Les mécanismes de l'activation ovocytaire.

Abstract

L' activation ovocytaire, qui emane d' un point localise de l' oeuf, fait suite a l' adherence et a la fusion des deux gametes et amorce le developpement embryonnaire en provoquant le signal calcique indispensable au reveil metabolique de l' oeuf. Elle pourrait faire intervenir des ligands spermatiques et des recepteurs ovocytaires (lies aux proteines G, a une activite tyrosine-kinase et PLCy, ou de la famille des integrines). Cependant, l' interaction entre les membranes des gametes est court-circuitee lors de l' ICSI (intracytoplasmic sperm injection) et suggere l' existence de facteur(s) spermatique(s) activateur(s). Parmi eux ont ete decrits l' oscilline, une forme tronquee de c-kit (recepteur a activite tyrosine-kinase), un facteur associe au materiel perinucleaire et une phospholipase C. Une autre hypothese est celle de " la bombe a calcium", selon laquelle le spermatozoide fusionne permettrait l' entree de calcium dans l' oeuf. Les resultats obtenus chez la souris et chez l' oursin plaident en faveur de l' une et de l' autre de ces hypotheses, peut-etre finalement non exclusives.

Fertilization results in a series of well-coordinated events, including specific recognition between species, adhesion and fusion between gametes, that finally leads to egg activation. In most deuterostomes, a transient rise in intracellular free calcium occurs during egg activation and is necessary for early embryonic development. In the light of recent data obtained in various biological models, from invertebrates to mammals, we discuss how this "calcium signal" is generated from the point of sperm/egg interaction. Different hypotheses have been proposed to explain egg activation. The sperm may bind to a receptor on the egg plasma membrane, linked to G-proteins, tyrosine kinase and PLCgamma, or belonging to the integrins family. Another possibility is that an activating sperm factor enters the egg after fusion establishes cytoplasmic continuity. Among the putative factors isolated from sperm extracts have been: oscillin, a truncated form of c-kit (a receptor with tyrosine-kinase activity), an unidentified factor associated to the perinuclear material, and a phospholipase C. Finally, in the "bomb calcium hypothesis", the sperm would allow the transit of calcium from the external medium into the egg once it has fused with the egg. Results argue for or against each model, which may not be exclusive.