Physiology of the Ovary PDF

SINDH AGRICULTURE UNIVERSITY FACULTY OF ANIMAL HUSBANDRY AND VETERIANRY SCIENCES DEPARTMENOF ANIMAL REPORDUCTION BY Dr. Shahzeen Reg No: 2K24-SO-11 PHYSIOLOGY OF THE OVARY By Shahzeen Reg no# 2k24-S0-...

SINDH AGRICULTURE UNIVERSITY FACULTY OF ANIMAL HUSBANDRY AND VETERIANRY SCIENCES DEPARTMENOF ANIMAL REPORDUCTION BY Dr. Shahzeen Reg No: 2K24-SO-11 PHYSIOLOGY OF THE OVARY By Shahzeen Reg no# 2k24-S0- 11  PHYSIOLOGY OF OVARY The ovary is the primary female reproductive organ and has two important functions: 1. producing the female reproductive cell (the egg or ovum) 2. producing the hormones estrogen and progesterone. The cow has two ovaries and are oval to bean-shaped organs that are 1—1.5 inches long and located in the abdominal cavity. The ovary produces the egg by a process called oogenesis. Oogenesis is cyclic known as estrous cycle (average 21 days). During the estrous cycle, two prominent structures are present within the ovary: the follicle and corpus luteum. The ovary contains several thousand tiny structures called primary follicles. Each follicle consists of a germ cell surrounded by a layer of flattened cells, only a small percentage of primary follicles continue through the secondary and tertiary follicular phases and ultimately undergo ovulation. This germ cell has the potential to mature into an egg if the follicle completes development. However, most of the primary follicles never develop. Rather, they die, are absorbed by the ovary, and are replaced by newly formed primary follicles. The relatively few primary follicles that develop completely do so through a series of phases. Many layers of cells are added to the single layer of cells surrounding the egg in the primary follicle, forming a central cavity. As the follicle and cavity grow larger, the egg becomes attached to the back of the follicle (opposite the ovulation site) by a stalk of cells. As the follicle continues to grow rapidly, the side opposite the egg bulges from the surface of the ovary and becomes very thin. Once the follicle reaches this mature state it is called a Graafian follicle (dominant follicles). At ovulation, the thin portion ruptures to release the contents of the follicle, including the egg and the corpus luteum, which has the important function of producing progesterone. Following ovulation, the cells that developed within the follicle undergo a differentiation process. This process is called luteinization and gives rise to the second prominent ovarian structure, the corpus luteum (pl. corpora lutea).  The CL goes through a maturation and regression cycle much the same as the follicle. A blood clot-type structure known as a corpus hemorrhagicum forms in the cavity left by the ruptured follicle and is transformed into a CL by Day 5 of the cycle (Day 0 = estrus). The CL is fully functional from Day 5 to Day 15 of the cycle and then begins to regress if the female does not become pregnant. The CL regresses and no longer secretes progesterone as the follicle of the next estrous cycle begins to develop. As the CL regresses further, it becomes known as the corpus albicans and remains visible on the ovary for several subsequent cycles.  The dynamic development and regression of the corpus luteum and follicles are a continual process in the normal cycling cow until she becomes pregnant. In a pregnant female, CL regression does not occur, and the cyclic activity stops until after calving. A cow generally remains anestrous (does not cycle) for a period of time following calving. The length of the postpartum anestrous period is on average around 60 days, but this is highly variable among cows based on age, nutrition, lactation, presence of a suckling calf, environmental stress, and numerous other factors.  Occasionally the CL does not regress normally (persistent CL) even though the animal does not become pregnant. This requires diagnosis and treatment by a veterinarian. Abnormally short estrous cycles (7 to 11 days) can occur. This condition appears to be caused by either no corpora lutea being formed, or if one is formed, it is non-functional and progesterone levels remain low.  THE HORMONES OF FEMALE REPRODUCTION Reproduction in the female is controlled by numerous hormones secreted from specialized glands called endocrine glands. These secretions are produced in the glandular cells and pass into the blood and lymph systems for transport to specific parts of the body where they produce their function. The hormone, estradiol (or estrogen) is produced by the granulosa cells of the follicle. Estradiol has various effects, including: Development and function of the secondary sex organs Receptivity to mating during and other behaviors associated with estrus Rate and type of growth, especially relative to deposition of fat Regulation of puberty attainment and length of the postpartum anestrous period Progesterone: secreted by the corpus luteum, suppresses ovulation of a dominant follicle and the associated secretion of high levels of estradiol. Therefore, elevated levels of progesterone prevent a cow from coming into heat. Progesterone is necessary for preparing the uterus to receive the fertilized ovum and maintains the proper uterine environment for continuation of pregnancy. Estradiol and progesterone are not separate in their functions since both must be present for certain processes to occur. For example, the ratio of the concentration of estradiol to progesterone dictates the onset and duration of behavioral estrus. Development of the uterus is initiated by estradiol and completed by progesterone. Estradiol causes contraction of the uterus near the time of estrus and ovulation, which aids in sperm transport. Progesterone has a quieting effect on the uterus so that there are no contractions which might disturb pregnancy.  The production of ovarian hormones is under direct influence of gonadotropic hormones produced by the anterior pituitary. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) are secreted from the pituitary and travel through the blood to the ovary. The release of FSH and LH is mediated by gonadotropin-releasing hormone (GnRH) by the hypothalamus. FSH stimulates the recruitment, growth, and development of follicular waves, while LH supports continued development of a dominant follicle. Additionally, a surge in LH release around the time of onset of estrus causes the follicle to initiate the ovulatory process and develop into a corpus luteum.  This cyclic process continues every 21 days in a normal-cycling cow, but changes if conception occurs. Following pregnancy and a period of anestrus, the estrous cycle once again continues. References: Reproductive anatomy and physiology of cattle 2020/11/05, article Duetscher, G.H.1980, reproductive tract anatomy and physiology of the cow (G80-537-A)