VET ENDO | Corpus Luteum (CL): Formation & Regression | Student Output

#VetEndo #Endocrinology #CorpusLuteum
In this educational clip, the group of Blessie del B. Purca, Jubilee Nile C. Namia (speaker), Windell Gaye C. Juntilla, and Ruzz S. Balasa [4-DVM students] discuss the physiology behind the formation and regression of the corpus luteum.
The corpus luteum is remarkable in that it is critically important for reproduction in numerous species (mammals and some reptiles), but exists only as a transient structure that arises from the ovarian follicle after ovulation. Marcello Malpighi first coined the term corpus luteum (yellow body) in 1681. Regnier de Graaf (1641-73) was the first to describe and provide detailed illustrations of the corpus luteum present in the ovaries of pregnant rabbits, where it was noted that they possessed “globular bodies.” The essential role of the corpus luteum was not defined until 1901 when Ludwig Frankel demonstrated that rabbits could not maintain a pregnancy when all of the corpora lutea were removed. It was subsequently discovered in 1929 that extracts obtained from corpora lutea were able to replace the function of the ovaries once removed. Around this same time, several groups purified and defined this luteal-derived steroid hormone and named it progesterone by joining the terms “progestin” and “luteo-sterone.” In the intervening decades, studies focused primarily on understanding the cellular processes that allow for the development of the corpus luteum following ovulation, its ability to produce large quantities of the steroid hormone progesterone, its regression in non-fecund cycles, as well as luteal regulation of pregnancy.
Since the corpus luteum forms from the antecedent ovulatory follicle, the number of corpora lutea are proportional to the number of ovulatory follicles that form and ovulate through each reproductive cycle. Thus, in poly-ovulatory species, numerous corpora lutea are generated per cycle; whereas most ovine, bovine, and primate species develop a single follicle that yields one corpus luteum per reproductive cycle. Species have also evolved several different mechanisms that control the lifespan of the corpus luteum in the nonfertile cycle as well as how pregnancy affects luteal survival. Mammals can be divided into three major categories based on how long the corpus luteum functions after ovulation in the non-fecund ovarian cycle and how luteal lifespan is affected by pregnancy onset and gestation, including the following: (1) ultrashort-lived corpora lutea, (2) short-lived corpora lutea, and (3) long-lived corpora lutea.
The absence of pregnancy leads to rapid luteolysis in species that have short-lived corpora lutea. However, there are important distinctions between primates and ungulates in terms of the processes that determine the length of the luteal phase as well as how maternal recognition of pregnancy is mediated. In the ungulate corpus luteum, antral follicle growth occurs during the luteal phase, which results in a very short (2-3 days) and the well-defined interval between luteolysis and the development of the next ovulatory follicle. In most primates, however, small antral follicles do not develop until after luteolysis, resulting in a relatively long follicular phase (∼ 1-2 weeks). Another distinguishing feature between ungulates and primates is that luteolysis in the former is dependent on a uterine-derived luteolytic signal that orchestrates corpus luteum regression; whereas, in primates, hysterectomy does not alter the functional lifespan of the corpus luteum, indicating the absence of a uterine luteolytic signal. Lastly, while early pregnancy in ungulates and some New World monkeys prevents the synthesis of a uterine luteolytic signal, an embryonic hormone (chorionic gonadotropin) is produced by the conceptuses of Old World monkeys, apes, and women that sustains corpus luteum progesterone synthesis.
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