CHAPTER 1

CUTANEOUS STRUCTURE

• Paragraphs in italic type indicate the authors notes.
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SKIN STRUCTURE OF THE EMBRYO

In the earliest days of fetal life, the epidermis is a single sheet of cells, which is doubled between the fifth and sixth week .The outer layer, is the periderm and the inner layer is the stratum germanitivum. The stratum germinatum is responsible for the development of most epithelial structures of the skin such as the basal cell layer, and eccrine sweat glands. The primary epithelial germ cells give rise to the sebaceous glands, apocrine glands and hair follicles.

Stratum Malpighi appears in the four months of fetal life.

STRUCTURE OF INFANT SKIN

The skin of babies has a smooth vulnerable appearance and the histologic features are mainly like adult skin with some variations. The epidermis has the same structure but the stratum corneum is less tough in infants. The main differences include immaturity of collagen, hair follicles, and sebaceous glands where these become more modified with increase of age.

The dermoepidermal adhesion in babies is less than in adults. This explains the stronger skin reactions in infants and young children in response to certain stimuli such as in bullous papular urticaria due to insect bites or other stimuli.

The heat regulation mechanism in infants differs than in adult skin. The differences are mainly due to variations in area-to-body volume, ratio of skin vascular reactivity and the tendency of loosing heat by sweating where, all of these can lead to more loss of heat in infants after minor stimuli.

PHYSIOLOLOGICAL CHANGES OF NEONATAL SKIN AND ITS APPENDAGES

A variety of skin lesions commonly seen in the newborn are regarded as "physiological".

At birth the skin is covered with a whitish greasy film, which is the vernix caseosa. The vernix may cover the entire skin surface or it may be present only in body folds such as the groins. It normally dries rapidly and starts to flake off within a few hours after birth.

Different color changes may appear in the newborn. Golden yellow staining occurs in hemolytic diseases of the newborn and post maturity. Bile pigments staining of the vernix and muconium occurs in fetal distress.

Vivid color difference may appear along the midline at some time during the first week of life. This phenomenon occurs when the baby is lying on its side, the upper half of the body becomes pale while the lower half has a deep red color with a sharp midline demarcation between the two.

Peripheral cyanosis (or acrocyanosis) is a feature particularly seen in the full term newborn.

Generalized hyperemia is usually known as erythema neonatorum, which fades spontaneously within few days.

Scaling of the skin of the newborn occurs in most normal neonates. Superficial cutaneous desquamation is often physiological.

One or two solitary blisters or erosions are occasionally present at birth on the fingers, lips or forearms. These are due to vigorous sucking in utero; hence the term sucking blister is often applied.

Sebaceous glands hyperplasia is usually considered a physiological event in the newborn, reflecting the influence of maternal androgens.

Multiple, uniform, pinpoint yellowish papules appear mainly on the nose, cheeks, upper lip forehead, genitalia and the limbs. The lesion usually clears spontaneously without treatment

Sweat ducts - papules may appear on the face and genitalia due to temporary obstruction of the sweat ducts, which usually disappear during the first weeks of life.

These changes are related to maternal and placental hormones.

Hair - lanugo hair covers the skin of the newborn that may be pigmented. This usually sheds within few weeks.

The Female Genitalia - a mucoid vaginal discharge is common in the female newborn. Vaginal epithelium desquamates to leave a more normal infantile mucosa ,  this desquamation may be accompanied by a creamy white discharge. Frank withdrawal bleeding may occur from the uterus on the third or fourth day, usually lasting 2 - 3 days.

The male genitalia appear similarly large and well developed at birth.

Breast
Both sex show hypertrophy of the mammary glands at birth. After a few days , the breasts may become engorged and show lactation ‘witch‘s milk‘. The swelling subsides during the second week and usually becomes undetectable by the end of the fourth week. In some girls, however, it may be more persistent predisposing to infection due to stagnant milk, leading to mastitis and abscess formation.

STRUCTURE OF ADULT HUMAN SKIN

EPIDERMIS
 
The epidermis is  purely a cellular structure without blood vessels or nerves.

The different layers of the epidermis include the stratum corneum, stratum granulosum, stratum malpighii (prickle cell layer), stratum spinosum, (ret mucosum) and the basal layer (stratum germinativum). In palmoplantar skin there is an additional zone, the stratum lucidum between the stratum granulosum and the stratum corneum.

Stratum Granulosum (granular layer) - lies on the top of the prickle cells and consists of rows of flattened nucleated cells, which contain keratohyaline granules.

Stratum Malpighi (prickle cell layer) - lies on the top of the basal layer and following its direction. This is a very important layer and is involved in most of the pathological conditions of the skin.

Stratum Germinativum (Basal Layer) - is the deeper layer of the epidermis and is composed of a single row of columnar cells arranged vertically to the basement membrane. The cells of the basal layer become modified to form all other cells of the epidermis.

The epidermis is maintained by the division of the germinate cells present in the basal layer and differentiate later to keratinocytes in about seven weeks.

EPIDERMAL CELLS

Epidermal cells are composed of keratinocytes, melanocytes,mast , Langerhans cells and undefined cells.

1. Keratinocytes

As the keratinocytes move upwards reaching the granular layer of the epidermis, loose their nuclei. Keratinocytes become flattened and heaped on the skin surface as horny dead cells. The thickness of this layer varies according to age and different body sites.

Vitamin A is found to have an important role in the development of keratinocytes.

Fetal epidermis, with incomplete keratinization, has abundant glycogen, which is present in the outer root sheath of active hair follicles. Glycogen is found to increase after epidermal injury, which has a direct relation to cellular metabolism.

Keratinocytes possess A and B blood group antigens and share with the red blood cells the same antibodies that are absorbed selectively in some immune reactions.

Functions of keratinocytes

• Synthesis of the fibrillar proteins giving the stratum corneum its toughness.

• Secretion of a large number of cytokines, which have an important effect on lymphocyte and granulocyte function.

• Synthesis of a wide range of growth factors that have an important role in wound healing. The factors controlling synthesis and secretion of these factors may be important in the pathogenesis of skin disease.

2. Langerhans cells - have immunological functions, which can provide traps for contact antigens and present them to T-cells.

3. Melanocytes - are dendritic cells derived from the neural crest and lie on the basement membrane. Melanocytes synthesize melanin from phenylalanine via tyrosine by series of reactions catalyzed initially by tyrosinase giving the skin its color. Pigmentation is related more to melanin synthesis than to the number of skin melanocytes.

DERMIS (corium)

The dermis is a dense fibrous layer beneath the epidermis. It is composed mainly of fibrous tissue, elastic fibres and muscle fibres. The upper surface of the dermis is the papillary layer that is a finger like projection into the epidermis and contains the terminal capillaries and sensory nerve endings. Dermoepidermal junction is undulating where ridges of the epidermis, known as ret ridges, project into the dermis. The junction provides mechanical support for the epidermis.

The entire surface of the dermis is beset with numerous papillae which are more numerous in some locations such as areola of the nipple and terminal phalanges. Most inflammatory diseases of the skin involve the papillary dermis.

The deeper part of the dermis is the reticular layer, where injury below that level leads to scarring after healing.

The dermis contains and supports blood vessels, lymph vessels, nerves, hair follicles, glands and muscle elements.

DERMAL CELLS

The cells of the dermis are derived from the reticulum cell, the primitive mesenchymal cell. The cells of the dermis include :

1. Fibroblasts: form the fibrous tissue and the matrix (ground substance).
   Functions of fibroblasts:

• Production of collagen and elastic tissue.

• Synthesize of mucopolysaccharides .

• Metabolize cholesterol and steroids.

2. Histiocytes: are part of the reticulo-endothelial system. Histiocytes are large phagocytic cells either fixed to the interstitial tissues or wandering .

3. Mastocytes: are specialised cells that synthesize histamine and heparin. Mast cells are numerous in the papillary layer of the epidermis, in the beds of capillaries, in the reticular layer and in the areolar tissue that surrounds the cutaneous appendages .

4. Lymphocytes: have an important role in the immune reactions.

5. Plasma cells: form Immunoglobulins (Ig) antibodies.

6. Eosinophils: inhibit the action of histamine.
    

COMPONENTS OF THE DERMIS

The dermis is composed of:

1. Collagen fibers: The dermis is composed mainly of collagen fibers, which are synthesized by fibroblast. The thickness of collagen depends on different factors such as age, sex and body sites. The collagen layer is organized into a smooth superficial layer under the epidermis at the level of papillae and is coarse in the deeper layers. The collage fibers give the skin its toughness and elasticity.

2. Elastic fibers: are associated with the collagen fibers and both are surrounded by mucopolysaccharides.

3. Ground Substance: The base of the dermis is a supporting matrix or ground substance which is composed of polysaccharides and protein which are linked to produce macromolecules with a remarkable capacity for holding water in their domain. Within and associated with this matrix are two kinds of protein fibers: collagen, which has a great tensile strength and forms the major constituent of the dermis, and elastin, which forms only a small proportion of the bulk. 

The dermis contains the specialized sensory organs and the skin appendages. Below the dermis is a fatty layer known as the subcutaneous tissue.

BLOOD SUPPLY OF THE SKIN

The dermis has a very rich blood supply, though no vessels pass the dermoepidermal junction.

SKIN INNERVATION

Cutaneous enervation depends upon dermal nerve network, which has special sensory nerve endings.

The motor innervation of the skin is autonomic. This includes a cholinergic component to the eccrine sweat glands and adrenergic components to both the eccrine, apocrine glands, to the smooth muscle, the blood vessels and to the erector pilorum muscles.

The sensory nerve endings are of several kinds: some are free, some terminate in the hair follicles and others have expanded tips.

The special sensory nerve endings in the dermis are of several different types mainly, the Vater-Pacini corpuscles and the Meissner corpuscles.

Merkel‘s corpuscles are found on the sides of the tongue. Krause‘s corpuscles are found on the conjuctiva while Ruffins corpuscles are present on the soles.

THE HAIR

Hair is present on all parts of the human skin except the palms, soles, red portions of the lips, ungual phalanges and glans penis.

Types of Hair

The outer root sheath extends from the epidermis to the hair bulb.

THE NAILS

The nails are convex, translucent horny plates that are composed of:

1. The body - the exposed part of the nail.

2. The free edge - the anterior extension from the body.

3. Nail root - the portion of the nail extending under the skin.

4. Nail lanula - the whitish crescent near the base of the nail.

5. Nail bed - part of the epidermis on which the nail lies.

6. Nail matrix - the part of the nail below the nail root.

7. Nail folds - the folds that bound the nail posteriorly and laterally.

8. Eponychium - the crescent shaped thin membrane that stretches for a short distance over the lanula.

9. Hyponychium - the horny layer under the free edge of the distal nail plate.
    

Fig. 7. Nail

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