Histology notes I2 for second self control test PDF

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Lior OnnHistology I2ndSCT:Adipose Tissue.Cartilage.Bone: Development andGrowth.Muscular Tissue.The Histology of Blood Vessels,Blood and Bone Marrow.Production of Blood Cells.*These notes are based on: Ross Histology book 7th edition, lectures slides,and notes from class (taught by Professor Antal).G...

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Histology I nd

2 SCT: Adipose Tissue. Cartilage. Bone: Development and Growth. Muscular Tissue. The Histology of Blood Vessels, Blood and Bone Marrow. Production of Blood Cells. *These notes are based on: Ross Histology book 7th edition, lectures slides, and notes from class (taught by Professor Antal). Good luck! Lior Onn

Adipose Tissue Overview • •

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Specialized connective tissue for energy homeostasis. Specialized cells: Adipocytes (found in loose connective tissue) 1. Specialized for storage of lipid 2. Generate ATP 3. Produce hormones 4. Produce cytokines 5. “Sense” energy balance in body In order to meet the body’s energy demands when nutrient supplies are low, adipose tissue stores excess energy within lipid droplets in the form of triglycerides (the body has limited capacity to store carbs and protein). How? Takes fatty acids from the blood and converts them into triglyceride within the adipocytes. This energy can be rapidly released for use at other sites in the body when needed. In the event of food deprivation, triglycerides are an essential source of both water and energy. Adipocytes secrete paracrine and endocrine substances to regulate energy metabolism àadipose tissue is considered a major endocrine organ. Active contribution in the regulation of energy balance. There are both white adipose tissue and brown adipose tissue, based on color in living state. White is predominant in adult humans, while brown is present in fetus and diminishes during first decade. Each adipocyte is surrounded by basal lamina which connects the adipocyte to stromal elements of adipose tissue. Stroma of adipose tissue contains undifferentiated mesenchymal cells, blood vessels, reticular fibers, reticular cells, mast cells, macrophages, and more. The stroma cells are: reticulocytes, macrophages, mesenchymal cells. Generate ATP and/or heat, metabolic water (hump of camel!). Forms cushions around joints and internal organs (mechanical and structural function). Located primarily in skin as subcutaneous fat, and in the peritoneal tissue of the abdominal cavity as visceral fat. Adipocytes develop from mesenchymal cells during embryonic life, and can be generated from adult mesenchymal stem cells (MSC) residing in the adipose tissue stroma during extra-uterine life. Staining: oil red O (lipid staining dye) or heavy metal salts (OsO4) osmium:

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White Adipose Tissue (unilocular)(univacuolar) Function of White Adipose Tissue • Predominant type in adult humans. • Forms “panniculus adiposus” or “hypodermis” layer in connective tissue under the skin. • Thermal conductivity of adipose tissue is only half that of skeletal muscleàthermal insulation against cold, by reducing the rate of heat loss. • Provides cushioning of vital organs, such as: GI, heart, eyeballs, kidneys. • Both sexes have a mammary fat pad. In females it is important for the lactating breast function. • Leptin, a hormone involved in the regulation of energy homeostasis, is exclusively secreted by adipocytes. It’s a satiety factor, active in endocrine signaling between brain and adipose. • Adipocytes synthesize and secrete other hormones as well: 1. Angiotensinogen (AGE) (vasoconstrictor) 2. Adiponectin (increases insulin sensitivity, fatty acid oxidation) 3. Resistin (obesity, diabetes; insulin resistance) 4. Steroid hormones. • Obesity increases the secretion of growth factors and cytokines.

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Differentiation of Adipocytes • • • • •

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Differentiate from mesenchymal stem cells under the control of PPARγ/RXR (transcription factors). Specific cell type derived from undifferentiated mesenchymal stem cells associated with the adventitia of small venules. Initially develop from stromal-vascular cell along the small blood vessels in the fetus and are then free of lipids. Primitive fat organs: collections of proliferating early lipoblasts and proliferating capillaries. Early lipoblasts look like fibroblasts (elongated configuration, multiple cytoplasmic processes, abundant endoplasmic reticulum and Golgi apparatus). During differentiation, small lipid inclusions appear at one pole of cytoplasm and an external lamina. Mid-stage lipoblast: oval configuration, excessive concentration of vesicles and small lipid droplets around nucleus, glycogen particles in periphery. Late lipoblasts: large, spherical, much sER and little rER, small lipid droplets have come together to form a single large lipid dropletàunilocular adipocytes or mature lipocytes. This large lipid droplet pushes the nucleus to the side of the cell.

Structure of Adipocytes and Adipose Tissue •

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Nucleus is flattened and displaced to one side of the lipid mass, with cytoplasm ring around the lipid (the thin strands that separate adjacent adipocytes represent the cytoplasm of both cells, as well as the ECM). During histological sectioning, lipid is lost through extraction by organic solvents such as xylene. Adipose tissue is richly supplied with blood vesselsàcapillaries are found at the angles of the meshwork where adjacent adipocytes meet Adipocytes are surrounded by reticular fibers (type III collagen), secreted by the adipocytes themselves. Shown in silver stains. Lipid mass in adipocytes is not membrane bounded, but has a layer of vimentin filament around itàseparation of the hydrophobic content of the lipid droplet from the hydrophilic cytoplasm matrix

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Regulation of Adipose Tissue •

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Brain-gut-adipose axis: interconnected hormonal and neural signals coming from adipose tissue, alimentary tract, and CNS. Regulates appetite, hunger, satiety, and energy homeostasis. Amount of an individual’s adipose tissue is determined by two physiological systems: short-term weight regulation and longterm weight regulation: 1. Short-term weight regulation: controls appetite and metabolism daily. Associated peptide hormones: a. Ghrelin (appetite stimulant, acts on hypothalamus). Produced by gastric epithelium à makes anterior lobe of pituitary gland release growth hormones. b. Peptide YY (PYY) (appetite suppressant, acts on hypothalamus). Produced by the small intestine. 2. Long-term weight regulation: controls appetite and metabolism on a continual basis (months or years). Associated hormones: a. Leptin: levels of leptin mRNA are elevated in obese humans àtheir adipocytes are resistant to leptinàleptin doesn’t reduce amount of adipose tissue in obese. Protects body against weight loss during food deprivation. Appetite regulation. b. Insulin: regulates blood glucose levels; enhances conversion of glucose into triglyceride (of the lipid droplets) by adipocytesàaccumulation of adipose tissue. Increases insulin sensitivity. Stimulates lipid synthesis enzymes and inhibits hormonesensitive lipaseàblocks release of fatty acids. c. Thyroid hormone, glucocorticoids, hormones of pituitary gland. Mobilization: stimulation by neural or hormonal mechanismsàbreak down of triglycerides into glycerol and fatty acids. Fatty acids pass through the adipocyte cell membrane to enter capillary (carrier protein: albumin). The FAs then pass to other cells through the blood and are used as metabolic fuel. Neural mobilization: important during fasting and severe cold. Norepinephrine leads the activation of lipases that split triglycerides, which is an early step of mobilization of lipids. Hormonal mobilization involves hormones and enzymes that control fatty-acids release from adipocytes. Examples: o Insulin, thyroid hormones, adrenal steroids, glucagon, and growth hormone. o Elevated levels of TNF-alpha have been shown as a cause in insulin resistance associated with obesity and diabetes.

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Brown Adipose Tissue = BAT (multilocular)(multivacuolar) • • • • • •

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Smaller cells. Contain numerous fat droplets. Nucleus is in an unusual position within the cell, not flattened (as it is in white adipose) In H&E the cytoplasm is shown as empty vacuoles cause the lipid is lost during preparation à resemble epithelial cells rather than CT cells Contain: numerous mitochondria with large amounts of cytochrome oxidase à brown color of the cells. Abundant in newborns (important in inhibiting heat loss + avoiding lethal hypothermia - a major risk for premature babies). The amount decreases as body grows but remains widely distributed along the first decade of life (in cervical, axillary, paravertebral, mediastinal, sternal, and abdominal regions). Disappears in adults from most sites, except for regions around the kidney, adrenal glands, large vessels (aorta), regions of the neck, back, and thorax. Subdivided into lobules by CT partitions Differentiate from mesenchymal stem cells under PRDM16/PGC-1 (transcription factors) in the presence of catecholamines. They activate UCP-1 gene à controls brown fat differentiation and encodes a mitochondrial protein called uncoupling protein (UCP-1) or Thermogenin. UCP-1 (found in inner mitochondrial membrane): used to uncouple the mitochondrial respiratory chain = uncouples the oxidation of fatty acids from the production of ATP à generating heat rather than ATP. Thermogenesis: Metabolism of lipid in brown adipose tissue generates heat. Non-shivering thermogenesis: when oxidized, adipose tissue produces heat to warm the blood flowing through the brown fat on arousal from hibernation. Maintenance of body temperature in the cold. In non-hibernating animals and humans: lipid is mobilized, and heat is generated by brown adipocytes when they are stimulated by sympathetic nervous system. Hibernating animals: have large amounts of brown adipose tissue. Metabolic activity is regulated by norepinephrine: increased blood levels of norepi à brown adipose tissue expands.

Transdifferentiation of adipose tissue •

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Transdifferentiation: white-to-brown and brown-to-white adipocyte transformations, in response to the thermogenic needs of an organism. Browning phenomenon: 3-5 days of exposure to chronic cold temperatures à increased thermogenic needs of an organism à mature white adipocytes can transform into brown (UCP-1 positive) adipocytes (to generate body heat) Energy balance is positive à the body requires an increase of triglyceride storage capacity à brown adipocytes are able to transform into white adipocytes. Mice with abundant brown adipose (either natural or induced via browning) are resistant to obesity and type 2 diabetes. Beige is the transitional kind of adipose tissue, between brown and white (prof. antal).

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Exercisingà muscle cells produce irisine àstimulation of browning: Other triggers of transdifferentiation include reprograming of adipose tissue genes by activating specific transcription factors (“master regulators”) and growth factors, such as fibroblast growth factor-21 (FGF-21).

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Extras from the lecture slides: • • • •

There is also “Beige” adipose tissue: Obesity leads to inflammation in adipose tissue à increase the probability of various diseases, including cancer. Inflamed adipose tissue à insulin resistance + malfunction of skeletal muscle fibers in obesity. Both white and brown have basal lamina

Slides 1. Hairy skin, H&E: • Adipose layer is seen underneath the CT (white adipose). • Light microscope resolution is 200 nm à plasma membrane cannot be seen (~8 nm thick) à the circle around each adipocyte is NOT membrane à the circles are cytoplasm of adipocytes, pushed to the sides by the large lipid droplet occupying the cell. Narrow ring, pressed against (unseen) cell membrane. • Lipid droplet is stored in the cytoplasm without a vesicle à no surrounding structure. Sits in cytoplasm. • Peripheral nuclei, pushed by lipid droplet.

2. Adrenal gland (suprarenal), H&E: • The gland has an adipose capsule surrounding it à adipose tissue is shown in the slide! • In the midst of the white adipose tissue we can see an area of brown adipose tissue: o Small, several lipid droplets, randomly distributed o Nuclei are in variable positions in the cytoplasm o Eosinophilic cytoplasm is seen in between the lipid droplets

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Cartilage Overview • • •

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Form of connective tissue Mesenchymal origin: mesenchymal cells à osteochondroprogenitors à chondroblasts à chondrocytes. Two main components of cartilage tissue: 1. Specialized cells, chondrocytes (3-5% of tissue). Chondrocytes are the ONLY cells of the cartilage! 2. Highly specialized extracellular matrix (secreted by chondroblasts and chondrocytes): (95% of tissue) a. fibers b. ground substance à attract water à highly hydrated (water makes up 60-80% of total mass)! Chondrocytes and ECM are adapted to the mechanical demands of the localization of cartilage. Cartilage is avascular (except for some parts of fibrous cartilage in menisci) à nutrients and oxygen received via diffusion. Large ratio of glycosaminoglycans (GAGS) to type II collagen fiber à diffusion of substances between blood vessels in the surrounding CT and chondrocytes. Presence of specific cartilage matrix is essential to keep chondrocytes differentiated. Extracellular matrix is solid and firm, but also somewhat pliable (flexible). Main tissue in the development of fetal skeleton and most growing bone. Low capacity for regeneration. Three main types, distinguished by characteristics of their matrix: 1. Hyaline cartilage: matrix containing type II collagen fibers, GAGs, proteoglycans, and multi-adhesive glycoproteins. 2. Elastic cartilage: whatever there is in hyaline cartilage + elastic fibers and elastic lamellae. 3. Fibrous cartilage: whatever there is in hyaline cartilage + abundant type I collagen fibers. Perichondrium: outer surrounding of most collagen tissue (there are exceptions). Has two layers: 1. Stratum fibroelasticum: outer, fibrous 2. Stratum chondroblasticum: inner, cellular Cartilage is NOT innervated. Perichondrium is innervated. Characteristics of chondrocytes: 1. Only cell type in cartilage. 2. Rich in small proteoglycans 3. Secretes the ECM 4. Anaerobic metabolism 5. Do not proliferate in adults, but have a long-life span 6. Stores glycogen and lipids. No chondroblasts in cartilage, only chondrocytes à create collagen fibrils only for maintenance à unable to create collagen in large amounts (as would be needed in healing) à little to no cartilage regeneration.

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Hyaline Cartilage •

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Rigid, resistant to compression à shock absorber (found in articular cartilage). Also found in organs of airways (trachea slide), nasal septum. Homogeneous, amorphous (shapeless) matrix. Appears glossy. In the cartilage matrix are spaces called lacunae, and within these lacunae there are: 1. Chondrocytes. 2. Newly produced ground substance Provides low-friction surface, lubrication of synovial joints, and distributes applied forces to the underlying bone. Limited repair capacity, but shows no abrasive (grinding, rubbing) wear over a lifetime. Exception: articular cartilage, which may break down with age. ECM is produced by chondrocytes, and contains three major classes of molecules: 1. Collagen molecules: major protein of the matrix. Four types, aka cartilage-specific collagen molecules: a. Type II constitute the bulk of the fibrils b. type IX facilitates fibril interaction with the PGs. c. type XI regulates fibril size. d. type X organizes fibrils into hexagonal lattice which is crucial for the mechanical function of the cartilage. Type VI collagen is also found. 2. Proteoglycans: contains 3 GAGs: a. Hyaluronan b. chondroitin sulfate à PG monomer c. keratin sulfate à PG monomer Most important proteoglycan monomer is aggrecan à many aggrecans associate with one hyaluronan à PG aggregate. Because of the presence of the sulfate groups, aggrecan molecules have a large negative charge with an affinity for water molecules. Other PGs are found as well, but don’t form aggregates. 3. Multiadhesive glycoproteins: AKA- “noncollagenous” and “nonproteoglycan-linked glycoproteins”. Influence interactions between chondrocytes and matrix molecules. Anchorin CII, tenascin, and fibronectin. Highly hydrated with intracellular water (60%-80%). Huge volume of water bound to aggrecan-hyaluronan aggregates à extreme resistance to compressive forces. Changes in water content occur during joint movement or due to pressure. Cartilage undergoes continuous internal remodeling, as the cells replace matrix molecules lost through degradation. Chondrocytes can be distributed either singularly or in clusters called Isogenous groups (chondron) (representing a cell that has recently

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divided). Chondrons of hyaline cartilage contain 3-4 cells in average (prof. Antal) and they are the cellular unit of the cartilage. Secrete metalloproteinases, enzymes that degrade cartilage matrix, allowing the cells to expand and reposition themselves within the growing isognenous groups. Cytoplasm appearance varies according to chondrocyte activity: active in matrix production à protein synthesis à basophilia. Because proteoglycans of hyaline cartilage contain high concentration of sulfate groups, ground substances stain basophilic (metachromasia). The matrix doesn’t stain homogenouslyà 3 different regions are described based on the staining property of the matrix (seen in the figure of the chondron, shown above): 1. Capsular (pericellular) matrix: a. Ring of densely (basophilic) stained matrix immediately around each chondrocyte. (ECM molecules > collagen molecules à strong basophilic staining). b. Contains highest concentration of sulfated proteoglycans, hyaluronan, biglycans, and multiadhesive proteins. c. Contains almost exclusively type VI collagen, with type XI collagen as well. 2. Territorial matrix: (TM) a. Surrounds the isogenous group. b. Randomly arranged network of type II collagen, with some type XI as well. c. Less sulfated proteoglycans à stains less intensely than capsular matrix. d. More collagen than in pericellular matrix à another reason for lighter staining than pericellular (still more ECM molecules than collagen!). 3. Interterritorial matrix: (IM) a. Surrounding the territorial matrix b. Occupies space between the isogenous groups. c. Collagen molecules > ECM molecules à even lighter staining than territorial matrix! (many GAGs but proportionally they’re less). Precursor of bones that develop via endochondral ossification (lecture slide): 1. Process in which much of cartilage is replaced by bone. 2. Remaining cartilage serves as growth site, epiphyseal growth plate (epiphyseal disc). Epiphyseal growth plate is made of hyaline cartilage and it increases Epiphysial plate is shown in the following imageà Perichondrium: a dense connective tissue (DCT) composed of cells that are indistinguishable from fibroblasts. Surrounds hyaline cartilage. 1. Contains: chondroblasts (immature chondrocytes). 2. Surrounds cartilage of internal organs, ear and nose, and glands. 3. Allows growing and certain regeneration of cartilage. Serves as a source of new cartilage cells. 4. Has two layers: a. Stratum fibroelasticum, outer fibrous layer: dense regular CT. Extremely thick and long collagen fibers + low amounts of fibrocytes. b. Stratum chondroblasticum, inner cell-rich (chondroblastic) layer. Source of young chondrocytes. 5. Not present in fibrocartilage and not in articular cartilage (prof. Antal). Hyaline cartilage of articular joint surfaces do not possess perichondrium.

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The articular surfaces of movable joints is called articular cartilage, and is a remnant of the original hyaline cartilage template of developing bone. It’s actually a specially layered hyaline cartilage. Articular cartilage is 2 to 5 mm thick and is divided into four zones: 1. Superficial (tangential) zone: pressure-resistant. Elongat...