Perikaryal projections of developing spinal ganglion neurons in the chick demonstrated by scanning electron microscopy.

The perikaryal projections of sensory ganglion neurons in chick embryos were observed by scanning electron microscopy after removal of the connective tissues and satellite cells by enzymatic digestion treatment. The perikaryal projections were seen not only on the surface of the perikarya but also on the surface of the stem processes. The projections were up to 3 microm in length, and their transverse diameters ranged between 0.12 and 0.24 microm from incubation (embryonic) day 10 to posthatching day 2. On days 6 and 8 of incubation, thicker projections with transverse diameters of 0.24-0.9 microm were observed transiently in addition to those described above, and some of them looked like vestiges of neuronal processes during development. The thin projections emerging mainly in the later developmental stages increased in number as spindle-shaped bipolar neurons differentiated into (pseudo)unipolar cells. Morphometric analysis revealed that the density of perikaryal projections correlated well with the shape and size of each neuron; thin perikaryal projections were more numerous on those of mature pseudounipolar neurons than on the surface of premature ganglion neurons, and they increased in number as the individual ganglion cell bodies grew larger. The neuronal shape- and size-dependent increase in perikaryal projections during development may support the hypothesis that perikaryal projections are structural devices for increasing neuronal surface areas and possibly the efficiency of metabolic activities.