The effector of Hippo signaling, Taz, is required for formation of the micropyle and fertilization in zebrafish
Fig 8
Model of micropylar cell specification in zebrafish.
In late stage II/early stage III wild type oocytes, a particular follicle cell (FC) on the top of the animal pole, the micropylar cell (MC), expresses high level of Taz, predominantly in nucleus. Taz is expressed in FC and the oocyte cortex at a much lower level than in MC. The conical MC depresses the developing vitelline envelope (VE) and forms a shallow invagination. At middle and late stage III, the leading tip, enriched with Taz and F-actin, is formed in MC, and protrudes into the developing VE. Moreover, α-Tubulin accumulates in the cytoplasmic extension of MC, and concurrently, F-actin bundles deposit on oocyte cortex to form a leading edge. The dynamic cytoskeleton in MC and oocyte might facilitate perforation of MC into oocyte to form a micropyle. During MC development, the expression level of Taz gradually decreases, and the nucleus in MC, composed of two closely juxtaposed nuclei, becomes less clear. In taz mutants, Taz protein is not detected and no MC is formed, and this process likely does not happen.