A large field of investigation that includes the study of all changes associated with an organism as it progresses through the life cycle. The life cycles of all multicellular organisms exhibit many similarities. That is, as an organism progresses from one generation to the next there is a series of common processes: for example, gametogenesis, fertilization, embryogenesis, cell differentiation, tissue differentiation, organogenesis, maturation, growth, reproduction, senescence, and death.
Analysis of all of the events associated with an organism as it progresses through its life cycle employs a multiplicity of approaches. Tremendous strides have been made in describing at the molecular level the developmental process of cell differentiation. However, the molecular control mechanisms which regulate cell differentiation are not known. Tissue and organ differentiation, as well as morphogenesis, are processes which have been described in detail for many situations, but little is known about the physical and chemical nature of the mechanisms involved. A complete understanding of the development of an organism will require an appreciation and comprehension of the changes which occur at all levels of organization as an organism traverses its life cycle.
The major unifying theme in biology is evolution. Not only has evolution led to the wide variety of organisms now present on Earth, but also evolution has modified the initial processes and patterns of development to the diversity of types currently encountered. This evolution of developmental parameters in multicellular organisms began as single-celled organisms became multicellular. The development of a multicellular organism entails a host of problems not faced by a single-celled organism. For example, cells in one part of the aggregate must coordinate their activities with cells in other parts, nutrients and oxygen must be provided to all cells, and water balance must be maintained.
Developmental biologists have focused on two central areas: the processes and associated mechanisms by which cells become different, that is, cell differentiation; and the processes and associated mechanisms by which patterns are created, that is, morphogenesis.
Current theories state that cells become different by expressing different genes. Thus, a liver cell is different from a muscle cell, not because it contains different genes or genetic information, but because it expresses different sets of genes. This explanation of cell differences is based upon the results of three types of experimental analysis. (1) Some plant cells are totipotent; that is, for tobacco, carrot, and a few other plant species, it has been demonstrated that a single cell (not a gamete) can divide and undergo morphogenesis to form a fertile plant. (2) Nuclei from some differentiated animal cells are totipotent. That is, a nucleus from a differentiated cell can be injected into a mature egg which has had its nucleus removed or destroyed, and the injected nucleus can direct normal development of the organism. (3) The sequences of nucleotides in the DNA of all cells in an organism appear to be the same; that is, DNA-DNA hybridization of DNA from different cell types indicates that the different cell types do not have unique DNA base sequences. Since these results indicate that all cells contain the complete genome for an organism, different cell types appear to arise as a result of the expression of unique sets of genes in each cell type. See also Cell differentiation; Developmental genetics; Gene action; Somatic cell genetics.
Initially the cells of a developing embryo are not restricted in their developmental potential or fate, but as embryogenesis proceeds, a cell's developmental potential becomes restricted or fixed. Restriction of developmental fate is called determination. Two mechanisms have been identified that bring about determination. The first involves the presence of unique factors, called cytoplasmic determinants, which are products of the maternal genome and are located in specific areas of some animal eggs. The cells which come to contain these determinants differentiate along specific pathways. The second mechanism is induction, a process by which two tissues interact so that one or both differentiate along specific pathways. A classic example of induction is the action of mesoderm on the overlaying ectoderm in the frog embryo at the time of gastrulation. The mesoderm acts on the ectoderm, causing it to form the neural plate. Only ectoderm of a certain developmental age is capable of responding to the mesoderm, and this ectoderm is said to be competent. See also Embryonic induction.
Developmental biologists have gained substantial insights into the molecular bases for determination in model organisms such as Drosophila. At least three sets of cytoplasmic determinants (maternal gene products) are present in the fly egg: determinants for germ-cell formation, determinants controlling dorsal-ventral polarity, and determinants for the anterior-posterior polarity. Some of these determinants are messenger ribonucleic acids (mRNAs) coding for proteins which are transcriptional regulators (that is, proteins that regulate gene activity).
Morphogenesis involves the production of form and structure by integrating the differentiation of many different cells and cell types into specific spatial patterns. This higher level of organization has been difficult to investigate in terms of establishing mechanisms. The processes of determination, competence, and induction are involved. One of the greatest challenges faced by developmental biologists is to bridge the gap between genes and patterns. It is clear that patterns are a result of gene activity, but the relationship between genes and patterns in most organisms is not well understood. See also Animal morphogenesis; Plant morphogenesis.
