Richard P. Harvey

epicardin: A novel basic helix-loop-helix transcription factor gene expressed in epicardium, branchial arch myoblasts, and mesenchyme of developing lung, gut, kidney, and gonads

We report the cloning, chromosomal localization, and analysis of the expression pattern of epicardin, a member of the basic helix‐loop‐helix (bHLH) family of transcription factors. Within its bHLH domain, the human and murine epicardin genes were most similar to paraxis, a bHLH gene important for segmentation of embryonic paraxial mesoderm. In situ hybridization studies revealed strong epicardin expression in murine embryos at 9.5 days postcoitum (dpc) in a region of the septum transversum at the base of the heart known as the proepicardial organ. This mesenchymal structure extends villous projections from which epicardial precursor cells emerge and migrate out over the surface of the myocardium. Strong expression was seen in individual migratory cells and clusters at 9.5 dpc and in a continuous epicardial cell layer in more mature hearts. Also from 9.5 dpc, epicardin transcripts were seen in endocardial cushions of the atrioventricular canal and outflow tract, in skeletal myoblasts within branchial arches and in condensing mesenchyme of gut, kidney, urinary tract, gonads, spleen, and lung. Northern analysis showed that expression persisted in mature visceral organs and heart, but was transient in skeletal muscle. The central role played by bHLH factors in pathways for tissue determination in the embryo suggests a function for epicardin in specification of select mesodermal cell populations associated with heart, cranial skeletal muscle, gut, and urogenital system. Dev. Dyn. 1998;213:105–113.

EXPRESSION OF NK-2 CLASS HOMEOBOX GENE NKX2-6 IN FOREGUT ENDODERM AND HEART

NK-2 class homeobox genes are candidate patterning and lineage regulators in diverse organisms. We report here the embryonic expression pattern of murine member, Nkx2-6. In keeping with its vertebrate relatives, Nkx2-6 was transcribed in ventrolateral embryonic structures. Expression was first detected at E8.0 in endodermal walls of the foregut pocket, tissue destined to become pharyngeal floor. From E8.0-10.5, transcripts were concentrated in pharyngeal pouches and juxtaposed arch ectoderm and mesoderm, as well as in more caudal gut segments. Expression was also seen at opposite poles of the developing heart from E8-8.5 in posterior myocardial progenitors, then sinus venosa and dorsal pericardium, and from E9.5 in outflow tract myocardium.

Mesoderm‐specific expression of the divergent homeobox gene Hlx during murine embryogenesis

We have determined the expression pattern of the divergent homeobox gene Hlxduring post‐implantation mouse development, utilizing in situ hybridization. Expression was mesoderm‐specific and occurred in a complex tissue distribution. Transcripts were first detected at 9.5 days post coitum (p.c.) in splanchnic mesoderm of the midgut and hindgut region, then during organogenesis, prominently in mesenchyme of the developing liver, gall bladder, and intestines, as well as their mesenteric tissues. In the foregut, lung mesenchyme became positive from 10.5 days p.c. Hlx transcripts were also detected in a subset of skeletal myogenic cells: those within branchial arches from 9.5 days p.c. and within limb buds from 12 days p.c. Hlx was not expressed in myogenic cells which are derived from the myotome and populate the trunk. However, from 10 days p.c., expression was seen in a region of the sclerotome immediately adjacent to the myotome and corresponding to precursors of the ribs and vertebral neural arches. In the anterior‐posterior aspect of the developing sclerotome, Hlx expression was out of register with original segmental boundaries (intersomitic fissures), a pattern consistent with a classical hypothesis that the developing vertebral column undergoes resegmentation. Hlx expression was also observed in vibrissae, pericardium, snout mesenchyme, and meningeal epithelium. Overall, expression of Hlx in only a subset of individual lineage progenitors and at know sites of inductive tissue interactions, suggests that the gene regulates local patterning or growth through cell:cell signalling at those embryonic sites.

MyoD protein expression in Xenopus embryos closely follows a mesoderm induction-dependent amplification of MyoD transcription and is synchronous across the future somite axis

The MyoD-related genes code for key regulators of skeletal muscle commitment and differentiation. In this study, expression of MyoD protein has been examined during Xenopus development. Protein is first detected in presumptive mesoderm at early gastrulation, directly following a dramatic increase in MyoD transcription that occurs in response to mesoderm induction. The pattern of expression resembles the muscle fate map at this time. Protein accumulates synchronously along the future somite axis, with no evidence of a spatial regulation which would explain the anterior/posterior wave of myogenic differentiation that follows MyoD expression. During gastrulation, the highest levels of MyoD are in cells next to the developing notochord, suggesting a role for the notochord in induction or maintenance of MyoD expression. After muscle differentiation, MyoD protein is degraded with a half-life of several hours, leading to very low expression in mature somites. These studies support a role for MyoD in induction of muscle mesoderm, but also point to the multi-layered regulation of these events.