Index
The skin is an organ that provides the outer protective envelope for all body parts.
The skin is the largest organ in the body. It is a waterproof, airtight, and flexible barrier between the environment and internal organs.
It is a mosaic of cells stuck together, and its thickness depends on its location in the body. The skin is divided into three layers, the epidermis, the dermis, and the subcutaneous layer.
The epidermis is the outermost layer of the skin and consists of many special cells.
The epidermis is an elastic layer on the outside that is continuously regenerated; this layer is shed and constantly replaced every 15 to 30 days.
Structurally, the epidermis is only about a tenth of a millimeter thick, but it comprises 40 to 50 rows of stacked squamous epithelial cells.
The epidermis does not have blood or blood vessels; it is an avascular region of the human body.
The cells present in the epidermis absorb all the nutrients through the diffusion of fluids from the dermis.
In most of the body, the epidermis is arranged in 4 different layers.
There is a fifth layer of epidermis present on the surface of the palms of the hands and the surface of the soles of the feet, where the skin is thicker than on the rest of the body.
Embryology
The skin begins to develop in the fourth week of fetal life.
It begins initially as a thick layer of single-cell ectoderm, under which the mesoderm cells proliferate and divide.
Fingernails and toenails are keratinous structures and arise only from the ectoderm.
Specialized structures such as sweat glands (apocrine and eccrine), hair follicles, and sebaceous glands develop from the epidermis and grow downward as invaginations penetrate the dermis.
Males have thicker skin all over their bodies than females.
The skin on the soles of the feet and the palms of the hands are thickest, and the eyelids and post-auricular skin are the thinnest.
Variations in thickness depend on dermal thickness, as epidermal thickness remains relatively constant throughout the body.
The epidermis is a layer that arises from the superficial ectoderm and is colonized by melanocytes, Langerhans cells (dendritic cells), and Merkel cells (pressure sensing receptors).
It is devoid of blood vessels and relies on the dermis below to draw blood and nutrients and remove waste.
Features
The epidermis plays an essential role in protecting the skin. Among the main functions are:
Thermoregulation
The skin helps maintain body temperature.
When the body is hot, there is vasodilation (widening of the blood vessels) on the skin’s surface. This cools the skin by allowing heat to escape.
There is constriction (narrowing of the blood vessels) when it is cold. This allows less heat to escape, helping to conserve temperature.
Metabolism
When it’s hot, or you exercise, your skin’s sweat glands excrete salts and proteins into the water.
Once on the surface of the skin, sweat evaporates into the air.
This cools the skin and helps control body temperature.
Sensation
There are many nerve endings and receptors that detect changes in the skin.
This allows you to feel everyday objects, feel pain, differentiate heat from cold, and feel pressure.
Protection
As the skin covers the entire body and is a continuous layer, it acts as a barrier and protects the body from mechanical and chemical injury, and protects against bacteria, viruses, and parasitic infections.
The pigment in the epidermis also plays a vital role in protecting the skin against ultraviolet radiation and preventing it from drying out.
It also prevents the loss of water and body fluids. It keeps the internal environment of our body stable.
Synthesis of vitamin D
When exposed to the sun’s rays, the skin produces vitamin D3. This is essential for building solid and well-formed bones.
Layers of the epidermis
The epidermis is subdivided into five layers.
Stratum corneum
The outermost layer of the skin is the stratum corneum. The stratum corneum is made up of many rows of cells.
The cells in the stratum corneum layer are known as corneocytes (or horny cells) which are thin, flat, dead cells without nuclei whose protoplasm has changed to keratin (horny substance).
Corneocytes are composed primarily of keratin proteins, which provide structural strength to the stratum corneum and allow water absorption.
The cells lie flattened on top of each other like fish scales, protecting the underlying layers.
Dead keratinocytes are constantly shed from the surface of the stratum corneum and are replaced by cells that come from the deeper layers.
A protein envelope surrounds the stratum corneum’s corneocytes (mature keratinocytes).
They are arranged in layers about 20 cells thick (the number of cells in this layer varies by location), which remain attached due to the corneodesmosomes and surrounding lipids.
The total lifespan of epidermal cells is about 40 days.
Many cells are tightly packed together, allowing the skin to be rugged and waterproof.
This layer is essential as they serve as an effective barrier against any chemicals, insects, and bacteria that can damage the living cells just below them.
The structure of the stratum corneum may seem simple, but it plays a crucial role in maintaining the structural integrity and hydration of the skin.
This layer not only ensures the continuous production of new skin cells but also provides the body with vital protection against viruses, bacteria, parasites, and any other form of pathogen or toxin.
The skin’s barrier functions are highly dependent on this layer, including fighting infection, chemical effects, daily wear and tear, and dehydration.
When corneocytes shed this layer due to degradation, the process is known as desquamation.
It takes two weeks for cells to migrate from the stratum basalis to the stratum corneum.
The cells present in this layer are the largest and most abundant in the entire epidermis.
Lucid stratum
In the thick skin of the hands and feet, there is a layer of skin superficial to the stratum granulosa known as the stratum lucid.
The stratum lucid comprises several rows of clear, dead keratinocytes that protect the underlying layers.
The name stratum lucid comes from the Latin for precise, which is “lucid” or “transparent layer,” which describes the transparency of the cells themselves.
This layer is approximately four keratinocytes thick.
It is the second layer from the top; the cells are distinctively grouped, and some may show flattened nuclei.
Cells are full of eleidin. Eleidin is a clear intracellular protein, a transformation product of the amino acid complex.
Its primary function is to reduce the friction between the stratum corneum and the stratum granulosa.
Granular layer
Just superficial to the spinous stratum is the granular stratum is the fine layer.
The stratum granulosa is made up of 3 to 4 layers of cells.
It is the third layer from the top; it consists of living cells of defined shape where mature anucleated keratinocytes reside with their cytoplasmic granules.
Lipids, which are initially polar, are located within the cytoplasm of these cells and extrude to form a barrier on the cell surface, where they become nonpolar.
Cells in the stratum granulosa, or granular layer, have lost their nuclei and appear as flattened cells that contain dark clumps of cytoplasmic material.
This layer has a lot of activity as keratin proteins and lipids work together to create many of the cells responsible for the skin’s protective barrier.
In the stratum granulosa, keratinocytes produce waxy lamellar granules to waterproof the skin.
Keratinocytes in the stratum granulosa are so far from the dermis that they begin to die from a lack of nutrients.
Spinous stratum
This fourth layer contains cells that transform from columnar to polygonal (multifaceted), have a spiny appearance (keratinocytes), and contain nuclei.
Less developed keratinocytes sit in the stratum spinosum and are connected via desmosomes.
The spiny appearance of this layer is due to cell contraction (fixation artifact for histology slides), resulting in spine-like desmosomes.
Langerhans cells are also found in this layer.
They arise from the bone marrow and are dendritic (antigen-presenting) cells that fight infection.
They are found in multiple layers of the skin (basal, spinous, and granular) but are more abundant in the stratum spinosum.
The spinous layer is located just above the basal stratum and is between 5 and 10 cells thick.
Cells that move into the spinous layer (also known as the spiny cell or the squamous cell layer).
The cells in this layer are responsible for producing keratin, the fibrous protein that gives skin, hair, and nails their toughness and waterproof properties.
This is the layer of the stratum spinosum that is superficial to the stratum basalis. Langerhans cells are found here, along with many rows of spiny keratinocytes.
The spines are cell projections called desmosomes that form between keratinocytes to hold them together and resist friction.
basal stratum
The stratum basalis is the deepest layer of the epidermis. The stratum basalis is where keratinocytes are dividing and growing, where the keratinocyte stem cells responsible for producing all the cells of the epidermis can be found.
Cells of the stratum basalis include cuboidal keratinocytes, melanocytes, and Merkel cells.
Melanocytes arise from neural crest cells and produce melanin (the pigment that gives skin its color).
The epidermal layer consists of a single row of keratinocytes attached to the underlying dermis by papillae, called basal cells.
These cells are constantly produced in this layer and push the cells that have already formed into the other layers of the epidermis.
When keratinocytes arise from the stratum basalis, they gradually ascend to the stratum corneum in a process that lasts 14 days.
As the basal cells move into the upper layers, they also flatten out, die, and shed to make room for the newer cells.
The stratum basalt is separated from the next layer, the dermis, by a basement membrane, a layer made of collagen and proteins.
Keratinocytes are attached to the basement membrane below by hemidesmosomes.
Cells present in the epidermis
The epidermis is made up of several specialized types of cells. There are main groups of cells in the epidermis, such as:
Keratinocytes
They represent about 90% of the cells of the epidermis.
When keratinocytes develop, they begin to produce and store the protein keratin.
Keratin gives it resistance and impermeability to keratinocytes.
They are cuboidal or columnar cells; they contain a nucleus and carry cholesterol.
They are the primary cells of the epidermis formed by cell division in the lower or basal layer and then migrate upwards for approximately four weeks to the outer surface (stratum corneum), where it is shed.
Corneocytes: dead keratinocytes flatten and together form the outermost layer of the epidermis; they are called stratum corneum or horny layer. This protective layer is continually flaking off.
Melanocytes
They represent about 8% of the cells of the epidermis. They are the second most numerous cell type in the epidermis.
Melanocytes are responsible for producing a pigment called melanin that protects the skin from ultraviolet radiation from burns caused by the sun and gives the skin its color.
Melanocytes are branched or dendritic structures and their dendrites are used to transfer pigment granules (melanin) to adjacent epidermal cells.
All humans have the same number of melanocytes.
The difference in skin color occurs because, in darker skin, the melanocytes produce more pigment.
The melanin pigment protects the cells of the epidermis and the tissues of the dermis from sun damage.
Fair-skinned people are more susceptible to developing sun-damaged skin because melanocytes produce less melanin (skin pigment).
Langerhans Cells
They represent about 8% of the cells of the epidermis.
Langerhans cells have an immunizing function, detect and fight pathogens that intend to enter the body through the skin.
Langerhan cells are part of the immune system.
In addition to being responsible for the skin’s immunity, they provide responses to allergies.
Merkel cells
Merkel cells make up less than 1% of all cells in the epidermis.
Merkel cells make up a disk arranged along with the deepest layer of the epidermis.
They connect to the nerve endings present in the dermis and detect light touch.
Merkel cells are pressure receptors found in this layer and sensory nerves for tactile understanding and discrimination.
Meissner corpuscles detect light touch and low-frequency vibrations and are primarily on the fingertips.
Other structures
Hair follicles, sweat glands, sebaceous (oil) glands, and apocrine glands develop from epidermal cells, but their deeper parts extend into the dermis.
The glands open to the surface of the skin through small ducts.
Hair grows from the hair follicle, found on all skin except the palms and soles.
Nails are specialized plates of hard keratin that develop from the epidermis that lines the small bones at the fingers’ ends and toes.
Melanin
It is a brown pigment secreted by melanocytes in the basal layer, part of the amino acid tyrosine; the production is due in part to heredity and, in part, to hormonal secretions; exposure to ultraviolet rays increases the production of melanin.
Its primary function is to protect the skin against the harmful effects of sunlight.
Melanin is also found in the retina, uveal tract, and hair follicles.
The melanin produced here also accumulates within the organelles (melanosomes), which anchor the pigment to the surrounding keratinocytes.
They are within keratinocytes and are located within the cytoplasm as granules. Melanoma (a type of skin cancer) arises from these cells.
Adrenocorticotropic hormone, melanocyte-stimulating hormone, and estrogens promote melanin production.
Ethnic differences in skin color arise from the size of the melanosomes rather than the number of cells.
Melanosomes typically reduce in number with age.
Keratinization
Keratinization begins in the spinous stratum and ends in the granular stratum.
Moisture is lost from the cells, they begin to lose their nuclei, keratohyalin granules form, and eventually, the cells die.
This process accelerates due to age, poor circulation, diseases such as psoriasis, and poor nutrition, among others.
Associated conditions
Skin cancer
There are many types of skin cancer, depending on the type of cell and the nature of the tumor.
Basal cell carcinomas are rodent ulcers (due to their dark gnawing appearance) and arise from the stratum basalis.
Squamous cell carcinomas arise from squamous cells found in the stratum spinosum.
Melanoma arises from melanocytes and is cancer. Treatment usually involves surgical excision.
Moles or moles
These are the result of the benign proliferation of melanocytes.
They can be congenital or acquired.
People with whiter skin tend to have more moles.
Surgical skin flaps
Plastic surgeons use the skin in all their operations.
Skin grafts can cover defects at other sites (near or distant) and are transplanted into the vascular bed of the fault.
Split-thickness skin grafts (epidermis and a layer of dermis) can be taken and laid out in a network as a formation for use at a distant site.
The donor site recovers a few weeks later.
Ehlers-Danlos syndrome
This collagen disease usually causes symptoms such as stretchy skin, hypermobile joints, or fragile skin.
Symptoms and type of disease depend on the kind of collagen affected.
Psoriasis
This is an autoimmune condition that produces pink, scaly plaques throughout the body.
There is a vital genetic component, and treatment includes phototherapy, immunosuppressants, topical agents, and alternative therapies.
Eczema
This is also known as atopic dermatitis, and it is essentially a dry inflammation of the skin.
The skin develops patchy areas of dryness. The condition is linked to other hypersensitivity conditions, for example, asthma and hay fever.
Treatment includes emollients and steroid creams.
Burns
Burns vary based on location, depth, and surface area.
Treatment includes fluid replacement (Parkland’s formula for the amount of fluid depends on the surface area of the burn), skin grafts, and antibiotics to reduce the risk of wound infection.
Burn patients lose fluid rapidly due to exposed tissue.
