Salivary glands: a paradigm for diversity of gland development.

The major salivary glands of mammals are represented by three pairs of organs that cooperate functionally to produce saliva for the oral cavity. While each type of gland produces a signature secretion that complements the secretions from the other glands, there is also redundancy as evidenced by secretion of functionally similar and, in some cases, identical products in the three glands. This, along with their common late initiation of development, in fetal terms, their similarities in developmental pattern, and their proximate sites of origin, suggests that a common regulatory cascade may have been shared until shortly before the onset of overt gland development. Furthermore, occasional ectopic differentiation of individual mature secretory cells in the "wrong" gland suggests that control mechanisms responsible for the distinctive cellular composition of each gland also share many common steps, with only minor differences providing the impetus for diversification. To begin to address this area, we examine here the origins of the salivary glands by reviewing the expression patterns of several genes with known morphogenetic potential that may be involved based on developmental timing and location. The possibility that factors leading to determination of the sites of mammalian salivary gland development might be homologous to the regulatory cascade leading to salivary gland formation in Drosophila is also evaluated. In a subsequent section, cellular phenotypes of neonatal and adult glands are compared and evaluated for insights into the mechanisms and lineages leading to cellular diversification. Finally, the phenomena of proliferation, repair, and regeneration in adult salivary glands are reviewed, with emphasis on the extent to which the cellular diversity is reversible and which cell type other than stem cells has the ability to redifferentiate into other cell types.