Intravital Multiphoton Imaging of the Bone and Bone Marrow Environment

Abstract

Over the last two decades, numerous advances in our understanding of bone cell biology and bone mineral homeostasis have been achieved. As a dynamic connective and supportive tissue, bone is constantly sensing and responding to both external mechanical forces and internal systemic and local signals. A variety of intravital imaging approaches have been investigated to identify molecular and cellular processes and to decipher signaling pathways involved in the cellular communication between different types of bone cells that form bone multicellular units. Furthermore, bone multicellular units interact with cells of the immune and hematopoietic system to maintain bone homeostasis. Bone-forming osteoblasts and bone-degrading osteoclasts are situated on the endosteal surface of bone influencing the dynamic remodeling and the regeneration of bone tissue. Osteocytes are found at very unique locations in the bone, closely surrounded by bone matrix, forming a cellular network through their interconnected dendritic processes. Bone marrow cells fill the numerous large cavities inside the bones with various blood cell lineages arising from hematopoietic stem and progenitor cells. A highly complex and interconnected network of arterial vessels and sinusoidal capillaries span through the bone marrow spaces forming an interface between the blood circulation and the bone marrow which allows cell trafficking between both compartments. Live imaging of animals using multiphoton microscopy represents a powerful approach to address the cellular behaviors of bone and bone marrow cells over time and space in their natural tissue microenvironment. The in vivo environment is crucial, because the dynamic behavior of cells is critically influenced by many tissue factors including extracellular components, cytokine and growth factor gradients, and fluid forces, such as blood flow. The review article focuses upon recent advances in multiphoton imaging technologies as well as novel experimental approaches in the understanding of the dynamic molecular and cellular mechanisms underlying bone tissue homeostasis, remodeling, and regeneration under physiological and pathological conditions.

Inside Out Integrin Activation Mediated by PIEZO1 Signaling in Erythroblasts

“…The consequences of these forces on haematopoiesis are beginning to emerge. For instance, it has been shown that hydrostatic pressure improves the clonogenic potential and CD34+ cell number in ex-vivo cultures (Kim and Bixel, 2020). PIEZO1 may be one of the sensors that could be involved in perceiving these forces and integrating them into a cellular response, for instance through integrin activation as shown here.…”

“…The semi-solid bone marrow is occupied with a heterogeneous pool of cells that will perceive and induce particular cellular programs upon shear forces from fluid movements, tensile strain, hydrostatic pressure and other physical forces (Meyer et al, 2016;Kim and Bixel, 2020). The consequences of these forces on haematopoiesis are beginning to emerge.…”

Osteoimmunology drives dental implant osseointegration: A new paradigm for implant dentistry

“…In turn, the bone marrow is the natural reservoir of MSCs. These cells maintain a stable bone mass and then bone homeostasis through life by differentiating to the osteoblastic lineage, able to secrete bone matrix [ 13 , 33 , 40 ]. Beside it, MSCs can differentiate into the chondrogenic and adipogenic lineages [ 3 ].…”

“…These interactions include heterogeneous cell populations (immune cells, bone, and vascular cells) [ 10 , 11 ]. The specific interaction between immune and bone cells is called the “osteoimmune system” [ [12] , [13] , [14] , [15] ]. In turn, osteoimmunology has become a fundamental discipline for the study of several inflammatory diseases, the crosstalk between bone and the immune system, and the influence of the immune system on bone regeneration and consumption [ 7 , 16 , 17 ].…”

Intravital imaging of orthotopic and ectopic bone

“…Therefore, there are only limited areas where this imaging can be applied. However, the recently developed three-photon microscopy substantially improves the penetration depth and is expected to overcome these limitations with its resolution [ 35 ]. Moreover, because of the wide scattering of light, it is difficult to visualize inflamed tissues with effusion or tissues with bleeding using multiphoton microscopy.…”