The reproductive system in vertebrates is a most varied assemblage of primary and accessory organs and parts, which begin developmentally in a similar fashion but result in strikingly different forms in the adult. In recent years the study of reproduction in animals (theriogenology) has made great strides, and as a result of our new understanding we are now able to manipulate the system in many ways such as to facilitate artificial insemination, egg or embryo transfer, freezing and storage of eggs and embryos and cloning of various species. For explanations of developmental processes in domestic animals see Noden and de Lahunta (1985). For an overall view of the reproductive system from fish to humans there is nothing better than Marshall’s Physiology of Reproduction. The most recent revision of Marshall’s by Lamming (1990-1992) devotes one volume to a consideration of reproductive cycles and female anatomy, a second to reproductive structures and functions of the male, and a third to pregnancy and lactation. Other books on the reproductive system include Austin and Short (1982), Segal et al. (1973), and Cupps (1991). For information on the reproductive habits, cycles, and gestations of mammals of the world, including canids, reference should be made to Asdell’s Patterns of Mammalian Reproduction: A Compendium of Species-Specific Data by Hayssen and van Tienhoven (1993).
Embryolocic Characteristics Of The Urocenital System
The development of the mammalian urogenital system is briefly summarized here. For specific details see the textbooks of embryology by Noden and de Lahunta (1985), Latshaw (1987) and McGeady et al (2009). The homologic characteristics of the structures in the male and female are shown in Box Homologies of Genital Organs in Male and Female Mammals.
Homologies of Genital Organs in Male and Female Mammals
|Appendix testis||Abdominal ostium of uterine tube|
|Proper ligament of testis, lig. of tail of epididymis||Proper ligament of ovary, round ligament of uterus|
|0s penis||0s clitoridis (inconstant)|
|Glans penis||Glans clitoridis|
|Corpus spongiosum||Vestibular bulb|
|Corpus cavernosum||Corpus cavernosum|
|Scrotal raphe||Dorsal commissure of labia|
|Prepuce||Fold of fossa clitoridis|
In the classical evolutionary approach to the development
of the urinary system there is the formation of three kidneys, the pronephros, mesonephros, and metanephros, that develop in sequence from cranial to caudal along the urogenital ridge of intermediate mesoderm.
The pronephros is rudimentary and nonfunctional in mammals. It consists of seven to eight pairs of pronephric tubules that form briefly adjacent to somites 7 to 14. Simultaneously a duct forms from the mesothelium of the adjacent somatopleura and grows caudally on the edge of the intermediate mesoderm to enter the metenteron (hindgutcloaca). At this stage this duct is called the pronephric duct. The pronephric tubules do not usually join this duct in the dog. Following this, adjacent to somites 9 to 26, 70 to 80 pairs of mesonephric tubules develop that are more extensive and on one end form a renal corpuscle with a glomerulus that develops from branches of the aorta. Venous drainage is provided by branches of the various cardinal veins. The other end of these embryonic nephrons attaches to the pronephric duct, which changes its name to the mesonephric duct. As these mesonephric nephrons develop, the pronephric tubules and the adjacent pronephric duct degenerate. Compared with other domestic animals, the size of the canine mesonephros is relatively small. The functional period of the mesonephros is brief and degeneration from cranial to caudal commences as formation of the metanephric kidney is initiated.
Development of the metanephros begins with an outgrowth from the mesonephric duct close to its entrance into the urogenital sinus. This follows the partitioning of the hindgut by the urorectal septum into the rectum and urogenital sinus. This evagination is the ureteric bud and this occurs adjacent to somites 26 through 28. The ureteric bud grows dorsally into the intermediate mesoderm where it expands to form the structure that will be the renal pelvis. Further branching forms papillary ducts and collecting tubules. Each of the collecting tubules induces the adjacent intermediate mesodermal cells to form a cluster of cells that will develop into a nephron. Each nephron is associated with a glomerulus developed from branches of the aorta that supply the metanephric intermediate mesoderm. The metanephric duct that originated as the ureteric bud becomes the ureter, which enters the portion of the urogenital sinus that gives rise to the bladder. A modified mesonephros is the functional kidney of anamniotes (fish and amphibians). The metanephros is the functional kidney of adult amniotes (reptiles, birds, and mammals). Gersh (1937) investigated the correlation of structure and function in the developing mesonephros and metanephros.
The genital system develops simultaneously with this development of the urinary system. The first indication is the proliferation of intermediate mesoderm on the medial side of the middle of the developing mesonephros. This is the genital ridge. The proliferating “gonadal” mesodermal cells here are invaded by migrating primary germ cells. Further development of the gonad depends on the presence or absence of the Y chromosome and its sex determining SRY gene in these intermediate mesodermal cells. A testis develops if these cells have a Y chromosome with this gene. An ovary develops if there are only X chromosomes in these genital ridge cells.
Prior to this gonadal sex determination, two duct systems are present: the mesonephric duct that developed with the embryonic urinary system and an adjacent duct that formed on the border of the intermediate mesoderm and grew caudally to enter the urogenital sinus. This is the paramesonephric duct, which is not joined by any mesonephric tubules (). The caudal ends of the two paramesonephric ducts fuse medially before entering the urogenital sinus. Further development of these duct systems depends on the gonadal sex and the production of endocrine substances in the male gonad.
In the male these endocrine substances induce the caudal mesonephric tubules to form efferent ductules in the testis and the caudal mesonephric duct to form the epididymis and ductus deferens. An additional testicular endocrine inhibits the development of the entire paramesonephric duct, which degenerates. In the female the absence of this paramesonephric duct inhibitory substance permits the paramesonephric ducts to form the uterine tubules, the uterus and the cranial vagina. The mesonephric duct degenerates in the female. Remnants of these degenerating duct systems include in the female adj acent to the ovary the paroophoron (mesonephric tubules) and the epoophoron (mesonephric tubules and duct). In the floor of the caudal vagina is the vestigial ductus deferens (Gartner’s ducts — caudal mesonephric ducts). In the male adjacent to the testis, the remnant of the mesonephric duct is the appendix epididymis and the remnant of the mesonephric tubules is the paradidymis. Remnants of the paramesonephric ducts in the male include the appendix testis and the uterus masculinus (prostatic utricle) in the seminal colliculus.
The development of external genitalia also depends on the presence or absence of endocrine secretions from the male gonad. In the female, the urogenital sinus gives rise to the bladder, urethra, caudal vagina, vestibule, and vulva. In the male, in addition to the bladder and pelvic urethra, the testicular endocrine substances induce the formation of the penis and penile urethra from the genital tubercle and the prostate from the pelvic urethra. The genital tubercle in the female forms the clitoris.
Although prenatal mammary development has been studied in carnivora as well as in many other species of mammals, the developing mammae of the fetal dog have not been investigated extensively. The mammary ridge is present at 25 days of gestation () and by the thirtieth day has differentiated into five pairs of nipples (). Turner and Gomez (1934) have studied the development of the gland during the estrous cycle, pregnancy, and pseudopregnancy. A few workers have studied mammary growth in dogs as influenced by estrogen and progesterone ().
Male mammae, and female mammae from birth until the approach of the first estrus, consist of small primary ducts extending a short distance below the base of the nipple. During estrus, the duct system of the gland grows rapidly and the alveolar system develops. Marshall and Hainan (1917) reported that within a week after estrus slow growth of the tissues of the gland (a few ducts surrounding the nipple) changes to a period of rapid development in the pregnant animal. The growth phase appears to be completed between day 30 and 40 after initiation of estrus. There is then a gradual increase in the size of the gland, owing to secretory activity of the alveolar epithelial cells.
Turner and Gomez (1934) made a detailed study of the gross and microscopic glandular changes during pregnancy. Ten days after conception the growth of the gland is grossly perceptible. At 20 days, the peripheral borders of adjoining glands in each row begin to unite and to extend toward the midventral line. The glandular systems on each side of the midline always remain intrinsically separate, however. On microscopic examination, the connective tissue stroma is reduced, adipose cells are present, and the growth of the duct system is very marked. At 30 days, a typical duct and lobule system is present, as well as anlagen of alveoli. Individual alveoli with lumina are seen at 40 days, and for the next 20 days there is a gradual enlargement of the gland owing to initiation of secretion by the alveolar epithelial cells. Within 1 day after parturition, the alveoli become greatly reduced, compared with the total amount of parenchyma. Changes in the mammary gland during pseudopregnancy are essentially identical to those of pregnancy, except that secretory activity at 60 days is less well developed.
Approximately 10 days after parturition, the size of the mammae is greatly reduced, the lobule-alveolar structures being affected sooner than the duct system. By 40 days the lobule-alveolar system is largely degenerated, and the ducts are shrunken. After cessation of lactation, the mammary gland in the dog regresses to a simple duct system.