Intrafollicular concentrations of steroids and steroidogenic enzymes in relation to follicular development in the mare.

This study investigates how the changes in both the concentration of follicular fluid steroids and the expression of steroidogenic enzyme in granulosa cells during the follicular phase relate to the size and physiological status of the follicle in a mare. The findings show that the dominant follicle experiences a significant increase in the concentrations of progesterone and estradiol-17beta during growth, but a subsequent decrease during maturation.

Research Methodology

• The researchers employed ultrasound-guided follicular punctures to recover follicular fluid and cells either during the emergence of the dominant follicle, at the end of dominant follicle growth, or at the preovulatory stage post-gonadotropin injection.
• The relative amounts of steroidogenic acute regulatory protein (StAR), P450-side chain cleavage (P450(scc)), 3beta-hydroxysteroid dehydrogenase (3betaHSD), 17alpha-hydroxylase, and aromatase in cells were evaluated using semiquantitative Western blot and densitometry.
• Additionally, follicular fluid was tested for cholesterol concentrations using a colorimetric assay and for progesterone, testosterone, and estradiol-17beta concentrations via radioimmunoassay (RIA).

Key Findings

• Progesterone and estradiol-17beta concentrations significantly increased in the dominant follicle during its growth phase.
• Following gonadotropin injection, the maturation of the follicle was marked by a decrease in estradiol-17beta concentrations and a further increase in progesterone concentrations.
• Granulosa cells in the dominant follicles showed increased levels of StAR, P450(scc), 3betaHSD, and aromatase during the follicle’s growth and conversely, decreased levels during maturation.
• Lower levels of steroids and steroidogenic enzymes including StAR, P450(scc), 3betaHSD, and aromatase along with progesterone and estradiol-17beta were observed in granulosa cells from subordinate follicles compared to dominant ones.
• The study found no remarkable relationship between the steroidogenic activity in the follicles and the capability for their enclosed oocytes to complete meiosis in vitro.

Conclusion

• The study’s findings provide critical insights into the changes in steroid concentrations and enzyme expressions throughout the follicular phase in relation to the size and physiological status of the follicle. This could be useful in further understanding the mechanisms behind fertility and ovulation in mares.