 8, remnant of mesonephros (future epididymis); 9, urorectal septum; 10, anal membrane; 11, urogenital membrane.
The bladder then appears as a widening that is continued cranially by the allantoic duct and caudally by an undilated urethra.
The allantoic duct or urachus (Fig. 5.3/6) can be followed through the umbilical opening to an extraembryonic expansion (the allantois) in which urine accumulates and which is discarded at birth. The part of the duct within the fetus then shrivels and is finally represented only by the cicatrix or scar on the apex of the bladder. The caudal part of the primordium is transformed into the urethra—the entire urethra in the female but only the short pelvic urethra in the male (in which the penile urethra develops with the genital system). The definitive positions of the openings of the mesonephric and metanephric ducts result from the incorporation of their lower ends within the larger passage. The rearrangement brings the opening of the metanephric duct (ureter) into the bladder and situates that of the mesonephric duct (deferent duct) more caudally within the urogenital sinus (see Fig. 5.5). In this process the mesoderm of the mesonephric duct provides the epithelium of the dorsal trigonal region of the bladder (p. 172) and the hindgut endoderm that of the remaining part. The outer layers of the bladder wall differentiate from local mesoderm.
FIG. 5.6 This series of schematic drawings depicts the connections between developing nephrons (1) and branches (2) of the ureteric bud. Note the dichotomous division of the drainage system (ureteric bud).