Lars-peter Kamolz (editor) - Dermal Replacements in General, Burn, and Plastic Surgery: Tissue Engineering in Clinical Practice

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Lars-Peter Kamolz and David Benjamin Lumenta (eds.) Dermal Replacements in General, Burn, and Plastic Surgery 2013 Tissue Engineering in Clinical Practice 10.1007/978-3-7091-1586-2_1 Springer-Verlag Wien 2013

1. Skin: Architecture and Function

Gerd G. Gauglitz 1, 2 and Jrgen Schauber 1

(1)

Department of Dermatology and Allergy, Ludwig Maximilian University, Munich, Germany

(2)

Scar Clinic, Department of Aesthetics, Department of Infectious and Sexual Transmitted Diseases, Department of Dermatology and Allergy, Ludwig Maximilian University, Frauenlobstr. 9-11, Munich, 80337, Germany

Gerd G. Gauglitz

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Abstract

The skin is the largest organ of the human body. It covers approximately 12 m2 of surface area and accounts for around 1216 % of an adults body weight. In direct contact with the outside environment, the skin provides the following essential functions:

1.1 Introduction

The skin is the largest organ of the human body. It covers approximately 12 m2 of surface area and accounts for around 1216 % of an adults body weight. In direct contact with the outside environment, the skin provides the following essential functions:

• Retention of moisture and prevention of loss of other molecules
• Barrier function against harmful external influences
• Immune function and protection of the body from microbes
• Sensory function (e.g. heat, cold, touch, pressure, vibration, tissue injury)
• Endocrine function (e.g. vitamin D production)
• Regulation of body temperature

To understand cutaneous biology, wound healing and skin diseases, it is critical to be aware of the defined structures and functions of normal human skin. Human skin consists of a stratified, cellular epidermis and an underlying dermis of connective tissue which together form the cutis (Fig. ).

Fig. 1.1

Structure of the skin (From Nakagawa )

The dermalepidermal junction is undulating and ridges of the epidermis, known as rete ridges, project into the dermis. The junction provides mechanical support for the epidermis and acts as a partial barrier against larger molecules.

Below the dermis, a fatty layer, the panniculus adiposus, exists, usually designated as subcutaneous fat. It is separated from the rest of the body by a vestigial layer of striated muscle, the panniculus carnosus (Fig. ).

Two variants of human skin can be differentiated: Glabrous skin (non-hairy skin), found on the palms and soles, is grooved and its surface shows individually unique configurations by continuously alternating ridges and sulci, known as dermatoglyphics. It is characterised by a thick epidermis divided into several well-marked layers including a compact stratum corneum, by the presence of encapsulated sense organs within the dermis and by a lack of hair follicles and sebaceous glands. Hair-bearing skin, on the other hand, has both hair follicles and sebaceous glands but lacks encapsulated sense organs. Hair-bearing skin shows a wide variation between different body sites.

The motor innervation of the skin is autonomic and includes a cholinergic component to the eccrine sweat glands and adrenergic components to both the eccrine and apocrine glands, to the smooth muscle and the arterioles and to the arrector pili muscle. Several sensory nerves are found in skin: Some show free endings, some terminate in hair follicles and others have expanded tips. Only in glabrous skin some nerve endings are encapsulated.

1.2 Epidermis

The epidermis is the outer layer of the skin and acts as the bodys major barrier against a hostile environment. The epidermis is a multilayered epithelium composed of 45 layers depending on the respective region of skin. In humans, it is thinnest on the eyelids and thickest on the palms and soles. The layers in ascending order are the basal/germinal layer (stratum basale/germinativum), the spinous layer (stratum spinosum), the granular layer (stratum granulosum), the clear/translucent layer (stratum lucidum) and the cornified layer (stratum corneum).

The epidermis is aneural and avascular, nourished by diffusion from the dermis, and contains keratinocytes, melanocytes, Langerhans cells, Merkel cells and inflammatory cells. Keratinocytes are the major resident cells constituting 95 % of the epidermis.

1.2.1 Stratum Germinativum or Basal Layer

The innermost layer of the epidermis which lies adjacent to the dermis comprises mainly dividing and non-dividing keratinocytes which are attached to the basement membrane by hemidesmosomes. As keratinocytes divide and differentiate, they move from this deeper layer to the surface. Melanin-producing melanocytes make up a small proportion of the basal cell population. These cells are characterised by dendritic processes which stretch between relatively large numbers of neighbouring keratinocytes (Bensouilah and Buck ). Merkel cells are also found in the basal layer predominantly in touch-sensitive sites such as the fingertips and lips. They are closely associated with cutaneous nerves and seem to be involved in light touch sensation.

1.2.2 Stratum Spinosum

As basal cells reproduce and mature, they move towards the outer layer of skin, initially forming the stratum spinosum. Intercellular bridges, the desmosomes, which appear as prickle at a microscopic level, connect the cells. Langerhans cells are dendritic, immunologically active cells derived from the bone marrow and are found on all epidermal surfaces but are mainly located in the middle of this layer. They play an important role in immune reactions of the skin, acting as antigen-presenting cells (Bensouilah and Buck ).

1.2.3 Stratum Granulosum

Continuing their transition to the surface the cells continue to flatten, lose their nuclei and their cytoplasm appears granular at this level.

1.2.4 Stratum Lucidum

The stratum lucidum represents a layer of 34 rows of dead flat transparent cells and is only found in the skin of palms and soles.

1.2.5 Stratum Corneum

The final result of keratinocyte maturation is found in the stratum corneum which is made up of layers of hexagonal-shaped, non-viable cornified cells also known as corneocytes. In most areas of the skin, there are >10 layers of stacked corneocytes with the palms and soles having the thickest stratum corneum. Each corneocyte is surrounded by a protein envelope and is filled with water-retaining keratin proteins. The cellular shape and orientation of the keratin proteins adds strength to the stratum corneum (Bensouilah and Buck ). Surrounding the cells in the extracellular space are stacked layers of lipid bilayers. The resulting structure provides the natural physical and water-retaining barrier of the skin.

The different epidermal compartments undergo constant cellular turnover to replace dead or damaged cells. This homeostatic process is thought to involve several types of stem cells each located in a specific epidermal region and contributing to the maintenance of a discrete compartment of the skin (Blanpain and Fuchs ).

The epidermal turnover time is about 1 month. The keratinocytes reach the interface of the granular layer and the stratum corneum after about 2 weeks. Another 2 weeks are normally required for the corneocytes to reach the surface of the stratum corneum and be shed into the environment. In psoriasis, this turnover time is dramatically decreased. Derangements in the process of keratinisation are responsible for a wide variety of disorders, including ichthyoses and palmoplantar keratoderma.

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