Ultrastructural development and cellular kinetics in intestinal epithelium

Despite the increased knowledge in the various fields of molecular biology the regulation of cell proliferation and differentiation in higher organisms has remained an intrigueing, complex problem in developmental biology. In some instances growth during embryonic or postnatal development is characterized by on increase in cell size without on accompanying increase in cell number like for instance in the postnatal development of cerebellum (Leblond1 1964). In most cases, however, growth is accompanied by an increase in cell number and a concurrent production of intercellular substance. In these so-called 11 expanding cell populations" the ability to divide is not restricted to a certain fraction of the population but nearly all cells ore able to proliferate as for instance during development or regeneration of liver. Cell renewing systems are characterized by an equilibrium between cell production and cell loss. Such systems are not only present during foetal development but they also occur in adult life. Examples are skin, mucous membranes, testis, bone marrow and intestinal epithelium. Such systems generally consist of a proliferative cell comportment, in which stem cells maybe present, a differentiation compartmP-nt where specific cellular characteristics are being developed and a functional compartment. These cell renewal systems have frequently been used for the study of regulation processes involved in cell proliferation and differentiation. More insight in these regulatory mechanisms are likely to contribute to the understanding of malignant cell growth.