Histology Exam One

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132 terms · Descriptions of cells for UWSVM 1st Year Veterinary Histology course up to exam One

Neutrophils

Key Features: Granules are present but may be hard to identify (do not stain well). Dark-staining, lobed nucleus. (Segmented Neutrophil)
Key Function: Antibacterial Activities

Band Neutrophil

Young, immature neutrophil. Non-segmented nucleus without lobes.

Eosinophil

Key Features: Specific granules pack cytoplasm and stain very darkly eosinophilic. Lobed nucleus. Similar cell cize to Neutrophil.
Key Function: Inflammatory reactions (parasitic worms/allergy)

Basophil

RARE. Key features: Granules stain so darkly basophilic you cannot see the lobed nucleus underneath. Similar cell size as neutrophil.
Key Function: Controversial?

Lymphocytes

Key Features: Do not have specific granules. Fairly round, Heterochromatic nuclei. Little cytoplasm.
Key Function: Organization/Control of Immune response Antibody Production.

Monocytes

Key Features: Deviate from round nuclei (kidney-shaped). Have more cytoplasm than lymphocytes. Often have clear vacuoles in cytoplasm.
Key Function: Become macrophage cells (Phagocytes, Osteoclasts, Astrocytes)

Erythrocytes

Red Blood Cells. Key Features: Biconcave disk shape. Hemoglobin binds esoin, eosinophilic stained cells.
Key Function: Storing and transporting Oxygen.

Platelets

Key Feature: Much smaller than erythrocytes, no nuclei. Amorphous in 2D slides. Contain serotonin in dense-core granules required for function.
Key Function: Blood clotting.

Thrombocytes

Key Features: Intact nucleated cells with similar cytoplasmic contents as mammalian platelets.
Key Function: Functional equivalent to mammalian platelets.

Epithelia (5 general characteristics)

-Form sheets/tubes
-Little intercellular space between cells
-Attached to subjacent connective tissue by basement membrane
-Cover body surfaces and separate internal tissue from cavity lumen
-Do not contain blood vessels.

Epithelia (4 FUNCTIONS)

Protection, Absorption, Secretion, Sensory Perception.

Basement Membrane

Composed of:
Basil Lamina- immediately subjacent to epithelial cells. Made up of type IV collagen. Not visible on light microscope. FUNCTION: establish polarity of epithelial cells, Macromolecule exchange
Fibrous Lamina- 2nd layer, underneath basil lamina. Fibrous layer secreted by fibroblasts. Visible with light microscope. FUNCTION: anchor basal lamina to underlying conn. tissue.

Basil Lamina

immediately subjacent to epithelial cells. Made up of type IV collagen. Not visible on light microscope. FUNCTION: establish polarity of epithelial cells, Macromolecule exchange

Fibrous Lamina

2nd layer of basement membrane, underneath basil lamina. Fibrous layer secreted by fibroblasts. Visible with light microscope. FUNCTION: anchor basal lamina to underlying conn. tissue.

Endothelium

Specialized simple squamous epithelium that lines heart, blood and lymphatic vessels.

Mesothelium

Specialized simple squamous that covers internal organs, lines major serous cavities of body.

Simple Squamous Epithelium

Key Features: single layer of flattened cells, centrally located nucleus. Fried egg in 3D.

Simple Cuboidal Epithelium

Shaped like cubes, roughly as wide as they are tall.
LOCATIONS: sweat glands and some kidney tubules.

Simple Columnar Epithelium

Sheet of Columnar Cells taller than they are wide. KEY FEATURE: All nuclei line up in a row at same level in BASAL aspect of cell.
LOCATION: Small intestine.

Stratified Squamous

Most superficial cells are typical flattened squamos morphology while more basal cells in the stack may be rounded and cuboidal. Naming layers of stratified epithelium always goes by morphology of superficial layer.

Keratinization

Wet vs Dry.
Wet- Most superficial cells, though still squamous in morphology, retain their nuclei and do not die.
Dry- Most superficial cells die and lose nuclei, forming layers of SQUAMES.

Squames

Found in dry-keratinized epithelium. Non-living scale-like cells essential only contain keratin. Dog footpad slide.

Transitional Epithelium

Characterized by balloon cells. Most superficial cells are large, balloon-shaped cells. Thickness of transitional epithelium varies from many cell layers to just a few. LOCATIONS: urinary bladder, ureter, parts of urethra.

Pseudostratified columnar epithelium

All cells are in contact with basal membrane but do NOT ALL EXTEND to lumenal edge of the epithelium. Appears stratified because nuclei do not all line up at the same level. LOCATIONS: trachea, upper resp system, ductus deferens.

Goblet Cells

Interspersed among simple columnar and pseudostratified epithelial cells. Thinner at base and bulbous at apex. Appears to be empty on slide. Mucus-containing secretory cells.

Connective Tissue Functions

-Shape/Form
-Support/Protection: bones, fibrous CT
-Connections/Anchors: tendons, ligaments. Sheaths, joint capsules.
-Storage: lipids energy storage. H20/electrolytes in ground substance.
-Transport: nutrients from blood to tissues/organs, removal of waste products
-Defense: Immunologic lymphocytes, phagocytic macrophages
-Regeneration/Wound Healing: Macrophage removal of damaged tissue

Loose fibrous connective tissue

Made up of cells and collagen fiber bundles with much white space (ground substance) in between wispy fibers.

Dense fibrous connective tissue

Less ground substance present. Cells interspersed amongst variably sized collagen bundles.
Regular CT: collagen fiber bundles run parallel to one another. EXAMPLE- tendons.
Irregular CT: Not as regularly arranged, fibers may run in various directions.

Procollagen

Precursors to collagen. modified by ER and Golgi to form tropocollagen.

Reticular Fibers

branching aggregates of type III collagen fibrils. Synthesized by fibroblasts and reticular cells. LOCATION: lymph nodes, spleen, tonsils

Elastic Fibers

constructed of ELASTIN, sometimes in association with FIBRILLIN. Interacts with Collagen II for added strength. LOCATION: large elastic arteries and ligaments.

Ground Substance

Semifluid gel in between cells/fibers in fibrous CT. Contains glycoproteins and glycosaminoglycans. GAGs attach to short protein core and the combination is called a proteoglycan.

Hyaluronic Acid

Unique GAG in ground substance because it uses linker protein to link to same protein core as other GAGs.

Fibroblasts

Cells of fibrous connective tissue. Produce collagen, elastic, and reticular fibers and various glycoproteins in ground substance. KEY FEATURES: oval nuclei with one or more prominent nucleoli. Widely and randomly dispersed.

Reticular Cells

stellate cells, produce reticular fibers (type III collagen). FUNCTION: Support lymphatic tissue and hematopoetic tissue. Must be identified primarily by location. Look like fibroblasts.

Leukocytes (in CT)

Will appear smaller in CT slides than they do in blood smears because they are not flattened.

Macrophages

FUNCTION: remodeling CT after trauma, normal CT turnover, and immune protection. ACTIVE macrophages use PHAGOCYTOSIS or ENDOCYTOSIS.

Syncytium

Two macrophages fused together to form single cell with multiple nuclei. Osteoclasts are syncytiums.

Plasma Cell

KEY CHARACTERISTICS: Yoshi egg nuclei. Have large Golgi (clear zone near nucleus).
FUNCTION: Synthesize and secret large amounts of "antibodies".

Mast Cells

Differentiate from blood-borne stem cells. FUNCTIONS: blood anticoagulation, chemotactic signaling to eosinophils, signaling nerves.
KEY FEATURES: Packed with lg numbers of lg granules containing proteoglycans with GAGs and non-GAG molecules. Special stains allow us to see HEPARIN (sulfated GAG that is a strong anti-coagulant).

Adipocytes

Store large amounts of fat. Almost every organ in body except nervous system has adipose tissue. Two types: unilocular and multilocular

Unilocular Adipocytes

"white fat" Key FEATURES: "Chicken wire". Cell is round, contains one large fat droplet that displaces nucleus and cytoplasm to the side. Size varies greatly. Adipose tissue is highly vascularized. Each adipocyte cradled in network of reticular fibers (type III) synthesized by both adipocytes AND fibroblasts.

Multilocular Adipocytes

"Brown fat". FUNTION: Rapid thermogenic response to cold stress. Largely present in neonates and diminish with age.
KEY CHARACTERISTICS: Generally smaller than unilocular adipocytes. Contains large number of mitochondria rich in cytochromes (brown coloring).

UCP-1

Present in mitochondria of multilocular adipocytes. Uncouples oxidative respiration from production of ATP and diverts energy to heat.

Lipoblast

Differentiates from precursor mesencymal cells. Relatively small with few cytoplasmic lipid droplets. Gives rise to multilocular and unilocular adipocytes.

Cartilage

Resilient, supportive form of connective tissue resides inside of loose CT for support (pinna, trachea, epiglottis) or serves as gliding surface for articulated surface of two bones (joints). Precursor cells to many types of bone. CONTAINS NO blood vessels or nerves.

Chondrocytes

Cells of cartilage. Surrounded by large amounts of amorphous, translucent ground substance. Remain grouped together in ISOGENOUS GROUPS. Located within a space called a LACUNA.

Isogenous group

Groups of divided chondrocytes that stay grouped together in cartilage.

Lacuna

Space where chondrocytes are located. Clear zone in lacuna is likely an artifact of chondrocyte shrinkage within the lacuna.

Hyaline Cartilage

Matrix with TYPE II collagen fibers. LOCATION: Rings of trachea, articular ends of growth plates of bones.

Elastic Cartilage

matrix of both TYPE II collagen and ELASTIC fibers. LOCATION: Pinna of ear, epiglottis of throat.

Fibrocartilage

LOCATION: interface between bone and tendons, or bone and ligaments - require high tensile strength.
CHARACTERISTICS: Thick collagen bundles separated by lines of chondrocytes. Small amt of ground substance. Contains TYPE I collagen.

Perichondrium

Cartilage can be surrounded by and attached to fibrous connective layers. Immediately adjacent layer is the perichondrium.
TWO LAYERS:
Fibrovascular Layer- contains fibroblasts and small blood vessels, source of nutrition for cartilage.
Chondrogenic Layer- contains chondrogenic stem cells (chondroblasts)

Chondroblasts

Chondrogenic stem cells. Relatively more basophilic than chondrocytes and have little to no adjacent matrix.

Appositional Growth

addition of new chondrocytes at the surface of cartilage.

Interstitial Growth

Division of chondrocytes within the cartilage matrix into isogenous groups.

Osteoprogenitor Cells

FUNCTION: Stem cells capable of differentiating into bone tissue. Produce OSTEOBLASTS, fibroblasts, adipocytes, and chondroblasts. CHARACTERISTICS: Oblong nucleus, tapered at ends, lightly basophilic. LOCATION: many places throughout bone.

Osteoblasts

derived from osteoprogentior cells. CHARACTERISTICS: Cuboidal to polygonal in shape, basophilic cytoplasm reflecting large rER content indicating highly active cell. Often a pale-staining Golgi can be seen. FUNCTION: Create bone tissue de novo, trap themselves in matrix the secrete, develop into OSTEOCYTES.

Osteocytes

Osteoblasts that have become trapped in their own matrix and differentiate to osteocytes. FUNCTION: Maintain strength and integrity of bone. CHARACTERISTICS: Reside in lacuna (like cartilage) however nutrients/metabolites must be received through cell-to-cell connections housed in numerous tiny channels called CANILICULI.

Caniliculi

Numerous tiny channels in the bone matrix that house cell to cell connections for oesteocytes and thus allow osteocyte processes to contact one another through gap junctions for nutrient/metabolite exchange.

Osteoclasts

NOT derived from osteoprogenitor. Instead, derived from Monocyte-Macrophage line. CHARACTERISTICS: very large, multi-nucleated cells formed by fusion of monocytes. Cytoplasm is markedly eosinophilic and has ruffled border formed by in-foldings of plasma membrane. FUNCTION: break down bone after senescence or trauma to an osteocyte. LOCATION: Resorption/erosion lacunae in bone tissue.

Resorption/Erosion (Howship's) Lacuna

Small depression in the bone tissue where Osteoclasts reside.

Hydroxyapatite crystals

Primary inorganic consituent of bone. Align orderly with collagen and impregnate ground substance to give bone its strength.

Ossification

Osteoblasts lay down osteoid around them. First step in bone formation regardless of location, ultimate structural organization, type of bone growth, or type of developmental bone formation.

Osteoid

newly synthesized collagen fibrils and ground stubstance.

Calcification

Process by which surrounding osteoid and ground substance become mineralized. Once osteoid is laid down, osteoblasts secrete membrane-bound vesicles containing alkaline phosphatase. Eventually vesicles burst saturating tissue in hydroxyapatite crystals.

Alkaline Phosphatase

Secreted by osteoblasts during calcification. Precipitates calcium and phosphate ions from the blood within the vesicle.

Compact Bone

compact/cortical. Quite strong because of density. Present in cortex of long bones where tensile strength is needed.

Spongy Bone

Spongy/cancellous. Less strong, present in marrow cavity of long bones. Instead of being solid, it is composed of lattice-work of thin interconnecting Spicules or Trabeculae

Marrow Cavity

Or medulla. Inner portion of bone.

Spicules/Trabeculae

Make up lattice-work in Spongy bone.

Periosteum (bone)

Dense connective tissue capsule surrounds bone on all sides. Has two layers: Fibrovascular and Osteogenic

Fibrovascular Layer (Bone Periosteum)

Outermost peripheral later, predominantly dense fibrous CT contains large number of vessels for maintenance/repair of healthy bone. Contains sensory nerve endings as well. Fibers run parallel to bone surface.

Sharpey's fibers

Where tendons/ligaments attach to bone. Penetrate periosteum and are continuous with colagen fibers of extracellular matrix of bone.

Osteogenic Layer (Bone Periosteum)

Between outer surface of bone and fibrovascular layer. Only seen when bone is actively growing. Osteoprogenitor cells in this layer assume a larger cuboidal shape and become osteoblasts during bone growth.

Endosteum

Lines the compact bone facing the marrow cavity and trabeculae of spongy bone within the cavity. Contains thinner layer of fibrous CT than periosteum makes it hard to differentiate layers.

Woven Bone

Primary bone. All bone begins as woven bone. Collagen fibrils produced by osteoblasts are randomly arranged into interlacing bundles. Template for formation of mature form of bone.

Lamellar Bone

Secondary bone. Once calcified, woven bone is converted to lamellar bone. Highly organized in arrangement and usually considered mature form of bone. Formed on the scaffold of calcified woven bone.

Osteon

Cylinder of bone tissue formed by layering of many concentric lamellae around a central canal (Osteonal/Haversian canal). Basic structural unit of mature bone.

Lamella

single concentric ring of calcified bone matrix that encircles osteonal canal. Collagen fibers of each lamellae run parallel to one another but perpendicular to adjacent lamellae. Provides tensile strength.

Osteonal (Haversian) canal

contains small blood vessels, lymphatic capillaries, very small amt of loose CT, osteoclasts, macrophages, osteoprogenitor stem cells. At center of the osteon, aligned parallel to long axis of bone.

Perforating (Volmann's) canals

Run perpindiular to long axis of the bone to connect osteonal canals in horizontal plane. Ensures a rich blood supply.

Interstitial Lamellae

Unusually-shaped pieces of lamellar bone located between fully formed osteons. Remnants of former osteons replaced during remodeling.

Circumferential Lamellae

Gives bone its smooth exterior. No osteons at exterior and interior surfaces of bone. Two or three lamalle are layered next to each other and follow the various contours of a bone's surface.

Endochondral Bone formation

Cartilage model replaced by bone. Nutrient artery penetrates the diaphysis of a cartilage model and provides osteoprogenitor cells.

Epiphyseal Growth

Disc of cartilage retained for growth in length. Located between primary and secondary centers of ossification. As more cartilage is produced by epiphyseal growth plate, it is progressively calcified. Calicified cartilage is replaced by woven bone and then lamellar bone.

Reserve Zone

Zone of epiphyseal growth plate. Contains non-proliferating chondrocytes in hyaline cartilage that do not actively participate in growht. Anchor epiphyseal growth plate to bony epiphysis.

Proliferative zone

Zone of epiphyseal growth plate. Actively dividing chrondrocytes. Chondrocytes line up in long columns (Stacks of coins) of cells, parallel to long axis of bone. Production of cartilage matrix here lengthens the bone.

Hypertrophic Zone

Zone of epiphyseal growth plate. Maturing chondrocytes, in longitudinal columns. No longer dividing, enlarge and acquire glycogen (pale-staining cytoplasm). Thin intervening matrix.

Zone of Calcified Cartilage

Zone of epiphyseal growth plate. Chrondocytes secrete alkaline phosphatase to calcify thinning matrix. As remaining cartilage matrix becomes calcified by deposition of hydroxyapetite crystals, hypertophied chondrocytes die.

Ossification Zone

At the distal-most end of the epiphyseal growth plate. Osteoprogentitor cells invade calcified cartilage scaffold via capillary network. Region corresponds to the metaphysis. Osteoprogenitor cells differentiate into osteoblasts and lay down osteoid on the surface of calcified cartilage WHILE calcified cartilage is resorbed by osteoclasts.

Myofiber

Extremely, large cylindrical cell. Multinucleated. Nuclei located at periphery of myofiber subjacent to sarcolemma.

Satellite Cells

Mononucleated spindle shaped cells closely apposed to the outside of myofiber sarcolemma. Reside between sarcolemma and basal lamina. ONLY SEEN with ELECTRON MICROSCOPE.

Myotendon Junction

Collagen of a tendon interdigitates with collagen fibers of connective tissue investment (epi-/peri-/endo-mysium) and the basal lamina of myofibers to form myotendon junction.

Epimysium

Dense irregular connective issue sheath of collagen fibers and fibroblasts that surrounds the entire muscle at the gross level.

Perimysium

Extensions of the epimysium that penetrat into interior of muscle subdividing it into many smaller bundles of myofibers called fasicles.

Endomysium

Thinner collagenous layer that surrounds each individual myofiber that make up a fasicle. Can't see well in H+E stain.

Sarcomere

Functional Unit of Contraction. Composed of myofilaments actin and myosin arranged such that they overal in a characteristic array. Extends from Z line to Z line and has alternating light and dark pattern reflecting amount of overlap of actin and mysoin filaments. Has I band, A band, H band.

Sarcoplasmic Reitculum

System of membranous channels that are modified in muscle tissue into an extensive, anastomosing netwok for calcium storage. Penetrates into core of myofiber to segregate myofilaments into cylindrical bundles called myofibrils. FUNCTION: Ensure every myofilament has access to Ca++ for contraction.

T-tubules

Tubular infoldings of sarcolemma that penetrate into cell interior, running perpendicular to long axis of cell. FUNCTION: carries wave of depolarization from surface to interior of cell so that sarcomeres deep within myofiber contract at approx the same time as those next to sarcolemma.

Triad

membrane association of two closely apposed cisternae of sarcoplasmic reticulum with a single t-tubule juxtaposed between them.

Endocardium

Innermost lining of the heart. Simple squamos epithelium.Resembles intima of blood vessels. Consists of endothelium and variable loose CT. Small nerve fibers, blood vessels, and Purkinje fibers.

Myocardium

Thickest layer of cardiac muscle, analogous to the media of a blood vessel. Forms substance of heart wall. Thicker in ventricles than atria.

Epicardium

Similar in structure as the endocardium, except it is the outermost layer that covers surface of the heart. Comprised of mesothelium and variably sized layer of loose CT that contains small nerve fibers, blood vessels, lympathics, adipose, and Purkinje.

Cardiac Myocytes

Elongated cell that branches to associate with branches of adjacent myocytes. Contains SINGLE, centrally-located nucleus. Occasionally two may be observed. STRIATED. UNIQUE FEATURE: specialized intercellular junctions that show up as thick lines between cells (intercalated disks). Comprise bulk of muscle in myocardium.

Intercalated Disks

specialized intercellular junctions in cardiac myofibers that show up as thick lines between cells

Purkinje Fibers

shorter in length and larger in diameter than regular cardiac myoctes. Have on or often two euchromatic nuclei surrounded by very pale cytoplasm (rich in glycogen). Found in ALL THREE LAYERS of the heart. FUNCTION: Initiate and coordinate rythmic contraction of the heart.

Cardiac Skeleton

Skeleton or framework of dense CT that allows heart to have shape and support for contraction. Cardiac muscle is laid down over this dense CT framework.

Diad

Special association (in cardiac myocytes) of one cisterna of saroplasmic reticulum and one t-tubule.

Zonula/Fascia occludens

Cardiac Myocyte specialization. Site where actin filaments of terminal sarcomeres are anchored.

Desmosome

Cardiac Myocyte membrane specialization needed for strength in binding cells together. Keeps cardiac cells from tearing apart during contraction.

Gap Junction

Cardiac Myocyte important membrane specialization that allows ionic continuity between adjacent myocytes and permits rapid spread of electrical impulses throughout the heart.

Smooth Muscle

Long, spindle-shaped. Single elongated cigar-shaped euchromatic nucleus in center. Each cells surrounded by basil lamina. Many cells together form layers or sheets that comprise major component of walls of hollow organs.

Cellular Specializations for Smooth Muscle

NO t-tubules, NO sarcomere, lacks striations. Individual cell is the functional unit of contraction. Sarcoplasmic reticulum is SPARSE. Gap junctions. Dense bodies

Dense Bodies

dense structures found ONLY in SR of smooth muscle associated with inner surface of sarcolemma. FUNCTION: attachment site for filaments that form an intra-cellular network for organization of contractile proteins.

Neuromuscular Junction

Where motor nerve meets surface of myofiber.

Junctional Folds

Part of neuromusuclar junction. At muscle cell surface, axon lies in a special indentation formed by enfoldings of the sarcolemma.

Synaptic Vesicles

Small membrane bound vesicles that reside in large numbers within nerve terminal

Synaptic Cleft

interval of space separating nerve terminal from junctional folds.

Acetlcholinesterase

Breaks down excess Ach in synaptic cleft. Associated with the basal lamina of the myofiber that etends into junctional folds at neuromuscular junction.

Tunica Intima

Innermost layer lining the lumen of a blood vessel. Avascular consists of two parts:
Endothelium- single layer of simple squamos and its basil lamina. Provides semi-permeable layer between blood plasma and interstitial fluid. Crucial to blood clotting, maintaining vascular tone, and growth of blood vessels.
subendothelial layer- loose CT immediately subtending endothelium.

Tunica Media

immediately subjacent to the tunica intima. Consists of concentrically layered sheets of smooth muscle with some collagen (type III), elastic fibers, and proteoglycans interspersed in between. Thickness varies with type/size of vessel: CAPILLARIES have NO tunica media. Large Artery has THICK tunica media.

Internal Elastic Lamina

Layer of elastin present in muscular arteries, arterioles, and large veins. Provides boundary between the tunica intima and the tunica media. Sometimes has fenestrae that are perforations that allow for diffusino of substances between tunica intima and tunica media.

Tunica Adventitia

Ill-defined loose CT tissue layer of variable thickness that blends with surrounding perivascular CT. Composition mostly elastic and collagen (TYPE I) fibers. Typically thinner than the media in arteries, thicker than the media in veins. Capillaries and Venules DO NOT HAVE adventitia.

External Elastic Lamina

may form boundary between tunica media and tunica adventitia in muscular arteries and large veins. Similar in function to the internal elastic lamina but it somewhat thinner.

Vasa Vasorum

"Vessels of the vessels" Capillaries that supply the tunica media if it is too thick to be adequately supplied through diffusion. May also be present in adventitia.

Nervi Vasorum

"Nerves of the vessels". Found in the tunica media and adventitia. Fine efferent sympathetic nerve fibers. Walls of arteries are more profusely innervated than veins.

Elastic Arteries

Large conducting arteries like the aorta and its main branches. FUNCTION: carry blood AWAY from the heart, and modulate the pressure fluctuations between systole and diastole. Elastic Recoil. Results in continuous, rather than intermittent, flow throughout arterial system.
CHARACTERISTICS: Intima- nucleus of endothelial cells may appear to bulge into luman. Internal Elastic Lamina is inconspicuous.
Media- THICK layer, more than FORTY layers of smooth muscle cells and collagen fibers. NO well-defined external lamina.
Adventitia- Often contains nervi and vasa vasorum

Muscular Arteries

medium size, major distributing vessels, most named arteris of body. Control blood flow to all major organs.
Intima- contains a few smooth muscle cells.
Media- 10-40 muscles layers. External elastic lamina is more prominent in muscular arteries.
Adventitia- usually thinner than media. Contains adipose cells, nervi vasorum, vasa vasorum, and lymphatic vessels.

Arterioles

SMALLEST arterial vessels that supply capillary beds. Usually less than 500 micrometers in diameter. MAIN source of peripheral resistance in the arterial system.
Intima- thin subendothelial layer.
Media- 1-2 concentrically arranged layers of smooth muscle.
Adventitia- thin, blends w/ surrounding CT

Large Veins

Intima- Well-developed, typical endothelium and subendothelial layer. May or may not be internal elastic lamina at periphery.
Media- Thin w/ only few layers of smooth muscle, abundant connective tissue.
Adventitia- very thick and well developed, large longitudinal bundles of smooth muscle in largest veins.

Medium/Small veins

Most have valves composed of two cusps formed by projections of tunica intima from opposite endothelial surfaces that appose in the lumen of the vein.

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