In vitro maturation is slowed in prepubertal lamb oocytes: ultrastructural evidences

Area of Science:

• Reproductive Biology
• Cell Biology
• Developmental Biology

Background:

• In vitro maturation (IVM) offers an alternative to IVF, potentially reducing risks associated with ovarian hyperstimulation.
• Human IVM is limited by the scarcity of immature oocytes for morphological studies.
• The ovine model presents a viable option for IVM research due to its monovular nature.

Purpose of the Study:

• To investigate morphological differences between prepubertal oocytes (PO) and adult oocytes (AO).
• To determine if developmental competence in PO is linked to morphological changes during IVM.
• To analyze ultrastructural changes during IVM in a sheep model.

Main Methods:

• Light and transmission electron microscopy were used to examine COCs from lambs and adult sheep.
• Oocytes were analyzed at retrieval and after 7, 19, and 24 hours of IVM.
• Meiotic progression was assessed concurrently with morphological analysis.

Main Results:

• Prepubertal oocytes (PO) exhibited distinct ultrastructural features, including trans-zonal projections and fewer cortical granules (CGs) compared to adult oocytes (AO).
• PO showed delayed nuclear maturation (GVBD-MI transition) and cytoplasmic alterations, such as discontinuous CG distribution, compared to AO.
• While PO reached the MII stage after 24h IVM, ultrastructural differences persisted, suggesting underlying developmental competence issues.

Conclusions:

• Ultrastructural analysis revealed cytoplasmic alterations in PO during IVM that correlate with delayed nuclear maturation.
• These morphological changes may explain the reduced developmental competence observed in prepubertal oocytes.
• Findings from the sheep model offer insights for improving human IVM technology and have implications for zootechny.