Skin Structure and Blood Composition. 1. Introduction. 2. Overview of the Skin. 3. Functions of the Skin. 4. Structure of the Skin. 5.

Transcription

1 Skin Structure and Blood Composition 1. Introduction 2. Overview of the Skin 3. Functions of the Skin 4. Structure of the Skin 5. Epidermis 6. Layers of the Epidermis 7. Dermis 8. Loose Connective Tissue of the Dermis 9. Subcutaneous Tissue Layer 10. Blood Functions 11. Blood Components: Dissolved 12. Blood Components: Cellular 13. Blood Components: Cellular (Cont) 14. Blood Components: Cellular (Cont) 15. Figure 7. Blood Composition 16. References

2 Introduction Many types of injury, and also surgical incisions, result in damage to the skin. The body is usually capable of closing these wounds spontaneously to restore the original functions of its protective covering as quickly as possible. This process involves a variety of repair mechanisms in the individual layers of the skin and a multiplicity of cells, some originating from the blood. To help us understand these processes, let us first take a brief look at the structure of healthy skin and the components of blood. Learning Objectives After completing this module, you will be better able to: Describe the structure and functions of the skin. Discuss the three layers of the skin and their sub-layers and functions. Explain the components of blood and their role in wound healing.

3 Overview of the Skin The skin is the largest organ of the body and performs many functions. Healthy skin provides a barrier against pressure, friction, chemicals, heat, cold, ultraviolet radiation, radiation and microorganisms. The skin is essential for maintaining the body's fluid balance, providing thermoregulation and communicating external stimuli to the body via touch, pressure, temperature and pain receptors. In addition, we externalize our emotional state through the skin: we blush, turn pale, our hair stands on end and we emit scents (pheromones).

4 Functions of the Skin The need to perform numerous and diverse tasks accounts for the intricate structure of the skin. Figure 1. Diagram of the Functions of the Skin Move your mouse over icons in the diagram to reveal associated descriptors. Note: Some interactive figures in these modules may take a little while to download, especially the first time you view them. Please be patient and click only once if the space below appears blank.

5 Structure of the Skin The skin is made up of three layers of tissue: epidermis (outermost layer) dermis subcutaneous tissue. Together, these layers form the body's outer covering. Figure 2. Diagram of the Structure of the Skin Position your cursor over the magnifying glass to the side of Figure 2 and use it to highlight areas you are particularly interested in. Position your cursor over the magnifying glass once more when you are ready to continue. If you search slowly and carefully, you will be able to locate 10 descriptors of different skin structures within the diagram!

6 The Epidermis The epidermis has an average thickness of only about 0.1 mm and is normally composed of four different layers: horny layer (stratum corneum) granular layer (stratum granulosum) prickle-cell layer (stratum spinosum) basal layer (stratum basale). Where exposure to friction is greatest, such as in the palms of the hands and soles of the feet, the epidermis has five layers. This extra layer is called the clear layer (stratum lucidum) situated between the horny layer and the granular layer. The prickle-cell and basal layers consist of living cells continually produced through mitotic division. These cells replenish cells shed by the horny layer. The prickle-cell and basal layers are referred to jointly as the germinative layer. The other three layers make up the cornification layer. The clear layer, normally found in the thick skin of the palms and soles, is absent in thin skin. The clear layer consists of three to five rows of clear, flat, dead cells. The flattened cells become increasingly keratinized as they move through the horny layer and are eventually shed at the skin surface. This process of continual shedding and replacement renews the entire epidermal layer in approximately 27 days. Figure 3 on the next page shows the layers of the epidermis.

7 Layers of the Epidermis Figure 3. Diagram of the Layers of the Epidermis Move your mouse slowly over the diagram to view 10 descriptors of structures in the epidermis. Reminder: Figures may take a little while to download, especially the first time you view them. Please be patient if the space initially appears blank.

8 The Dermis The dermis is the layer that gives the skin its tensile strength and elasticity. These characteristics are derived from the high proportion of loosely interwoven connective tissue fibers (collagen and elastin). Histologically, two layers are distinguished: the papillary layer (stratum papillare) the reticular layer (stratum reticulare). The dermis is firmly intermeshed with the epidermis via the connective tissue papillae of the papillary layer. These papillae are interspersed with fine capillary loops which provide the nutrient supply of the epidermis. The papillary layer also contains numerous free nerve endings branching into the epidermis as well as heat, cold and touch receptors (Meissner's cells). The free connective tissue cells comprise fibroblasts, macrophages, mast cells, lymphocytes, plasma cells, eosinophils and monocytes. The free space between the cellular and fibrous elements is filled with a gelatinous fluid in which the cells are able to move freely. The mobile fibroblasts differentiate into fibrocytes which link via elongated processes to form a three-dimensional network. These fibrocytes synthesize components of the intercellular fluid (e.g., hyaluronic acid) as well as collagen and elastin fibers which are interspersed within the network of cells. The other free cells of the connective tissue are components of the endogenous defense system and play a major role in inflammation and in the immune regulation of the skin. Click on the seven words highlighted in color in the previous paragraph to reveal more detail. These cells and skin structures are also shown in Figure 4 on the next page. The reticular layer (stratum reticulare) contains fewer free cells than the papillary layer. The collagen fibers within this layer form a dense network running parallel to the body surface. Between this collagen mesh, the fibers of the elastin connective tissue branch out, giving the skin its extensibility. The direction of maximum extensibility is indicated by Langer's lines. This is why incisions made perpendicular to these lines cause gaping wounds. In surgical operations, therefore, incisions should be made along these lines wherever possible to improve the cosmetic result. Special modifications of the skin, hair follicles, and sebaceous, sweat and scent glands are embedded in the dermis.

9 Loose Connective Tissue of the Dermis Figure 4. Section of Loose Connective Tissue of the Dermis

10 Subcutaneous Tissue Layer The dermis merges into the subcutaneous tissue layer without a clear boundary. The loose connective tissue of the subcutaneous layer is interspersed with many of the firm fibers of the dermis which anchor the skin to the underlying structures, e.g., the fascia or periosteum. If these retaining bands are few in number, the skin moves on its substrate to create a skinfold. If the fibrous bands are highly developed and numerous, for example on the soles of the feet or the scalp, the skin is almost immovable. The entire subcutaneous tissue layer contains pads of fat which are either readily mobilizable fat stores or nonmobilizable structural fatty cushions providing insulation against heat loss and reducing pressure on underlying structures. Below the subcutaneous tissue layer is the general fascia. Below the general fascia, depending upon the part of the body, is muscle, fat, bone or cartilage. Figure 5 is a diagram of tissue layers underlying the abdominal wall. Figure 5. Skin of the Abdominal Wall with Underlying Tissue Layers Mouse over the numbers on the diagram below to reveal the annotations.

11 Blood Functions In addition to supplying the organs and tissues with oxygen and nutrients and transporting endogenous messenger substances and enzymes, the blood has two further important tasks associated with wound healing. It contains cells of the defense system that recognize and remove foreign particles that have invaded the body, and it contains components of the coagulation system that seal leaks resulting from injury. Functions of the blood are diagrammed in Figure 6. Figure 6. Functions of the Blood

12 Blood Components Dissolved Components Blood plasma is a slightly yellowish fluid containing 90% water. Dissolved in it are many proteins (7-8%) such as albumins which maintain the oncotic pressure in the blood and act as transport proteins for water-insoluble materials and globulins which play a role as antibodies (e.g., IgG, IgA, IgM) in humoral immunity. The plasma also contains: nutrients (proteins, fats, sugars) inorganic salts metabolic waste products (especially urea) enzymes hormones Fibrinogen is a component of plasma essential for blood coagulation. It is a ß2-globulin that is normally present at a concentration of 2-4 g/l.

13 Blood Components Cellular Components Blood cells make up about 45% of the blood. Erythrocytes (also known as red blood cells) are the most numerous cells in the blood at 4-5 million/μl. They take the form of a flat, biconcave disk with a diameter of 7-8 μm (1 μm = 1/1000 mm), are non-nucleated and very flexible. Red blood cells contain hemoglobin and are responsible for oxygen and CO 2 transport. They are produced in the bone marrow and are broken down in the spleen and liver. They have a lifetime of about 120 days. Leucocytes (also known as white blood cells) are present in the blood in much smaller numbers, /μl. Their diameter is more than twice that of the erythrocytes. White blood cells always have a nucleus and exhibit amoeboid movement. They are produced in the bone marrow and mature in the different lymphatic organs (spleen, lymph nodes, tonsils, bone marrow, thymus) into cells with a variety of functions and structural appearance. They have a life span of four days. Non-specific defense is provided by cells referred to as phagocytes. These cells can recognize, engulf and digest foreign organisms such as fungi, bacteria and viruses. They include: granulocytes mononuclear phagocytes or monocytes. Granulocytes (11-14 μm) (also known as polymorphonuclear leucocytes) are the most numerous phagocytic cell type. They circulate in the blood and derive their name from their granular cell inclusion (granulae) easily recognizable under a light microscope. When foreign organisms invade the body, granulocytes leave the blood capillaries, migrate into the affected area and eliminate the invader by phagocytosis. The mononuclear phagocytes are also involved in general defense activities. Depending upon their localization, we distinguish between monocytes (in the blood) and macrophages (in the tissue). Macrophages, as their name suggests, are the largest phagocytic cells with a diameter of μm. In addition to destroying invading microorganisms, they also remove degenerating or aging endogenous tissue.

14 Blood Components Cellular Components (Continued) A more specific and developed form of the general defense system (specific immunity) is provided by lymphocytes. Lymphocytes are specific defense cells because they possess structures on their cell membrane that allow them to recognize specific pathogens (antigens) which they eliminate rapidly, and selectively, after contact. Stem cells of the lymphocytes, like those of other leucocytes, are produced in the bone marrow. During embryonic development they migrate into the lymphatic organs. Here they mature to produce two different types of lymphocytes: T-lymphocytes B-lymphocytes. The maturation of the T-lymphocytes depends on the thymus gland (thus the name T-lymphocyte). Here they differentiate further into the following agents of cell-mediated defense: killer cells which release toxins, cytolytic enzymes or complement factors and thereby selectively destroy invading pathogens; helper cells which, after contact with the antigen, assist the production of antibodies in the B-lymphocytes and activate phagocytic cells by emitting chemotactic agents; and suppressor cells that inhibit the activity of other lymphocytes and thereby regulate the immune response. B-lymphocytes mature in the lymphatic tissue of the intestine and liver. In birds, this function is performed by a rectal gland, the bursa of Fabricius, hence the name B-lymphocyte. B-lymphocytes have specific antigen receptors on their cell surface and, in the event of infection, can transform into memory cells (large plasma cells) after stimulation by the respective antigen. These cells constitute the immunological memory since they "memorize" the antigen and, in the event of a repeat infection, immediately trigger the appropriate immune response. Plasma cells produce an antibody which is released into the blood. Here they bind to their target antigen and either render it harmless directly or "mark" it for destruction by the phagocytic cells.

15 Blood Components Cellular Components (Continued) Platelets (150, ,000 /μl) are not cells in the true sense. The non-nucleated disk-shaped platelets have a diameter of μm. They can clump together, i.e., aggregate, in response to specific stimuli. Once platelets have aggregated, platelet factors are released and these initiate blood coagulation.

16 Blood Composition Figure 7. Composition of the Blood Move your mouse over sections of the diagram to view six annotations.

17 Recommended Resources Wound management is a rapidly evolving field. The editors of the UW Wound Academy strongly recommend consulting multiple sources, including the following publications, to continually update your knowledge and verify current approaches to treatment and prevention when making wound management decisions. Texts Journals Baranoski S and Ayello E (eds) (2016). Wound Care Essentials: Practice Principles, Fourth Edition. Philadelphia: Wolters Kluwer Bryant RA and Nix DP (eds) (2016). Acute and Chronic Wounds: Current Management Concepts, Fifth Edition. St. Louis, MO: Elsevier/Mosby Doughty D and McNichol L (2016). Wound, Ostomy and Continence Nurses Society Core Curriculum: Wound Management. Philadelphia: Wolters Kluwer Krasner DL (ed) (2014). Chronic Wound Care: The Essentials -- A Clinical Source Book for Healthcare Professionals, Fourth Edition. Malvern, Pennsylvania: HMP Communications LLC Sussman C and Bates-Jensen B (eds) (2012). Wound Care: A Collaborative Practice Manual for Health Professionals, Fourth Edition. Philadelphia: Wolters Kluwer/Lippincott Williams and Wilkins Advances in Skin and Wound Care: The International Journal for Prevention and Healing Journal of WOCN (Wound Ostomy and Continence Nursing) OWM -- Ostomy Wound Management Today's Wound Clinic Wounds: A Compendium of Clinical Research and Practice References The following reference was used in developing the original version of this module for the Global Wound Academy by Smith and Nephew, Inc. with kind permission of the authors, Peter D. Asmussen and Brigitte Söllner. Permission to independently review, edit, update and publish the module as part of the UW Wound Academy was given to Continuing Nursing Education at the University of Washington School of Nursing (UWCNE) by Smith and Nephew, Inc in 2014 as part of an unrestricted educational grant to expand access to wound management continuing education. This module was last edited in Asmussen PD and Söllner B (1993). Principles of Wound Healing. Germany (Stuttgart): Druckerei Kohlhammer.