Chapter 2: Anatomy and Physiology of Wound Healing
Wound healing is one of the most remarkable biological processes of the human body. Every day, the skin is exposed to mechanical injury, environmental hazards, microorganisms, and chemical insults. Under normal circumstances, the body responds rapidly by initiating a highly coordinated sequence of cellular and biochemical events that restore tissue integrity and protect against infection. In individuals with diabetes mellitus, however, this natural healing process is significantly disrupted. The result is delayed wound healing, chronic ulcer formation, and an increased risk of severe complications.
A thorough understanding of the anatomy of the skin and the normal physiology of wound healing is essential for appreciating why diabetic foot ulcers are so difficult to treat and how electrophysical therapies may assist in restoring the healing process.
The skin is the largest organ of the human body and serves as the first line of defense against the external environment. It performs numerous vital functions, including protection against pathogens, regulation of body temperature, prevention of fluid loss, sensation, vitamin D synthesis, and immune surveillance. Healthy skin also plays a critical role in maintaining internal homeostasis.
The skin consists of three major layers: the epidermis, the dermis, and the hypodermis or subcutaneous tissue.
The epidermis is the outermost layer and is primarily composed of keratinocytes. It forms a waterproof protective barrier that shields the body from microorganisms, ultraviolet radiation, and physical injury. The epidermis contains melanocytes, which produce melanin to protect against ultraviolet damage, Langerhans cells that participate in immune defense, and Merkel cells involved in touch sensation. Although the epidermis lacks blood vessels, it receives nutrients by diffusion from the underlying dermis.
Beneath the epidermis lies the dermis, a thicker connective tissue layer that provides structural strength and elasticity. The dermis contains collagen fibers, elastin fibers, fibroblasts, blood vessels, lymphatic vessels, sweat glands, sebaceous glands, hair follicles, and numerous sensory nerve endings. Fibroblasts are particularly important because they synthesize collagen, elastin, and extracellular matrix proteins that are essential for wound repair.
The deepest layer, the hypodermis, consists primarily of adipose tissue and connective tissue. This layer cushions underlying structures, stores energy, insulates the body, and contains larger blood vessels and nerves. In diabetic foot ulcers, damage may extend through all three layers and even involve muscles, tendons, joints, and bone.
When the integrity of the skin is disrupted, the body immediately initiates a sophisticated healing response involving numerous cells, signaling molecules, enzymes, and growth factors. Wound healing is traditionally divided into four overlapping phases: hemostasis, inflammation, proliferation, and remodeling.
The first phase, hemostasis, begins within seconds after injury. Blood vessels constrict to reduce blood loss, while platelets adhere to the damaged vessel walls and aggregate to form a temporary clot. Platelets release numerous growth factors, including platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-β), vascular endothelial growth factor (VEGF), and epidermal growth factor (EGF). These signaling molecules initiate the healing cascade by attracting inflammatory cells and stimulating tissue repair.
The fibrin clot serves not only to stop bleeding but also as a temporary scaffold that supports the migration of inflammatory cells and fibroblasts into the wound. This provisional matrix forms the foundation upon which new tissue will develop.
The inflammatory phase begins almost immediately after hemostasis and usually lasts for several days. During this stage, neutrophils are the first immune cells to arrive at the wound site. Their primary function is to destroy invading microorganisms, remove cellular debris, and reduce the risk of infection. After approximately forty-eight hours, macrophages become the dominant inflammatory cells.
Macrophages play a central role in successful wound healing. In addition to removing bacteria and dead tissue, they release cytokines and growth factors that stimulate fibroblast proliferation, collagen production, angiogenesis, and epithelial cell migration. Without adequate macrophage function, wounds cannot progress efficiently into the next stage of healing.
Inflammation is often misunderstood as being harmful; however, controlled inflammation is essential for effective tissue repair. Problems arise only when inflammation becomes excessive or prolonged, as commonly occurs in chronic diabetic wounds.
The proliferative phase generally begins within three to five days after injury and may continue for several weeks. During this phase, fibroblasts migrate into the wound and begin synthesizing collagen, glycosaminoglycans, and extracellular matrix proteins that provide structural support for new tissue formation.
At the same time, angiogenesis occurs. New capillaries grow into the wound, supplying oxygen and nutrients that support rapidly dividing cells. This newly formed vascular tissue gives healthy healing wounds their characteristic bright red appearance, known as granulation tissue.
Keratinocytes migrate across the wound surface in a process known as epithelialization. These cells proliferate until the wound surface is completely covered with new epithelium. Simultaneously, myofibroblasts contract the wound edges, reducing wound size and accelerating closure.
The final phase of wound healing is remodeling, also referred to as the maturation phase. This stage may continue for several months or even years after apparent wound closure. During remodeling, immature type III collagen is gradually replaced by stronger type I collagen. Collagen fibers become increasingly organized along lines of mechanical stress, significantly improving tissue strength.
Although scar tissue never achieves the full tensile strength of uninjured skin, it can eventually regain approximately seventy to eighty percent of its original strength under optimal conditions.
Successful wound healing depends upon a delicate balance among cellular activity, growth factors, adequate blood supply, oxygen delivery, immune function, and mechanical stability. Any disturbance affecting these factors can delay or prevent healing altogether.
Diabetes disrupts virtually every phase of this healing process. Persistent hyperglycemia impairs platelet function, alters inflammatory responses, reduces macrophage activity, inhibits fibroblast proliferation, decreases collagen synthesis, suppresses angiogenesis, and delays epithelialization. Consequently, wounds that would normally heal within a few weeks may remain open for months or even years.
The systematic review that forms the basis of this ebook emphasizes that peripheral neuropathy and ischemia are the major causes of diabetic foot ulceration. Neuropathy results in loss of sensation and impaired microvascular circulation, while ischemia limits oxygen and nutrient delivery to healing tissues. Together, these abnormalities contribute significantly to delayed wound repair.
Neuropathy affects wound healing in several ways. Loss of protective sensation allows repeated trauma to occur without the patient’s awareness. Motor neuropathy produces muscle imbalance and foot deformities that increase localized pressure. Autonomic neuropathy reduces sweating, leading to dry, cracked skin that is more susceptible to injury and infection.
Poor circulation further complicates healing by reducing tissue oxygenation. Oxygen is essential for collagen synthesis, bacterial killing by neutrophils, angiogenesis, and cellular metabolism. Inadequate oxygen delivery severely compromises each of these processes, prolonging wound healing and increasing the likelihood of infection.
Diabetes also impairs immune function. Neutrophils exhibit reduced chemotaxis, phagocytosis, and bacterial killing, while macrophages demonstrate diminished activity. As a result, diabetic wounds are highly susceptible to persistent infection, biofilm formation, and chronic inflammation.
Another important abnormality is reduced fibroblast activity. Fibroblasts are responsible for producing collagen and extracellular matrix proteins that form the structural framework of healing tissue. Hyperglycemia decreases fibroblast proliferation and collagen synthesis, resulting in fragile granulation tissue and delayed wound closure.
Angiogenesis, the formation of new blood vessels, is equally affected. Reduced production of vascular endothelial growth factor and endothelial dysfunction limit new capillary formation, depriving healing tissues of oxygen and nutrients. The systematic review notes that electrophysical modalities have been investigated because they may enhance fibroblast activity and angiogenesis, potentially improving tissue repair in chronic diabetic wounds.
In addition to biological abnormalities, mechanical factors significantly influence wound healing. Continuous pressure, friction, and shear forces disrupt newly formed tissue and repeatedly damage healing wounds. Effective pressure offloading therefore remains a fundamental component of diabetic foot ulcer management regardless of the use of advanced therapies.
The local wound environment also plays a critical role. Excessive wound exudate, necrotic tissue, bacterial colonization, and inadequate moisture balance all interfere with normal healing. Modern wound care aims to maintain an optimal moist environment while minimizing bacterial burden and preserving healthy tissue.
Understanding these complex physiological processes provides the scientific foundation for electrophysical therapy. Modalities such as electrical stimulation, therapeutic ultrasound, low-level laser therapy, and electromagnetic field therapy are designed to influence many of the cellular mechanisms involved in wound repair. By stimulating fibroblast proliferation, enhancing angiogenesis, improving local circulation, reducing inflammation, and promoting tissue regeneration, these therapies may help chronic diabetic wounds progress through the normal stages of healing.
Although conventional wound care remains the cornerstone of treatment, advanced therapeutic technologies continue to evolve as researchers seek more effective methods to accelerate healing, reduce complications, and prevent lower-limb amputation in individuals living with diabetes.
The next chapter explores the pathogenesis of diabetic foot ulcers in greater detail, examining how neuropathy, vascular disease, infection, biomechanical abnormalities, and metabolic disturbances interact to produce one of the most challenging chronic wounds encountered in modern clinical practice.
