Download - Egg Membranes and Placentae
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Egg Membranes and Placentae
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ANAMNIOTES -AMNIOTES
• In vertebrates distinction betweenthose with extra embryonicmembranes 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 betweenembryonic and extraembryonic
• As body forms, border epithelia formfolds - BODY FOLDS– surround embryo and isolate it from
yolk
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1st - head fold2nd - lateral folds both sides3rd - tail fold undercuts tail
folds create composite membranes
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Becomesumbilicalcord
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‘Land’ Egg
Problem =desiccation
pore
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Amnion = water
• Formation of amnionprovides aqueousenvironment
• Cells of amnion secreteand absorb water
• Formation occurs withformation of chorion
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Chorioallantois = Gas Exchange
• Second problem - gasexchange
• Chorioallantoic membranewith shell to maximize gasexchange
• But…….recent work alsoshows 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 embryonicmembrane to form
• mediates nutrition• derived from
endodermal cellsthat grow over yolkto 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 exchangeduring development– chondrichthyan fishes– gymnophione amphibians– salamanders
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Integumental Modifications
• Pericardial trophoderm– highly vascular belly wall for nutrient and gas exchange
• Pericardial sac in teleost fish can form– pericardial amnion (pseudoamnion)– pericardial chorion (pseudochorion)
• Trophotaeniae - teleost fish– modifications of hindgut– functions in gas exchange and nutrient transfer
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What is a placenta?
• “Structural modification of the maternalorganism and/or embryo that facilitatesexchange between them”
• Addresses needs of developing embryo:– Gas exchange– Waste exchange– Nutrients
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Why a placenta?• Oviparous species
– conduct gas / waste exchange– Obtain nutrients from yolk
• Viviparous species– Must ‘communicate’ with external environment
via maternal tissues– Degree of maternal / embryo separation varies
across taxa– Evolved independently numerous times
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Embryo RetentionAdvantages
• Predator avoidance• Maternal homeostasis• Precociality• Distribute cost of reproduction• Colonization of new habitats
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Embryo RetentionDisadvantages
• Maternal susceptibility to predation• Reduced fecundity (brood size)• Maternal requirements increased / offspring• Reduced # of reproductive opportunities
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Embryonic Nutrition
• Lecithotrophy – embryo relies onendogenous yolk reserves;– Uptake of acellular yolk by inner yolk sac
surfaces• (e.g., sharks, gymnophiones, reptiles, birds)
– Uptake of acellular yolk by gut• (e.g., sharks)
http://courses.washington.edu/chordate/453photos/urogenital_photos/shark_urogenital_photos.htm
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Embryonic Nutrition
• Lecithotrophy – embryo relies on endogenousyolk reserves;– Breakdown of cellular yolk and direct uptake by blood
vessels• (anurans, urodeles)
http://academic.reed.edu/biology/professors/sblack/research-salamander.html
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Embryonic Nutrition• Matrotrophy – nutrients provided by maternal organism
as needed• Enteric
– Imbibement of maternal secretions– Intrauterine cannibalism– Hindgut uptake
• Integumental– Uptake by body surfaces– Uptake by extra-embryonic membranes
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Lecithotrophy vs. Matrotrophy• Most ovulparous and zygoparous species are
lecithotrophic• Most embryoparous species are matrotrophic• Exceptions
– Lecithotropic: external brooding frogs– Matrotrophic: embryoparous sharks
• Determined by % change in embryo organic mass– 20-40% loss of organic mass in lecithotrophic
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Enteric Exchange• Uterine secretions – histotroph or uterine milk• Trophonemata – uterine extensions• Trophotaeniae – hypertrophy of intestinal lining• Oophagy, Embryophagy & Uterophagy
– Some sharks, teleosts, and urodeles
From Hamlett 1993. Environ. Biol. Fishes 38: 253-267.
Stingray embyro, with yolk stalk (st) and yolk sac (ys) still attached, resides in the uterus (ut) adorned with secretory trophonemata (t). From Hamlett et al. 1993.
www.viviparos.com/Galeria/A%20splendens.htm
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Integumental Exchange• Dermatrophy involves hypertrophy of
epithelial surfaces– Fins, gills, pericardial sac– Some sharks, teleosts, & amphibians
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Reptilian Placentation• Choriovitelline –
• ectoderm, vascular mesoderm & yolk sac endoderm;transitory
• Omphaloplacenta –• bilaminar layer of ectoderm & yolk sac endoderm;
some squamates• Omphalallantoic –
• omphalopleure associated with expanding allantois;snakes
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Reptilian Placentation• Chorioallantoic –
• fusion of allantoic mesoderm & extraembryonicsomatopleure (ectoderm & mesoderm of thechorion)
• Highly vascularized• Used for
• Gas exchange• Waster exchange• Steroid synthesis• Nutrient exchange
• Considered homologous to mammalian placenta
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Reptilian Chorioallantoic Placenta
Chorioallantoic placentaof the lizard Mabuya
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Mammalian Placentation• Transitory placentae exist in most species
– Yolk sac– Chorionic– Chorioamniotic
• Chorioallantoic – formed by chorion, allantois;some marsupials and all eutherians
• Chorioallantoamniotic – final placenta ofartiodactyls, great apes, & humans
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Fetal
Maternal
Uterine Connective Tissue
Chorionic Connective Tissue
Blood Vessel
Blood Vessel
•Epithelialchorial
Uterine EpitheliumChorionic Epithelium
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Fetal
Maternal
Uterine Connective Tissue
Chorionic Connective Tissue
Blood Vessel
Blood Vessel
•Syndesmochorial
Chorionic Epithelium
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Fetal
Maternal
Uterine Connective Tissue
Chorionic Connective Tissue
Blood Vessel
Blood Vessel
•Endotheliochorial
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Fetal
Maternal
Uterine Connective Tissue
Chorionic Connective Tissue
Blood Sinus
Blood Vessel
•Hemochorial
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•Embryo travels down tube and enters uterus•Embryo interacts with wall of uterus
–Implantation–Maternal recognition of pregnancy
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•Trophoblast•expands and forms placentaltissues•2 types
•Cytotrophoblast•Syncytiotrophoblast
•Cyto - main region of theplacenta•Syncytio- invasive tissue
•Uterine Response•Decidualization•Inflammation response -endometrium overgrowsembryo
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•Inflammation response• endometriumovergrows embryo•decidualization
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Placental Shapes
• Diffuse– Horses, camels, pigs, dolphins, whales
• Zonary– Carnivores (raccoon, dog, cat)
• Cotyledonary (placentomes)– Cows, sheep
• Discoid– Primates, rodents, rabbits, insectivores
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Discoid Zonary
Cotyledonary Diffuse
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Human Hemochorial