1. Anatomy of Skin Structures and Functions
Burn 2nd Lecture
Anatomy of Skin Structures and Functions

2. FUNDAMENTAL AND KNOWLEDGE (Medical Approaches)
Normal Anatomy of Skin Structures and Functions:
The following part has been presented under the following headings:
Normal Skin:
The skin is the largest organ of the body, comprising approximately 15 percent of body weight. Averaging over 2 m in surface area and yet in most places is less than 2 mm in thickness.
Anatomically, the skin consists of two distinct layers of tissues: the epidermis, which is the outermost layer and the deeper layer, termed the dermis, a third layer involved in the anatomic consideration of the skin is the subcutaneous fat cell layer directly under the dermis and above muscle facial layer
(Fig. 1).
The skin is the organ which envelops and covers the body. The skin with its associated glands and specialized derivatives and the mammary glands form the integumentary system.

3. The Skin Layers Consist of the Following Layers:
1. The Epidermis Layers
The epidermis is a thin layer composed of sequamous epithelial cells which are moved upward as they form. The epidermis consists of four distinct layers: the horny layer, or stratum corneum, the granular layer or stratum granulosum, the spinous layer or stratum spinosum and the basal cell layer or stratum basale. A fifth layer, the stratum lucidum, is found in the skin on the soles of feet and the palms of the hands as a thin eosinophilic band that is located at the base of the stratum corneum.
The epidermis and its appendages (hair and sweat and sebaceous glands) are derived from embryonic ectoderm.
The stratum corneum is the outermost layer of the epidermis and serves to protect the body from the infection. The second layer of the epidermis is the stratum granulosum; this layer consists of flattened nucleated cells containing distinctive cytoplasmic inclusions called kerato hyalin granules and serves as a transition layer between the stratum corneum and lower layer of the epidermis

4. The stratum spinosum or prickle cell layer and this layer consists about 10 rows of cells that fit closely together and connected by desmosons and is comprised of several layers of a polyhedral type cell that lie above the germinal layers of cells. The stratum basale or basal cell layer of feratinocytes is the germinativc layer of the epidermis. The basal feratinocytes are structurally cubaidal or low column in shape with a basal surface that conforms to the supporting dermal tissue.
The keratinocytes contain various cytoplasmic structures and organelles, such as mitochondria, endoplasmic reticulum, rosettes of ribosomel, (iolgi complexes and prominent nucleoli. They are attached to each other by desmosomes, which are highly developed cellular attachments located at the periphery of each keratinocytes.

5. The epidermis is normally in a constant balance between the production keratinocytes and desquamation of the outer layers of the stratum corneum. The keratinocytes cells will differentiate in the direction toward the surface of the epidermis to produce the cornfield surface layer.

6. Epidermal Structures: Melanocytes
Melanocytes are dendritic cells that produce pigment granules known as melanosomes, which contain the pigmenl melanin. They are located at the dermo epidermal junction, although there are melanacytes located in the dermis.
There is about 1 melanocyte for every 5 epidermal cells on the face, but only 1 melanocyte for every 20 cells on the buttocks. The normal ratio of melanocytes to keratinocytes varies between 1: 4 to 1: 10 depending on the region of the body.
Melanin functions is to protect the epidermis from the harmful effects of ultraviolet light by forming a protective cap around the keratinocyte nuclei to prevent ultraviolet damage to the DNA in the nucleus.

7. Langerhans cells: The langerhans cells are another type of dendritic cell found in the epidermis. They are similar to melanocytes, except they lack pigment. They are active as phagocytes. Langerhans cells assist in one of the primary functions of the skin, to protect the internal structures from potential infectious agents in the external environment and they may be damaged by excessive exposures to ultraviolet light. 2. The dermis layers: The dermis or corium is the thicker fibrous portion of the skin which provides strength and elasticity. It is the location of the glands of the skin and the hair follicles. The dermis is divided into superficial papillary layer and a deep reticular layer.

8. The papillary layer presses against the epidermis and intense it at intervals with finger-like projections, called the papillae of dermis.
The papillae consist of an aerolary connective tissue core which carries capillaries, nerve fibers and sensory end organs into more intimate contact with epidermis.
In the reticular layer the connective tissue is formed into a dense interlacing network of elastic fibers and bundles of collagenous fibers. The collagenous bundles are very coarse in the depth of the dermis. The boundary of the dermis with tela subcutanea is marked by the presence of fat and the rearrangement of collagenous bundles into vertical strands. The dermis is 20 to 30 times thicker than the epidermis. It is comprised primarily of interwoven collagen and elastic fibres, which provide the skin with tensile strength one of resistance to deformation. The predominantly parallel orientation of normal collagen in the dermis is different then the whorls of collagen typically seen in the scar tissue that results from burn injury.

9. Dermal structures: Smooth Muscle:
The arrector pilorum muscles arise in the dermis and insert up on the hair folicles below the sebaceous duct.
The muscle is able to cause the hair follicle to elevate when contracted, causing "goose flesh" of the skin. This muscle contraction results in an increase in the metabolic rate of the skin and is an attempt by the skin to assist in maintaining the necessary thermoneutral environment of the body during exposure to cold.
3. Hairs:
Hairs are keratin shafts formed at the depths of tubular in growths of the epidermis called the hair follicles. The expended end of the follicle (bulb), located in the dermis, is intended by a connective tissue papilla which bears blood vessels and sensory receptors. The hair follicles of pockets of epithelium which are continuous with the superficial epidermis.

10. The hair follicle can livided into 3 positions: The infundifilulum which is the most superficial and primarily epidermal, the isthmus which comprises the tissue between the entry into the follicle of the sebaceous glands and the insertion of the effector pili muscle, and the inferior portion which gives rise to the hair bulb including the hair papila and matrix.
4. glands of the skin:
Two types of glands are found in the skin, sebaceous glands and sweat glands.
a- Sebaceous glands
These kinds of glands cluster in lobules adjacent to hair follicles in the dermis and open into the follicles, which arc used as excretory ducts. These glands are arising from an epithelial band from the outer hair root sheath at the junction of the infundifulum and isthmus of the hair follicle. An oily product, sebum is produced by holocrine secretion in which the cells become filled with fat droplets and die.

11. b- Sweat glands:
Sweat glands are long tubular gland whose coiled secretory Portions are located in the dermis or even in the tela subcutanea. A long straight duct connects the secretory body of the gland to the surface of the skin where an opening forms one of the many pores scattered a top the papillary ridges.
5. Blood vessels and microcirculation:
The blood supply to the skin is derived from the cutaneous branches of the subcutaneous musculocutaneous arteries.
The branches from the cutaneous arteries become a distinictive small-vessel plexus, which lies in the deep layer the reticular dermis Small arteries and arterioles then ascend toward the surface as small perforating arteries into the papillary dermis, where they branch again into terminal arterials (Fig. 2)

12. Fig. (2): Schematic description of the microcirculation units of the skin. The arterial vessels are shown in white and the veins are in black. (A) The papillary dermis; (B) The reticular dermis; The pilosebaceous apparatus An eccrine sweat gland and duct. (I) Identifies some arteriovenous communications at different levels in the skin and subcutaneous tissue.

13. 6. Lymphatic vessels: The Structure of the lymphatic vessels consists of a single endothelial layer containing system of valves and a discontinuous layer laminar material. The function of lymphatic vessels within the skin is to transport particulate and liquid matter from the extravascular compartment of the skin . the collecting and transportation vessels descend to the lower level of the reticular dermis and accompany the venous blood vessels away from the skin.

14. 7. Nerve supply to the skin:
The skin contains sensory organs for all of the principle sensations of touch, pain, warmth, cold, itch, and pressure.
The skin serves as one of the major sensory organs of the body and provides constant feedback to the nervous system about the nature and stress of the human environment. The skin may be innervated with more than one million afferent nerve fibres.
It has identified an extensive branching of myelinated sensory nerve endings into two main plexuses in the dermis. The lower most plexus is deep in the reticular layer, which is oriented in a parallel fashion with the surface of the skin.

15. Sensory Receptors:
Figure (3) presents a diagram of the orientation and variety of1 sensory organs found in the skin. Because of the importance of the sensations that the skin transmits to the nervous system and the impact that the environment has these sensory organs, each structure will be summarized in table (1).
8. Functions of the Skin:
Transmits sensory feedback from environment.
Functions as part of the immune system to prevent organisms from gaining exposure to internal organs.
Serves as a water barrier.
Serve as a barrier to prevent the penetration of foreign objects, material and radiation.
Regulates heat exchange and assist in body temperature homeostasis.
Provides a cosmetic covering for personal Identity
Excretions of some waste products.
Resists mechanical stresses and formation of vitamin D'

16. Table (1): The structure, location, and functions of sensory receptors in the epidermis and dermis of human skin.
Structure
Location
Function
Free terminal
Epidermis, dermis
Sensations of itch and pain; fine discrimination for sensory stimuli in the digits.
Merkel endings
Epidermis ridge
Mechanoreceptors for the sensation touch.
Meissner
touch
corpuscles
Papillary dermis of palm of hand and sole of foot
Sensation of touch and relative amounts of pressure associated with sensation of touch.
Paccinian i orpuscles
Deep layers of palmar dermis palmar subcutaneous tissue, and near periosteum of proximal phalanges.
Sense of vibration, signals local changes in blood flow.
Pilo-Ruffini
Encircling the hair follicles
Slow-reacting mechanoreceptors that responds to changes in pressure sensed by hair follicles; Ruffini end organs also responsible for sensation warmth.
Krause end
bulb
Superficial layers of the dermis
Sensation of cold

17. Fig (3): Diagrammatic section showing various nerve ending In the skin of the fingertip

18. PATHOPHYSIOLOGICAL CONSIDERATIONS OF THE SKIN
Skin is a wonderful barrier that keeps the outside out and the inside in. Skin is one of the largest organs of the body, comprising 16% of the total body weight, contains the nerve endings of sensory nerves. Skin is one of the main excretory organs in the body, so it plays an essential role in the regulation of body temperature; also it protects the deeper from injury and guards against the invasion of micro-orgasms
Burn injuries vary in severity, depending on the amount of the total body surface area "TBSA" that has been damaged. Even the smallest burn causes discomfort that can be relived by rapid first aid. The severe or dangerous burn involving more than 30 % of the (TBSA) may be life threading.
Burn injury primarily results in disruption and destruction of the normal protective functions of the skin.

19. Skin is composed of epidermis and the corium (dermis or the true skin)
Skin is composed of epidermis and the corium (dermis or the true skin). The epidermis and its appendages (hair, sweat and sebaceous glands) are derived from the embryonic ectoderm while dermis is derived from the mesoderm. Epidermis is the most superficial part of the skin and it's composed of stratified squamous epithelial cells laid on top of each other like the bricks in a wall. Epidermis differ in its thickness from one part of the body to another, it's more thick on the palms of hands and the soles of feet.
The epidermis is composed of two zones, horny zone and greminative zone, the horny zone is composed of three strata, Stratum corneum, stratum lucidum and stratum granulosum. The germinative zone is composed of two layers, the prickle cell layer and the basal cell layer. The stratum corneum is the most superficial layer which contains a horny substance known as keratin, which protects the deep layers, while the cells of the stratum lucidum arc responsible for replacing the cells of the stratum granulosum contains granules which are the first from of keratin

20. Prickle cell layer acquires its name from the spiky or the thorny appearance of its intercellular bridges which connect the adjacent cells, also prickle cell layer contains the langerhans cells and the keratinoblasts. Langerhans cells are a modified macrophages which originate in the bone marrow and migrate to the epidermis, so langerhans cells represent the first line of immunological defence against the environmental antigens. While keratinoblasts are responsible for columnar cells which arc anchored to the basement membrane, basal cell layer contains the melanoblasts which are responsible for the production of the melanin pigment, that gives the colour of the skin.

21. The epidermal appendages are the sweat glands, nails, hair and the sebaceous. Sweat glands are of two types, the eccrine and the apocrine. Eccrine sweat glands are present all over the body and play an important role in the regulation of body temperature, while the apocrine sweat glands which become active at puberty are present in axillae and the ano-genital regions, and they are responsible for the body odour. Sebaceous glands are part of the pilo-sebaceous unit, and secrete a lipid rich secretion called the sebum into the hair follicle. The sebaceous gland is attached to the hair follicle just above the attachment of the arrector pili muscle, which is an involuntary muscle and its contraction results in hair erection and elevation of the skin around hair, particularly in cold, fear and horrible conditions giving the goose llesh appearance and horripilation.

22. The dermis is formed of connective tissue (C
The dermis is formed of connective tissue (C.T) which is present under the epidermis. It is made up of two layers which arc continuous with each other, the papillary layer and the reticular layer. The papillary layer is formed of a loose C.T. under the papillae of the basement membrane, papillary layci Is rich in nerve endings, while the reticular layer is formed of dense C.T fibers, and its continuous with the papillary layer but deeper to it. Dermis has a structural elements and cellular elements: Structural elements as the collagenous fibers and the elastic fibers while the cellular element as the fibroblasts, mast cell and the macrophages Collagenous and elastic fiber gives strength and elasticity respectively, fibroblasts are responsible for the synthesis of the C.T. matrix of the dermis so fibroblasts are very important in the repair of burn wound. Mast cells are specialized secretory cells present throughout the dermis, which release heparin as anticoagulant plus the chemical mediators, histamine, bradykinin and prostaglandins which can induce the chemical theory of pain via stimulating the polymodal chemosensitive pain receptors.

23. Macrophages are phagocytic cells which originate in the bone marrow, then migrate to the dermis, they act as a scavengers of the cell debris and necrotic tissues. So dermis and its elements play an essential role in the process of wound and burn healing. Macrophages has an important role in the inflammatory phase of wound healing, while fibroblasts are responsible for the collagen (glue) formation in the fibroplastic phase of the wound healing.
Disruption and destruction of the skin as a result of burn injury, affects the following skin functions; Fluid retention or prevention of the essential body fluids loss, protection of the body from the micro-organisms invasion, protection of the body from the ultraviolet damage via the protective function of the melanin, and the sensory perception function of the skin. Vitamin D3 formation from its precursor in the skin, protection of the sebum which stops and prevents the bacterial conniption over the skin and the regulation of body temperature.

24. Horripilation and the Arrector pili muscle
A small bundle of smooth involuntary muscle fibers called the arrector pili muscle, which extended from just beneath the epidermis (origin) and is attached to the side of the follicle at an angle (insertion). Arrector pili muscle is supplied by adrenergic nerves (pilo motor nerves), and are responsible for the erection oi hairs during cold or emotional stress giving the goose flesh appearance of horipilation
Horripilation = goose flesh appearance: sympathetic adrenergic (Pilo motor nerves) in fear, cold and Horrible condition → involuntary contraction of the arrector pilorum muscles → hair erection with elevation of the skin around hair.

25. Sebaceous glands
The sebaceous gland is attached to the follicle just above the point of attachment of the arrector pili muscle.
Sebaceous glands are part of the pilo sebaceous unit, and their lipid rich secretion (sebum) flows through a duct into the hair follicle to keep the hair in a soft & shiny condition. Sebum contains 7-dehydrocholestrol substance.
The dermis
The dermis is a layer of connective tissue lying beneath the epidermis and forming the bulk of the skin, so it is called the corium or the true skin. Dermis is deep part of the skin which contains small blood vessels called capillaries, sensory nerve ending, sweat glands and their ducts, hair follicle and hairs, sebaceous glands and the arrector pili muscle.
Dermis & epidermis interdigitate via downward epidermal projections called rete ridges, and upward dermal projections called the dermal papillae.

26. Dermal structures (structural features of the dermis & its cellular elements) A - structural elements B-cellular elements 1- Collagen network 1- Fibroblasts 2- Elastin fibers 2- Mast cells 3- Macrophages

27. Collagen network: It is the main structural feature of the dermis (net work of collagen fibers).
Elastin fibers: Some elastin fibers are included in the collagen fibers. This network of the collagen fibers & elastin fibers give the dermis great strength & elasticity.
Fibroblasts: are responsible for the synthesis of the connective tissue matrix of the dermis.
Mast cells: They are specialized secretory cells present throughout the dermis, but more numerous around blood vessels & appendages. Cytoplasm of mast cells contains granules which contains mediators as histamine and prostaglandins chemotactic factors which can induce the chemical theory of pain receptors ++++, as in cases of hypersensitivity reactions degranulation of the mast cells release of the chemotactic factors as histamine & prostaglandins.

28. Macrophages: They are phagocytic cells which originate in the bone marrow, then migrate to dermis, they act as scavengers of cell debris & extra cellular material.
N.B.: Finally under the dermis there is a layer of subcutaneous fat (insulator), which separates the skin from the underlying fascia & muscles.

29. Functions of the skin 1. Fluid retention: (Prevent the loss of the essential body fluids):With an intact stratum corneum we lose about 500 ml of water daily via diffusion through skin. Nevertheless stratum corneum makes this diffusion of water into the environment very difficult. In the absence of stratum corneum we lose a significant amount of water (10 fold) to the environment & rapidly become dehydrated. 2. Protect: The body against the invasion of microorganisms.

30. 3-Protect: The body against damage from ultraviolet via the protective effect of melanin. 4-Sensory Perception: Skin is a huge sensory receptor, perceiving heat, cold, pain, temp, light touch, pressure and even tickle. 5-Vitamin D3 formation: Within the sebum a fatty substance called 7-dehydrocholestrol -> on its exposure to ultraviolet rays of the sun -» Vit D (cholicalciferol) which is absorbed via blood stream & then utilized within the body for the proper development of bone tissues and for the proper utilization of Ca+ & phosphorus. 6-Sebum: Has a protective action also, as skin prevent the bacterial invasion, while the oily secretion of the sebaceous glands (sebum) stop & prevent bacterial colonization over the skin by the harmful bacteria. 7-Subcutaneous fat: Which is beneath the dermis act as a poor conductor of heat (insulator), so it helps the skin and the dermis in their functions as insulators against the environmental heat or cold

31. Skin & regulation of body temperature:
Skin participates in thermo regulation as an insulator against extremes of temperature as well as by retaining or eliminating heat created by the body. Human beings are warm-blooded animals and the body temperature is maintained at an average of 36 °C (98.4 f) in health, with slight variations from 0.5 °C to 1 °C.
Temperature is lowest in the morning, during sleep and highest at the afternoon. Core temp, is regulated via a temperature sensitive area in the hypothalamus, which is influenced by the temp, of the blood perfusing it. The rich network of capillaries (small blood vessels) in the dermis, by dilatation -> transmits & eliminate a large quantity of heat, conversely, by constriction and 4- blood flow, this same network of vessels will retrain heat.
So skin response to cold -> vasoconstriction with skin blood flow, and the response to hot is vasodilatation -» loss of heat from the body to the environment. Perspiration (sweating) helps to cool the body by sweat Evaporation -> allowing heat elimination.

32. Thermoregulatory system
Peripheral skin receptor.
Central receptor present in hypothalamus, measure brain blood temperature and this central receptor is called the thermoregulatory center, which is adjusted at 37°C (set point).
If body temp, is below the set point ->• create the anti drop mechanism (vasoconstriction + shivering + adrenaline + and thyroxin release).
If body temp, is above the set point -> create the anti-rise mechanism (vasodilatation + sweating + i muscle tone).
Anti drop mechanism -> sympathetic +++ -> V.C. of skin blood vessels -» shivering in cold = T muscle tone -> thyroxin & adrenaline release.

33. Counter - current heat exchange The superficial venous blood is shifted to the deep venous plexuses (venacomitantes) so heat passes from the arteries to the veins. So venous blood is warmed as it returns to the heart, while arterial blood is cooled as it passes to the skin, so heat is conserved in the body core like the retentivity (retentiveness) power or property of the magnetism. (So core temperature will remain constant).

34. Integumentary system dysfunctions (burn types and causes):
Burns are caused by exposure to excessive heat as flame, hot surfaces (Contact burns), scalding liquids (Scald burns) friction, electricity and chemical burns, heat destroys tissues by coagulation, thermolysis and evaporation of proteins. Exposure to irradiation may also result in skin destruction.
Mild and moderate heating during the application of the therapeutic modalities will result in a reversible process, where tissues revert back to the pre-lasting temperature immediately following the application. So this reversible heating process is valuable from the therapeutical point of view, as this process will follow the temperature law of van't Hoff, which stated that,

35. for every rise of 10°C of the tissue, the rate of oxidation in this tissue is increased 2.5 times the normal. Dehydration following the second and third degree of burns, resulting from the increased capillary permeability with the loss of colloid, water and electrolytes, is a reversible process which can be corrected by intravenous fluid resuscitation to restore the intravascular volume and prevent the hypovolemic shock. In the other hand, excessive heat burns results in the three irreversible processes, protein coagulation, thermolysis and evaporation. Protein coagulation is an irreversible process and thermolysis which is the melting process from heat still irreversible, while evaporation is the transformation of the liquid into gaseous state, this evaporation process is not easily reversed.

36. Burns classification according to the depth of the injury:
Burns are classified into four degrees:
First degree: In which burns are superficial injuries, involving only the outer layers of epithelium. The wound is red, erythematous, hypersensitive and painful, but there is no tissue destruction, spontaneous healing occurs in about 3 weeks.
Second degree: In which burns are erythematous, oedematous, hypersensitive and painful. Second degree burn can be subdivided into a superficial second degree in which there is blistering and destruction of the superficial layers of the epidermis, and a deep second degree burn affecting and destroying the deep epidermal layers until the basement membrane. In the superficial second degree regeneration and healing is possible, but the deep second degree burn with complete epidermal destruction and partially dermal destruction is in need of skin grafting like the third degree burn.

37. Third degree: In which there is complete epidermal and dermal destruction, burn wound is dry and insensitive to pain and is in need of skin grafting.
Fourth degree (Char degree): In which burn destroys and involves the underlying tissues, subcutaneous fat, muscles, nerves, bones and joints.

38. C) Burns classification according to the extent or the percentage of TBSA:
Burns may be classified into 3 groups according to the percentage of TBSA, minor burns involve less than 10% of the TBSA, major burns involve more than 10% of the TBSA, and dangerous burns involve more than 30% of the TBSA the estimation of the TBSA percentage is implemented by the application of the rule of nines, which is the most common clinical method, in which the body is divided into areas, each one representing approximately 9% or 18% of the TBSA, as follows: head and neck represent 9%, each upper limb represents 9%, each lower limb represents 18%, anterior trunk represents 18% posterior trunk represents 18% and the perineum represents only 1%.