Structural components and morphogenetic mechanics of the zebrafish yolk extension, a developmental module.

The yolk extension (YE) appears to be a novel developmental module that has been inserted into the phylotypic period of teleostean development, specifically in the order Cypriniformes. The zebrafish YE informs the study of morphogenetic movements reshaping ventral tissues because (1) this trait is easily visible, so disruptions are easy to score; (2) its ontogenesis occurs quickly; and (3) the yolk cell isolates the tissues elongating the ventrum from the rest of the embryo, serving as a three-dimensional in vivo "tissue culture." We determined that three histological compartments comprise the structural components of the YE: (1) the internal yolk cell; (2) the mesendodermal mantle external to the yolk cell; and (3) the external embryonic integument, consisting of an embryonic epidermis plus enveloping layer cells. These structural components interact with one another in a hierarchical manner, resulting in the morphogenesis of the elongated and tubular embryonic zebrafish ventrum as the cylindrical YE forms. Time-lapse videomicroscopy and experimental manipulation show that the yolk mass is a cohesive, viscoelastic foam, which resists compression. Moreover, as the mesodermal mantle participates in tubulation of the posterior trunk, Kupffer's Vesicle, the organ of laterality in teleosts, separates from the posterior pole of the yolk syncytial layer. Additionally, the embryonic integument becomes contractile over the posterior yolk cell, constricting the yolk mass to form the YE. These findings constitute an initial assessment of the morphogenetic mechanics underlying formation of the YE developmental module in zebrafish.