Ultrastructural comparison of in-vitro and in-vivo matured human oocytes

Study question: Is the ultrastructure of the cytoplasm of human in-vitro and in-vivo matured oocytes comparable? Summary answer: Overall, the ultrastructure of human in-vitro matured oocytes is comparable to that of in-vivo matured controls, although following in vitro maturation (IVM) mitochondria-smooth endoplasmic reticulum (M-SER) complexes are partly replaced by mitochondria-vesicles (M-V) aggregates. What is known already: Immature oocytes retrieved from antral follicles of patients undergoing IVM treatment can achieve meiotic maturation in vitro, fertilize and develop into embryos able to implant and give rise to viable pregnancies. However, nothing is known on the ultrastructure of IVM oocytes. Study design, size, duration: The ultrastructure of in-vitro matured oocytes (n = 7) was compared with that of in-vivo matured oocytes (n = 10) by light and transmission electron microscopy (LM, TEM). The study was carried out over a period of 18 months. Participants/materials, setting, methods: Immature cumulus cell-enclosed oocytes, retrieved from mid-sized antral follicles of women requiring IVM treatment, were matured in vitro for 30 h. Mature oocytes were obtained from age-matched women undergoing full ovarian stimulation. In-vitro and in-vivo matured oocytes were fi xed and analysed by LM and TEM. Main results and the role of chance: In-vitro matured oocytes showed general features comparable to in-vivo matured controls. All oocytes had normal ooplasm showing uniform distribution of organelles. M-SER aggregates and M-V complexes were commonly found in in-vivo matured oocytes. Large M-V complexes partially replaced M-SER aggregates in IVM oocytes. Mitochondria appeared morphologically unaffected by IVM. Cortical granules appeared typically stratifi ed in a single, mostly continuous row just beneath the ooplasm in all oocytes. Microvilli were well preserved after IVM. Vacuoles were only occasionally found in all oocytes and, if present, they were frequently associated with lysosomes. The morphological features of the MII spindle and the fi rst polar body of in-vitro matured oocytes were comparable to those shown by control oocytes. Limitations, reason for caution: Although informative and based on a wellestablished methodology, the study should be extended to larger number of oocytes and different maturation conditions.Wider implications of the fi ndings: Ultrastructural analysis offers an objective approach for the comparison of organelle structure and distribution in invitro and in-vivo matured oocytes. The present data confi rm that following IVM the overall oocyte cytoplasmic architecture is well preserved, although subtle differences in comparison to in vivo-matured controls encourage a further refi nement of IVM protocols.