An ultrastructural study of human epithelial rests of Malassez maintained in a differentiated state in vitro

Healthy human periodontal ligaments (PDL), obtained from the extracted teeth (premolars and third molars), were cultivated for 1-35 days, using a multi-purposes culture chamber (MPCC) equipped with various transparent membranes. The resting state of the epithelial rests of Malassez (ERM), similar to their in vivo counterparts, appeared as small islands or strands with scant cytoplasm containing poorly developed organelles. This state was most effectively maintained in MPCC with a cellophane sheet. MPCC with a Sartorius membrane filter permitted proliferation and emigration of ERM. Proliferating ERM were characterized by more profiles of rough endoplasmic reticulum and free ribosomes, new formation of actin-containing microfilaments, less prominent tonofilaments and desmosomes and loss of gap junctions. Most of these ultrastructural changes are manifested in epithelial cells during wound healing. The emigrating ERM from PDL explants, as well as occasional proliferating ERM within explants, consisted of two cell types--outer basal-like cells, as described above, and inner tonofilament-rich prickle-like cells, suggesting a propensity for differentiation of ERM. The results show the possibility of controlling the growth and differentiation of ERM through the MPCC culture environment.