Vessel-associated stem cells from skeletal muscle: From biology to future uses in cell therapy

Table 1 Origin and main characteristics of possible muscular progenitors

Satellite cells
	Ref.	Results
Myoblasts/SCs	43-48	Dystrophin restoration in nude/mdx mice after transplantation
		In clinical trials: either negative or poor results; main problems:
		Inability to cross vascular barrier and limited intramuscular migration/need of multiple intramuscular injections
		Immunosuppression needed to prevent cells rejection and apoptosis
		Poor efficiency of cells graft: very high number of transplanted cells needed
		DMD clinical trial, best result: restoration of dystrophin expression: 26%-30%
BMSCs/circulating AC133+ cells
	49-52	Ability to undergo myogenic differentiation but only minimal effects in vivo
		Possibility of intra-arterial injection
		Activation induced by muscle inflammation/damage
HSC	53	Purification of CD45+Sca1+c-Kit+Lin- fraction with myogenic potential
MSC	54,55	Myogenic potential of MSC but poor muscle recovery in vivo
bmSP	5	Isolated through Hoechst 33342 exclusion
		Purification of CD45+c-Met+CD43+Lin- fraction
		Poor muscle incorporation rates
AC133+ cells	56,57	Circulating human hematopoietic/endothelial progenitors endowed with myogenic potential (upon co-culture with myoblasts)
		Advantage: circulating ability
		Ability to incorporate in SCc niche and participate in muscle regeneration to a poor extent
Other skeletal muscle progenitors
MDSCs	58-63	Population of mesodermal origin, isolated from skeletal muscle based on temporal differences in binding collagen pre-coated plates
		Identification of CD34+/-Bcl+Sca1+ fraction with myogenic potential and ability to bind to muscular capillary network after intra-arterial injection
		Expression of L-selectin (ligand for MAdCAM-1) by CD34- fraction probably accounting for their homing ability
		Migration and regenerative properties influenced by pathology, age and sex
mSP	5-7,64,65	Isolated from muscle through Hoechst 33342 dye exclusion
		Differences with bmSP:
		Both Sca1+Lin- but mSP are c-Kit- and CD43-
		mSP: ability to enter SCs niche
		Differences with SCs:
		Expression of Sca1/class of Sca-1 positive cells associated with blood vessels with high degree of plasticity
		Presence in Pax7null mice
		Ability to flow through small vessels
		2 different fractions:
		CD45+: hematopoietic origin and preferential differentiation
		CD45-: somitic origin, greater myogenic potential (in co-culture with myoblasts/under stimulation of Wnt pathway)
Possible muscular progenitors of other origin
ADSCs	66-68	Close relationship with myogenic cells (same mesodermal origin, inverse relation skeletal muscle/adipose tissue size, myoblasts/SCs capable to convert into adipose tissue)
		CD13+CD44+CD73+CD90+ and stromal vascular fraction: in vivo and in vitro myogenic potential; myogenic potential enhanced by forced MyoD expression
		Advantages: easy availability, immune-privileged behavior, strong expansion ex vivo
EPCs	3,25,26,28	Identification of CD34+CD144+Flk1+CD45-;CD56-;fraction with myogenic potential (contribution to muscle regeneration in vivo)

SCs: Satellite cells; BMSCs: Bone marrow stem cells; HSCs: Hematopoietic stem cells; MSCs: Mesenchymal stem cells; bmSP: Bone marrow side population; mSP: Muscle side population; MDSCs: Muscle derived stem cells; ADSCs: Adipose-derived stem cells; EPCs: Endothelial progenitor cells.