Follicular Dynamics in Ruminants

Follicular Dynamics in Ruminants

• Follicles in ruminants develop in a wave-like pattern, emerging due to periodic surges of FSH.
• Dominant follicles are determined by a decrease in FSH and an increase in LH responsiveness.
• Anovulatory follicular waves persist until an LH surge occurs; the duration of the estrous cycle correlates positively with the number of follicular waves.
• Progesterone suppresses LH secretion and influences the growth of dominant follicles, while the interwave interval is inversely linked to FSH levels.
• Follicular dominance is most pronounced during the initial and final waves of the estrous cycle.
• Conditions such as pregnancy and seasonal anestrus lead to regular FSH surges and the emergence of non-ovulatory follicular waves.

Corpus Luteum (CL) Formation and Function

• The CL, formed from the Graafian follicle post-ovulation, demonstrates significant mass increase, particularly in ewes, where it grows 20-fold in 12 days.
• Cell types in the mature CL include large and small luteal cells, fibroblasts, and endothelial cells—making it highly vascularized.
• Luteinization can be observed before ovulation in certain species, like the bitch.
• Hormone regulation of the CL formation involves prolactin and LH, with an important role of FSH in maintenance.
• Luteal regression occurs due to PGF2α, released episodically from the uterus, with oxytocin stimulating both PGF2α release and further oxytocin secretion.
• As the CL ages, sensitivity to luteolytic PGF2α increases, while its early stage remains unresponsive.

Mare Reproductive Cycle

• Fillies commonly enter estrus in their first spring but typically do not foal until over 3 years old.
• The mare exhibits seasonal breeding, with regular cycles occurring from spring through autumn, while some breeds cycle year-round under optimal conditions.
• Thoroughbred racing influences breeding schedules, with foaling dates standardized from January, causing many mares to breed when fertility is low.
• Transitional phases in estrus can show irregular cycles, with a lack of correlation between sexual behavior and ovarian activity.

Equine Estrous Cycle Characteristics

• The average equine cycle lasts 20-23 days, with oestrus typically lasting 6 days and dioestrus lasting 15 days.
• Ovulation often coincides with the end of estrous, while the presence of large follicles indicates imminent ovulation.
• First post-partum estrus occurs 5-10 days after foaling, often shorter in duration.
• Left ovary has a slight predominance for ovulation, with historical data reflecting 52.2% of ovulations occurring from this side.

Signs of Estrus in Mares

• Mares display restlessness, frequent urination, and raising of the tail in the presence of a stallion during estrus.
• The vulva may appear swollen with mucoid discharge, and non-receptive mares actively resist stallion advances.
• Professional management involves teasing mares approximately 15-16 days after the end of the last oestrus.

Endocrine Changes During the Estrous Cycle

• FSH secretion exhibits a biphasic pattern with surges at roughly 10-12 day intervals; the second surge occurs just before the subsequent ovulation.
• Unique to mares, LH shows a gradual increase without sudden surges, maintaining elevated levels around ovulation.
• Estrogens peak during estrus, while progesterone levels coincide with the physical changes of the CL.

Ovarian Structure in Cows

• Cows maintain polyoestrous cycles throughout the year under domestic conditions.
• The size of the ovaries remains relatively stable initially in the interestrus phase but may slightly enlarge due to the presence of both the mature and regressing CLs.
• Old scarred corpora lutea from previous pregnancies appear as whitish projections, typically around 0.5 cm in size, and may persist for several months.

Explore the intricate processes of follicular dynamics and CL formation in ruminants. This quiz covers the development of follicles, hormonal influences, and the implications of various physiological states on reproductive cycles. Test your understanding of these critical biological concepts.