Osteoarthritis is widely recognised as a complex, multifactorial disease with a significant genetic predisposition. In canine populations, the manifestation and severity of osteoarthritis is also influenced by various lifestyle factors, including diet and physical activity levels. In veterinary contexts, osteoarthritis is frequently classified as secondary, emerging subsequent to primary joint abnormalities, such as dysplasia, cranial cruciate ligament rupture or patellar luxation, as well as primary trauma. These primary conditions are hypothesised to precipitate the onset of osteoarthritic changes. The precise epidemiological data regarding the proportion of canines that develop secondary osteoarthritis because of these predisposing factors remain undetermined.
The prevalence of osteoarthritis in dogs ranges from 6.6% (based on primary care data) to 20% (based on referral data) in the UK dog population. Estimates from North America report prevalence from 20% in dogs older than one year up to 80% in dogs older than eight years, based on radiographic and clinical data from referral clinics (Anderson et al, 2018).
Treatment options for canine osteoarthritis
Treating canine osteoarthritis often requires a multimodal approach. Stem cell techniques have been developed out of the necessity to enhance veterinary ability to best treat patients, where there is no simple solution to what is considered a chronic, complex disease process. Stem cell therapy may be used alone or in combination with a variety of other treatment modalities, which should be considered in every case. These include, but are not limited to:
Reviews and current evidence for the use of these modalities and treatment options are described by Pye et al, 2022; 2024. Several resources, such as the Aim-OA multimodal system (AIM OA, 2024) and a 5-stage grading system developed by Canine Arthritis Management (2024) are available to serve as guidance as to when to introduce one or more of these modalities. Table 1 summarises the stages of osteoarthritis and some treatment options. It may be helpful to note that management options for late-stage disease are limited (arthrodesis, joint replacement, amputation and euthanasia) and that treatment modalities should be tailored to each individual to prevent or delay the onset of late-stage disease.
| Disease stage | Main pathological changes | Focus for management | Management strategy |
|---|---|---|---|
| Pre-clinical | Cartilage only |
Prevention |
Diet |
| Early clinical | Cartilage |
Disease modification |
Additionally: Disease-modifying drugs |
| Established clinical | Established cartilage damage |
Pain management |
Additionally: |
| Late-stage | As the established clinical stage |
Chronic pain control |
Additionally: |
| End-stage | Immobile joint |
Reduce pain |
Surgical salvage (joint replacement/arthrodesis) |
A brief history of stem cell therapy
Stem cell therapy involves the use of stem cells to repair or replace damaged cells, tissues or organs, and has potential applications in a wide range of conditions. The history of stem cell therapy spans more than a century, with key discoveries contributing to its evolution as one of the most promising areas of regenerative medicine. Stem cell therapy is currently an active area of clinical research, including in areas such as Parkinson's disease, diabetes, heart disease, multiple sclerosis and Alzheimer's disease.
Three-dimensional bioprinting and tissue engineering using stem cells are emerging fields, with the potential to create replacement tissues and organs for transplantation. Gene editing technologies have further enhanced the possibilities for stem cell therapies by allowing precise modifications to stem cells before they are used therapeutically.
The history of stem cell therapy in dogs is closely tied to the progress made in human and equine medicine. Research into stem cell therapy has focused on mesenchymal stem cells, which are multipotent stromal cells capable of differentiating into various cell types, including bone, cartilage, muscle and fat. These cells were initially studied for their potential to treat musculoskeletal injuries in horses. By the early 2000s, stem cell treatments were being used in practice to treat equine tendon and ligament injuries, and this laid the groundwork for extending similar therapies to dogs for orthopaedic conditions.
Although stem cell therapy in dogs has primarily been used for orthopaedic conditions, research has expanded to other areas, such as cardiac conditions, kidney disease and autoimmune disorders. The potential of stem cell therapy in treating canine cognitive dysfunction has also been evaluated (Chandrasekaran et al, 2021). Emerging research has also focused on the use of stem cells to modulate the immune system in conditions like diabetes mellitus (Gooch et al, 2019), inflammatory bowel disease (Pérez-Merino et al, 2015) and atopic dermatitis (Villatoro et al, 2018), where the immunomodulatory properties of mesenchymal stem cells have shown promise in reducing inflammation and managing symptoms.
Categories of stem cells
Stem cells are classified based on their origin and potential to differentiate into various cell types (Table 2). There are four main categories of stem cells:
| Type | Origin | Potency | Application |
|---|---|---|---|
| Embryonic | Inner cell mass of blastocysts | Pluripotent | Research, cloning and regenerative therapies, though their use is ethically controversial |
| Adult | Found in various tissues of the adult body, such as bone marrow, fat, skin and brain | Multipotent, can develop into a limited range of cell types related to their tissue of origin | Bone marrow transplants and regenerative treatments for specific tissues |
| Induced pluripotent stem cells | Somatic (adult) cells that have been genetically reprogrammed to an embryonic stem cell-like state | Pluripotent | Disease modelling, drug testing and potential regenerative therapies, offering an alternative to embryonic stem cells without the associated ethical concerns |
| Perinatal stem cells | Derived from perinatal tissues, including the placenta, umbilical cord and amniotic fluid | Typically multipotent, although research suggests they may have pluripotent characteristics | Research and potential therapies, particularly in regenerative medicine |
| Mesenchymal stem cells | Commonly bone marrow, but also in fat and umbilical cord tissue | Multipotent, with the ability to differentiate into bone, cartilage, fat and muscle cells | Regenerative medicine, particularly in osteoarthritis and bone and cartilage repair, and for immunomodulatory properties. |
| Haematopoietic stem cells | Bone marrow, peripheral blood and umbilical cord blood | Multipotent, specifically giving rise to various types of blood cells | Bone marrow transplants for treating blood disorders such as leukaemia and lymphoma |
| Neural stem cells | Specific regions of the brain and spinal cord | Multipotent, capable of differentiating into neurons, astrocytes and oligodendrocytes | Research into neurodegenerative diseases and potential therapies for conditions like Parkinson's and spinal cord injuries |
| Epithelial stem cells | Epithelial tissues of the skin and intestine | Multipotent, contributing to the regeneration of epithelial layers | Wound healing and research into tissue regeneration |
| Trophoblast stem cells | Derived from the early embryo, specifically from the cells that wil form the placenta | Multipotent, with the potential to differentiate into various types of placental cells | Research into pregnancy and placental development |
Effects of stem cells
Mesenchymal stem cells have been extensively studied for their therapeutic potential because of their unique properties. The effects of mesenchymal stem cells in osteoarthritis are multifaceted:
The mode of action of mesenchymal stem cells
There are two known primary modes of action of stem cells, when injected either locally (for example via the intra-articular route) or parenterally (intravenous route):
Donor cells secrete growth factors that signal to the recipient's cells to change their behaviour. This signalling from one cell to another is called the paracrine effect, and in this way, donor cells do not have to persist or populate damaged recipient tissue, to achieve tissue regeneration.
Damaged patient cells secrete cytokines and regulatory proteins that act as mediators to generate an immune response that attracts the donor cells. In turn, the donor cells secrete their own chemotactic proteins that stimulate the patient's stem cells and help to reduce inflammation, promote cell proliferation and increase vascularisation and blood flow into areas of damaged tissue. Paracrine effect cells also secrete factors that inhibit the death of recipient cells because of injury or disease, and are able to ‘dampen’ the immune response that occurs during autoimmune disease (Najar et al, 2010).
This paracrine effect is considered the most important aspect of the regenerative process provided by the therapeutic use of stem cells. It is also thought to be responsible for the long-term effects of stem cell therapy where mesenchymal stem cells have a long-term effect on tissue regeneration, even though the cells themselves are short-lived (Abe et al, 2020), and it has been proposed that mesenchymal stem cells should now be known as ‘medicinal signalling cells’ (Caplan, 2017).
Mesenchymal stem cell homing
Besides their complex mechanisms of immunomodulation, one of the key advantages of mesenchymal stem cell-based therapies is their ability to home to damaged tissue (Ullah et al, 2019).
When tissues are damaged, mesenchymal stem cells are naturally released into the circulation (as capillary pericytes), where they migrate to the site of injury and secrete molecules with immune-modulating, angiogenic and anti-apoptotic effects to create a microenvironment that promotes regeneration. Mesenchymal stem cells can home to damaged tissue and act as a ‘drug store’ to aid in repair or tissue regeneration (Caplan, 2009).
Homing can be systemic or non-systemic. In systemic homing, they are administered or endogenously released into the bloodstream and undergo a multi-step process to exit circulation and migrate to an injury site. The process of systemic homing can be divided into five steps: tethering and rolling, activation, arrest, transmigration or diapedesis, and migration (Ullah et al, 2019). In non-systemic homing, mesenchymal stem cells are transplanted locally at the target tissue and guided to the site of injury via a chemokine gradient.
Sources of stem cells
Stem cells can be harvested from various sources (Table 3). In clinical situations, stem cells are isolated from the patient (Table 4) from either adipose tissue or bone marrow, which are good sources of mesenchymal stem cells (Zhu et al, 2008). It is possible to isolate stem cells from peripheral blood, but these circulating cells are very low in numbers for the volume of blood sample required and as a result, cell culture and doubling time is very slow. Stem cells may also be derived from dental pulp, umbilical cord, placenta and synovium (Humenik et al, 2022). Adipose tissue-derived autologous mesenchymal stem cells are currently the preferred source for clinical use in the dog (Voga et al, 2020). Once an adipose sample has been collected, mesenchymal stem cells may be isolated and cultured in vitro, or prepared patient-side as a stromal vascular fraction for intra-articular injection (Table 5).
| Autologous stem cells |
|---|
|
|
| Allogeneic stem cells |
|
|
| Zoogeneic stem cells |
|
|
| Bone marrow aspiration |
|---|
|
|
| Adipose tissue harvesting |
|
|
| Peripheral blood collection |
|
|
| Umbilical cord collection |
|
|
| Dental pulp extraction |
|
|
| Synovial fluid and synovial membrane harvesting |
|
|
| Laboratory-cultured mesenchymal stem cells | Patient-side stromal vascular fraction | |
|---|---|---|
| Advantages | Can request specific cell numbers depending on number and size of joint(s) to be treated Does not contain other cell types Can carry out cell count Can carry out sterility testing Can carry out cell viability testing Can store cells for future use (-180ºC) | Adipose collection and intra-articular injection of cells may be carried out in the same visit Less expensive |
| Disadvantages | Takes 14–28 days for cell culture Requires second patient visit Requires second anaesthetic or sedation More expensive | Less stem cells/ml Contains other cell types such as endothelial cells, pericytes and immune cells (Franklin et al, 2018) Cannot carry out cell count Cannot carry out sterility testing Cannot carry out cell viability testing Requires specialised in-house lab equipment Requires specialised training Cannot store cells for future use |
Routes of administration
Adipose-derived stem cells for osteoarthritis therapy can be administered intra-articularly, intravenously or via a scaffold. Intra-articular administration directly into the joint space of the affected joint is the most commonly adopted method in studies and in clinical practice to date (Kriston-Pál et al, 2017; Daems et al, 2019; Olsson et al, 2021).
Intravenous administration
As mesenchymal stem cells are known to have homing capacities (Ullah et al, 2019), studies have investigated the effectiveness of intravenous administration (Olsen et al, 2019) and consider that this route may be useful because the mesenchymal stem cells may interact with the immune system more than locally delivered cells.
Based on labelled mesenchymal stem cell detection studies, the principle of mesenchymal stem cells homing after intra-articular and intravenous administration seems to be promising for animals with osteoarthritis. However, there is a discrepancy between study results after intra-articular and intravenous mesenchymal stem cell administration in dogs.
Mesenchymal stem cell homing capacities were assessed by a model-based study on autologous bone marrow-derived mesenchymal stem cells, which performed fluorescence analysis. Labelled mesenchymal stem cells were detected in new cartilage 2 and 8 weeks after intra-articular injection, confirming local or non-systemic mesenchymal stem cell homing (Mokbel et al, 2011a). These results are comparable to a similar model-based study in donkeys which assessed the homing capacity of intra-articular injected, labelled, autologous bone marrow-derived mesenchymal stem cells where fluorescence microscopy confirmed the incorporation of labelled mesenchymal stem cells in newly formed cartilage 1, 2 and 6 months post-treatment (Mokbel et al, 2011b).
In a study on intravenous injection of allogeneic adipose tissue-derived mesenchymal stem cells, those labelled with cell membrane dye were only occasionally detected in synovial fluid (Olsen et al, 2019). In rabbit and equine studies, homing of intravenous autologous and allogeneic mesenchymal stem cells to areas of tissue injury was also confirmed (Ueda et al, 2017; Mund et al, 2020). Olsen et al (2019) suggested pulmonary trapping, early assessment of the synovial fluid, attachment to synovial membrane and inadequate cell labelling as possible causes of insufficient mesenchymal stem cell homing detection on intravenous administration. To bypass problems associated with lung capillaries when stem cells are introduced intravenously, where mesenchymal stem cells are relatively large cells (with an average size of 30 μm in comparison to pulmonary capillaries which are on average, only 14 μm in diameter) causing mechanical entrapment of mesenchymal stem cells in the lungs, intra-arterial and intraperitoneal routes of administration have been trialled (Leibacher and Henschler, 2016; Wang et al, 2016).
Scaffold administration
Scaffolds are tissue-engineered cartilage and bone, transplanted into osteochondral defects as a template for new tissue formation and organisation. Mesenchymal stem cells can be seeded on these scaffolds (Yang et al, 2011; Duan et al, 2013; Qiang et al, 2014). Based on a matched-pair study, mesenchymal stem cell administration on a scaffold was more beneficial to clinical and arthroscopic outcomes in humans with stifle osteoarthritis than mesenchymal stem cell administration via intra-articular injection alone (Kim et al, 2015).
Conclusions
Stem cell therapy has undergone significant development from its theoretical origins to clinical applications, and regenerative medicine holds great promise for treating a wide array of conditions and diseases. Despite challenges, the potential for stem cell therapy to revolutionise medicine remains immense. In dogs, stem cell therapy may be used as part of a multimodal approach to best manage osteoarthritis where there is no simple generic solution to what is considered a chronic, complex disease process.