CHAPTER 1
Physiology of Wound Healing

Christine Theoret, DMV, PhD, Diplomate ACVS

Chapter Contents

• Summary
• Introduction
• Skin anatomy
• Phases of wound repair
  • Hemostasis/coagulation
  • Inflammation
  • Cellular Proliferation
    • Fibroplasia
    • Angiogenesis
    • Epithelialization
  • Matrix synthesis and remodeling (also referred to clinically as the maturation phase)
• Mediators of wound repair
• Conclusion
• References

Summary

Prior to undertaking the management of a wound, the underlying biology of wound healing must be understood so that the best approach at the correct time can be selected, and so that problems with healing, if they arise, are recognized.

This chapter aims to provide an update on the physiologic, cellular, biochemical, and molecular aspects of wound repair.

Introduction

This chapter is reprinted, in a modified form, from Equine Surgery, 3rd edition, Theoret CL, Wound repair, pp. 44-62, Copyright (2005),1 with permission from Elsevier.

A vital trait of living organisms, continually subjected to insults from the environment, is their capacity for self repair. Whether the injury is from surgery or accidental, it generates an attempt by the host to restore continuity to tissue. Two processes are involved in healing: regeneration and repair. Regeneration entails the replacement of damaged tissue with normal cells of the type lost and is only possible in tissues with a sustained population of cells capable of mitosis, such as epithelium, bone, and liver. Conversely, repair is a "stop-gap" reaction designed to re-establish the continuity of interrupted tissues with undifferentiated scar tissue. Repair is, therefore, an inferior method of healing, producing scar tissue that is less biologically useful than the tissue it replaced, and that may adversely affect adjacent normal tissues. When complications of wound healing arise, the final result is even worse.

Accidental wounds occur commonly in horses and exert a significant welfare concern and financial burden on the equine industry. A large study by the United States Department of Agriculture's National Animal Health Monitoring System found, in 2006, that injury/wound/trauma was the most common medical condition affecting horses, with a prevalence of 4.7% in equids 6 months of age and older.2 Injury/wound/trauma was the leading cause of death of foals less than 6 months old, accounting for 24% of deaths, while for horses at least 6 months old, it accounted for 16% of deaths and was the leading cause of mortality, after old age.2 A study in Mexico conducted specifically on a population of working equids found a prevalence of 20.6% for cutaneous pathologic conditions; among these, skin wounds (abrasions, lacerations, abscesses) were the most prevalent (6.8%).3

Figures are also concerning in Europe. A study conducted in the UK found that wounds were the most common type of injury reported by horse owners, accounting for roughly half of all injuries occurring over a 12-month period.4 Another study found that wounds accounted for 21.6% of veterinary treatments of injured polo ponies in the UK.5 Horses in the southern hemisphere do not seem to fare any better: wounds ranked as the third most common medical condition encountered by equine practitioners in Australia and New Zealand and ranked second, after colic, as the most common cause of death or euthanasia.6,7

Finally, skin trauma/wounds are a frequent cause of morbidity in athletic horses. A study in Thoroughbred racehorses has shown that 70% of injuries leading to early retirement are the result of a musculoskeletal injury, of which 7% are associated with wounds or lacerations.8

The objective of repair is to re-establish an epithelial cover and to recover the integrity, strength, and function of the skin. Partial-thickness cutaneous wounds (e.g., abrasions and erosions) heal primarily by migration and proliferation of epidermal cells from the remaining underlying epithelium, as well as from the adnexal structures (i.e., hair follicles and sweat and sebaceous glands), with little participation of inflammatory or stromal cells. In contrast, second-intention repair of full-thickness cutaneous wounds hinges on four coordinated and interrelated phases (Figure 1.1). Partitioning the process into discrete phases suggests simplicity while, in reality, healing is exquisitely complex. The phases rely on interactions between multiple cellular types, their surrounding matrix, and the soluble mediators that govern the numerous activities required to rebuild the tissue. Moreover, the interactions are not static but rather in a state of constant flux, resulting in a microenvironment that is continually evolving as the wound heals.10

Figure 1.1 Temporal profile of synchronized phases and gain in tensile strength of healing cutaneous wounds. Solid lines show the healing profile of laboratory animals while superimposed shaded areas show the profile of healing full-thickness wounds on the limb of horses. It should be noted that the timescale is suggestive and depends on the size and extent of the wound.

Source: Modified by Marco Langlois (Faculté de médecine vétérinaire, Université de Montréal) from Stashak & Theoret 2014.9 Reproduced with permission of Elsevier.

Before veterinarians can positively influence wound healing, they must understand its mechanisms so that they select the appropriate techniques of wound management. In fact, Hippocrates once said, "Healing is a matter of time, but it is sometimes also a matter of opportunity."11

Skin anatomy

The skin is the largest organ and serves key functions including physical protection, sensation, temperature regulation, and insulation. It is composed of two compartments - the epidermis and the dermis (Figure 1.2a). In the horse, the epidermis consists of five layers of keratinocytes: the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum (Figure 1.2b). Additional epidermal components, referred to as skin appendages, include hair follicles, sweat glands, sebaceous glands, and hooves/nails. Although 90-95% of the cells populating the epidermis are keratinocytes, this compartment also includes melanocytes, Langerhans cells, and Merkel cells. Epidermis is attached to the dermis at the level of the basement membrane, a thin, glycoprotein-rich layer composed primarily of laminin and type IV collagen. This attachment is through hemidesmosomes, which physically attach the basal cells of the epidermis to the underlying dermis, as well as by vertically oriented type VII collagen anchoring fibrils, which bind the cytoskeleton.12

Figure 1.2 (a) Diagram of a cross-section of skin, showing the epidermal and dermal compartments. (b) Diagram of the layers of the epidermis of horse skin.

Source: Stashak & Theoret 2014.9 Reproduced with permission of Elsevier.

The dermal compartment consists of two regions, the papillary dermis and the reticular dermis. This compartment is composed of dense, fibroelastic connective tissue and constitutes the bulk of the skin. The epidermis projects into this underlying connective tissue via extensions known as rete pegs or ridges. A network of collagen fibers provides tensile strength to the dermis, and elastin and glycosaminoglycans (GAGs) ensure resilience. Collagen type I is the major collagen of the dermis (~62%) whereas collagen type III comprises ~15% of the dermis.13 The fibroblast is the principal type of cell found in the dermis; perivascular mast cells and tissue macrophages are also found within the dermis. The connective tissue supports these cells and also a network of nerves, epithelial glands, keratinizing appendages, and a microvascular and lymphatic system. Indeed, only the dermal compartment is vascularized; nutrients reach the epidermis by diffusion.

In the horse, the thickness of the skin varies according to body site. For example, in the Dutch warmblood, the skin on the head, neck, and ventral abdomen is relatively thin, measuring between 1.73 mm (±0.16) and 2.03 mm (±0.2) whereas the skin on the limbs is slightly thicker, measuring an average of 2.83 mm (±0.27) for the forelimb and 2.89 mm (±0.24) for the hindlimb, depending on the specific anatomic location (e.g., the skin over the dorsal coffin joint is particularly thick, measuring 4.54 mm [±0.39]).14

The subcutaneous tissue (i.e., tissue just deep to the skin) is also known as the hypodermis or superficial fascia and is not considered part of the skin. It is comprised of loose connective tissue; approximately half of the body's fat stores are located in this region. The hypodermis anchors the skin to the underlying organs and allows the skin to move relatively freely. It also acts as a shock absorber and insulates the deeper body tissues from heat loss.

Phases of wound repair

Hemostasis/coagulation

The first phase of wound healing begins immediately upon injury, is completed within hours, and is...