Evidence of a phenotypically determined ductal cell lineage in mouse salivary glands

The submandibular salivary gland of mice contains a parenchymal element, the granular duct, which matures peripubertally from the striated ducts. Granular duct cells also differentiate from intercalated ducts in the adult mouse submandibular gland. Using preproNGF-A as a signature protein of mature granular duct cells, this study inquired if phenotypic determination might have occurred earlier than the first signs of cellular differentiation. Results from RT-PCR indicate the presence of preproNGF-A transcripts at all postnatal stages of development of the submandibular glands, as well as in adult sublingual glands. The preproNGF-A transcript was also detected prenatally as early as embryonic day 17 in the submandibular/sublingual complex. Using an antibody directed specifically against the "pre" peptide, immunocytochemistry showed preproNGF-A localized in the granular ducts and striated ducts of the adult submandibular gland. In addition preproNGF-A was detected throughout the first order branches of the intercalated duct system. In the neonatal gland, preproNGF-A was found in the large tubules that differentiate to the striated ducts. The early appearance of preproNGF-A in the histological lineage that sequentially gives rise to striated ducts and then to granular ducts suggests that this lineage is phenotypically determined as early as birth. An undifferentiated stage of the phenotypically determined lineage also appears to be retained in the intercalated duct system to provide progenitors for subsequent differentiation in the adult gland. Throughout development of the sublingual gland, preproNGF-A was detectable in the striated ducts or in their predecessors, suggesting that they may also represent a phenotypically determined cell lineage similar to that of the submandibular gland.