Ferenc Husvéth (2011)
Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon Egyetem
The ovarian corpus luteum is a temporary endocrine organ with progesterone as its primary secretory product. a corpus luteum forms at the site of each ovulated follicle (Fig. 12.10.), so litter-bearing animals may have multiple corpora lutea on an individual ovary.
Figure 12.10. Figure 12.10.: Functional corpus luteum. The corpus luteum is now a mixture of large luteal cells,LLC (formerly granulosal cells) and many small luteal cells, SCL (formerly thecal cells). In some cases, there is a remnant of the follicular antrum that forms a small cavity in the centre of the corpus luteum. (Senger 2003)
Sometimes during ovulation small blood vessels rupture, and the cavity of the ruptured follicle fills with a blood clot, a corpus hemorrhagicum. Whether or not a corpus hemorrhagicum forms, the granulose cells lining the empty follicular cavity begin to multiply under the influence of LH and form a corpus luteum, or yellow body. The granulose cells also continue to undergo luteinization. Most luteal cells are derived from granulose cells, but some cells in the corpus luteum are derived from theca interna. Although a mature follicle and a fully formed corpus luteum are about the same size, they can be differentiated by sight or palpation. The follicle is a sac filled with fluid that has the appearance and feel of a blister, while the corpus luteum looks and feels solid.
Blood progesterone levels increase as corpora lutea grow and develop after ovulation. When corpora lutea are fully deceloped, progesterone level secretion is maximal and plasma levels stabilize (Fig. 12.11). If fertilization of the ova does not occur and a pregnancy is not established, the corpora lutea
Figure 12.11. Figure 12.11.: Typical concentrations of progesterone in the systemic circulation of mares during the estrous cycle and early pregnancy. Black bar represents estrus.(McKinnon et al., 2011)
Corpus luteum regression entails apoptotic death of luteal cells, their removal, and the replacement of the corpus luteum with connective tissue forming a corpus albicant. If a pregnancy is established, maternal recognition of pregnancy occurs, and regression of the corpus luteum is prevented. This process and the role of corpus luteum during pregnancy are discussed later in more detailed.
The basic function of progesterone during this part of the estrous cycle is to prepare for a pregnancy. Progesterone increases uterine gland secretion and inhibits uterine motility to promote implantation and maintain any pregnancy. Progesterone also promotes mammary gland development. High levels of progesterone act on the hypothalamic-adenohypophyseal axis to inhibit further LH secretion. If a successful pregnancy is not established, the corpora lutea must undergo regression (luteolysis) to permit the animal to continue the estrous cycle. The humoral signals between the uterus and ovary that initiate luteolysis differ among species. In most domestic species (mare, cow, ewe, sow), prostaglandin F2α (PGF2α) is the humoral signal used by the nonpregnant uterus to stimulate luteolysis. The non-pregnant uterus increases PGF2α synthesis; releases are increased after ovulation at times appropriate for the species (e.g., 10 days for sow, and 14 days for ewes; and; and luteolysis occurs shortly thereafter. Luteolysis can be induced in cattle by administering analogs of PGF2α at any point in the estrous cycle as long as a corpus luteum is intact and functioning. The removal of the corpus luteum permits rapid development of new follivles and ovulation in about 3 days. The use of PGF2α to induce ovulation and estrus at a predictable time is a management tool to synchronize the estrous cycles of groups of animals.