Immunohistochemistry of myoepithelial cells during development of the rat salivary glands

Using a battery of monoclonal antibodies specific for rat proteins, immunoh istochemistry was carried out on the developing myoepithelial cells (MECs) of the rat major salivary glands. The proteins examined were alpha-smooth m uscle actin (alpha SMA), h1-calponin (calponin), keratin 14 (K14), beta sub unit of S-100 protein (S-100 beta), vimentin and glial fibrillary acidic pr otein (GFAP). The MECs exhibited immunoreactivity for aSMA, calponin and K1 4, but not that for S-100 beta, vimentin and GFAP. Immunoreactivity for aSM A appeared in the MECs from the time when the microfilaments were initially deposited in these cells, i.e., at 20 days in utero in the sublingual and submandibular glands and at birth in the parotid gland. Calponin immunoreac tivity was seen 1 day earlier than aSMA. The appearance was almost at the s ame time as the onset of the MEC differentiation in each gland. A small num ber of the MECs expressed weak K14 immunoreactivity from the time when the acinus-intercalated duct structure was established, i.e., at 21 days in ute ro in the sublingual gland, at 5 days after birth in the perotid gland and after 5 weeks post-natally in the submandibular gland. In addition, K14 imm unoreactivity was observed in the basal cells of the striated and excretory ducts. The first appearance of K14 in these cells again coincided with the emergence of the duct system in each gland, i.e., at 20 days in utero in t he sublingual gland, at 21 days in utero in the submandibular gland and at 3 days after birth in the parotid gland. Finally, the MECs in all the gland s were found to redistribute as the acini matured. As the acini grew rapidl y during the weaning period in the parotid and the sublingual glands, the M ECs ceased to surround the acini. Thereafter, they disappeared from the aci ni in the parotid gland, whereas they reappeared in the sublingual gland. I n the submandibular gland, the MECs were confined to the terminal tubules u ntil 4 weeks after birth. Thereafter, the acini were established and invest ed by the MECs. In conclusion, immunohistochemistry of calponin and alpha S MA is a useful tool for identification of the MEC during its earliest diffe rentiation, which has hitherto been possible only electron microscopically. In addition, it is suggested that the MEC is heterogeneous and the functio nally differentiated MEC appears after weaning around acini of the mucous a nd seromucous glands.