Uterogestation and placentation in elasmobranchs

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AbstractElasmobranch fishes utilize a variety of means to provide nourishment for their developing young. All employ internal fertilization and sperm storage within the female genital tract. Some elasmobranchs, including all the skates and some sharks, are oviparous. In these species, fertilized eggs are enclosed in a tough egg case that is secreted by the nidamental or shell gland. The female lays the egg cases and development is entirely dependent on the yolk stores sequestered in the yolk sac. Upon oviposition, the embryo weighs less than the fertilized egg. The majority of elasmobranchs are viviparous, however, and utilize a variety of strategies to provide nourishment and satisfy respiratory demands of the developing young. Some sharks simply retain their young in the dilated posterior segment of the oviduct. In its simplest form, the maternal uterus does not provide any additional nutrients to the embryos. Other elasmobranchs develop secretory uterine villi that produce nutrient histotroph to supplement oocyte yolk stores. Uterine secretions find their zenith in the stingrays. Following yolk depletion, the uterine lining hypertrophies into secretory appendages termed trophonemata. The process by which the uterine secretions, also known as uterine milk or histotroph, are elaborated resembles the production of breast milk in higher vertebrates, and the milk is rich in protein and lipid. Concurrent with growth of the embryos, the vascular bed of the trophonemata enlarges to form sinusoids that project out to the surface to form a functional respiratory membrane. In lamnoid sharks, following yolk use, the embryos develop precocious dentition and feed on intrauterine eggs and siblings. There is generally one fetus per uterus and it grows to enormous proportions of up to 4 feet in length. In placental sharks the yolk sac is not withdrawn to become incorporated into the abdominal wall. Instead, it lengthens to form an umbilical cord and the yolk sac becomes modified into a functional epitheliochorial placenta. © 1993 Wiley‐Liss, Inc.