Developmental Patterns: What’s a Protostome and a Deuterostome? Early Development
cleavage • Fertilized Egg 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_Cleavage.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