M. F. Hopgood

Changes in Protein Synthesis and Degradation Involved in Enzyme Accumulation in Differentiating Liver

The growth of a mammal from embryo to adult is accompanied by differentiation so that non-specific cells eventually give rise to specific tissues. As part of this developmental process a series of sequential changes occur with the gradual attainment of the mature function characteristic of a tissue. The foetal rat liver, for example, although somewhat similar histologically to adult liver, has very few of the specific functions carried out by the mature organ. Gluconeogenesis, urea formation, amino acid degradation, glucuronide formation, sugar phosphate transformations and drug detoxication reactions are absent or essentially so in the foetal rat liver, and appear during development at birth or at weaning (Schimke and Doyle, 1970; Ballard, 1971). Although little is known about the natural stimuli for the appearance of these pathways, it is a general finding that the pathways become functional when rate-limiting enzymes appear for the first time. Thus an essential problem in understanding development is to resolve the factors responsible for specific gene expression: how are the respective genes repressed during the early growth of foetal liver and what changes initiate gene expression at a particular developmental stage?