The pattern of MyoD and contractile protein localization in primary epaxial myotome reflects the dynamic progression of nascent myoblast differentiation

Somite unit chronometry to analyze teratogen phase specificity in the paraxial mesoderm

Phase specificity, the temporal and tissue restriction of teratogen-induced defects during embryonic -development, is a poorly understood but common property of teratogens, an important source of human birth defects. Somite counting and somite units are novel chronometric tools used here to identify stages of paraxial mesoderm development that are sensitive to pulse-chase exposure (2 to >16 h) to 5-bromodeoxyuridine (BrdU). In all cases, it was the presomitic mesoderm (PSM) that was sensitive to BrdU induced segmentation anomalies. At high concentration (1.0 × 10(-2) M BrdU), PSM presegment stages ss-IV and earlier were irreversibly inhibited from completing segmentation. At low concentration (2.6 × 10(-6) M), BrdU induced periodic focal defects that predominantly trace back to PSM presegments between ss-V and ss-IX. Phase specificity is characteristic of both types of segmentation anomalies. Focal segmentation defects are phase-specific because they result from disruption of 2-3 presegments in the PSM while adjacent -rostral and caudal presegments are (apparently) unaffected. Irreversible inhibition of segmentation is also phase-specific because only PSM presegments ss-IV or earlier were affected while older segments (ss-III to ss-I) were able to complete segmentation. The presegments predominantly affected have not yet passed the determination front, the point at which the segmentation clock establishes somite rostro-caudal -polarity. Somite unit chronometry provides a means to identify specific PSM presegment stages that are susceptible to induced segmentation defects and the biological processes that underlie that vulnerability.

Somite cell cycle analysis using somite-staging to measure intrinsic developmental time

Somite stages were employed as units of intrinsic developmental time to measure cell doubling rate and other cell cycle parameters of chick forelimb level somites. Somite cell nuclei doubled over an interval corresponding to approximately 7+ somite stages (7+ ss; approximately 11 hr) and approximately 24 new primary myotome cells are born per somite stage ( approximately 16/hr). FACS analysis of DNA content in dissociated paraxial mesoderm cells indicated that slightly more than half are in G1/G0 phase of the cell cycle and that the average combined length of the S phase and G2 phase intervals is approximately 3 ss ( approximately 4.5 hr). A wavefront of increased mitotic nuclei per segment coincident with somite budding potentially reflects a surge in the number of cells entering S phase 3 ss earlier as each PSM segment becomes unresponsive to FGF signaling as it passes through the determination front.

A NLRR-1 gene is expressed in migrating slow muscle cells of the trout (Oncorhynchus mykiss) embryo

NLRR-l (neuronal leucine-rich repeat-l) is a transmembrane protein that functions as a cell adhesion molecule regulating morphogenesis. A previous study in the mouse reported that the somitic expression of NLRR-1 is restricted to the dorsal lip of the dermomyotome that gives rise to the epaxial muscle. In this study, we report the expression of a NLRR-1 gene in the trout-developing somite. Whole mount in situ hybridization showed that NLRR-l transcript accumulated in a rostro-caudal wave in the adaxial slow muscle cells, which are initially found deep in the somite, immediately adjacent to the notochord. No labelling was observed in the segmental plate from which somites form. As somites mature along an anteroposterior axis, the NLRR-l-positive adaxial cells exhibited an apparent migration radially to the lateral surface of the myotome where they ultimately form the peripheral slow muscle fibres. These observations show that a NLRR-1 gene is expressed in a subpopulation of myogenic cells of the trout embryo, but the anatomical location and the fate of this subpopulation are distinct from those of the NLRR-1 positive myogenic cells in amniotes. NLRR-l was also transcribed in distinct areas of the developing nervous system including the telencephalon, the optic tectum, the cerebellum, the neural tube, the retina, and the branchial arches.

Expression of the avian gene cNOC2 encoding nucleolar complex associated protein 2 during embryonic development

Genetic information that directs a cell during different phases of embryogenesis is locked up in the genome. Therein is contained the road map for growth, proliferation, differentiation and morphogenesis. The cellular transportation machinery plays a major role to ensure that all the components for transcription and translation are available at the right place at the right time. Nucleolar complex associated protein2 (NOC2) has a highly conserved UPF0120 domain, and is an element involved in ribosome transportation from the nucleoplasm to the cytoplasm. However, its gene expression pattern is still unknown. We chose the developing chick embryo to investigate the possible involvement of avian NOC2 (cNOC2) in developmental processes, particularly neurogenesis and myogenesis. For this purpose, we constructed a fragment of chicken cNOC2, which contains the UPF0120 domain coding sequence, into pDrive vector, and performed in situ hybridization on chicken embryos of different stages with this gene probe. A dynamic expression pattern of cNOC2 transcripts can be seen beginning as early as from stage HH7 until stage HH32. Using in situ hybridization we could detect that cNOC2 transcripts were expressed ubiquitously, but prominent expression could be found in the neural tissue, the somites and in the developing limbs. Comparison of cNOC2 gene expression with the proliferation marker gene cPCNA, muscle specific marker genes cMyf5 and cMyoD in single or double in situ hybridisation show that cNOC2 is expressed in the myotome, similar to cMyf5 and cMyoD, but not like cPCNA, which is hardly detectable in the myotome. Our results suggest that cNOC2 is involved in the development of neural tissue, somites and limbs.