Innervation de type neurosécréteur dans la glande sternale de Kalotermes flavicollis (Isoptera). Étude ultrastructurale

Abstract

The sternal gland of Kalotermes flavicollis secretes a trail pheromone dissolved in a lipid substance. The gland appears as an epidermal thickening in the anterior part of the fifth abdominal sternite. Two types of cells may be recognized: glandular cells with many lipidic droplets and columnar cells with a brush border beneath the cuticle. There are also about thirty campaniform sensilla. An ultrastructural study discloses nervous efferent fibres in the anterior part of the sternal gland. These fibres are easily distinguished from the sensory fibres by the following features: glial cells, which surround the axon before reaching the gland, end in the connective layer, and naked axons run between the gland cells to the base of the brush border. In the axoplasm there are electrondense granules (probably neurosecretory), 900 to 1500 Å in diameter. Both at the level of the connective layer and in the gland near the columnar cells, ‘synaptoid vesicles’ are aggregated close to the axon membrane in ‘synaptoid areas’ where the neurotransmitter may be released. Such a ‘neurosecretomotor junction’ probably plays a rôle in the control of secretion. It is tentatively suggested that efferent nerve fibres influence the release of pheromene. Then the pheromone is mixed with the continuously produced lipid component before being laid down on the ground.