During teleost fertilisation, sperm fertilises the oocyte through the micropyle, a channel traversing the vitelline membrane at the animal pole. This crucial structure is formed by a specialised micropylar cell (MC) in the follicular epithelium that surrounds the oocyte, but many aspects of MC specification and differentiation remain incompletely understood. Now, Virginie Lecaudey and colleagues reveal that the Hippo pathway effector Taz is a key regulator of MC development in zebrafish. Females carrying a mutation in the gene wwrt1 (which encodes Taz) are infertile because their eggs cannot be fertilised. These eggs lack a discernible micropyle, which is not due to a failure in animal-vegetal polarity in the oocyte but does result from the failure of MC differentiation. Taz protein is strikingly and specifically enriched in the MC and its precursor, beginning before any morphological changes in the cell. Taz is thus the first bona fide marker of the MC and an essential regulator of its differentiation. The high level of Taz correlates with elevated TEAD-dependent transcriptional activity in the MC. Finally, the authors describe dynamic changes in both cell junctions and the cytoskeleton that accompany MC differentiation and attachment to the oocyte. This report therefore reveals multiple molecular aspects of MC development, as well as demonstrating an unexpected role for the Hippo pathway in fertilisation.
