Gene expression for LH receptor, 17α-hydroxylase and StAR in the theca interna of preantral and early antral follicles in the bovine ovary

Abstract

The onset of gene expression for three proteins that play pivotal roles in theca interna function, namely the LH receptor (LH-R), cytochrome P450 17a-hydroxylase (17aOH) and the steroidogenic acute regulatory protein (StAR), was determined. Ovaries were obtained on day 9 of the oestrus cycle from mature synchronized dairy cows (n 5 5) and gene expression in preantral and antral follicles up to 4 mm in diameter was evaluated by in situ hybridization. LH-R and 17aOH mRNAs were observed first, in the theca interna of large preantral follicles (type 4), concurrent with its morphological differentiation. StAR mRNA appeared later during follicular growth, in follicles >1 mm in diameter (type 6). LH-R and 17aOH mRNAs were found exclusively in the thecal cells, whereas StAR mRNA appeared in thecal cells, granulosa cells of late atretic follicles and oocytes. In early atresia, thecal cells expressed all three mRNAs, and their expression decreased gradually as atresia progressed. Atresia in granulosa cells was characterized by massive apoptosis of periantral, but not peribasal cells, that differentiated into luteallike cells expressing StAR. In summary, our study suggests that in spite of the presence of 17aOH, a key enzyme in steroidogenesis, the ability to produce steroids by bovine follicles smaller than 1 mm in diameter must be very limited due to the absence of StAR protein.During the early stages of atresia, thecal cells remain morphologically and functionally healthy, and continue to express all three studied mRNAs.

Morphological classification of bovine ovarian follicles

“…The early decline of the theca in these follicles is also consistent with reduced levels of thecal-derived androstenedione and testosterone in the follicular fluid compared with healthy and antral atretic follicles of the same size (Irving- Rodgers et al 2003b). It has been suggested that the basal atretic follicles develop from antral atretic follicles later in the process of atresia (Braw-Tal & Roth 2005); however, in the absence of an intermediate phenotype between antral and basal atresia as would be expected if one morphed into the other, together with the restriction of basal atresia to sizes !5 mm, and with two distinct phenotypes of healthy follicles that correspond to the same size ranges as for the two atretic phenotypes, it is more likely that these atretic follicle phenotypes are separate identities.…”

“…apoptosis as observed by DNA fragmentation or karyorrhexis. Examples of why this is important in follicles include the observation in bovine follicles of morphological dead pyknotic nuclei that can be TUNEL negative (Braw-Tal & Roth 2005) or not have DNA ladders by COMET (van Wezel et al 1999a). Using the latter two methods would not have identified all the dead cells.…”

Atresia Folicular en Ovarios de Prochilodus lineatus

“…Se señaló que la atresia puede presentarse en cualquier momento del desarrollo oocitario (Ganias et al, 2008 ;Leonardo et al;Miranda et al), sin embargo en cautividad, lo más común es que aparezca en las fases de vitelogénesis y preovulatoria (Bromage & Cumaranatunga, 1988;LinaresCasenave et al, 2002;Miranda et al). En algunas especies, los factores más importantes que determinan el destino hacia la ovulación o la atresia de los folículos son la capacidad de respuesta a gonadotropinas, la capacidad de producir estradiol (Bao & Garverick, 1998;Braw-Tal & Roth, 2005;Garverick et al, 2002;Webb et al, 1999), la participación del sistema de factor de crecimiento insulinoide (IGF) (Rivera & Fortune, 2003a, 2003b) y el medio endocrinológico en el que se desarrollan (Webb et al).…”

Effect of the metabolic environment at key stages of follicle development in cattle: focus on steroid biosynthesis

“…The rate-limiting step in steroidogenesis is the transport of cholesterol from the outer to the inner mitochondrial membrane, where it is converted to pregnenolone by cytochrome P450 cholesterol side chain cleavage complex. STAR, a mitochondrial phosphoprotein, has been identified as the critical cholesterol transport protein (14,75) and has been extensively studied across numerous species (10,83,85). In bovine follicles, the expression of STAR is restricted to the theca interna (3,73); however, it is acquired in granulosa cells following luteinization (3,61).…”