The role of cell lineage in development.

Studies of the role of cell lineage in development began in the latter part of the 19th century, fell into decline in the early part of the 20th, and were revived about 20 years ago. This recent revival was accompanied by the introduction of new and powerful analytical techniques. Concepts of importance for cell lineage studies include the principal division modes by which a cell may give rise to its descendant clone (proliferative, stem cell and diversifying); developmental determinacy, or indeterminacy, which refer to the degree to which the normal cleavage pattern of the early embryo and the developmental fate of its individual cells is, or is not, the same in specimen after specimen; commitment, which refers to the restriction of the developmental potential of a pluripotent embryonic cell; and equivalence group, which refers to two or more equivalently pluripotent cell clones that normally take on different fates but of which under abnormal conditions one clone can take on the fate of another. Cell lineage can be inferred to have a causative role in developmental cell fate in embryos in which induced changes in cell division patterns lead to changes in cell fate. Moreover, such a causative role of cell lineage is suggested by cases where homologous cell types characteristic of a symmetrical and longitudinally metameric body plan arise via homologous cell lineages. The developmental pathways of commitment to particular cell fates proceed according to a mixed typologic and topographic hierarchy, which appears to reflect an evolutionary compromise between maximizing the ease of ordering the spatial distribution of the determinants of commitment and minimizing the need for migration of differentially committed embryonic cells. Comparison of the developmental cell lineages in leeches and insects indicates that the early course of embryogenesis is radically different in these phyletically related taxa. This evolutionary divergence of the course of early embryogenesis appears to be attributable to an increasing prevalence of polyclonal rather than monoclonal commitment in the phylogenetic line leading from an annelid-like ancestor to insects.