Epiblast lumenogenesis is not a mammalian-specific trait.

Epiblast lumenogenesis is a hallmark of mammalian embryogenesis, and crucial for the subsequent processes of anterior-posterior patterning and gastrulation. Based on avian model-organisms, reptile epiblasts are thought to form a monolayered flat disc to undergo these developmental events. Here, we report that the squamate, veiled chameleon (Chameleo calyptratus), exhibits epiblast lumenogenesis prior to anterior-posterior patterning. Using SEM, immunofluorescence, and histology techniques, we demonstrate that chameleon epiblast lumenogenesis occurs via a purse-string-like mechanism involving the formation and constriction of concentric rings of supracellular actin cables around the epiblast. Through expression analyses of Nodal1, Nodal2, Cerberus, Lefty, Brachyury, Wnt3A, and Bmp2, and immunostaining for Brachyury, we uncovered a Wnt3A- and Brachyury-positive ring at the edge of the epiblast concomitant with lumenogenesis. Furthermore, our data suggest that anterior-posterior patterning in veiled chameleons may occur independently of Cerberus and Lefty. These processes result in chameleon embryos exhibiting human embryo-like morphology, despite 300 million years of evolutionary separation. Collectively, we show that pre-gastrulation epiblast lumenogenesis is not mammalian-specific but has also evolved in some non-avian reptiles.