Chapter 12 Steps in the Elaboration of Collagen by Odontoblasts and Osteoblasts

Abstract

This chapter describes how various morphological methods are used to clarify the steps in the elaboration of the procollagen precursor of collagen. Both the odontoblasts associated with dentin and the osteoblasts associated with bone are active collagen producers. Five types of collagen have been identified; the most common, type I, which is present in dentin and bone, is examined in this chapter. The production of type I collagen by a producer cell starts in ribosomes with the synthesis of two types of randomly coiled polypeptides, the pro α l(I) and pro α2 chains. The pro α chains consist of three parts: a central region made up of repetitive units composed of three amino acids (glycine, proline or hydroxyproline, and a third, variable, one) and two different groups of amino acids at each end, referred to as the NH 2 - and COOH-terminal propeptides. When the synthesis of a pro α chain is completed, it is released into the lumen of a rough endoplasmic reticulum (RER) cisterna. At a later stage, the COOH terminals of two pro α1(I) chains and one pro α2 chain bind to one another through disulfide bonds; the central region then begins to coil into a helix and continues doing so till the three NH 2 terminals become associated. The procollagen molecule is then released from the producer cell, and enzymes remove the two terminal propeptides. The remaining central portion constitutes the type I collagen molecule. This is the unit making up the bulk of dentin and bone matrix and the collagen fibrils of connective tissue.