Cardiovascular disease typically is associated with dysfunction of the coronary vasculature and microvasculature. The study of cardiovascular disease typically involves imaging of the large coronary vessels and quantification of cardiac blood perfusion. These methods, however, are not well suited for imaging of the cardiac microvasculature. We used the optical histology method, which combines chemical optical clearing and optical imaging, to create high-resolution, wide-field maps of the cardiac microvasculature in ventral slices of mouse heart. We have demonstrated the ability of the optical histology method to enable wide-field visualization of the cardiac microvasculature in high-resolution and anticipate that optical histology may have significant impact in studying cardiovascular disease.

Abstract. Fluorescence microscopy is commonly used to investigate disease progression in biological tissues. Biological tissues, however, are strongly scattering in the visible wavelengths, limiting the application of fluorescence microscopy to superficial (<200 μm) regions. Optical clearing, which involves incubation of the tissue in a chemical bath, reduces the optical scattering in tissue, resulting in increased tissue transparency and optical imaging depth. The goal of this study was to determine the time-and wavelength-resolved dynamics of the optical scattering properties of rodent brain after optical clearing with FocusClear™. Light transmittance and reflectance of 1-mm mouse brain sections were measured using an integrating sphere before and after optical clearing and the inverse adding doubling algorithm used to determine tissue optical scattering. The degree of optical clearing was quantified by calculating the optical clearing potential (OCP), and the effects of differing OCP were demonstrated using the optical histology method, which combines tissue optical clearing with optical imaging to visualize the microvasculature. We observed increased tissue transparency with longer optical clearing time and an analogous increase in OCP. Furthermore, OCP did not vary substantially between 400 and 1000 nm for increasing optical clearing durations, suggesting that optical histology can improve ex vivo visualization of several fluorescent probes.

Cerebral amyloid angiopathy (CAA) is a neurovascular disease that is strongly associated with an increase in the number and size of spontaneous microbleeds. Conventional methods of magnetic resonance imaging for detection of microbleeds, and positron emission tomography with Pittsburgh Compound B imaging for amyloid deposits, can separately demonstrate the presence of microbleeds and CAA in affected brains in vivo; however, there still is a critical need for strong evidence that shows involvement of CAA in microbleed formation. Here, we show in a Tg2576 mouse model of Alzheimer's disease, that the combination of histochemical staining and an optical clearing method called optical histology, enables simultaneous, co-registered three-dimensional visualization of cerebral microvasculature, microbleeds, and amyloid deposits. Our data suggest that microbleeds are localized within the brain regions affected by vascular amyloid deposits. All observed microhemorrhages (n = 39) were in close proximity (0 to 144 µm) with vessels affected by CAA. Our data suggest that the predominant type of CAA-related microbleed is associated with leaky or ruptured hemorrhagic microvasculature. The proposed methodological and instrumental approach will allow future study of the relationship between CAA and microbleeds during disease development and in response to treatment strategies.