THE ULTRASTRUCTURE AND INNERVATION OF MUSCLES CONTROLLING CHROMATOPHORE EXPANSION IN THE SQUID, LOLIGO-VULGARIS

Squid chromatophores are organs of colour change, consisting of a pigm ent sac opened by contraction of 10-24 radial muscle fibres. The ultra structure and innervation of these muscle fibres were examined by elec tron microscopy and diagramatic reconstructions made on the basis of s erial ultra-thin sections. At the proximal end of the fibre, nearest t he pigment sac a cortical myofilament zone surrounds 2 cores containin g mitochrondria; further along the fibre these merge to form one centr al core. The myofilament zone forms a groove containing a nerve bundle consisting of 2 to 4 axons per muscle fibre. The axons are surrounded by glial cell processes, and either originate from a neighbouring fib re, or join the fibre at some point along its length. Axons twist arou nd each other, forming a series of synapses with the muscle fibre. As many as 6-37 synapses exist along the length of each muscle fibre; the mean synapse interval is 9.05 mu m, but the largest may be 123 mu m A t the distal end of the muscles, the nerve is located towards the midd le of the fibre, which it penetrates as the muscle splits up. Electron -lucent vesicles are present in all synaptic regions, but electron-den se vesicles are only found towards the distal end of the fibre. There is thus a possibility that more than one neurotransmitter is present i n the nerves innervating chromatophores. Electron-lucent and dense-cor ed vesicles are not colocalised.