The development of form and pattern in animals. Animals have complex shapes and structural patterns which are faithfully reproduced during the embryonic development of each generation. Morphogenesis takes place by the generation of progressively more complex structures from a single cell: the fertilized egg, or zygote. The zygote divides repeatedly to form a multicellular embryo, within which groups of cells undergo structural and functional specialization (differentiation) in the precise spatial patterns that are recognized as tissues and organs.
Cell differentiation involves the differential expression of genes in the nuclear deoxyribonucleic acid (DNA) which code for the production of proteins specifying the structure and function of each cell type. During cleavage, nuclei divide equivalently so that all cells of the embryo receive the total complement of genes contained in the DNA of the zygote nucleus. However, cells in different regions of the embryo contain cytoplasm which differs in composition. The cytoplasmic composition of a cell controls which genes will be expressed in each region of the embryo, resulting in a patterned differentiation. Initially, different cell types arise because cells come to occupy unique positions; some cells are on the inside and others on the outside of the group of cells produced by the early divisions of the zygote. Interactions between subpopulations of cell types thus produced further alter regional cytoplasmic compositions, resulting in new patterns of gene expression. By means of many such interactions, all of the 200 or so different cell types of an animal body gradually emerge in the proper spatial patterns. See Cell differentiation, Deoxyribonucleic acid (dna), Gene action
The structural patterns of animals and their parts exhibit polarity; that is, they display structural differences along one or more axes. In many animals, the overall polarities of the embryo are established during oogenesis and the period between fertilization and the first cleavage. For example, as the amphibian egg grows in the ovary, its posterior half becomes laden with yolk, and a dark pigment is deposited in the cortical cytoplasm of its anterior half. The line between the poles of these two halves defines the anterior-posterior axis of the future embryo. See Fertilization, Oogenesis, Ovum
The dorsal-ventral axis develops perpendicular to the anterior-posterior axis, and is established by a reorganization of the zygote cytoplasm initiated by the events of fertilization. This axis can form in any of the planes which contain the anterior-posterior axis. The plane in which it actually forms is determined by the meridian at which the sperm enters the egg. Cytoplasmic reorganization occurs after fertilization. A tongue of yolky cytoplasm is formed on the elevated side as the heavy yolk flows down under the influence of gravity, and a new gray crescent appears on this side, its dorsal midline coinciding with the plane in which the anterior-posterior axis was tilted. The dorsal-ventral axis established by sperm entry becomes determined shortly before the first cleavage.
Two major cell types are formed during cleavage of the amphibian egg: large, yolky endoderm cells, and smaller, pigmented ectoderm cells, including the cells formed from the gray crescent region.
At the mid-blastula state, when the embryo consists of several thousand cells, an inductive interaction takes place between the endoderm cells and the gray crescent cells, causing the latter to become mesoderm cells. During gastrulation, these three cell types are rearranged to form three concentric layers, with ectoderm on the outside, endoderm on the inside, and mesoderm in between. The ectoderm develops into the skin epidermis and nervous system, endoderm into the organs of the alimentary tract, and mesoderm into muscles, skeleton, heart, kidneys, and connective tissue. See Blastulation, Gastrulation, Germ layers
Prior to gastrulation, the prospective organ regions of the ectoderm and endoderm are not yet determined. Mesodermal organ regions, however, are highly self-organizing under these conditions. Ectodermal and endodermal organ regions become determined during and after gastrulation by the inductive action of the mesoderm. For example, dorsal mesoderm normally invaginates and stretches out along the dorsal midline where it differentiates as notochord and trunk muscles. The ectoderm overlying the dorsal mesoderm differentiates as the central nervous system. See Fate maps (embryology)
Once induced, organ regions can themselves induce additional organs from undetermined tissue. For example, the retina and iris of the eye develop from a vesicle growing out of the forebrain. This vesicle induces a lens from the overlying head ectoderm, and the lens then induces the cornea from head ectoderm to complete the eye. By means of such cascades of inductive interactions, all the organs of the body are blocked out. See Embryonic induction
Once determined, an organ region constitutes a developmental system, called a morphogenetic field, which specifies the detailed pattern of cell differentiation within the organ. Cells differentiate in patterns dictated by their relative positions within the field. It is generally accepted that graded molecular signals, or cues, are the basis of this positional information. It is proposed that the source of the signals is a set of boundary cells which define the limits of the field. All the cells of a field thus derive their positional information from a common set of boundary cells.
Most fields become inactive after the pattern they specify begins to differentiate; the ability to form normal organs after removal or interchange of cells is then lost. However, in some animals, the fields of certain organs can be reactivated by loss of a part, even in the adult. The missing part is then redeveloped in a process called regeneration. See Regeneration (biology)
Different kinds of cells secrete molecules of protein and protein complexed with carbohydrate which constitute specific types and patterns of extracellular matrix. The matrix stabilizes tissue and organ structure, guides migrating cells to their proper locations, and is a medium through which cell interactions take place. Cell interactions take place at cell surfaces, the molecular composition of which is distinct from one cell type to another, allowing them to recognize one another. These differences are reflected in varying degrees of adhesivity between different kinds of cells, and between cells and different kinds of extracellular matrix. Differential adhesivity is the property upon which cell migration, clustering, and rearrangement is based, and is thus important in creating the conditions for cell-cell and cell-matrix interactions. See Cellular adhesion
Another important mechanism for the development of complex patterns and shapes is the differential growth of organs and their parts. Differential growth is evident as soon as embryonic organ regions begin to develop. During the early part of their development, the growth rates of organs are controlled by intrinsic factors. At later stages of development, including postnatal life, organ growth is largely under the control of hormones secreted by cells of the endocrine glands.
Programmed cell death (apoptosis) is an important feature in the shaping of such structures as the head, limbs, hands, and feet of some animals. A striking example is foot development in ducks and chickens. Ducks have webbed toes while chickens do not. This difference results because as the chicken leg bud grows and forms the toes, the cells between the developing digits die. Various grafting and culturing experiments have indicated that it is a cell's relative position in the limb bud which establishes its fate to die at some later time in development. See Developmental biology, Molecular biology