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 cav ity. While each type of gland produces a signature secretion that comp lements 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 be fore the onset of overt gland development. Furthermore, occasional ect opic differentiation of individual mature secretory cells in the ''wro ng'' gland suggests that control mechanisms responsible for the distin ctive 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 sa livary glands by reviewing the expression patterns of several genes wi th known morphogenetic potential that may be involved based on develop mental timing and location. The possibility that factors leading to de termination of the sites of mammalian salivary gland development might be homologous to the regulatory cascade leading to salivary gland for mation in Drosophila is also evaluated. In a subsequent section, cellu lar phenotypes of neonatal and adult glands are compared and evaluated for insights into the mechanisms and lineages leading to cellular div ersification. Finally, the phenomena of proliferation, repair, and reg eneration in adult salivary glands are reviewed, with emphasis on the extent to which the cellular diversity is reversible and which cell ty pe other than stem cells has the ability to redifferentiate into other cell types.