The Integumentary System - The Skin

Consists of the skin and its derivatives including glands, hair and nails. The skin is also referred to as a keratinized epithelial membrane.

Structure of Skin

I.  The Epidermis – Consists of a keratinized stratified squamous epithelium.

     Cell Types – The epidermis contains a number of different cell types:

     A.  Keratinocytes – Most abundant cell type.

  1. Produce keratin, a tough, fibrous protein.
  2. Cells are held together by desmosomes.
  3. Organized into layers.
    1. Stratum germinativum – Cells found just above the basement membrane which undergo rapid cell divisions. Also called the stratum basale. Basal cell carcinoma originates here.
    2. Stratum spinosum is a middle portion of the epidermis several cells thick. The name derives from the appearance of these cells after they are fixed and processed for sectioning. The cells shrink but remain attached to each other by desmosomes giving the cells a "spiny appearance".
    3. Stratum granulosum is a layer just above the spinosum in which large amounts of keratohyalin is being synthesized. Granules of this basophilic protein can be seen in stained sections.
    4. Stratum lucidum - The keratinocytes in this layer are filled with keratohyalin and a fibrous protein, keratin. Keratin is tough, durable and water resistant. Keratin is also the chief component in hair and nails.
    5. Stratum corneum is the outermost layer of the epidermis composed of flattened, dead, keratin-filled cells. These dead cells are constantly being flaked off and replaced from below by cells dividing in the germinativium. This dead layer provides protection from abrasion, strong chemicals, microbial invasion and dehydration.

     B.  Other cell types in the epidermis include:

  1. Langerhans cells are T lymphocytes (cells that mount an immune response) located within the stratum spinosum. These cells will initiate an immune response against microbes and epidermal skin cancers.
  2. Merkel cells are located among the cells of the germinativium. They are sensitive to touch.
  3. Melanocytes are also located in the germinativium often under the basal cells just above the basement membrane. They manufacture a brown pigment called melanin. Melanin is packaged within vesicles called melanosomes in the cytoplasm of the melanocytes. These vesicles are transferred into the overlying keratinocytes, darkening the skin. Melanin is a helpful material in the skin. It tends to surround the nucleus of the keratinocyte protecting the DNA of that cell from damage due to ultraviolet light from the sun. It also seems to neutralize damaging chemicals called free radicals that accumulate in traumatized tissues. Cancer of the melanocytes, melanoma, is an extremely dangerous and malignant neoplasm.

 

Changes in Human Skin Color

  1. Tanning Effect – see previous comments concerning the action of melanocytes.
  2. Reddening of skin occurs due to an increased blood flow in the underlying dermis. The increasedblood flow may be caused by:
  1. The release of the vasoactive chemical Histamine due to trauma in the skin (inflammation).
  2. An increase in body temperature which leads to a vasodilation of blood vessels in the dermis.
  3. Strong emotional states will lead to an increase in blood flow in the dermis.

      3.  Blanching of Skin ("turning pale") occurs due to decreased blood flow to the skin. This decreased blood         flow may be caused by:

  1. A sudden drop in blood pressure – shock.
  2. Hypothermia
  3. Emotional states – fear, panic.

     4.  Cyanosis – bluish color in skin due to sustained reduction in blood supply to the skin. Hemoglobin in blood        becomes depleted of oxygen and appears blue through the layers of skin.

     5.  Jaundice – When liver function is interrupted due to cirrhosis, liver cancer or blockage of bile flow, yellow       bile pigments and bilirubin accumulate in the skin and whites of the eyes.

     6.  Tumor of the pituitary gland or Addison’s disease leads to an over production of melanocyte stimulating       hormone (MSH). MSH darkens the skin.

     7.  Vitiligo – In this condition an individual loses their melanocytes. It is thought to be due to an autoimmune       disease in which the immune system mistakenly attacks and destroys the body’s melanocytes.

II.  The Dermis – Located below the epidermis it contains all of the accessory organs of epidermal origin, such as, hair follicles and glands. It also has extensive networks of blood vessels, lymphatic vessels nerve endings and nerve fibers. The dermis consists of two major layers:

  1. The Papillary layer contains loose (areolar) connective tissue with a rich supply of blood capillaries. It also contains the nerve endings for touch and pain.
  2. The Reticular layer contains dense irregular bundles of collagen, elastic and reticular fibers. These fibrous bundles blend into the papillary layer above and into the underlying subcutaneous layer.

A.  Dermal Blood Supply – When arteries supplying the skin reach the subcutaneous layer, they form a network of branches called the cutaneous plexus. Branches from this plexus supply:

  1. The subcutaneous fat
  2. Various structures in the dermis such as hair follicles and sweat glands.
  3. The papillary plexus – capillary beds that follow the contours of the epidermal-dermal boundary.

Interruptions in this circulatory flow can result in epidermal and dermal deterioration and necrosis,
e.g., decubitis ulcers and diabetic foot.

B. Innervation of the skin – The skin is richly innervated. The functions of these nerves include:

  1. Control of blood flow through the skin.
  2. Adjusting the rate of glandular secretions.
  3. Sensory reception – The sensory receptors of the skin respond to two basic types of stimulation, mechanical change and pain.
    1. Mechanoreceptors of the skin
    1. Meissner’s corpuscles – located within the dermal papillae, they respond to light touch.
    2. Pacinian corpuscles – located deep within the reticular layer, Pacinian corpuscles are sensitive to deep pressure and vibrations.
    3. Merkel’s disks – are specialized nerve endings found at the ends of nerve fibers. These disks are closely associated with the epidermal Merkel’s cells and respond to fine touch.
    4. Ruffini’s corpuscles – are located deep in the reticular layer where they respond to distortion or stretching of the skin.

                   b.  Nociceptors – Pain receptors are especially abundant in the upper skin, joint capsules, the                      periosteum of bone and the walls of  blood vessels. Very few pain receptors are located in visceral                      organs or deep tissues. There are three types of pain receptors:

          i.  Those sensitive to temperature extremes.
         ii.  Those sensitive to mechanical damage.
        iii.  Those sensitive to chemicals, e.g., metabolites from traumatized tissues such as             aracidonic acid and prostaglandins.

      Very strong pain stimuli will excite all three types of receptors. For example, severe trauma of any kind (burns, cuts, corrosive chemicals) will often be described as "burning" or acute pain.

III.  Accessory Structures of the Skin
A.  Hair Follicle – A complex organ which is the source of the hair. Hair follicles project deep into the dermis and subcutaneous fat from the surface of the skin. Each hair follicle consists of the following:

1.  Hair Papilla – a small clump of connective tissue, capillaries and nerves which supports the growth of the hair.

2.  Hair Bulb – consists of the epithelium that surrounds the papilla. This structure is an invagination of the epidermis. Cells from the basal layer of the bulb divide and are pushed up into the root of the hair.

3.  Hair shaft – begins at a point about midway between the papilla and the skin surface.

4.  In cross section, the cells of the hair form several concentric layers including:

    a.  The medulla – the core of the hair made up of cells containing soft flexible keratin.
    b.  The cortex – the outer portion of the hair containing hard keratin which stiffens the hair.
    c.  The cuticle – a dead layer of cells on the surface of the hair.

             5.  A cross section of the follicle wall or Hair sheath surrounding the hair shows the following layers:

    a.  The internal root sheath – immediately surrounds the hair root
    b.  The external root sheath – contains all the cell layers of the epidermis
    c.  The glossy membrane – a thickened basement membrane found at the base of the external root sheath.
    d.  A connective tissue sheath surrounds the entire hair follicle from the hair bulb to just under the dermal papillary area.

  6.  The Arrector pili are a small bands of smooth muscle which extend from the connective tissue      sheath of the follicle and anchor in the papillary layer of the dermis. This muscle is stimulated to     contract by strong emotional states or cold temperatures. These stimuli operate through the
   nervous system to cause the hairs to become erect. This response is associated with a bunching
   up of the skin ("goose bumps"). In animals with long hair, this response produces an insulating
   blanket of air trapped within the coat of fur. In humans, this response has no real function.

B.  Glands in the Skin – The skin contains a number of exocrine glands

1.  Sebaceous glands

    1. Holocrine glands which discharge an oily secretion called sebum.
    2. The gland cells originate in the periphery of the gland. As they mature, the cells manufacture sebum, a mixture of triglycerides, cholesterol, proteins and electrolytes.
    3. As the cells reach the opening or lumen of the gland, they rupture releasing their product (holocrine secretion).
    4. There are two types of sebaceous glands:
      i.  Simple branched alveolar glands – empty their secretion into the follicle of a hair.
      ii.  Sebaceous follicles – large sebaceous glands that are connected directly to the epidermis and are not associated with a hair. They are found in the skin of the face, back, chest and nipples.
    5. Sebum functions by lubricating the skin and retarding the growth of bacteria.

2.  Apocrine glands

    1. Located in the armpits, groin and around the nipples.
    2. They produce a sticky, cloudy, odorous secretion into a hair follicle.
    3. These secretions become intensified at the time of puberty under the influence of the nervous and endocrine systems.

3.  Merocrine (eccrine) sweat glands

    1. Very numerous. In the adult, the skin may contain 2 to 5 million merocrine glands per square inch. Palms and the soles of the feet have the highest concentration.
    2. Merocrine glands are smaller than the apocrine glands.
    3. They produce a watery sweat containing electrolytes, lysozymes, antibodies and other ingredients.
    4. The functions of these glands include:
    5. i.  Removing heat from body’s surface to lower body temperature. During periods of extreme sweating, the rate of perspiration may exceed 1 gallon of water / hour.
      ii.  Excretion of water, electrolytes and nitrogenous wastes.
      iii.  Protection from chemical and microbial

     C.  Nails – are epidermal derivatives composed of dead, tightly compressed cells filled with keratin.
                                              (Review the basic structure of a nail – optional topic)

IV.  Functions of the Skin

A.  Regulation of body temperature

  1. During hyperthermia
    1. Blood vessels (arteries) in the skin dilate due to nerve stimulation from the brain.
    2. Blood flow to the skin increases.
    3. Warm water moves from the blood to the merocrine sweat glands by filtration.
    4. This warm sweat now moves to the skin’s surface by the duct from the gland.
    5. The warm sweat evaporates from the skin’s surface taking excess body heat with it.
    6. Body temperature comes down and sweating is reduced (negative feedback).

      2.   During hypothermia

    1. Blood vessels in the skin constrict due to nerve stimulation from the brain. This reduces blood flow to the skin.
    2. Radiant heat loss is reduced from the skin.
    3. Sweat production decreases.
    4. Body heat is retained in the trunk and head or "core" of the body.
    5. Shivering, an involuntary contraction of skeletal muscles due to nerve stimulation, helps to generate body heat. This is the result of the breakdown or "burning" of glucose in the skeletal muscles.

B.  Protection from ultraviolet light

  1. Ultraviolet light penetrates the epidermis and stimulates the melanocytes to increase their production of melanin.
  2. Melanocytes inject packages of melanin called melanosomes into cells of the lower epidermis. As these darkened cells divide, they are pushed to the top of the epidermis. Eventually, the entire epidermis becomes a "sunscreen" blocking out much of the harmful ultraviolet light.

C.  Protection from infection by microbes

  1. The tough impervious epidermis forms a barrier to microbes, as long as it is intact.
  2. The formation of sweat and sebum flushes away microbes on the surface of the skin, as well as those in sweat gland pores and hair follicles. The pH of sweat and sebum tends to retard the growth of bacteria. Sweat also contains lysozymes and antibodies which attack microbes.
  3. The blood-rich dermis reacts quickly to microbial invasion through the activities of various cells. Tissue basophils release chemicals which increase the blood supply to an infected region of skin. Phagocytic cells such as the macrophages and neutrophils physically attack microbes. The combined reaction of these cells is referred to as inflammation.

D.  Protection from excessive water loss or dehydration. The tough, keratinized epidermis forms a watertight cover on the body keeping body water in.

E.  Production of Vitamin D - Some of the ultraviolet light striking the skin passes through the melanin sunscreen and causes a chemical change in the blood leading to the formation of vitamin D. This vitamin is required for the body to absorb calcium from the intestine. Calcium is essential for the development of teeth and bones.