Calcium transport in nonmammalian vertebrates.

The title of this paper commemorating the contributions made by Professor Urist has an interesting bearing upon basic skeletal tissue biology. This is because the calcium-binding proteins (vitellogenins), upon which Professor Urist and Schjeide have focused much of their attention in non-mammalian vertebrates, although produced by the liver and present in the blood plasmas of non-mammalian vertebrates during vitellogenesis, are, in effect, substitutes for the protein casein present in the milk of mammalian vertebrates. Vitellogenins (180,000-250,000 daltons) appear to be produced solely for deposition in the yolk of the egg so that the calcium they carry (considerably more than is associated with casein of milk) and the amino acids of which they are comprised can be utilized during embryonic development. In many instances the progeny of non-mammalian vertebrates emerge from the shell as miniatures of the adult, capable of rapid movement and thus requiring a well developed skeletal as well as muscular system. Vitellogenins are not found in any other cells (phagocytes excepted) other than hepatocytes wherein they are made, nor are they present in the intercellular matrix of developing or remodeling bone. In non-vitellogenic females and in males of non-mammalian vertebrates, they are absent from the blood plasma altogether, so that nonionized calcium therein is solely bound to such proteins as albumin and the lipoproteins.(ABSTRACT TRUNCATED AT 250 WORDS)