17. Egg Membranes Placenta '10

17. Egg Membranes Placenta '10 - Egg Membranes and...

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Egg Membranes and Placentae
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ANAMNIOTES -AMNIOTES • In vertebrates distinction between those with extra embryonic membranes and those without – ANAMNIOTES – without (e.g. fish, amphibians) – AMNIOTES – with (e.g. reptiles, birds, mammals)
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Extraembryonic Membranes – Found in amniotes – Evolve with reptilian 'land' egg • the amniote egg – 4 distinct membranes • yolk sac • amnion • allantois • chorion
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Development of membranes • Initially - no distinction between embryonic and extraembryonic • As body forms, border epithelia form folds - BODY FOLDS – surround embryo and isolate it from yolk
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1 st - head fold 2 nd - lateral folds both sides 3 rd - tail fold undercuts tail folds create composite membranes
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Becomes umbilical cord
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‘Land’ Egg Problem = desiccation pore
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Amnion = water • Formation of amnion provides aqueous environment • Cells of amnion secrete and absorb water • Formation occurs with formation of chorion
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Chorioallantois = Gas Exchange • Second problem - gas exchange • Chorioallantoic membrane with shell to maximize gas exchange • But…….recent work also shows it is an endocrine organ – A primitive endocrine placenta?
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Allantois = waste removal 3rd problem – store or remove waste Reptiles and birds store – allantois stores waste – Forms as evagination of hindgut – attached to hindgut via allantoic stalk • highly vascularized and lies next to yolk stalk
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Yolk Sac • 1st extra embryonic membrane to form • mediates nutrition • derived from endodermal cells that grow over yolk to enclose it
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Other Extraembryonic Membranes • Anamniotes also have ‘extraembryonic’ membranes
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External Gills • Hypertrophy of gills for gas exchange during development – chondrichthyan fishes – gymnophione amphibians – salamanders
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This note was uploaded on 05/19/2011 for the course ZOO 4926 taught by Professor Staff during the Summer '08 term at University of Florida.

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17. Egg Membranes Placenta '10 - Egg Membranes and...

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