## 1 Introduction  
Soft tissue musculoskeletal disorders are common in horses and typically include acute traumatic injuries such as tendonitis and desmitis, as well as chronic or degenerative conditions such as tendonopathies and desmopathies. Acute lesions are often part of a chronic disease process and can be misinterpreted as isolated acute disorders. In many cases, what initially appears to be an acute injury may actually represent an episodic flare-up within a well-established clinicopathologic framework of chronic tendonopathy or desmopathy [1; 2; 3]. The most commonly affected soft tissue structures in the equine musculoskeletal system include the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), and suspensiony ligament (SL) [4]. Injuries to these structures account for approximately 10% or more of equine lameness cases [5] and represent a significant economic loss to the equine industry [6]. The exact causes of tendonopathy and desmopathy remain unclear, with multiple contributing factors, including aging, obesity, cumulative fatigue, and prior traumatic injuries, among others [1; 7; 8]. Interestingly, proinflammatory and catabolic cytokines such as interleukin-1 beta (IL-18), tumor necrosis factor alpha (TNF-\(\alpha\)), and interferon gamma (IFN-\(\gamma\)) have been immunolocalized in horses with tendonopathy but not in healthy horses [9]. This suggests that these mediators may play roles in perpetuating inflammation, disrupting the metabolic balance of the extracellular matrix, and inducing aberrant cell apoptosis in injured tendons and ligaments [1]. Traditionally, tendon and ligament injuries have been treated with a variety of medical, surgical, and physical therapy approaches aimed at alleviating pain, reducing inflammation, promoting healing, and restoring function [10]. However, in some cases, the healing process may result in fibrosis or scar formation [11]. Over the past 20 years, several regenerative medicine approaches using orthobiologics--such as bone marrow aspirate concentrates (BMACs), mesenchymal stem cells (MSCs), platelet-rich plasma (PRP), and related hemocomponents--have become increasingly popular for the treatment of joint and soft tissue injuries in horses [7; 12; 13]. One of the major beneficial effects claimed for orthobiologic products is their potent anabolic and anti-inflammatory paracrine effects mediated by growth factors (GFs) such as transforming growth factor beta 1 (TGF-\(\beta_{1}\)), platelet-derived growth factors (PDGFs), vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), insulin-like growth factors (IGFs), and others, as well as anti-inflammatory cytokines such as IL-4 and IL-10 [7; 14]. When injected into injured tendons or ligaments, these mediators promote the inhibition of nuclear factor kappa B (NF-\(\kappa\)B), thereby reducing inflammation, enhancing stem cell trafficking and differentiation, stimulating angiogenesis, and supporting extracellular matrix synthesis [15; 16]. PRP is perhaps one of the simplest, most versatile, and cost-effective orthobiologics for treating tendon and ligament injuries in horses [17; 18]. This hemocomponent is primarily considered a suspension of \"live platelets and leukocytes\" in plasma [19], with varying concentrations of these cells, depending on the method of preparation used to procure this orthobiologic. Consequently, PRP can be classified into different types based on the leukocyte concentration, including leukoreduced PRP (pure PRP; P-PRP) or leukocyte-rich PRP (L-PRP) [14]. P-PRPs are typically defined as blood-derived components containing low concentrations of platelets, with platelet counts comparable to or exceeding those found in whole blood. They also contain minimal to no leukocytes. In contrast, L-PRPs are characterized by the presence of measurable leukocyte levels and significantly higherplatelet concentrations [20,21].