Influence of the corpus luteum on cumulus-oocyte complexes competence in equines.

Overview

• This study investigated how the presence and location of the corpus luteum (CL) in horse ovaries affects the quantity and developmental competence of cumulus-oocyte complexes (COCs).
• The research examined differences in oocyte quantity, quality, gene expression, and maturation rates in ovaries with CL, ovaries opposite a CL, and ovaries without CL.

Background

• The corpus luteum (CL) is a structure that forms on the ovary after ovulation and produces hormones, particularly progesterone.
• COCs consist of an oocyte (egg cell) surrounded by cumulus cells, which support oocyte development and maturation.
• Understanding how the CL influences oocyte quality and maturation is important for improving equine reproductive technologies.

Study Design

• Equine ovaries were collected from a slaughterhouse and sorted into three groups:
  • CL+ : Ovaries containing a corpus luteum.
  • CL- : Ovaries without a CL but located opposite an ovary with a CL.
  • NCL : Ovaries without any CL.
• COCs were isolated from these ovaries and assessed for:
  • Quantity recovered per ovary.
  • Morphology, including the state of cumulus cell expansion.
  • Nuclear maturation rates after in vitro maturation (IVM), focusing on progression to metaphase II.
  • Intrafollicular progesterone concentrations.
  • Expression of genes associated with oocyte quality and development: BAX, BMP-15, ODC1, GDF9, AURKA in both cumulus cells and oocytes.

Key Findings

• COC Quantity:
  • Lower number of COCs were recovered from CL+ ovaries (average 2) compared to NCL ovaries (average 4.9).
  • CL- ovaries had an intermediate COC recovery rate (average 3.5) and were not significantly different from either group.
• COC Morphology:
  • COCs from CL- ovaries showed a much greater proportion of expanded cumulus cells (74.4%), indicating more advanced cumulus cell development.
  • In contrast, CL+ and NCL groups had relatively low proportions of expansion (10% and 13.1%, respectively).
• Nuclear Maturation:
  • A higher percentage of COCs from CL+ ovaries reached metaphase II after IVM (58.3%) compared to the NCL group (42%).
  • CL- group fell in between (47.2%) and was not statistically different from the other groups.
• Progesterone Levels:
  • Intrafollicular progesterone concentration was highest in CL+ ovaries (574 ng/mL), followed by CL- (485 ng/mL), and lowest in NCL (392 ng/mL).
  • The significant difference was mainly between CL+ and NCL groups.
• Gene Expression:
  • Expression of BMP-15 (an oocyte-specific growth factor) was higher in oocytes from ovaries with a CL (both CL+ and CL-) compared to those without (NCL).
  • Expression of GDF9 (another oocyte developmental gene) was higher in cumulus cells from CL+ and CL- groups compared to NCL.
  • Other genes (BAX, ODC1, AURKA) were analyzed but no particular results were highlighted in the abstract.

Interpretation and Implications

• The presence of the corpus luteum appears to affect the follicular environment positively by increasing intrafollicular progesterone, which is associated with enhanced expression of important developmental genes in COCs.
• Although fewer COCs were recovered from ovaries with a CL, these oocytes showed higher nuclear maturation rates, indicating higher developmental competence.
• The greater expansion of cumulus cells in the contralateral ovary (CL-) suggests it may create an environment conducive to cumulus cell differentiation without the direct presence of a CL.
• These findings suggest the CL influences oocyte quality and maturation through hormonal and molecular mechanisms, which can be important for optimizing assisted reproductive techniques in horses.

Conclusions

• The corpus luteum presence is linked with improved equine oocyte maturation and expression of key quality markers, likely mediated by progesterone levels within follicles.
• This study highlights the importance of considering ovarian state in reproductive interventions and may guide better timing and selection of oocytes for in vitro procedures.