Some features of the fine structure and histochemistry of planarian subepidermal gland cells

Summary

The conspicuous subepidermal gland cells, namely eosinophiles, basophiles and rhabdite forming cells, in Planaria vitta, have been studied using a variety of histochemical methods for proteins, amino acids, polysaccharides, lipids, nucleic acids and calcium, in addition to common histological methods. Several fixation methods and freeze-drying have been employed. The eosinophilia of certain of the gland cells has been investigated by blocking amino acid end- and side-groups and by the staining capacity at different pH's. The fine structure of the secretion granules has been elucidated by means of electron microscopy.

The eosinophilic granules are very coarse and up to 1.5 μ long and 0.8 μ wide. They are predominantly composed of protein and give positive reactions for tyrosine, arginine and cysteine-cystine and a negative reaction for tryptophan. They possibly also contain histidine. The eosinophilia is only slightly affected by acetylation or nitrosation and is only slightly decreased at pH 11.8, so the presence of arginine is probably the basis for the eosinophilia. The granules probably also contain phospholipid. The fine structure of the granules is rather unique for secretion granules. They are not surrounded by a membrane and are built up of light and dark striated structures, very regular in appearance and unperforated. The striation is mainly oriented transverse to the long axis of the granules. Parallel with this axis a much finer striation is found, bounding rectangular compartments. The coarse, dark striations are about 175–250Å wide.

The basophilic cells may be divided into at last three types. The most commonly encountered contain granules 0.2–0.3 μ in diameter. They are selectively stained by aldehyde-fuchsin after permanganate oxidation. The granules are negative to tests for protein and amino acids. They are PAS-positive and become intensely metachromatic and stainable by alcian blue at pH 2.9 after sulfation. These and several other observations point to the presence of neutral and/or acid mucopolysaccharides. Glycogen is not present. Another type of basophilic cells contains RNA in great amounts. The basophilic granules appear homogeneous on the electron micrographs.

The rhabdites are up to 5 μ long and 1 μ wide. They are negative to tests for lipid, nucleic acid and polysaccharides. They are intensely eosinophilic even at pH 11.8. They are probably composed of proteins, but only one amino acid was found reacting to a noteworthy degree: cysteine. It was not possible to elucidate the basis for the strong eosinophilia. The presence of ionic calcium was ruled out. The rhabdites are very electron-dense and on the electron micrographs appear homogeneous and not invested by a membrane.