Growth and transformation of the small intestinal mucosa--importance of connective tissue, gut associated lymphoid tissue and gastrointestinal regulatory peptides.

Physiologically, the architecture of the small intestinal mucosa is maintained by a delicate balance between cell production in the crypt compartment, enterocyte migration along the villi, and extrusion of mature epithelial cells from the tip of the villi into the lumen.' Migration and maturation of the lining cells are affected in response to various types of stress and under conditions of mucosal damage and repair. This is also true for the tissue components beneath the lining cells. On the other hand, although the equilibrium of the lining cells may be disturbed by a great variety of various types of stress, the response of the mucosa is rather uniform and restricted to three distinct mucosal patterns only-namely, hyper-trophy, hypotrophy, and mucosal transformation of the hyperregenerative type (Fig. 1).' In these mucosal patterns alterations of the absorptive cells and the zonation of villi and crypts have been studied extensively. The regulation of cell proliferation and differentiation in this system is, however, still not well understood although the importance of some factors such as luminal nutrition and the ornithine decarboxylase system have been recognised.'4 Furthermore, the role of regulatory peptides has attracted much attention, though with few concrete results, and the implications of the interaction between the lining cells, the mesenchyme, and the gut associated lymphoid tissue with respect to cell proliferation and maturation have only recently gained interest.' It is the purpose of this contribution to briefly describe the three principal mucosal responses to various types of stress and also to discuss some aspects of the connective tissue, the gut associated lymphoid tissue and the regulatory peptides in the control of intestinal mucosal growth and their perspectives in future research as far as the regulation of mucosal growth is concerned. It is not intended, however, to be complete within the frame given, but rather to focus on areas of our own interest in the subject. One of the prerequisites of the striking small intestinal mucosal potential to change its structure and function is the fact that each villus possesses numerous crypts with an enormous proliferative potential. The average number of crypts per villus in upper human jejunum amounts to 8&3 per villus.' In man, 23 of 33 cell positions of the lower part of the crypt cell column belong to the proliferative compartment , whereas the upper third represents the matura-tion area in which the crypt cells obtain their functional competence.7 The …