The uterus serves for the conduction of sperm to the uterine tube for the fertilization of the ovocyte, and for the conduction, implantation, and nourishment of the developing young. Hypertrophy of the tunica mucosa (endometrium) forms with the fetal membranes a placenta to serve as a source of embryonic and fetal nourishment ().
The uterus consists of a neck (cervix uteri), a short body (corpus uteri), and two horns (cornua uteri dextrum et sinistrum). It is Y-shaped and communicates with the uterine tubes cranially and the vagina caudally (). Its size varies considerably, depending on age, previous pregnancies, stage of the estrous cycle, and whether the animal is currently pregnant. In females that have never had young (nulliparous) the uterine horns of a 25-pound dog average 10 to 14 cm in length and 0.5 to 1 cm in diameter. They diverge from the body of the uterus at a point 4 to 5 cm cranial to the symphysis pelvis. The uterine body is 1.4 to 3 cm long and 0.8 to 1 cm in diameter. The cervix averages 1.5 to 2 cm long. A small caudal portion of the cervix may protrude into the vagina. The diameter of this intravaginal cervix is approximately 0.8 cm. The gravid uterus in the latter third of pregnancy may occupy any portion of the abdominal cavity, and the uterine horns frequently change position in relation to one another, although they are somewhat limited by the suspensory and round ligaments. During uterine distention, the horns bend upon themselves and may rest entirely upon the ventral wall the abdomen.
The uterine horns are usually of the same size and unite at an acute angle with the body of the uterus. The cranial end of each uterine horn is connected to the ovary by the proper ligament of the ovary. The uterine tube opens via the tubouterine junction into the cranial end of the uterine horn.
The body of the uterus is usually located in both the pelvic and the abdominal cavities. Generally, the largest portion is in the abdomen, and in multiparous bitches the entire uterine body may be located cranial to the brim of the pelvis. The body extends from the point of convergence of the uterine horns to the cervix (). An internal partition, the uterine velum (velum uteri) projects approximately 1 cm into the body of the uterus, separating the horns, and it has been claimed that this feature causes an alternation of births from right and left horns. The partition is not discernible externally. The ventral border of the cervix attaches to the uterine wall cranial to its dorsal attachment and the canal of the cervix is directed caudoventrally from uterus to vagina. Therefore the cervix lies diagonally across the uterovaginal junction. Consequently, the internal orifice of the cervical canal (ostium uteri internum) faces almost directly dorsally, whereas the external orifice (ostium uteri externum) is directed ventrally toward the vaginal ventral wall. The external uterine orifice opens on a hillock projecting into the vagina on a dorsal median fold (). The cervical canal averages 0.5 to 1 cm in length and is closed during pregnancy by a mucus plug.
Uterus: Spaces and Folds
The rectogenital pouch, an extension of the peritoneal cavity between the rectum and the uterus, is continuous with the pararectal fossa. The vesicogenital pouch extends between the urinary bladder and the uterus with its attached broad ligament.
The broad ligaments, containing some fat and unstriped muscle, attach the uterus and ovaries to the body wall. The mesometrium is that part of the broad ligament that attaches the uterus to the dorsolateral body wall. The mesovarium and mesosalpinx have been discussed in the descriptions of the ovary and of the uterine tube. The mesometrium begins on a transverse plane through the cranial end of the uterine horn and extends caudally as far as the cranial end of the vagina. It is attached peripherally to the lateral pelvic wall. The medial surfaces of the uterine horns are connected to each other for approximately 1 cm by a triangular-shaped double layer of peritoneum, the genital fold (plica genitalid) (). The mesometrium and the lateral ligament of the bladder fuse at their attachments to the pelvic wall.
The round ligament of the uterus (ligamentum teres uteri) is attached to the cranial tip of the ipsilateral uterine horn and is a caudal continuation from the proper ligament of the ovary. These two ligaments are remnants of the embryonic gubernaculum testis in the male. These ligaments consist largely of smooth muscle, allowing for stretching during pregnancy. The round ligament runs in the free edge of the peritoneal fold given off from the lateral surface of the mesometrium. It extends caudally, ventrally, and mesially, toward the deep inguinal ring. In most bitches the round ligament, with its investment of peritoneum, the vaginal process, passes through the inguinal canal and terminates subcutaneously in or near the vulva. The vaginal process is accompanied in its course through the inguinal canal by the genitofemoral nerve, the external pudendal artery and vein, and fat. The fascial layers enveloping the vaginal process are the same as those described with the vaginal tunic of the male.
The tunica muscularis (myometrium) consists of a thin, longitudinal outer layer and a thick, circular inner layer of involuntary muscle. Within the circular layer, close to its junction with the longitudinal layer, is a vascular layer containing blood vessels, nerves, and circular and oblique muscle fibers. The circular layer is especially thick in the region of the cervix. In describing the process of labor in the bitch, Rudolph and Ivy (1930) discuss the action of uterine musculature in detail. Briefly, the fetus is advanced by a strong circular contraction that progresses like a cylindrical band, and by a longitudinal shortening. The “retreat” of the fetus is prevented by a persistent longitudinal contraction. In the body of the uterus, transverse circular contraction, with some longitudinal shortening, moves the fetus into the vagina. Contraction of the abdominal muscles, as well as of the vaginal musculature, causes the final expulsion of the fetus. Reynolds (1937) describes uterine motility, during estrus, as being a series of simple myometrial contraction waves, because intermediation of an intrinsic innervation is not essential to it. Verma and Chibuzo (1974) found that sectioning the hypogastric and pelvic nerves did not alter the frequency or the amplitude of uterine contractions.
For an account of birth in the dog see Naaktgeboren and Slijper (1970); for endocrine parameters of pregnancy and parturition see Concannon (1991). For an account of fetal development and associated hypertrophy of the uterus see site.
Evans (1961) reporting on successive cesarean operations of Beagles at various stages of gestation, found that within 10 minutes of the removal of the conceptus (fetus and its fetal membranes) there was considerable contraction of the uterus. Within 5 days after the operation, involution and healing of the uterus was well advanced, and by the next pregnancy there was little or no evidence of the uterine incision when chromic gut was used for closure. In those instances where evidence of previous uterine incisions was visible, it was clear that subsequent implantations were not located at the same sites. Serial removals of embryos did not appear to interfere with subsequent reproductive activity. One Beagle bitch that had four fetuses removed from the right uterine horn at 37 days postin-semination gave birth at 63 days to four normal pups. This dog later whelped two litters, one of five pups and another of eight.
The tunica mucosa (endometrium)\ is the thickest of the three uterine tunics. Facing the lumen of the uterus is a layer of low columnar epithelium with cells that are periodically ciliated. Simple branched tubular glands are present in the lamina propria. Opening into the uterine cavity, these glands are generally very long and are separated by shorter, inconstant glands or crypts. The long glands in the bitch show relatively little branching or coiling, in contrast with those of the mare or cow. They generally traverse the entire thickness of the endometrium. Grossly, the mucosal surface of the uterus is reddish in color and may be smooth or contain low longitudinal ridges that obliterate the uterine cavity in the nonpregnant state. The cervical canal does not contain the relatively high mucosal folds observed in other domestic animals, but is closed in pregnancy by a collagenous plug.
Uterus: Vessels and Nerves
The uterus is supplied with arterial blood via the ovarian and uterine arteries. See Del Campo and Ginther (1974) for the details of vascularization as seen in cleared tissues. The origin of the ovarian arteries from the aorta has been discussed with the description of the ovaries and of the uterine tubes. The uterine branch of the ovarian artery anastomoses with the uterine artery, one of the principal branches of the vaginal artery. The artery enters the mesometrium at the level of the cervix (). On entering the broad ligament, the artery lies relatively close to the body of the uterus. It diverges from the uterine horn until it approaches the cranial extremity of the horn, where it anastomoses with the uterine branch of the ovarian artery. The uterine artery ramifies in the wall of the uterus and in the mesometrium. Branches supply both sides of the uterine horn.
The uterine and ovarian veins follow a course similar to that of the arteries, except at their terminations. The right ovarian vein empties into the caudal vena cava at the level of the right ovary, whereas the left enters the left renal vein. Both ovarian veins are very tortuous in their course between the peritoneal layers of the broad ligament.
Studying the physiologic aspects of uterine circulation during pregnancy, Reynolds (1949) found two distinct phases in the adjustment of the uterine vessels to the shape and size of the conceptus. First, there is progressive stretching of the blood vessels, and, second, the blood vessels during the latter part of gestation separate from one another without increase of length. Burwell and his coworkers (1938) found that blood pressure in the femoral and uterine veins was elevated during pregnancy in the dog. However, pressure in the uterine vein is higher than in the femoral vein. The uterine veins drain into the caudal vena cava. Two of the principal functions served by rhythmic uterine contractions during estrus, according to Fagin and Reynolds (1936), may be production of an increased volume flow of blood through enlarged, hyperemic vessels and removal of any edematous fluid. The lymphatics from the uterus pass to the hypogastric and lumbar lymph nodes. The vagina possesses a dense plexus of lymphatics in its tunica propria.
The uterus receives sympathetic and parasympathetic innervation via the pelvic plexus. Sympathetics reach the pelvic plexus as right and left hypogastric nerves. The parasympathetics reach the pelvic plexus via the pelvic nerves. Visceral afferent fibers reach the uterus via the pelvic nerves and the pelvic plexus.