Bioengineering the Bone Marrow Vascular Niche

Abstract

The bone marrow (BM) tissue is the main physiological site for adult hematopoiesis. In recent years, the cellular and matrix components composing the BM have been defined with unprecedent resolution, both at the molecular and structural levels. With the expansion of this knowledge, the possibility of reproducing a BM-like structure, to ectopically support and study hematopoiesis, becomes a reality. A number of experimental systems have been implemented and have displayed the feasibility of bioengineering BM tissues, supported by cells of mesenchymal origin. Despite being known as an abundant component of the BM, the vasculature has been largely disregarded for its role in regulating tissue formation, organization and determination. Recent reports have highlighted the crucial role for vascular endothelial cells in shaping tissue development and supporting steady state, emergency and malignant hematopoiesis, both pre- and postnatally. Herein, we review the field of BM-tissue bioengineering with a particular focus on vascular system implementation and integration, starting from describing a variety of applicable in vitro models, ending up with in vivo preclinical models. Additionally, we highlight the challenges of the field and discuss the clinical perspectives in terms of adoptive transfer of vascularized BM-niche grafts in patients to support recovering hematopoiesis.

Keywords: bioengineering; bioprinting; bone marrow; endothelial cells; manufacturing; microfluidics; organoid; vascular niche.

FIGURE 1

Bone marrow vascular structure. (A) Scheme of the organization of BM vasculature at different magnifications. The BM vasculature is highly interconnected with the tissue matrix, with small arterioles covered by pericytes entering the marrow from the bone and merging with the venous system via a network of sinusoidal capillaries. The vascularized marrow space is populated by hematopoietic and stromal cells embedded in a unique extracellular matrix. (B) Representative Z-stack two-photon microscopy image of vascular organization in the mouse calvarial bone marrow, with Nestin-GFP peri-arteriolar pericytes (green), TRITC-dextran labeled vascular lumen (red) and second harmonic generation (SHG) bone matrix (blue).

FIGURE 2

Manufacturing vascularized BM systems. (A) Implantation of mesenchymal and endothelial cell-loaded hydrogels, sponges or printed scaffolds subcutaneously in immunodeficient mice. The resulting vasculature will be anastomosed to mouse host vasculature. (B) Generation of a hollow tube in a 3D matrix through the use of sacrificial material; the channel walls can be seeded with ECs afterward. (C) Cell-loaded hydrogel islets are seeded with a layer of ECs and placed in a microfluidic channel with a continuous flow, forming an inverted vascularized system (Khan et al., 2012). (D) Formation of organ-on-chip systems with two parallel hollow tubes separated by a porous membrane, one containing tissue specific cells and the other coated with ECs and perfused with medium to serve as a functional vascular system (Huh et al., 2013). (E) Generation of self-assembled and perfusable vascular network in a microfluidic chip (see dedicated Box 1).

FIGURE 3

Evolution of system complexity over time. Evolution in the organization complexity of engineered BM models with: (A) 2D co-cultures (feeder cells), (B) 3D co-culture within a hydrogel, (C) introduction of a source for medium renewal with either perfusion or the creation of a hollow tube, thus generating compartments within the tissue, (D) introduction of vasculature (self-assembled or engineered) bringing contact-dependent interaction in addition to compartmentalization, (E) introduction of a bone environment generating a segmented endosteal and vascular regions, with different ECM and cellular contents as well as different physical properties.

FIGURE 4

Bioengineered vascularized BM niche unit. Schematic representation of the essential elements to be reproduced in an engineered vascularized BM niche-on-a-chip. These would include different matrix for bone and marrow space, as well as an intricate net of specialized vessels in close proximity with stromal and hematopoietic components.