We analyzed characteristic features of the parvalbumin-immunostaining within the lumbosacral spinal cord of newborn kittens. In contrast to the adults, parvalbumin-immunostaining was mainly revealed for the sensory fibers located within the dorsal horns and in the medial part of the intermediate gray matter. The location of these fibers partially resembles the location of Clarke's nuclei, but lasted throughout the total length of the lumbar spinal cord and merged with the presumptive Stilling’s nuclei in the sacral region. Therefore, in newborns, in contrast to adults, the parvalbumin-immunostaining proprioceptive fibers seem like a single unit. We propose that with maturation, this system is restructured because of the spread of the neuronal and neuropil elements of the lumbar enlargement responsible for the locomotor control. As a result, two local nuclear complexes: Clarke’s and Stilling’s are retained. A single population of parvalbumin-immunostaining neurons in newborns are premotor interneurons located around the lamina IX. These neurons are characterized by the low or absent NeuN-immunostaining. We believe that this neurochemical feature may be inherent for these cells.
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