Segmentation in staged human embryos: the occipitocervical region revisited.

The first seven somites, the rhombomeres, and the pharyngeal arches were reassessed in 145 serially sectioned human embryos of stages 9-23, 22 of which were controlled by precise graphic reconstructions. Segmentation begins in the neuromeres, somites and aortic arches at stage 9. The following new observations are presented. (1) The first somite in the human, unlike that of the chick, is neither reduced in size nor different in structure, and it possesses sclerotome, somitocoel and dermatomyotome. (2) Somites 1-4, unlike those of the chick, are related to rhombomere 8 (rather than 7 and 8) and are caudal to pharyngeal arch 4 (rather than in line with 3 and 4). (3) Occipital segment 4 resembles a developing vertebra more than do segments 1-3. (4) The development of the basioccipital resembles that of the first two cervical vertebrae in that medial and lateral components arise in a manner that differs from that in the rest of the vertebral column. (5) The two groups of somites, occipital 1-4 and cervical 5-7, each form a median skeletal mass. (6) An 'S-shaped head/trunk interface', described for the chick and unjustifiably for the mouse, was not found because it is not compatible with the topographical development of the otic primordium and somite 1, between which neural crest migrates without hindrance in mammals. (7) Occipital segmentation and related features are documented by photomicrographs and graphic interpretations for the first time in the human. It is confirmed that the first somite, unlike that of the chick, is separated from the otic primordium by a distance, although the otic anlage undergoes a relative shift caudally. The important, although frequently neglected, distinction between lateral and medial components is emphasized. Laterally, sclerotomes 3 and 4 delineate the hypoglossal foramen, 4 gives rise to the exoccipital and participates in the occipital condyle, 5 forms the posterior arch of the atlas and 6 provides the neural arch of the axis, which is greater in height than the arches of the other cervical vertebrae. Medially, the perinotochord and migrated sclerotomic cells give rise to the basioccipital as well as to the vertebral centra, including the tripartite column of the axis. Registration between (1) the somites and (2) the occipital and cervical medial segments becomes interrupted by the special development of the axis, the three components of which come to occupy the height of only 2 1/2 segments.