Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
Cellular Mechanisms of Development

The Organism of any multicellular animal can be viewed as a clone of Cells derived from a single Cell: the fertilized egg. Consequently, somatic cells are generally genetically identical, yet they differ phenotypically: some become Muscle cells, others Neurons, still others Blood Cells, and so on. Within the organism, cells of different types are arranged in a strictly ordered manner, giving the body its characteristic shape. All traits of an organism are determined by The nucleotide sequence in its genomic DNA, which is replicated in every cell. All cells receive the same genetic "instructions," but interpret them taking into account time and context, ensuring that each cell performs its specific function within the multicellular community.

Multicellular Organisms are often highly complex, yet they are constructed using a remarkably limited repertoire of cellular activities. Cells grow, divide, and die; they adhere, migrate, and change shape. They differentiate, meaning they begin or cease the Synthesis of specific substances encoded by The Genome. Cells secrete substances into their environment or display them on their surface, influencing The activity of neighboring cells. These forms of cellular behavior constitute the universal foundation of animal development. In this chapter, we will attempt to explain how the execution of Various Forms of cellular activity at the right time and place results in The formation of an intact organism.

Rather than tracing the complete development of a single organism step by step from beginning to end, we will examine various aspects of developmental cell behavior, illustrating General Principles with Examples from animals in which these processes are most clearly manifested. We will discuss how and through what forces cell movements drive embryo formation, how the spatial pattern of differentiation unfolds under the control of the cells' own genes and intercellular interactions, and how differentiated cells originating in different PARTS OF THE embryo arrive at their final destinations to form complex Tissues and Organs. All these questions will be explored using The Development of amphibians, sea urchins, mice, flies, birds, cockroaches, and nematodes as examples. Plant development will be discussed in Chapter 20.