A ROLE FOR REGULATED SECRETION OF APICAL EXTRACELLULAR-MATRIX DURING EPITHELIAL INVAGINATION IN THE SEA-URCHIN

Epithelial invagination, a basic morphogenetic process reiterated thro ughout 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 acti n microfilaments at the apical end of epithelial cells constrict to yi eld a curved epithelial sheet, has been widely invoked to explain inva ginations during embryogenesis. We have reevaluated this hypothesis in two species of sea urchin by examining both natural invagination of t he vegetal plate at the beginning of gastrulation and invagination ind uced precociously by Ca2+ ionophore. Neither type of invagination is p revented by cytochalasin D. In one species, treatment with A23187 thre e hours before the initiation of invagination resulted in the depositi on of apical extracellular matrix at the vegetal plate, rather than in vagination. This apical matrix contains chondroitin sulfate, as does t he lumen of the archenteron in normal gastrulae. When the expansion of this secreted matrix was resisted by an agarose gel, the vegetal plat e buckled inward, creating an archenteron that appeared 3-4 hours prem aturely. Pretreatment with monensin, which blocks secretion, inhibits both Ca2+ ionophore-stimulated folding and natural invagination, demon strating that secretion is probably required for this morphogenetic ev ent. These results indicate that alternatives to the purse-string hypo thesis must be considered, and that the directed deposition of extrace llular matrix may be a key Ca2+-regulated event in some embryonic inva ginations. A bending bilayer model for matrix-driven epithelial invagi nation is proposed in which the deposition of hygroscopic material int o a complex, stratified extracellular matrix results in the folding of an epithelial sheet in a manner analagous to thermal bending in a bim etallic strip.