Label-free measurement of wall shear stress in the brain venule and arteriole using dual-wavelength third-harmonic-generation line-scanning imaging

Polarization multiplexed 2200-nm soliton generation and its application to 3-photon microscopy

The 2200-nm window has recently been demonstrated as the longest excitation window for deep-tissue multiphoton microscopy (MPM). So far, MPM at this window exclusively uses a soliton laser source based on soliton self-frequency shift (SSFS). In order to boost the multiphoton signal level at this window, here we demonstrate a polarization multiplexed soliton source based on orthogonal polarized SSFS in a polarization maintaining large mode area (PM LMA) fiber. Using this technology, the effective pulse repetition rate, i.e., the pulse number per unit time, is twice higher than the 1550-nm pump laser with a fixed repetition rate. We further demonstrate comparative third harmonic generation (THG) and 3-photon fluorescence (3PF) imaging with this polarization multiplexed soliton source and single polarization soliton source. The ∼2 times higher 3-photon signal of the former indicates that this soliton multiplexing technology is promising for MPM at the 2200-nm window.

Tension at the gate: sensing mechanical forces at the blood-brain barrier in health and disease

Microvascular brain endothelial cells tightly limit the entry of blood components and peripheral cells into the brain by forming the blood-brain barrier (BBB). The BBB is regulated by a cascade of mechanical and chemical signals including shear stress and elasticity of the adjacent endothelial basement membrane (BM). During physiological aging, but especially in neurological diseases including multiple sclerosis (MS), stroke, small vessel disease, and Alzheimer's disease (AD), the BBB is exposed to inflammation, rigidity changes of the BM, and disturbed cerebral blood flow (CBF). These altered forces lead to increased vascular permeability, reduced endothelial reactivity to vasoactive mediators, and promote leukocyte transmigration. Whereas the molecular players involved in leukocyte infiltration have been described in detail, the importance of mechanical signalling throughout this process has only recently been recognized. Here, we review relevant features of mechanical forces acting on the BBB under healthy and pathological conditions, as well as the endothelial mechanosensory elements detecting and responding to altered forces. We demonstrate the underlying complexity by focussing on the family of transient receptor potential (TRP) ion channels. A better understanding of these processes will provide insights into the pathogenesis of several neurological disorders and new potential leads for treatment.

Comparison of the penetration depth in mouse brain in vivo through 3PF imaging using AIE nanoparticle labeling and THG imaging within the 1700 nm window

3-Photon microscopy (3PM) excited at the 1700 nm window features a smaller tissue attenuation and hence a larger penetration depth in brain imaging compared with other excitation wavelengths in vivo. While the comparison of the penetration depth quantified by effective attenuation length le with other excitation wavelengths have been extensively investigated, comparison within the 1700 nm window has never been demonstrated. This is mainly due to the lack of a proper excitation laser source and characterization of the in vivo emission properties of fluorescent labels within this window. Herein, we demonstrate detailed measurements and comparison of le through the 3-photon imaging of the mouse brain in vivo, at different excitation wavelengths (1600 nm, 1700 nm, and 1800 nm). 3PF imaging and in vivo spectrum measurements were performed using AIE nanoparticle labeling. Our results show that le derived from both 3PF imaging and THG imaging is the largest at 1700 nm, indicating that it enables the deepest penetration in brain imaging in vivo.

The Cousa objective: a long-working distance air objective for multiphoton imaging in vivo

Multiphoton microscopy can resolve fluorescent structures and dynamics deep in scattering tissue and has transformed neural imaging, but applying this technique in vivo can be limited by the mechanical and optical constraints of conventional objectives. Short working distance objectives can collide with compact surgical windows or other instrumentation and preclude imaging. Here we present an ultra-long working distance (20 mm) air objective called the Cousa objective. It is optimized for performance across multiphoton imaging wavelengths, offers a more than 4 mm2 field of view with submicrometer lateral resolution and is compatible with commonly used multiphoton imaging systems. A novel mechanical design, wider than typical microscope objectives, enabled this combination of specifications. We share the full optical prescription, and report performance including in vivo two-photon and three-photon imaging in an array of species and preparations, including nonhuman primates. The Cousa objective can enable a range of experiments in neuroscience and beyond.