Skin Histology PDF

THE SKIN DR. ADEKEYE ADESHINA DEPARTMENT OF ANATOMY 2024 EDITION  Outlines 2  Introduction  Layers of skin  Epidermal derivatives  Functions of skin  Importance of SSKE  Cells of the Epidermis  Layers of the Epidermis  Medical Appl...

THE SKIN DR. ADEKEYE ADESHINA DEPARTMENT OF ANATOMY 2024 EDITION  Outlines 2  Introduction  Layers of skin  Epidermal derivatives  Functions of skin  Importance of SSKE  Cells of the Epidermis  Layers of the Epidermis  Medical Application 12/4/2024  QUESTIONS 3  Importance of Epithelialization in wound healing  What is Keratinization  What do y0u understand about desquamation 12/4/2024 4 12/4/2024  INTRODUCTION 5  The skin is the largest single organ of the body, typically accounting for 15%-20% of total body weight  In adults, It presents 1.5-2 m2 of surface to the external environment.  It is also known as the integument (covering) or cutaneous layer. 12/4/2024  LAYERS OF SKIN 6  The skin is composed of two parts  Epidermis: an epithelial layer of ectodermal origin  Dermis: a layer of mesodermal connective tissue. 12/4/2024  EPIDERMAL DERIVATIVES 7 Epidermal derivatives include:  Hairs  Nails  Sebaceous glands  Sweat glands 12/4/2024  8  Beneath the dermis lies the subcutaneous tissue or hypodermis (Gr. hypo, under + derma,skin),  Subcutaneous tissue a loose connective tissue layer usually containing pads of adipocytes.  The subcutaneous tissue binds the skin loosely to the underlying tissues and corresponds to the superficial fascia of gross anatomy. 12/4/2024  FUNCTIONS OF SKIN 9  The specific functions of the skin fall into several broad categories. 1. Protective: It provides a physical barrier against thermal and mechanical insults such as friction and against most potential pathogens and other material.  Microorganisms that do penetrate skin alert resident lymphocytes and antigen-presenting cells (APCs) in skin and an immune response is mounted. 12/4/2024  10  Skin is also a permeability barrier against excessive loss or uptake of water, which has allowed for terrestrial life.  Skin’s selective permeability allows some lipophilic drugs such as certain steroid hormones and medications to be administered via skin patches. 12/4/2024  11 2. Sensory: Many types of sensory receptors allow skin to constantly monitor the environment, and various skin mechanoreceptors help regulate the body’s interactions with physical objects. 12/4/2024  12 3. Thermoregulatory: A constant body temperature is normally easily maintained as a result of skin’s insulating components (eg, the fatty layer and hair on the head) and its mechanisms for accelerating heat loss (sweat production and a dense superficial microvasculature). 12/4/2024  13 4. Metabolic: Cells of skin synthesize vitamin D3, needed in calcium metabolism and proper bone formation, through the local action of UV light on the vitamin’s precursor.  Excess electrolytes can be removed in sweat, and the subcutaneous layer stores a significant amount of energy in the form of fat. 12/4/2024  14  Sexual signaling: Many features of skin, such as pigmentation and hair, are visual indicators of health involved in attraction between the sexes in all vertebrate species, including humans.  The effects of sex pheromones produced by the apocrine sweat glands and other skin glands are also important for this attraction. 12/4/2024  15  The dermal-epidermal interdigitations are of the peg-and socket variety in most skin, but they occur as well-formed ridges and grooves in the thick skin of the palms and soles, which is more subject to friction. 12/4/2024  16  These ridges and the intervening sulci form distinctive patterns unique for each individual, appearing as combinations of loops, arches, and  whorls, called dermatoglyphs, also known as fingerprints and footprints. 12/4/2024  Parts of skin 17 AP: This is a tiny muscle that attaches to the base of a hair follicle at one end and to dermal tissue on the other end. In order to generate heat when the body is cold, the arrector pili muscles contract all at once, causing the hair to "stand up straight" on the skin. Inderbir-Singh, 12/4/2024  EPIDERMIS 18  What epithelium lines the outermost layer of the skin? 12/4/2024  EPIDERMIS 19  The epidermis consists mainly of a stratified squamous keratinized epithelium composed of cells called keratinocytes.  The cell constitute about 90% of the epidermis. Note: When found in the basal layer (stratum basale) of the skin, they are sometimes called Basal cell (basal 12/4/2024 keratinocytes)  PRIMARY FUNCTION OF KERATINOCYTES 20 Formation of a barrier against environmental damage by: heat UV radiation water loss pathogenic bacteria, fungi, parasites and viruses Keratinocytes participate in the repair and wound healing 12/4/2024  21  Pathogens invading the upper layers of the epidermis cause keratinocytes to produce proinflammatory mediators particulary as chemokines such as CXCL10 or CCL2which attract monocytes, natural killer cell 12/4/2024  OTHER EPIDERMAL CELLS 22 ❑ There are also three much less abundant epidermal cell types:  Pigment-producing melanocytes  Antigen-presenting Langerhans cells  Tactile epithelial cells called Merkel cells 12/4/2024  Melanocytes 23  The color of the skin is the result of several factors, the most important of which are the keratinocytes’ content of melanin and carotene and the number of blood vessels in the dermis. 12/4/2024  24 Eumelanins are brown or black pigments produced by the melanocyte.  Eumelanin is a specialized cell of the epidermis found among the cells of the basal layer and in hair follicles.  The similar pigment found in red hair is called pheomelanin. 12/4/2024  25  Ultrastructurally a melanocyte has numerous small mitochondria, short cisternae of RER and a well- developed Golgi apparatus. 12/4/2024  26 (b) Diagram of a melanocyte shows the irregular cytoplasmic processes between neighboring keratinocytes for transfer of melanin. (c) Ultrastructurally, a melanocyte is located on the basal lamina (BL) and has well- developed Golgi complexes (G) producing the vesicles in which melanin is synthesized. As they fill, these vesicles become melanin granules (MG), which accumulate at the tips of the dendritic cytoplasmic extensions (CE) (a) In light microscopy melanocytes (M) typically appear as before transfer to rounded, pale-staining or clear cells just above the dermis (D). keratinocytes 12/4/2024  MEDICAL APPLICATION 27 ❑ Albinism is a congenital disorder producing skin hypopigmentation due to a defect in tyrosinase or some other component of the melanin-producing pathway.  An acquired condition called vitiligo involves skin depigmentation, often only in affected patches, due to the loss or decreased activity of melanocytes. 12/4/2024  28  The causes of melanocyte loss are not clear, but they may include environmental, genetic, or autoimmune conditions. 12/4/2024  Alopecia (Areata; totalis) 29  Alopecia areata is a condition that causes hair to fall out in small patches, which can be unnoticeable. These patches may connect, however, and then become Alopecia areata/ noticeable. Spot Baldness  The condition develops when the immune system attacks the hair follicles, resulting in hair loss 12/4/2024  30  Alopecia totalis is a skin condition that causes hair loss  It causes complete baldness of the scalp.  When loss of all body hair occur, it is known as alopecia universalis 12/4/2024  31 ❑ Melanocytes can normally proliferate in skin to produce moles, or benign melanocytic nevi of various types.  Changes in the size or appearance of moles are sometimes indicative of dysplasia that can progress further to malignant melanoma. 12/4/2024  Langerhans Cells 32  Antigen-presenting cells (APCs) is called Langerhans cells.  It is derived from monocytes, represent 2%-8% of the cells in epidermis and are usually most clearly seen in the spinous layer.  Langerhans cells bind, process, and present antigens to T lymphocytes in the same manner as immune dendritic cells in other organs. 12/4/2024  33  Microorganisms cannot penetrate the epidermis without alerting these dendritic cells and triggering an immune response.  Langerhans cells, along with more scattered epidermal lymphocytes and other APCs in the dermis, comprise a major component of the skin’s adaptive immunity. 12/4/2024  34  Various epidermal features participate in both innate and adaptive immunity 12/4/2024  Merkel Cells 35  Merkel cells or epithelial tactile cells are sensitive mechanoreceptors essential for light touch sensation.  They are abundant in highly sensitive skin like that of fingertips and at the bases of some hair follicles.  Merkel cells originate from the same stem cells as keratinocytes.  They are characterized by small, Golgi-derived dense-core neurosecretory granules containing peptides. 12/4/2024  LAYERS OF EPIDERMIS 36 ❑ The epidermis consists of four layers of keratinocytes (or five layers in thick skin) from the dermis:  The basal layer (stratum basale)  The spinous layer (stratum spinosum)  The granular layer (stratum granulosum)  The stratum lucidum  The stratum corneum 12/4/2024 Layers (strata) of epidermis in thick skin. 37 12/4/2024  38  Epidermis forms the major distinction between thick skin , found on the palms and soles and thin skin found elsewhere on the body. 12/4/2024 Layers of epidermis in thin skin. 39 The interface between dermis and epidermis in thin skin is held together firmly by interlocking epidermal ridges or pegs (EP) and dermal papillae (DP). The dermis (D) of thin skin is more cellular and well vascularized than that of thick skin, with elastin and less coarse bundles of collagen. The epidermis usually shows only four layers in thin skin: the one-cell thick stratum basale (B) containing most mitotic cells; the stratum spinosum (S) where synthesis of much keratin and other proteins takes place; the stratum granulosum (G); and the stratum corneum (C), consisting of dead squames composed mostly of keratin. (H&E) 12/4/2024  LAYERS OF EPIDERMIS 40 ❑ The epidermis consists of four layers of keratinocytes (or five layers in thick skin) from the dermis:  The basal layer (stratum basale)  The spinous layer (stratum spinosum)  The granular layer (stratum granulosum)  The stratum lucidum  The stratum corneum 12/4/2024 a. The basal layer (stratum 41 basale) is a single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction.  Hemidesmosomes in the basal cell membranes join these cells to the basal lamina, and  Desmosomes bind the cells of this layer together in their lateral and upper surfaces. 12/4/2024  42 The stratum basale is characterized by intense mitotic activity and contains, along with the deepest part of the next layer, progenitor cells for all the epidermal layers.  Presence of basal stem cells for keratinocytes. 12/4/2024  43  The human epidermis is renewed about every 15-30 days, depending on age, the region of the body, and other factors.  An important feature of all keratinocytes in the stratum basale is the cytoskeletal keratins, intermediate filaments about 10 nm in diameter. 12/4/2024  44  During differentiation, the cells move upward and the amount and types of keratin filaments increase until they represent half the total protein in the superficial keratinocytes. 12/4/2024  MEDICAL APPLICATION 45  Friction blisters are lymph-filled spaces created between the epidermis and dermis of thick skin by excessive rubbing, as with ill-fitting shoes or hard use of the hands.  If continued, such activity produces protective thickening and hardening of the outer cornified epidermal layers, seen as corns and calluses. 12/4/2024  46 b. The spinous layer (stratum spinosum) is normally the thickest layer, especially in the epidermal ridges.  It consists of generally polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins. 12/4/2024 Keratinocytes of the stratum spinosum. 47 (a) A section of epidermal stratum spinosum of (b) TEM of a single spinous keratinocyte with surface projections (arrows).showing cells with numerous short cytoplasmic projections (arrow). (b) TEM of a single spinous keratinocyte with surface projections (arrows). 12/4/2024  48  Just above the basal layer, some cells may still divide and this combined zone is sometimes called the stratum germinativum.  The keratin filaments assemble here into microscopically visible bundles called tonofibrils that converge and terminate at the numerous desmosomes holding the cell layers together. 12/4/2024  49  The cells extend slightly around the tonofibrils on both sides of each desmosome (and the extensions elongate if the cells shrink slightly during histologic  processing), leading to the appearance of many short “spines” or prickles at the cell surfaces. 12/4/2024  50  The epidermis of thick skin subject to continuous friction and pressure (such as the foot soles) has a thicker stratum spinosum with more abundant tonofibrils and desmosomes 12/4/2024  MEDICAL APPLICATION 51  In adults, one-third of all cancers originate in the skin.  Most of these derive from cells of the basal or spinous layers, producing, respectively, basal cell carcinomas and squamous cell carcinomas.  Fortunately, both types of tumors can be diagnosed and excised early and consequently are rarely lethal. 12/4/2024  52 c. The granular layer (stratum granulosum) consists of three to five layers of flattened cells, now undergoing the terminal differentiation process of keratinization.  Their cytoplasm is filled with intensely basophilic masses called keratohyaline granules. 12/4/2024  53  These are dense, non–membrane-bound masses of filaggrin and other proteins associated with the keratins of tonofibrils, linking them further into large cytoplasmic structures. 12/4/2024 Stratum granulosum and stratum lucidum: thick skin. In keratinocytes 54 moving upward from the stratum spinosum (S), differentiation proceeds with the cells becoming filled with masses of protein called keratohyaline granules. Cells with these basophilic granules make up the stratum granulosum (G), Smaller organelles called lamellar granules undergo exocytosis in this layer, secreting a lipid- rich layer around the cells which makes the epidermis impermeable to water. The cells leaving the stratum granulosum, still bound together by desmosomes, undergo terminal differentiation and in thick skin appear as a dense, thin layer called the stratum lucidum (L). In the most superficial stratum corneum (C), the cells have lost nuclei and cytoplasm, consisting only of flattened, keratinized structures called squames bound by hydrophobic, lipid-rich intercellular cement. 12/4/2024  55 d. The stratum lucidum is found only in thick skin.  It consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes.  Nuclei and organelles have been lost, and the cytoplasm consists almost exclusively of packed keratin filaments embedded in an electron-dense matrix. 12/4/2024  56 e. The stratum corneum consists of 15-20 layers of squamous, keratinized cells filled with birefringent filamentous keratins. By the end of keratinization, the cells contain only amorphous, fibrillar proteins with plasma membranes surrounded by the lipid-rich layer. 12/4/2024  57  These fully keratinized or cornified cells called squames are continuously shed at the epidermal surface as the desmosomes and lipid-rich cell envelopes break down. 12/4/2024  MEDICAL APPLICATION 58  In the chronic skin condition called psoriasis, keratocytes are typically produced and differentiate at accelerated rates, causing at least slight thickening of the epidermal layers and increased keratinization and desquamation. 12/4/2024  59  Psoriasis is caused by overactive T lymphocytes that trigger an autoimmune reaction in the skin, which can also lead to inflammation with redness, irritation, itching, and scaling, with a defective skin barrier. 12/4/2024 60 12/4/2024  MEDICAL APPLICATION 61  Merkel cells are of clinical importance because Merkel cell carcinomas, though uncommon, are very aggressive and difficult to treat.  Merkel cell carcinoma is 40 times less common than malignant melanoma but has twice the mortality of that disease. 12/4/2024  DERMIS 62  The dermis is the layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis).  The thickness of the dermis varies with the region of the body and reaches its maximum of 4 mm on the back. 12/4/2024  63  The surface of the dermis is very irregular and has many projections (dermal papillae) that interdigitate with projections (epidermal pegs or ridges) of the epidermis, especially in skin subject to frequent pressure, where they reinforce the dermal-epidermal junction. 12/4/2024  64  A basement membrane always occurs between the stratum basale and the dermis, and follows the contour of the interdigitations between these layers.  Nutrients for keratinocytes diffuse into the avascular epidermis from the dermal vasculature through the basement membrane. 12/4/2024  65 The dermis contains two sublayers with indistinct boundaries:  Thin papillary layer  Thick reticular layer 12/4/2024 66 A section of thin skin stained for elastic fibers shows the extensive distribution of these darkly stained fibers among the eosinophilic collagen bundles. 12/4/2024  67  The thin papillary layer consists of loose connective tissue, with types I and III collagen fibers, fibroblasts and scattered mast cells, dendritic cells, and leukocytes.  From this layer, special fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis (anchoring fibrils).  This layer is so called because it constitutes the major part of the dermal papillae. 12/4/2024  68  The dermis also contains epidermal derivatives such as the hair follicles and sweat and sebaceous glands.  There is a rich supply of nerves in the dermis.  Presence of afferent nerve endings 12/4/2024  SENSORY RECEPTORS 69  With its large surface and external location, the skin functions as an extensive receiver for various stimuli from the environment.  Diverse sensory receptors are present in skin, including both simple nerve endings. 12/4/2024 Tactile receptors 70 12/4/2024  71 The unencapsulated receptors include the following:  The Merkel cells, each associated with expanded nerve endings, which function as tonic receptors for sustained light touch and for sensing an object’s texture.  Free nerve endings in the papillary dermis and extending into lower epidermal layers, which respond primarily to high and low temperatures, pain, and itching, but also function as tactile receptors. 12/4/2024  Encapsulated receptors 72  The encapsulated receptors are all phasic mechanoreceptors, responding rapidly to stimuli on the skin.  Four are recognized in human skin, although only the first two are seen in routine preparations:  Meissner corpuscles  Lamellated (pacinian) corpuscles  Krause end bulbs  Ruffini corpuscles 12/4/2024  73  ASSIGNMENT: FIND OUT THE ROLES OF ENCAPSULATED RECEPTORS 12/4/2024 74 (a) Meissner tactile corpuscles (TC) are specialized to detect light touch and are frequently located in dermal papillae (DP), partially surrounded by epidermis (E). (b)Lamellated (pacinian) corpuscles (PC) detect coarse touch or pressure and are large oval structures, frequently 1 mm in length, found among adipose tissue (A) deep in the reticular dermis or in the subcutaneous tissue. 12/4/2024  75  Meissner corpuscles are elliptical structures, 30- 75 μm by 50-150 μm, consisting of sensory axons winding among flattened Schwann cells arranged perpendicular to the epidermis in the dermal papillae  They initiate impulses when light-touch or low- frequency stimuli against skin temporarily deform their shape.  They are numerous in the fingertips, palms, and soles but decline slowly in number during aging after puberty. 12/4/2024  76  Lamellated (pacinian) corpuscles are large oval structures,  approximately 0.5 mm by 1 mm, found deep in the  reticular dermis and hypodermis, with an outer capsule  and 15-50 thin, concentric lamellae of flattened Schwann  cells and collagen surrounding a highly branched, unmyelinated  axon 12/4/2024  77  Lamellated corpuscles are specialized for sensing coarse touch, pressure (sustained touch), and vibrations, with distortion of the capsule amplifying a mechanical stimulus to the axonal core where an impulse is initiated. 12/4/2024  78  Krause end bulbs are simpler encapsulated, ovoid structures, with extremely thin, collagenous capsules penetrated by a sensory fiber.  They are found primarily in the skin of the penis and clitoris where they sense low frequency vibrations. 12/4/2024  HAIR 79  Ruffini corpuscles have collagenous, fusiform capsules anchored firmly to the surrounding connective tissue, with sensory axons stimulated by stretch (tension) or twisting (torque) in the skin. 12/4/2024  HAIR 80 (a) The diagram shows major parts of a hair and its follicle (b) A longitudinal section of a hair root and (c) The outermost layer of the hair is the thin bulb cuticle 12/4/2024  HAIR 81  Hairs are elongated keratinized structures that form within epidermal invaginations, the hair follicles. 12/4/2024  Factors affecting the hair 82  The color, size, shape, and texture of hairs vary according:  Age  Genetic background  Region of the body. 12/4/2024  AREA DEVOID OF HAIR 83  All skin has at least minimal hair except  the glabrous skin of the palms  Soles  Lips  Glans penis  Clitoris  Labia minora. 12/4/2024  84  The growing hair follicle has a terminal dilation called a hair bulb.  A dermal papilla inserts into the base of the hair bulb and contains a capillary network required to sustain the hair follicle. 12/4/2024  85  Keratinocytes continuous with those of the basal epidermis cover the dermal papilla.  These cells form the matrix of the elongating hair root; the part of a hair extending beyond the skin surface is the hair shaft. 12/4/2024