AN ULTRASTRUCTURAL POSTNATAL DEVELOPMENTAL STUDY OF THE INTEGUMENT OF THE PIG (SUS SCROFA)

The main objective of this investigation was to describe the normal ultrastructure of the pig integument (sampled from the proximal forelimb, lumbar region) from one through fourteen weeks, and to compare it with the human integument. Some structural differences were seen. A large bundle of smooth muscle fibers spanned the triad of hair follicles. This muscle does not conform to the classical description of the arrector pili muscle. Therefore, it was identified as an interfollicular muscle. Nerve endings were seen among the muscle fibers. Ultrastructural investigation showed short interdigitating projections on adjacent cells. Otherwise this muscle was similar to smooth muscle in other areas. Nerves were frequently found in the reticular dermis, primarily around hair follicles. Several bare axons were noticed at all ages. The presence of dark and clear vesicles in subepidermal nerve endings would suggest autonomic innervation. Intraepidermal nerve fibers were not revealed with Bodian\u27s protargol method but were seen by electron microscopy. These were only found at low resistance skin points. Basal cell projections were seen in pig integument of one to fourteen weeks. The magnitude of these projections tended to increase slightly with age. Thickenings of basal lamina opposite the hemidesmosomes of the epidermal-dermal junction were noted. Intercellular space among the epidermal cells varied from minimal to large with a very slight change with age. Numerous mitochondria were often situated beneath the nuclei of the stratum basale cells. An unusual feature of mast cells was the presence of a pericellular matrix, resembling a thick basal lamina surrounded by collagen fibers. This matrix was more prominent at seven weeks and older. A unique finding in the reticular dermis of the lumbar region was a structure resembling a large Meissner\u27s corpuscle. It consists of several lamellae of cells surrounded by a capsule. Apocrine sweat grands and ducts were found in the regions studied. Ducts ran parallel to the hair follicle and emptied alongside the follicle at the surface. Ultrastructure of the apocrine ducts was similar to that described in humans. Merkel cells were not seen. Atypical Langerhans cells were commonly found in the stratum basale and lower and upper stratum spinosum layers. The characteristic Langerhans granules were not seen and thus these cells could possibly be classified as indeterminate cells. Except for the finding of intraepidermal nerves only at low resistance points, these points were not different from control points. In conclusion, the pig integument was similar at the light and electron microscopic level to the human integument. Significant postnatal developmental changes were not seen