Anatomy of the Skin
The skin is the largest organ and covers the external surface of the human body. It protects us from ultraviolet radiation, from variations in temperature, pathogens and other toxins. Our skin is also our connection with the outside world and contains sensory receptors and nerves that enable us to experience tactile sensation and temperature variations. It can also give us our sense of racial identity and ethnicity. This article will look at the anatomy of the skin including its layers and components. We will also discuss function and clinical relevance.
The skin is also referred to as the integument, and consists of the epidermis and dermis, two closely associated layers. Beneath the skin we have the subcutaneous layer, which contains fat, vessels and other structures. Skin is constantly shedding and being replaced by epithelial cells which divide frequently.
The skin starts developing in the fourth week of foetal life. It begins initially as a single cell thick layer of ectoderm, below which the cells of the mesoderm proliferate and divide. Fingernails and toenails are keratinous structures, and arise from the ectoderm alone. Specialised structures such as sweat glands (apocrine and eccrine), hair follicles and sebaceous glands develop from the epidermis, and grow downwards as invaginations into the dermis. Males have thicker skin all over the body than females, with the skin of the soles and palms the thickest, and eyelid and post auricular skin being the thinnest. Variations in thickness are dependent on dermal thickness, as epidermal thickness remains relatively consistent throughout the body.
This layer arises from the surface ectoderm and is colonized by melanocytes, Langerhans cells (dendritic cells), as well as Merkel cells (pressure sensing receptors). It is devoid of blood vessels and relies on the dermis underneath for blood, nutrients and waste disposal.
The stratum corneum is composed of corneocytes (mature keratinocytes), each of which is surrounded by a protein envelope. They are arranged in around 20 cell thick layers (number of cells in this layer varies by location), which remains together due to corneodesmosomes and surrounding lipids. The barrier functions of the skin are largely dependent on this layer, and include fighting infection, chemical effects, daily wear and tear as well as dehydration. When corneocytes are shed from this layer due to corneodesmosome degradation, the process is referred to as desquamation. It takes cells two weeks to migrate from the stratum basale to the stratum corneum. Cells that sit in this layer have no nuclei, and are the largest and most abundant in the whole epidermis.
The name stratum lucidum comes from Latin for clear, which is ‘lucid.’ This layer only exists on the palms and soles, where the thickest skin of the body can be found. This layer is approximately 4 keratinocytes thick.
The stratum granulosum is the granular layer where the mature anucleated keratinocytes reside with their cytoplasmic granules. Lipids, which are initially polar, are located within the cytoplasm of these cells, and are extruded to form a barrier on the cell surface, where they become non-polar.
Less developed keratinocytes sit in the stratum spinosum, and are connected through desmosomes. The spinous appearance of this layer is due to the shrinking of the cells (artefact of fixation for histology slides), which results in desmosomes resembling spines. Langerhans cells are also located in this layer. They arise from the bone marrow, and are dendritic cells (antigen presenting cells) that fight infection. They are found in multiple layers of the skin (basale, spinosum and granulosum) but are most abundant in the stratum spinosum.
The stratum basale is where the keratinocytes are dividing and growing, and therefore where the keratinocyte stem cells can be found. They are attached to the basement membrane underneath by hemidesmosomes. As keratinocytes arise from the stratum basale, they gradually ascend to the stratum corneum in a process that takes 14 days. This layer also contains melanocytes, which arise from neural crest cells, and produce melanin (the pigment which gives skin its colour).
Melanin is also found in the retina, uveal tract and hair follicles. The melanin produced here also accumulates within organelles (melanosomes), which then anchor the pigment to the surrounding keratinocytes. They end up within the keratinocytes themselves and sit within the cytoplasm as granules. Melanomas (type of skin cancer) arises from these cells. ACTH, MSH and oestrogens promote the production of melanin. Ethnic differences in skin colour arise from the size of the melanosomes rather than the cell number. Melanosomes decline in number with age.
Pressure receptors (Merkel cells) are also found in this layer, together with sensory nerves for tactile discrimination/acuity.
Meissner’s corpuscles detect light touch and low frequency vibrations and are found mainly in the fingertips. The Pacinian corpuscles lie within the deep dermis and detect pressure and high frequency vibrations. Cutaneous nerves detect sensation, and Krause bulbs and Ruffini corpuscles detect temperature (cold and heat respectively). Each spinal nerve supplies a given dermatome, or strip/area of skin. They can be tested clinically to look for lesions in the nerves or the nerve roots.
The thin and undulating basement membrane is composed of two layers, that lies between the stratum basale and the dermis. It is formed by the lamina lucida (it is thin and lies right below the stratum basale) and the lamina densa (it is deeper and lies adjacent to the dermis). It regulates the movement of molecules and ions at the dermal-epidermal junction and anchors the epidermis to the dermis. Proteins from this layer have been found to speed up the process of epithelial cell division. The layer can also act as a barrier to slow the spread of cancers arising in the epidermis to the dermis and beyond.
This layer of skin arises from the mesoderm, and contains blood vessels, sensory receptors, fibroblasts and collagen. Its primary function is to support and sustain the epidermis.
This is the more superficial of the two dermal layers, and is composed of loose connective tissue that is filled with capillaries, collagen and elastic fibers. There are a number of dermal papillae that arise from this surface and form an irregular layer. They act to increase the surface area between the dermis and epidermis for the transfer of oxygen and nutrients up to the epidermis.
The deeper reticular dermis is thicker and is composed of thicker connective tissue. This matrix is bathed in a fluid containing a number of molecules including mucopolysaccharides and glycoproteins. The deepest border of the reticular dermis is irregular and is superficial to the subcutaneous layer, which acts as a cushion beneath the skin.
Within the dermis, the major cell type is the fibroblast. It gains its name from its ability to form procollagen (which is transformed into collagen by cleavage and cross linkage) and numerous elastic fibers. In terms of weight, collagen provides 70% of the weight of the dermis. Type 1 collagen makes up 85% of this, with type 3 collagen forming the remainder. Collagen provides high strength as well as resistance to shearing and mechanical forces. Elastic fibers enable the skin to return to its original shape after deformation, and also has resistance to deformation of the skin.
These are additional structures that lie within the dermis, and are lined with epithelial cells that enable them to divide and replicate swiftly. In cases of epithelial damage, these appendages act as a source of additional epithelial cells.
These are also known as holocrine glands. They produce sebum (which is composed of lipids such as triglycerides, wax esters, squalene and free fatty acids among other substances). It lubricates the skin, maintains moisture and protects against friction. Sebaceous glands are found all over the body (highest concentration in the face and scalp), except for the palms and feet (sole and dorsum).
Eccrine sweat glands
They are found all over the body (aside from the lip border, the external ear and parts of the external genitalia i.e. glans penis and labia majora). This gland produces sweat that cools the body when it evaporates. A coiled duct that connects to the epidermis forms the gland and the temperature center in the hypothalamus regulates sweat formation.
These are found in the axilla and anogenital area and begin working during puberty. The sweat secreted by apocrine glands only produces odour when in contact with the bacterial skin flora. The mammary gland is essentially a highly modified apocrine gland. Modified versions are also found in the ear (ceruminous glands) and the eyelid (Moll’s glands).
Vasculature and lymphatics
The major arteries of the body form complex vascular networks that perfuse the skin. They also aid with thermoregulation and supply the skin with oxygen and nutrients.
There are lymphatic vessels that arise from the interstitial spaces of the dermal papillae. They drain to the deeper plexi (dermal and subdermal), which join to form larger channels and ultimately enter the subclavian vein to reach the venous circulation.