Life Cycles – Sea Urchin Development Biology 4361 – Developmental Biology September 13, 2006Sperm Structure Head – compacted nucleus acrosome – modified lysosome Midpiece – mitochondria Residual cytoplasm Tail axoneme CentrioleFertilizationSea Urchin Fertilization unfertilized egg fertilized egg = zygote raised fertilization envelope blastomeresCleavage postfertilization development mitosis, cell division no growth provides: raw material for development division of embryonic cytoplasm equal unequalCleavage Patterns Holoblastic (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, amphioxus Spiral annelids, molluscs, flatworms Bilateral tunicates Rotational mammals Displaced Radial amphibiansSea Urchin Cleavage holoblastic radialUnequal 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 medoderm small micromeres ciliated band larval spiculesGastrulation (gastr “stomach”) formation of independent cell layers raw material for morphogenesis formation of a body cavity arche nteron – 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) formation of three tissue 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 mesenchyme)Mesenchyme & Epithelium mesenchyme – loosely organized; undifferentiated; mostly mesodermal eithelial – tightly connected in sheets or tubes; intercellular connectionsMorphogenic
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