yolk-sac placenta

anatomy

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July 15, 2026

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The yolk-sac placenta is a structure found in some sharks that resembles a placenta. It consists of the vascularized embryonic yolk sac wall closely associated with the maternal uterine or oviducal wall. Its primary function is to nourish the developing embryo.1

In broader terms of embryonic development, the yolk sac is a membranous bag connected to the embryo by a stalk. Blood vessels in its walls transport yolk products to the embryo. As the yolk is used, the yolk sac shrinks and is absorbed into the embryo's body. In some viviparous elasmobranchs (sharks and rays), the yolk sac serves as a respiratory and nutritive membrane in contact with the uterine lining. When stored yolk is depleted, the yolk sac can function as a placenta, absorbing nutrients. Yolk-sac placentas are also common in marsupials with short gestation periods and in lizards.23

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animal reproductive system
Provisions for the developing embryo
The yolk-sac placenta does not invade maternal tissues, but intimate interlocking folds may occur between the two. The chorioallantoic membranes of reptiles and mammals exhibit many degrees of intimacy with maternal tissues, from simple contact to a deeply rooted condition (deciduate placentas). Chorioallantoic or chorionic placentas…
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The yolk-sac placenta does not invade maternal tissues, but intimate interlocking folds may occur between the two. The chorioallantoic membranes of reptiles and mammals exhibit many degrees of intimacy with maternal tissues, from simple contact to a deeply rooted condition (deciduate placentas). Chorioallantoic or chorionic placentas…
Read More