What are the layers of the skin?

Layers of the skin:

With the average surface area of 2m² (meter square) in the adult, the skin is the largest organ in the human body. It has 3 layers: (From outside to inward)

• The epidermis
• The dermis
• The subcutis /hypodermis

Structure of the skin (from Dermatology Secrets Plus 4th Edition; Fig: 1-1)

The Epidermis:

The epidermis is a compound tissue consisting mainly of a continuously self-renewing, keratinized, stratified squamous epithelium. The epidermis is only 0.1-0.2 mm thick with the exception on the palms and soles where it can extend up to several mm.

Cells in the Epidermis:

• Keratinocyte: They are the principal cells of the skin. They make up approximately 90% of the epidermal cells. They originate from the proliferating basal layer composed of germinative cells. Keratinocytes produce structural proteins (keratin, loricrin, filaggrin, desmoglein, desmoplakin), synthesize vitamin D under the influence of UVB, secrete a variety of cytokines (interleukins, IFN-γ, TNF-α), produce antimicrobial peptides (β-defensin and cathelicidins). They migrate from the basal layer up to topmost layers (stratum corneum) and during the journey, they differentiate and ultimately shed out. It takes around 30 days for keratinocytes to migrate from the basal layer to the skin surface in the normal skin but is considerably faster in some disease condition e.g. psoriasis

• Non-keratinocytes cells: Remaining 10%

• Melanocytes: Pigment-forming cells from the embryonic neural crest and reside in the basal layer in a ratio of approximately 1 for 10 keratinocytes.  They synthesize the pigment melanin (from tyrosine), which is transferred into surrounding keratinocytes to give protection against the UV radiation. Racial differences in skin colour are due to variation in melanin production, not melanocytes numbers.
• Langerhans cells: Immature antigen-presenting dendritic cells derived from the bone marrow. They form a network across the suprabasal layer (stratum spinosum) and play a role in antigen presentation and in immunoregulation.
• Lymphocytes: May be present with Langerhans cell in stratum spinosum layer
• Merkel cells: They are found in the basal layer. These may function as sensory mechanoreceptors and are associated with nerve endings. They are numerous on fingertips and in the oral cavity.

Layers of the epidermis:

The epidermis can be divided into 5 layers from deep to superficial as follows:

• Stratum basale (basal layer): Contains keratinocytes, melanocytes and Markel cells. Occasionally Langerhans cells may be found.
• Stratum spinosum (spinous or prickle cell layer): Beside keratinocytes, Langerhans cells and the occasionally associated lymphocyte are found here. It is the thickest layer of the skin
• Stratum granulosum (granular layer): Contain 3-4 layers of flattened keratinocytes. In this layer, keratohyalin granules are formed and bind to the keratin filaments to form large electron-dense masses within the cytoplasm that gives this layer its “granular appearance”.
• Stratum lucidum (clear layer): Some books do not consider it as a separate layer. They are present only in thick skin (sole and palm)
• Stratum corneum (cornified layer): The transformed keratinocytes which have lost their nuclei and organelles, now known as corneocytes are found here. It is basically a dead layer.

Epidermal layers ((from Dermatology Secrets Plus 4th Edition; Fig: 1-2)

The deep three of these layers are metabolically active compartments through which cells pass and change their form as they progressively differentiate. The more superficial layers of cells undergo terminal keratinization or cornification, which involves not only structural changes in keratinocytes but also alterations in their relationships with each other and with non-keratinocytes, and molecular changes within the intercellular space. 

Epidermal appendages:

• Hair follicles
• Sebaceous gland
• Sweat gland
• Nails

The dermis:

It is over 1 mm thick on the inner forearm and around 4 mm thick on the back. Fibroblasts are the predominant cells here and they produce collagen, elastin and reticulin fibres. They are again divided into two parts: (From outside to inward)

• Papillary dermis
• Reticular dermis

The subcutis/hypodermis:

It consists predominantly of adipose tissue. It contains about 80% of all body fat.

Source:

• Kumar & Clark’s Clinical Medicine 9th Edition: 1337
• Gray's anatomy- 40th Edition: 145
• Dermatology Secrets Plus 4th Edition: 6
• Davidsons Principles and Practice of Medicine 23 edition: 1212