AbstractThe objective of this study was to provide a detailed account of the morphogenesis and early cytodifferentiation of the hamster cheek pouch. Although the newborn “cheek pouch” is used for in vitro studies of the effects of retinoids and carcinogens, its rudimentary structure has not been adequately described. Complete paraffin serial sections of the heads of 14‐ and 15‐day fetuses were cut in three planes to determine the location and shape of the earliest pouch rudiments. Complete paraffin serial sections were prepared from pouch rudiments dissected from hamsters at birth and at daily intervals from 3 to 12 days postnatal. Semithin Epon sections were examined by light microscopy and ultrathin sections by transmission electron microscopy. The pouch can appear in the fetus as two solid epithelial ingrowths from the lining of the oral cavity. They are the margins of an ingrowing sheet of oral epithelium which becomes leaflike at about the time of birth, as it grows caudad into the tissue of the cheek. The central cells of the ingrowth accumulate large quantities of glycogen before differentiating as a stratum spinosum 5 days after birth. Within the stratum spinosum, groups of cells containing keratohyalin granules initiate the stratum granulosum. Keratinized cells appear within the stratum granulosum areas. Spaces appear between keratinized cells, and the spaces coalesce to form the pouch cavity between 7 and 12 days postnatal. Soon afterward, this cavity opens to the oral cavity to make a pouch, and the ultrastructure of the cheek pouch epithelium closely resembles that of th
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