Keratinocytes stratum spinosum

The stratum granulosum is the most superficial cell layer of the viable epidermis and contains highly differentiated keratinocytes. The lamellar bodies, which have been formed in the stratum spinosum, migrate to the apical periphery of the uppermost granular cells and eventually fuse with the membrane of the keratinocyte. Via exocytosis their content is extruded into the intercellular spaces at the stratum granulosum-stratum corneum interface. The lipids derived from the lamellar bodies are essential for the formation of the stratum corneum barrier. 

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 spinosum (prickly layer) is composed of several layers of keratinocytes which are starting to exhibit histological and biochemical changes that mark the beginning of the differentiation process. The shape of the keratinocytes has become irregular and enzymes responsible for lipid synthesis are present. 

Human keratinocytes express cholinergic receptors in the cells of the stratum basale, the stratum spinosum and the stratum granulosum. They use ACh, among other things, to stick together. They also synthesize, store, degrade and release ACh. 

The keratinocyte touch should be positive a gloved finger slides easily over the corneocytes (dead cells) that are still present in the area surrounding the abraded zone, but brakes on the hydrated ker-atinocytes (stratum spinosum) that have been laid bare by the sandpaper. The first appearance of pinpoint bleeding signals that the sandpaper has reached the basal layer. Abrasion that goes any deeper can cause permanent complications. 

Keratinocytes of the epidermis are arranged into four or five layers. The deepest layer is the stratum basale, which is a single layer of dividing cells. The stratum spinosum is next and consists of 8-10 layers of polyhedral keratinocytes. Above this layer is the stratum granulosum, composed of three to five layers of keratinocytes that have lost their... 

The epidermis is an epithelium consisting of inner viable epidermis, a living hydrophilic layer, and outer nonviable epidermis, a hydrophobic layer made from dead cells. It is differentiated into stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale In the direction of dermis (Fig. 16.2). The viable epidermal layer has a thickness of about 0.02 to 0.2 mm. It is composed of many layers of keratinocytes, a widespread distribution of melanocytes, Langerhans cells, dendritic T cells, epidermotropic lymphocytes and Merkel cells, and a number of catabolic enzymes such as esterases, proteases, phosphatases, nucleotidases and lipases [Walters and Roberts, 2002 Barry, 2001). The outer and nonviable epidermis, namely, stratum corneum, is about 10 to 20 pm thick [Gregor and Ulrich, 2010). The stratum corneum Is deemed to be the major obstacle of drug permeation. 

A dramatic ( 75%) reduction in size and number of desmosomes was observed in a freeze-fracture analysis of keratinocyte membranes in patients with psoriasis treated with etretinate (Kitajima and Mori, 1983). This reduction in desmosomal size and number (Williams and Elias, 1981), which was particularly evident in the stratum comeum but also observed in the stratum spinosum of both lesional and nonlesional skin, appeared to be of sufficient magnitude to contribute significantly to the keratolytic effect of retinoids in hyperkeratotic disorders. Other factors, for example, decreased tonofrlaments and decreased glycocalyx cohesion, may also be contributory. This effect also has been observed in vitro as retinoids cause increased shedding of squames from stratified cultures of human foreskin epidermal cells (McGuire et al., 1982 Kubilus, 1980). 

The fine granular, amorphous, mucous-like deposits induced by treatment with oral retinoids began to appear both within and between keratinocytes in the upper stratum spinosum and persisted into the stratum comeum. By transmission electron microscopy, no evi nce of active secretion or endocytosis was noted. 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

In Fig. 2.16 it shows the skin composition in layers, the stratum comeum, granular, spinosum, and basal. The epidermis uppermost layer, the stratum comeum, has keratinocytes tightly linked by des-mosomes in a hydrophobic cellular matrix. This sfructure CTeates an anatomical barrier against irritants, alleigens, and shear force. In addition to the physical barrier, there are mediators of resistance such as leucocytes, cytokines, complanent cascade, and host defense peptides [AMPs]. 

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