The ovary, or female gonad, is a paired oval organ, attached by a mesovarium to the body wall and the mesosalpinx. The mesovarium proximale extends from the body wall to the origin of the mesosalpinx, and the mesovarium distale extends from the origin of the mesosalpinx to the ovary and forms part of the wall of the ovarian bursa. The ovary lies caudal to the kidney () and contains all of the ovocytes that the female will ovulate in her lifetime (). It is also the source of several hormones. In a 25-pound dog, an ovary averages 1.5 cm in length, 0.7 cm in width, 0.5 cm in thickness, and 0.3 g in weight. In its normal position, an ovary may be described as having tubal and uterine extremities, a free border, and medial and lateral surfaces. The tubal end is nearest the infundibulum. The uterine end is the end attached to the uterus by the proper ligament of the ovary. The ovary is smooth in appearance before estrus, which occurs for the first time between 6 and 9 months of age. In multipa-rous bitches (several litters) the surface may be rough and nodular. At times the ovaries are dissimilar in size, in which case the left ovary is usually larger.
A laparoscopic study of the ovary of a cycling bitch was made by Wildt et al. (1977). In a sexually mature, 25-pound dog, the left ovary is located approximately 12 cm caudal to the middle of the thirteenth rib and 1 to 3 cm caudal to the corresponding kidney. Typically, it lies between the abdominal wall and the descending colon. The right ovary is located approximately 10 cm caudal to the last rib of the right side. The ventral border and medial surface of the ovary are in contact with the mesovarium. In a young animal it lies ventral to the adipose capsule of the right kidney and dorsal to the descending duodenum. In animals that have undergone numerous pregnancies, both right and left ovaries shift caudally and ventrally Frequently fat is deposited within the mesosalpinx partially obscuring the ovary.
In addition to the mesovarium, the ovary has two other liga-mentous attachments. The suspensory ligament of the ovary (lig. suspensorium ovarii) is attached cranially to the middle and ventral thirds of the last one or two ribs. Caudally, it attaches to the ventral aspect of the ovary and mesosalpinx, lying between the opening of the ovarian bursa and the ascending uterine tube (). The suspensory ligament lies between the two layers of peritoneum in the free border of the mesovarium, which is the cranial portion broad ligament. It is continued caudally by the proper ligament of the ovary (lig. ovarii proprium). This in turn attaches the uterine end of the ovary to the cranial end of the uterine horn. There it is continuous with the round ligament of the uterus, which extends caudally toward the inguinal canal and passes through it to be wrapped by the vaginal process before it ends near the vulva. Both the proper and the suspensory ovarian ligament are composed of connective tissue mixed with smooth muscle fibers.
The ovary has a medulla and a cortex. The medulla (medulla ovarii [zona vasculosaj) contains blood vessels, nerves, lymphatics, smooth muscle fibers, and connective tissue fibers. The cortex (cortex ovarii [zona parenchymentosa]) consists of a connective tissue stroma that contains a large number of follicles. For a review of ovarian structures in several mammals, including the dog, see Mossman and Duke (1973). The most comprehensive work is that of Zuckerman (1971).
The connective tissue condenses to form the tunica albu-ginea around the ovary. The tunica albuginea is covered by visceral peritoneum referred to as the superficial epithelium of the ovary. (The latter term replaces the unsuitable but much used name germinal epithelium)) Follicles are present deep to the tunica albuginea. A primordial follicle consists of an ovocyte (ovocytus) and its surrounding granulosa cells enclosed in a basement membrane that separates the follicle from the ovarian stroma.
With each estrus a number of follicles mature. The granulosa cells at first form a single cuboidal layer around the ovocyte, which constitutes a primary follicle (folliculus ovaricusprimarius). With further maturation, several layers of granulosa cells are formed around the ovocyte. Eventually, a cavity filled with follicular fluid forms within the granulosa cell mass. Such a follicle is designated as a tertiary or vesicular follicle (folliculus ovaricus tertiarius fvesiculosusj), formerly termed a Graafian follicle. At one end of the cavity there is a hillock, cumulus oophorus, which contains the maturing ovocyte. In intimate contact with the ovocyte is a clear membrane, the zona pellucida. This is surrounded by a layer of radially arranged granulosa cells, the corona radiata. As the follicular fluid (liquor follicularis) increases, the follicle migrates to the periphery of the ovary. When the follicle ruptures, the ovocyte is released into the ovarian bursa and swept by ciliary action into the infundibulum of the uterine tube (). Not all follicles that begin to mature proceed to ovulation. The vast majority of follicles degenerate at different stages of development throughout life, as they do in all mammals ().
After ovulation, relatively slight hemorrhage occurs in the ovary. The follicular cavity tinged by blood is called a corpus hemorrhagicum and as this is resorbed, a corpus luteum () is formed from the granulosa and theca interna cells. If fertilization does not take place, the corpus luteum gradually degenerates into a connective tissue scar, the corpus albicans. If the ovocyte is fertilized, the corpus luteum remains fully developed throughout pregnancy and produces progesterone. After parturition, it regresses. Involution of the corpus luteum again allows vesicular follicles to mature.
An ovulated ovocyte is visible to the naked eye when held in fluid against a dark background. Recent birth-control studies using the ovocytes of several animals have resulted in a patent being issued in 1991 to Bonnie Dunbar of Baylor College of Texas for a genetically engineered vaccine based on proteins from the zona pellucida, which surrounds the ovocyte. When the protein is injected, the animal produces antibodies that bind to the zona pellucida and prevent sperm from entering the ovocyte. Clinical trials of dogs have been authorized by the U.S. Food and Drug Administration.
Ovaries: Vessels and Nerves
The ovary is supplied with blood through the ovarian artery. Homologous to the testicular artery of the male, the ovarian artery arises from the aorta approximately one-third to one-half the distance from the renal arteries to the deep circumflex iliac arteries. Usually the right ovarian artery arises slightly cranial to the left. The degree of uterine development determines the tortuosity and size as well as the position of the artery. In a nulliparous animal the artery extends laterally almost at right angles from the aorta, whereas in late pregnancy it is drawn cranioventrally, along with the ovary, by the enlarged, heavy uterus. In addition to supplying the ovary, the ovarian artery supplies branches to the adipose and fibrous capsules of the kidney. In addition, small tortuous branches supply the uterine tube and uterus. Caudally, the uterine branch of the ovarian artery anastomoses with the uterine artery, a branch of the vaginal artery (formerly urogenital artery). Through this anastomotic connection, the uterine artery may be considered as a supplementary source of arterial blood to the ovary. The arteries to the ovary supply the parenchyma of the medulla and cortex as well as the thecae of the follicles. Capillary loops become extensive during follicular enlargement but recede or disappear during corpus luteum regression.
The right and left ovarian veins have different terminations. The right vein drains into the caudal vena cava, whereas the left enters the left renal vein. Similar to the corresponding arteries, the uterine vein and ovarian vein anastomose between the peritoneal layers of the broad ligament. The ovarian vein receives a tributary that comes from the medial edge of the suspensory ligament of the ovary and the lateral surface of the kidney. In some instances, the vein will also anastomose with the deep circumflex iliac vein. The arteries and veins of the ovaries in the dog have been studied by Del Campo and Ginther (1974). The lymphatics drain into the lumbar lymph nodes. Polano (1903) has demonstrated the ovarian lymphatics of the dog.
The nerve supply to the ovaries is from the sympathetic division of the autonomic nervous system. The nerves reach the ovaries by way of the renal and aortic plexuses, which receive axons from the fourth, fifth, and sixth lumbar sympathetic trunk ganglia. They accompany the ovarian artery to the ovary. The ovarian blood vessels receive an abundant sympathetic nerve supply, but, according to Kuntz (1919), the ovarian follicles and interstitial secretory tissue are devoid of sympathetic innervation. Chien et al. (1991) investigated the origins of the sympathetic nerves innervating the ovary of the dog using the horseradish-peroxidase technique. Their findings indicated that the ovary receives sensory nerve fibers from the spinal ganglia of thoracic 10 to lumbar 4 segments. The highest concentrations of labeled neurons were at the thoracolumbar junction, T-13 toL-3. It has been suggested that nerves reaching the ovary play a role in follicle maturation and ovulation.
Ovaries: Anomalies and Variations
The ovaries may be hypoplastic, displaced, or completely missing. Rarely, an ovary may descend through the inguinal canal, in the manner of a testis, to rest in the vulvar region. Follicular cysts rarely occur. According to McEntee and Zepp (1953), ovarian tumors are relatively infrequent in the dog. Wenzel and Odend’hal (1985) reviewed the literature on the development and presence of the rete ovarii, the homologue of the rete testis. The rete ovarii of the adult dog appears as a mass of tubules in the hilus of the ovary, and the authors conclude that the rete ovarii is important in the control of meiosis and contributes cells to the ovarian follicle. As is true of most hollow remnant structures, cysts may develop from rete remnants.