Development and distribution of enkephalin-immunoreactive elements in the chicken spinal cord

The development and distribution of methionine-enkephalin-immunoreactive elements were studied in the chicken spinal cord with the indirect immunofluorescence method. Methionine-enkephalin-like immunoreactivity was first detected in the chick spinal cord at embryonic stages 29-30 (incubation day 6). Before stage 35 (day 9), it was mainly observed in fibres almost throughout the white matter. Subsequently, fibres containing the peptide appeared in the ventral half of the gray matter, but mostly in the lateral portion of the neck of the dorsal horn. From stage 40 (day 13 or 14), fibres were especially noticed in laminae 1 and 2, and in the area dorsal to the central canal. In particular, many enkephalin-immunoreactive perikarya were observed in several spinal areas during this period. Such a distribution of both enkephalin-immunoreactive fibres and perikarya remained visible at later embryonic stages, but labelled cells gradually decreased in number and disappeared after hatching. With colchicine treatment, however, a similar distribution of the peptide was found in the spinal cord of adult chickens. As in the embryo, enkephalin-immunoreactive perikarya were mainly observed in the lateral portion of the neck of the dorsal horn, in lamina 1, and in the nucleus of the dorsolateral funiculus throughout the spinal cord. At the thoracic level, many were also located ventral to the central canal. Enkephalin-immunoreactive fibres increased notably in the gray matter of adult chickens. They mainly occurred in laminae 1 and 2, in the lateral portion of the neck of the dorsal horn, and in the area around, especially dorsal to, the central canal. In contrast, enkephalin-immunoreactive fibres decreased in the white matter and they were mainly observed in the dorsolateral funiculus, in Lissauer's tract, and in the lateral funiculus adjacent to the gray. The distribution of enkephalin-immunoreactive fibres was generally comparable at all spinal levels examined. In addition, examination of post-hatched chickens showed virtually the same results as in the adult.