Flow Dynamics and HSPC Homing in Bone Marrow Microvessels

Abstract

Measurements of flow velocities at the level of individual arterial vessels and sinusoidal capillaries are crucial for understanding the dynamics of hematopoietic stem and progenitor cell homing in the bone marrow vasculature. We have developed two complementary intravital two-photon imaging approaches to determine blood flow dynamics and velocities in multiple vessel segments by capturing the motion of red blood cells. High-resolution spatiotemporal measurements through a cranial window to determine short-time dynamics of flowing blood cells and repetitive centerline scans were used to obtain a detailed flow-profile map with hemodynamic parameters. In addition, we observed the homing of individual hematopoietic stem and progenitor cells and obtained detailed information on their homing behavior. With our imaging setup, we determined flow patterns at cellular resolution, blood flow velocities and wall shear stress in small arterial vessels and highly branched sinusoidal capillaries, and the cellular dynamics of hematopoietic stem and progenitor cell homing.

Keywords: blood flow velocities; bone marrow; deep tissue imaging; hematopoietic parameters; hematopoietic stem cell; intravital imaging; microvasculature; stem cell homing; wall shear stress.

Graphical abstract

Figure 1

Experimental Setup and Procedure for Intravital Imaging of the Skull BM Microvasculature (A) Custom-made mouse imaging setup with a head immobilization device using a titanium ring glued on the mouse skull and a skull holder fixed on a custom-made plate. The skull holder eliminates movement artifacts during imaging. (B) The skin (s) is removed over the circular area of interest. Both a 6-mm round glass coverslip (cs) and a titanium fixation ring (fr) are mounted on the skull (sk) with dental acrylic (da). A circular area inside the ring (typically ∼3–4 mm in diameter) can be visualized using two-photon laser imaging reaching an imaging depth of ∼250–300 μm, bone marrow (bm), calcified bone (b). (C) Cross-section showing GFP⁺ (green) endothelium and SHG⁺ (blue) bone tissue in calvarium (frontoparietal bone) of Flk1-GFP transgenic mice. (D–I) Representative xy images collected at different imaging depths showing bone vessels in the outer compact bone (D and E), sinusoidal capillaries in the BM cavities with trabecular bone (F–H), and inner compact bone with few bone vessels (I). (J) Image showing maximum intensity projection of GFP⁺ bone vessels and BM sinusoidal capillaries. Corresponding 3D representation is shown in Movie S1. (K) Schematic showing curved shape of calvarial bone with compact bone (cp), trabecular bone (tb), and BM cavities (bmc).

Figure 2

Identification of Bone Vessel Subtypes in the BM Compartment with Arterial and Sinusoidal Vessel Markers (A) Maximum intensity projection of tile scans showing interconnecting network of Emcn⁺ arterial vessels and sinusoidal capillaries. (B) Maximum intensity projection of tile scans showing CD31^hi (red) GFP⁺ (green) arterioles and CD31^low sinusoids of Efnb2-GFP mouse calvarium. (C) Emcn^low arteriole (arrowhead) connects to Emcn^hi sinusoidal capillaries (arrows). (D) Both arterioles (arrowhead) and sinusoidal capillaries (arrows) are GFP⁺ (green) in Flk1-GFP mice. (F and G) CD31^hi (red) Efnb2-GFP⁺ (green) arterioles (arrowheads) connect to BM microdomains formed by interconnecting CD31^low sinusoidal capillaries (arrows). (E and H) SHG signals (blue) indicate bone tissue surrounding the BM cavities. Note that one single plane for SHG is shown. Due to the curved shape of the skull SHG planes were slightly tilted.

Figure 3

Microvascular Blood Flow Dynamics in the BM Compartment (A) In vivo two-photon imaging of arterial vessels and sinusoidal capillaries using intravenously injected TexasRed dextran (red) and real-time movies of regional microvascular blood flow dynamics (boxed areas 1–4). (B–E) Blood flow dynamics in arterial vessel and down-stream sinusoidal vessel segments were recorded (B). Representative image sequences of vessel segments 1–4; boxed areas in (A) are shown in (C)–(E). Arrowheads (A) and arrows (B) indicate direction of blood flow. Corresponding real-time movies are shown in Movies S2, S3, and S4. Data are from one representative animal, n = 4 animals.

Figure 4

Microvascular Blood Flow Profile in the BM Compartment (A and B) In vivo two-photon imaging of the BM compartment showing the microarchitecture of blood vessels (red) and GFP⁺ arterial vessels (green) in Efnb2-GFP mice. (C and D) Microvascular blood flow velocities of depicted vessel segments were determined using repetitive line-scans. Note that arterial vessels of small caliber (segments 1–3) progressively transform into post-arterial (segments 4–6), intermediate (segments 7 and 8), and thin-walled sinusoidal capillaries with a wider lumen (segments 9–19). Sinusoidal capillaries finally drain blood into a larger collecting venule (segments 20–22). Arrowheads point to cells adhering to the endothelium (C). Color coding in (D): red, arterial vessel; light red, post-arterial vessel; gray, intermediate capillary; light blue, sinusoidal capillaries, blue, collecting vein. Bars represent mean values ± SD of one representative animal, 10–22 vessels segments per animal, n = 4 animals.

Figure 5

Microvascular Blood Flow Profile of a Terminal BM Compartment (A and C) In vivo two-photon imaging of a terminal BM compartment showing an arterial and post-arterial vessel (segments 1–3) connecting to an intermediate capillary (segment 4) that drains into a sinusoidal capillary (segments 6 and 7) and an interconnected network of downstream sinusoidal capillaries (segments 8–16) visualized by intravenous (i.v.) injection of TexasRed-dextran (red). GFP⁺ (green) labels the endothelium in Flk1-GFP transgenic mice. (B) SHG signals (blue) indicate calcified bone tissue surrounding the BM cavities. (D) Schematic showing principle of line-scan measurements for blood flow measurements (top). Representative line scans from six vessel segments (bottom). (E and F) Blood flow velocities (E) and RBC volume flow (F) of 16 vessel segments indicated in (C). Color coding in (E) and (F): red, arterial vessel; light red, post-arterial vessel; gray, intermediate capillary; light blue, sinusoidal capillary; blue, collecting vein. Bars represent mean values ± SD from one representative animal, 11–16 vessels segments per animal, n = 4 animals.

Figure 6

Homing of HSPCs to BM Sinusoidal Capillaries (A) Micrograph showing velocities of individual HSPC tracks during rolling and transient adhesion, slow rolling and adhesion, slow migration, and transmigration 0.5–3 hr after intravenous injection of fluorescently labeled HSPCs. After 24 hr, HSPC reside in the BM cavity in proximity to sinusoidal capillaries. Zoom-in view is on the right. (B–D) Velocity-time graphs of two representative tracks for rolling and transient adhesion (B, T1 and T2 in A showing slow rolling and adhesion; C, T3 and T4 in A showing slow rolling and adhesion; D, T5 and T6 in A showing slow migration). Bars represent mean values ± SD, 15 vessels segments were analyzed from n = 7 animals. (E and F) HSPC transmigration and local cellular interaction in the BM compartment. (E) Fluorescently labeled BM lineage-depleted HSPC (green) transmigrated across the Enmc⁺ endothelium of BM sinusoids (red) after rolling and a short adhesion phase as visualized by in vivo two-photon imaging; time in minutes (Movie S9). Red arrowheads follow luminal adhering HSPC to a perivascular position close to the sinusoidal endothelium. Yellow arrowheads point toward adhering or slowly migrating HSPCs. Note that transmigration event is initiated by a single protrusion that projects through the endothelium. Within ∼1.5 min, the HSPC translocates its cell body through the endothelium to a perivascular position in the BM cavity. Note that two transmigration events from two animals are reported (Figure S3C). (F) Several HSPCs (green) reside in the BM compartment in close proximity to Enmc⁺ sinusoidal capillaries (red). Most HSPCs reside stationary or migrate very slowly and show local cell membrane protrusions behavior (arrows) indicating active cell interactions; time is in minutes (Movie S10). (G) Schematic illustrating the multistep process of HSPC homing with mean velocities for individual steps. Bars represent mean values ± SD, 15 vessels segments were analyzed from n = 7 animals.