Development (biology)

I

Introduction

Development (biology), branch of biology concerned with describing and understanding how a fertilized egg or spore or bud turns into an adult organism. More inclusive than embryology, the term also encompasses such processes as regeneration of limbs in many animals and vegetative propagation as found in higher plants. In addition, biologists are interested in the relationship between the processes of development and those of aging.

Sexual reproduction requires a single-cell stage (see Cell). If large multicellular size has adaptive advantages, then the life cycle must necessarily include a period of development from the single cell to the mature form. The process of development has three components: growth (size increase), morphogenetic movement (the shaping of patterns and forms), and differentiation (the change from undifferentiated to specialized structures).

II

Growth

The synthesis of new protoplasm constitutes growth; that this synthesis has occurred is shown by adults being larger than their fertilized egg. In multicellular organisms cell size remains within strict limits; therefore the increase in protoplasm is accompanied by successive cell divisions. In bacteria and similar one-celled organisms, cell division is the means of reproduction, the two daughter cells beginning new lives. In multicellular organisms, dividing cells remain in aggregate and assemble in different ways. In animals such as vertebrates, first the egg cell cleaves, and then the cells multiply through continuous synthesis of protoplasm and repeated cell divisions to form the component cells of all the tissues of the body. The same is true of plants, with one important difference: Plant cells are contained in hard walls, and therefore the structures produced as a result of growth are rigid, such as trunks, branches, and leaves. Because of this rigidity, plant growth is confined to certain softer growing zones called meristems, consisting of unspecialized tissue cells that continue to form different plant parts. Such embryonic tissues are characteristically found at the tips of shoots, at nodes, and as a layer of cells (cambium) in the stems and roots.

III

Morphogenetic Movements

Formative cell movements may occur with or without growth. When cells move and grow simultaneously, the process is called morphogenesis. Morphogenetic movements are the rule in multicellular animals, but they are generally absent in plants because of the hard cell wall. In the development of a vertebrate, the first important morphogenetic movement is gastrulation, a cell movement that can occur in a number of ways but invariably results in an embryo with two cell layers formed from one. Subsequent morphogenetic movements are numerous, such as the aggregation of cells to form limb buds or the migration of the primordian germ (sex) cells to the region of the gonads (testes or ovaries).

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IV

Differentiation

During or after growth and morphogenetic movements, cells become different from one another in chemical composition and structure. In plants, for instance, some cells may turn into phloem or xylem cells in the main stem; in animals, a group of embryonic cells called stem cells give rise to all of the specialized cells of the mature animal. This cell differentiation may occur in groups of cells to form tissues (such as muscles or nerves), which in turn form organs (such as the heart or brain). Some differentiation seems to occur only after a fixed number of cell divisions have taken place, as in the formation of blood cells or germ cells (sperm and ova); in other cases, differentiation proceeds independently of cells division. Many differentiated cells, such as red blood cells or nerve cells, lose the power of cell division. Other cells not only can continue to differentiate, they are even capable of dedifferentiating, that is reverting an earlier cell type; examples are regenerating liver and muscle cells. Embryonic stem cells are said to be pluripotent, meaning that they are capable of developing into any of the specialized tissues that make up the mature animal.

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