Diffuse Optical Spectroscopy and Imaging to Detect and Quantify Adipose Tissue Browning

Abstract

Adipose (fat) tissue is a complex metabolic organ that is highly active and essential. In contrast to white adipose tissue (WAT), brown adipose tissue (BAT) is deemed metabolically beneficial because of its ability to burn calories through heat production. The conversion of WAT-resident adipocytes to “beige” or “brown-like” adipocytes has recently attracted attention. However, it typically takes a few days to analyze and confirm this browning of WAT through conventional molecular, biochemical, or histological methods. Moreover, accurate quantification of the overall browning process is not possible by any of these methods. In this context, we report the novel application of diffuse reflectance spectroscopy (DRS) and multispectral imaging (MSI) to detect and quantify the browning process in mice. We successfully demonstrated the time-dependent increase in browning of WAT, following its induction through β-adrenergic agonist injections. The results from these optical techniques were confirmed with those of standard molecular and biochemical assays, which measure gene and protein expression levels of UCP1 and PGC-1α, as well as with histological examinations. We envision that the reported optical methods can be developed into a fast, real time, cost effective and easy to implement imaging approach for quantification of the browning process in adipose tissue.

Non-invasive investigation of adipose tissue by time domain diffuse optical spectroscopy

“…Optical techniques have obtained interesting initial results suggesting their high potential for the non-invasive investigation of adipose tissue. For instance, initial attempts have demonstrated the usability of diffuse optical spectroscopy to discriminate white from brown fat on mice [12,13]. The increased activity of mitochondria in brown fat could provide a spectral absorption and scattering signature to track adipose tissue transformation.…”

Diffuse reflectance spectroscopy accurately discriminates early and advanced grades of fatty liver in mice

“…24,29 A reduction of the absolute slope in the range of 500 to 550 nm is also noticeable as steatosis grows (−0.62 nm −1 for MCC, −0.30 nm −1 for MCD2w, and −0.04 nm −1 for MCD8w) that could be used as a differentiation parameter. 30 However, the uncertainties of the slope values are high, increasing with the tissue fat content for the MCD8w group. This result is consistent with the observed histological images, such as the ones depicted in Fig.…”

Elucidating the contribution of Rayleigh scattering to the bluish appearance of veins

“…[40][41][42][43] Due to the strong absorptive properties of cutaneous melanin in the blue region of the light spectrum, only a negligible amount of light in this region is likely to reach the hypodermis. 25 Experiments by Dinish et al 44 show that 90% to 95% of light in the red and green regions of the light spectrum are reflected by white adipose tissue. Accordingly, the majority of the visible light that reaches the hypodermis is likely to be scattered by the droplets of lipids and remitted into the reticular dermis.…”

“…2.3, that the amount of light in the blue region reaching this layer is negligible due to the cutaneous pigmentation 25 and most of the light in the green and red regions is reflected back. 44 Thus, by reflecting the visible light that may reach the hypodermis, a strategy also employed by several relevant works on skin color reproduction (e.g., the interested reader is referred to the works by Cotton and Claridge 10 and by Doi and Tominaga 52 ), one can minimize the introduction of undue computational overhead in the simulations without reducing the fidelity of the simulations. Accordingly, we assume that visible light reaching the hypodermis is diffusely reflected back into the reticular dermis.…”