The role of the notochord in vertebral column formation

Changes in gravitational force affect gene expression in developing organ systems at different developmental times

Background

Little is known about the affect of microgravity on gene expression, particularly in vivo during embryonic development. Using transgenic zebrafish that express the gfp gene under the influence of a β-actin promoter, we examined the affect of simulated-microgravity on GFP expression in the heart, notochord, eye, somites, and rohon beard neurons. We exposed transgenic zebrafish to simulated-microgravity for different durations at a variety of developmental times in an attempt to determine periods of susceptibility for the different developing organ systems.

Results

The developing heart had a period of maximum susceptibility between 32 and 56 hours after fertilization when there was an approximately 30% increase in gene expression. The notochord, eye, somites, and rohon beard neurons all showed periods of susceptibility occurring between 24 and 72 hours after fertilization. In addition, the notochord showed a second period of susceptibility between 8 and 32 hours after fertilization. Interestingly, all organs appeared to be recovering by 80 hours after fertilization despite continued exposure to simulated-microgravity.

Conclusion

These results support the idea that exposure to microgravity can cause changes in gene expression in a variety of developing organ systems in live embryos and that there are periods of maximum susceptibility to the effects.

Conditioned medium differentially regulates matrix protein gene expression in cells of the intervertebral disc

STUDY DESIGN

Matrix protein gene expression was determined for cells of the anulus fibrosus (AF) and nucleus pulposus (NP) regions of the intervertebral disc when cultured in AF or NP cell-conditioned medium.

OBJECTIVES

To investigate changes in mRNA levels for type I collagen, type II collagen and aggrecan in cells of the AF and NP in response to cell-conditioned medium.

SUMMARY OF BACKGROUND DATA

Cells of the intervertebral disc have been shown to respond to exogenous soluble mediators such as the growth factors TGF-beta and IGF-1. Little is known of their biologic response to endogenous factors that may be secreted locally or by cells of neighboring regions.

METHODS

Porcine cells were cultured for 48 hours in alginate gel in the presence or absence of conditioned medium. Gene expression for aggrecan and collagens was quantified using real-time reverse transcriptase-polymerase chain reaction. RESULTS.: AF cell gene expression was generally stimulated by the conditioned medium of either AF or NP cells. In contrast, the notochordal cell-containing NP cells showed little change in gene expression with either source of conditioned medium.

CONCLUSIONS

Cells of the NP and AF secrete soluble factors in culture at similarly effective doses to stimulate matrix protein gene expression in AF cells of the intervertebral disc. Unlike AF cells, however, NP cell gene expression was not stimulated by any conditioned medium, suggesting that differences exist in the responsiveness of cells of notochordal (NP) and fibrocartilaginous (AF) phenotypes. Understanding these differences between cells of the intervertebral disc may reveal unique stimulatory factors important to repair and regeneration of the degenerated intervertebral disc.

A central role for the notochord in vertebral patterning

The vertebrates are defined by their segmented vertebral column, and vertebral periodicity is thought to originate from embryonic segments, the somites. According to the widely accepted 'resegmentation' model, a single vertebra forms from the recombination of the anterior and posterior halves of two adjacent sclerotomes on both sides of the embryo. Although there is supporting evidence for this model in amniotes, it remains uncertain whether it applies to all vertebrates. To explore this, we have investigated vertebral patterning in the zebrafish. Surprisingly, we find that vertebral bodies (centra) arise by secretion of bone matrix from the notochord rather than somites; centra do not form via a cartilage intermediate stage, nor do they contain osteoblasts. Moreover, isolated, cultured notochords secrete bone matrix in vitro, and ablation of notochord cells at segmentally reiterated positions in vivo prevents the formation of centra. Analysis of fss mutant embryos, in which sclerotome segmentation is disrupted, shows that whereas neural arch segmentation is also disrupted, centrum development proceeds normally. These findings suggest that the notochord plays a key, perhaps ancient, role in the segmental patterning of vertebrae.

Perlecan, the multidomain heparan sulfate proteoglycan of basement membranes, is also a prominent component of the cartilaginous primordia in the developing human fetal spine

The aim of this study was to localize perlecan in human fetal spine tissues. Human fetal spines (12-20 weeks; n=6) were fixed in either Histochoice or 10% neutral buffered formalin, routinely processed, paraffin-embedded, and 4-microm sagittal sections were cut and stained with toluidine blue, H&E, and von Kossa. Perlecan, types I, II, IV, and X collagen, CD-31, aggrecan core protein, and native and delta-HS 4, 5 hexuronate stub epitopes were immunolocalized. Toluidine blue staining visualized the cartilaginous vertebral body (VB) rudiments and annular lamellae encompassing the nucleus pulposus (NP). Von Kossa staining identified the VB primary center of ossification. Immunolocalization of type IV collagen, CD-31, and perlecan delineated small blood vessels in the outer annulus fibrosus (AF) and large canals deep within the VBs. Perlecan and type X collagen were also prominently expressed by the hypertrophic vertebral growth plate chondrocytes. Aggrecan was extracellularly distributed in the intervertebral disk (IVD) with intense staining in the posterior AF. Notochordal tissue stained strongly for aggrecan but negatively for perlecan and types I and II collagen. Type I collagen was prominent in the outer AF and less abundant in the NP, while type II collagen was localized throughout the IVD and VB. The immunolocalization patterns observed indicated key roles for perlecan in vasculogenic, chondrogenic, and endochondral ossification processes associated with spinal development.

Sox5 and Sox6 are required for notochord extracellular matrix sheath formation, notochord cell survival and development of the nucleus pulposus of intervertebral discs

The notochord has major roles in vertebral column formation: indirectly by inducing sclerotome cell differentiation; and directly by forming the nucleus pulposus of intervertebral discs. Sox5 and Sox6 encode Sry-related HMG box transcription factors that act redundantly to promote chondroblast differentiation in all cartilages of the mouse embryo. We show that Sox5 and Sox6 are expressed in the notochord cell lineage and required for notochord late development. In Sox5(-/-)/Sox6(-/-) embryos, the notochord formed a typical rod-like structure. It fulfilled its inductive functions, as indicated by expression of sonic hedgehog and sclerotome specification. However, the notochord failed to become surrounded with an extracellular matrix sheath. This phenotype was associated with a downregulation of extracellular matrix genes, including the genes for collagen 2, aggrecan and perlecan in both notochord cells and surrounding chondrocytic cells of presumptive inner annuli and vertebral bodies. The mutant notochord then underwent an aberrant, fatal dismantling after sclerotome cell migration. Its cells became removed first from intervertebral spaces and then from vertebral bodies, and it progressively underwent apoptosis. Meanwhile, the development of inner annuli and vertebral bodies was dramatically impaired. Consequently, the vertebral column of Sox5(-/-)/Sox6(-/-) fetuses consisted of a very deficient cartilage and was devoid of nuclei pulposi. In Sox5(-/-)/Sox6(+/-) and more severely in Sox5(+/-)/Sox6(-/-) embryos, the notochord sheath was thinner, but cells survived. By birth, nuclei pulposi were rudimentary, and its cells poorly swelled and still expressing sonic hedgehog. Hence, Sox5 and Sox6 are required for notochord extracellular matrix sheath formation, notochord cell survival and formation of nuclei pulposi. Through these roles and essential roles in cartilage formation, they are central transcriptional regulators of vertebral column development.

Changes in gravitational force induce alterations in gene expression that can be monitored in the live, developing zebrafish heart

Little is known about the effect of microgravity on gene expression, particularly in vivo during embryonic development. Using transgenic zebrafish that express the gfp gene under the influence of a beta-actin promoter, we examined the affect of simulated-microgravity on GFP expression in the heart. Zebrafish embryos, at the 18-20 somite-stage, were exposed to simulated-microgravity for 24 hours. The intensity of GFP fluorescence associated with the heart was then determined using fluorescence microscopy. Our measurements indicated that simulated-microgravity induced a 23.9% increase in GFP-associated fluorescence in the heart. In contrast, the caudal notochord showed a 17.5% increase and the embryo as a whole showed only an 8.5% increase in GFP-associated fluorescence. This suggests that there are specific effects on the heart causing the more dramatic increase. These studies indicate that microgravity can influence gene expression and demonstrate the usefulness of this in vivo model of 'reporter-gene' expression for studying the effects of microgravity.

Origin and early evolution of vertebrate skeletonization

Data from living and extinct faunas of primitive vertebrates imply very different scenarios for the origin and evolution of the dermal and oral skeletal developmental system. A direct reading of the evolutionary relationships of living primitive vertebrates implies that the dermal scales, teeth, and jaws arose synchronously with a cohort of other characters that could be considered unique to jawed vertebrates: the dermoskeleton is primitively composed of numerous scales, each derived from an individual dental papilla; teeth are primitively patterned such that they are replaced in a classical conveyor-belt system. The paleontological record provides a unique but complementary perspective in that: 1) the organisms in which the skeletal system evolved are extinct and we have no recourse but to fossils if we aim to address this problem; 2) extinct organisms can be classified among, and in the same way as, living relatives; 3) a holistic approach to the incorporation of all data provides a more complete perspective on early vertebrate evolution. This combined approach is of no greater significance than in dealing with the origin of the skeleton and, combined with recent discoveries and new phylogenetic analyses, we have been able to test and reject existing hypotheses for the origin of the skeleton and erect a new model in their place.

Segmental relationship between somites and vertebral column in zebrafish

The segmental heritage of all vertebrates is evident in the character of the vertebral column. And yet, the extent to which direct translation of pattern from the somitic mesoderm and de novo cell and tissue interactions pattern the vertebral column remains a fundamental, unresolved issue. The elements of vertebral column pattern under debate include both segmental pattern and anteroposterior regional specificity. Understanding how vertebral segmentation and anteroposterior positional identity are patterned requires understanding vertebral column cellular and developmental biology. In this study, we characterized alignment of somites and vertebrae, distribution of individual sclerotome progeny along the anteroposterior axis and development of the axial skeleton in zebrafish. Our clonal analysis of zebrafish sclerotome shows that anterior and posterior somite domains are not lineage-restricted compartments with respect to distribution along the anteroposterior axis but support a ‘leaky’ resegmentation in development from somite to vertebral column. Alignment of somites with vertebrae suggests that the first two somites do not contribute to the vertebral column. Characterization of vertebral column development allowed examination of the relationship between vertebral formula and expression patterns of zebrafish Hox genes. Our results support co-localization of the anterior expression boundaries of zebrafish hoxc6 homologs with a cervical/thoracic transition and also suggest Hox-independent patterning of regionally specific posterior vertebrae.