ABSTRACT
INTRODUCTION The basement membrane is one of the characteristic structures of the differentiated epithelium and is important for the development of epithelial polarity. The most useful definition of the basement membrane is probably still a morphological one, i.e. that of the basal lamina as observed in the electron microscope. It is a thin sheetlike extracellular matrix network, found at all borders between epithelia or endothelia and the underlying stroma. This extracellular matrix layer is present also in other types of tissues e.g. surrounding muscle and fat cells and ensheathing peripheral nerves (Vracko, 1974). It appears that the basement membranes are more heterogeneous than has been assumed. As they are present at such different locations, they also differ in their morphology, in their molecular composition, and in their functional properties (Figure 1).
