The purpose of this research was to explore the in situ anatomic and
mechanical environment of disc cells. Laser scanning confocal microscopy was
used to characterize three-dimensional morphology of intervertebral disc
cells, micromechanical deformation and interaction with extracellular matrix,
and functional intercellular communication. Bovine coccygeal discs were used
for both the anatomic and micromechanical investigations.
Anulus fibrosus cells had a complex morphology with sinuous processes woven
into the extracellular matrix, particularly in the outer aspect of the anulus
where they were also interconnected via functional gap junctions. They were
also found in an extensive pericellular matrix of type-VI collagen, joining as
many as ten cells into linear cell arrays that could be extracted from the
matrix as functional units. Mechanically, collagen fibril sliding was
demonstrated to govern cell mechanics and strain transfer in the anulus
fibrosus during loading activities. Lamellar cells were largely protected from
direct tensile strains in the matrix, with minimal intercellular strains.
However, intercellular strains between lamellar cells in adjacent arrays were
large, illustrating shearing between linear cell arrays. Appreciable shear was
observed across the lamellar cell bodies as well as to the cellular processes
woven into the matrix. These findings demonstrated the morphologic and
micromechanical complexity of anulus fibrosus cells. The knowledge of the in
situ environment of disc cells will provide a base to investigate the
mechanical implications of disc degeneration on the cellular environment and
to better understand how mechanical and genetic risk factors can impact the
cells that are essential to maintaining the intervertebral disc.