Title

Investigations into the Origin of Muscle Satellite Cells and the Myogenic Capacity of Limb Endothelial Cells

Date of Completion

January 2011

Keywords

Biology, Cell|Health Sciences, Human Development

Degree

Ph.D.

Abstract

The direct progenitors of the principle cell type responsible for skeletal muscle growth and repair, the muscle satellite cell, has not been fully elucidated. Here we have utilized a permanent, genetic labeling system based on Cre/lox recombination, in order to permanently track the fate of likely candidate precursor cells. Candidate precursor cells that have been directly assayed include myoblasts and endothelial precursor cells. MyoD icre knockin mice were used to assay for myoblast contribution, and Tie2cre mice were used to assay for endothelial cell precursor contribution. We show that limb, body wall, diaphragm and extra-ocular muscle satellite cells are derived from a population of MyoD positive myoblasts. There is no contribution from endothelial cell precursors. Additionally, by utilizing MyoD-null animals we find that satellite cells are derived solely from a MyoD expressing population of myoblasts.^ While the satellite cell remains the preeminent cell type responsible for muscle repair, alternative cell types have been described. By the use of Cre/lox lineage tracing, we identify a subpopulation of endothelial cells that have historically expressed MyoD. Upon isolation, this endothelial cell subpopulation is Pax7, and MyoD negative, while it is positive for CD31 and VE-Cadherin. ^ Isolation of these cells by FAGS reveals that this population is myogenic in cell culture, and progressively shuts down the expression of endothelial cell markers while the expression of myogenic markers turn on. Direct comparison in proliferation conditions in vitro of the myogenic endothelial cell population to FACS isolated satellite cells reveals a time delay in myogenic fusion within the former. However, when both populations are placed in differentiation medium, at equal densities, the disparity in fusion is greatly reduced. The identification of a myogenic endothelial cell population, that has historically expressed MyoD, allows for future investigations into these cells' ability for in vivo muscle repair. ^