A role for regulated secretion of apical extracellular matrix during epithelial invagination in the sea urchin

Epithelial invagination, a basic morphogenetic process reiterated throughout embryonic development, generates tubular structures such as the neural tube, or pit-like structures such as the optic cup. The 'purse-string' hypothesis, which proposes that circumferential bands of actin microfilaments at the apical end of epithelial cells constrict to yield a curved epithelial sheet, has been widely invoked to explain invaginations during embryogenesis. We have reevaluated this hypothesis in two species of sea urchin by examining both natural invagination of the vegetal plate at the beginning of gastrulation and invagination induced precociously by Ca2+ ionophore. Neither type of invagination is prevented by cytochalasin D. In one species, treatment with A23187 three hours before the initiation of invagination resulted in the deposition of apical extracellular matrix at the vegetal plate, rather than invagination. This apical matrix contains chondroitin sulfate, as does the lumen of the archenteron in normal gastrulae. When the expansion of this secreted matrix was resisted by an agarose gel, the vegetal plate buckled inward, creating an archenteron that appeared 3-4 hours prematurely. Pretreatment with monensin, which blocks secretion, inhibits both Ca2+ ionophore-stimulated folding and natural invagination, demonstrating that secretion is probably required for this morphogenetic event. These results indicate that alternatives to the purse-string hypothesis must be considered, and that the directed deposition of extracellular matrix may be a key Ca(2+)-regulated event in some embryonic invaginations. A bending bilayer model for matrix-driven epithelial invagination is proposed in which the deposition of hygroscopic material into a complex, stratified extra-cellular matrix results in the folding of an epithelial sheet in a manner analagous to thermal bending in a bimetallic strip.