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Segmentation in staged human embryos: the occipitocervical region revisited
Fabiola Müller1, Ronan O'Rahilly
1School of Medicine, University of California, Davis, California, USA.
Journal of Anatomy
|October 8, 2003
Summary
Human embryonic development reveals distinct early segmentation patterns in somites and rhombomeres, differing from chick models. This study details occipital and cervical vertebral development, highlighting unique median skeletal mass formation and the absence of an
Area of Science:
- Developmental Biology
- Human Embryology
- Comparative Anatomy
Background:
- Early human embryonic development involves complex segmentation of somites, rhombomeres, and pharyngeal arches.
- Understanding these processes is crucial for identifying congenital anomalies and evolutionary relationships.
Purpose of the Study:
- To reassess the segmentation of the first seven somites, rhombomeres, and pharyngeal arches in human embryos.
- To provide detailed graphic reconstructions and photomicrographs of human occipital and cervical segmentation.
- To compare human segmentation patterns with those observed in other species, particularly the chick.
Main Methods:
- Serial sectioning of 145 human embryos (stages 9-23).
- Precise graphic reconstructions of 22 embryos.
- Detailed analysis of somite, rhombomere, and pharyngeal arch development.
- Photomicrographic documentation of occipital segmentation.
Main Results:
- Human embryos exhibit distinct segmentation patterns from stage 9, with somites 1-4 related to rhombomere 8 and caudal to pharyngeal arch 4.
- The first human somite is structurally similar to subsequent somites, unlike in chicks.
- Occipital segment 4 shows vertebral-like development; basioccipital development differs from the rest of the vertebral column.
- Distinct median skeletal masses form from occipital (1-4) and cervical (5-7) somites.
- The 'S-shaped head/trunk interface' observed in chicks is absent in humans due to topographical differences.
- Lateral sclerotomes contribute to specific structures like the hypoglossal foramen and occipital condyle.
- Medial components form the basioccipital and vertebral centra, with the axis exhibiting unique tripartite development.
Conclusions:
- Human embryonic segmentation, particularly of the occipital and cervical regions, displays unique characteristics compared to avian models.
- Detailed documentation of human occipital segmentation provides novel insights into early vertebral development.
- The study clarifies the topographical relationships between somites, neural crest, and the otic primordium in human embryos.