The differentiation of the fibroblast was followed in the dental pulp of continuously growing incisors of the guinea pig. Based on the ultrastructure, the processes of differentiation of the fibroblasts might be conveniently broken into three stages; Stage I – period of early differentiation, Stage II – period of maturation and functioning, and Stage III – period of regression.
During Stage I the cell had structural characteristics shared by other less differentiated cells. The endoplasmic reticulum was poorly developed, showing vesicular to tubular profiles. The ribosomes were abundant but were mostly distributed in free form. Mitochondria were small and had irregular interiors.
Stage II was characterized by a striking development of the rough-surfaced endoplasmic reticulum, which appeared in various shapes and sizes. The Golgi complex was enlarged, and contained some fibriller materials in dilated portions of its membraneous elements. Other features described in the cytoplasm of fibroblasts from other sources were confirmed.
Stage III was characterized by the decrease in size and number of various cytoplasmic constituents and was considered to represent cells in the state of regression.