Histology and immunohistochemistry of the developing juxta-oral organ in mice

The juxta-oral organ is a persistent bilateral organ in the mammalian bucca. It consists of epithelial parenchyma and surrounding mesenchymal sheath with connective tissue rich in nerve fibers and sensory receptors between them. The organ develops from the embryonic oral epithelium and is separated from the oral mucosa. The morphology of this organ is very different in mice compared to humans. Although several reports have described the histology of the juxta-oral organ in rodents, its developmental changes have rarely been reported. In the present study, histological development and ultrastructure of the juxta-oral organ, with special attention to its epithelial parenchyma and mesenchymal sheath, were investigated in mice during several developmental stages. Immunohistochemical staining for pan-cytokeratin (CK) on serial paraffin sections of the head allowed easier detection of this organ. The juxta-oral organ was innervated by the buccal nerve from an early stage of organogenesis at E13.5. The organ was located adjacent to the fascia of the masticatory muscles during all developmental stages examined. The parenchymal cells were immunohistochemically positive for p63 and CK14 in both newborn and adult mice, suggesting that these epithelial cells possess characteristics of the basal cells of the stratified epithelium. Furthermore, the parenchyma was ensheathed by layers of mesenchymal cells with perineurial characteristics as shown by immunohistochemical staining and electron microscopy. Unique sensory endings and bundles of elastic fibers were observed between the epithelial parenchyma and mesenchymal sheath. The present findings concerning the structure of mouse juxta-oral organ, especially that of epithelial parenchyma and mesenchymal sheath provide baseline knowledge for the further understanding of the unique morphological features, developmental changes and functional details of this organ.