Atiq Durrani

Intercellular signaling pathways active during intervertebral disc growth, differentiation, and aging.

Study Design. Intervertebral discs at different postnatal ages were assessed for active intercellular signaling pathways. Objective. To generate a spatial and temporal map of the signaling pathways active in the postnatal intervertebral disc (IVD). Summary of Background Data. The postnatal IVD is a complex structure, consisting of 3 histologically distinct components, the nucleus pulposus, fibrous anulus fibrosus, and endplate. These differentiate and grow during the first 9 weeks of age in the mouse. Identification of the major signaling pathways active during and after the growth and differentiation period will allow functional analysis using mouse genetics and identify targets for therapy for individual components of the disc. Methods. Antibodies specific for individual cell signaling pathways were used on cryostat sections of IVD at different postnatal ages to identify which components of the IVD were responding to major classes of intercellular signal, including sonic hedgehog, Wnt, TGFβ, FGF, and BMPs. Results. We present a spatial/temporal map of these signaling pathways during growth, differentiation, and aging of the disc. Conclusion. During growth and differentiation of the disc, its different components respond at different times to different intercellular signaling ligands. Most of these are dramatically downregulated at the end of disc growth.

Postnatal Growth, Differentiation, and Aging of the Mouse Intervertebral Disc

Study Design. This study follows postnatal intervertebral disc (IVD) growth and differentiation in the mouse. Objective. To initiate use of the mouse as a model system for postnatal IVD differentiation and growth, and to serve as a basis for assaying changes caused by disease or genetic or experimental perturbation. Summary of Background Data. Lower back pain caused by disc degeneration is one of the most common clinical conditions worldwide. There is currently no completely effective treatment, largely because of a lack of basic knowledge of the molecular and cellular controls of disc growth, differentiation, and maintenance after birth. Methods. Conventional histology of decalcified IVDs, differential interference contrast, polarizing optics, immunocytochemistry, laser capture microscopy followed by molecular analysis of the dissected cells by reverse transcriptase polymerase chain reaction. Results. There is a single postnatal growth spurt in the mouse IVD, between birth and 9 weeks of age. Cell proliferation was found in the nucleus pulposus (NP) and anulus fibrosus (AF) only until 3 weeks of age. Most of the postnatal growth of the IVD is due to accumulating extracellular matrix. NP cell numbers decline steadily after 2 weeks of age, because of apoptosis. Laser capture microscopy was used to dissect NP cells from the disc, and showed that these cells express markers of the embryonic notochord. The postnatal AF appears initially as a continuous structure surrounding the NP. This structure differentiates, during the first 2 postnatal weeks, to form the mineralized, but nonossified endplate over the surfaces of the vertebral growth plates, and the mature fibrous AF (fAF) passing between adjacent vertebrae. The fact that the mature fAF and the endplate form from an originally continuous layer of cells explains the anatomic relationship between these 2 structures, in which the fAF inserts into the vertebral endplate. Conclusion. Growth of the IVD takes place during the first 9 postnatal weeks, although cell proliferation ceases after 3 weeks. After birth, the early postnatal IVD differentiates into 3 tissue types, the NP, the fAF between the vertebrae, and the mineralized endplates over the surfaces of the vertebrae.

Could junctional problems at the end of a long construct be addressed by providing a graduated reduction in stiffness? A biomechanical investigation.

Study Design. The effect of long, rigid fixation on adjacent level hypermobility was investigated in a human cadaver model with and without a transitional posterior dynamic stabilization (PDS) device placed at the last caudal level. Objective. To evaluate if PDS devices are useful in the setting of spinal deformities to restore increased adjacent level motions, which occur in long constructs. The hypothesis is that load-sharing benefits of these devices will be most suitable in long constructs and may reduce thoracolumbar junctional effects. The PDS device evaluated has a compressive spacer and flexion-dampening bumper. Summary of Background Data. Mechanical factors such as excessive mobility, increased disc height due to instrumentation, and abnormal loading are thought to accentuate distal level problems, which occur in extended instrumentation. Specifically adjacent level degeneration and distal junctional kyphosis are known to occur in these cases. Methods. Seven cadaver spines were tested from T7 to L3. Long instrumentation was applied in 2 rigid groups, R1: Rigid (T8–L2) and R2: Rigid (T8–L1), and PDS to the last caudal level of each, RP1: Rigid (T8–L1) + PDS (L1–L2), and RP2: Rigid (T8–T12) + PDS (T12–L1). Range of motion was evaluated at surgical and distal adjacent levels after displacement controlled loading in a spine tester. Results. Distal adjacent level motion was increased after 5- and 6-level rigid fixation in flexion-extension, lateral bending, and axial rotation. Most of the increases were seen in axial rotation and lateral bending. Replacing the last caudal instrumented level with the PDS test device was able to alleviate hypermobile conditions of the adjacent noninstrumented level, closer to intact (24%, 12% reduction in RP2, RP1, respectively). Conclusion. Reduction of hypermobility caused by extended arthrodesis may represent a new and ideally suited function for PDS devices. Mechanically, the devices were seen to kinematically restore abnormal distal motion, especially with placement of the PDS at the thoracolumbar junction.

Intercellular signaling pathways active during and after growth and differentiation of the lumbar vertebral growth plate.

Study Design. Vertebral growth plates at different postnatal ages were assessed for active intercellular signaling pathways. Objective. To generate a spatial and temporal map of the major signaling pathways active in the postnatal mouse lumbar vertebral growth plate. Summary of Background Data. The growth of all long bones is known to occur by cartilaginous growth plates. The growth plate is composed of layers of chondrocyets that actively proliferate, differentiate, die and, are replaced by bone. The role of major cell signaling pathways has been suggested for regulation of the fetal long bones. But not much is known about the molecular or cellular signals that control the postnatal vertebral growth plate and hence postnatal vertebral bone growth. Understanding such molecular mechanisms will help design therapeutic treatments for vertebral growth disorders such as scoliosis. Methods. Antibodies against activated downstream intermediates were used to identify cells in the growth plate responding to BMP, TGF&bgr;, and FGF in cryosections of lumbar vertebrae from different postnatal age mice to identify the zones that were responding to these signals. Reporter mice were used to identify the chondrocytes responding to hedgehog (Ihh), and Wnt signaling. Results. We present a spatial/temporal map of these signaling pathways during growth, and differentiation of the mouse lumbar vertebral growth plate. Conclusion. During growth and differentiation of the vertebral growth plate, its different components respond at different times to different intercellular signaling ligands. Response to most of these signals is dramatically downregulated at the end of vertebral growth.