Meeting Abstract

35.3  Jan. 5  Invagination of ascidian endoderm is preceded by apical contraction, but driven by active shortening of cells SHERRARD, K. M.; University of Washington kmsherra@u.washington.edu

Invagination is a fundamental mode of morphogenesis whereby a sheet of cells autonomously generates forces causing itself to buckle inwards. Contraction of the apical (outer) cell cortex is the most commonly invoked mechanism; however, the means by which cells actively deform to drive invagination have not been fully documented in any system. I performed a detailed case-study of endoderm invagination in ascidians using time-lapse microscopy of intact embryos and explants, labeling of the cell surface with fluorescent microspheres, experimental manipulations of adhesivity and cortical contractility, and antibody labeling. The data reveal that ascidian invagination is a two-step process: apical contraction coinciding with placode formation followed by basolateral contraction that shortens the endoderm cells and actually generates the invagination. This mechanism is only possible because the apical cortex of endoderm cells and their adhesive connections are sufficiently strong to resist apical expansion. Because invaginations are typically preceded by placode formations and act on columnar cells which shorten during the invagination, apical-basal shortening by basolateral contraction may be a general mechanism of invagination.