Stratum corneum
Encyclopedia
The stratum corneum is the outermost layer of the epidermis, consisting of dead cell
s (corneocytes) that lack nuclei
and organelle
s. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection
, dehydration
, chemicals and mechanical stress. Desquamation
, the process of cell shedding from the surface of the stratum corneum, balances proliferating keratinocyte
s that form in the stratum basale. These cells migrate through the epidermis towards the surface in a journey that takes approximately fourteen days.
is replaced by a layer of ceramide
s which become covalently linked to an envelope of structural proteins (the cornified envelope).
This complex surrounds cells in the stratum corneum and contributes to the skin's barrier function. Corneodesmosomes (modified desmosome
s) facilitate cellular adhesion by linking adjacent cells within this epidermal layer. These complexes are degraded by proteases, eventually permitting cells to be shed at the surface. Desquamation and formation of the cornified envelope are both required for the maintenance of skin homeostasis. A failure to correctly regulate these processes leads to the development of skin disorders.
Cells of the stratum corneum contain a dense network of keratin
, a protein
that helps keep the skin hydrated by preventing water
evaporation
. These cells can also absorb water, further aiding in hydration, and explaining why humans and other animals experience wrinkling
of the skin on the finger
s and toes ("pruning") when immersed in water for prolonged periods. In addition, this layer is responsible for the "spring back" or stretchy properties of skin. A weak glutenous protein bond pulls the skin back to its natural shape.
The thickness of the stratum corneum varies throughout the body. In the palms of the hand
s and the sole
s of the feet this layer is typically thicker, since these regions require additional protection in order to grasp objects and avoid injury. In general, the stratum corneum contains 15 to 20 layers of dead cells. The stratum corneum has a thickness between 10 and 40 μm.
In reptile
s, the stratum corneum is permanent, and is replaced only during times of rapid growth, in a process called ecdysis
or moulting. This is conferred by the presence of beta-keratin
, which provides a much more rigid skin layer.
In the human forearm, about 1300 cells/cm2/hr are shed.
LEKTI causes the debilitating disorder Netherton syndrome
.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s (corneocytes) that lack nuclei
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
, dehydration
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
, chemicals and mechanical stress. Desquamation
Desquamation
Desquamation , also called skin peeling, is the shedding of the outermost membrane or layer of a tissue, such as the skin.-Skin:Normal, nonpathologic desquamation of the skin occurs when keratinocytes, after moving apically over about 14 days, are individually shed unnoticeably...
, the process of cell shedding from the surface of the stratum corneum, balances proliferating keratinocyte
Keratinocyte
Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes"...
s that form in the stratum basale. These cells migrate through the epidermis towards the surface in a journey that takes approximately fourteen days.
Function
During cornification, the process whereby living keratinocytes are transformed into non-living corneocytes, the cell membraneCell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
is replaced by a layer of ceramide
Ceramide
Ceramides are a family of lipid molecules. A ceramide is composed of sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of cells. They are one of the component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer...
s which become covalently linked to an envelope of structural proteins (the cornified envelope).
This complex surrounds cells in the stratum corneum and contributes to the skin's barrier function. Corneodesmosomes (modified desmosome
Desmosome
A desmosome , also known as macula adherens , is a cell structure specialized for cell-to-cell adhesion...
s) facilitate cellular adhesion by linking adjacent cells within this epidermal layer. These complexes are degraded by proteases, eventually permitting cells to be shed at the surface. Desquamation and formation of the cornified envelope are both required for the maintenance of skin homeostasis. A failure to correctly regulate these processes leads to the development of skin disorders.
Cells of the stratum corneum contain a dense network of keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
, a protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
that helps keep the skin hydrated by preventing water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
. These cells can also absorb water, further aiding in hydration, and explaining why humans and other animals experience wrinkling
Wrinkle
A wrinkle is a fold, ridge or crease in the skin. Skin wrinkles typically appear as a result of aging processes such as glycation or, temporarily, as the result of prolonged immersion in water. Wrinkling in the skin is caused by habitual facial expressions, aging, sun damage, smoking, poor...
of the skin on the finger
Finger
A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates....
s and toes ("pruning") when immersed in water for prolonged periods. In addition, this layer is responsible for the "spring back" or stretchy properties of skin. A weak glutenous protein bond pulls the skin back to its natural shape.
The thickness of the stratum corneum varies throughout the body. In the palms of the hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
s and the sole
Sole (foot)
The sole is the bottom of the foot.In humans the sole of the foot is anatomically referred to as the plantar aspect. The equivalent surface in ungulates is the hoof.- Human sole :...
s of the feet this layer is typically thicker, since these regions require additional protection in order to grasp objects and avoid injury. In general, the stratum corneum contains 15 to 20 layers of dead cells. The stratum corneum has a thickness between 10 and 40 μm.
In reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, the stratum corneum is permanent, and is replaced only during times of rapid growth, in a process called ecdysis
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
or moulting. This is conferred by the presence of beta-keratin
Beta-keratin
β-keratin or beta-keratin is rich in stacked β pleated sheets, in contrast to alpha-keratin, a fibrous protein rich in alpha helices.β-keratin is found in reptiles...
, which provides a much more rigid skin layer.
In the human forearm, about 1300 cells/cm2/hr are shed.
Skin disease
An inability to correctly maintain the skin barrier function due to the dysregulation of epidermal components can lead to skin disorders. For example, a failure to modulate the activity of kallikreins via the disruption of the protease inhibitorProtease inhibitor (biology)
In biology and biochemistry, protease inhibitors are molecules that inhibit the function of proteases. Many naturally occurring protease inhibitors are proteins....
LEKTI causes the debilitating disorder Netherton syndrome
Netherton syndrome
Netherton syndrome is a severe, autosomal recessive form of ichthyosis associated with mutations in the SPINK5 gene. It is named after E.W. Netherton.- Characteristics :...
.
See also
- Epidermis
- Stratum granulosumStratum granulosumThe stratum granulosum is a thin layer of cells in the epidermis. Keratinocytes migrating from the underlying stratum spinosum become known as granular cells in this layer...
- Stratum spinosumStratum spinosumThe stratum spinosum is a layer of the epidermis found between the stratum granulosum and stratum basale. This layer is also referred to as the "spinous" or "prickle-cell" layer. This appearance is due to desmosomal connections of adjacent cells. Keratinization begins in the stratum spinosum....
- Stratum basaleStratum germinativumThe stratum basale is the deepest layer of the five layers of the epidermis, which is the outer covering of skin in mammals. The stratum basale is a continuous layer of cells...