Origins and genetic control of the external cell layer

The external cell layer (ECL) is a single cell layer located lateral to the myotome and directly underneath the skin of the zebrafish. It is considered to be the functional equivalent to the amniote dermomyotome due to its similar gene expression and myogenic properties despite it being morphologically dissimilar. The ECL is a long-lived structure and provides muscle progenitors for growth. The mechanism on how this occurs is unknown. This is particularly important since the zebrafish continues to grow throughout its life, and the ECL must have mechanisms to both replenish its cell numbers as well as provide sufficient muscle progenitors. This thesis examined the cellular basis of ECL formation and maintenance of the ECL during larval stages. I show the formation of the ECL can be fate mapped to specific regions within the nascent somite. We show the specification of ECL formation is controlled by meox1 and through the examination of the meox1 mutant, we uncover a new somitic compartment we term the endotome. The endotome compartment provides somatically-derived endothelial cells that positively influence hematopoietic stem cell induction. When ECL dynamics were examined during early larval stages, there was evidence for the ECL to be the zebrafish muscle stem cell compartment. Furthermore, this muscle stem cell compartment could be modulated with meox1 and integrin linked kinase activity.