Life Cycles – Sea Urchin Development Biology 4361 – Developmental Biology July 2, 2008Sperm Structure Head – compacted nucleus acrosome – modified lysosome Midpiece – mitochondria Residual cytoplasm Tail axoneme CentrioleFertilizationSea Urchin Fertilization unfertilized egg fertilized egg = zygote raised fertilization envelope blast omeresCleavage postfertilization development mitosis, cell division no growth provides: raw material for development division of embryonic cytoplasm equal unequalCleavage Patterns Holob lastic (complete cleavage) Meroblastic (incomplete cleavage) Isolecithal Mesolecithal Telolecithal Centrolethical Radial (echinoderms, amphioxis Spiral (annelids, molluscs, flatworms) Bilateral (tunicates) Rotational (mammals, nematodes) Radial (amphibians) Bilateral (cephalopod molluscs) Discoidal (fish, reptiles, birds) Superficial (most insects)Holoblastic Cleavage Radial echinoderms, amphioxusSea Urchin Cleavage holoblastic radial cleavageUnequal Cytoplasmic Distribution animal vegetal Xenopus egg4 th – unequal cleavage between animal and vegetal hemispheres 2 nd meridional 3 rd equatorial 1 st cleavage meridional pronucleiEmbryogenesis Fate Maps oral ectoderm aboral ectoderm vegetal plate/ endoderm skeletogenic mesoderm vegetal plate mesoderm small micromeres ciliated band larval spiculesGastrulation gastr “stomach” formation of independent cell layers raw material for morphogenesis formation of a body cavity archenteron – primitive gut formation of mouth (protostomes) or anus (deuterostomes) proto – 1 st deutero – 2 nd stome openingSea Urchin Gastrulation PMCs primary mesenchyme cells (mesoderm equivalent in urchins) Gastrulation formation of three tissue layers (germ layers) and the archenteron (primitive gut)Sea Urchin Gastrulation 2Sea Urchin DevelopmentSea Urchin Development Deuterostome – mouth from 2 nd opening vertebrates, echinoderms, etc. Protostome – mouth from 1 st opening (blastopore) arthropods, mematodes, annelids, etc. ectoderm oral ectoderm endoderm (primary
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