6.2: Anatomy of the Skin

Anatomy of the Skin

Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single structure to perform unique and critical functions. The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue. The deeper layer of skin is well vascularized (has numerous blood vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from the brain.

Above: Skin cross section diagram showing the epidermis with melanocytes (pigment cells), dermis, and hypodermis (fatty tissue) layers as well as glands, hairs, and blood vessels embedded in these tissue layers.

The skin is also called the cutaneous membrane. There are two types of skin: thin skin that is covered with hair (also contains sebaceous glands) and thick skin that has no hair. Thick skin, as the name suggests has extra tissue and cell layers in the epidermis compared to thin skin. The skin is composed of two main layers the epidermis and the dermis. Skin has an associated layer below the dermis called the hypodermis.

Above: Microscopic cross sections of the skin. Image A shows a skin cross section at lower magnification which shows that the dermis comprises a larger part of the skin than the epidermis. Image B shows a skin cross section at higher magnification. The divide between the epidermis and the dermis is based on the differences in tissue type. You can clearly see the difference between the stratified squamous epithelial tissue in the epidermis (darker pink) and the connective tissue in the dermis (lighter pink).

The Epidermis

The epidermis is composed of keratinized (contain a protein called keratin; keratin is a rigid structural protein that also helps to waterproof the skin), stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Most of the skin can be classified as thin skin. Thick skin is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum.

Above: This figure shows a diagram and microscopic tissue with labels indicating the five different layers that may be present in the epidermis of the skin. All five layers are present in thick skin, but in thin skin the stratum lucidum layer is absent. You can see that here we are zoomed in on the epidermis and only a small part of the underlying dermis is shown.

The layers of epidermis tissue are describe below from deep to superficial:

• Stratum basale: This deepest layer of the epidermis is composed mainly of one layer of proliferating and non-proliferating keratinocytes (keratin-containing cells), attached to the basement membrane by hemidesmosomes. Melanocytes are present in this layer (melanin-containing cells aka pigment cells), connected to numerous keratinocytes through dendrites. Merkel cells are also found in the stratum basale (large numbers of touch-sensitive receptors are found in fingertips and lips).
• Stratum spinosum: Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes. Langerhans cells (immunologically active cells) and melanin are found in the middle of this layer.
• Stratum granulosum: layer in which keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier.
• Stratum lucidum: A clear/translucent layer of dead keratinocytes, only present in thick skin.
• Stratum corneum: Superficial layer of cells, composed of 10 - 30 layers anucleated keratinocyte, with the palms and soles having the most layers. Most of the barrier functions of the epidermis localize to this layer.

The Dermis

The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over”) and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures, such as hair follicles, sebaceous glands (oil glands), sweat glands, free nerve endings, pressure receptors (Pacinnian corpuscles), and arrector pili muscles capable of moving the hair follicles to make hair "stand on end." The dermis is made of two layers of connective tissue composed of an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts. The papillary layer (superficial layer of the dermis) consists of areolar connective tissue. The reticular layer (deep layer of the dermis) consists of dense irregular connective tissue. These layers serve to give elasticity and flexibility to the integument.

Above: Microscopic image of the layers of the dermis. This stained slide shows the two components of the dermis—the papillary layer and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other, making the border between the two somewhat indistinct. The dermal papillae extending into the epidermis belong to the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer.

Hypodermis

The hypodermis, also known as the subcutaneous layer, is a layer beneath the skin. It invaginates into the dermis and is attached to it with collagen and elastin fibers. It is largely composed of adipocytes that accumulate and store fats. These cells are grouped together in lobules separated by connective tissue. The hypodermis acts as an energy reserve. The energy-rich fats contained in the adipocytes can be brought into circulation via the venous route (through veins) during intense energy expenditure or when energy-rich molecules are in low supply. The hypodermis participates passively in thermoregulation since the fat stored in adipocytes act as a heat insulators.

Attributions

• "Anatomy and Physiology I Lab" by Victoria Vidal is licensed under CC BY 4.0
• Anatomy The Skin - NCI Visuals Online.jpg by National Institutes of Health and is in the public domain.
• "BIOL 250 Human Anatomy Lab Manual SU 19" by Yancy Aquino, Skyline College is licensed under CC BY-NC-SA 4.0