Mouse-chick chimera: an experimental system for study of somite development

Chick Models and Human-Chick Organizer Grafts

The combination of affordability, large size, and ease of access at almost every stage of development renders the chick an excellent model organism for studying vertebrate development. Not only is it a great system in and of itself, but these qualities make it a great host for interspecies chimera experiments. In this chapter we highlight some notable examples of mammalian-chick chimeras, and show how one can for instance use the chick to push mammalian stem cell experiments further to learn about the behavior and capabilities of these cells in vivo. In particular, here we present the methodology necessary for transplantation of human embryonic stem cell (hESC)-derived "gastruloids" stimulated to generate a human organizer into the chick embryo. In these human-chick chimeras, the human organizer cells self-organize to contribute directly to notochord-like tissue and indirectly induce host chick cells to generate neural tissue.

The chicken as a model for embryonic development

The traditional strength of chicken embryos for studying development is that they are readily manipulated. This has led to some major discoveries in developmental biology such as the demonstration that the neural crest gives rise to almost the entire peripheral nervous system and the identification of signalling centres that specify the pattern of structures in the central nervous system and limb. More recently with the burgeoning discovery of developmentally important genes, chicken embryos have provided useful models for testing function. Uncovering the molecular basis of development provides direct links with clinical genetics. In addition, since many genes that have crucial roles in development are also expressed in tumours, basic research on chickens has implications for understanding human health and disease. Now that the chicken genome has been sequenced and genomic resources for chicken are becoming increasingly available, this opens up opportunities for combining these new technologies with the manipulability of chicken embryos and also exploiting comparative genomics.

Vertebral body integrity: a review of various anatomical factors involved in the lumbar region

The body of the vertebra can be affected in the majority of the conditions involving the lumbar spine. Multiple references, both books and periodicals, have been reviewed, and the anatomical factors responsible for the vertebral body integrity in the lumbar spine have been included under the following important areas, namely, morphology, development, genetics, microscopic examination using histology, structural architecture, blood supply, neuromuscular control, and biomechanics.

INTRODUCTION

The anatomy provides a three-dimensional frame work to support the interaction between the physiological and pathological alterations. The body of the vertebra can be affected in a majority of acute or chronic conditions involving the lumbar spine. The etiology of these conditions is multifactorial, which has been dealt with in previous studies sporadically. This study aims to review and incorporate the important anatomical factors which can influence the integrity of vertebral bodies in the lumbar region and manifest as low back pain.

METHODS

Multiple references, both books and periodicals, have been reviewed for the literature. Electronic databases, including Medline and PubMed, were used to collect the latest information. They were finally arranged in an anatomical framework for the article. An attempt has been made to cover these relevant issues in an integrated way in the article and have been structured into introduction, morphology, development, genetics, microscopic examination using histology, structural architecture, blood supply, neuromuscular control, biomechanics, and conclusion. The aforementioned anatomical aspects, some of which have received less attention in the literature, may be helpful to clinicians for restoring the mobility, stability, and load bearing capacity of the lumbar spine as well as planning better management strategies, especially for the chronic low back pain.

RESULTS

In our article all the anatomical factors affecting the integrity of vertebral body, including the morphology, development, genetics, growth and ossification, blood supply, specifically in the lumbar region, have been described, which were not covered earlier. The limitations of this review is its wide dimensions; hence, there are fair scopes of missing many relevant facts, as all of them cannot be compiled in a single article. We have attempted to confine our views to different anatomical domains only, this is our second limitation. Additional studies are required to incorporate and discuss the uncovered relevant scientific details.

CONCLUSIONS

The integrity of the body of the lumbar vertebra is multifactorial (Fig. 8). The vast spectrum of the anatomical domain influencing it has been summarized. The evolution of erect posture is a landmark in the morphology of human beings and the lumbar lordosis, which has also contributed to the gross design of the vertebral body, is one of the most important adaptations for axial loading and bipedal movements. The role of metamerism in the evolution of vertebrate morphology is repeated in the development of spine. The body of the vertebra is intersegmental in origin, which results in dual vascular and nerve supply, both from superior and inferior aspects of the body of the lumbar vertebrae. The vertebral body ossifies from three primary centers, one for centrum, which will form the major portion of body, and the other two for neural arches. The cartilaginous growth plate is mainly responsible for the longitudinal vertebral growth. Regional differentiation of the vertebral column, and the definite pattern of the structure of the different vertebra, is regulated by a large number of genetic factors, including the Hox genes. The vertebral body design therefore provides the requirements for optimal load transfer by maximal strength with minimal weight. Bone mineral density (BMD), bone quality, microarchitecture, and material properties are the important factors that contribute to bone strength. BMD is highly heritable; bone mineral distribution and architecture are also shown to be under strong genetic influence. All the aforementioned factors finally integrate to ensure mainly the mobility, stability, and load bearing capacity of the lumbar spine.

Cell-type-specific transcriptomics in chimeric models using transcriptome-based masks

Regulatory networks involving different cell types control inflammation, morphogenesis and tissue homeostasis. Cell-type-specific transcriptional profiling offers a powerful tool for analyzing such cross-talk but is often hampered by mingling of cells within a tissue. Here, we present a novel method that performs cell-type-specific expression measurements without prior cell separation. This involves inter-species transplantation or chimeric co-culture models among which the human mouse system is frequently used. Here, we exploit the sufficiently divergent transcriptomes of human and mouse in conjunction with high-density oligonucleotide arrays. This required a masking procedure based on transcriptome databases and exhaustive fuzzy mapping of oligonucleotide probes onto these data. The approach was tested in a human-mouse experiment, demonstrating that we can efficiently measure species-specific transcriptional profiles in chimeric RNA samples without physically separating cells. Our results stress the importance of transcriptome databases with accurate 3' mRNA termination for computational prediction of accurate probe masks. We find that most human and mouse 3'-untranslated region contain unique stretches to allow for an effective control of cross-hybridization between the two species. This approach can be applied to xenograft models studying tumor-host interactions, morphogenesis or immune responses.

Photoreceptor cells from mouse ES cells by co-culture with chick embryonic retina

Degeneration of photoreceptors is a consistent and common endpoint in retinal diseases. Herein, we report the efficient induction of photoreceptor-like cells from mouse embryonic stem (ES) cells using chick embryonic retina tissue. Undifferentiated mouse ES cells were initially cultured in a preferential condition into a neural lineage, and ES cells were then co-cultured with chick embryonic day 6 (E6) retina tissues. After a 10-day co-culture, approximately 20% of the mouse ES derivatives became immuno-positive for rhodopsin. RT-PCR analysis demonstrated expression of the transcription factor crx and a distinct increase of rod photoreceptor-specific markers, IRBP and recoverin, after the start of the co-culture. These results indicate that co-culture of ES cells with chick embryonic retina tissue is a useful and efficient method for the induction of photoreceptor-like cells.

Assembly of trunk and limb blood vessels involves extensive migration and vasculogenesis of somite-derived angioblasts

Vascular development requires the assembly of precursor cells into blood vessels, but how embryonic vessels are assembled is not well understood. To determine how vascular cells migrate and assemble into vessels of the trunk and limb, marked somite-derived angioblasts were followed in developing embryos. Injection of avian somites with the cell-tracker DiI showed that somite-derived angioblasts in unperturbed embryos migrated extensively and contributed to trunk and limb vessels. Mouse-avian chimeras with mouse presomitic mesoderm grafts had graft-derived endothelial cells in blood vessels at significant distances from the graft, indicating that mouse angioblasts migrated extensively in avian hosts. Mouse graft-derived endothelial cells were consistently found in trunk vessels, such as the perineural vascular plexus, the cardinal vein, and presumptive intersomitic vessels, as well as in vessels of the limb and kidney rudiment. This reproducible pattern of graft colonization suggests that avian vascular patterning cues for trunk and limb vessels are recognized by mammalian somitic angioblasts. Mouse-quail chimeras stained with both the quail vascular marker QH1 and the mouse vascular marker PECAM-1 had finely chimeric vessels, with graft-derived mouse cells interdigitated with quail vascular cells in most vascular beds colonized by graft cells. Thus, diverse trunk and limb blood vessels have endothelial cells that developed from migratory somitic angioblasts, and assembly of these vessels is likely to have a large vasculogenic component.