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  • 1
    Electronic Resource
    Electronic Resource
    Subcellular organization of the yolk syncytial-endoderm complex in the preimplantation yolk sac of the shark, Rhizoprionodon terraenovae (1987)
    Springer
    Cell & tissue research 247 (1987), S. 275-285 
    Details
    ISSN: 1432-0878
    Keywords: Yolk sac ; Endoderm ; Fetal membranes ; Endocytosis ; Shark, Rhizoprionodon terraenovae
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The structure of the yolk syncytial-endoderm complex of the preimplantation yolk sac of the shark is examined by light- and transmission electron microscopy. The yolk syncytium is bounded by a membrane that is anchored to the plasmalemma of adjacent endoderm cells by desmosomes. Enlarged nuclei, rough endoplasmic reticulum, Golgi complexes, mitochondria, and other cellular organelles populate the syncytium. Microtubules and filamentous elements are also observed free in the syncytium. Yolk is present as pleomorphic droplets, the profiles of which are generally spherical but may be vesicular, especially at the periphery of large yolk droplets. Occasionally, large yolk droplets have a paracrystalline configuration. Small yolk droplets are modulated through the Golgi complex of the yolk syncytium, and it is suggested that acid hydrolases are added there. Small yolk droplets released from the maturing face of the Golgi complex are sequestered in membrane-limited packets. The membrane of the packets fuses with the membrane enveloping the yolk syncytium and the yolk droplets are released into the yolk syncytialendoderm interspace. Subsequently, the yolk droplets are endocytosed by the endoderm. Yolk droplets disperse and fuse to form the large irregular yolk inclusions of the endoderm. Yolk metabolites are transported out of the endoderm through the yolk sac endothelium. The yolk sac endoderm thus mediates the transfer of metabolites from the yolk mass to the extraembryonic circulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00218309
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  • 2
    Electronic Resource
    Electronic Resource
    Ontogeny of the umbilical cord and placenta in the Atlantic sharpnose shark,Rhizoprionodon terraenovae (1993)
    Springer
    Environmental biology of fishes 38 (1993), S. 253-267 
    Details
    ISSN: 1573-5133
    Keywords: Yolksac ; Fetal membranes ; Viviparity ; Elasmobranchs
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Synopsis The Atlantic sharpnose shark is a viviparous anamniote that develops an epitheliochorial yolksac placenta. Initially, contents of the yolksac nourish the embryos. Yolk is partially digested in the yolk syncytial-endoderm complex and subsequently transferred to the vitelline circulation. Yolk is also transported by ciliary activity of the yolk stalk ductus to the fetal gut for digestion. When embryos are 4.0cm in length, vascular ridges, termed appendiculae, develop on the yolk stalk. As yolk stores are depleted, the yolksac differentiates into the fetal portion of the placenta and the uterus abutting the yolksac differentiates into the maternal portion of the placenta. The yolk stalk differentiates into an elongate umbilical cord. The uterine epithelium produces secretions that are positive by the periodic acid-Schiff and alcian blue methods and metachromatic when stained with toluidine blue. Uterine capillaries are continuous and the surface epithelium is active both in secretion and transport of nutrients. When the embryos are 7–10cm in length, appendiculae are elongate, branched and populated by separate microvillar and granulated cells. Appendiculae may function as a paraplacental nutrient absorptive organ and be involved in the regulation of osmolarity of periembryonic fluids. The fetal placenta has two functional regions: a proximal portion that is presumed to function as a steroid producing organ and a distal portion that effects nutrient and metabolic exchange between the mother and fetus. Characteristics of the fetal placenta include endocytotic activity, crystalline-like cytoplasmic bodies and fenestrated capillaries. Fetal and maternal components of the placenta are separated by the egg envelope.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00842921
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    Paper (German National Licenses)
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