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Invited Review Unorthodox Myogenesis

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Invited Review Unorthodox Myogenesis Histol Histopathol (1997) 12: 755-760 Histology and 001: 10.14670/HH-12.755 Histopathology http://www.hh.um.es From Cell Biology to Tissue Engineering Invited Review Unorthodox myogenesis: possible developmental significance and implications for tissue histogenesis and regeneration G. Cossu Department of Histology and Medical Embryology, University of Rome "La Sapienza", Rome, Italy Summary. During the last few years several reports have th ere is no such cell as an undiffe rentiated cell : in a described the occurrence of skeletal myogenesis in cell s given area of an embryo, at a given developmental stage, derived from embryonic, fetal and perinatal tissues that each cell is already di ffe rent fro m cell s located in usuall y do not contribute to skeletal muscle in the adult di ffe rent areas as well as from its ancestors and progeny. vertebrate body. Although in most cases fa te may still be experimentall y Afte r a brie f descriptio n of c urre nt ideas on changed, cell s already express a subset of genes typical, myogenic determination in higher vertebrates, three a ltho ug h pe rhaps no t unique, o f a pa rtic ula r examples of this uno rthodox m yogenesis will be develo pme nta l stage and locatio n. Up to few years described: 1) the occurrence of myogenesis in chick ago we knew very few of these genes. With the epiblast ce ll s, c ultured in isola tion in serum-f ree explosive progress of mo lecul ar e mbryolog y, new me dium ; 2) the presence of c e ll s endow e d with developmental genes are continuously identified and myogenic potential in the embryonic mouse neural tube; their developmental pattern of expression described. An and 3) th e occurre nce of spo nt aneous o r induced emerging picture begin s to appear where each cell , or myogenesis in mesenchymal cells during fe tal and post­ ra the r each g ro up of ce ll s , e xpresses a unique natal life. combination of genes, such as ho meogenes and other A possible e mbryologica l basis fo r uno rthodox transcriptio n facto rs, g rowth facto rs a nd cyto kines, myogenesis, in relati on to gastrulati o n and mo rpho­ extracellular matri x and adhesion molecules, each of genetic fields, is then presented. It is also proposed that which is certainly al so expressed in many other pl aces unorthodox myogenesis may represent a compensatory and times. As a result, a progressive specificati on of mechanism for higher vertebrates that have lost much of positio n a nd fa te is o bta ined leading eventuall y to the regeneration potential of lower vertebrates. terminal di fferentiation. For example a newly fo rmed mesodermal cell (w hi ch already expresses mesoderm­ Key words: Myogenesis, Determination, Histogenesis, specific genes such as brachiury, noggin, etc.) may find Differentiation, Myogenic genes itself closer to the ax ia l structures (no tocho rd and mesoderm) and thus adopt a paraxial fate, ending up in somites. Later the dorsal part of the somite is specified Introduction as dermomyoto me and finall y a choice between the fi broblastic and the myogenic lineage will be made. Di ffe rentiatio n is commonl y defined as a process The avail ability of new molecular marke rs has w hi c h leads a cell to express a reperto ire of genes allowed us to investigate the expression of specific gene specific and characteristi c of the tissue where the cell produc ts a t th e s ing le cell leve l, d uring ti ssue belongs. However we can presently study with some histogenesis and regeneration. Several of these studies confid ence onl y terminall y differentiated cells. In this have unexpectedl y revealed the occurrence of (or the c ontext it is impo rta nt to dis ting uis h te rmina l po te ntia l fo r) s ke le ta l muscle di ffe re ntiati o n in a di ffe rentiation from all the previous steps th at lead the significant number of cells belonging to tissues where progeny of a blastomere to progressive diversification muscle no rma ll y does no t form. Furthe rmo re this from the other cells of the embryo. Indeed, as pointed process has been observed at inappropriate time, either out by Holtzer many years ago (Holtzer et aI. , 1973), before gastrulation or aft er the end of organogenesis. In this review I will briefl y discuss current ideas on Offprint requests to: Dr. Gu iul io Cossu , Institute of Histology and myogenic determinati on in mammals, then describe General Embryology, University of Rome .. La Sapienza», 14 Via A. three di ffe rent examples of unorthodox myogenesis, and Scarpa, 00161 Italy then I will discuss possible deveJopmental signi ficance Unorthodox myogenesis in mammals in terms of plasticity and regenerative potential of epiblast cells with adjacent tissue will inhibit mammalian tissues. myogenesis (George-Weinstein et al., 1996). One Unorthodox myogenesis is conceptually distinct interpretation of these experiments therefore is that from trans-differentiation, a phenomenon by which an epiblast cells even prior to their entry into the primitive already differentiated cell can be induced to change the streak and specificatioq as mesoderm, are already repertoire of gene expressed and to express genes typical programrned for myogenesis and that in the absence of of a different tissues. In higher vertebrates, this situation repression exerted by the in vivo context, they will is mainly related to pathology (metaplasia), although differentiate into muscle (Fig. 1). Notch has been trans-differentiation from smooth to skeletal muscle has proposed as a possible mediator of this inhibitory been recently demonstrated at the single cell leve1 in the mechanism (Kopan et al., 1994). If this is the case, then post-natal mammalian esophagus (Patapoutian et al., the influence of structures surrounding the early somites 1995). Trans-differentiation is not discussed in this is not to induce myogenesis but to relieve its repression. review. By PCR analysis, MyoD messenger RNA is detectable in the chick epiblast and in the mouse embryo prior to The origin of myoblasts in mammals somitogenesis (Kopan et al., 1994; George-Weinstein et al., 1996). The recent claim that, in Drosophila, wingless Skeletal myoblasts of the vertebrate body (with the is another ligand of notch (Couso and Martinez-Arias, exception of the head) are derived from somites, 1994), potentially competing with delta, raises the segrnented blocks of paraxial mesoderm which form in a possibility that in vertebrates the Wnts (homologues of cranio-caudal sequence along the neural tube and the wingless) produced by the neural tube or dorsal notochord (Christ and Ordhal, 1995). Among the cells of ectoderm, relieve repression of myogenesis by blocking the somite, only those which are located in the dorsal the notch receptor. Even if the receptor for Wnt has been domain, termed dermomyotome, are specified as recently identified as Fri.z.de (Bhanot et al., 1996), a myoblasts (and dermal fibroblasts) while cells located in protein different from Notch, a cross-talk between the the ventral domain, the sclerotome, will form cartilage two receptors may anyway relieve this repressive action. and bone. Myogenesis depends upon activation of either Whatever the case in molecular terms, the natural myf-5 or MyoD , the two upstream genes of this family tendency of embryonic cells towards myogenesis may of muscle specific transcnption factors (Rudnicki et al., explain the peculiar capacity of MyoD to activate 1993). Recent work has shed light on the role of adjacent myogenesis in non muscle cells and should be kept in tissue such as neural tube, notochord and dorsal mind when discussing the unorthodox origin of ectoderm, which appear to activate different myogenic myogenic cells. programs in responding mesodermal cells. Specifically the neural tube activates myogenesis in the dorso-media1 Misplaced cells and lineage lnfidellty half of paraxial mesoderm (fated to give rise to epaxial muscles: Ordhal and Le Douann, 1991) through a myf-5- Spontaneous myogenic differentiation of cells from dependent pathway, while the dorsal ectoderm can the brain has been documented a number of times activate myogenesis in the dorso-lateral precursors of (examples quoted in Tajbakhsh et al., 1994) but it was hypaxial muscles (such as limb and body wall) through a only through insertion of the reporter gene LacZ into the Myo D-dependent pathway. In this case, lateral myf-5 locus that it was possible to unequivocally identify mesoderm delays the positive effect of the ectoderm myf-5 expressing cells in the nervous system and to show suggesting that this tissue produces an inhibitory signal that these cells co-express neural and muscle markers (Cossu et al., 1996a) to maintain the cells in a committed and yet undifferentiated state during the migration to their final destination. Candidate molecules for this Myogenesis by default complex signaling activity include Sonic hedgehog and Wnts as positive signals (Munstenberg et al., 1995), and BMP4 as a possible inhibitor (reviewed in Cossu et al., 1996b). Myogenesis as a default process In apparent contrast with what is exposed above, recent observations suggest a "default" tendency of embryonic cells toward myogenesis, which should be therefore repressed in vivo until proper time and place. In contrast with their normal fate, the great majority of cells from the chick epiblast layer, cultured in vitro, will Flg. 1. A schernatic representation of the repressive influence of cell-cell undergo myogenesis, even when grown at low clonal interaction on the default tendency to rnyogenesis of chick pre-gastrula density in protein-free medium.
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