Overview

When a multicellular organism develops from a fertilized egg, four genetically regulated processes are at work: cell proliferation, programmed cell death, differentiation, and association of functionally related cells.

Genetic mechanisms of development are of two kinds, differential gene expression and changes in genome structure. Differential gene expression – spatial and temporal variation in the rates of synthesis of gene products – is by far the more prevalent. Gene expression, often triggered by signals from outside the cell, is regulated at the level of transcription, RNA processing, or protein synthesis.

Basic Developmental Processes

Cell Proliferation

Cell proliferation via cell cycling (Chapter 19) is universal. In plants and animals, cells proliferate when the organism grows, replaces dead cells, metamorphoses (remodels the body during development), or regenerates lost body parts. In the first 270 days or so of human development, cell number increases exponentially from 1 to ~1014; the rate of cell proliferation far exceeds the rate of cell death. By conservative estimate, 99.9% of human cells die and are replaced, making a person's lifetime cell number ~1017, although the true figure may be orders of magnitude greater than this.

The G1→S transition, blocked by RB-E2F, is critical to cell proliferation (Chapter 19); once a cell passes the R point it is committed to enter S phase, and passage through a full cycle normally occurs. External signaling molecules influence the G1→S transition: mitogens stimulate the transition and growth inhibitors block it.