Expression of steroidogenic enzyme and gonadotropin receptor genes in bovine follicles during ovarian follicular waves: a review.

Abstract

Ovarian follicular development in cattle is characterized by waves of growth during the prepubertal and postpartum periods and during estrous cycles. Each wave of follicular growth is characterized by recruitment of a cohort of follicles 4 to 5 mm in diameter. From the cohort, one follicle is selected for continued growth and becomes dominant. If luteolysis occurs during the growth phase of dominant follicles, final maturation and ovulation occurs. If luteolysis does not occur during the growing and maintenance phase of follicles, the fate is atresia. Changes in mRNA expression for the gonadotropin receptors (FSHr and LHr), key steroidogenic enzymes (cytochrome P450 side chain cleavage [P450scc], cytochrome P450 17alpha-hydroxylase-[P450c17], cytochrome P450 aromatase [P450arom], and 3beta-hydroxysteroid dehydrogenase [3beta-HSD]), and growth factors (IGF-I and -II) and their binding proteins (IGFBP) have been associated with different stages of follicular growth and atresia. In general, expression of mRNA for the gonadotropin receptors, steroidogenic enzymes, and steroidogenic acute regulatory protein (StAR) increase with progressive follicular development and is highest when dominant follicles approach maximum size. Expression of mRNA declines rapidly and becomes low or undetectable in atretic follicles. The IGF-I (granulosal cells) and IGF-II (thecal cells) are increased, whereas IGFBP-2 (granulosal cells) is reduced, in dominant follicles. Recruitment of a cohort of follicles is associated with initiation of expression of mRNA for P450scc and P450arom in granulosal cells. Selection of dominant follicles is associated with expression of mRNA for LHr and 3beta-HSD in granulosal cells. Thus, changes in gene expression likely are important to recruitment, selection, dominance, and atresia in ovarian follicles.

A simple mathematical model of the bovine estrous cycle: Follicle development and endocrine interactions

“…This FSH rise induces the emergence of the next follicular wave [27]. Follicle growth is associated with increased mRNA expression for LH and FSH receptors and steroidogenic enzymes, which regulates the follicles ability to produce steroid hormones [6]. Approximately two days after cohort recruitment, one follicle of 8-9 mm in diameter is selected to grow to 15 mm [6].…”

“…Follicle growth is associated with increased mRNA expression for LH and FSH receptors and steroidogenic enzymes, which regulates the follicles ability to produce steroid hormones [6]. Approximately two days after cohort recruitment, one follicle of 8-9 mm in diameter is selected to grow to 15 mm [6]. The growing dominant follicle secretes increased amounts of Inh, which suppresses FSH release, thereby inhibiting the growth of subordinate follicles.…”

“…If P4 remains at luteal levels, the dominant follicle goes into regression. Follicle regression is associated with down-regulation in mRNA expression of receptors for LH and FSH and steroidogenic enzymes, resulting in declined E2 production [6]. Therefore, neither the LH surge nor ovulation take place [3,6].…”

“…Follicle regression is associated with down-regulation in mRNA expression of receptors for LH and FSH and steroidogenic enzymes, resulting in declined E2 production [6]. Therefore, neither the LH surge nor ovulation take place [3,6]. The dominant follicle that is present at the onset of CL regression becomes the ovulatory follicle [3,6].…”

“…Therefore, neither the LH surge nor ovulation take place [3,6]. The dominant follicle that is present at the onset of CL regression becomes the ovulatory follicle [3,6]. In the preovulatory follicle, the LH peak induces a shift from E2 to P4 production [18] by changing the expression of steroidogenic enzymes [6,72].…”