Growth, differentiation and ribonucleic acid content of the fetal rat spinal ganglion cells in culture

Abstract

AbstractObservations were made on 324 cultures of spinal ganglia which were excised from fetal rats between the tenth and eighteenth day of gestation. The cultures were maintained up to 81 days in vitro. As a control sixteen Sprague‐Dawley rats from two litters were sacrificed at different ages. RNA content of the cell bodies in both normal and cultured spinal ganglia was determined by the Edström method. The volume of the cell bodies was determined by the optical reconstruction method of Micklewright. Spinal ganglion cells in culture develop at a much slower pace than in situ, but show the normal sequential order of cytodifferentiation. A few days after explantation, the cells increase rapidly in size and a large fibrillogenous zone becomes apparent around the centrosome. All of the neurofibrillar network of the cell and the axon seem to derive from this zone. At this stage the Nissl material is found mainly at the periphery of the cell body since the fibrillogenous zone is essentially free from Nissl granules. The initial development of the zone in culture is accompanied by no or a very small increase in the cellular RNA content present at the time of explantation though the cell volume increases greatly. In later stages, the fibrillogenous zone disappears slowly as a separate entity while the Nissl bodies invade the entire soma of the cell. When this is accomplished the neurofibrillar network becomes complex, the Nissl bodies form distinct patterns, the growing fibers start to myelinate. The cells increase only slowly in size. After four to ten weeks in culture the typical spinal ganglion cell is a monopolar neuron which is morphologically advanced to various degrees. However, some less common morphological forms can be seen occasionally in culture as is also the case in normal material.Our cytochemical determinations suggest that a growth‐curve relationship exists in normal material between the RNA content of the cell and the age of the animal up to about eighth to eleventh day after birth. The largest increase in RNA content in normal material occurs in the second week after birth when nearly adult values are reached. In culture, a similar relationship exists between the age of the culture and the RNA content of the cells though the data points are scattered considerably. In sharp contrast to the normal material, however, the slope of the growth‐curve is very much less steep and no plateau is apparent up to 81 days in vitro. Since the initial increase in cell volume in culture is due to a formation of the fibrillogenous zone and is not accompanied by a proportional increase in RNA it seems likely that the increase in RNA content of the cell during normal development is primarily due to the formation and differentiation of Nissl material.