Ovary
Ovaries are paired organs suspended in the abdomen with the mesovarium in most animals. In ruminants, it remains close to the ventral abdominal wall, slightly above to mid of the pelvic inlet; in sows, positioned at the ventrolateral side of the pelvic inlet and generally remains within the intestines; in dogs and cats, located at the dorsal part of the abdomen behind the kidneys below 3-4 lumbar vertebrae; in mare, below the 4-5 lumbar vertebrae.
The actual potion of the ovaries is variable depending on the animal’s age, parity and breeding season. The shape of ovaries is generally oval or almond-shaped in almost all mammals and bean-shaped in horses, and berry shaped in the sow. The size of the ovary depends upon several factors, like, the number and size of the ovum, seasons (in the case of the seasonal breeder) and age. Size enlarges during the breeding season and the early stage of life, immediate to puberty. Most domestic females have a larger and heavier right ovary than the left ovary (Table 21.1). The free surface of the ovary is physiologically active where follicles and corpus luteum develop.21.2.1 Structure
The outmost layer of the ovary is made of a single layer of cuboidal cells called germinal epithelium, which is continuous with the peritoneum lining the body cavity. The type of germinal epithelium cells is changed from squamous to cuboidal with the advancement of age. Immediately below the epithelium cells, the dense irregular connective tissue layer or tunica albuginea is situated, made of fewer cells and closely packed fibres. Beneath the tunica albuginea is the ovarian cortex that contains follicle and corpus luteum in various stages of development and regression. Fibroblasts, collagen, reticular fibres, blood vessels, nerves, lymphatic vessels, and smooth muscle form the connective tissue of the ovarian cortex. The central part of the ovary is called the medulla.
The ovarian cortex is concerned with ova and hormone production; hence, it is considered the functional unit of the ovary. The ova are released from the entire surface of the ovary in most species, except the horse. The ova are released at one edge of the ovary over its surface, having a long groove in the horse called the ovulation fossa. The medulla comprises blood vessels, lymphatics, nerves, loose areolar connective tissue and various elastic and reticular fibres. Its structure remains static during various phases of the reproductive cycle. It has a narrow depression along the mesovarian edge, the hilum. It acts as the passage of blood vessels, lymphatics and nerves to the ovary. Small masses made of some solid tubules or cords, the rete ovarii, are found near the hilum.The right ovary is rudimentary in birds and usually contains only medullary tissue, and the left ovary is functional. The developing follicles appear like a cluster of tiny grapes. The weight of the ovary of chickens (Gallus domesticus) is about 0.3-0.5 g in young and about 60 g in adults, and the same in quail (Coturnix coturnix japonica) is 0.06-0.09 g and 10-12 g, respectively. The ovaries of amphibians, reptiles, birds and monotremes have large medullary spaces with fluid-filled cavities called lacunate.
21.2.2 Function
Primary functions of the ovaries are gametogenic and hormone production. It produces the female gamete (ova or oocyte, in singular ovum) and various hormones, including oestrogen, progesterone, relaxin, activin and inhibin, under the influence of follicle-stimulating hormone (FSH) and luteinising hormone (LH). The ovarian hormones control the reproductive cycle, the activity of the reproductive tract and secondary sex characteristics.
21.3