Skin
Anatomy of the Skin
What is skin?
The skin is the body's largest and heaviest organ. In adults, the skin covers an area of 1.5 to 2.0 m², and it is responsible for about 15% of body weight. The skin consists of two layers: the epidermis and the dermis. Below the dermis is the hypodermis, which isn't part of the skin but is usually studied along with the skin.
Epidermis: Stratified squamous epithelium.
Dermis: Deep connective tissue. Hypodermis: Connective tissue below the dermis; not actually part of the skin. |
The skin is divided into two classifications: thick skin or thin skin.
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Thick skin: Covers the palms, soles, and corresponding surfaces of fingers and toes. Just its epidermis is about 0.5 mm thick.
- Has a thick surface layer of dead cells called the stratum corneum.
- Has sweat glands but no hair follicles or sebaceous glands.
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Thin skin: Covers the rest of the body. Its epidermis is 0.1 mm thick.
- Has hair follicles, oil glands, and sweat glands.
The Epidermis
Cells of the Epidermis
1. Stem Cells: Undifferentiated cells.
2. Keratinocytes: The majority of epidermal cells.
3. Melanocytes: Cells in the epidermal layer of the skin that produce melanin.
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4. Tactile (Merkel) Cells: Relatively few in number.
5. Dendritic (Langerhans) Cells: Immune cells that originate in the bone marrow but migrate to the epidermis and epithelia of the oral cavity, esophagus, and vagina.
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Layers of the Epidermis
4. Stratum lucidum: A thin zone right above the stratum granulosum. It is seen only in thick skin. Its keratinocytes are packed closely together, and a clear protein named eleiden holds them together. The cells have no nuclei or other organelles. This zone has a pale, bland appearance with cell boundaries that are unclear.
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The Dermis
What is the dermis?
The dermis is the connective tissue layer beneath the epidermis. It is composed mainly of collagen, but it also contains elastic fibers, reticular fibers, fibroblasts, and other cells that are typically found in fibrous connective tissue. The dermis is supplied with blood vessels, cutaneous glands, and nerve endings. Hair and nail roots are both embedded here, and facial skeletal muscles also attach to dermal collagen fibers. The dermis has two layers. |
Layers of the Dermis
The Hypodermis
What is the hypodermis?
The hypodermis is the layer beneath the skin. It is also called subcutaneous tissue. The boundary between the hypodermis and the dermis isn't clear, but the hypodermis usually has more areolar and adipose tissue. It pads the body and binds skin to the tissue beneath it. Drugs are injected here because the hypodermis absorbs them quickly. |