life11e_ch19

Development involves four distinct but overlapping processes

As an organism progresses from a fertilized egg to a mature adult, four processes occur:

Determination sets the developmental fate of a cell—what type of cell it will become—even before any characteristics of that cell type are observable. For example, the mesenchymal stem cells described in the story opening this chapter look unspecialized, but their fate to become connective tissue cells has already been determined.
Differentiation is the process by which different types of cells arise, leading to cells with specific structures and functions. For example, mesenchymal stem cells differentiate to become muscle, fat, tendon, or other connective tissue cells.
Morphogenesis (Greek for “origin of form”) is the organization and spatial distribution of differentiated cells into the multicellular body and its organs.
Growth is the increase in size of the body and its organs by cell division and cell enlargement.

Determination and differentiation occur largely because of regulation of gene expression, a topic explored in Chapter 16. You’ll see many of the mechanisms described in that chapter here. The cells that arise from repeated mitoses in the early embryo may look the same superficially, but they soon begin to differ in terms of which of the thousands of genes in the genome are expressed.

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Morphogenesis involves not just differential gene expression but also intercellular signaling (see Chapter 7). Morphogenesis proceeds in several ways, influenced by:

Cell division—a key event in both plants and animals
Cell expansion—especially important in plant development, in which a cell’s position and shape are constrained by the cell wall
Cell movements—very important in animal morphogenesis
Apoptosis (programmed cell death)—essential in organ development

Growth occurs by cell enlargement. In some cases, cell enlargement is coupled to cell division, so the average cell size remains the same as the tissue grows; in other cases (especially in plant tissues), cells enlarge without dividing, so the average cell size increases. Growth continues throughout the individual’s life in some organisms, but reaches a more or less stable end point in others.