The spatial distribution of the cranial paraxial mesoderm and the neural crest cells during craniofacial morphogen-

Craniofacial development is intrinsically related to segmentation along the primary body axis (Hunt and Krumlauf, 1991; Hunt et al., 1991a; Thorogood, 1993). In mammalian embryos, segmentally arranged cranial structures such as the branchial arches and nerve ganglia are built upon this primary metamerism (Keynes and Stern, 1984; Lumsden and Keynes, 1989; Puelles and Rubenstein, 1993). Like most elements that constitute the head and face, the branchial arches are populated by both the paraxial mesoderm and the neural crest cells (Couly et al., 1993). The paraxial mesoderm forms the craniofacial muscles, some skeletal elements and vascular tissues, while cranial neural crest cells form elements of the peripheral nervous system, connective tissues and the cartilage (Le Douarin, 1982; Noden, 1988; Kimmel et al, 1991). The cranial paraxial mesoderm is organised as a meristic pattern of loosely packed cell clusters called somitomeres (Meier, 1979; Meier and Tam, 1982; Tam and Meier, 1982; Jacobson, 1993). There is an orderly cranial to caudal distribution of the somitomeric mesoderm in the mouse, such that each branchial arch is derived from a set of two consecutive somitomeres (Tam and Trainor, 1994; Trainor et al., 1994). This dual contribution correlates with the ‘two segment periodicity’ that describes the derivation of branchial arch motor neurons from consecutive pairs of rhombomeres. The origin of neural crest cells in the branchial arches and nerve ganglia is related dynamically to the segmental organisation of the hindbrain (Wilkinson et al., 1989; Krumlauf, 1993). Hindbrain segmentation is a direct consequence of lineage restriction and differential activity of a series of regulatory genes (Lumsden, 1990; Fraser et al., 1990; Thorogood, 1993). A number of homeobox genes such as the Hoxb group of genes display precise rostral limits of expression that correspond with the boundaries of rhombomeres (Wilkinson et al., 1989). Neural crest cells which originate from alternate segments of the hindbrain (rhombomeres 2, 4 and 6) have been shown to express a set of Hoxb genes characteristic of their rhombomeric origins (Hunt et al., 1991a; Wilkinson et al., 1989). The crest cells migrate in discrete streams colonising the first three branchial arches in a pattern consistent with their craniocaudal orders in the hindbrain (Hunt et al., 1991b,c; Lumsden et al., 1991; Serbedzija et al., 1992). Unlike the rhombomeres, lineage restriction and molecular heterogeneity of cells in different somitomeres have not been identified. What is clear however, is that the segmental organisation of the cranial paraxial mesoderm and the hindbrain has profound effects on the differentiation and patterning of the craniofacial tissues (Noden, 1983b; Couly et al., 1992; Trainor et al., 1994). Comparisons of the fate maps of the somitomeres and the 2569 Development 121, 2569-2582 (1995) Printed in Great Britain © The Company of Biologists Limited 1995