4. Skin & wound healing

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General Training Module The Human Skin & Tissue Healing RK


SKIN The skin is considered to be the largest organ of the body It covers the body completely and is continuous with the membranes lining the body orifices. It consists of several layers: Epidermis Dermis (Corium) Subcutaneous fat underlying skin

3 main layers of the skin:

3 main layers of the skin


SKIN It performs several basic functions: Protects the body from injuries Protects from invasive microbes Act as a sensory organ and has nerve endings for touch, pressure, pain as well as temperature (Cold, warmth and Heat) Regulates body temperature Formation of vitamin D


Sebaceous Glands Subcutaneous Connective Tissue


Epidermis Epidermis is the most superficial layer of the skin Made of several layers It is a Keratinized Stratified (Squamous) Epithelium Its thickness varies in different parts of the body – thickest in the palms and soles It has no blood vessels or nerves


Papillae ?


Epidermis Most superficial layer only has enucleated (cells that has no nuclei) dead cells that are thin and flattened . Their cytoplasm contains keratin, a fibrous protein that protects the cells from stressors and damages. These cells are constantly being rubbed off and shredded off and replaced by cells from lower layers.


Epidermis The bottom most layer of cells replicate constantly to replace the cells of the layer above in such a way that each layer above the other is pushed to the surface to replace the cells that are lost from the most superficial layer. In between the basal most and the superficial layer are several layers through which cells change in shape while reaching the surface during which they are also enucleated and keratinized to suit the surface layer that needs to withstand the stress.


Epidermis The surface of the epidermis has ridges, caused by projections of the dermis below into the epidermis which are called papillae. These ridges make the finger prints which are unique to each individual


Epidermis There are special cells called melano-cytes in the basal layer of the epidermis that secretes a dark pigment called melanin that gives the dark color of the skin. Melanin is absorbed by surrounding epithelial cells and darkened by the sunlight which also promotes its secretions There are several other factors that affect the skin color


Epidermis Body hair, ducts from sweat and sebaceous glands in the dermis pass through the epidermis to the surface of the skin.


Dermis The dermis lies underneath the epidermis and has collagen and elastic fibers with areolar and fat tissues (connective tissues). The structures in the dermis are: Hair: roots, follicles, hairs Arrectores pilorum (involuntary muscles attached to hair follicles) Sebaceous gland Sweat glands and their ducts Sensory nerves Blood vessels Lymph vessels


Dermis bulb root

Dermis – Hair follicles:

Dermis – Hair follicles Hair follicles: are down growths of the epidermis into the dermis and contains hair. They are made of dead cells with keratin deposits The part of the hair above the surface of the skin is called the shaft the rest the root The color of hair depends on the amount of melanin present.

Dermis – Glands:

Dermis – Glands There are two types of glands in Dermis. They are made up of the Epithelial cell layer invagination of the epidermis Sebaceous glands Sweat glands

Dermis – Sebaceous Glands:

Dermis – Sebaceous Glands Sebaceous glands : They secrete a fluid called sebum into the hair follicle or to the skin surface. It is an oily substance that keeps hair soft, pliable and shiny. On the surface of the skin it provides water proofing, prevents drying and cracking of skin and act as a germicidal agent.

Dermis – Sweat glands:

Dermis – Sweat glands Sweat glands : Glands composed of epithelial cells that are coiled and lay within the dermis. Their ducts open into the skin surface or hair follicles. They are most numerous in the palms, soles, axillae and groins. Sweat glands that open into hair in the axillae secrete an odourless fluid if decomposed by surface bacteria gives an unpleasant ordour .

Dermis – Sweat glands:

Dermis – Sweat glands The main function of the secretions of the sweat glands that open into the body surface is thermoregulation. Because evaporation of sweat from the body surface uses body heat and the amount produced is governed by the thermoregulatory centers in the hypothalamus.

Dermis – Arrector Pilorum:

Dermis – Arrector Pilorum Arrector Pilorum : is a bundle of smooth muscle attached to the hair in the dermis. Contraction of them causes erection of hair causing “Goose Flesh”. They are innervated and stimulated by sympathetic nerve fibers in response to fear and cold. When the hair is erected a thin layer of air traps in between them protecting heat loss Their contractions collectively generates considerable amount of heat too that is also important in thermoregulation.

Dermis – Nerve Endings:

Dermis – Nerve Endings Sensory Nerve Endings : Skin is a very important sensory organ through which individuals become aware of their environment. It has sensory nerve endings that are sensitive to touch, pain, pressure and temperature changes. They are conveyed to the central nervous system by sensory cutaneous nerves and information is travelled to and perceived in the sensory area of the cortex in the brain.

Dermis – Blood & Lymph:

Dermis – Blood & Lymph Blood & Lymph Vessels : There are no blood vessels in the epidermis. Blood vessels in the dermis supply hair follicles, sweat and sebaceous glands and other structures in the dermis. The epidermis gets nutrients from the blood vessels in the dermis through diffusion across the extracellular fluid. In addition there are lymph vessels which also drain the lower layers of the epidermis in addition to the dermis below.


Nails Nails are derived from the same cells that make up the epidermis and hair. They consist of hard keratinized dead cells

Functions of the skin:

Functions of the skin Protection: As a physical barrier Against invasion of microbes Sensory nerve endings initiating reflex actions in response to unpleasant and/or painful stimuli

Functions of the skin:

Functions of the skin Formation of vitamin D: A precursor substance for the vitamin D is present in the skin. Ultraviolet light from the sun converts this precursor into vitamin D. Vitamin D is then circulated in the blood which is used in formation and maintenance of bone along with calcium and phosphorus. Any excess vitamin is stored in Liver.

Functions of the skin:

Functions of the skin Regulation of body temperature (THERMOREGULATION): Body temperature is maintained at a fairly constant level for the normal cellular functioning To ensure that the body temperature is maintained constantly a fine balance between the heat production and heat loss inside the body needs to be maintained This regulation is achieved by the part of the brain called the Hypothalamus.


Hypothalamus in the Brain Regulates the Body Temperature Environmental Temperature = 27 – 30 Celsius degrees Internal Environment = Body Temperature = 98.4 Fahrenheit 37 Celsius Degrees HEAT LOSS HEAT PRODUCTION SKIN Thermoregulation is the Process by which the Body Maintain its Temperature amidst a fluctuating and lower Environmental temperature

Thermoregulation (Control of body temperature):

Thermoregulation (Control of body temperature) Thermo-receptors on the skin keep the brain informed about the surrounding temperature Brain also receives information of the body temperature from the blood it receives Depending on these two information (body and the environmental temperature) the brain carry out the control of the temperature through several mechanisms

Heat Production:

Heat Production Cells use raw materials from nutrients for their work. These nutrients we get from our food are glucose, amino acids and fatty acids. When they are metabolized (used by the cells in their chemical reactions) some of the energy is released as heat. Therefore most active organs both physically and chemically produce the most amount of heat.

Heat Production:

Heat Production The main such heat producing organs and tissues are: The skeletal muscles during physical activity, The liver (where a lot of chemical actions of the body takes place) Digestive organs – smooth muscle contractions in their walls and During metabolism of food in all cells

Heat Loss:

Heat Loss Mostly from the skin Small amounts are lost from: The expired air Urine and Feces Only the heat loss from the skin could be regulated to maintain the body temperature but not the heat loss from other sources

Heat loss through the skin:

Heat loss through the skin Environmental temperature is the temperature of the surrounding and is usually below the body temperature. Therefore there is a general tendency to lose temperature from the body to the surrounding. This occurs via the skin surface. Usually those areas that are open. Since air is a poor heat conductor when clothes are worn, the air trapped between clothes and skin insulates against heat loss from the body.

Control of heat loss:

Control of heat loss Mechanisms involved: Nervous control – hypothalamus through ANS – sweat glands Connections to the vasomotor center in the medulla Vasomotor control – medulla oblongata Vasodilation – through ANS (Heat loss through radiation, conduction & Convection) Vasoconstriction – through ANS Sweat Glands’ activity – ANS Insensible water loss (Heat Loss is through evaporation)

Control of heat loss:

Control of heat loss Vasomotor control: If heat is to be lost the vasomotor centre dilates the small arteries and arterioles of the skin increasing circulation hence the heat loss by way of radiation (direct radiation from the exposed areas of the skin), conduction (-by the clothes we wear) and convection (from the areas exposed through convection currents) occurs. If heat is to be conserved the opposite occurs - vasoconstriction

Control of heat loss:

Control of heat loss Evaporation of sweat The heat is lost when water in the sweat secreted by the sweat glands of the skin is evaporated to the atmosphere . Therefore the hypothalamus through the autonomic nervous system increases the secretion of sweat when heat loss is required. Sweating occurs when the sweat production rate is higher than the rate of evaporation of sweat Insensible water loss In addition loss of heat occurs when insensible water loss takes place through evaporation. That is loss of water from the deeper layers the skin through diffusion to the surface

Tissue Healing:

Tissue Healing

Tissue Healing:

Tissue Healing Wound Healing Healing of Nerves Healing of muscles Healing of Bones

Wound Healing:

Wound Healing There are two types of wound healing: Primary Healing or Healing by first intention: Occurs when the tissue damage is minimal and the edges of the wound are in close proximity. Absent to minimal car. Secondary Healing or healing by second intention: Occurs when there is extensive tissue damage or when the edges of the wound are not in close proximity like in the case of large ulcers and after chronic inflammation. May leave a scar following secondary healing

Primary Healing:

Primary Healing There are several key stages in Primary Healing Injury Bleeding from the site of injury Formation of a blood clot to stop bleeding Acute Inflammation – It is the normal tissue response to injury. It is part of the first line of defense of the body It is non-specific that is the response does not change based on the injurious agent.

Acute Inflammation:

Acute Inflammation Main features of Acute Inflammation Pain Swelling Redness Warmth or heat Loss of function See Video on Acute Inflammation

Acute Inflammation:

Acute Inflammation Main features of Acute Inflammation are caused by t he chemical mediators released by the cells of the injured tissue ( eg : Histamine released by mast cells) These chemicals act on the blood capillaries at the injured site. They increase the permeability of the vessels – that is the vessels become more transparent to cells and fluid inside them In addition they dilate capillaries increasing the blood flow to the injured tissue.

Acute Inflammation:

Acute Inflammation Pain is due to stimulation of sensory nerve endings Increased blood flow cause redness and heat Increased permeability cause more fluid and phagocytic cells to come out of the blood vessels to the site of injury causing swelling. This is called fluid extravasation and the fluid that comes out is called the Exudate There is Loss of function of the tissue due to collective effects of these

Primary Healing:

Primary Healing Scavenging Phase - Phagocytes migrate to the site out of tissue capillaries They remove dead material, the blood clot, and microbes if present from the injury site Proliferative phase - Epithelial cells in the epithelium and fibroblasts in the connective tissue of the dermis start to divide initiating the healing process The Epithelium grow across the blood clot that fills the gap

Primary Healing:

Primary Healing

Primary Healing:

Primary Healing Remainder of the clot above the epithelial growth makes the scab Scab is removed gradually as the epithelialization (epithelial cell proliferation) progress upwards to restore the full thickness of the skin Fibroblasts (cells of the connective tissues of the dermis of the skin) deposits collagen and the fibrous tissue formed unite the edges of the wound After sometime the inflammation resolves Then the fibrous tissue that was laid in excess gradually reduced to leave only a pale thin line. - Remodeling Phase

Nerve Injury & Healing:

Nerve Injury & Healing Neurones in the brain, spinal cord or ganglia are not replaced when they injure and die. However if an axon of a peripheral nerve is damaged or cut - it may regenerate if its cell remains intact. The regeneration depends on if the two cut endings are in alignment to each other Restoration of function depends on, if the new nerve ending successfully reestablish the contact with the effector organ

Nerve injury:

Nerve injury

Nerve Healing:

Nerve Healing Additionally when an axon is cut closer to the end organ and the distal part dies Restoration of nerve supply to the end organ may be accomplished by outgrowths of new nerve endings from another adjacent axon In this way too restoration of function could be established – This is the purpose of neurorehabilitation

Muscle Injury & Healing:

Muscle Injury & Healing Muscle fibers could be injured accidentally or cut during surgery When the injury is slight the small gap in the muscle fiber is bridged by outgrowths from the surviving ends of the fiber completely restoring the integrity and function. – Primary Healing If the injury is extensive the growth of muscle fibers across the gap may not be complete. The healing would be incomplete with scar tissue formation at the site – Secondary Healing When this happen joints may contract and movements may be restricted

Healing of Bone:

Healing of Bone Similar to other tissue type healing First the fracture site is filled with the blood clot and tissue debris Acute inflammatory process is initiated Scavenger cells come to the site and remove the blood clot and tissue debris New tissue is formed with new blood vessels Large number of osteoblasts start depositing bone forming now an irregular bony callus at the site Remodeling takes place with the help of osteoclasts that remove excess callus and forms medullary cavity in the bone replacing the bones original shape.

Callus formation and remodeling:

Callus formation and remodeling

Factors that delay healing:

Factors that delay healing Infection Deficient blood supply Other systemic conditions like diabetes mellitus Malnutrition Old age Suppressed immunity

End of Unit 1 – Part C:

End of Unit 1 – Part C

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