Histology of the Skin - PSU School of Medicine - Year 1

HISTOLOGY & GENETICS T03 HG201A | Dr. Maynie Lugasan-Nigos | PSU School of Medicine Year 1 | Batch 2028 09/02/2024...

HISTOLOGY & GENETICS T03 HG201A | Dr. Maynie Lugasan-Nigos | PSU School of Medicine Year 1 | Batch 2028 09/02/2024 HISTOLOGY OF THE SKIN OUTLINE I. Epidermis II. Dermis A. Thick and thin skin A.Papillary layer B. Strutures of Layers B.Reticular layer 1. St. Corneum III. Hypodermis 2. St. Lucidum IV. Skin appendages 3. St. Granulosum A. Hair 4. St. Spinosum B. Nails 5. St. Basale C. Glands C. Cells of epidemis V. Skin homeostasis D. Sensory receptors VI. Wound healing EPIDERMIS Figure 3.1 Thick vs thin skin Derived from ectoderm Avascular ○ The stratified squamous epidermis lacks microvasculature. ○ Its cells only received nutrients and oxygen by diffusion from dermis. Maintains normal thickness through desquamation Composed of stratified squamous epithelium Most common epidermal cell type: ○ KERATINOCYTE Other 3 less abundant epidermal cell types: ○ Melanocyte (pigment-forming) ○ Langerhans cells (antigen-forming) LAYERS OF THE SKIN ○ Merkel cells (Tactile epithelial cells) Stratum Basale/ stratum germinativum Stratum spinosum/ spinous layer/ prickle cell layer Stratum granulosum/ granular layer THICK AND THIN SKIN Stratum lucidum/ clear layer THICK SKIN Stratum corneum layer/ horny layer Has all five epithelial strata Stratum corneum has many layers of cells 4 PRINCIPAL TYPES OF CELLS IN EPIDERMIS Found in areas subject to friction or pressure 1. KERATINOCYTES Found in hairless areas (palm of the hands, soles of 90% of the epidermal cells the feet, fingertips) Arranged in 4 to five layers THIN SKIN Produce keratin In most regions of the body the epidermis has 4— Produced by Keratinocytes stratum basale, stratum spinosum, stratum KERATIN - tough, fibrous protein that helps granulosum, and a thin stratum corneum. protect the skin and underlying tissues from Covers the rest of the body abrasions, heat, microbes, and chemicals More flexible than thick skin LAMELLAR GRANULES - release water-repellent Each stratum contains fewer layer of cells sealant that decreases water entry and loss and Hair is only found in thin skin inhibits the entry of foreign materials 2. MELANOCYTES 8% of epidermal cells Produce the pigment melanin Their long, slender projections extend between the keratinocytes and transfer melanin granules to them MELANIN - yellow-red or brown-black pigment that contributes to skin color and absorbs damaging ultraviolet (UV) light ○ Once inside keratinocytes, the melanin granules cluster to form a protective veil over the nucleus, on the side toward the skin surface. - HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 1  HISTOLOGY OF THE SKIN TRANS #3 What is the reason for this action? This action shield the nuclear DNA from damage of UV Light Are melanocytes free from damage of the UV light? Although their melanin granules effectively protect keratinocytes, melanocytes themselves are particularly susceptible to damage by UV light. 3. INTRAEPIDERMAL MACROPHAGES/ LANGERHANS CELLS From red bone marrow Migrated to the epidermis Immune responses mounted against microbes that invade the skin ○ Help other cells of the immune system recognize an invading microbe and destroy it. Figure 3.4 Photomicrograph showing layers of thick skin 4. MERKEL CELLS/ TACTILE EPITHELIAL CELLS the least numerous of the epidermal cells located in the deepest layer of the epidermis, where they contact the flattened process of a sensory neuron (nerve cell), a structure called a tactile disc or Merkel disc Tactile epithelial cells and their associated tactile discs detect touch sensations. Figure 3.5 Stratum granulosum & st. lucidum: thick skin (left) stratum spinosum and stratum basale (right) Figure 3.6 Epidermal Strata of Thick Skin Figure 3.2 Types of Cells in the Epidermis EPIDERMAL STRUCTURES (LAYERS) MICROGRAPHS OF THE LAYERS Table 3.1 Features of Layers of the Epidermis Layer # of layers Characteristic Function Distinct Cells & Fibers Process Stratum 20-30 layers anucleated/ water cells lose Corneum/ (thickest) dead/ flattened barrier in their nuclei Horny Layer squamous epidermis and Cornified keratinocytes cytoplasm Layer protects against worn away at friction and thick skin microbial invasion flakes off at thin skin Stratum 2-3 thin flattened limited to proteins are Lucidum/ translucent eosinophilic thick skin dispersed Clear Layer layer keratinocytes through held by tonofibril Figure 3.3 Layers of epidermis in thin skin desmosomes bundles hence clearness HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 2  HISTOLOGY OF THE SKIN TRANS #3 Stratum 2-5 layers of Lamellar produce the free In the stratum granulosum: Granulosum flattened, granules keratohyalin ribosomes Keratohyalin granules are distinct features Granular Layer diamond- granules within the shaped keratinocytes Granules contain the two major intermediate begin to filament—associated proteins → filaggrin and synthesize trichohyalin keratohyalin granules that Appearance of the granules + expression of become the filaggrin in the keratinocytes → clinical marker distinctive for the initiation of the final stage of apoptosis feature of the cells Keratinization Filaggrin and trichohyalin function as Stratum 8-10 layers Keratinocyte, produce keratinocytes promoters in the aggregation of keratin Spinosum Langerhans keratin in this layer filaments into tonofibrils to initiate the Spinous Layer Cells and fibers are larger conversion of granular cells into cornified Prickle Cell Melanocytes than those of Layer the stratum cells Occurs in 2 to 6 hours, the time it takes for the cells to leave the stratum granulosum and Stratum single layer Basal cells and produce represented enter the stratum corneum Basale Merkel cells cells of the by a single Germativum Melanocytes most layer of cells In the stratum corneum: Germinativum superficial that Cells are exfoliated or desquamated from surface strata rests on the → involves degradation of cells’ desmosomes basal lamina contain stem 2. MELANOCYTES cells from which new Found among the basal keratinocytes of skin and hair cells follicles Synthesize brown-black pigments called eumelanin and yellow-red pigments called pheomelanin CELLS OF EPIDERMIS 4x more numerous and active in thin skin Pale-staining, rounded cell bodies attached by 1. KERATINOCYTES hemidesmosomes to the basal lamina Long irregular cytoplasmic extensions from each Predominant cell type of epidermis melanocyte cell body course among the cells of the Originate in the stratum basale: basal and spinous layers Contain numerous free ribosomes, scattered intermediate (keratin) filaments, a small Golgi apparatus, mitochondria, and rough endoplasmic reticulum (rER) Cytoplasm of immature keratinocytes appears basophilic (purple stain) in histologic sections because of the large number of free ribosomes In the stratum spinosum: Synthesis of keratin filaments continues → grouped into bundles called tonofibrils Cytoplasm appears eosinophilic (pink stain) Upper part of s. Spinosum: free ribosomes within the keratinocytes begin to synthesize keratohyalin granules Fig 3.8 Melanocytes (M) typically appear as rounded, pale-staining cells just above the dermis (D) Also produce membrane-bound lamellar granules (membrane-coating granules) Fig 3.9 TEM shows the large nucleus of a melanocyte (M) on Fig 3.7 (a) A section of the epidermal spinous layer of thick the basal lamina (BL), large Golgi complexes (G), dark melanin- skin showing keratinocytes with numerous short cytoplasmic containing granules (MG), cytoplasmic extensions (CE), and projections (arrow). (×400; PT) keratinocytes (K) (b) TEM of a single spinous keratinocyte with surface projections (arrows). (×8400) HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 3  HISTOLOGY OF THE SKIN TRANS #3 3. LANGERHANS CELLS Dendritic cells that appear most clearly in the spinous layer Comprise a major component of the skin’s adaptive immunity Cytoplasmic processes extend between keratinocytes of all layers Fig 3.12 Schematic diagram of epidermal cell differentiation and replacement Table 3.2 Keratinization process in each layers Fig 3.10 Immunostained skin shows Langerhans cells (yellow) abundant in hair follicles (F) and throughout the superficial layers of epidermis (E) Layer Keratinization process 4. MERKEL CELLS Stratum Dead cells have a hard protein envelope, contain Epithelial tactile cells, low-threshold mechanoreceptors corneum keratin, and are surrounded by lipids. in basal epidermal layer Essential for sensing gentle touch Stratum Cells are dead and contain dispersed keratohyalin. Higher cell percentage in thick skin (e.g. fingertips) lucidum Resemble the surrounding keratinocytes by light microscopy but contain few if any melanosomes Stratum Keratin granules accumulate and a hard protein granulosu envelope forms beneath the plasma membrane; m lamellar bodies release lipids; cells die. Stratum Keratin fibers and lamellar bodies accumulate. spinosum Stratum Cells divide by mitosis and some of the newly basale formed cells become the cells of the more superficial strata. MELANIN FORMATION Table 3.3 Melanosome formation Stage I Tyrosinase is synthesized in the rough ER, processed through the Golgi apparatus, and accumulates in vesicles Fig 3.11 TEM: mass of Golgi-derived dense-core cytoplasmic Stage II Melanin synthesis begins in the ovoid granules (G) in direct synaptic contact with the expanded, disc- like ending of an unmyelinated sensory nerve fiber (N) Stage III Matrix has been organized into parallel filaments on INSET: Granules are similar in morphology and content to granules of other neuroendocrine cells which polymerized melanin is deposited and accumulates DISTINGUISHING MELANOCYTES FROM MERKEL AND Stage IV Mature melanin granule has lost enzymatic activities LANGERHANS CELLS and has the internal matrix filled with melanin. Melanocytes are difficult to distinguish from Merkel cells by routine microscopy. Langerhans cells are also rounded, poorly stained cells but Melanin are typically located more superficially than melanocytes, in the spinous layer. Group of pigments responsible for skin, hair, and eye color Protection against ultraviolet light from the sun KERATINIZATION Amount of melanin differs in different location Cells move from the deeper epidermal layers to the Production is determined by genetic factors, exposure surface, the cells change shape and chemical to light, and hormones composition Cells become filled with keratin HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 4  HISTOLOGY OF THE SKIN TRANS #3 Clinical Correlation ○ 30-75 µm by 50-150 µm Albinism - a recessive genetic trait causing inability to ○ Consist of sensory axons winding among flattened produce tyrosinase resulting in deficiency or absence of Schwann cells pigment in the skin, hair, and eyes. ○ Detect: light touch low frequency stimuli ○ Numerous in fingertips, palms and soles but decline slowly in number during aging after puberty Fig 3.13 Melanosome formation Fig 3.14 Meissner and lamellated (pacinian) corpuscles.(a) Meissner's tactile corpuscles (TC) detect light touch, located in dermal papillae. (DP) partially surrounded by epidermis (E) elliptical, 150 µm long. (b) SENSORY RECEPTORS Lamellated (pacinian) corpuscles (PC) detect coarse touch or pressure, Surface receptor large oval structures, 1mm in length found among adipose tissue Detect gentle pressure, temperature and vibrations Lamellated (pacinian) corpuscles Dendrites- detect pain ○ Found in Deep dermal receptor deep reticular dermis Naked dendrites that wind around the bases of hair hypodermis follicles ○ Specialized for sensing Detect motion of the hairs course touch Pacinian corpuscles- respond to strong pressure pressure (sustained touch) vibrations and vibrations ○ Large oval structures, 0.5 mm by 1mm ○ With an outer connective tissue capsule that UNENCAPSULATED RECEPTORS amplifies a mechanical stimulus to the axonal core Merkel cells when distorted ○ Associated with expanded nerve endings ○ 15-50 thin, concentric lamellae of flattened Schwann ○ Tonic receptor for cells and collagen surrounding highly branched sustained light touch unmyelinated axon sensing an object’s texture ○ Found in connective tissues located deep in the Free nerve endings body where they produce sensation of pressure ○ Tactile receptors located in: when surrounding tissue is distorted Papillary dermis, to wall of the rectum Lower epidermal layers urinary bladder ○ Respond to: Krause end bulbs high low temperatures ○ Simpler encapsulated, ovoid structures pain and itching ○ Has extremely thin collagenous capsules penetrated fine touch by sensory fiber ○ Tactile receptors ○ Found in skin of Root hair plexuses penis ○ Location: surrounds base of hair follicles clitoris ○ Web of sensory fibers ○ Sense low frequency vibrations ○ Detects movements of the hairs Ruffini corpuscles ○ Have collagenous fusiform capsules anchored firmly ENCAPSULATED RECEPTORS to the surrounding connective tissues ○ Sensory axons stimulated by ○ phasic mechanoreceptors that respond rapidly to stretch (tension) stimuli on the skin. twisting (torque) Meissner corpuscles ○ Frequently located in dermal papillae, partially surrounded by epidermis ○ Elliptical structures HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 5  HISTOLOGY OF THE SKIN TRANS #3 Fig 3.15 Receptors DERMIS STRUCTURE Layer of Connective Tissue that supports the epidermis and binds it to subcutaneous tissue. Dermal papillae ○ Projects into the undersurface of the epidermis ○ Finger-like connective tissue protrusions. Dermal ridges ○ Tend to have parallel arrangement, with dermal papillae located between them. ○ Forms a distinctive pattern that is genetically Fig 3.17 Stained slides show two dermal layers– the papillary unique to each individual and is reflected in the layer and the reticular layer appearance of epidermal grooves and ridges on the surface of the skin. ○ These patterns are the basis of science of PAPILLARY LAYER dermatoglyphics or fingerprint and footprint Thin and superficial layer beneath the epidermis identification. Consist of Loose Connective Tissue Hemidesmosomes The collagen fibers located in this part are not thick as ○ Strengthens the attachment of epidermis to those in the deeper portion. underlying connective tissue. ○ Predominant collagen network type: Type I and The Dermis is composed of 2 Layers: Type III PAPILLARY LAYER ○ Type IV collagen - insert into basal lamina, RETICULAR LAYER helping bind dermis to epidermis Also consists of: ○ Fibroblast, scattered mast cells, dendritic cells, and leukocytes. Contains blood vessels - that serve but do not enter epidermis Contains nerve process - that either terminate in the dermis or penetrate the basal lamina to enter the epithelial compartment. RETICULAR LAYER Although its thickness varies in different parts of the body, it is always considerably thicker and less cellular than the papillary layer. Consist of Dense Irregular Connective Tissue Mainly consist of bundles of type I collagen A network of elastic fibers is also present, providing elasticity to the skin. Between collagen and elastic fibers are abundant proteoglycan rich dermatan sulfate. Tension lines Fig 3.16 (H&E)–stained specimen from human skin shows the ○ The collagen and elastic fibers are not randomly two chief layers of the skin—the epidermis (Epi) and dermis oriented but form regular lines of tension in the (Derm). skin called Langer’s lines. ○ Skin incisions made parallel to Langer’s lines heal with the least scarring. HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 6 HISTOLOGY OF THE SKIN TRANS #3 Sensory afferent nerve fibers ○ Form a network in the papillary dermis and around hair follicles, ending at the epithelial and dermal receptors. Autonomic effector nerves ○ To dermal sweat glands and smooth muscle fibers in the skin of some areas are postganglionic fibers of sympathetic ganglia. No parasympathetic innervation is present HYPODERMIS also termed ○ superficial fascia ○ subcutaneous layer ○ subcutis ○ panniculus no specific layers not considered part of the integument deep to dermis composed of: Fig 3.18 Slide Stained for elastic fibers shows the extensive ○ areolar (loose) connective tissue distribution of these darkly stained fibers (RED ARROWS) binds the skin loosely to subjacent organs, making it possible to slide over among the eosinophilic collagen bundles. them ○ adipose connective tissue BLOOD AND LYMPHATIC VESSELS panniculus adiposus Both dermal regions contain a rich network of blood adipocytes vary in number and size and lymphatic vessels. according to body region and nutritional The nutritive vessels form two major plexuses: state ○ Between the papillary and reticular dermal layers major energy storage site and provides lies the microvascular subpapillary plexus, from insulation which capillary branches extend into the dermal ○ extensive vascular supply papillae and form a rich, nutritive capillary network promotes rapid uptake of drugs injected just below the epidermis. into this tissue (ex: insulin) ○ A deep plexus with larger blood and lymphatic Arrector pili muscles vessels lies near the interface of the dermis and Formed by the Individual/bundles of smooth muscle cells that originate in the hypodermis the subcutaneous layer. Connect the deep part of hair follicles to the more Arteriovenous anastomoses or shunts superficial dermis ○ Located between the two major plexuses Contraction causes goosebumps or goose flesh ○ For thermoregulatory function Cold conditions: The shunts decrease (erection of hairs and puckering of skin) blood flow in the papillary layer to Pacinian corpuscles minimize heat loss Can be seen in the lower part of hypodermis Warm Conditions: Increase flow to Large, slightly oval structures with a layered or facilitate heat loss in warm environments. lamellated pattern PANNICULUS CARNOSUS A thin layer of striated muscle Deep to the hypodermis Well defined in the skin of the face, neck and scalp Constitutes the platysma muscle muscles of facial expression Fig 3.19 Blood vessels in the dermis. NERVE SUPPLIES The dermis also has a rich nerve supply. HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 7  HISTOLOGY OF THE SKIN TRANS #3 ○ Isthmus extends from the infundibulum to the level of insertion of the arrector pili muscle. ○ Follicular bulge protrudes from the hair follicle near the insertion of the arrector pili muscle and contains epidermal stem cells. ○ Inferior segment in the growing follicle is of nearly uniform diameter except at its base, where it expands to form the bulb. The base of the bulb is invaginated by a tuft of vascularized loose connective tissue - dermal papilla. ○ Other cells forming the bulb, adjacent to dermal papilla, are collectively referred to as the hair matrix, which consists simply of matrix cells (rapidly dividing and differentiating, responsible for hair growth). INTERNAL ROOT SHEATH ○ Multilayered cellular covering that surrounds the deep part of the hair. ○ 3 layers of internal root sheath: Henle’s layer consists of an outer single layer of cuboidal cells. These cells are in direct contact with the outermost part of the hair follicle, which represents a Fig 3.20 Thick skin from a fingertip, showing the hypodermis downgrowth of the epidermis and is (Hy) containing sweat glands (SG) in the upper dermis, and designated the external root sheath. pacinian corpuscles (PC) and nerve bundles (N) in the lower Huxley’s layer consists of a single or dermis. double layer of flattened cells that form the middle plate of the internal root sheath. TIPS IN DIFFERENTIATING HYPODERMIS TO DERMIS Internal root sheath cuticle consists of Look at the dermal papillae. squamous cells whose outer free surface Pag hindi well defined yung ridges, hypodermis is the lighter portion and dermis is the slightly darker portion (kasi lower faces the hair shaft. magnification). Take into account din which body part. In the 3 layers of the hair: picture above, fingertip skin sya so makikita mo agad yung Medulla forms the central part of the shaft and hypodermis. contains a column of large, loosely connected Pag well defined yung ridges tapos kita mo 2 layers, you're probably looking at the dermis where papillary layer is the keratinized cells containing soft keratin. The medulla concentrated portion w/ thinner fibers (loose CT) and reticular is present only in thick hairs. layer is the one w/ thicker fibers and may spaces sya (dense Cortex is the largest layer. It is located outside the irregular CT) Take note anong tissue composition per layer: medulla and is composed of cortical cells filled with ○ Papillary - AREOLAR( LOOSE) CT hard keratin intermediate filaments. Each filament is ground substance, fibers sparse, thin surrounded by an amorphous space containing ○ Reticular - DENSE IRREGULAR CT fibers thicker, many, may spaces keratin-associated proteins (KAPs) - responsible ○ Hypodermis - ADIPOSE CT for forming the rigid hair shaft by extensive cross- fat cells!! dami white white linking of keratin intermediate filaments by disulfide bonds. The cortex determines texture, elasticity, and the color of hair. SKIN APPENDAGES Cuticle is the outermost layer of the hair. It contains Hair, Nail, Glands several layers of overlapping, semi transparent keratinized squamous cells. These cells resemble HAIR fish scales or roof tiles with their free edges lying away from the hair follicle. The cuticle Present over almost the entire body EXCEPT palms and protects the hair from physical and chemical soles, lips, and region around the urogenital orifices. damage and determines its porosity. Hair distribution is influenced by sex hormones. KERATINIZATION ○ Occurs shortly after the cells leave the matrix in a HAIR FOLLICLE region called the keratogenous zone in the lower Responsible for the production and growth of hair. third of the follicle. Represents an invagination of the epidermis into the ○ As the cortical cells pass through the keratogenous dermis a.k.a external root sheath. zone, they differentiate, extrude their organelles, Contains specially organized keratin built into long and become tightly packed with cross-linked keratin tubular structures. intermediate filaments. Hair follicles have generous blood and nerve supplies. ○ By the time the hair emerges from the follicle, it is 4 regions of hair follicle: entirely keratinized as hard keratin. ○ Infundibulum extends from the surface opening of ○ The internal root sheath, consisting of soft keratin, the follicle to the level of the opening of its does not emerge from the follicle with the hair but is sebaceous gland. It is part of the pilosebaceous broken down at about the isthmus level where canal, the route for sebum discharge. sebaceous secretions enter the follicle. HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 8 HISTOLOGY OF THE SKIN TRANS #3 ○ A thick basal lamina, called the glassy membrane, NAILS separates the hair follicle from the dermis. Plates of keratinized cells containing hard keratin. Surrounding the follicle is a dense irregular Nail plate - slightly arched fingernails and toenails, rests connective tissue. on nail beds. Nail beds - consist of epithelial cells that are continuous with the stratum basale and stratum spinosum of the epidermis. Nail root - buried in a fold of epidermis and covers the cells of the germinative zone or matrix. Matrix contains stem cells, epithelial cells, melanocytes, Merkel’s cells, and Langerhans’ cells. The stem cells of the matrix regularly divide, migrate toward the root of the nail, and there differentiate and produce the keratin of the nail. The constant addition of new cells at the root and their keratinization account for nail growth. As the nail plate grows, it moves over the nail bed. Lunula - the crescent-shaped white area near the root of the nail. It derives its color from the thick, opaque layer of partially keratinized matrix cells in this region. Eponychium or cuticle - edge of the skin fold covering the root of the nail. The cuticle is also composed of hard keratin; therefore, it does not desquamate. Figure 3.21 Photomicrograph of H&E–stained section of thin skin from human scalp. HAIR GROWTH CYCLE ○ Growth stage (Anagen) Follicle regeneration with active hair growth; Hair is formed by matrix cells that differentiate, become keratinized, and die; Hair grows longer as cells are added at the base of the hair root. ○ Catagen Follicle rests in the dermal papilla and stops production of new cells, melanin production ceases, and the hairshaft is ejected. ○ Resting stage (Telogen) Figure 3.23 Photomicrograph of a sagittal section of distal Lasts for approximately 4 months phalanx with a nail. Follicle remains quiescent in its shortened state and awakens to regenerate in new GLANDS Anagen phase Sebaceous glands ○ Exogen/New Anagen stage Eccrine glands Hair shedding Apocrine glands Melanin is produced by melanocytes within the hair bulb Mammary gland matrix Aging → white hair SEBACEOUS GLANDS Structure: Pear-shaped, alveolar glands. Function: ○ Secretes sebum, an oily substance. ○ Moisturizes and waterproofs hair. Location: ○ Typically attached to hair follicles. ○ Positioned near the arrector pili muscle, aiding in hair standing upright. Unique Characteristics: Surrounded by an extensive capillary plexus. Figure 3.22 Hair Growth Cycle HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 9  HISTOLOGY OF THE SKIN TRANS #3 Fig 3.24 Sebaceous Gland under microscope Fig 3.26 Apocrine Gland under microscope ECCRINE GLANDS Structure: Long tubules extending from the epidermis MAMMARY GLANDS into the dermis/hypodermis. Structure: Alveoli containing two layers of cells. ○ Secretory portion appears as tightly wound, cross- ○ Inner cuboidal epithelium and sectioned tubules. ○ outer myoepithelial cells. Function: Function: ○ Produces sweat, which helps regulate body ○ Produces milk through tubuloalveolar glands. temperature. Location: Alveoli connect via lactiferous ducts to the Location: Found throughout most of the skin. base of the nipple. Additional Features: ○ Encased by myoepithelial cells that contain actin filaments. ○ Nerve input causes contraction of myoepithelial cells, expelling sweat. Fig 3.27 Mammary Gland under microscope SKIN HOMEOSTASIS The cell production and cell death must be carefully balanced to maintain homeostasis. As new cells are produced, some must be removed, and vice versa. CELL DEATH Fig 3.25 Eccrine Gland under microscope May occur as a result of acute cell injury— necrosis or; Internally coded suicide program— apoptosis APOCRINE GLANDS Necrosis Structure: Larger than eccrine glands with straight, ○ Accidental cell death, a pathologic process narrow ducts. ○ Occurs when cells are exposed to unfavorable Function: physical or chemical environment (hypothermia, ○ Produces a thicker secretion, often linked to body hypoxia radiation, low pH, trauma) odor. ○ Triggers the inflammatory response Location: Runs parallel to hair follicles, frequently ○ Two Characteristic Features: opening into the pilosebaceous canal. Cell Swelling Lysis Apoptosis ○ A mode of cell death that occurs under normal physiologic conditions HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 10  HISTOLOGY OF THE SKIN TRANS #3 ○ A rapid cellular activity that shrinks and eliminates defective and unneeded cells PHASES OF DEEP WOUND HEALING ○ “Cellular Suicide” Inflammatory phase ○ Unlike necrosis, apoptotic cells do not rupture and ○ Blood clot forms in the wound and loosely unites does not trigger inflammatory response the wound edges ○ Involves inflammation, a vascular and cellular response that helps eliminate microbes, foreign material, and dying tissue in preparation for repair. ○ The vasodilation and increased permeability of blood vessels associated with inflammation enhance delivery of helpful cells. These include Neutrophils– phagocytic white blood cells Monocytes– which develop into macrophages that phagocytize microbes; Mesenchymal cells– which develop into fibroblasts. Migratory phase Fig 3.28 Necrosis and Apoptosis ○ The clot becomes a scab, and epithelial cells migrate beneath the scab to bridge the wound. WOUND HEALING ○ Fibroblasts migrate along fibrin threads and begin The skin has a good capacity for repair which is important synthesizing scar tissue (collagen fibers and as it is an exposed and easily damaged organ glycoproteins) Stages of Cutaneous Wound Healing ○ Damaged blood vessels begin to regrow. 1. The process begins with blood quickly clotting at ○ During this phase, the tissue filling the wound is the wound site, releasing platelet-derived growth called granulation tissue. factors and other substances; inflammation Proliferative phase typically lasts 2-3 days ○ Characterized by extensive growth of epithelial Before this phase is completed, cells beneath the scab epithelialization begins as cells of the ○ Deposition by fibroblasts of collagen fibers in epidermal basal layer remove their random patterns desmosomes and hemidesmosomes and ○ Continued growth of blood vessels. migrate laterally beneath the blood clot, Maturation phase which becomes an increasingly ○ The scab sloughs off once the epidermis has been dessicated eschar or scab. restored to normal thickness. 2. Macrophages and neutrophils enter the wound as ○ Collagen fibers become more organized inflammation begins, and epithelial cells from cut edges of the stratum basale begin to migrate ○ Fibroblasts decrease in number beneath and through blood clot ○ Blood vessels are restored to normal 3. Under the influence of growth factors and hydrolytic enzymes released in part from macrophages, FIBROSIS fibroblasts proliferate and produce much new Process of scar tissue formation collagen to form “granulation tissue” containing Hypertrophic scar– raised scar that remains within the many new growing capillaries boundaries of the original wound 4. The epidermis gradually reestablish continuity over Keloid/Cheloid scar– raised scar that extends within the the wound site, but excessive collagen usually boundaries of the original wound remains in the dermis as scar tissue Scar tissue is lighter in color than normal skin because: ○ Collagen fibers are more densely arranged ○ Has decreased elasticity ○ Fewer blood vessels ○ Skin appendages and sensory reseptors are less REFERENCES Ross, M. H., & Pawlina, W. (2010). Histology: A text and atlas: With correlated cell and molecular biology (9th ed.). Philadelphia: Lip pincott Williams & Wilkins. Mescher, A.L. (2021) Junqueira’s Basic Histology Text & Atlas. 16th Edition, McGraw Hill. Department of Anatomy and Neurobiology and the Office of Faculty Affair (2024) Digital Histology. Virginia Commonwealth University School of Medicine and ALT Lab - VCU. https://digitalhistology.org Young, B. et. al. (2014) Wheater’s Functional Histology: A Text and Color Atlas. 6th edition. Elsevier. Fig 3.29 Stages of Cutaneous Wound Healing HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 11  HISTOLOGY OF THE SKIN TRANS #3 APPENDIX Fill in the blanks in the following statements. (a) 1, 2, and 3 1. The epidermal layer that is found in thick skin but not in thin skin (b) 1, 3, and 4 is the ________________. (c) 1, 2, 3, and 4 2. The most common sweat glands that release a watery secretion (d) 2, 3, and 4 are _____ sweat glands; modified sweat glands in the ear are (e) 1, 3, and 5 _____ glands; sweat glands located in the axillae, groin, areolae, and beards of males and that release a slightly viscous, lipid-rich Ans: 5-12) C, E, A, C, B, E, A, C, 13) 3, 5, 4, 1, 6, 11, 2, 8, 9, 10, 7 secretion are _____ sweat glands. Indicate whether the following statements are true or false. 13. Match the following: 3. An individual with a dark skin color has more melanocytes than a (1) Merkel cells fair-skinned person. (2) callus 4. In order to permanently prevent growth of an unwanted hair, you (3) keratinocytes must destroy the hair matrix. (4) Langerhans cells (5) melanocytes Ans: 1) st. lucidum 2) eccrine, ceruminous, apocrine 3) false 4) true (6) free nerve endings (7) sebaceous glands Choose the one best answer to the following questions. (8) lamellar granules 5. The layer of the epidermis that contains stem cells undergoing (9) pacinian (lamellated) corpuscles mitosis is the (10) vernix caseosa (a) stratum corneum. (11) arrector pili (b) stratum lucidum ___ (a) produce the protein that helps protect the skin and (c) stratum basale. underlying tissues from light, heat, microbes, and many chemicals (d) stratum spinosum. ___ (b) produce a pigment that contributes to skin color and (e) stratum granulosum. absorbs ultraviolet light 6. The substance that helps promote mitosis in epidermal skin cells i ___ (c) cells that arise from red bone marrow, migrate to the (a) keratohyalin. epidermis, and participate in immune responses (b) melanin. ___ (d) cells thought to function in the sensation of touch (c) carotene. ___ (e) located in the dermis, they function in the sensations of (d) collagen. warmth, coolness, pain, itching, and tickling (e) epidermal growth factor. ___ (f) smooth muscles associated with the hair follicles; when 7. Which of the following is not a function of skin? contracted, they pull the hair shafts perpendicular to the skin's (a) calcium production surface (b) vitamin D synthesis ___ (g) an abnormal thickening of the epidermis (c) protection ___ (h) release a lipid-rich secretion that functions as a water- (d) excretion of wastes repellent sealant in the stratum granulosum (e) temperature regulation ___ (i) pressure-sensitive cells found mostly in the subcutaneous 8. To expose underlying tissues in the bottom of the foot, a foot layer surgeon must first cut through the skin. Place the following layers in ___ (j) a fatty substance that covers and protects the skin of the the order that the scalpel would cut. (1) stratum lucidum, (2) stratum fetus from the constant exposure to amniotic fluid corneum, (3) stratum basale, (4) stratum granulosum, ___ (k) associated with hair follicles, these secrete an oily (5) stratum spinosum. substance that helps prevent hair from becoming brittle, prevents (a) 3, 5, 4, 1, 2 evaporation of water from the skin's surface, and inhibits the growth (b) 2, 1, 5, 4, 3 of certain bacteria (c) 2, 1, 4, 5, 3 (d) 1, 3, 5, 4, 2 14. Match the following: (e) 3, 4, 5, 1, 2 (1) subcutaneous layer (hypodermis) 9. Aging of the skin can result in (2) papillary region (a) an increase in collagen and elastic fibers. (3) reticular region (b) a decrease in the activity of sebaceous glands. (4) epidermis (c) a thickening of the skin. ___ (a) deep region of the dermis composed primarily of dense (d) an increased blood flow to the skin. irregular connective tissue (e) an increase in toenail growth. ___ (b) composed of keratinized stratified squamous epithelial 10. Which of the following is not true? tissue (a) Albinism is an inherited inability of melanocytes to produce melanin. ___ (c) not considered part of the skin, it contains areolar and (b) Striae occurs when the dermis is overstretched to the point of tearing. adipose tissues and blood vessels; attaches skin to underlying (c) In order to prevent excessive scarring, surgeons should cut parallel to tissues and organs the lines of cleavage. ___ (d) superficial region of the dermis; composed of areolar (d) The papillary layer of the dermis is responsible for fingerprints. connective tissue (e) Much of the body's fat is located in the dermis of the skin. 11. A patient is brought into the emergency room suffering from a Ans: 14) 3, 4, 1, 2, 15) 4, 3, 2, 1 burn. The patient does not feel any pain at the burn site. Using a 15. Match the following and place the phases of deep wound gentle pull on a hair, the examining physician can remove entire hair healing in the correct order: follicles from the patient's arm. This patient is suffering from what (1) proliferative phase type of burn? (2) inflammatory phase (a) third degree (3) maturation phase (b) second degree (4) migratory phase (c) first degree ___ (a) epithelial cells migrate under scab to bridge the wound; (d) partial-thickness formation of granulation tissue (e) localized ___ (b) sloughing of scab; reorganization of collagen fibers; blood 12. Which of the following statements are true? vessels return to normal (1) Nails are composed of tightly packed, hard, keratinized cells of the epidermis that form a clear, solid covering over the dorsal surface of the ___ (c) vasodilation and increased permeability of blood vessels to terminal end of digits. deliver cells involved in phagocytosis; clot formation (2) The free edge of the nail is white due to the absence of capillaries. ___ (d) extensive growth of epithelial cells beneath scab; random (3) Nails help us grasp and manipulate small objects. deposition of collagen fibers; continued growth of blood vessels (4) Nails protect the ends of digits from trauma. (5) Nail color is due to a combination of melanin and carotene. HG201A TRANSCRIBERS: Plete, Cait, Escote, Gamba, Manila, Padasas, Rodriguez M, Sarmiento, Taha, Toledo, Villas 12