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Description

Tissue repair in mice depleted of resident macrophages or lacking certain macrophage functions is defective hiv infection after 1 year symptoms generic 200 mg monuvir visa. Epithelial cells, fibroblasts, and endothelial cells are also sources of chemokines. Chemokine-mediated activation of fibroblasts at the site of tissue injury temporally coincides with the influx of macrophages, typically within a few days after the initial injury. The matrix is also well hydrated, and therefore conducive to diffusion of oxygen and amino acids. These myofibroblasts promote contraction of tissue undergoing repair, which accelerates lesion resolution but sometimes results in tissue distortion. The purpose of this response is obvious: to reestablish the blood supply required for supplying essential nutrients to regenerating parenchymal cells and proliferating fibroblasts. Provided tissue injury has ceased, endothelial buds form tubular structures that merge with the intact capillary network and interweave with newly deposited collagen fibers. Hence, in life, young granulation tissue is typically pink, somewhat gelatinous, quite friable, and bleeds easily. Granulation tissue is more suited for scar formation than parenchymal cell regeneration, but over time may be partially remodeled so that partial resolution is possible. With type I collagen maturation, cross-linking condensation and reduced vascularization allow tissue contraction to proceed beyond that mediated by myofibroblast function. Electron photomicrograph of a portal triad from a rat with xenobiotic-induced periportal hepatic damage leading to fibrosis. Note the increase in fibroblasts (1), increased deposition of collagen fibers (2), and influx of macrophages (3) within the interstitial space, which is expanded by edema fluid [clear spaces (4)]. Epithelial cells lining a bile duct (5) are enlarged, indicating a hypertrophic response due to increased cell mass. For a comprehensive insight regarding how tissue repair versus regeneration occurs in a damaged tissue, refer to Chapter 15: Digestive System. Apoptosis In contrast to necrosis, which usually provokes a robust inflammatory response, the tissue response to apoptosis is quite benign. The molecular basis for this targeting is complex and appears to depend on a number of factors. Photomicrograph of atrophied acinar pancreas in a Fischer 344 (F344) rat 4 days after extensive apoptosis due to administration of the pancreatic toxicant crambene. Small acini (arrows) containing shrunken, non-functional acinar cells are scattered throughout the section. Numerous histiocytic macrophages (starbursts) occupy the loose preexisting lobular stroma between acini. Tissue macrophages (M) containing ingested apoptotic bodies (small arrows) appearing as inclusions are present as well. Other external macromolecules apparently recognized by macrophages include lysophosphatidyl choline, calreticulin, and annexin. Phagocytosis of an apoptotic body after engulfment by a macrophage has been described as a "big gulp" to contrast with the "zipper" type of engulfment that typically precedes phagocytosis of bacteria; the latter is characterized by tight binding of the macrophage membrane to specific ligands on the bacterial membrane. The precise mechanisms involved in digestion of the apoptotic body are uncertain, but the overall process depends on gradual acidification and protease degradation of the phagosome. The net effect of is to block antigen presentation by those macrophages with engulfed apoptotic bodies. Furthermore, while apoptotic bodies remain in their phagosomes, macrophages have markedly reduced phagocytic capability. In total, phagocyte-derived release of intracellular inflammatory chemokines, those that enhance endothelial permeability and activate or attract neutrophils, is minimal. Once the cause of apoptosis is addressed, for example, by removing a noxious insult or adding a trophic hormone, full regeneration is usually achievable provided a residual precursor cell population survives. Regeneration potential is particularly high in tissues with constant cell turnover such as lymph nodes, intestinal epithelium, and liver. However, in those tissues with so-called permanent cell populations, such as the retina, significant apoptosis will result in a permanent lesion, retinal atrophy, with some loss of function. Hyperplasia As part of the regenerative process subsequent to widespread cell loss, cell proliferation is necessary and often recognizable temporarily as compensatory hyperplasia. However, compensatory hyperplasia can predispose to an ominous molecular event, namely the permanent establishment, that is, fixation, of genetic mutations that can lead to cancer. Hyperplasia in this context is discussed in Chapter 6, Carcinogenesis: -catenin bridge to the actin cytoskeleton.

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Effects of Endothelial Cell Function and Damage on Blood Vessel Activity Vascular endothelial cells serve as a protective barrier in blood vessel walls and serve as an active source for the synthesis antivirus windows 8.1 buy monuvir visa, metabolism, uptake, storage, and degradation of a number of vasoactive substances. When endothelial cells are destroyed, the vessels lose the ability to relax on exposure to most of these dilator substances. In addition, the loss of functional endothelial cells seems to transform normal vasodilator responses into potent vasoconstrictor activity. A substance that damages or destroys endothelial cells to the extent that vasodilatory responses are altered could conceivably cause significant decreases in blood flow and subsequent tissue damage in certain organs. Evaluation of Vasotoxic Effects Xenobiotic-induced vascular dysfunction or injury may be systemic or localized to a particular organ or vascular bed. Studies of vascular function and structure have been done using a variety of in vivo and in vitro methods. Physiologic Methods for Testing Blood Flow Measurements Measurement of blood flow can be done with electromagnetic or ultrasonic Doppler flow meters. In addition, the probes can be chronically implanted so measurements can be made continuously or over long periods of time without the interference of anesthetics. The electromagnetic flow meter and the Doppler ultrasonic techniques provide moment-to-moment phasic flow information. This type of information is important in situations where the vascular effects of a substance either are transient or where changes occur over a period of time. These techniques have been used successfully in monitoring blood flows from large arteries such as the aorta as well as from smaller vessels such as the renal, mesenteric, femoral, carotid, and coronary arteries. Dilution Methods Measurements of organ perfusion can be made with the aid of dilution methods utilizing the Fick principle. All methods use an indicator substance that may be an inert gas, a natural metabolite, a nondiffusable dye, or a radioactive tracer. The indicator substance is injected into the blood and its concentration measured by an appropriate detector at a downstream sampling site. The indicator dilution method is useful for the measurement of mean organ blood flow if the indicators used are removed from the blood by these organs. For example, the liver removes bromsulfalein and the kidneys remove p-aminohippurate while the brain and heart absorb nitrous oxide. Direct Measurement of Blood Flow Direct in vivo measurement of blood flow is possible by using a semiisolated dog biceps muscle preparation. The advantage of this preparation is that the principle components (muscle, blood vessels, and nerve) are all part of a single system. A nearly identical type of biceps muscle preparation uses the cat as the experimental model. Measurement of blood flow and vascular reactivity in these preparations are most reliable under normal flow conditions. This preparation allows monitoring of the effects of vasoactive substances on the resistance and capacitance vessels at times when the circulation to the area is compromised. Perfusion of isolated hindlimb preparations at a constant rate allows a means of detecting substance-induced alterations in vascular resistance. This technique has allowed identification of substances that possess vasoactive effects. However, it is not possible to determine whether this activity is due to a direct or indirect effect on the vascular smooth muscle. Direct Observations of Blood Vessels Direct microscopic observations of microvascular beds can be achieved in easily transilluminated thin membranes. Studies of this type are usually limited to acute experiments in anesthetized animal preparations such as the hamster cheek pouch, the rat and mouse ear, or mesenteric beds. A transparent chamber for use in the rabbit ear considerably lengthens the period for direct observation of substance effects on blood vessels in unanesthetized animals. This technique allows measurement of changes in vessel wall thickness, lumen diameter, and total vessel diameter in both isolated tissues and the intact vascular beds. In Vitro Methods for Detecting Vascular Toxicity Mechanistic insights into a particular vascular effect may be aided by studies in an isolated system rather than on the entire vascular bed. Ex vivo and in vivo rat mesenteric preparations have been used in vascular studies. Though in vitro preparations allow a more controlled analysis of the potential toxic vascular effects, results obtained from isolated or even perfused tissues do not always completely represent the action of the xenobiotic on the vasculature in the intact organism, particularly if cellular components of the blood are involved in the activity of the substance. Isolated Vascular Muscle Strip In vitro studies can be conducted on strips obtained from large arteries of laboratory animals such as rabbits and rats.

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Some hepatocellular carcinomas are characterized by rarer cellular arrangements including sheets antiviral research abbreviation buy discount monuvir on-line, glands, and mixed cholangiocellular patterns. Occasionally, some hepatocellular neoplasms may have the typical appearance of adenomas except for one or few select areas within the lesion, where a nodule of neoplastic hepatocytes compresses the adjacent adenoma tissue, and contains a trabecular pattern and/or possesses a distinct, usually more basophilic, cytoplasm staining. Current diagnostic conventions designate these lesions as hepatocellular carcinomas. In rats and mice, hepatocellular neoplasms may occur spontaneously and are often observed as incidental findings in aging untreated animals in standard laboratory environments. However, in mice, some strains such as the C3H are known to have a fairly high incidence (30%À50% in males) while other strains such as the C57Bl/6 have a fairly low incidence (,5% in males). Rats occasionally have spontaneous hepatodiaphragmatic nodules of the median lobe, with the incidence varying by strain. They are composed of hepatic lobular parenchyma, often with hepatocytes possessing unusual linear chromatin patterns. A rare lesion that may be confused with hepatocellular neoplasia is glandular metaplasia of hepatocytes. This lesion may have a variable appearance, with formation of a few too many glands of diverse size that are lined by cells resembling hepatocytes or cuboidal epithelial cells which resemble bile duct epithelium. The partial replacement of hepatic parenchyma by glandular structures with features resembling hepatocytes has been observed in chronic studies of some pentachlorobiphenyls. Hepatoblastoma is a distinct form of hepatic neoplasm that has been described in aging mice as a spontaneous lesion, or in some instances attributed to a test article. Among aged untreated control B6C3F1 mice, the incidence of hepatoblastoma was approximately 2% in males and,1% in females. Hepatoblastomas are usually detected in mice with concurrent hepatocellular adenoma or carcinoma. When associated with a test article, affected treatment groups with an increased incidence of hepatoblastoma often also have an increased incidence of hepatocellular neoplasia. The lesions typically include endothelial-lined cystic spaces that contain blood and/or eosinophilic material. Central areas of necrosis with hemosiderin-laden macrophages, calcium deposits, and cholesterol crystals may also be present. On occasion, there are focal areas with cells arranged in an embryonal-like organoid pattern or form rosettes and ribbons. In some studies, focal areas of squamous differentiation or osteoid differentiation have been noted. Biliary Neoplasia Neoplasms of the biliary epithelium may be apparent by gross inspection. Typically, they form raised, pale, white to beige masses that may be cystic or contain areas of necrosis. Cholangioma (biliary adenoma) is a benign lesion of bile duct epithelium that is characterized by a distinct border and composed of numerous irregular bile ducts. Cholangiomas have scant connective tissue stroma and may have a thin connective tissue capsule. Although the individual bile ducts may be slightly irregular in shape, the epithelium lining these ducts is usually homogeneous in cell size and shape. Cystic cholangiomas are characterized by irregular cystic cavities usually lined by a uniformly low cuboidal to squamous epithelium. Cholangiofibroma Cholangiofibroma is a benign liver tumor that has some characteristics in common with the cholangiocarcinoma. The cholangiofibroma has an extensive collagenous connective tissue stroma surrounding atypical bile ducts composed of several different cell types. Most cells have bile duct epithelial characteristics, including a basophilic cytoplasm and vesicular nucleus, and goblet cells are observed frequently. In addition, cells resembling Paneth cells and enterochromaffin cells are not infrequent. The duct-like structures are usually filled with necrotic cellular debris from sloughed epithelial cells and mucous substances. In the center of cholangiofibromas, the bile duct epithelium may be completely denuded from the surface of the gland-like structure, leaving a mucus-filled cyst surrounded by connective tissue. Cholangiocarcinoma Cholangiocarcinomas are malignant neoplasms of biliary epithelium.

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Severe disturbances in maturation sequence may be seen histologically as changes in the ratio of proliferating cells to maturing cells antiviral medication for mono monuvir 200mg otc, generally 1:4 in rodents. Asynchronous maturation observed histologically should be confirmed by cytologic evaluation. Erythroblastic islands may be evaluated histologically, though they are not always readily apparent without the aid of immunohistochemistry. The more mature erythroid cells within these islands are smaller and more hyperchromatic than the larger immature erythroid cells with large round nuclei and deeply basophilic cytoplasm. Histologically, immature myeloid cells are large, with bean-shaped nuclei and finely granular, vesicular chromatin. These are the largest and thus easiest cells to identify with abundant pale eosinophilic cytoplasm and multilobulated nuclei. It may include a quantitative assessment (differential count of 100À500 cells) to characterize alterations in lineage proportions (including M:E ratio) or incomplete/asynchronous maturation of a given lineage. Flow Cytometry the biggest limitation to quantitative evaluation of cytologic specimens is that relatively few cells are counted. Flow cytometry is an investigative tool that can be applied to any hemolymphatic tissue including peripheral blood, lymph node, and spleen. Although technically challenging, it allows for high-throughput, reproducible, and precise differential counting of many cells (10,000À35,000). Flow cytometry cannot be used to obtain a complete differential count or to assess cytomorphology, and thus cannot replace cytologic assessments. Therefore cytospin preparations of cell suspensions should always be prepared simultaneously. In Vitro Techniques In vitro methods are useful to investigate mechanisms and to screen compounds in early discovery toxicology. In vitro cytogenetic analysis can be used to investigate genotoxicity, clastogenesis, myelodysplasia, and leukemogenesis. Clonogenic assays evaluate stem cell to proliferation and differentiation in selective media when exposed to potential toxicants. Colony-forming assays are time consuming to perform and rely on the subjective and technically demanding manual enumeration of colonies. Animal Models Animal models have historically relied on naturally occurring susceptibilities or exposure to classic hematotoxicants. The hematopoietic system can also be studied in zebrafish (Danio rerio) embryos, which are easy to manipulate genetically, permeable to water-soluble chemicals, and amenable to highthroughput screening. Less commonly, they may be unpredictable and nondose-related events that are idiosyncratic or attributable to hypersensitivity (Type B). Xenobiotics may target a broad spectrum of blood cells or cells of a specific cell type (erythroid, myeloid, platelet) or stage of maturity (stem cell, progenitor, maturing, circulating). Injury to uncommitted stem cells generally produces aplastic pancytopenia (aplastic anemia), while injury to committed myeloid or erythroid progenitors produces agranulocytosis or pure red cell aplasia, respectively. Death of maturing cells may produce maturation arrest and/or increased proportions of immature cells. Altered nuclear maturation with spared protein synthesis and cytoplasmic maturation, results in macrocytosis, megalocytosis, or granulocyte hypersegmentation. Agents that inhibit heme synthesis (ethanol, isoniazid, pyrazinamide, cycloserine, chloramphenicol, copper chelation/deficiency, zinc, lead, trichloroethylene, and gallium arsenide) cause iron and ferritin to precipitate in mitochondria of maturing erythroid cells, producing siderocytes. Mechanisms of hematopoietic injury are similar between species, and hematopoietic toxicity in animals holds high concordance with and predictivity for human toxicity. However, species differences do modulate the hematotoxic effects of some compounds. For example, mice and cats are particularly susceptible to the hematotoxic effects of nitrosoureas while rats, guinea pigs, and sheep appear to be somewhat resistant. Cytosine arabinoside and busulfan induce thrombocytopenia in a wide variety of animal species, but not humans. Classes of common hematotoxic agents include antineoplastic, antimicrobial and immunosuppressive chemotherapeutics, kinase inhibitors, antibiotics, antiretrovirals, antifungals/parasiticides, hormones and hormone antagonists, cytokines, environmental contaminants, physical injury, heavy metals, mycotoxins, and plant toxins (Table 13. Circulating blood cells are particularly prone to direct injury from high exposures to circulating xenobiotics.

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