Prenatal development of myoepithelial cell of human submandibular gland observed by immunohistochemistry of smooth muscle actin and rhodamine-phalloidin fluorescence.

Immunostaining of monoclonal antibody (MoAb) of smooth muscle actin in paraffin sections and fluorescence of actin-specific phalloidin in cryostat sections were utilized to demonstrate the myoepithelial cells in prenatal and adult salivary glands of humans. In the early developmental stage (10-18 weeks) MoAb actin was weakly positive in the basal cells of the gland epithelium, and the positivity gradually accentuated at the basal portions of the terminal ducts and acini as the gestational period advanced. In the early intermediate developmental stage (19-24 weeks) the polyhedral myoepithelial cells were arranged in the basal portions of the acini and intercalated ducts. At this stage the myoepithelial cells produced phalloidin-positive spindle cytoplasmic processes. In the late intermediate developmental stage (25-32 weeks) the myoepithelial cells became flattened and formed dendritic processes to surround the acini and intercalated ducts. In the late developmental stage (33-40 weeks) numerous myoepithelial cells with well developed dendritic processes were demonstrable in the acini and intercalated ducts. In conclusion, it was found that the myoepithelial cells began to develop at 15-16 weeks of gestation when the acinar cells were still immature. The primitive myoepithelial cells were polyhedral in shape to form compact basal layer beneath the developing acinar cells during 19-24 weeks of gestation. In late gestational period the myoepithelial cells almost matured like the dendritic ones of adult salivary glands. However, the myoepithelial cells were never demonstrated in the striated and excretory ducts of the fetal salivary glands as opposed to its normal presence in the adult salivary glands. A possible aging process of myoepithelial cells was discussed in accordance with the histogenesis of transformed myoepithelial cells of salivary gland tumors.