Acetylcholinesterase (AChE) activity, a marker of neural differentiation, was histo-chemically localized in embryos and larvae of Mytilus galloprovincialis Lamarck and M. edulis L. (Mollusca: Bivalvia). The results show that: (1) the first AChE-active cells develop as two bilaterally symmetrical, postero-dorsal pioneer sensory neurons and their supporting superficial cells; (2) the pathways of their pioneering longitudinal axons are the same as those of two bilaterally symmetrical nerve cords which differentiate later in a postero-anterior direction; (3) in the conchostoma larva the posterior neurons and their associated superficial AChE-active cells give rise to two ciliary sensory-like organs which are the earliest rudiments of pedal ganglia and primary byssus glands; (4) three pairs of posttrochal gangliar rudiments, most probably visceral, parietal and pleural ganglia, develop on the same longitudinal nerve cords in the trochophore larva; (5) starting from the veliger stage, the presumptive foot rotates on the ventral side and migrates forwards, so that this gangliar chain is bent and the viscero-parietal ganglia become localized dorso-posteriorly to the pedal ganglia; (6) the cerebral ganglia differentiate together with the apical organ and the nerve network of the velum after the first pedal nerve rudiments have been detected.

Histochemical methods for cholinesterase activities were applied to embryos and early larvae of the marine mussel Mytilus galloprovincklis Lamarck (Mollusca: Bivalvia: Mytilidae) and demonstrated that the cerebral ganglia, which are generally accepted as the first differentiating ganglia, appear after two bilaterally symmetrical nerve rudiments have developed in a posterior position prior to the trochophore stage. They are intimately associated with non-nervous superficial cells in two structures which have features of sensory organs.

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Membrane acetylcholinesterase activity is considered to be a marker for a cholinergic system. When temporarily expressed in differentiating cells other than the nervous or muscular ones, it may play a role in morphogenesis. In the lichen Parmelia caperata (L.) Ach., acetylcholinesterase is histochemically localized mainly in the cell walls and/or membranes of both symbionts just where they proliferate and form well-organized propagation structures, the soredia. The enzyme activity is first detected in a few algae undergoing aplanosporogenesis and later in medullary hyphae that reach the dividing algae by elongating perpendicularly to the thallus surface. This histochemical pattern that is associated with algal proliferation and oriented hyphal growth is characteristic of early morphogenesis of the soredia; when fully differentiated, they consist of an inner dividing alga and an outer hyphal envelope, both showing cholinesterase activity. Substrate specificity and inhibitor sensitivity of the histochemical staining indicate an acetylcholinesterase-like activity. However, extracts of the thallus areas where soredia develop give four bands of cholinesterase activity on disc electrophoresis: the two cathodal bands have the characteristics of acetylcholinesterase, the others of pseudocholinesterase. One of the latter hydrolyses propionylthiocholine very rapidly. The findings suggest that in lichen symbiosis, a cholinergic-like system participates in regulating morphogenetic processes such as cell division, oriented tip growth and alga-fungus membrane interactions. Environmental stimuli, particularly light, might trigger the development of soredia by modulating the activity of the cholinergic mechanism.

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