SALIVARY GLANDS:A PARADIGMFOR DIVERSITY OFGLANDDEVELOPMENT

Themajor salivary glands ofmammals arerepresented bythree pairs oforgans that cooperate functionally topro- ducesaliva fortheoral cavity. While eachtype ofgland produces asignature secretion that complements thesecretions from theother glands, there isalso redundancy asevidenced bysecretion offunctionally similar and, insomecases, identical prod- ucts inthethree glands. This, along with their commonlate initiation ofdevelopment, infetal terms, their similarities indevel- opmental pattern, andtheir proximate sites oforigin, suggests that acommonregulatory cascade mayhavebeenshared until shortly before theonset ofovert gland development. Furthermore, occasional ectopic differentiation ofindividual mature secretory cells inthewrong" gland suggests that control mechanisms responsible forthedistinctive cellular composition of eachgland also share manycommonsteps, withonlyminor differences providing theimpetus fordiversification. Tobegin to address this area, weexamine here theorigins ofthesalivary glands byreviewing theexpression patterns ofseveral genes with knownmorphogenetic potential that maybeinvolved based ondevelopmental timing andlocation. Thepossibility that fac- tors leading todetermination ofthesites ofmammalian salivary gland development might behomologous totheregulatory cascade leading tosalivarv gland formation inDrosophila isalsoevaluated. Inasubsequent section, cellular phenotypes of neonatal andadult glands arecompared andevaluated forinsights into themechanisms andlineages leading tocellular diver- sification. Finally, thephenomena ofproliferation, repair, andregeneration inadult salivary glands arereviewed, withempha- sisontheextent towhichthecellular diversity isreversible andwhich cell typeother thanstemcells hastheability toredif- ferentiate into other cell types.