Axonal and dendritic development of substantia gelatinosa neurons in the lumbosacral spinal cord of the rat.

In the present study, developing neurons of the substantia gelatinosa (SG) are examined at short interval sequential stages from 15 days of gestation through 20 days postpartum. Rapid Golgi preparations are utilized to examine axonal and dendritic development and toluidine blue preparations are employed to study the overall growth pattern of SG cells by measuring changes in mean cell body area. Results show that there are two maturation periods, which involve two separate groups of SG neurons. The sequence and pattern of development for each group is different. The first period occurs prenatally and involves the axonal and dendritic development of presumptive projection and propriospinal neurons. In classical terminology, these cells can be classified as limiting, large and small central, and transverse cells. These neurons have axons that enter the white matter and their dendritic arbors develop through a relatively simple process of elongation and branching. The second maturation period occurs postnatally and involves the development of presumptive nonprojection intrinsic neurons that have axons which remain within the gray matter. These neurons are identified as islet, stalk, inverted stalklike, and vertical cells. Unlike projection or propriopinal neurons, the intrinsic nonprojection neurons sprout numerous short, beaded dendrites that radiate from the cell body in a starlike fashion. Starshaped cells undergo a metamorphosis involving a rearrangement of dendrites along adult dendritic patterns. Measurements taken from toluidine blue preparations indicate that the nonprojection intrinsic population makes up the greatest percentage of SG neurons, as evidenced by a marked increase in the size of the average SG nerve cell during the second maturation period.