Fate restriction in limb muscle precursor cells precedes high-level expression of MyoD family member genes

The mechanisms by which pluripotent embryonic cells generate unipotent tiss ue progenitor cells during development are unknown. Molecular/genetic exper iments in cultured cells have led to the hypothesis that the product of a s ingle member of the MyoD gene family (MDF) is necessary and sufficient to e stablish the positive aspects of the determined state of myogenic precursor cells: i.e., the ability to initiate and maintain the differentiated state (Weintraub, H., Davis, R., Tapscott, S., Thayer, M., Krause, M., Benezra, R., Blackwell, T. K., Turner, D., Rupp, R., Hollenberg, S, et al, (1991) Sc ience 251, 761-766), Embryonic cell type determination also involves negati ve regulation, such as the restriction of developmental potential for alter native cell types, that is not directly addressed by the MDF model. In the experiments reported here, phenotypic restriction in myogenic precursor cel ls is assayed by an in vive 'notochord challenge' to evaluate their potenti al to 'choose' between two alternative cell fate endpoints: cartilage and m uscle (Williams, B, A. and Ordahl, C, P, (1997) Development 124, 4983-4997) , Two separate myogenic precursor cell populations were found to be phenoty pically restricted while expressing the Pax3 gene and prior to MDF gene act ivation. Therefore, while MDF family members act positively during myogenic differentiation, phenotypic restriction, the negative aspect of cell speci fication, requires cellular and molecular events and interactions that prec ede MDF expression in myogenic precursor cells. The qualities of muscle for med by the determined myogenic precursor cells in these experiments further indicate that their developmental potential is intermediate between that o f myoblastic stem cells taken from fetal or adult tissue (which lack mitoti c and morphogenetic potential when tested in vivo) embryonic stem cells (wh ich are multipotent), We hypothesize that such embryonic myogenic progenito r cells represent a distinct class of determined embryonic cell, one that i s responsible for both tissue growth and tissue morphogenesis.