The follicles of cattle, pigs, sheep and horses begin to form LH receptors when they grow to 8mm, 5-6mm, 4mm and 25mm respectively. When the follicles of various animals grow to 8mm, 5-6mm, 4mm and 25mm, the dynamics can be observed with a veterinary B-ultrasound machine. Among all female animals, the earliest to form LH receptors are the granulosa cells in the selective follicles. The selective follicles of cattle appear 3 days before the appearance of the follicular development wave; the selective follicles of pigs occur on the 14th-16th day of the estrus cycle; and the dominant follicles of sheep appear 12-24h after the functional corpus luteum dissolves. The number of follicles selected by different animals is also different. For example, cattle select about 1 out of 5 follicles, and pigs select about 12 out of 50 follicles. This process can also be observed by veterinary B-ultrasound.

The growth of 4-9mm follicles in cattle requires a higher concentration of FSH. The peripheral blood FSH concentration level appears in waves during the estrus cycle, and each follicle wave is just before the peak of follicle development. The large amount of estrogen secreted before ovulation can act on the hypothalamus and pituitary gland through a positive feedback pathway, causing a large amount of GnRH secretion, which leads to the appearance of the LH preovulatory peak; after 2 weeks of treatment with GnRH agonist, the peak of LH concentration was suppressed, and the growth and development of follicles remained at 7.9mm; then further treated with GnRH agonist for 2 weeks, not only the peak of LH concentration was suppressed, but the concentration level of FSH also decreased, and no follicles larger than 4mm grew and developed on the ovaries. The growth and development of follicles of 4-9mm in cattle requires a certain concentration of FSH, while the growth and development of follicles above 9mm requires LH pulses. Similarly, weak LH receptor mRNA was detected in the granulosa cells of pig recruitment and selection follicles, while strong FSH receptor mRNA was expressed in the selection follicles; but LH receptor mRNA was expressed more strongly in the dominant follicles. In other words, the recruitment and selection of follicles mainly depend on FSH, and after that, the selected dominant follicles turn to depend on LH.

link: https://www.bxlimage.com/nw/152.html