Stratum basale

Fig. 18.3. A cercaria of Schistosoma mansoni penetrating the skin epidermis. The parasite has traversed the outer skin layer, stratum corneum (SC), and is halfway through the underlying dermis, stratum basale (SB). Movement through these layers is facilitated by proteases, most particularly an elastase, which is secreted from the cercarial acetabular glands. The cercaria is moving, as indicated by the arrow, into the inner skin layers where it will meet and enter a venule.

The innermost layer of the epidermis, the stratum basale, consists of a single layer of columnar-shaped, undifferentiated stem cells. Mitosis of these cells constantly renews the epidermis and this proliferation compensates for the loss of dead stratum corneum cells (corneocytes) from the skin surface. As the cells produced by the basal layer move upward, they alter morphologically as well as histochemically to form the outermost layer, the stratum corneum. Over a 4- to 5-week period the entire epidermis is renewed [5]. 

Epidermis can be divided into stratum basale (SB), stratum spin-osum (SS), stratum granulosum (SG), and stratum corneum (SC). The epidermal cell layers are interconnected by desmosomes.45,48 A cross section of the skin epidermis is shown in Fig. 2.4. 

Stratum basale is a single layer composed of stem cells and their derivative cells. It is attached to the basement membrane by hemidesmo-somes.48 The cells in this layer are columnar or cuboidal in shape and characterized by large nuclei (high nuclear-cytoplasmic ratio) and keratin filaments (tonofilaments). The basal layer contains keratins K14 and K15, melanocytes (which are pigment-forming cells), Langerhans... 

Cutaneous biotransformation is mostly associated with the stratum basale layer where there can be phase I and phase II metabolism. However, the skin is not very efficient, compared to the liver. The epidermal layer accounts for the major portion of biochemical transformations in skin, although the total skin activity is low (2-6% that of the liver). Where activity is based on epidermis alone, that layer is as active as the liver or, in the case of certain toxicants, several times more active. For some chemicals, metabolism can influence absorption, and transdermal delivery systems of drugs utilize this activity. For example prodrug such as lipid esters are applied topically, and cutaneous esterases liberate the free drug. These basal cells and extracellular esterases have been shown to be involved in detoxification of several pesticides and bioactivation of carcinogens such as benzo(a)pyrene. For rapidly penetrating substances, metabolism by the skin is not presently considered to be of major significance, but skin may have an important first-pass metabolic function, especially for compounds that are absorbed slowly. 

Fig. 12.3. Epidermal measurements, mitotic figures, and apoptotic keratinocytes in a chronic proliferative dermatitis mutant (Sharplncpdm/Sharplncpdm) mouse. Routine hematoxylin- and eosin-stained paraffin histologic sections can be used to determine proliferation rates based on mitotic index (number of mitotic figures, circled in the figure, in the stratum basale per 1000 cells) or the presence and numbers of apoptotic epidermal keratinocytes (dotted arrows) when present. Epidermal thickness can be measured at high dry magnification (40x) to include the malpigian, living cell, layer (M), the stratum corneum thickness (SC), or the full thickness of the epidermis (M+SC).

FIGURE 6.1 Illustration of calcium gradient in epidermis based on literature data (proton induced x-ray emission analysis of calcium in sectioned human skin) (Malmqvist et al., 1987). SB, stratum basale/basallayer SS, stratum spinosum SG, stratum granulosum SC, stratum corneum. 

The transformation of the fluid- to gel-state lipids is achieved by the catabolism of the pro-barrier lipids and the subsequent synthesis of the barrier lipids. In stratum basale, up to 60% of the lipids... 

Epidermis Stratified squamous epithelium Stratum corneum Stratum spinosum Stratum basale Basement membrane Provides resistance to shear injury... 

Stratum Corneum Stratum Spinosum Stratum Basale... 

Figure 3.7. Structure of skin. Skin is composed of two layers the dermis and epidermis. Epidermis is a stratified squamous epithelium containing stratum corneum, stratum spinosum, and stratum basale (basal cell layer). The dermis contains papillary and reticular layers and a subcutaneous layer containing fat, blood vessels, and skeletal muscle.

The stratum basale or basal layer is responsible for the continual renewal of the epidermis (a process occurring every 20-30 days). Proliferation of the stem cells in the stratum basale creates new keratinocytes which then push existing cells towards the surface. During this upward transit, given that the epidermis is... 

Appendageal structures commonly found within the skin are the hairs, hair follicles, associated sebaceous glands, apocrine and eccrine sweat glands, and arrector pili muscles. Hairs are formed by epidermal invaginations. These keratinized structures traverse the dermis and may extend into the hypodermis. The free part of the hair above the surface of the skin is the hair shaft, and the part deep within the dermis is the hair root, which forms an expanded knob-like structure called the hair bulb. This is composed of a matrix of epithelial cells in different stages of differentiation. Hair is composed of three concentric epithelial cell layers the outermost thin cuticle, a densely packed keratinized cortex, and a central medulla of cuboidal cells. The hair follicle consists of four major components (1) internal root sheath (internal root sheath cuticle, granular layer, pale epithelial layer) (2) external root sheath (several layers similar to the epidermis) (3) dermal papilla (connective tissue) and (4) hair matrix (comparable to the stratum basale of the epidermis). 

Figure 35.5. Light micrograph of an isolated perfused porcine skin flap irradiated with 1260mJ/cm2 depicting sunburn cells in the stratum basale layer (arrows) (560x). (Source Modified from Monteiro-Riviere, N. A., et al., Photodermatol. Photoimmunol. Photomed. 10, 235-243,1994.)...

Transport of vitamin D3 away from the dermal junction of skin is accomplished by a 52 kDa serum vitamin D-binding protein (DBP). Serum DBP is a member of the a-fetoprotein-albumin super family [36], DBP has high affinity for vitamin D3, but does not bind to its precursors or the products of previtamin D3 side-reactions, lumisterol and tachysterol [37], Accumulation of 7-dehydrocholesterol in skin occurs in sebaceous glands at the malpighian layer of the epidermis, mostly in the stratum spinosum and stratum basal... 

The epidermis consists of five principal layers and is an area of both intense biochemical activity and differentiation. These layers are the stratum comeum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The stratum corneum (horny layer) is the uppermost layer of the epidermis and the skin. The stratum corneum is composed of dead keratinocytes, which are called corneocytes, and has an abundance of keratin and lipid structures [8], The stratum comeum is considered the rate-limiting barrier for the diffusion of chemical compounds across the skin. The stratum lucidum (clear layer) is composed of two to three layers of dead flattened keratinocytes which appear translucent under a microscope and are present only in thick glabrous skin. 

The stratum basale is the deepest layer of the epidermis and is composed mainly of keratinocytes with melanocytes making up approximately 10% of the cell population. The stratum basale is one cell layer thick and is a layer of rapid cell division where keratinocytes are rapidly dividing and giving rise to the uppermost layers of the epidermis. 

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