Fine structural and histochemical studies on salivary glands of Peripatoides novae-zealandiae (Onychophora) with special reference to acid phosphatase distribution

The Onychophora feed on small arthropods and produce saliva when ingesting prey. Although saliva undoubtedly helps to liquefy the food its constituents have not yet been fully described. The salivary glands, two long tubes of glandular epithelium, are known to secrete a powerful protease, however, besides other enzymes and mucus. In Peripatoides novae-zealandiae there are protein-secreting cells of three types, referred to here as columnar, cuboidal and modified cells, and mucus cells. The anterior two-thirds of the gland show most cell diversity, while the posterior region consists mainly of columnar cells. These are the most numerous elements overall and they probably secrete salivary protease. In thick resin sections the granules of all protein-secreting cells stain strongly with methylene blue. Those of columnar cells are markedly uneven in size and accumulate distally, eventually filling the cytoplasm. More proximal Golgi regions may be discernible. Mucus cells are all of one type and their secretion droplets are stained lightly by methylene blue. The electron microscope shows that distal microvilli, desmosomes and septate junctions are common to all gland cells. In columnar cells, secretory material is contributed by Golgi complexes and by rough endoplasmic reticulum. Early secretory vacuoles containing dense material are seen in the concavity of Golgi regions. They are precursors to larger condensing vacuoles whose contents have a more flocculent appearance, and which may attain 3–4 μm in diameter. These evolve into secretory granules, usually of uneven texture, which are up to 2–5 μm in diameter. Histochemical tests for acid phosphatase show moderate amounts of enzyme throughout the gland. In whole mounts and sections the strongest reaction is in a band of cuboidal cells along the anterior median border. Columnar cells show a diffuse cytoplasmic reaction towards the base and sometimes distal to the nucleus, and mucus cells may also react strongly round the nucleus. Cytoplasm near the lumen shows little reaction. The secretory granules do not appear to contain active enzyme. Under the electron microscope a positive reaction for acid phosphatase is seen in lysosomal derivatives near the base and lateral periphery of gland cells. These bodies are probably autophagic vacuoles and they may contain membranous whorls and possibly old secretion granules. Acid phosphatase is involved also in the elaboration of new secretory granules in both columnar and mucus cells. Dense reaction product is found in a system of interconnected tubules and cisternae near the innermost face of the Golgi complex, which is interpreted as GERL. Acid phosphatase is present in the peripheral zone of adjacent early secretory vacuoles, and interconnections occur between GERL and secretory vacuoles. It is suggested that GERL tubules containing the enzyme may fuse with early secretory vacuoles and release acid phosphatase at their periphery. The acid phosphatase reaction is negative in large condensing vacuoles and most secretory granules. These findings are consistent with what is known from mammalian cells, including those of salivary glands.