Porth's Essentials of Pathophysiology, 4e

1143

Structure and Function of the Integumentum

C h a p t e r 4 5

Keratinized cells

Stratum corneum

Stratum lucidum

Stratum granulosum

Stratum spinosum (prickle cells)

Stratum germinativum (basal cells)

Melanocyte

referred to as prickle cells because they develop a spiny appearance as their cell borders interact. The third layer, the stratum granulosum, consists of three to five layers of flattened keratinocytes. The nuclei and organelles of these cells begin to degenerate and keratin filaments become more apparent. A distinctive feature of cells in this layer is the appearance of dark staining granules of a protein called keratohyalin, which serve as a support for the keratin filaments. The stra- tum lucidum, the fourth layer, which lies just superficial to the stratum granulosum, is a thin, transparent layer mostly confined to the palms of the hands and soles of the feet. It consists of transitional cells that retain some of the functions of living skin cells from the layers below but otherwise resemble the cells of the stratum corneum. The top or surface layer of the epidermis is the stratum corneum. The cells in the stratum corneum are made up of numerous layers of dead keratinized cells. The interi- ors of the cells contain mostly keratin and keratohyalin. These cells are continuously shed and replaced by cells from the deeper strata. The stratum corneum is the layer that varies most in cell layers and thickness. It ranges from 15 layers of cells in areas such as the face to 25 or more layers on the arm. Specialized areas, such as the palms of the hand and soles of the feet, have 100 layers or more. Melanocytes Melanocytes are dendritic cells found scattered among the basal cells in the stratum germinativum. They func- tion to produce pigment granules called melanin , the substance that is responsible for skin color, tanning, and protection against ultraviolet radiation. During embryonic life, melanocyte precursor cells migrate from the neural crest and enter the develop- ing epidermis. A specific functional association is then established in which one melanocyte maintains an asso- ciation with a given number of keratinocytes. This asso- ciation is called the dermal melanin unit . The ratio of FIGURE 45-2. Epidermal cells.The basal cells undergo mitosis, producing keratinocytes that change their size and shape as they move upward, replacing cells that are lost during normal cell shedding.

melanocytes to keratinocytes varies in different parts of the body and is constant in all races. As dendritic cells, melanocytes are round to columnar cells with long undulating processes that extend between the keratinocytes of the stratum spinosum (Fig. 45-3). Melanocytes synthesize melanin from the amino acid tyro- sine in the presence of an enzyme called tyrosinas e that is activated by ultraviolet light. Synthesis of melanin occurs in membrane-bound structures called premelanosomes, which are derived from the Golgi complex. As melanin accumulates in the premelanosomes, they are transformed into mature melanin granules, called melanosomes. Premelanosomes are concentratednear the golgi apparatus, and nearly mature and mature melanocytes among the basal cells of stratum germinativum. Melanosomes and their melanin content are transferred to neighboring kera- tinocytes by pigment donation. This process involves the phagocytosis of the tips of the melanocyte process by kera- tinocytes. After pigment donation, the melanocyte process elongates and receives more melanosomes, and the pro- cess is repeated. Thus, a single melanocyte is able to pro- vide melanin to all of the associated keratinocytes in its epidermal melanin unit. Skin color is determined by the type, number, and size of the melanosomes transferred into the surrounding keratinocytes. Dark-skinned and light-skinned people have approximately the same number of melanocytes, but the production and packaging of pigment are different. In dark-skinned people, larger melanin-containing mela- nosomes are produced and transferred individually to the keratinocyte, whereas in light-skinned people, smaller melanosomes are produced and then packaged together in a membrane before being transferred to the keratinocyte. There are two major forms of melanin: eumelanin and pheomelanin. Exposure to the sun’s ultraviolet rays FIGURE 45-3. The melanocytes, which are located in the basal layer of the skin, produce melanin pigment granules that give skin its color.The melanocytes have threadlike, cytoplasm-filled extensions that are used in passing the pigment granules to the keratinocytes.