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Module 1: Anatomy & Physiology of the Skin
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DERMIS

The dermis is the thickest layer of the skin. It ranges in thickness from 2-4mm and tends to be very thick on the palms of the hands and soles of the feet and thin in areas such as the eyelids. It is also thicker on the back (posterior) than the front (anterior) of the body.

The dermis is composed of connective tissue containing collagen and elastic fibres that make it tough and elastic. When the skin is overstretched, rupture of elastic fibres occurs, resulting in permanent striae or stretch marks, such as those that may be found in pregnancy, obesity, or weight loss. Collagen is the protein that gives skin its tensile strength, although this ability declines with age, when wrinkles may develop.

Fibroblasts, macrophages, and mast cells are the main cells found in the dermis (Tortora & Derrickson, 2013).

Structures within the skin

  • Blood Vessels
  • Arector Pili Muscle
  • Sebaceous Gland
  • Sweat Gland
  • Sweat Pore
  • Nerve
  • Hair Follicle
  • Meissner's Corpuscle
  • Hair Shaft
  • Pacinian Corpuscle
  • Lymph Vessel
  • Stratum Corneum

    • There are several structures found in the dermis.

    • Blood vessels (capillaries) - supply oxygen and nutrients to sweat glands and the dermis.
    • Lymph vessels - the lymphatic system is responsible for the transportation of particulate and liquid material, such as protein, from the extracellular compartment of the dermis. Lymph vessels have broad lumens, with single-cell thick walls that transport fluid away from the skin to the lymph nodes, maintaining homeostasis in the tissues.
    • Sensory nerve endings - skin is an important sensory organ. Incoming stimuli are received by sensory receptors in the dermis and transmitted to the brain (Tortora & Derrickson, 2013).
    • Sweat (sudoriferous) glands - there are two types of sweat gland:
      • Eccrine sweat glands are important in body temperature regulation. A human has between two and three million eccrine sweat glands covering almost all the body surface (Tortora & Dickinson, 2013). They are most abundant on the palms of the hands and soles of the feet. The eccrine glands secrete water, electrolytes, lactate, urea and ammonia (Wound Care Resource, 2015). Sweat is conveyed directly to the surface of the skin via a secretory coil deep in the dermis.
      • Apocrine sweat glands are found in the axillae and genital area. Specialised apocrine glands are also found in the milk glands of the breast and the wax gland of the ear. Apocrine glands also consist of a secretory coil and are situated in the dermis, however, in the hair follicle rather than directly onto the skin surface. These glands lie dormant until puberty. Body odour is the result of the action of bacteria on the secretions released from the apocrine glands (Tortora & Derrickson, 2013).
    • Hairs (pili) - hair is a protein filament that grows from follicles found in the skin. Hair follicles are formed by the down-growth of the epidermal cells into the epidermis or subcutaneous tissue. At the base of the follicle is a structure called the bulb. As cells at the bulb multiply and are pushed away from their source of nutrients, they die and become keratinised - this is how hair is formed. Hair above the skin is called the shaft, while hair within the dermis is called the root. Arrector pili muscles are smooth muscle fibres attached to the hair follicles. They contract under stress, e.g. fright and cold, and the hairs have the ability to move vertically.
    • Sebaceous (oil) glands - secrete an oily substance called sebum into hair follicles that help to keep hair from drying out. They are absent in the palms of the hands and soles of the feet and vary in size and shape throughout the rest of the body. Together, the epidermis and the dermis form the cutaneous layer.