Satellite Cell Plasticity in the Chicken Embryo

Abstract

During embryogenesis, mononucleated myoblasts fuse, forming myotubes that subsequently develop into mature myofibers. Myonuclei are not capable of division. Postnatal muscle growth occurs through an increase in myofiber size, not by an increase in myofiber number. Myofiber number does not change, adult myonuclei are postmitotic and cannot synthesize DNA. In postnatal vertebrates the increase in myofiber size (muscle growth) is directly related to myonuclei. The source of these additional nuclei is the satellite cell. Satellite cells are defined by their morphological criteria as occupying grooves between the basal lamina and sarcolemma of the muscle fiber. Satellite cells define a unique lineage of myogenic progenitors that arise late in development, and they are required for postnatal muscle growth and repair. Satellite cells express myogenic specific genes, thus representing committed stem cells. That are distinct from their daughter myoblasts. Despite repeated challenges to the muscle, there is recurrent regeneration of the satellite cell population without exhausting the supply of myogenic cells suggesting that satellite cells have the capacity for growth and self-maintenance.

Satellite cells exhibit the characteristics of stem cells because a stem cell is not differentiated, displays a capacity for self-renewal throughout the lifetime of an organism, and has the potential to give rise to differentiated progeny. In the past, satellite cells were believed to be unipotent, giving rise to only one type of differentiated progeny. The myogenic precursor cells. However, current research suggests that satellite cells are not a homogeneous committed pool of myogenic stem cells, but rather a heterogeneous population with different subpopulations. Whether there are different subpopulations of satellite cells with each population having specific or multiple roles, or one population of myogenic precursor cells with the ability to "multitask" and adapt or "transdifferentiate" into many roles. If there is a subpopulation of myogenic precursor cells, in addition to the satellite cell, from what cell lineage does this pool of myogenic precursor cells arise and what is their developmental capacity?

This study investigated the plasticity of satellite cells in the chicken embryo.

And the question that we set out to answer; "Is the chicken satellite cell plastic"? Can the satellite cell from adult chicken take on new identities?

The objective of the first experiment was to demonstrate that microinjection of somites in Stage 10 (HH) embryos could result in viable chicks. The goal of the experiment was to develop a method to study satellite cell plasticity at the generally accepted site of embryonic origin, and to develop methods that will provide insight about the effect to embryonic manipulation on posthatch muscle. A successful method was accomplished where 54% of the embryos from the stage 10 injections, 19% from the stage 12 injections, and 62% from the stage 15 injections hatched.

The objective of the second experiment was to test the hypothesis that avian skeletal muscle satellite cells can/cannot participate in cardiogenesis in avian embryonic hearts. Freshly isolated skeletal muscle cells and cultured skeletal muscle cells were used prior to transplanting satellite cells into embryonic hearts. The findings showed that transplanted skeletal muscle cells from adult tissue can integrate into avian embryonic tissue.

The objective of the third experiment was the production of chick chimeras utilizing myoblast transplantation methodology, to determine the plasticity of satellite cells in skeletal muscle. The positive control for this experiment was the production of somatic chimeras from Barred Plymouth Rock blastodermal cell transfer to recipient Stage X, White Leghorn embryos. In the study, blastoderm injection into irradiated recipients; 62.5% of somatic chimeras survived to feather color stage, ED 15.

The objective of this experiment was to produce chick chimeras utilizing myoblast transplantation to demonstrate the plasticity of satellite cells in skeletal muscle.

Survival in irradiated embryos injected with myoblasts and blastodermal cells (treatments 1 and 2) was 9.4% and survival in non-irradiated embryos injected with myoblasts and blastodermal cells (treatments 3 and 4) was 17.7%. Irradiated injected embryos (treatments 1 and 2) and non-irradiated injected embryos were subjected to chi-square statistical analysis to determine if irradiation effects embryonic survival. Numbers of dead and live irradiated stage X embryos subjected to treatments 1 and 2, myoblast and blastodermal cell injection, were subjected to chi-square analysis showing that irradiation is inconsequential to injected embryonic survival. The result of the high mortality (90.5%) in irradiated treatments 1 and 2 and non-irradiated treatments 3 and 4 (82.2%) could possibly be the result of cell concentration.

Irradiation and high myoblast cell concentration could possibly account for this high embryonic mortality (92.9%). No somatic chimeras were produced after the injection of myoblasts, but somatic chimeras were produced with blastodermal cell injections.