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Ultrastructural characteristics of muscle fibers and neuromuscular contacts were investigated during two stages of embryogenesis of the pulmonate snail Lymnaea stagnalis. The first muscle cells appear as early as during metamorphosis (50-55% of embryonic development), whereas previously, in the trochophore/veliger stages (25-45%), muscular elements cannot be detected at all. The first muscle fibers contain large amounts of free numbers, a well-developed rER system and only a few irregularly arranged contractile elements. The nucleus is densely packed with heterochromatine material. At 75% adult-like postmetamorphic stage, the frequency of muscle fibers increases significantly, but, bundles of muscle fibers cannot yet be observed. Furthermore the muscle cells are characterized by large numbers of free ribosomes and numerous rER elements. Fine axon bundles and single axon processes, both accompanied by glial elements, can already be found at this time. Axon varicosities with different vesicle and/or granule contents form membrane contacts with muscle fibers, but without revealing membrane specialization on the pre- or postsynaptic side. The late development of the muscle system and neuromuscular contacts during Lymnaea embryogenesis correlates well with the maturation of different forms of behavior of adult, free-living life, and also with the peripheral appearance of chemically identified components of the embryonic nervous system of central origin.

Ultrastructure of neuromuscular contacts in the embryonic pond snailLymnaea stagnalisL.