Barrier inhibition of a temporal neuraxial influence on early chick somitic myogenesis.

Skeletal myogenesis in the chick embryo first occurs in the somite. Somites are transient, paired mesodermal structures adjacent to the neural tube. Somites form from the segmental plate mesenchyme at approximately 90-min intervals. We identify somitic myogenic cells by using confocal microscopy to detect the muscle specific intermediate filament protein, desmin, in whole mount chick embryo preparations. The appearance of desmin in somitic cells does not occur at a constant interval after the somite has formed. The rate of chick somitic myogenic onset, as evidenced by detection of desmin, is approximately 1.5 times faster than the rate of somitogenesis (Borman and Yorde [1994] J. Histochem. Cytochem. 42:265-272). Somitic myogenesis does not appear to be directly linked to somitogenesis but instead may be regulated by some influence external to the somite. Here we have specifically addressed the issue of whether an impermeable barrier placed between the neuraxis and the somites can prevent the onset of somitic myogenesis. When tantalum foil barriers are placed medial to the caudalmost 3-5 somites of embryos having up to 20 somites total (stage 13), the predominant result is an inhibition of myogenic cells lateral to the barrier. Conversely, when the tantalum foil is placed medial to the caudal somites of an embryo having 21 somites (stage 14) or more, desmin is detected lateral to the barrier in most cases. There is a temporal influence originating in the neuraxis which plays a role in the onset of somitic myogenesis. Although the nature of this interaction between the neuraxis and the somites is not yet clear, we have defined a precise temporal location within the developing embryo at which this tissue interaction is taking place.