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BIOLOGY 205/SECTION 7 DEVELOPMENT-LILJEGREN Lecture 4 Gastrulation 1. GASTRULATION is a complex series of cell movements that: a. rearranges cells, giving them new neighbors. These rearrangements put cells in a new environment, with the potential to receive new signals. b. results in the formation of the 3 GERM LAYERS (do not confuse with germline) that will form most of the subsequent embryo: ECTODERM, ENDODERM and MESODERM. 2. While all animals gastrulate , gastrulation can seem very different in different organisms (mostly due to geometric differences due to yolk content and distribution), but like cleavage the similarities in terms of mechanism and outcome suggest that the same controls operate across species. 3. Changes in the shape or adhesive properties of single cells in concert can rearrange the entire embryo . a. Cell shape changes, mediated by cytoskeleton lead to indentation of a flat sheet of cells, which can then lead to invagination of a tube or ball of cells. Contraction of the adhesion belt b. Other changes in cellular shape drive elongation or shortening of a flat sheet of cells- Can provide motive force for complex rearrangements. c. Cells can alter their surface proteins & thus alter relative stickiness to different groups of neighbors or to ECM secreted by those neighbors. Changes in adhesion lead to cell migrations etc. 4. Changes in cell shape or adhesiveness occur in specific regions of the body. i.e. individual cells have individual identities or fates very early 5. Just as with cleavage, if we understand the possible ways cells can move and rearrange, we can mix and match to get gastrulation in different organisms. a. Individual cells move by: i. MIGRATION - movement of individual cells over other cells or matrix ii. INGRESSION - movement of individual cells or small groups from an epithelium into a cavity b. Groups of cells move by: i. INVAGINATION - local inward buckling of an epithelium ii. INVOLUTION - inward movement of a cell layer around a point or edge
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