A note on presynaptic dendrites in explant cultures of embryonic chick tectum.

Explants from embryonic chick tectum maintained in culture for at least two weeks contained presynaptic dendrites, characterised by focal presynaptic specialisations in addition to the usual fine structural features of dendrites. The explants also contained bouton-like structures with both pre- and postsynaptic specialisations, interpreted as the terminal parts and appendages of the presynaptic dendrites. The dendrites and their appendages engaged in various serial synapses including triplet and reciprocal synapses. Although it is known that cells which do not normally have presynaptic dendrites may be induced to develop them under abnormal developmental or pathological conditions, the development of presynaptic dendrites in the present material is more likely to be a manifestation of organotypic development, since the adult avian tectum normally contains cells with presynaptic dendrites.