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Developmental Patterns: What’s a Protostome and a Deuterostome? Early Development • Fertilized Egg cleavage Blastula reorganization • Blastula Gastrula What Characterizes Cleavage? • Outcome – large egg divided into typical small cells – large increase in number of cells, chromatin, surface membrane • Characteristics – – – – – no growth cell cycle: rapid S, little G1 shape the same except for cavity (blastocoele) use of storage reserves (from oogenesis) embryonic gene activity relatively unimportant Cleavage Patterns I • Holoblastic – complete cleavage • Isolecithal – evenly distributed yolk • Mesolecithal – slight polarization of yolk Cleavage Patterns II • Meroblastic – incomplete division • Telolecithal – more yolk at one end • Centrolecithal – yolk in the center What Characterizes Gastrulation? • Outcome – increase in overt differentiation – morphogenetic changes • Characteristics – – – – – slowing of cell division little if any growth increasing importance of embryo gene activity cell movements and rearrangements establishment of basic body plan In What Ways Do Cells Move? Gastrulation • Common Processes – – – – – – changes in cell contacts motility shape cell-cell communication interactions of cells with extracellular matrix all precise and orderly The Body Plan • Gastrula establishes basic pattern – ectoderm, endoderm and mesoderm – primary germ layers primordial organ rudiments – germ layers bring cells to new positions to • self-differentiate in correct spatial relationships • be induced by new neighbors Early Development of Selected Invertebrates • Sea Urchin – echinoderm, deuterostome • Snail – spiralian, protostome • Tunicate – chordate, deuterostome • Nematode – C. elegans, protostome What’s A Sea Urchin? • • • • • • Echinoderm Adult has 5-fold symmetry Small radially and completely cleaving egg Moderate yolk Easy to work with in lab Lots of gametes Patterns of Sea Urchin Cleavage • Radial, holoblastic • First three cleavage planes perpendicular • Divisions are equal • Fourth division is unequal 4 micro, 4 macro and 8 mesomeres 3-08_01_SU_First_Cleavage.mov 3-08_02_SU_Second_C leavage.mov 3-08_03_Normal_Sea_Urchin.mov SU Fate Map I Where Do Layers Originate? wrong SU Fate Map II Where Do Layers Originate? Fate Map of 64-Cell Stage and Tiers