Patterning and Positional Information

Abstract

Abstract Pattern formation is a key process in embryonic development (1). When a fertilized egg divides, the resulting cells become different and give rise to a variety of structures such as the main body and its appendages. These structures have a well-defined spatial organization-the vertebrate limb is a good example—which is brought about by the process of pattern formation. There are three other related processes: (1) cell differentiation is the process by which different specialized adult cell types such as muscle, cartilage, and neurons are generated; (2) morphogenesis, or a change in form, such as neural tube formation; and (3) growth—it is a general feature of pattern formation that it is initially only specified for any organ on a small scale—rarely greater than more than 100 cell diameters. These processes are related to one another but, in general, pattern formation is the primary event in that it specifies the other processes. Thus patterning in vertebrate limb development specifies where cartilage will form and how it will grow; it also specifies ectodermal cells such as the apical ridge, which controls the overall form of the limb.