Myo/Nog cell regulation of bone morphogenetic protein signaling in the blastocyst is essential for normal morphogenesis and striated muscle lineage specification.

Cells that express MyoD mRNA, the G8 antigen and the bone morphogenetic protein (BMP) inhibitor noggin (Nog) are present in the epiblast before gastrulation. Ablation of "Myo/Nog" cells in the blastocyst results in an expansion of canonical BMP signaling and prevents the expression of noggin and follistatin before and after the onset of gastrulation. Once eliminated in the epiblast, they are neither replaced nor compensated for as development progresses. Older embryos lacking Myo/Nog cells exhibit severe axial malformations. Although Wnts and Sonic hedgehog are expressed in ablated embryos, skeletal muscle progenitors expressing Pax3 are missing in the somites. Pax3+ cells do emerge adjacent to Wnt3a+ cells in vitro; however, few undergo skeletal myogenesis. Ablation of Myo/Nog cells also results in ectopically placed cardiac progenitors and cardiomyocytes in the somites. Reintroduction of Myo/Nog cells into the epiblast of ablated embryos restores normal patterns of BMP signaling, morphogenesis and skeletal myogenesis, and inhibits the expression of cardiac markers in the somites. This study demonstrates that Myo/Nog cells are essential regulators of BMP signaling in the early epiblast and are indispensable for normal morphogenesis and striated muscle lineage specification.

Cells that express MyoD mRNA in the epiblast are stably committed to the skeletal muscle lineage.

The epiblast of the chick embryo contains cells that express MyoD mRNA but not MyoD protein. We investigated whether MyoD-positive (MyoDpos) epiblast cells are stably committed to the skeletal muscle lineage or whether their fate can be altered in different environments. A small number of MyoDpos epiblast cells were tracked into the heart and nervous system. In these locations, they expressed MyoD mRNA and some synthesized MyoD protein. No MyoDpos epiblast cells differentiated into cardiac muscle or neurons. Similar results were obtained when MyoDpos cells were isolated from the epiblast and microinjected into the precardiac mesoderm or neural plate. In contrast, epiblast cells lacking MyoD differentiated according to their environment. These results demonstrate that the epiblast contains both multipotent cells and a subpopulation of cells that are stably committed to the skeletal muscle lineage before the onset of gastrulation. Stable programming in the epiblast may ensure that MyoDpos cells express similar signaling molecules in a variety of environments.

MyoD-positive epiblast cells regulate skeletal muscle differentiation in the embryo.

MyoD mRNA is expressed in a subpopulation of cells within the embryonic epiblast. Most of these cells are incorporated into somites and synthesize Noggin. Ablation of MyoD-positive cells in the epiblast subsequently results in the herniation of organs through the ventral body wall, a decrease in the expression of Noggin, MyoD, Myf5, and myosin in the somites and limbs, and an increase in Pax-3-positive myogenic precursors. The addition of Noggin lateral to the somites compensates for the loss of MyoD-positive epiblast cells. Skeletal muscle stem cells that arise in the epiblast are utilized in the somites to promote muscle differentiation by serving as a source of Noggin.

Noggin producing, MyoD-positive cells are crucial for eye development.

A subpopulation of cells expresses MyoD mRNA and the cell surface G8 antigen in the epiblast prior to the onset of gastrulation. When an antibody to the G8 antigen was applied to the epiblast, labeled cells were later found in the ocular primordia and muscle and non-muscle forming tissues of the eyes. In the lens, retina and periocular mesenchyme, G8-positive cells synthesized MyoD mRNA and the bone morphogenetic protein inhibitor Noggin. MyoD expressing cells were ablated in the epiblast by labeling them with the G8 MAb and lysing them with complement. Their ablation in the epiblast resulted in eye defects, including anopthalmia, micropthalmia, altered pigmentation and malformations of the lens and/or retina. The right eye was more severely affected than the left eye. The asymmetry of the eye defects in ablated embryos correlated with differences in the number of residual Noggin producing, MyoD-positive cells in ocular tissues. Exogenously supplied Noggin compensated for the ablated epiblast cells. This study demonstrates that MyoD expressing cells serve as a Noggin delivery system to regulate the morphogenesis of the lens and optic cup.

Epiblast cells that express MyoD recruit pluripotent cells to the skeletal muscle lineage.

Embryonic stem cells are derived from the epiblast. A subpopulation of epiblast cells expresses MyoD mRNA and the G8 antigen in vivo. G8 positive (G8pos) and G8 negative (G8neg) populations were isolated by magnetic cell sorting. Nearly all G8pos cells switched from E- to N-cadherin and differentiated into skeletal muscle in culture. G8neg cells were impaired in their ability to switch cadherins and few formed skeletal muscle. Medium conditioned by G8pos cells stimulated skeletal myogenesis and N-cadherin synthesis in G8neg cultures. The effect of conditioned medium from G8pos cultures was inhibited by bone morphogenetic protein (BMP) 4. Treatment of G8neg cells with a soluble form of the BMP receptor-IA or Noggin promoted N-cadherin synthesis and skeletal myogenesis. These results demonstrate that MyoD-positive epiblast cells recruit pluripotent cells to the skeletal muscle lineage. The mechanism of recruitment involves blocking the BMP signaling pathway.

Tracking and ablating subpopulations of epiblast cells in the chick embryo.

The early chick embryo contains subpopulations of cells that express lineage-specific transcription factors. We have developed protocols to examine the role of these cells during development that involve labeling them for cell tracking purposes and ablating them within the epiblast. The procedures take advantage of the fact that subpopulations of epiblast cells differentially express cell surface antigens recognized by monoclonal antibodies. Embryos are removed from the shell and incubated on the yolk with an antibody. Cells that bind the antibody are either tagged with a fluorescent secondary antibody or lysed with complement. For long-term analyses, embryos are returned to a host shell and placed in an incubator. This method of whole embryo manipulation ex-ovo and incubation in-ovo supports normal development into the fetal period.

NeuroM and MyoD are expressed in separate subpopulations of cells in the pregastrulating epiblast.

Epiblast cells form skeletal muscle and neurons in culture and some express mRNA for the skeletal muscle specific transcription factor MyoD in vivo. The following experiments were designed to determine whether the neurogenic transcription factor NeuroM is expressed in the epiblast and if NeuroM and MyoD are present in separate subpopulations of epiblast cells that can differentiate into neurons and muscle, respectively. In situ hybridization revealed that NeuroM was present in the anterior region of the pregastrulating epiblast. Some cells with NeuroM were proliferating and expressed two molecules present in neurogenic cells, NCAM and the Zn-12/HNK-1 carbohydrate. The G8 antibody labeled cells with MyoD but not NeuroM. When G8 positive cells were isolated by magnetic cell sorting and placed in culture, nearly all differentiated into skeletal muscle in serum free medium. A subpopulation of cells isolated with antibodies that bound to cells expressing NeuroM formed neurons when cultured in medium supplemented with sera and embryo extract. These experiments demonstrate that NeuroM and MyoD are present in separate subpopulations of cells in the pregastrulating epiblast. Epiblast cells with NeuroM are more dependent on exogenous factors to differentiate than those with MyoD.

Visualizing the Needle in the Haystack: In Situ Hybridization With Fluorescent Dendrimers.

In situ hybridization with 3DNA trade mark dendrimers is a novel tool for detecting low levels of mRNA in tissue sections and whole embryos. Fluorescently labeled dendrimers were used to identify cells that express mRNA for the skeletal muscle transcription factor MyoD in the early chick embryo. A small population of MyoD mRNA positive cells was found in the epiblast prior to the initiation of gastrulation, two days earlier than previously detected using enzymatic or radiolabeled probes for mRNA. When isolated from the epiblast and placed in culture, the MyoD mRNA positive cells were able to differentiate into skeletal muscle cells. These results demonstrate that DNA dendrimers are sensitive and precise tools for identifying low levels of mRNA in single cells and tissues.