page_title: Epiboly
section_title: General movements
hierarchical_section_title: Epiboly / Zebrafish / General movements
caption_attribution_description: English: This is a cartoon of a zebrafish embryo at sphere stage (~4 hours post fertilization), showing the three cell layers that are present in the embryo.
context_page_description: Epiboly describes one of the five major types of cell movements that occur in the Gastrulation stage of embryonic development of some organisms. Epibolic movement is the way in which a layer epithelial cells spreads. This can be achieved in multiple ways.
  When undergoing epiboly, a monolayer of cells must undergo a physical change in shape in order to spread. Alternatively, multiple layers of cells can also undergo epiboly as the position of cells is changed or the cell layers undergo intercalation. While human embryos do not experience epiboly, this movement can be studied in sea urchins, tunicates, amphibians, and most commonly zebrafish.
context_section_description: Epiboly in zebrafish is the first coordinated cell movement, and begins once the embryo has completed the blastula stage. At this point the zebrafish embryo contains three portions: an epithelial monolayer known as the enveloping layer (EVL), a yolk syncytial layer (YSL) which is a membrane-enclosed group of nuclei that lie on top of the yolk cell, and the deep cells (DEL) of the blastoderm which will eventually form the embryo's three germ layers (ectoderm, mesoderm, and endoderm).  The EVL, YSL, and DEL all undergo epiboly. 
  Radial intercalation occurs in the DEL. Interior cells of the blastoderm move towards the outer cells, thus "intercalating" with each other. The blastoderm begins to thin as it spreads toward the vegetal pole of the embryo until it has completely engulfed the yolk cell. The EVL also moves vegetally during epiboly, increasing its surface area as it spreads. Work in the ray-finned fish Fundulus has shown that no large rearrangements occur in the EVL; instead, cells at the leading edge of the EVL align and constrict. The YSL also moves towards the vegetal pole, spreading along the surface of the yolk and migrating slightly ahead of the blastomeres. Once epiboly is complete, the DEL, EVL, and YSL have engulfed the yolk cell, forming a closure known as the blastopore.
caption: Cartoon of a 4-hour post fertilization zebrafish embryo, before the initiation of epiboly
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