Development (biology) - biology.

Development (biology) - 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). 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. V REGENERATION A striking phenomenon of cell differentiation that occurs commonly in plants and in many animals is the replacement of lost parts by regeneration, the rebuilding of differentiated tissues. In general, more complex animals have fewer powers of regeneration than the simple forms that appeared earlier in the evolutionary chain. Cnidarians such as the freshwater hydra show spectacular power to regenerate all parts of the body; fish and amphibians can often regenerate fins or limbs; mammals can regenerate the liver and the blood cells (the latter normally being in a constant state of differentiation and destruction). VI REGULATIVE AND MOSAIC DEVELOPMENT An undifferentiated mass of embryonic cells can also be divided, and a complete organism will grow from each separated portion. This process is called regulation or regulative development. In a famous experiment at the close of the 19th century, the German embryologist Hans Driesch cut a very early sea urchin embryo longitudinally, and each half embryo produced a dwarf but normal larva. At about the same time two American embryologists, Edmund Beecher Wilson at Columbia University and Edwin Grant Conklin at Princeton University, showed that in mollusks, worms, and sea squirts such an operation produced two abnormal half embryos. They called this mosaic development, and they contrasted it to Driesch's regulative development. The differentiation process seems to start earlier in mosaic eggs than in regulative eggs; mosaic eggs are said to be determined relatively early. VII ANALYSIS OF DEVELOPMENT The attempt to understand the mechanisms of development is a field of active research, both because of its own intrinsic interest and importance and because of its relevance to the problem of abnormal growth, or cancer. The starting point of modern developmental biology is the study of gene action--that is, the molecular basis of the synthesis of the most important chemical constituents of cells, namely proteins. Portions of the deoxyribonucleic acid (DNA; the molecules of the genes) in the cell nucleus are now known to code for particular proteins. The DNA strand produces a complementary substance called messenger ribonucleic acid (RNA), which leaves the nucleus and attaches to small bodies in the cytoplasm called ribosomes. The messenger RNA is processed by the ribosomes like a tape, and the appropriate amino acids are brought together to form specific proteins. These proteins may be structural, or they may be enzymes. An enzyme is a protein that is a catalyst, specifically encouraging the conversion of one substance into another. For example, pepsin is an enzyme that breaks up a protein into its constituent amino acids. Other enzymes build up such substances as cellulose, starch, fats, and vitamins. Other proteins may become hormones, which are chemical messengers between cells. Insulin, a hormone that controls the amount of sugar mobilized from starch, is a good example of a protein hormone. VIII CONTROL OF GROWTH Understanding the molecular machinery within cells gives biologists a direct basis for understanding growth, because growth is the synthesis of new protoplasm, and biologists know the basic mechanism of this synthesis. One key gap, however, exists in this knowledge. Biologists want to know not only how substances are synthesized but also how growth is controlled so that the proportions of an animal or plant remain consistent from generation to generation. The direction and amount of growth, which are responsible for shape and size, clearly are also genetically controlled. The way in which this control is exerted, however, is an active field of research involving the study of chemical messengers that stimulate and inhibit cell division and that are asymmetrically distributed so as to control the direction of growth. IX CONTROL OF MORPHOGENETIC MOVEMENTS The control of morphogenetic movements is also an active field of research. Biologists are attempting to discover how molecules cause cells to move in certain directions and produce consistent shapes from generation to generation. Two methods are being studied intensively: chemotaxis and cell adhesion. In chemotaxis, or chemotactic sensing, cells are attracted to or repelled by a substance according to its level of concentration, or its chemical gradient. As with all scientific research, an ulterior, unanswered question exists: Although chemical gradients may explain some morphogenetic movements, how can the distribution of the chemical substance in the first place be explained? The other method of molecular control of shape in moving cells is cell adhesion. If members of a group of moving cells have different abilities to adhere to one another, they will arrive at a stable shape that can be predicted from the forces of adhesion between the different kinds of cells. Possibly some cells also differ in adhesive force on different parts of their surface. Study of the surface chemistry of cells to learn how the cells achieve these different forces of mutual adhesion is an active field of research. X CONTROL OF DIFFERENTIATION Perhaps the greatest concentration of research efforts is on the control of differentiation. Scientists of the past century have understood that different regions of the cytoplasm of mosaic eggs contain different substances and that these substances are somehow responsible for the differentiation. In modern terms, the cytoplasm of a cell may contain substances that control which genes will be expressed in the nucleus. Generally each nucleus of each cell in an embryo contains all the genetic information needed to make a whole organism, but parts of this genetic information are selectively turned on or off, depending on the individual cell's role in the whole organism. Researchers believe that the development of an embryo is greatly influenced by so-called homeotic genes, which interact with networks of other genes to determine the position of various body parts. Homeotic genes contain sequences called homeoboxes, which can be found in organisms ranging from sea urchins to humans. Studies of homeoboxes are shedding light not only on embryonic development but also on the evolutionary relationships among various animals. Biologists know more about the signals in regulative embryos, largely through the work of the German embryologist Hans Spemann in the early 20th century. In the famous experiments for which he received the Nobel Prize in physiology or medicine, he showed that a special region of the amphibian gastrula (the two-cell-layer embryonic stage) induced the tissue above it to differentiate into the main axis of the embryo. He called this region the "organizer" under an initial misapprehension that it was responsible for the shape of the axis. Later, scientists showed that the region simply sends out a chemical agent that stimulates specific gene action in the cells of the overlying tissues. The assumption now is that many secondary chemical messages between the different types of cells help to shape the main axis of the embryo. XI TIMING IN DEVELOPMENT A convenient way to consider how controlled development is achieved is to treat it as a process that consists of synthesizing a particular substance at a particular time and at a particular place. The first (synthesis) and the last (localization) have already been discussed, and the important phenomenon of timing must now be added. The timing of some aspects of development involves a rigid sequence: Event B cannot occur before A, nor can C occur before B, and so on. The idea goes back to Aristotle and is often referred to as epigenesis. Development unfolds because of a sequence of events, each one the direct cause of the next. In the early history of embryology this idea was supported by William Harvey, famous for his discovery of the circulation of blood, but opposed by Charles Bonnet, a remarkable Swiss biologist who believed that all forms of life are static, or preformed. This early controversy of preformation versus epigenesis now seems a word battle hiding ignorance, because development has the elements of both ideas. Another aspect of the timing of development is the relative time of appearance of major structures in the developing organism. Certain events may be speeded up or slowed down, and the time of appearance of one structure relative to the appearance of other structures may be altered. For example, some amphibians, while still retaining the physical form of larvae, will produce mature gametes. This alteration in the timing of events in the development of the sex organs relative to the rest of the body structures is called neoteny and is thought to be important in some major evolutionary changes, as in the development of the brain in humans. Contributed By: John Tyler Bonner Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.