We explored the relationship in chick embryos between somitogenesis and the onset of somite myogenesis by immunodetection of the muscle-specific intermediate filament protein desmin. Early somite desmin expression was detected by whole-mount in situ confocal microscopy. No detectable somite desmin was observed in embryos of 15 somites (Stage 12) or younger. In embryos having between 16 and 26 somites (Stages 12-15), desmin could be detected in somites positioned increasingly more caudal in the embryo. Finally, in embryos of 27 somites (Stage 16) and older, somite desmin expression was consistently present in all but the caudal-most six somites. Although the rate of somite formation is fairly constant, the rate of observed somite desmin expression progressing caudally in the embryo is greater initially than the rate of segmentation. After an embryo has formed about 27 somites, the rate of desmin appearance parallels the rate of segmentation at a distance of about six somites. This result suggests that very early somite myogenesis is not linked to somitogenesis.

Expression of the muscle specific intermediate filament protein, desmin, is an early marker for chick somitic myogenesis. Somites are transient, paired, mesodermal structures adjacent to the neural tube which are formed very uniformly in a cranial to caudal fashion. The developmental somitic expression of desmin in vivo has been reported previously (Holtzer et al. [1991] "Frontiers in Muscle Research." New York Elsevier Science, pp 187-207; Borman and Yorde U9941 J. Histochem. Cytochem. 42265-272). Here we explore the ability of those somitic cells which are desmin negative in vivo to successfully carry out a myogenic program of development in the absence of the surrounding embryonic microenvironment. Somites which are known to be overtly desmin negative in the embryo were explanted and cultured on collagen gels for 4 days. Immuno-detection of desmin identified a population of somites that could support desmin positive cells in vitro as well as a population of somites that could not. The cranially located somites must remain in the embryo for a greater length of time than the caudally positioned somites prior to each being able to express desmin in vitro. In embryos of many ages there is also a population of somites unable to support desmin expression in vitro. The rate at which this ability to support somitic desmin expression in vitro progresses caudally in the embryo is significantly greater than the rate at which somites form. Notably, the detected expression of desmin in somites in vitro is parallel to the rate at which overt expression of desmin in vivo is detected. The implication for these observations with regard to the regulation of somitic myogenesis is discussed. o

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 [19941 J. Histochem. Cytochem. 42265-272). 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 131, 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 orig inating 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.

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