The skin is the largest organ of the body, and is involved in sensation, defence, thermoregulation and other functions. It is the most common site of cancer in humans although many malignancies are captured early in the course of disease. The structure and lymphatic drainage of skin are of particular importance.
The skin can be divided into two components:
- The skin itself is usually considered to be the epithelial epidermis and the underlying dermis
- Skin appendages are structures that are found in skin that contribute to various functions, such as hair follicles, sweat glands and fingernails.
There are two types of skin. Thin skin, which covers most of the body, is notable for a thinner stratum corneum than the other layers of epidermis. The stratum corneum in thick skin is broader than the other epidermal layers. Thick skin is found in the palms and soles.
The external surface of the eyelids contains the thinnest skin (epidermis only several cells thick) whereas the skin on the palms may be up to 150 μm thick. If taken together, the epidermis and dermis of the back skin is 4 mm thick.
The skin has five primary functions:
- Protection - The skin acts as a barrier against toxic insults, including physical, thermal or biological. It prevents these agents from reaching deeper, more critical structures.
- Thermoregulation - The skin is highly vascular, and blood can be directed to or from the dermis by the autonomic nervous system. This allows the body to lose (or gain) heat from the environment
- Sensation - The skin contains a number of different sensory bodies that can distinguish touch, pain, temperature and stretch. They provide essential feedback to the brain regarding the environment
- Metabolism - Although not a major function, vitamin D synthesis can occur in the skin in response to sunlight. Electrolytes can also be excreted via sweat.
- Reproduction - The skin and appendages contain visual and other non-verbal forms of sexual signalling, such as apocrine sweat glands.
The skin contains two large arterial plexuses. The first lies at the junction of the dermis and hypodermis. This communicates readily with the second plexus at the junction between the rete and papillary dermis. From the second plexus, capillaries ascend into the papillae the supply the overlying epidermis. The blood supply to individual portions of skin is beyond the scope of this section.
Venous blood collects in plexuses aligned with those of arterial blood.
Lymphatic vessels accompany the other vascular structures. These vessels converge to form larger channels that return to regional lymph nodes.
The skin receives rich innervation from somatic sensory as well as autonomic nerves. Somatic sensory inputs are detected by a variety of receptors that are located in all layers of the skin. The most important cell in oncology is the Merkel cell (see epidermis), which is thought to give rise to Merkel cell carcinoma as well as detect light touch.
Potential Routes of Malignant Spread
Neoplasms arising in the epidermis (the majority) must first penetrate the basement membrane to be considered malignant. They may continue to evade locally, and often become ulcerated centrally due to loss of blood supply.
Regional lymph nodes are commonly involved in Merkel cell carcinoma and less frequently in squamous cell carcinoma. The exact nodes involved depends on the skin location and lymphatic drainage. Malignancies of the head and neck travel to the cervical ring of nodes; those of the arms pass to axillary nodes, those from the legs to inguinal nodes. The lymphatic drainage of the trunk is less well defined but lymph from above the umbilicus typically passes to the axillary nodes whereas that from below passes to inguinal nodes. The skin of the buttocks may, rarely, pass lymph through to internal iliac nodes along the gluteal vessels.