Somitic-vertebral correlation and vertebral levels in the human embryo

Somitic and vertebral interrelationships and levels were studied in 84 human embryos of stages 9-23 (3-8 postovulatory weeks). The first four somites are occipital, the occipitocervical junction is at somites 4/5, and eight somites are involved in the cervical region: X, Y, Z, and C. 3-7. By stage 17 the total number of occipitovertebral "units," namely 38 or 39, is attained. Resegmentation (Neugliederung) of sclerotomes is not supported. A new scheme of somitic/vertebral correlation is proposed in which somites and centra are in register. Differential growth of the regions of the vertebral column was calculated, and it was found that the percentages of the total column occupied by the various regions vary from one stage to another. The cervical and coccygeal regions decrease, the thoracic and lumbar regions increase, and the sacral region remains more or less constant during embryonic development. The following structures descend with reference to the vertebral column during the embryonic period proper: roots of lower limbs, thyroid gland and thymus, tracheal bifurcation, lungs, heart, diaphragm, abdominal arteries, mesonephroi, and suprarenal glands. The gonads may descend slightly. The scapulae and the separation point between the trachea and the esophagus remain at a fairly constant level. The metanephroi ascend. The migration of many of these structures (e.g., the heart, diaphragm, and metanephroi) is much more marked in the embryonic period than later although it continues during the fetal and postnatal periods. The conus medullaris ascends during the fetal period. Anomalies of migration that affect such organs as the thyroid gland, gonads, and metanephroi are discussed.