Electron Microscope Studies on the Carpal Organ of the Pig

Abstract

The eccrine sweat gland located in the subcutaneous tissue at the palmar side of carpal or metacarpal regions of forelegs of the pig (the carpal organ) was studied with the electron microscope. Besides the description of its fine structures, dicussions on the secretion mechanism of this gland are presented in this paper. The results obtained are summarized as follows.
1. Two different cell types of the glandular epithelium which were previously pointed out by light microscopy (KITAMURA, 1957), namely the dark and clear cell, are clealy distinguished from each other under the electron microscope. The designation of two cell types may depend on the previous light microscopic observation, though appearances by light- and electron microscopy may often be reverse.
2. Within the cytoplasm of the dark cell, there exist a great number of secretory granules or vacuoles. The electron density of these granules or vacuoles is so low that the electron optical appearance of this cell type is rather clear, in contrast to the image by light microscopy of stained preparations, in which the secretory granules are always darkly tinted. The secretory granule is surrounded by a dense smooth membrane (probably one type of the smooth surfaced variety of the endoplasmic reticulum). As it grows and ripens, its internal substance shows a marked decrease in density and the fusing of neighbouring granules occurs, then it converts into the secretory vacuole.
3. A small number of mitochondria with high electron density are present in the dark cell cytoplasm. The rough surfaced variety of the endoplasmic reticulum is occasionally observed to be localized at the infranuclear zone. Furthermore, small dense particles (microsomes) and microvesicles are abundant within the cytoplasm, especially in the interstices among the secretory grauules. It was forcibly convinced from many electron micrographs that granules of varying sizes and densities might be transitional forms from one to another of each of the microsomes, microvesicles, secretory granules and secretory vacuoles.
The GOLGI apparatus consists of GOLGI vesicles, several pairs of GOLGI membranes and GOLGI vacuoles, which are probably produced either by the expantion of GOLGI vesicles or by the terminal vacuolation of flattened sacs, viz. GOLGI double membranes. It is difficult, in some cases, to distinguish the GOLGI vacuole from the secretory granules.
4. The secretory granules of the clear cell are generally opaque to the electron beam and are small in number. These often situate in the vicinity of the crescent-shaped endoplasmic reticulum (rough surfaced). Somtimes, extremely dense granules with irregular outline were observed; they might be identified as fat droplets.
5. As compared with the dark cell, far more numerous mitochondria are contained in the clear cell. They have marked limiting membranes and internal ridges (cristae mitochondriales), both of which are electron dense and show a double membraned structure. There have been recognized various transitional steps from the mitochondria to the secretory granules of the clear cell.
6. In the clear cell cytoplasm, were observed to be scattered many microsomes and thread-like or tubular structures. The latter may be considered to be produced by chaining of microsomes, and sometimes they aggregate in a compact mass of peculiar crescent shape, which is found to surround the nucleus at the supra- and paranuclear sides maintaining a roughly uniform distance apart the nucleus. This crescent-shaped structure was found under the ordinary light microscope to be strongly basophilic. The constituent elements of such a crescent-shaped mass possess the characteristic of typical endoplasmic reticulum (rough surfaced); but in the case that thread-like or tubular structures are dispersed throughout the cytoplasm, the characteristics of the endoplasmic reticulum are obscure in such distributed bodies.