Objectives: We showed previously that the main excretory duct (MED) epithelium of the rat submandibular gland has light, dark, tuft and basal cells, i.e., an adult-type cell composition, around postnatal day 7. The present study was designed to investigate in more detail the cell composition of the rat submandibular gland MED from the late prenatal to early postnatal period. Methods: Wistar rats between embryonic day (ED) 18 and postnatal day (PD) 7 were used. Samples were prepared for scanning and transmission electron microscopy. Results: From ED 18 to PD 6, the rat submandibular gland MED epithelium was principally composed of cells with microvilli about 300 nm in length, cells with microvilli about 150 nm in length, and basal cells, although rarely cells with microvilli exceeding 800 nm in length were observed. The microvilli of the former and the latter cells were shorter than the microvilli of the dark and the light cells respectively. The diameter of the microvilli was also less than that of adult-type microvilli. After postnatal day 7, the MED epithelium gradually changed to the adult type. Conclusions: These findings show that from the late prenatal to early postnatal period, the rat submandibular gland MED epithelium is composed of cells with microvilli that differ in size from those of the adult-type epithelium.

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