Structure & function of the skin and lymphatic systems

The integumentary system:

• Protects against environmental hazards and helps control body temperature.
• Consists of the skin, hair, nails and various glands.
• It is the most visible organ system of the body and contributes to 12-15% of total body weight.
• Has 2 major components: the cutaneous membrane and the accessory structures.
• Is the outer covering of the body.
• Made up of multiple layers of epithelial tissues and guards the underlying structures and organs.

The two parts of the integumentary system are:

The cutaneous membrane

• or skin is an organ composed of the superficial epithelium, or epidermis
• underlying connective tissues of the dermis with a blood supply.

Accessory structures

• Hair
• nails 
• Variets of exocrine glands

• The Epidermis consists of a stratified squamous epithelium of several different cell layers. Thick skin on the hands and feet contains 5 layers. Only 4 layers make up thin skin which covers the rest of the body.
• The term thick and thin refers to the relative thickness of the epidermis only, not to that of the integument as a whole.
• The epidermis is relativelt thin, tough outer layer, mostly made up of keratinocyte cells. These cells are constantly regenerating to create a new skin layer.
• The outermost portion is the stratum corneum, this is relatively waterproof and when undamaged acts as a barrier.
• Throughout the basal layer of the qpidermis are melanocytes, these produce the pigment melanin, one of the main contributors to skin colour.
• The epidermis also contains the Langerhans cells which are part of the skins immune system.
• The epidermis is made up of different layers: in order from the basement membrane towards the free surface are the: Stratum germinativum, 3 intermediate layers (the Stratum spinosum, stratum granulosum and stratum lucidum) and the stratum corneum.

• The dermis lies beneath the epidermis. It has 2 major components: a superficial papillary layer and a deeper reticular layer. 
• The dermis is touch and elastic. 
• It is formed from connective tissues and the matrix contains collagen fibres interlaced with elastic fibres.
• Collagen fibres bind water and give the skin its tensile strength, but this ability declines with age.
• Fibro-blasts, microphages and mast cells are the main cells found in the dermis.
• Underlying its deepest layer there is areolar tissue and varying amounts of adipose (fat) tissue.
• Thick layer of fibrous and elastic tissue made mostly of collagen, elastin and fibrillin.
• Gives the skin its flexibility and strength.

The structures of the dermis are:

• blood vessels
• lymph vessels 
• sensory (somatic) nerve endings
• sweat glands and their ducts
• hairs, arrector pili muscles and sebaceous glands

• Below the dermis lies a layer of fat that helps insulate the body from heat and cold, providing a protective padding and serves as an energy storage area.
• The fat layer varies in thickness, from a fraction of an inch on the eyelids to serveral inches on the abdomen and buttocks in some people.
• An extensive network of connective tissue fibres that attatches the dermis to the subcutaneous layer. 
• It is important in stabilizing the position of the skin relative to underlying tissues, such as skeletal muscles or other organs, while permitting independent movement.
• The subcutanious layer consists of loose connective tissue and many fat cells. 
• The hypodermis is quite elastic, below its superficial region with its large blood vessels, the hypodermis contains no vital organs and few capillaries.
• The lack of vital organs makes subcutaneous injections a useful method for administering drugs using a hypodermic needle.

The skin completely covers the body and is continuous with the membranes lining the body orifices.

The skin

• Protects the underlying structure from injury and from invasion by microbes
• Contains sensory (somatic) nerve endings of pain, temperature and touch
• Is involved in the regulation of body temperature.
• Forms a relatively waterproof layer provided mainly by its Keratinised epithelium which protects the deeper and more delicate structures.

• The melanin in the skin protects it from harmful UV rays.
• Keratin, a type of protein found in the skin, helps promote elasticity and supports the structure that forms the skin.
• The skin protects organs, inner tissues and bones from being exposed to the outside world.
• More exact examples are protection against sun damage (pigment melanin), physical trauma and bacterial invasion.
• The skin forms a relatively waterproof layer provided mainly by its Keratinised epithelium, which protects the deeper and more delicate structures.

As an important non-specific defense mechanism it acts as a barrier against:

• invasion by micro-organisms
• chemicals
• physical agents e.g. mild trauma, ultraviolet light
• dehydration
• The epidermis contains specialised immune cells called Langerhans cells, which are a type of microphage. They phagocytose (eat) intruding antigens and travel to lymphoid tissue, where they present antigen to T-lymphocytes, thus stimulating an immune response.
• Due to the presence of the sensory nerve endings in the skin the body reacts by reflex action (withdrawal) to unpleasant or painful stimuli, protecting it from further injury

• When you become hot, your skin releases sweat from the sweat glands to help cool down the skins surface.
• The blood vessels in the dermis skin fill with blood when you are cold, making you warmer.
• The release or retention of heat depends on the temperature outside of the body. This is one of the crucial functions of the skin.
• Body temperature remains fairly constant at about 36.8'c across a wide range of environmental temperatures ensuring that the optimal range for enzymes activity required for metabolism is maintained.
• When body temperature rises, heat loss from the body occurs through the skin. Small amounts are lost in the expelling of air, urine and faeces. In evaporation the body is cooled as heat converts to water in sweat to water vapors. In radiation exposed parts of the body radiate heat away from the body. In conduction clothes and other objects in direct contact with the skin take up the heat. In convection air passes over the exposed parts of the body is heated and rises, cool air replaces it and convection currents are set up. 
• When the body temperatures falls some energy released during metabolic activity is in the form of heat and the most active organs produce most heat. Skeletal muscles contract and produces a large amount of heat, shivering also involves skeletal muscles contraction which increases heat production when there is the risk of the body temperature falling below normal. The hair follicles on our body stand erect and raise the skin around the hair. Erect hairs trap air, which acts as an insulating layer, warming the body, expecially when accompanied by shivering. 
  

• Certain cells in the skin work closely with the immune system to fight off bacteria, viruses and other unkown substances that come into contact with the skin.
• The skin prevents them from penetrating by providing a protective barrier. Those cells include epidermal dendritic cells, phagocytic cells and langerhans cells.

• The elastic properties of the skin and the tissues underneath it allows the body to grow and change.
• They also make it much easier to move.
• The recoil properties of the skin (elastin fibres) means that it bounces back to its original form.

• The body needs to get rid of the waste products somehow. Some of these wastes can be excreted through the skin, such as urea, water, uric acid and ammonia.
• The skin acts as a gateway to the outside.

The skin is a minor excretory organ for some substances including:

• sodium chloride in sweat; excess sweating may lead to low blood sodium levels (hyponatraemia)
• urea, expecially when kidney function is impared
• aromatic substances e.g. garlic and other spices.

• This function lies in the synthesis of Vitamin D
• Skin abdorbs Vitamin D from the sun
• An organic chemical in your skin reacts with the sunlight and gives you essential vitamin.
• During exposure to sunlight 7- dehydrocholesterol in the skin absorbs UV B radiation and is converted to pre-vitamin D3 which in turn isomerozes into vitamin D3.

• Sensory receptors are nerve endings in the dermis that are sensitive to touch, pressure, temperature or pain.
• Stimulation generates nerve impulses in sensory nerves that are transmitted to the cerebral cortex.
• Some areas have more sensory receptors that others causing them to be especially sensitive e.g. lips and fingertips
• These nerve endings in turn then react to the sensations the skin feels, alerting your brain to the new feeling in an instant.

Hair follicule:

• Hairs are formed by a downgrowth of epidermal cells into the dermis or subcutaneous tissues, called hair follicles.
• At the base of the follicle is a cluster of cells called the papilla or bulb.
• The hair is formed by multiplication of cells of the bulb and as they are pushed upwards, away from their source of nutrition, the cells die and become keratinised.
• The part of the hair above the skin is the shaft and the remainder, the root.
• The Arrector pili are little bundles of smooth muscle fibres attatched to the hair follicles. Contraction makes the hair stand erect and raises the skin around the hair causing "goose flesh".
• The hair is also made up of Medulla, Cortex and Cuticle

Subaceous glands (oil glands).

• These consists of secretory epithelial cells derived from the same tissue as the hair follicles.
• They secrete an oily substance sebum into the hair follicles and are present in the skin in all parts of the body except the palm of the hands and the sole of the feet. They are most numerous in the skin of the scalp, face, axillae and groins. 
• In regions of transition from one type of superficial epithelium to another, such as lips, eyelids, *******, labia minora and glans penis, there are sebaceous glands that are independent of hair follicles, secreting sebum directly onto the surface.
• Sebum keeps the hair soft and pliable and gives it a shiny appearance, while lubricating the hair shaft.
• On the skin it provides some waterproofing and acts as a bactericidal and fungicidal agent, preventing infection. 
• It also prevents drying and cracking of the skin, especially when exposed to heat and sunshine.
• Also contains Holocrine gland are glands that secrete whole cells that have completely broken down for elimination from the body.
• Sebaceous follicle which lubricates the skin and makes the skin waterproof.

• Age-related changes in the skin usually become noticeable in the late forties
• Most of these changes occur in the dermis
• Collagen and elastic fibres degenerate, and wrinkles form
• The skins immune responsiveness decreases as Langerhans cells decrease in numbers 
• Adipose tissue changes causing cellulite and abdominal apron
• Thinning fat layer means less warmth
• Decreased hair follicles resulting in hair loss, baldness
• Decreased melanin which causes the hair to lose is pigment and turn grey
• Decreased nail growth which can cause fungal infections
• Decreased sebum production causes the pores to enlarge
• Decreased eccrine and apocrine resulting in the skin drying and reduses the cooling effect

The skin repairs in 4 stages:

• Phase 1: Hemostasis
• Phase 2: Defensive/inflammatory 
• Phase 3: Proliferative
• Phase 4: Maturation

• The process of hemostasis it to stop the bleeding, halts the blood loss through the walls of damaged vessels, this begins at the onset of injury.
• At the same time it establishes a framework for tissue repair, the blood clotting system forms a dam to block the damaged vessel.
• Hemostasis has 3 phases: the vascular phase, the platelet phase and the coagulation phase.
• Hemostasis is a complex chain reaction in which many things happen at once, and all of them interact to some degree. The end process is clot retraction.
• vascular phase of hemostasis last for around 30 minutes after the injury occurs. The endothelial cells contract and release endothelins, which stimulate smooth muscle contraction and endothelial division. The endothelail cells become "sticky" and adhere to platelets and each other.
• platelet phase of hemostasis begins with the attatchment of platelets to sticky endothelial surfaces, to the basement membrane, to exposed collagen fibers and to each other. As they become activated, platelets release a variety of chemicals that promote aggregation, vascular spasm, clotting and vessel repair.
• coagulation or blood clotting involves a complex sequence of steps leading to the conversion of circulating fibrinogen into fibrin. As the fibrin network grows, blood cells and additional platelets are trapped in the fibrous tangle, forming a blood clot that seals off the damaged portion of the vessel.
• clot retraction once the fibrin mesh work has formed, platelets and red blood cells stick to the fibrin strands. The platelets then contract, and the entire clot begins to undergo clot retraction which can take over 30-60 minutes.
• An enzyme called thrombin is at the centre and it initiates the formation of fibrin mesh which strengthens the platelet clumps into a stable clot.

• Defensive/inflammatory phase, focuses on destorying bacteria and removing debris- essentially prepating the wound bed for the groeth of new tissue.
• The cut surface becomes imflamed and blood clot and cell debris fill the gap between them in the first few hours where Phagocytes including macrophages and fibroblasts migrate into the clot.
• Phagocytes begin to remove the clot and cell debris stimulating fibroblast activity.
• Fibroblasts secrete collagen fibres which begin to bind the surface together.
• During Inflammatory phase, a type of white blood cells called neutrophils enter the wound to destroy bacteria and remove debris.
• These cells oftern reach their peak population between 24 and 48 hours after injury, reducing greatly in numbers after three days.
• As the white blood cells leave, specialized cells called macrophages arrive to continue clearing debris.
• These cells also secrete growth factors and proteins that attract immune system cells to the wound to facilitate tissue repair. 
• These phases oftern last four to six days and is often associated with oedema, erythema (reddening of the skin), heat and pain.

The poliferative focuses on filling and covering the wound.

The poliferation phase features three distince stages:

• Filling the wound
• contraction of the wound margins
• coverinf the wound (epithelisation)

During the first stage, shiny, deep red granulation tissue fills the wound bed with connective tissue, and new blood vessels are formed.
During contraction the wound margins contract and pull toward the centre of the wound
In the third stage, epithelial cells arise from the wound bed or margins and begin to migrate across the wound bed in leapfrog fashion until the wound is covered with epithelium
The proliferative phase oftern lasts anywhere from 4 to 24 days.

The epidermis meets and grows upwards until full thickness is restored. The clot above the new tissue becomes the scab and separates after 3 to 10 days. Granulation tissue consisting of new capillary bude, phagocytes and fibroblasts, develops, invading the clot and restoring the blood supply to the wound. 

Fibroblasts continue to secrete collagen fibres as the clot and any bacteria are removed by phyagocytes.

• During maturation the granulation tissue is replaced by fibrous scar tissue. The new tissue slowly gains strength and flexibility.
• Here, collagen fibres reorganise, the tissue remodels and matures and there is an overall increase in tensile strength (though maximum strength is limited to 80% of the pre-injured strength).
• The maturation phase varies greatly from wound to wound, oftern lasting anywhere from 21 days to 2 years.
• The healing process is remarkable and complex, and it is also susceptible to interruption due to local and systemic factors, including moisture, infection and maceration (local), and age, nutritional statue, body type (systemic).

The Lymphatic system consists of:

• Lymph
• Lymph vessels
• Lymph nodes
• Lymph organs e.g. spleen and thymus
• Diffuse lymphoid tissue e.g. tonsils
• Bone marrow 

The lymphatic system- cells tissues and organs that play a central role in the bodys defence against pathogens.

Lymphatic system- consists of vessles (lymphatics) filled with lymph, connected to lymphatic organs

Lymphatic system is a network of tissues and organs that help rid the body of toxins, waste and other unwanted materials.

• The system is of primary importance in transporting fat from the intestines to the blood stream.
• It also removes and destroys interstitial fluid (toxic sunstances), regenerates tissues and maintains a healthy immune system.
• Lymph fluid is formed when high arterial pressure forces fluid out of the capillaries and into the tissues.

It is comprised of 3 main elements:

• Lymph fluid
• vesses
• lymph nodes

• The lymphatic system is a linear network of lymphatic vessels and secondary lymphoid organs.
• it is responsible for the removal of interstitial fluid from tissues
• it absorbs and transports fatty acids and fats as chyle from the digestive system
• it transports white blood cells to and from the lymph nodes into the bones
• Lymph nodes are located at intervals along the lymphatic system. Lymphoid tissue contains lymphocytes and other specialized cells and tissues that have immune system functions.

• Since the lymph is derived from the interstitial fluid, its composition continually changes as the blood and the surrounding cells continually exhange substances with the intersitial fluid.
• It is generally similar to blood plasma.
• Lymph returns proteins and excess interstitial fluid to the bloodstream.
• Lymph also transports fats from the digestive system (beginning in the lacteals) to the blood via chylomicrons.
• Bacteria may enter the lymph channels and be transported to lymph nodes where they are destoryed.
• Metastatic cancer cells can also be transported via lymph.
• Interstitial fluid forms at the arterial (coming from the heart) end of capillaries because of the higher pressure of blood compared to veins, and most of it returns to its venous ends and venules.
• The rest (up to 10%) enters the lymph capillaries as lymph
• Lymph when formed is a watery clear liquids with the same composition as the interstitial fluid.
• As it flows through the lymph nodes it comes in contact with blood and tends to accumulate more cells (particulary lymphocytes) and proteins.
• Tubular vessels transports lymph back to the blood, ultimately replacing the volume lost during the formation of interstitial fluid. These channels are the lymphatic channels, or simply lymphatics.

• Lymphatic vessels are structures of the lymphatic system that transports fluid away from tissues
• Mingled among the blood capillaries throughout your body is another network of tiny, thin-walled vessels called lymphatic capillaries.
• As part of the lymphatic system, lymph vessles are complementary to the cardiovascular system.
• Lymphatic capillaries are designed to pick up the fluid that leaks into your tissues from the bloodstream and return it to your circulatory system
• Lymph flows inside specialized channels, referred to as the conduit system.
• Begin as blind end lymphatic capillaries in tissues
• Lymph empties into the venous system, at the thoracic duct and at the right lymphatic duct.

• A lymph node is a kidney shaped organ of the lymphatic system, and of the adaptive immune system that is widely present throughout the body.
• They are linked by the lymphatic vessels as a part of the circulatory system.
• Lymph nodes are major sites of B and T lymphocytes, and other white blood cells.
• Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles and cancer cells.
• In the lymphatic system the lymph node is a secondary lymphoid organ.
• A lymph node is enclosed in a fibrous capsule and is made up of an outer cortex and an inner medulla.
• Lymph nodes also have clinical significance. They become inflamed or enlarged in various diseases which may range from trivial throat infections to life threatening cancers.

• Also called leukocytes are the cells of the immune system that are involved in protecting the body against both infectious diseases and foreign invaders.
• All leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell.
• Leukocytes are found throughout the body, including the blood and lymphatic system.
• Five different and diverse types of leukocytes exist.
• All white blood cells are nucleated but are otherwise distinct in form and function.
• White cells are best classified into 2 major lineages: the myeloid leukocytes and the lymphocytes.

Three classes of Lymphocytes:

• T Cells- Thymus dependant
• B Cells- Bone marrow derived
• NK Cells- Natural killer

Mainly produced and storied in the lympoid tissues:

• Tonsils
• Spleen
• Thymus

Some Lymphocytes are produced in the bone marrow (red)

Lymphocytes continuously migrate between lymphoid tissues and the blood.

Other important lymphoid organs include:

• The thymus: located in the thoracic cavity, just under the neck. It is made up of 2 lobes of lymphoid tissue. Each lobe has a medulla (the inner region of an organ or tissue) surrounded by a cortex. The cortex is where immature lymphocytes first go to become T cells, but their maturation finishes in the medulla.
• The spleen: located in the upper-left part of the abdomen. It is ticked up under the ribs, so you generally cant palpate it. The spleen's main function is to filer the blood. It removes old or damaged red blood cells, which are phagocytized by macrophages. The spleen also detects viruses and bacteria and triggers the release of lymphocytes.
• The tonsils: masses of lymphoid tissue found in the back of the throat and nasal cavity. They're part of the immune system, so they help fight infections.
• The appendix: a pouch of lymphatic tissue that's attached to the large intestine. It's located in the lower-right area of the abdomen. Although its made of lymphatic tissue, the appendix doesnt appear to have much lymphatic function in humans, but it does release some mucus into the large intestine.
• Peyer's patch: These patches of lymphoid tissues are located in the mucosa and submucosa throughout the small intestine, although they're more concentrated in the ileum. Peyer's pathces contain mostly B cells.
• Lamina propria lymphocytes: This type of GALT (gut-associated lymphoid tissue) is located in the mucosa of the small intestine. It also contains mostly B cellls.