Ultrastructural similarities between Chlorohydra viridissima and human neurosecretory granules: a cytochemical study using the uranaffin reaction.

The uranaffin reaction is an ultrastructural cytochemical procedure which, in humans, stains the cores of true neurosecretory granules of all morphological types, and will not stain the granules of exocrine-type glands or lysosomes found in a variety of cell types. Positively-charged uranyl ions appear to react with concentrated polyphosphate groups and stain the nucleotides known to be present in the neurosecretory granules of higher organisms. In Chlorohydra viridissima, two morphological types of neurosecretory granules were identified within the epidermis and gastrodermis by routine electron microscopy; one granule type was small and displayed a central dense core with a surrounding clear halo, whereas, the other was larger and displayed an eccentric core with a loosely-associated granule membrane. Both types stained with the uranaffin reaction, but the characteristic granules of the exocrine-type digestive glandular cells and the mucous cells of the gastrodermis were uniformly uranaffin-negative. In the gastrodermis, a third granule type was identified which showed a larger uranaffin-positive core. The discovery of uranaffin-positive neurosecretory granules in Hydra suggests that formation of nucleotide-neurohormone complexes as a means for packaging and storage of neurohormones within the granule matrix, may have existed in primitive nervous systems.