CELL MOVEMENT IN ZEBRAFISH

Cell movement in Zebrafish during gastrulation

Cell movement in Zebrafish during gastrulation

Introduction

Beginning during the late blastula stage in zebrafish, cells located beneath a surface epithelial layer of the blastoderm undergo rearrangements that accompany major changes in shape of the embryo. We describe three distinctive kinds of cell rearrangements. Radial cell intercalations during epiboly mix cells located deeply in the blastoderm among more superficial ones. These rearrangements thoroughly stir the positions of deep cells, as the blastoderm thins and spreads across the yolk cell. Involution at or near the blastoderm margin occurs during gastrulation (Bell, 2002).

This movement folds the blastoderm into two cellular layers, the epiblast and hypoblast, within a ring (the germ ring) around its entire circumference. Involuting cells move anteriorwards in the hypoblast relative to cells that remain in the epiblast; the movement shears the positions of cells that were neighbors before gastrulation. Involuting cells eventually form endoderm and mesoderm, in an anterior-posterior sequence according to the time of involution. The epiblast is equivalent to embryonic ectoderm. Mediolateral cell intercalations in both the epiblast and hypoblast mediate convergence and extension movements towards the dorsal side of the gastrula.

By this rearrangement, cells that were initially neighboring one another become dispersed along the anterior-posterior axis of the embryo. Epiboly, involution and convergent extension in zebrafish involve the same kinds of cellular rearrangements as in amphibians, and they occur during comparable stages of embryogenesis.

Invagination

During gastrulation, embryonic cells rearrange dynamically to organize the basic body plan. A prominent feature of gastrulation is the formation of the archenteron, which begins with an inpocketing of the epithelial cell sheet at the vegetal pole of the sea urchin embryo. This epithelial bending is then transformed into a tube by cell migration, convergence, and extension movements. In deuterostomes, the archenteron later connects to a distinct invagination called the stomodaeum, the site of the future mouth. For the embryonic gut to form correctly, many cellular and molecular components must be carefully coordinated. The morphogenetic movements associated with gastrulation have received close scrutiny, most notably in Xenopus, Drosophila, and sea urchin embryos, and current efforts are directed toward detailed molecular explanations for the phenomena observed (Grosshans, 2000).

Gastrulation in the zebra fish is often divided into four stages, although each stage has its own complex cell biology. Ingression of skeletogenic primary mesenchyme cells (PMCs) from the vegetal pole occurs several hours before invagination of the archenteron is initiated. The blastopore forms by an inward bending of the vegetal plate. This is marked by the appearance of bottle cells and apical deposits of new extracellular matrix (ECM) and results in invagination of the archenteron.

The archenteron elongates and forms a tube that extends across the blastocoel in a process that is driven by both convergent extension movements and recruitment of additional cells. Finally, filopodia are extended by secondary mesenchyme cells (SMCs) at the tip of the archenteron toward the animal pole, and these provide a pulling force to complete elongation and help guide the gut toward the ...