DEVELOPMENT OF CALBINDIN-D28K IMMUNOREACTIVE NEURONS IN THE EMBRYONICCHICK LUMBOSACRAL SPINAL-CORD

The development of immunoreactivity for the calcium-binding protein ca lbindin-D28k (CaB) was investigated in the embryonic and hatched chick lumbosacral spinal cord. CaB-immunoreactive neurons were revealed in the dorsal and ventral horns as well as in the intermediate grey matte r from early stages of neuronal development. CaB immunoreactivity was first detected in large neurons in the presumptive dorsal horn at embr yonic day 5, while small neurons in the lateral dorsal horn were the l ast to appear, at embryonic day 10. We have identified and traced the morphological maturation of six CaB-immunoreactive cell groups, three in the dorsal horn and three in the ventral horn. In the dorsal horn t hese groups were (1) large neurons in the lateral dorsal horn (laminae I and IV), (2) small neurons in the lateral dorsal horn (lamina II), and (3) small neurons in the medial dorsal horn (lamina III). All thre e groups were present throughout the entire length of the lumbosacral spinal cord and showed persistent CaB immunoreactivity. In the ventral horn, CaB-immunoreactive neurons were classified into the following t hree categories: (1) Neurons dorsal to the lateral motor column (lamin a VII). These neurons were present exclusively in the upper lumbosacra l segments (LS1 - 3), and they showed steady CaB immunoreactivity duri ng their maturation. (2) Neurons at the dorsomedial aspect of the late ral motor column (at the border of laminae VII and IX). This populatio n of neurons was characteristic of the lower segments of the lumbosacr al cord (LS5 - 7) and presented transient CaB expression. (3) Neurons within the lateral motor column (lamina IX). These neurons were disper sed throughout the length of the lumbosacral spinal cord. They were th ree to four times more numerous in the upper than in the lower lumbosa cral segments, and their numbers declined throughout LS1 - 7 as the an imal matured. The characteristic features of the development of neuron s immunoreactive for CaB are discussed and correlated with previous ne uroanatomical and physiological studies concerning sensory and motor f unctions of the developing chick spinal cord.