The group of calcium-binding proteins known as the cal- bindins (CaBPs) plays a cardinal role in calcium metabo- lism, acting either as an intracellular facilitator of calcium diffusion or as an intracellular buffer of calcium (Wasser-

The group of calcium-binding proteins known as the calbindins (CaBPs) plays a cardinal role in calcium metabolism, acting either as an intracellular facilitator of calcium diffusion or as an intracellular buffer of calcium (Wasserman and Fullmer, 1983; Norman, 1990). The presence of CaBP-D28K in nerve terminals (Jande et al., 1981a,b; Baimbridge and Miller, 1982; Resibois et al., 1988a) suggests a further role for CaBP in neurotransmission. Production of CaBP in a number of tissues is mediated via vitamin Ddependent processes (Haussler et al., 1974; Henry and Norman, 1984). The CaBP group comprises a 28 000 Mr CaBP (CaBP-D28K), first found in avian intestine (Wasserman and Taylor, 1966) and a 9 000 Mr CaBP (CaBP-D9K), originally found in mammalian intestine (Kallfelz et al., 1967). The presence of CaBPs in numerous tissues of the body has been observed (for review see, Christakos et al., 1989). In most mammals, either CaBP-D28K or CaBP-D9K occur exclusively in a particular tissue (Christakos et al., 1989). Notable exceptions to this generalization are: the presence of both CaBPs in chondrocytes of epiphyseal growth plates (Balmain et al., 1991) and in ameloblasts of developing teeth (Berdal et al., 1991), the transient appearance of both CaBPs in the foetal rat kidney and the presence of both CaBPs in the adult mouse kidney (Rhoten et al., 1985). Recently, CaBP research has focussed on the time at which the CaBPs appear in the various embryonic tissues and reproductive organs, since this can then be correlated to specific events that mark normal development. In the rat reproductive system, CaBP-D9K has been localized in (a) the myometrium, (b) the endometrial stroma of the nonpregnant uterus and the transporting epithelium of the visceral yolk sac, (c) the intraplacental yolk sac (IPYS) (Delorme et al., 1983; Bruns et al., 1988) and the luminal epithelium of the maternal uterus in late gestation, and (d) the epithelial cells lining the fallopian tubes of the mature rat (Mathieu et al., 1989a,b). In the mouse, CaBP-D9K is present in the uterus of pregnant mice and in the endodermal cells of the IPYS (Bruns et al., 1985). During late gestation in rats and mice, both placental and maternal intestinal levels of CaBP-D9K increase in response to the calcium demands of the developing skeleton (Bruns et al., 1981). First appearance of CaBP-D9K in the yolk sac endoderm of the rat occurs on day E10 (Mathieu et al., 1989a,b), and in the mouse IPYS epithelium on day E17 (Bruns et al., 1985). Renal CaBP-D28K first appears in the distal convoluted tubules of the rat metanephros on day E19 of gestation (Chandler and Bucci, 1978). This is much later in development than chick renal CaBP-D28K which appears in the mesonephric duct on day E7, and in metanephric tubules on day E12 (Opperman et al., 1990). Duodenal CaBP-D9K 491 Development 116, 491-496 (1992) Printed in Great Britain © The Company of Biologists Limited 1992