Ontogenetic Variations in the Distribution of Ca and Mg in Skeletal Tissues of Vertebrates and Invertebrates
There are important ontogenetic patterns in the distribution of Ca and Mg in both invertebrate and vertebrate skeletal tissues. These variations need to be defined before either the physiological or environmental significance of fluctuations in the inorganic skeletal components can be assessed fully. For example, biomineralogists have long known that organisms tend to exclude Mg from their skeletons because Mg inhibits calcification. However, we have found clearly defined, inverse Ca and Mg oscillations within skeletal tissues over a range in frequencies that are difficult to explain simply on the basis of random substitutions of Mg for Ca in the biomineral lattice. Moreover, our data suggest that there are ontogenetic trends in the development of the inverse oscillations. With mixed results, marine biologists have used Mg content in the skeletons of various invertebrates as an index of water temperature and of changing composition of seawater over the past 600 million years. Yet our analyses of the shells of invertebrates, such as brachiopods, reveal age-mediated concentrations of elements that complicate such generalizations.
We describe here the results of electron microprobe analyses of the skeletal tissues of two different animals to exemplify the above: the shells of the living brachiopod, Terebratalia transversa, and incisors of the rapidly aging Brookhaven National Laboratory Strain of Swiss Webster mice (BNLSW mice). The outer shell layer of the brachiopod was traversed across a series of structural growth increments that resemble bivalve growth patterns known to be deposited under tidal influence. The incisors were traversed from the enamel to the margin of the pulp cavity, across concentric growth increments representing about 20–30 days of growth. In both the brachiopod shell and the BNLSW mouse incisor, Ca and Mg were seen to oscillate inversely over a range of temporal and spatial periodicities. In cases where the structural growth patterns are a response to environmental changes (e.g. tidal oscillations in the brachiopod), the elemental patterns are a chemical response to the same parameter. There is some evidence that the inverse oscillations do not appear early in the ontogeny of the mouse incisor. It is not yet clear that the oscillations are confined to later ontogeny of the brachiopod shell. These results underscore our contention that an ontogenetic framework will help the physiological and environmental implications of changing skeletal composition appear comprehensible.
KeywordsSkeletal Tissue Correlation Line Structural Growth Swiss Webster Mouse Ontogenetic Variation
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