## 1 Introduction  
Stem-cell-based therapies have the potential of becoming the future of medicine [1], personalised to the patient [2]. Embryonic stem cells (ESCs) have the highest differentiation potential, offering limitless possibilities for tissue replacement. However, sourcing embryonic stem cells poses ethical concerns, and current UK regulations restrict their availability to the first 14 days, so a more plentiful source of stem cells would be necessary for wide-scale uptake of stem cell therapy. Therefore, the more readily available adult stem cell populations, such as mesenchymal stem cells (MSCs), are of great interest when new therapies and technologies are developed [3]. MSCs derived from the bone marrow niche are multipotent, capable of differentiating into osteoblasts (bone), chondroblasts (cartilage), and adipoblasts (fat) [4]. These precursor cells are essential to the formation, maintenance, and repair of the skeleton. MSCs are identified by several cell surface markers including the presence of CD105, CD90, and CD73 and the absence of CD45, CD34, CD14, CD19, and HLA-DR [5]. MSCs grown in monolayer will typically begin to lose differentiation potential from around the 6 th passage, demonstrating the vital role cell signalling from other supporting cell types within the bone marrow microenvironment plays in maintaining stemness [6,7]. Within the bone marrow microenvironment, the MSC niche can be subdivided into the endosteal and the perivascular niches, where MSC populations are maintained by, and communicate via cytokines, with other cells, such as hematopoietic stem cells (HSCs) and later precursor cells such as osteoblasts and adipoblasts [8, 9, 10]. Upon appropriate signalling cues, MSCs mobilise and migrate out from the niche [11]. MSC mobilisation is often due to either (1) homing to sites of injury [12,13] or (2) homing to disseminated tumour cells [14]--both are of key interest to researchers in terms of regenerative medicine (i.e., replacement of lost skeletal tissue) and potential therapeutics for cancer. To date the precise mechanism of mobilisation remains poorly understood. To gain a deeper understanding of MSC behaviour within the bone marrow microenvironment, it is important to develop more accurate in vitro model systems. One key approach to bring this to fruition is the development of three-dimensional (3D) cell culture model systems. Unlike standard two-dimensional (2D) monolayer cultures, 3D models display nutrient and oxygen gradients, providing a more physiological representation. In addition to this, a more in vivo-like cell behaviour is encouraged, leading to longer culture life spans offering a useful platform for more accurate assessment of drug efficacy during cell cytotoxicity tests [15, 16, 17]. The 3D culture adopted in this study utilises MSC internalisation of magnetic nanoparticles (mNPs), prior to levitation in response to an external magnetic field (280 mT), which coerces magnetically labelled MSCs together to form cell aggregates or spheroids. It has been confirmed in multiple studies [18, 19, 20] that mNPs do not adversely affect cells and their presence allows cell imaging to be done quickly, without fixation and subsequent staining, as the mNPs are conjugated to, in this case, fluorescein molecules. The spheroids are then cultured within type 1 collagen gels (Figure 1), where MSC spheroids display stem cell-like properties, with cells entering a quiescent state, maintaining a population of multipotent cells capable of responding to modelled wound sites by migration and differentiation [18]. Injury or inflammation cause cellular release of cytokines, transforming growth factor-8 (TGF-8), tumour necrosis factor-\(\alpha\) (TNF-\(\alpha\)), several interleukins (IL) such as IL-1, IL-6, and IL-10, and the interferons, which all play an important role in controlling the immune response [21]. These cytokines also stimulate the recruitment of MSCs where they initiate tissue repair at the site of the wound [22].