Integumentary System 2: Dermis and accessory structures

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What is the dermis?

The dermis is a connective tissue layer divided into two layers; papillary and reticular.

The papillary layer is the more superficial layer composed of areolar tissue. It contains small capillaries, lymphatics and sensory neurons, as well as tactile corpuscles to detect light touch.

The reticular layer is composed of dense irregular connective tissue (collagen, elastic fibres, connective tissue proper). It contains larger blood vessels, lymph vessels and sensory neurons and receptors called lamellated corpuscles (pressure and vibration receptors). The reticular layer shows us 'skin turgor', which determines hydration.

NOTE: the hypodermis connects the dermis to underlying tissue. It is loose connective tissue with fat cells and large blood vessels. There is an absence of any vital organs or structures.

Hair

Hair, also termed pili, is composed of columns of dead keratinocytes held together by extracellular proteins. Parts of hair include:
  • Shaft: thin, superficial portion
  • Root: penetrates the skin
  • Follicle: connective tissue surrounding the column
  • Dendrites of hair root plexuses: surround each hair follicle, generating nerve impulses
Hair protects and insulates, guards openings against particles and insects, and is sensitive to very light touch. Colours in hair result in the degree of melanin in the the keratinised cells, although grey hair results from a progress decline in the synthesis of the tyrosinase enzyme.

Goose bumps are a result of the contraction of the arrective pili muscle, which is under the control of the sympathetic nervous system.

Sebaceous, Sweat and Ceruminous Glands

Sebaceous glands are connected to hair follicles and are the secretory portion in the dermis. They are absent in thick skin. They secrete sebum, which moistens hair, waterproofs and softens skin, inhibiting bacterial growth.

Sweat glands are divided into two categories; apocrine and merocrine sweat glands.
  • Apocrine sweat glands are limited in distribution to the skin of the axilla, areolae, pubis and beards (in males). The ducts run into hair follicles, which release sticky, cloudy secretions that may have an odour. Apocrine sweat glands do not begin to function until puberty.
  • Merocrine (eccrine) sweat glands are widely distributed on the body surface, particularly in the palms and soles of the feet. They discharge directly onto the skin surface, rather than from ducts which run into hair follicles. Merocrine sweat glands excrete water and electrolytes, unlike the sticky, cloudy and smelly secretions of apocrine sweat glands. Merocrine sweat glands function in heat loss.
Merocrine sweat glands are said to function in temperature homeostasis. Temperature homeostasis can be regulated by cutaneous blood flow; dilation of skin blood vessels releases heat and constriction of these vessels causes a rapid increase in core temperature. However, as mentioned, perspiration/sweating plays a role in regulating temperature. This is purely a sympathetic response:
  • Insensible perspiration occurs when interstitial fluid is lost by evaporation through the stratum corneum. It is continuous.
  • Sensible perspiration occurs when water is excreted by the sweat glands when triggered by heat or exercise.
Ceruminous glands are modified sudoriferous glands located in the external auditory canal. They produce a waxy substance called cerumen (ear wax), which impedes the entrance of foreign bodies.

What are nails?

Nails are plates of tightly packed, hard, keratinised epidermal cells. They consist of the nail body, free edge, and a nail root, which contains cells that undergo mitosis and allow nail growth. There are other parts to nails:
  • Lunula, an area of thickened stratum basale
  • Hyponychium (nail bed), an area of thickened stratum corneum that secures the nail to the tip of the digit
  • Eponychium (cuticle), a narrow band of epithelium occupying the proximal border of the nail
Nails grow through a nail matrix. In the epithelium deep to the nail root, cells divide by mitosis. The superficial cells of the matrix are then converted to nail cells. The average growth is 1mm per week.

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