|
The ability of optical imaging techniques such as optical coherence tomography (OCT) to non-destructively
characterize tissue-engineered constructs has generated enormous interest recently. Collagen gels are 3D structures that
represent a simple common model of many engineered tissues that contain 2 primary scatterers: collagen and cells. We
are testing the ability of OCT data to characterize the remodeling of such collagen-based constructs by 3 different types
of cells: vascular smooth muscle cells (SMCs), endothelial cells (ECs), and osteoblasts (OBs). Collagen gels were
prepared with SMCs, ECs, and OBs with a seeding density of 1×106 cells/ml; additionally, acellular controls were also
prepared. The disk-shaped constructs were allowed to remodel in the incubator for 5 days, with OCT imaging occurring
on days 1 and 5. From the OCT data, the attenuation and reflectivity were evaluated by fitting the data to a theoretical
model that relates the tissue optical properties (scattering coefficient and anisotropy factor) and imaging conditions to
the OCT signal. The degree of gel compaction was determined from the volume of the culture medium that feeds the
constructs. We found that gel compaction (relative to the acellular control) occurred in the SMC constructs, but not in
the OB or EC constructs. The optical property data showed that at day 5 the SMC constructs had an overall higher
reflectivity (lower g) relative to day 1, whereas there was no obvious change in reflectivity of the EC, OB constructs
and acellular controls relative to day 1. Moreover, there was a difference in the attenuation of the OB constructs on day
5 relative to day 1, but not in the other constructs. The apparent decrease in anisotropy observed in the SMC constructs,
but not in the OB and EC constructs and acellular controls, suggests that OCT is sensitive to the remodeling of the
collagen matrix that accompanies gel compaction, and can offer highly localized information on the construct
microstructure. The apparent increase in the scattering coefficient of the OB constructs is believed to be caused by a
higher rate of proliferation by these cell types relative to the others. Overall, these results suggest that the optical
properties of collagen gels contain information on both cell number and collagen gel microstructure.
|
