Histology and Sensory Innervation of Transitional and Mucosal Parts of the Lip of Monkey

Abstract

The lip of monkey is not different from the human counterpart in that it can be divided into the cutaneous, the transitional and the mucous parts and that the transitional part can be subdivided into the outer and the inner zones. The cutaneous part is covered by a common haired skin with sinus-hairs. The epithelium of the outer transitional zone is considerably thicker than the epidermis of the cutaneons part, is only very weakly hornified on its surface and is intruded by conspicuous papillae growing out of the propria. In this zone of the human lip sebaceous glands without hairs are frequently found, but in the simian lip, such glands are more often found provided with hairs, suggesting that this zone is not derived only from the mucous membrane but also from the haired skin as well. In the inner transitional zone, the epithelium is even thicker and the papillae are better developed, but the boundary between the inner and the outer zones is rather blurred in adult monkey. The epithelium in the mucous part is indeed still well developed, but the papillae are thinner and shorter and are arranged rather irregularly.
The development of the sensory fibres distributed in the transitional and the mucous parts of the lip of monkey shows a parallelism with that of the papillae, as was the case with human lips. Namely, the sensory fibres are developed in the inner zone of the transitional part where the papillae are in strongest development, followed by its outer zone where the epithelium is relatively poorer in development but the papillae are still powerfully formed, while in the mucous part with papillae reduced in size and length only in spite of the thick epithelium, the sensory fibres are in poorest development.
The sensory fibres coming into the outer transitional zone always end in the papillae in various types of terminations, as follows: 1. Unbranched and branched terminations formed by mediumsized fibres. The branched terminations are usually of simple type but somewhat more complex ones are not rare either. Their terminal fibres mostly end subepithelially, but frequently enough some are found to end in intraepithelial terminations. 2. Unbranched and branched terminations of peculiar form. In these the terminal fibres show the characteristics of conspicuous swelling and fibrillar expansion, markedly winding courses and blunt end points beneath the epithelium. 3. Glomerular terminations, mostly uncapsulated but more rarely covered by a thin connective tissue capsule. 1-3 sensory fibres are found running into the bulb containing a small number of specific nuclei, and their terminal fibres often show glomerular arrangement, but on many occasions they are found to end in fibrillar arrangement resembling that in MEISSNER's tactile bodies. The fibres forming such arrangement frequently show fibrillar expansions in their courses, 4. MEISSNER's tactile bodies found in the outer transitional zone in the portion adjacent to the inner zone. The very fine terminal fibres in such a body often show fibrillar expansions while running parallel to the epithelial surface or spiral-wise. The ellipsoid or spindle-form specific cell nuclei of limited number in the inner bulb also are arranged parallel to the fibres.
The sensory terminations in the inner transitional zone are even better developed than in the outer zone. Here the MEISSNER's tactile bodies are particularly well developed. The unbranched and branched terminations here originate in thick fibres, but the latter are comparatively simply branched and the terminations of Type II above with swollen ends of the terminal fibres are found here too. The intraepithelial fibres are less frequent than in the outer zone.
In the mucous part, the sensory fibres decrease abruptly in number and their terminations comprise only unbranched and simple branched ones, corpuscular terminations and intraepithelial fibres being found no more.