Histology of the Skin PDF

 Created by: v Seosan Faith v Musa Oluwatomiwa v Oyebanji Samson v Orimoloye Moses v Olanrewaju Rukayat v Adeyemi Ayomikun v Awoneye Emmanuel v Ejioye Henry v Ojo Rachel v Ogunfunminiyi Temitope v Ajibola Idris v Oyeleke Rachael v Alabi Bukola v...

 Created by: v Seosan Faith v Musa Oluwatomiwa v Oyebanji Samson v Orimoloye Moses v Olanrewaju Rukayat v Adeyemi Ayomikun v Awoneye Emmanuel v Ejioye Henry v Ojo Rachel v Ogunfunminiyi Temitope v Ajibola Idris v Oyeleke Rachael v Alabi Bukola v Onyia Susan v Ilesanmi Kofoworola v Adeola Adebusuyi v Akinniyi Dorcas v Adeniyi Maryam v Nnamani Samuel v Akinnuoye Abidemi  Outline Introduction Functions Layers Appendages Sensory receptors Nerve Endings Skin Repair  INTRODUCTION The skin is the largest single organ of the body, typically accounting for 15%-20% of total body weight and, in adults, presenting 1.5-2 m2 of surface to the external environment. Also known as the integument or cutaneous layer, the skin consists of a superficial layer, the epidermis, made up of stratified squamous epithelium; an epithelial layer of ectodermal origin, and a deeper layer, the dermis, made up of connective tissue of mesodermal origin. The surface of the epidermis is also often marked by elevations and depressions. These elevations form epidermal ridges which interdigitate with the dermal papillae to strengthen adhesion of the two layers I.e. the epidermis and dermis.  INTRODUCTION Epidermal derivatives include hairs, nails, and sebaceous and sweat glands. Beneath the dermis lies the subcutaneous tissue or hypodermis,a loose connective tissue layer usually containing pads of adipocytes. This tissue is sometimes described as a third layer of skin. It binds the skin loosely to the underlying tissues and corresponds to the superficial fascia of gross anatomy. Skin is elastic and can expand rapidly to cover swollen areas and, like the gut lining, is self-renewing throughout life. In healthy individuals injured skin is repaired rapidly. INTRODUCTION  FUNCTIONS Protective It provides a physical barrier to underlying tissues against thermal and mechanical insults such as friction, and against most potential pathogens and other material. The dark pigment melanin in the epidermis protects cell nuclei from ultraviolet (UV) radiation. Skin is also a permeability barrier against excessive loss or uptake of water, which has allowed for terrestrial life. The importance of this function is seen in persons who have lost extensive areas of skin through burns. One important cause of death in such cases is water loss. Skin’s selective permeability also allows some lipophilic drugs such as certain steroid hormones and medications to be administered via skin patches.  FUNCTIONS Sensory Function The skin is a very important sensory organ, containing receptors for touch and related sensations. These receptors allows the skin to constantly monitor the environment, and various skin mechanoreceptors help regulate the body’s interactions with physical objects. The presence of relatively sparse and short hair over most of the skin increases its sensitivity.  FUNCTIONS Thermoregulation A constant body temperature is normally easily maintained thanks to the skin’s insulating components (eg, the fatty layer and hair on the head) and its mechanisms for accelerating heat loss through sweat production and its dense superficial microvasculature. Blood flow through capillaries of the skin can be controlled by numerous arterio-venous anastomoses present in it. In cold weather blood flow through capillaries is kept to a minimum to prevent heat loss. In warm weather the flow is increased to promote cooling. In extreme cold, when some peripheral parts of the body (like the digits, the nose and the ears) are in danger of being frozen the blood flow through these parts increases to keep them warm. In warm climates cooling of the body is facilitated by secretion of sweat and its evaporation.  FUNCTIONS Metabolic function 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 ( 7- dehydrocholesterol) present in skin. Excess electrolytes can be removed in sweat, and the subcutaneous layer stores a significant amount of energy in the form of fat. 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.  LAYERS The skin consists of three main layers: 1. Epidermis: The outermost layer, primarily composed of keratinocytes (cells producing the protein keratin). It provides a protective barrier and undergoes constant renewal. 2. Dermis: Located beneath the epidermis, the dermis contains collagen and elastic fibers, blood vessels, nerves, sweat glands, and hair follicles. It contributes to the skin's strength, elasticity, and sensory functions. 3. Subcutaneous Tissue (Hypodermis): Situated below the dermis, this layer is mainly composed of adipose (fat) tissue. It serves as insulation, energy storage, and a cushion against external trauma. These layers work together to provide structural support, protection, and functionality to the skin.  Epidermis The epidermis consists mainly of a stratified squamous kera- tinized epithelium composed of cells called keratinocytes. There are also three much less abundant epidermal cell types: 1)pigment-producing melanocytes 2)antigen-presenting Langerhans cells 3)tactile epithelial cells called Merkel cells. The epidermis forms the major distinction between thick skin found on the palms and soles, and thin skin found elsewhere on the body. Like all epithelia, the stratified squamous epidermis lacks microvas-culature, its cells receiving nutrients and O, by diffusion from the dermis.  Epidermis From the dermis, the epidermis consists of four layers of keratinocytes (or five layers in thick skin: A)The basal layer (stratum basale) is a single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction. B)The spinous layer (stratum spinosum) is normally the thickest layer, especially in the epidermal ridges and consists of generally polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins. C)The granular layer (Stratum granulosum) consists of three to five layers of flattened cells, now undergoing the terminal differentiation process of keratinization.  Epidermis D)The stratum lucidum: found only in thick skin, consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes. E) The stratum corneum: consists of 15-20 layers of squamous, keratinized cells filled with birefringent filamentous keratins. Epidermis  Medical Application Skin Cancers 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.  Cells of the Epidermis Melanocytes 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. Eumelanins are brown or black pigments produced by the melanocyte,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. Melanocytes are neural crest derivatives that migrate into the embryonic epidermis stratum basale, where eventually one melanocyte accumulates for every five or six basal keratinocytes. Ultrastructurally a melanocyte has numerous small mitochondria, short cisternae of RER, and a well-developed Golgi apparatus.  Cells of the Epidermis Synthesis of Melanin Melanocytes are responsible for the synthesis of melanin. Eumelanin, the pigment produced by the melanocytes, is derived from the amino acid, tyrosine. Tyrosine is converted, by activity of the catalytic enzyme, tyrosinase, into dihydroxy-phenylalanine (DOPA) that is in turn converted into melanin. Melanin pigment is linked to a matrix of structural proteins and accumulates in vesicles derived from the Golgi complex until they form mature elliptical granules about 1-μm long called melanosomes. These melanosomes bud off from the Golgi complex and enter the dendrites of the melanocytes. At the ends of the dendrites, they are shed off from the cell and are engulfed by neighbouring keratinocytes. Although melanocytes produce melanosomes, the keratinocytes are the melanin depot and contain more of this pigment than the cells that make it. The color of skin is mostly influenced by melanin present. Prominence of melanin is more in dark-skinned individuals. Cells of the Epidermis  Cells of the Epidermis Langerhans Cells Antigen-presenting cells called Langerhans cells, 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. Microorganisms cannot penetrate the epidermis without alerting these dendritic cells and triggering an immune response. Cells of the Epidermis  Cells of the Epidermis Merkel Cells Merkel cells, or epithelial tactile cells, are low-threshold mechanoreceptors essential for sensing gentle touch. They are abundant in highly sensitive skin like that of fingertips and at the bases of some hair follicles. Joined by desmosomes to keratinocytes of the basal epidermal layer, present in both thick and thin skin, Merkel cells resemble the surrounding keratinocytes, with which they share a stem cell origin, but contain few, if any, mela-nosomes. Cells of the Epidermis  Medical Application Albinism 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. The causes of melanocyte loss are not clear, but they may include environmental, genetic, or autoimmune conditions. Merkel cell carcinomas 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  Dermis The dermis is the layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis).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. 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.  Dermis Clinical Anatomy Abnormalities of the dermal-epidermal junction can lead to one type of blistering disorder (bullous pemphigoid). Another type of blistering disorder (pemphigus) is caused by autoimmune damage to intercellular junctions between keratinocytes.  Dermis The dermis contains two sublayers with indistinct boundaries: A)The thin papillary layer, which 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 this layer, anchoring fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis. B)The underlying reticular layer is much thicker, 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.  Subcutaneous Tissue The subcutaneous layer consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them. This layer, also called the hypodermis or superficial fascia, contains adipocytes that vary in number in different body regions and vary in size according to nutritional state. The extensive vascular supply at the subcutaneous layer promotes rapid uptake of insulin or drugs injected into this tissue.  APPENDAGES Skin appendages are structures associated with the skin, and they include: 1.Sebaceous Glands 2.Sweat Glands 3.Hair follicles 4.Nails These appendages contribute to the overall functions of the skin, such as protection, thermoregulation, and sensory perception. APPENDAGES  Sebaceous Glands The sebaceous gland is defined as oil-producing glands that are found on the skin of humans and other mammals. These glands are microscopic organs (organs that require a microscope to be seen) and are found across the skin of the entire body, with some exceptions like the palms of the hands and the soles of the feet. The epithelial lining of the sebaceous gland is called the acrosyringium. This epithelium is unique to the sebaceous glands. It is made up of keratinocytes, which secretes sebum, and also contains other specialized cells such as Merkel cells involved in touch sensation. Pimples, also known as acne, is a skin condition that affects almost everyone sometime in their lives, and it is caused by the blockage and subsequent infection of these sebaceous glands. The face, upper neck, scalp, and chest have the greatest number of sebaceous glands in the body and so are also the regions most likely to be affected by acne!  Sebaceous Glands Location The sebaceous glands' location is the dermis, the layer of skin located just below the epidermis, which is the thin, outermost layer of skin. Similarly, the layer located beneath the dermis is called the subcutis (or the hypodermis) which is the innermost layer of skin that contains collagen cells and fat deposits. Sebaceous glands in the dermis are usually found in association with a hair follicle and connected by a channel called the sebaceous duct. While most sebaceous glands are associated with hair follicles, they are also found in non-hairy parts of the body like the nose, nipples, and eyelids, where the glands directly connect to the surface of the skin. On average, there are 3 thousand sebaceous glands found per square inch of skin.  Sebaceous Glands Functions Sebaceous glands are oil-producing glands that secrete mixtures of fats and cellular debris on the surface of the skin. Since these glands secrete a substance to the external surface of the body, they are considered a type of exocrine gland. The fatty substance secreted by the glands is called sebum, which plays many roles in maintaining the health of the skin and hair. By secreting fatty oils, sebaceous glands help: 1)Retain the flexibility of skin 2)Preserve the skin and hair moisture 3)Protect the skin and hair from too much moisture 4)Reduce damage from friction 5)Facilitate the cooling down of skin in hot conditions (in conjunction with sweat glands) 6)Protect skin from UV damage  Sebaceous Glands The production of sebum is affected by weather conditions and also by types of hormones called androgens which include testosterone. A surge in hormone levels during puberty also causes a surge in sebum production. This surge in hormones is therefore also associated with an increase in acne, a skin condition caused by blockages in sebaceous glands. There are two types of sebaceous glands: those connected to hair follicles and those directly connected to the surface of the skin.  Sebaceous Glands Sebum produced in some sebaceous glands can travel towards the hair follicle via a thin tube called the sebaceous duct. Once it reaches the hair follicle, it travels up the hair until it emerges onto the surface. In regions of the body with little or no hair, sebaceous glands are found directly connected to the surface of the skin, and the sebum produced is directly secreted onto the surface of the skin through pores. Sebaceous Glands  Sweat Glands Sweat glands are glands in the skin that produce sweat. The epithelial lining of the sweat glands is called the acrodermal sweat pore. Like the acrosyringium, the acrodermal sweat pore is made up of keratinocytes and other specialized cells, including secretory cells which produce sweat rather than sebum, as in the sebaceous glands. There are two main types: 1)Eccrine glands:which regulate body temperature 2)Apocrine glands:which are associated with hair follicles and become active during emotional stress.  Eccrine Sweat Glands Eccrine sweat glands are widely distributed in the skin and are most numerous on the foot soles. They are exocrine glands with its secretory components and ducts being coiled and possessing small lumens. The secretory part is generally more pale-staining than the ducts and consists of an unusual stratified cuboidal epithelium with three cell types: Pale-staining clear cells which are located on the basal lamina and produce sweat, Dark cells which are filled with strongly eosinophilic granules and line most of the lumen Myoepithelial cells on the basal lamina which moves watery secretion into the ducts.  Eccrine Sweat Glands Cells of the ducts (of eccrine sweat glands) absorb necessary electrolytes such as Na+ from the secreted water to prevent excessive loss. Each duct merges with the basal layer of the epidermis and sweat flows through them, passing through the five epidermal layers to an excretory sweat pore on the skin surface. It quickly evaporates and cools the area of skin and blood present there. Eccrine Sweat Glands  Apocrine Sweat Glands Apocrine sweat glands are largely confined to skin of the axillary and perineal regions, including the areola and nipple regions. Their development depends on sex hormones and is not complete and functional until after puberty. The secretory components of apocrine glands have much larger lumens than those of the eccrine glands and consist of simple cuboidal, eosinophilic cells with numerous secretory granules. The ducts of apocrine glands are similar to those of the eccrine glands, but they usually open into hair follicles at the epidermis. The secretions of apocrine sweat glands are viscous and contain proteins. They are odourless, but after bacterial decomposition they give off body odours that vary from person to person. The production of pheromones by apocrine glands is well established in many mammals and is likely in humans, although in a reduced or vestigial capacity. Sweat Glands  Hair and Nails Structures like hair and nails are known as "accessory structures" of the skin, because they're not essential for the skin's main functions like protection and temperature regulation. But they do serve important functions of their own. Hair helps to regulate body temperature and protects the skin from the sun, while nails help to protect the ends of our fingers and toes.  Nails The nails are hard plates on the dorsal surface of each distal phalanx. They contain keratinized stratified squamous epithelium. They are made up of several different parts. The visible part of the nail is called the nail plate, and it's made of a protein called keratin. The proximal part of the nail is the nail root, and it forms from a matrix of proliferating cells from the stratum basale. The nail root is covered by a fold of skin, from which the epidermal stratum corneum extends as the cuticle, or eponychium. This overlaps the nail plate at the nail root.  Nails The area underneath the nail plate is called the nail bed, and it contains blood vessels and nerves. It also has basal and spinous layers of the epidermis. The skin around the base of the nail is called the nail fold, and it helps to hold the nail in place. The crescent-shaped white area at the base of the nail is the lunula. The distal end of the nail plate becomes free of the nail bed at the epidermal fold called the hyponychium. The distal part of the nail plate, free of the nail bed, is called the free edge.  Nails The nails help to protect the ends of our fingers and toes in a few ways. First, they act as a hard, protective barrier that can help to prevent damage to the underlying skin and bone. Second, they provide a counterforce when our fingers or toes are pressed against an object, helping to reduce the amount of force that's transmitted to the underlying tissues. And finally, they help to prevent dirt, bacteria, and other foreign objects from entering the body through the ends of our fingers and toes. Nails  Hair Follicles Hairs are elongated keratinized structures that form within epidermal invaginations. They are present on the skin covering almost the whole body Hair is made up of three main parts: the hair shaft, the hair root, and the hair follicle. The hair shaft is the visible part of the hair that protrudes from the skin. The hair root is the part of the hair that's embedded in the skin, and it contains living cells that produce the hair shaft. The hair follicle is the pocket in the skin that houses the hair root. Each hair follicle also has a sebaceous gland, which produces an oily substance called sebum that helps to lubricate the hair and skin.  Hair Follicles The hair plays a role in thermoregulation by trapping a layer of air close to the skin. This layer of air is known as the boundary layer, and it acts as an insulator to help keep the body warm. The hair also helps to wick away sweat, which helps to evaporate heat from the body. In addition, hair helps to protect the skin from the sun's UV rays, which can cause the body to overheat. So, hair is actually a pretty important part of our body's thermoregulation system. Hair Follicles  Medical Application Alopecia Loss of hair to produce baldness or alopecia results from a complex combination of genetic and hormonal factors that is not well understood. Arresting mitotic activity in the hair matrix during cancer chemotherapy disrupts both the function and the structural integrity of hair follicles and usually leads to rapid, reversible alopecia  SENSORY RECEPTORS Sensory receptors are specialized structures within the body that respond to stimuli and transmit signals to the nervous system. Their histology varies based on the type of sensation they detect: 1)Mechanoreceptors: These respond to mechanical stimuli like pressure, touch, vibration, and stretch. Histologically, they often have encapsulated structures like Meissner's corpuscles for light touch or Pacinian corpuscles for deep pressure, along with free nerve endings spread throughout tissues. 2)Photoreceptors: Found in the eyes, these cells respond to light. They contain specialized structures like rods and cones in the retina, which convert light into electrical signals.  SENSORY RECEPTORS 3)Chemoreceptors: These detect chemical stimuli such as taste and smell. Taste buds in the mouth and olfactory receptor cells in the nose are examples of chemoreceptor structures. 4)Thermoreceptors: These respond to temperature changes. Histologically, they can be free nerve endings located in the skin and deeper tissues. 5)Nociceptors: These detect pain. They are also often free nerve endings present in various tissues and can respond to various stimuli that can potentially cause tissue damage.  NERVE ENDINGS Also known as sensory receptors.The skin contains various types of nerve endings that play a crucial role in sensing different sensations. They include: 1)Meissner's Corpuscles: These are light touch receptors located in the dermal papillae of hairless skin, such as the fingertips and lips. They are responsible for detecting fine touch and texture. 2)Merkel Discs: Found in the epidermis, Merkel discs are touch receptors that respond to sustained pressure and are involved in the perception of shapes and textures.  NERVE ENDINGS 3)Pacinian Corpuscles: These are deep pressure and vibration receptors located in the deeper layers of the skin. They are particularly sensitive to rapid changes in pressure and help detect sensations like deep touch and vibration. 4)Ruffini Endings: Ruffini endings are stretch receptors found in the dermis. They respond to skin stretch and contribute to the perception of skin deformation and joint position. 5)Free Nerve Endings: These are the most abundant nerve endings in the skin. They are responsible for detecting pain, temperature, and itch sensations. They are found throughout the skin, including the epidermis and dermis. NERVE ENDINGS  SKIN REPAIR As the skin is an exposed and easily damaged organ, it has a good capacity for repair. The process of cutaneous wound healing involves several overlapping stages that vary in duration with the size of the wound. These stages are classified into distinct phases as follows: 1.Inflammatory Phase: In this first phase, blood from cut vessels coagulates in the wound, releasing polypeptide growth factors and chemokines from the disintegrating platelets. Vessel becomes dilated with increased permeability. Neutrophils and macrophages infiltrate the area to remove debris and pathogens. This phase typically lasts for 2-3 days.  SKIN REPAIR 2. Proliferative Phase: Before the inflammatory phase is completed, epithelization begins as epidermal cells and fibroblasts begin to migrate and grow beneath the blood clot. The cell growth is stimulated by several growth factors released from macrophages and other cells, and cell migration is facilitated by proteases from the migrating cells and also macrophages. Blood vessels regrow, and proliferating fibroblasts produce collagen, forming a well-vascularized tissue in the dermis called granulation tissue, which gradually replaces the blood clot.  SKIN REPAIR 3.Remodeling Phase: In the final phase, the epidermis reestablishes continuity but has lost the ability to form new hair or glands. The granulation tissue undergoes remodeling and a more normal vasculature is reestablished. Scar tissue forms at the wound site due to the now, much more abundant and disorganized presence of collagen fibres and fibroblasts. SKIN REPAIR