L4 Histology of Connective Tissue, Skin & Appendages PDF

COURSE: INTRODUCTION TO BASIC SCIENCES COURSE CODE: MIBS65101 MBBS BATCH 30 YEAR 1 TERM 1 Lec 4(ANA) : Histology of Connective Tissue, Skin & Appendages Dr Nazma farhat MBBS Mphil Assistant professor...

COURSE: INTRODUCTION TO BASIC SCIENCES COURSE CODE: MIBS65101 MBBS BATCH 30 YEAR 1 TERM 1 Lec 4(ANA) : Histology of Connective Tissue, Skin & Appendages Dr Nazma farhat MBBS Mphil Assistant professor Anatomy Histology of Connective Tissue, Skin & Appendages Objectives: The objective of the lecture is to discuss the types of cells in connective tissue composition of extracellular matrix superficial and deep fasciae, their function and clinical importance skin and its appendages. Histology of Connective Tissue, Skin & Appendages Topic outcomes: At the end of the lecture, students should be able to: 4.1 Define connective tissue. Outline the distribution and functions of connective tissue. 4.2 Compare cells, ground substance and fibres of connective tissue. 4.3 Differentiate the superficial and deep fascia, their function and understand their clinical importance. 4.4 Describe the histology of different types of connective tissue and its areas of distribution. 4.5 Identify and explain the histology of skin and its appendages.  Tissue Definition of Tissue: Each tissue is an assemblage of similarly specialized cells united in performing a specific function. Composition of Tissue: cell + ECM (extra cellular matrix) Classification of tissue: Tissues are classified into four primary types: 1.Epithelial tissue 2.Connective tissue 3.Muscular tissue 4.Nervous tissue  Organ Organ: An organ is made up with various proportions and morphologies of cells/ tissue and ECM, with characteristic of that organ. Structure of an organ: Parenchyma: Parenchyma forms the functional components of organ. Stroma: Stroma forms the supporting structure. Characteristic features: 4 types of tissues CONNECTIVE TISSUE  Connective Tissue Definition: Connective tissue provides a matrix that supports and physically connects other tissues and cells together, to form the organs of the body. Origin of Connective tissue: Embryonic Connective Mesoderm mesenchyme Tissue  Functions of connective tissue Support: Connective tissue gives structural and mechanical support to the body by binding the cells and organs together. Packing: Loose areolar connective tissue fills the spaces between cells of various tissues and gives shape to the organ. Storage: Adipose tissue is the storehouse of energy (lipid) and about 9 calories can be liberated from every gram of adipose tissue. Loose areolar connective tissue stores water and electrolytes. Transport: The connective tissue matrix serves as a medium through which nutrients and metabolic wastes are exchanged between cells and blood. Repair: Connective tissue has great regenerative capacity following destruction caused by wound or infection. Myofibroblasts are involved in contraction of wound and fibroblasts are involved in laying down of matrix and fibres which fill the space formed by injury. The excess tissue formed during the repair process remains as a ‘scar’. Defense: Most of the cells of connective tissue are involved in the defense of the body either by phagocytosis of foreign body or by producing specific antibodies against antigen.  Connective Tissue - composition Connective Tissue Cells Extracellular Matrix 1. Fibroblast / Fibrocyte 2. Adipocyte 3. Macrophage- Fibers Ground Substance MPS 1. Collagen f 4. Mast Cell 2. Elastic f 5. Plasma Cell 3. Reticular f 6. Leucocyte Eosiniphilic Basophilic Water Proteoglycan, Neutrophilic Glycosaminoglycan(GAG), 7. Lymphocyte – Glycoprotein(Laminin, several types Fibronectin) Connective Tissue - composition  Connective Tissue - Classification The variety of connective tissue types in the body reflects differences in composition and amount of following substances in the extracellular matrix  the cells  Fibers  ground substance Together these are responsible for the remarkable structural, functional, and pathologic diversity of connective tissue.  Connective Tissue - Classification Connective Tissue Connective Tissue Embryonic CT Specialized CT Proper 1. Mesenchyme. Ex. Mesodermal 1. Reticular CT layer of Early Ex. Bone marrow, Liver, embryo Pancreas, Adrenal Loose 2. Mucoid(mucous). Dense CT Gland, all Lymphoid (areolar) CT Ex. Umbelical cord organ except thymus Ex: Lamina Propria 2. Adipose tissue 3. Cartilage D. Irregular CT D. Regular CT 4. Bone Ex: Dermis of Skin, Ex: Ligament, organ capsule, Tendon, aponurosis, 5. Blood Submucosa of GIT coroneal stroma Connective Tissue Connective tissue cells  Connective tissue cells Figure Cell Type Major Product or Activity Fibroblast(fibrocyte) Produce extracellular fiber & ground substance Plasma cells Produce antibodies Lymphocytes(several types) Has various immune and defense function  Connective tissue cells Figure Cell Type Major Product or Activity Eosinophilic leukocytes Modulate allergic/vasoactive reactions and defense against parasites Neutrophilic leukocytes Phagocytosis of bacteria Macrophages Phagocytosis of ECM components and debris; antigen processing and presentation to immune cells; secretion of growth factors, cytokines, and other agents  Connective tissue cells Figure Cell Type Major Product or Activity Mast cells and basophilic Produce pharmacologically leukocytes active molecules (eg, histamine) Adipocytes Store neutral fats  Connective Tissue Cell : Fibroblast Origin : Mesenchymal Cells Permanent / resident cell of connective tissue Characteristics / Traits Fibroblast Fibrocyte Functional state Active Inactive / resting / quiescent Shape Abundant irregular branched Spindle shaped & less process cytoplasm Rough endoplasmic reticulum more less Golgi complex Well developed Not so Nucleus Large , ovoid ,euchromatic Darker, hetarochromatic Nucleolus Prominent Connective Tissue Cell : Adipose/ fat cell Origin : Mesenchymal Cells Flattened eccentric Adipose Tissue: nucleus When adipose cells Narrow rim of cytoplasm predominate in connective tissue, it is Empty space. Large fat globule was called adipose tissue here Number: Single or in Group  Connective Tissue Cell : Macrophage / Histocyte Size : 10 – 30 micrometer in diameter Origin : Irregular Derived from blood monocytes surface Features: 1. Cell surface / membrane : irregular with pleats, protrusions and indentations produced due to active Small kidney pinocytosis and phagocytic activity. shaped / oval nucleus 2. Nucleus: a. Size: small b. Shape: oval or kidney shaped Dense c. Location: eccentrically placed irregular d. Content: rich in chromatin particle 3. Cytoplasm : filled with dense ingested particles 4. Golgi complex: well developed 5. Lysosomes: Many in number  Name of Macrophages in Different Organs Name of macrophage cells Residing organ Dust cell Alveoli of lung Kupffer cell Sinusoid of Liver Langerhans cell Epidermis of skin Microglia Tissue of Brain Monocyte Circulating Blood Osteoclast Bone (formed by fusion of several macrophages) Dendritic cells Lymph node , spleen Multinuclear Giant Cells In connective tissue under various (several fused macrophages) pathological condition  Connective Tissue Cell : Mast cell Origin : Progenitor cells in the bone Basophilic marrow secretory Shape of cell: Oval or irregular granules Diameter / Size : 7- 20 micro meter Centrally Location: placed small nucleus Closely associated with small blood vessels Arranged in group or as single cells Features: 1. Nucleus: small, centrally placed 2.. Cytoplasm : filled with basophilic densely stained secretory granules that cover / obscure central nucleus.  Connective Tissue Cell : Mast cell Basophilic secretory Features: granules Secretory granules of cytoplasm a. Size: 0.3 – 2.0 micro meter in diameter Centrally b. Shape : fine, closely packed placed small c. Character: display Metachromasia nucleus Means granules can change color of some basic dye (ex. Toludine blue) from blue to purple or red  Connective Tissue Cell : Plasma cell Origin : From B lymphocyte Basophilic cytoplasm Life span: 10-20 days Shape of cell: large / ovoid eccentrically Location: More in placed nucleus 1. loose connective tissue 2. Lymphatic tissue of respiratory & digestive tract A few present in most connective tissue  Connective Tissue Cell : Plasma cell Features: Central hetarochromatin 1. Cytoplasm : Basophilic. It has more RER & large Golgi apparatus 2. Nucleus: Shape: spherical Location: eccentrically placed Appearance: In the eccentrically  coarse chromatin clumps distributed placed nucleus Peripheral cartwheel peripherally in radial/ cartwheel pattern presentation pattern and one central mass of chromatin clump  compact peripheral regions of where - Hetarochromatin heterochromatin looks dark looks dark  there will be alternating area of - Euchromatin euchromatin looks lighter Extracellular matrix – Ground Substance The Ground Substance of the ECM is a highly hydrated (with much bound water), transparent, complex mixture of -- 1. three major kinds of macromolecules a) Glycosaminoglycans (GAGs) b) Proteoglycans c) Multiadhesive glycoproteins 2. Water Extracellular matrix – Ground Substance Function of Ground Substances: A. Fill the space between cells and fibers in connective tissue and allows diffusion of small molecules. B. Glycosaminoglycans - ( hyaluronic acid, heparan sulphate), are mucopolysaccharides that provide consistency and viscosity of the ground substance, which serves as a physical barrier to spread of infection. C. structural glycoproteins - fibronectin (in dermis), chondronectin (in cartilage) and laminin (in basement membrane), play an important role in adhesion of cells to the neighboring structures. D. Water of the ground substance allows the exchange of nutrients and metabolic wastes between cells and the blood supply.  Extracellular matrix – Fibers The fibrous components of connective tissue are elongated structures formed from proteins that polymerize after secretion from fibroblasts. There are three types of fibers - 1. Collagen fibers (formed by proteins of the collagen family) 2. Reticular fibers (formed by proteins of the collagen family) 3. Elastic fibers (formed by protein elastin)  Collagen fiber The collagens are proteins that form various extracellular fibers, sheets, and networks, all of which extremely strong and resistant to normal shearing and tearing forces. Collagen is the most abundant protein in the human body, representing 30% of its dry weight. Collagen is a key element of  all connective tissues  epithelial basement membranes  the external laminae of muscle and nerve cells Synthesis of collagen: by fibroblasts Chondroblasts – in cartilage Osteoblasts – in bone Smooth muscle – in blood vessels Odontoblasts – in the tooth Degradation: Collagen fibrils are degraded by collagenase enzymes produced by macrophages. Collagen on denaturation (boiling) gives gelatin.  Collagen fiber Subunits of type I collagen, the most abundant collagen, assemble to form (b) The large bundles of type I extremely strong fibrils, which are then collagen fibrils (C) appear as bundled together further by other acidophilic collagen fibers in collagens, into much larger structures connective tissues, where they may called collagen fibers. fill the extracellular space. (a) TEM shows fibrils cut longitudinally and transversely.  Collagen fiber A family of 28 collagens exists in vertebrates, numbered in the order they were identified. The most common types are as follows: Type I – found in dermis of skin, tendon, bone, dentin Type II – found in cartilage, vitreous body Type III – Skin, muscle, blood vessels, frequently together with type I , reticular fibres Type IV – found in basement membrane, basal and external laminae Type V – found in fetal tissues, skin, bone, placenta, most interstitial tissues  Reticular Fiber Reticular fibers: Type III collagen produces a network of delicate thin (diameter 0.5- 2 μm) reticular fibers for the support of different cells Reticular fibers contain up to 10% carbohydrate as opposed to 1% in most other collagen fibers Produced by : fibroblasts. Fibroblasts specialized for reticular fiber production in hematopoietic and lymphoid organs are called reticular cells Locations: Occur in the reticular lamina of basement membranes Surround adipocytes, smooth muscle and nerve fibers, and small blood vessels. Serve as the supportive stroma for the parenchymal secretory cells and rich microvasculature of the liver and endocrine glands. Abundant in the stroma of hemopoietic tissue (bone marrow), the spleen, and lymph nodes where they support rapidly changing populations of proliferating cells and phagocytic cells.  Reticular Fiber (a) adrenal cortex (b) lymph node Staining of Reticular fiber Seldom visible in hematoxylin and eosin (H&E) preparations Stain very dark with silver stains, termed argyrophilic (Gr. argyros, silver). Periodic acid–Schiff (PAS) positive due to the high content of sugar chains bound to type III collagen α chains. In these silver-stained sections of (networks of delicate, black reticular fibers are prominent.  These fibers serve as a supportive stroma in most lymphoid and hematopoietic organs and many endocrine glands.  The fibers consist of type III collagen that is heavily glycosylated, producing the black argyrophilia.  Cell nuclei are also dark, but cytoplasm is unstained.  Elastic fiber Elastic fibers or sheets called elastic lamellae, are composed of the proteins elastin and fibrillin, which are stretchable. Site of production: Elastin & fibrilin proteins are secreted from  fibroblasts  smooth muscle cells in vascular walls Properties of elastic fiber: Elastic fibers form sparse networks interspersed with collagen bundles in many organs those subject to regular stretching or bending. It has a rubber like properties. So add resilience to connective tissue. Degradation: Elastin resists digestion by most proteases, but it is hydrolyzed by pancreatic elastase.  Elastic fiber Location of elastic fiber: The stroma of the lungs - stretched or distended and return to their original shape during inspiration & expiration. In the wall of large blood vessels, especially arteries, elastin occurs as fenestrated sheets called elastic lamellae. ligamentum nuchae, ligamentum flava Staining properties: Elastic fibers and lamellae are not strongly acidophilic and stain poorly with H&E stained more darkly with orcein and aldehyde fuchsin.  Elastic fibers or lamellae (sheets) - staining mesentery. Dermis of skin aorta (a) spread preparation of nonstretched connective tissue in a mesentery. ( Hematoxylin and orcein) (b)the acidophilic collagen bundles of dermis. (Aldehyde fuchsin) (c)***Elastic lamellae in the wall of the aorta are more darkly stained, incomplete sheets of elastin between the layers of eosinophilic smooth muscle. (H&E) Elastic fiber structure – relaxed & stretched Elastin polypeptides have random-coil domains that straighten or stretch under force, and then relax. the cross-links between elastin subunits consist of Elastin the covalent, cyclic structure subunits/ ppolypeptides desmosine desmosine  Connective tissue proper Classification of connective tissue proper : Connective tissue proper is classified according to the amount of collagen present. 1. Loose connective tissue / areolar tissue 2. Dense connective tissue  Dense regular connective tissue Dense irregular connective tissue  Loose Connective tissue Location of Loose / areolar connective tissue  form a layer beneath the epithelial lining of many organs  fill the spaces between fibers of muscle and nerve. Composition of loose connective tissue: contains cells, fibers, and ground substance in equal parts. The most numerous cells are fibroblasts, other connective tissue cells are present with nerves and small blood vessels. Collagen fibers predominate, elastic and reticular fibers are present. moderate amount of ground substance delicate consistency, flexible and not resistant to stress  Dense Connective tissue Composition of Dense connective tissue: contains less cells – mostly fibroblast predominance of bundled type I collagen fibers over ground substance. The abundance of collagen here, protects organs and strengthens them structurally.  Dense irregular connective tissue Characteristic features: contains less cells – mostly fibroblast predominance of bundled type I collagen fibers over ground substance. Bundled collagen fibers appear randomly interwoven, with no definite orientation. The tough three-dimensional collagen network provides resistance to stress from all directions. Dense irregular and loose connective tissues are often closely associated Examples of dense irregular connective tissue the deep dermis layer of skin capsules surrounding most organs.  Dense regular connective tissue Characteristic features: Dense regular connective tissue consists mostly of type I collagen bundles and fibroblasts aligned in parallel for great resistance to prolonged or repeated stresses from the same direction. Fibrocytes with elongated nuclei lie parallel to the collagen fibers with cytoplasmic folds enveloping portions of the collagen bundles. very little ground substance very few blood vessels Colour: white in the fresh state.  Dense regular connective tissue Examples of dense regular connective tissue Tendons, the very strong and flexible cords connecting muscles to bones. Aponeuroses, which are sheet like tendons. Ligaments, bands or sheets that hold together components of the skeletal system.  Dense regular connective tissue Special features:  On their outer surface tendons and ligaments have a layer of dense irregular connective tissue that is continuous with the outermost layers of the adjacent muscles and bones. All regular connective tissue structures are poorly vascularized and repair of damage in this tissue is usually slow.  Specialized Connective Tissue - Reticular Tissue Composition of Reticular tissue:  abundant fibers of type III collagen - reticulin - form a delicate network, with specialized microenvironments for cells in hemopoietic tissue and some lymphoid organs (bone marrow, lymph nodes, and spleen)  Form passage for leukocytes and lymph.  Macrophages and dendritic cells are dispersed within these reticular tissues to monitor cells formed there or passing through and to remove debris. Produced by modified fibroblasts called reticular cells which partially cover the fibers.  Specialized connective tissue - Reticular tissue (a) Reticular fibers of type III collagen / reticulin are produced and enveloped by the reticular cells, forming an elaborate network through which interstitial fluid or lymph and wandering cells from blood pass continuously. (b) The micrograph shows a silver-stained section of lymph node in which reticular fibers are seen as irregular black lines. Reticular cells are also heavily stained and dark.  Specialized connective tissue - Mucoid tissue Mucoid (or mucous) connective tissue is gelatinous Composition:  abundant ground substance composed chiefly of hyaluronan  sparse collagen fibers  scattered fibroblasts **** many mesenchymal stem cells (which are being studied for their potential in regenerative medicine) Location: Wharton’s jelly : the principal component of the fetal umbilical cord. the vitreous chambers of eyes and pulp cavities of young teeth.  Specialized connective tissue - Adipose tissue Defination: Connective tissue in which fat-storing cells or adipocytes predominate is called adipose tissue. Adipose tissue normally represents 15%-20% of the body weight in men, more in women. Where found : isolated or in small groups within loose or dense irregular connective tissue. occur in large aggregates in adipose tissue or “fat” in many organs and body regions. Types: 1. White adipose tissue 2. Brown adipose tissue Function: storage depots for neutral fats, chiefly triglycerides (long-chain fatty acyl esters of glycerol) regulator of the body’s overall energy metabolism With a growing epidemic of obesity and its associated health problems, including diabetes and heart disease, adipocytes and adipose tissue now constitute a major area of medical research  Specialized connective tissue - Adipose tissue Origin: Mesenchymal stem cells differentiate into preadipocytes. White adipocytes  Unilocular - with one large lipid droplet occupying most of the cytoplasm. Brown adipocytes  multilocular (having many small lipid droplets) with numerous mitochondria.  Mitochondrial metabolism of lipid in brown adipocytes releases heat rather than ATP.  Cells functioning as brown adipocytes can also develop from beige adipocytes during adaptation to cold temperatures. Beige adipocytes have cytological features of both white and brown adipocytes.  Specialized connective tissue - Adipose tissue White Adipose Tissue White adipose tissue is found in many organs throughout the body, typically forming about 20% of the body weight in adults. Diameter : 50 to 150 μm Adipocytes of white fat are very large cells Each cell contain one large lipid droplet (they are unilocular), causing the nucleus and remaining cytoplasm to be pushed against the plasmalemma. Fatty acids are released from white adipocytes by lipase activity, when nutrients are needed and carried throughout the body on plasma proteins, such as albumin. Leptin (hormone) released from white adepocyte regulate eating behavior (target cells in the hypothalamus) Specialized connective tissue - Adipose tissue Specialized connective tissue - Adipose tissue Brown Adipose Tissue Brown fat comprises up to 5% of the newborn body weight but smaller amounts in adults. Adipocytes of this tissue are smaller than adipocytes of white fat  contain many small lipid droplets (they are multilocular)  contain many mitochondria  a central nucleus Adipocytes of brown fat metabolize fatty acid in their mitochondria for thermogenesis. Specialized connective tissue - Adipose tissue (a) Brown adipose tissue is shown here around a small blood vessel (BV) and adjacent white adipose tissue at the top of the photo. FASCIA  Fascia The skin is separated from the deeper structures (muscles and bones) by two layers of connective tissue  the superficial fascia/ subcutaneous tissue  the deep fascia Function of fascia: Fascias (L. fasciae) constitute the wrapping, packing, and insulating materials of the deep structures of the body. Fascia  Superficial Fascia The composition of superficial fascia: Loose irregular connective tissue Stored fat Contents of superficial fascia: sweat glands superficial blood vessels lymphatic vessels cutaneous nerves  The neurovascular structures of the skin (cutaneous nerves, superficial vessels) course in the subcutaneous tissue, distributing only their terminal branches to the skin.  Superficial Fascia Distribution of Superficial fascia: The thickness of the superficial fascia varies with the amount of fat in it. It is thinnest in the eyelids, the nipples and areolae of the breasts, and in some parts of the external genitalia there is no fat. In a well-nourished body, the fat in the superficial fascia rounds off the contours. Its distribution and amount vary in the sexes. The smoother outline of a woman’s figure due to the greater amount of subcutaneous fat is a secondary sex characteristic. Function of Superficial fascia: Subcutaneous tissue participates in thermoregulation, functioning as insulation, retaining heat in the body’s core. It also provides padding that protects the skin from compression by bony prominences, such as those in the buttocks.  Deep fascia Composition of Deep fascia: Dense regular connective tissue with flattened parallel fiber bundles, devoid of fat, that covers most of the body, parallel to (deep to) the skin and subcutaneous tissue. Its thickness varies widely. For example, in the face, distinct layers of deep fascia are absent. Distribution of Deep fascia: Extensions from its internal surface invest deeper structures  individual muscles - epimysium  neurovascular bundles - investing fascia  Deep fascia Distribution of Deep fascia:  In the limbs, groups of muscles with similar functions, usually sharing the same nerve supply, are located in fascial compartments.  These compartments are separated by thick sheets of deep fascia, called intermuscular septa, that extend centrally from the surrounding fascial sleeve to attach to the periosteum of bones. Deep fascia  Deep fascia Function of Deep Fascia 1. Deep fascial compartments contain or direct the spread of an infection or a tumor 2. Gives attachment (origin) to the underlying muscles. 3. Helps in venous return of blood from limbs Q : how deep fascia helps in venous return of blood from lower limbs? The deep fascia surrounding the fascial compartments in the limbs, limits the outward expansion of the bellies of contracting skeletal muscles. Blood is thus pushed out as the veins of the muscles and compartments are compressed. Valves within the veins allow the blood to flow only in one direction (toward the heart), preventing the backflow that might occur as the muscles relax. Thus, deep fascia, contracting muscles, and venous valves work together as a musculovenous pump to return blood to the heart, especially in the lower limbs where blood must move against the pull of gravity.  Deep fascia Musculovenous pump Muscular contractions in the limbs function with the venous valves to move blood toward the heart. The outward expansion of the bellies of contracting muscles is limited by deep fascia and becomes a compressive force, propelling the blood against gravity.  DEEP FASCIA Retinaculum : Near certain joints (e.g., wrist and ankle), the deep fascia becomes markedly thickened, forming a retinaculum. Function of retinaculum: hold tendons in place where they cross the joint during flexion and extension, preventing them from taking a shortcut, or bow stringing, across the angle created. SKIN & IT’S APPENDAGES  Skin Skin / integument (L. integumentum, covering) / cutaneous layer The largest single organ of the body 15%-20% of total body weight In adults present 1.5-2m² of surface to the external environment Layers of Skin: 1. The epidermis, an epithelial layer of ectodermal origin 2. The dermis, a layer of mesodermal connective tissue  Skin At the irregular junction between the dermis and epidermis, projections called dermal papillae interdigitate with invaginating epidermal ridges to strengthen adhesion of the two layers. Appendages of skin: Epidermal derivatives include  Hairs  nails  sebaceous and sweat glands  Functions of skin 1. Protection: Skin gives protection against mechanical trauma, invasion of microorganisms, evaporation (water loss) and ultraviolet rays (by melanin pigments). 2. Sensory perception: Skin is the largest sense organ of the body. It contains many receptors for general sensation (pain, touch, temperature and pressure). 3. Thermoregulation: It is mainly performed by glands (sweating) and also by blood vessels and adipose tissue. 4. Synthesis of vitamin D: Epidermis of skin is involved in synthesis of vitamin D from 7-dehydrocholesterol by the action of UV light. 5. Excretion: Skin acts as a minor excretory organ for certain catabolic nitrogenous waste products and water. 6. Storage: Skin acts as a storehouse for glycogen and cholesterol in the subcutaneous fat. 7. Absorption: Skin also absorbs certain lipid soluble substances, drugs/chemicals which are of therapeutic value. 8. Identification: Skin is useful in personal identification, especially in criminology—through dermatoglyphics - finger print.  Epidermis Composition / structure: Stratified squamous keratinized epithelium, mainly made of keratinocytes, other cells are melanocytes, Langerhans cells, Merkel’s cells. Thickness: varies from 0.1 mm to 1.4 mm. Nutrition: Is avascular and is nourished by diffusion. Innervation: Free nerve endings are seen in its basal layer. Renewal: Is renewed every 15–30 days depending on the region of the body, age and other factors. Epidermis  Cells of Epidermis 1. keratinocytes : Amount: Most abundant Cell type: stratified squamous keratinized epithelium 2. Melanocytes: pigment-producing cell 3. Langerhans cells: antigen-presenting cell 4. Merkel cells: tactile epithelial cell  Cells of epidermis: keratinocyte Function of Keratinocytes: protect from microbial invasion, shield UV exposure, and maintain adequate skin hydration by secreting, cytoskeletal keratins( intermediate filaments, about 10 nm in diameter)  Cells of epidermis: Melanocytes Arise from neural crest cells Melanocytes are located in the epidermal basal layer / between stratum basale and stratum spinosum  Synthesize melanin granules and transfer them into neighboring keratinocytes. Transfer occurs through many long, branching melanocyte processes that extend into the spinous layer Dark melanin pigment accumulate to protect nuclear DNA from UV damage. Melanin darkens skin color Cells of epidermis: Melanocytes  Cells of epidermis: Langerhans Cells Antigen-presenting cells of the skin(part of immune system of body) Found mainly in stratum spinosum Langerhans cells form a network through the epidermis, intercepting and sampling microbial invaders before moving to lymph nodes in an adaptive immune response. Dermis  Dermis Papillary layer(thin) includes the dermal papillae, consists of loose connective tissue, with types I and III collagen fibers, fibroblasts and scattered mast cells, dendritic cells, and leukocytes. From the papillary layer Anchoring fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis. Reticular layer (thick) consists of dense irregular connective tissue (mainly bundles of type I collagen), with more fibers and fewer cells than the papillary layer. A network of elastic fibers is also present, providing elasticity to the skin. Between the collagen and elastic fibers are abundant proteoglycans rich in dermatan sulfate.  Cutaneous Sensory Receptors Cutaneous Sensory Receptors Sensory receptors in the epidermis include free nerve endings, which detect pain and temperature extremes basal Merkel cells, light-touch (tactile) receptors associated with sensory fibers. Other cutaneous sensory structures include Meissner corpuscles, encapsulated elliptical mechanoreceptors that surround sensory axons and also detect light touch. Deeper in the dermis and subcutaneous layer are lamellated or pacinian corpuscles, which are ovoid and much larger than Meissner corpuscles, for detection of pressure or firm touch.  Glands of skin Skin includes three major types of exocrine glands. 1. Sebaceous glands are usually part of a pilosebaceous unit with a hair follicle and secrete oily sebum into the space around the hair root. 2. Thermoregulatory merocrine sweat glands empty their secretion onto the skin surface via sweat pores. 3. Apocrine sweat glands secrete a more protein-rich sweat into the follicles of hair in skin of the axillae and perineum.  Glands of skin Modified Glands of Skin 1. Mammary gland - Modified apocrine sweat gland 2. Ceruminous gland - in external acoustic meatus 3. Glands of Moll - in eyelid 4. Glands of Zeis - in eyelid 5. Tarsal or Meibomian gland - in eyelid (Modified sebaceous gland)  Types of skin Features Thick skin Thin skin Thickness of Epidermis is very Epidermis is very epidermis thin thick with a thick layer of stratum corneum Presence of hair Has hair Has no hair Location Found in all other Found in palm of parts of the body hand and sole of except palm and foot sole.  Types of skin : Thin skin Thin Skin /Hairy Skin: Presence of (i) thin epidermis made up of keratinized stratified squamous epithelium (stratum corneum is thin) (ii) hair follicles and sebaceous glands (iii) sweat glands in the dermis.  Types of skin : Thick skin Thick/ Nonhairy Skin Presence of (i) thick epidermis made up of keratinized stratified squamous epithelium (stratum corneum is very thick) (ii) absence of hair follicles and sebaceous glands (iii) presence of sweat glands in the dermis.  Hair Hairs are elongated / cylindrical, hard structures, made of fused dead keratinized cells. Development: surface epithelium invaginate in the dermis and form the hair follicle. The hair develop from this hair follicle. Location: Hair is found in all parts of the skin except palm, sole, lip, umbilicus, glans penis, clitoris, labia minora and distal phalanx. Parts of hair: 1. Shaft / Scapus: The visible projecting part of the hair 2. Root / radix: the invisible part embedded in the dermis 3. Hair follicle: The root of the hair is surrounded by a tubular invagination of the epidermis called hair follicle, from which hair arises. - Hair bulb - Hair papilla  Structure of Hair Layers of hair: from outside to inside 1. Cuticle 2. Cortex 3. Medulla Cuticle is the outer layer and is made of single layer of flat scale-like cells that overlap one another from below. Cortex lies deep to the cuticle and is composed of several layers of elongated cells. Cortex forms the main bulk of the hair. Medulla is found in the centre and is made of large vacuolated cells which are often separated by air spaces.  All the cells of the above layers of hair contain hard keratin and melanin pigment granules.  Cells of the hair bulb matrix proliferate, take up melanin granules, and undergo keratinization to differentiate as the three concentric layers of the hair.  Structure of Hair Follicle Hair follicle is the tubular invagination of the epidermis that surrounds the root of the hair. The deep expanded part of the follicle is called hair bulb which is made of pluripotent polyhedral matrix cells. Hair grows by differentiation and keratinization of cells of hair bulb. Melanocytes are also present in the hair bulb which transfer melanin granules into the cells of hair and are responsible for pigmentation of hair. The hair bulb is indented by vascular connective tissue of the dermis and is known as hair papilla.  Structure of Hair Follicle The hair follicle receives the duct of the sebaceous gland. It also gives attachment to a band of smooth muscle, called arrector pili muscle, below the level of sebaceous gland. Contraction of the muscle causes erection of hair resulting in goose skin, as occurs on exposure to cold or during emotions. Contraction also causes compression of sebaceous gland expressing sebum. The wall of the follicle has two coats  connective tissue sheath derived from dermis  epithelial or epidermal sheath derived from epidermis.  Structure of Hair Follicle The epithelial sheath consists of the following layers from outer to inner: 1. Glassy membrane : thickened basement membrane separating connective tissue sheath from epithelial sheath. 2. Outer epithelial root sheath : corresponds to and is continuous with stratum basale and stratum spinosum of epidermis. 3. Inner epithelial root sheath : corresponds to superficial layers of the epidermis and is present only below the level of sebaceous glands – made of three layers, namely, from outer to inner, Henle’s layer, Huxley’s layer and cuticle. – the cells of the cuticle of inner root sheath interlock with the cells of cuticle of hair. This arrangement helps to anchor the hair within the follicle.  Structure of root & bulb (b) A longitudinal section of a hair root and bulb shows the matrix, medulla, and cortex in the root and the surrounding epithelial and connective tissue sheaths. (c) The outermost layer of the hair is the thin cuticle, composed of shingle-like cells, of a hair shaft emerging at the stratum corneum  Information about hair Skin of foetus is covered by fine hair called lanugo (primary hair) which is shed at birth and is replaced by pale downy hair called vellus (secondary hair). Vellus is retained in most of the regions of the body except scalp, face, eyebrow, axilla and pubis, where it is replaced by coarse dark hair called terminal hair (influenced by sex hormone). Hair do not grow continuously but have a growth cycle [they have period of growth (anagen phase) followed by a period of rest (telogen phase). Hair growth is not affected by frequency of cutting or shaving. Growth rate of hair is approximately 1.5–2.2 mm per week. Hair grow faster between ages 26 and 46 years. Life span of hair varies from region to region; in scalp as long as 4 years, in axilla as short as 4 months. Greying or whitening of hair is caused by either failure of melanocytes to form pigment granules (congenital) or appearance of small air bubbles among the cells of the cortex and medulla of hair. The reflection of light in the air bubbles is responsible for the glistening or silvery appearance of white hair. Baldness is caused by – progressive atrophy of hair follicle with age – genetic factor – presence of androgenic hormone.  Nail Nail is a cornified plate of stratum corneum / hard plates of keratin, on the dorsal surface of each distal phalanx / terminal part of fingers and toes. The inferior surface of nail rests on nail bed which corresponds to stratum basale and stratum spinosum of the epidermis.  Nail - parts Nail Plate: Parts of nail plate are Nail Matrix: cells divide, move distally, and become keratinized and nail root is formed Nail root : The proximal part of the nail, nail root matures and hardens as the nail plate. Nail bed : contains only the basal and spinous epidermal layers , nail plate is bound to it Cuticle / eponychium : the root of nail is covered by a fold of skin ( extension of the epidermal stratum) Lunula : the crescent-shaped white area, which derives its color from the opaque nail matrix and immature nail plate below it Hyponychium: The distal end of the plate becomes free of the nail bed at the epidermal fold  Nail How nail grows? Continuous growth in the matrix pushes the nail plate forward over the nail bed (which makes no contribution to the plate) at a rate of about 3 mm/mo for fingernails and 1 mm/mo for toenails. The nearly transparent nail plate and the thin epithelium of the nail bed provide a useful window on the amount of oxygen in the blood by showing the color of blood in the dermal vessels. THANK YOU