NEUROSECRETION IN OLIGOCHAETES - A LESS STUDIED SUBJECT IN INVERTEBRATE RESEARCH

The concept of neurosecretion was first developed by Ernst Scharrer (1905-1965) and Berta Scharrer (1906-1995) between 1928 and 1937. The neurosecretory cells are neuronal in structure and glandular in function. Typically, the neurosecretory cells are “tear drop” in appearance with a long axon. The neurosecretory products (neurosecretion/neurohormone) are proteinaeous in nature which is consistent with the ultrastructure of the cells

In earthworm species, irrespective of their ecological categories, the neurosecretory cells in their distribution, structure and functions in the central nervous system are more or less similar in different species of earthworms. The central nervous system in tropical earthworms reveals mainly two types of neurosecretory cells: A type (outer cortical tier of deeply stained AF positive cells with distinct axonal processes) and B type (light stained cells lying in between A type cells and central fibrous neuropile). In addition to A and B cells, an “islet” of neurosecretory cells called S cells or Hubl cells are present at the base of circum esophageal connectives of subesophageal ganglia. Both A and B cells discharge their secretion (neurohormone) at the margin of highly vascularised fibrous neuropile called “zone of accumulation” which may be considered as “elementary neuro-haemal organ”. Neurosecretion in Oligochaetes has great role in food intake, osmoregulation, thermal acclimation, regeneration, copulation and cocoon production.