Tissues and structures (integumentary system; skin, Fascias and Bursae)

SKIN

Description
       The skin is the largest organ in the body, it is made up of two layers ; epidermis and dermis.

Epidermis : the epidermis is keratinized i.e it has a top superficial layer that provides a protective outer surface overlying it's regenerative and pigmented deep or basal layer. Epidermis has no blood vessels or lymphatics. The avascular epidermis is nourished by the underlying vascularized dermis. The various skin appendages - sweat glands, sebaceous glands, hair and nail are specialized derivatives of this epidermis which is ectodermal in origin.

Dermis : dermis is mesodermal in origin and consisting mainly of collagen fibers, together with some elastic tissues, blood vessels, lymphatics and nerve fibers. The fibers provide skin tone and account for the strength and toughness of skin.

Skin colour: the main factor determining the colour of the skin is the degree of pigmentation provided by the melanocytes in the basal layer of the epidermis. In darker skin, the melanocytes produce more pigment.

Apocrine glands : they are large, modified sweat glands confined to the axillae, areolae, periumbilical, genital and perianal region. Their ducts open into hair follicles or directly onto the skin surface. Their odourless secretion acquires smell through bacterial action. They are supplied by the adrenergic fiber in sympathetic nerve.

Sebaceous gland: it is a saccular structure in the dermis which opens into the side of hair follicles and directly onto the surface of the hairless skin of the lips, nipples, areolae, inner surface of the prepuce, glanpenis and labia minora. There are none on the sole but it is particularly large on the face.

Hair and nail : they are hard type of keratin formed from the hair matrix located at the base of the hair follicle, which extends deeply into the dermis and subcutaneous tissue. The lose their nuclei and form hard keratin as they move up inside the tubular epidermal sheath of the hair follicle. Contraction of the arrector pilli muscle at the base of the hair follicle, with a sympathetic innervation makes the hair 'stand on end', squeezing the sebaceous gland. Hair follicles are richly supplied by the sensory nerves.

Nail: it consists of nail plates lying on nail bed on the dorsum of the terminal segment of fingers and toes. Compacted keratin-filled squames form the nail plate. It has sensory nerves and blood vessels in the nail bed.

Tension lines: the predominant pattern of collagen fibers determine the characteristic tension and wrinkles lines in the skin. Tension line tend to be spiral longitudinally in the limbs and runs transversely in the neck and trunk. Tension lines at the elbows, knees, ankles and wrists are parallel to the transverse creases that appear when the limbs are flexed. The elastic fibers of the dermis deteriorate with age and are not replaced. The skin wrinkles and sags as it losses it's elasticity.

Arteries : the arteries of the skin are derived from a tangential plexus in the subcutaneous connective tissue. They dilate to fill superficial capillary beds to radiate heat or constrict to minimize surface heat loss.

The subcutaneous tissue : it is composed mainly of loose connective tissue and stored fat and contain sweat gland, superficial blood vessels, lymphatic vessel and cutaneous nerves. Subcutaneous tissue participates in thermoregulation, functioning as insulation, retaining heat in the body's core.

Skin ligaments : they are numerous small  fibrous bands that extends through the subcutaneous tissue and attach the deep surface of the dermis to the underlying deep fascia. The length and density of these ligaments determine the mobility of the skin over deep structure.

          CLINICALS

• When the blood is not carrying enough oxygen the skin appears bluish. This occurs because the oxygen carrying the hemoglobin of the blood is bright red when carrying oxygen appears deep, purplish blue when depleted of oxygen, as it does in the veins. This is known as CYANOSIS.

• Skin injury, exposure to excess heat, infections, inflammation, or allergic reactions may cause the superficial capillary bed to become engorged, making the skin look abnormally red. A sign called ERYTHEMA 

• In certain liver disorder, a yellow pigment called bilirubin build up in the blood, giving a yellow appearance to the whites of the eyes and skin, a condition called JAUNDICE 

• Skin incision and scarring : lacerations or incisions that parallel the tension lines usually heal well with little scarring because their is minimal disruption of fibers. However, a laceration or incision across the tension disrupts more collagen fiber, causing the wound to gape and it may heal with excessive (keloid) SCARRING. 

           FASCIAS 

Fascias constitute the wrapping, packing and insulating material of the deep structure of the body. 

  Superficial Fascias : the skin is connected to the underlying bone or deep Fascias by a layer of loose areolar connective tissue known as superficial Fascias. It is of variable thickness and fat content. The superficial Fascias is most distint on the lower abdominal wall, where it differentiates into two layers. 

  Deep Fascias : the limbs and body wall are wrapped in a membrane of fibrous tissue, the deep fascia. It varies widely in thickness, in the iliotibial tract of the fascia lata, it is very well developed, while over the rectus sheath and external oblique aponeurosis of the abdominal wall,it is so thin and is usually considered absent. It is entirely absent in the face and the ischioanal fossa. It is devoid of fat. In the limbs, group of muscles with similar functions, sharing the same nerve supply in facial compartment are separated by thick sheet of deep Fascia called intermuscular septa. The deep Fascia, contracting muscle and venous valves work together as musculovenous pump to return blood to the heart, especially in the lower limbs where blood moves against the pull of gravity.
  Subserous Fascia : it lies between the internal surface of the musculoskeletal walls and serous membranes lining the body cavity, e.g endothoracic, endoabdominal and endopelvic Fascia.

                      BURSAE

  Bursae are closed sac or envelopes of serous membrane (a delicate connective tissue membrane capable of secreting fluid to lubricate a smooth internal surface). Bursae are normally collapsed.

Types of Bursae
  Subcutaneous Bursae : occurs in the subcutaneous tissue between the skin and bony prominences, such as at the elbow or knee.
  Subfacial Bursae : lies beneath deep Fascia
  Subtendinous Bursae : facilitate the Movement of tendons over bone.
  Synovial tendon sheath : they are specialized type of elongated Bursae that wrap around tendons, usually enclosing them as they transverse osseofibrous tunnels that anchor the tendons in place. 

  Because Bursae are formed by delicate, transparent serous membrane and are collapsed, they are not easily noticed or dissected in the laboratory. It is possible to display Bursae by injecting and distending them colored fluid.
  Collapsed bursal sacs surround many organs {e.g heart, lung and abdominal viscera} and structures, {e.g portion of the tendons}
   The heart is surrounded by the pericardial sac. The lung is surrounded by the pleural sac and the abdominal viscera is surrounded by the peritoneal sac. The inner layer of the serous layer is called the visceral layer, the outer layer is called the parietal layer

Reference
Keith Moore and last's Anatomy