Cells derived from blood vessels of human skeletal muscle can regenerate skeletal muscle, similarly to embryonic mesoangioblasts. However, adult cells do not express endothelial markers, but instead express markers of pericytes, such as NG2 proteoglycan and alkaline phosphatase (ALP), and can be prospectively isolated from freshly dissociated ALP(+) cells. Unlike canonical myogenic precursors (satellite cells), pericyte-derived cells express myogenic markers only in differentiated myotubes, which they form spontaneously with high efficiency. When transplanted into severe combined immune deficient-X-linked, mouse muscular dystrophy (scid-mdx) mice, pericyte-derived cells colonize host muscle and generate numerous fibres expressing human dystrophin. Similar cells isolated from Duchenne patients, and engineered to express human mini-dystrophin, also give rise to many dystrophin-positive fibres in vivo. These data show that myogenic precursors, distinct from satellite cells, are associated with microvascular walls in the human skeletal muscle, may represent a correlate of embryonic 'mesoangioblasts' present after birth and may be a promising candidate for future cell-therapy protocols in patients.

Skeletal muscle damaged by injury or by degenerative diseases such as muscular dystrophy is able to regenerate new muscle fibers. Regeneration mainly depends upon satellite cells, myogenic progenitors localized between the basal lamina and the muscle fiber membrane. However, other cell types outside the basal lamina, such as pericytes, also have myogenic potency. Here, we discuss the main properties of satellite cells and other myogenic progenitors as well as recent efforts to obtain myogenic cells from pluripotent stem cells for patient-tailored cell therapy. Clinical trials utilizing these cells to treat muscular dystrophies, heart failure, and stress urinary incontinence are also briefly outlined.

skeletal muscle fibres form by fusion of mesoderm progenitors called myoblasts. After birth, muscle fibres do not increase in number but continue to grow in size because of fusion of satellite cells, the postnatal myogenic cells, responsible for muscle growth and regeneration. numerous studies suggest that, on transplantation, non-myogenic cells also may contribute to muscle regeneration. However, there is currently no evidence that such a contribution represents a natural developmental option of these non-myogenic cells, rather than a consequence of experimental manipulation resulting in cell fusion. Here we show that pericytes, transgenically labelled with an inducible Alkaline Phosphatase CreERT2, but not endothelial cells, fuse with developing myofibres and enter the satellite cell compartment during unperturbed postnatal development. This contribution increases significantly during acute injury or in chronically regenerating dystrophic muscle. These data show that pericytes, resident in small vessels of skeletal muscle, contribute to its growth and regeneration during postnatal life.

Reprogramming of somatic cells into neurons provides a new approach toward cell-based therapy of neurodegenerative diseases. A major challenge for the translation of neuronal reprogramming into therapy is whether the adult human brain contains cell populations amenable to direct somatic cell conversion. Here we show that cells from the adult human cerebral cortex expressing pericyte hallmarks can be reprogrammed into neuronal cells by retrovirus-mediated coexpression of the transcription factors Sox2 and Mash1. These induced neuronal cells acquire the ability of repetitive action potential firing and serve as synaptic targets for other neurons, indicating their capability of integrating into neural networks. Genetic fate-mapping in mice expressing an inducible Cre recombinase under the tissue-nonspecific alkaline phosphatase promoter corroborated the pericytic origin of the reprogrammed cells. Our results raise the possibility of functional conversion of endogenous cells in the adult human brain to induced neuronal fates.

Mesoangioblasts are recently identified stem/progenitor cells, associated with small vessels of the mesoderm in mammals. Originally described in the mouse embryonic dorsal aorta, similar though not identical cells have been later identified and characterized from postnatal small vessels of skeletal muscle and heart (not described in this unit). They have in common the anatomical location, the expression of endothelial and/or pericyte markers, the ability to proliferate in culture, and the ability to undergo differentiation into various types of mesoderm cells upon proper culture conditions. Currently, the developmental origin of mesoangioblasts, their phenotypic heterogeneity, and the relationship with other mesoderm stem cells are not understood in detail and are the subject of active research. However, from a practical point of view, these cells have been successfully used in cell transplantation protocols that have yielded a significant rescue of structure and function in skeletal muscle of dystrophic mice and dogs. Since the corresponding human cells have been recently isolated and characterized, a clinical trial with these cells is planned in the near future. This unit provides detailed methods for isolation, culture, and characterization of mesoangioblasts. Curr. Protoc. Stem Cell Biol. 3:2B.1.1-2B.1.29. C 2007 by John Wiley & Sons, Inc.Keywords: mesoangioblasts r pericytes r mesoderm progenitor cells r cell culture NOTE: Mesoangioblasts and pericyte-derived cells must be cultured under physiological O 2 conditions (5% O 2 , 5% CO 2 , 90% N 2 ; see Critical Parameters).