Evidence that sclerotomal cells do not migrate medially during normal embryonic development of the rat

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Abstract
During embryonic development the medial part of the somite disorganizes or breaks up into sclerotomal cells which appear to migrate medially to surround the notochord. To determine whether or not this migration takes place, distances were measured between the notochord and the adjacent neural tube and the somite or its remnant during the period of somite disorganization. Serially sectioned, normal 10.5-13.5 day (d) rat embryos were used. Only transverse sections through the middle of the 4th cervical (C-4) body segment were measured, corresponding to the level of somite No. 8 (10.5 days) or its dermatomyotome remnant (10.5-11.5 days) or spinal nerve C-4 (12.5-13.5 days). Measurements were taken at 6 stages from photographic montages, all of which were made at precisely the same magnification. The notochord was the central axial structure from which the measurements were determined. The changes in distance show that during the period of somite breakup the neural tube grows dorsally, away from the notochord which lies adjacent to its ventral surface. Simultaneously the somite remnant moves laterally and dorsally, maintaining its position relative to the overlying ectoderm and leaving behind a trail of sclerotomal cells. At each stage cell counts were made on the medial sclerotomal region of the C-4 segment. The average counts reveal that the total number of cells increases substantially over the 3 day period (42-7546) and the total number of mitoses (3.5-200) increases, while the mitotic index decreases (9.0-2.7). High proliferative activity is apparent in the medial sclerotomal cells throughout the 3 day period. Local proliferation of the trailing cells, which were left by the somite remnant as it moved dorsolaterally, apparently causes the subsequent increase in density of the perichordal tissue, rather than an influx of migrating cells. Instead of sclerotomal cells migrating medially toward the notochord, these cells probably retain their position relative to the notochord or central axis and the medial sclerotomal region forms as a result of the growth movements of the surrounding structures.