In the early development of many tissues and organs of complex, multicellular organisms, the action of one group of cells on another that leads to the establishment of the developmental pathway in the responding tissue. The groups of cells which influence the responding cells are termed the inducing tissue. Since specific inducing tissues cannot act on all types of cells, those cells which can respond are referred to as competent to react to the action of a specific inducer stimulus.
Embryonic induction is considered to play an important role in the development of tissues and organs in most animal embryos, from the lower chordates to the higher vertebrates.
Perhaps the first major induction phenomenon occurs during the final stages of gastrulation of most animal embryos. Following fertilization, the egg divides to form a multicellular blastula-stage embryo. The cells of the blastula then undergo a series of movements which generate a more complex embryo, the gastrula, which contains three major groups of cells: ectoderm, mesoderm, and endoderm. The mesoderm actually arises as cells move from the surface of the embryo to the inside. Once inside, they induce the cells which reside over them, the surface ectoderm cells, to develop into the neural tube. The neural tube eventually forms the central nervous system. The first induction event of early embryogenesis is called primary embryonic induction. The migratory cells which invaginate from the surface and induce the development of the neural tube are termed the embryonic organizer. The first step in the sequence of events termed primary embryonic induction is the acquisition by the mesoderm of neural inducing activity. Proteins such as fibro blast growth factor and activin, which belong to a category of so-called peptide growth factors, play key roles in programming the mesoderm cells to induce overlying ectoderm to differentiate into neural structures. See Gastrulation
The development of a large number of tissues and organs is influenced by embryonic inductions. Various eye structures (lens, optic cup, and so on), internal ear structures, as well as several tissues (for example, vertebral cartilage) emerge from cells which were acted upon by inducer tissues. See Nervous system (vertebrates)
Limbs, kidney, nasal structures, salivary glands, pancreas, teeth, feathers, and hair are organs which require inductive stimuli. It is not known whether a single common mechanism underlies each of those inductions. Many scientists believe that inductive interactions are mediated by cell-cell contacts; that is, the developmental information which is transferred from the inducing tissue is thought to reside at the cell surface of that tissue. Perhaps the surface of the responding tissue recognizes the signal molecules present on the surface of the inducing tissue. In other instances, a secreted protein might move among various cells or tissues and exert its effects on competent cells.
The principles of animal development also apply to plants. A greater role is, however, usually played by the diffusion of small-molecular-weight signal molecules rather than cell-cell contacts or protein growth factors. The earliest stages of plant embryo development involve groups of cells acquiring the competence to respond to inductive signals. Later in development, inductive signaling also becomes important. For example, in flowering plants the distance between nodes along the stem elongates, and lateral buds form below the shoot apex. The buds are believed to develop in response to a concentration gradient of signal molecules which exists along the stem. Thus, a process which is analogous to embryonic limb bud formation in animals is played out, and both plant and animal inductions can be conceptualized in similar terms. See Cell differentiation, Developmental biology, Embryology, Plant morphogenesis