Some morphological aspects of the innervation of a spider's silk gland

A structure in the ampullate silk gland of an orb-weaving spider, previously thought to be a pressure receptor, was studied by electron microscopy to clarify its sensory function. This lamellated 'bulbous region' is situated between the gland and the beginning duct, which consists of a layered, acellular matrix (muco-protein), surrounded by a flat epithelium and partly covered by connective tissue. The epithelial cells are densely filled by a mesh-work of microtubules and interconnected by extensive cell junctions, both indicating exposure to mechanical stress. The few nerve fibres found to supply the bulbous region do not exhibit specialized dendritic endings and provide no evidence for a receptor function. The abundance of translucent vesicles of 400-500 A (synaptic'?) and dense granules of 1000-1500 A (neurosecretory?) indicate efferent fibres, possibly controlling the secretion of the matrix. The function of the bulbous region remains to be investigated. Similar nerve fibres but with distinct synaptic foci were noted in the adjacent gland duct in large numbers. The neuro-glandular synapses have a characteristic location, namely, opposing the mesaxon-like invagination of the epidermal cells. These nerve fibres are believed to play a role in water uptake and/or secretion of the extra-cellular duct lining.