Chapter 636 Morphology of the Skin Joseph G. Morelli Epidermis The mature epidermis is a stratified epithelial tissue composed predominantly of keratinocytes. The function of the epidermis is protection of the organism from the external environment and the prevention of water loss. The process of epidermal differentiation results in the formation of a functional barrier to the external world. Keratinocytes are composed largely of keratin filaments. These proteins are members of the family of intermediate filaments. The predominant keratins expressed within the keratinocytes change with cellular differentiation. The epidermis comprises four histologically recognizable layers. The first or basal layer consists of columnar cells that rest on the dermal-epidermal junction. Basal keratinocytes are connected to the dermal-epidermal junction by hemidesmosomes. Basal keratinocytes are attached to themselves and to the cells in the spinous layer by desmosomal, tight, gap, and adherens junctions. The role of the basal keratinocyte is to serve as a continuing supply of keratinocytes for the normally differentiating epidermis as well as a reservoir of cells to repair epidermal damage. The second layer consists of 3 to 4 rows of spinous cells. Their role is to begin formation of the epidermal barrier and to initiate vitamin D synthesis. The third layer consists of 2 to 3 rows of granular appearing cells. Granular cells continue the process of epidermal barrier formation and prepare for the formation of the fourth layer or stratum corneum, which is composed of multiple layers of dead, highly compacted cells. The dead cells are composed mainly of disulfide-bonded keratins cross-linked by filaggrins. The intercellular spaces are composed of hydrophobic lipids, predominantly ceramides. As the stratum corneum is replenished, the old stratum corneum is shed in a highly regulated process. The normal process of epidermal differentiation from basal cell to shedding of stratum corneum takes 28 days. The epidermis also contains 3 other cell types. The melanocytes are pigment-forming cells, which are responsible for skin color and protection from ultraviolet radiation. Epidermal melanocytes are derived from the neural crest and migrate to the skin during embryonic life. They reside in the interfollicular epidermis and in the hair follicles and increase in number in the epidermis by mitosis or migration of additional cells into the epidermis. Melanocytes produce intracellular organelles (melanosomes) containing melanin. There is approximately 1 melanocyte per 36 keratinocytes in the epidermal melanin unit. The melanosomes are then transferred via melanocyte dendrites to the keratinocytes. Merkel cells are type I slow-adapting mechanosensory receptors for touch. Langerhans cells are dendritic cells of the mononuclear phagocyte system. They are recognized electron microscopically by a specific organelle, the Birbeck granule. These cells are derived from bone marrow and participate in immune reactions in the skin, playing an active part in antigen presentation and processing. The junction of the epidermis and dermis is the basement membrane zone. This complex structure is a result of contributions from both epidermal and mesenchymal cells. The dermal-epidermal junction extends from the basal cell plasma membrane to the uppermost region of the dermis. Ultrastructurally, the basement membrane appears as a trilaminar structure, consisting of a lamina lucida immediately adjacent to the basal cell plasma membrane, a central lamina densa, and the subbasal lamina on the dermal side of the lamina densa. Several structures within this zone act to anchor the epidermis to the dermis. The plasma membrane of basal cells contains electron-dense plates known as hemidesmosomes; tonofilaments course within basal cells to insert at these sites. The hemidesmosomes are composed of 180- and 230-kd bullous pemphigoid antigens, α6β4 and α3β1 integrins, and plectin. Anchoring filaments originate in the plasma membrane, primarily near the hemidesmosomes, and insert into the lamina densa. Anchoring fibrils, composed predominantly of type VII collagen, extend from the lamina densa into the uppermost dermis, where they insert into anchoring plaques. Dermis Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Rumination, Pica, and Elimination (Enuresis, Encopresis) Disorders Adolescent Pregnancy Neisseria gonorrhoeae (Gonococcus) Blastomycosis (Blastomyces dermatitidis) Stay updated, free articles. Join our Telegram channel Join Tags: Nelson Textbook of Pediatrics Expert Consult Jun 18, 2016 | Posted by admin in PEDIATRICS | Comments Off on Morphology of the Skin Full access? Get Clinical Tree
Chapter 636 Morphology of the Skin Joseph G. Morelli Epidermis The mature epidermis is a stratified epithelial tissue composed predominantly of keratinocytes. The function of the epidermis is protection of the organism from the external environment and the prevention of water loss. The process of epidermal differentiation results in the formation of a functional barrier to the external world. Keratinocytes are composed largely of keratin filaments. These proteins are members of the family of intermediate filaments. The predominant keratins expressed within the keratinocytes change with cellular differentiation. The epidermis comprises four histologically recognizable layers. The first or basal layer consists of columnar cells that rest on the dermal-epidermal junction. Basal keratinocytes are connected to the dermal-epidermal junction by hemidesmosomes. Basal keratinocytes are attached to themselves and to the cells in the spinous layer by desmosomal, tight, gap, and adherens junctions. The role of the basal keratinocyte is to serve as a continuing supply of keratinocytes for the normally differentiating epidermis as well as a reservoir of cells to repair epidermal damage. The second layer consists of 3 to 4 rows of spinous cells. Their role is to begin formation of the epidermal barrier and to initiate vitamin D synthesis. The third layer consists of 2 to 3 rows of granular appearing cells. Granular cells continue the process of epidermal barrier formation and prepare for the formation of the fourth layer or stratum corneum, which is composed of multiple layers of dead, highly compacted cells. The dead cells are composed mainly of disulfide-bonded keratins cross-linked by filaggrins. The intercellular spaces are composed of hydrophobic lipids, predominantly ceramides. As the stratum corneum is replenished, the old stratum corneum is shed in a highly regulated process. The normal process of epidermal differentiation from basal cell to shedding of stratum corneum takes 28 days. The epidermis also contains 3 other cell types. The melanocytes are pigment-forming cells, which are responsible for skin color and protection from ultraviolet radiation. Epidermal melanocytes are derived from the neural crest and migrate to the skin during embryonic life. They reside in the interfollicular epidermis and in the hair follicles and increase in number in the epidermis by mitosis or migration of additional cells into the epidermis. Melanocytes produce intracellular organelles (melanosomes) containing melanin. There is approximately 1 melanocyte per 36 keratinocytes in the epidermal melanin unit. The melanosomes are then transferred via melanocyte dendrites to the keratinocytes. Merkel cells are type I slow-adapting mechanosensory receptors for touch. Langerhans cells are dendritic cells of the mononuclear phagocyte system. They are recognized electron microscopically by a specific organelle, the Birbeck granule. These cells are derived from bone marrow and participate in immune reactions in the skin, playing an active part in antigen presentation and processing. The junction of the epidermis and dermis is the basement membrane zone. This complex structure is a result of contributions from both epidermal and mesenchymal cells. The dermal-epidermal junction extends from the basal cell plasma membrane to the uppermost region of the dermis. Ultrastructurally, the basement membrane appears as a trilaminar structure, consisting of a lamina lucida immediately adjacent to the basal cell plasma membrane, a central lamina densa, and the subbasal lamina on the dermal side of the lamina densa. Several structures within this zone act to anchor the epidermis to the dermis. The plasma membrane of basal cells contains electron-dense plates known as hemidesmosomes; tonofilaments course within basal cells to insert at these sites. The hemidesmosomes are composed of 180- and 230-kd bullous pemphigoid antigens, α6β4 and α3β1 integrins, and plectin. Anchoring filaments originate in the plasma membrane, primarily near the hemidesmosomes, and insert into the lamina densa. Anchoring fibrils, composed predominantly of type VII collagen, extend from the lamina densa into the uppermost dermis, where they insert into anchoring plaques. Dermis Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Rumination, Pica, and Elimination (Enuresis, Encopresis) Disorders Adolescent Pregnancy Neisseria gonorrhoeae (Gonococcus) Blastomycosis (Blastomyces dermatitidis) Stay updated, free articles. Join our Telegram channel Join Tags: Nelson Textbook of Pediatrics Expert Consult Jun 18, 2016 | Posted by admin in PEDIATRICS | Comments Off on Morphology of the Skin Full access? Get Clinical Tree
