The Epidermis

The epidermis has an average thickness of only about 0.1 mm and it 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 (straum lucidum), situated between the horny layer and the granular layer.

 

 

Figure 3. Diagram of the Layers of the Epidermis

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Dissolved Components

Blood plasma is a slightly yellowish fluid containing 90% water. Dissolved in it are many proteins (7-8%) such as albumins, which are responsible for maintaining the osmotic pressure in the blood and act as transport proteins for water-insoluble materials and globulins that play a role as antibodies (eg 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.

 

Cellular Components

Erythrocytes - Blood cells make up about 45% of the blood. Erythrocytes (red blood cells), which contain haemoglobin, are the most numerous 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 (Figure 7).

 

Figure 7. Composition of the Blood

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Figure 4. Section of Loose Connective Tissue of the Dermis

The reticular layer (stratum reticulare) contains fewer free cells than the papillary layer. The collagen fibres within this layer form a dense network running parallel to the body surface. Between this collagen mesh, the fibres 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, the hair follicles, sebaceous, sweat and scent glands, are embedded in the dermis.

Figure 2. Diagram of the Structure of the Skin

Position your curser over the magnifying glass to the side of Figure 2 and use it to highlight areas you are particularly interested in. Position your curser over the magnifying glass once more when you are ready to continue. If you search carefully, you will be able to locate 10 descriptors of different skin structures within the diagram!

Red blood cells contain haemoglobin and are responsible for oxygen and CO₂ 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

Leucocytes (also known as white blood cells) are present in the blood in much smaller numbers, 4000 - 11000 /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.

 

Non-specific defence is provided by cells referred to as phagocytes. These cells can recognise, 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 recognisable 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 the general defence activities. Depending upon their localisation, 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 12-20 µm. As well as destroying invading microorganisms they also remove degenerating or ageing endogenous tissue.

 

A more specific and developed form of the general defence system (specific immunity) is provided by the lymphocytes. These are specific defence cells since they possess on their cell membrane, structures allowing them to recognise specific pathogens (antigens), which they eliminate rapidly, and selectively, after contact.

Epidermis 

The prickle-cell and basal layer consist of living cells continually produced through mitotic division. These cells replenish the shed cells of the horny layer and the two 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. It 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.

 

 

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 "memorise" 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.

 

The platelets, (150,000 - 450,000 /µl) are not cells in the true sense. The non-nucleated disk-shaped platelets have a diameter of 2-3.5 µm. They can clump together, ie aggregate, in response to specific stimuli. 

 

Once platelets have aggregated, platelet factors are released and these initiate blood coagulation.

The content of the learning zone has been reproduced with kind permission of the authors Peter D.Asmussen and Brigitte Söllner (Peter D.Asmussen,Brigitte Söllner. Principles of Wound Healing. Germany (Stuttgart): Druckerei Kohlhammer.)

Blood Composition

As well as 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 defence system that recognise and remove foreign particles which have invaded the body and, in addition, the blood contains components of the coagulation system that seal leaks resulting from injury (Figure 6).

 

 

Figure 6. Functions of the Blood

Module Exam

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 defence:

 

• 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 
• supressor cells that inhibit the activity of other lymphocytes and thereby regulate the immune response.

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 fibres of the dermis, which anchor the skin to the underlying structures, eg the fascia or periosteum. If these retaining bands are few in number the skin moves on its substrate to create a skinfold. For example, on the soles of the feet or the scalp the skin is almost immovable since the fibrous bands are highly developed and numerous.

 

The entire subcutaneous tissue layer contains pads of fat, which are either readily mobilisable fat stores or non-mobilisable, structural fatty cushions providing insulation against heat loss, and reducing pressure on underlying structures. Below the subcutaneous tissue layer is the general fascia. Below that depending upon the part of the body, there is muscle, fat, bone or cartilage (Figure 5)

 

 

Figure 5. Skin of the Abdominal Wall with Underlying Tissue Layers

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