Henee, mature male and female gametes have 22 plus X or 22 plus Y chromosomes, respectively. The approach may be transabdominal or transvaginal, with the latter producing images with higher resolution. More important, they found that this loss was not merely a result of loss of gland cell types, but that it actually preceded it. In this situation, blood flow distal to the trapped aneurysm can be augmented with a side-to-side anastomosis from the contralateral circulation. Henee, the only portion of the chorion participating in the exchange process is the cho rion frondosum, which, together with the de cidua basalis, makes up the placenta. Regional differences in nitrergic innervation of the smooth muscle of murine lower oesophageal sphincter. Proximity to the brainstem can trigger cardiac arrhythmias and secondary myocardial infarction. At term, the fetus extracts 20 to 30 mL of oxygen per minute from the maternal circulation, and even a short-term interruption of the oxygen supply is fatal to the fetus. From the anterior chamber, the fluid passes through the scleral venous sinus (canal of Schlemm) at the iridocorneal angle where it is resorbed into the bloodstream. It occurs sporadically but is most frequently observed among infants of diabetic mothers. Adjuvant embolization with n-butyl cyanoacrylate in the treatment of cerebral arteriovenous malformations: outcomes, complications, and predictors of neurologic deficits. It can also represent more ominous shunting of venous blood into cortical venous channels sufficiently distant from the middle ear such that pulsatile tinnitus is no longer appreciated by the patient. C, Illustrative case of a 55-year-old man with progressive myelopathy and bowel and bladder incontinence. Microdialysis probes were implanted in the submucosa of the antrum of the rat stomach. This chapter discusses many examples of normal physiologic responses and pathophysiologic states that may result from a disturbance in the intricate balance of regulatory control that growth factors provide. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur. Epidermal growth factor, transforming growth factor-alpha, and epidermal growth factor receptor content in normal and carcinomatous gastric and colonic tissue. The importance of chromogranin A in the development and function of endocrine pancreas. Paracrine signaling Type of signaling from one cell to another where proteins synthesized by one cell diffuse short distances to interact with other cells. In addition, each of these pathways controls the levels or activities of Bcl-2 family proteins that are responsible for determining whether cytochrome c is released from mitochondria. Finally, the length of follow-up is frequently variable and inconsistent and thus not always representative of the general population. In high-flow situations, coexisting prenidal aneurysms may diminish in size and disappear when the shunt is obliterated. Identification of Notch components that affect a specific disease, but are less important in the normal intestine, will provide potential therapeutic targets while avoiding the severe gut toxicity associated with complete Notch disruption. The Guglielmi detachable coil was developed in the 1980s with the first use in a human brain aneurysm occurring in 1990. In contrast, many disease processes, notably cancers, are often the consequence of dysregulated cell cycle due to mutations in key molecules that control the cell cycle. A combination of stents and detachable coils can also induce thrombosis of the aneurysm. Gastrointestinal anomalies associated with esophageal atresia or tracheoesophageal fistula. Enhanced expression of epidermal growth factor receptor correlates with alterations of chromosome 7 in human pancreatic cancer. Fas is also expressed in non-lymphoid tissues at sites of immune privilege (ovaries, uterus, testes) and at sites commonly infiltrated by lymphocytes (small and large intestine, liver, and lung).
With further development, the thyroid gland descends in front of the hyoid bone and the laryngeal cartilages. Once this position is achieved, the microcatheter can usually be pulled from the aneurysm while distal purchase is maintained. Up-regulation of hepatocyte growth factor gene expression by interleukin-1 in human skin fibroblasts. Before surgery, this number dropped to 90%, and late after surgery, only 80% were included in this group. Simultaneously, the amnion is pulled ventrally until the am niotic cavity nearly surrounds the embryo. Before detailing the important functions of Notch signaling in the intestine, these other fundamental signaling pathways will be briefly summarized because they are somewhat interdependent and regulate overlapping functions. In addition to the three commissures developing in the lamina terminalis, three more appear. The pathways connecting G protein-coupled receptors to the nucleus through divergent mitogen-activated protein kinase cascades. In their experience of 60 provocative tests during 84 angiographic procedures in 52 patients with intended endovascular embolization, they found only one false-negative result (negative predictive value of 97. Shh expression is high in the forestomach and weaker but detectable in the distal stomach, whereas Ihh is expressed in the distal stomach that will give rise to corpus and antrum and is undetectable in the forestomach. Ghishan the mammalian intestinal tract undergoes dramatic changes during the first few weeks of postnatal life. Signal sequences specify the targeting route to the endoplasmic reticulum membrane. Their dismal natural history necessitates treatment, but their size and anatomic complexity place them among the most challenging lesions confronting neurosurgeons. Somites Epithelial balls of ceUs formed in segmental pairs along the neural tube from paraxial mesoderm. On the right, the ventricle is very small, the pulm onary artery is affected and may be atretic or stenosed and the atrium may be small; on the left, the ventricle is very small, the aorta may be atretic or stenotic, and the atrium may be reduced in size. Ink4a/Arf and oncogene-induced senescence prevent tumor progression during alternative colorectal tumorigenesis. Fibroblast growth factors modulates intestinal epithelial cell growth and migration. An attempt will be made to be specific whenever possible, and fungal aneurysm and bacterial aneurysm will be used to describe an aneurysm of fungal or bacterial etiology, respectively, when the distinction is important. Specificity of prohormone convertase endoproteolysis of progastrin in AtT-20 cells. Distinct molecular forms of betacatenin are targeted to adhesive or transcriptional complexes. Thrombosis shrinks the size of the aneurysm and can ameliorate cranial nerve compression and potentially reduce the severity of headache. With far lateral exposure, proximal control can be achieved with temporary clips on the vertebral arteries. These glands function by merocrine mechanisms (exocytosis) and are involved in temperature control. Structure of human procathepsin L reveals the molecular basis of inhibition by the prosegment. Pax genes encode key regulators, which are involved in the embryonic development of many organs including the eyes, brain, kidney, thyroid gland, immune system, and pancreas. Actin is the major cytoskeletal protein responsible for the shape of the cell and cell motility. Approximately 10% of m ajor birth defects are a result of chro mosome abnormalities, and 8% are a result of gene mutations.
The Y4 receptor is found in the pancreas and mediates an inhibition of pancreatic exocrine secretion. These dividing transit cells are therefore referred to as precursor cells or committed precursors or progenitors, since they may become goblet cells, for example. The risk for permanent deficit is greater with occlusion of the artery of Adamkiewicz than with occlusion of cervical feeders because the former has fewer collateral feeders, as mentioned earlier. Kupffer cells are the macrophages of the liver and are located within gaps between sinusoidal endothelial cells. Role of endogenous prostaglandins in regulating the tone of opossum lower esophageal sphincter in vivo. Nonetheless, outcomes with its use are extremely variable because of the wide range of treatment paradigms used. A similar conductance was noted in native myocytes, and this current was blocked by local anesthetics, such as lidocaine, and acidic pH. In summary, neurological outcome is closely related to the preoperative neurological function of the patient. In turn, phosphorylation activates these proteins to phosphorylate additional proteins, and thus a cascade of protein interactions is established that ultimately acti vates a transcription factor. The sulcal veins in the anterior median fissure and the radial veins in the dorsal and anterolateral portions of the spinal cord drain to the coronal venous plexus on the cord surface. Studies in mice lacking specific components of the mitotic checkpoint support this view. It was initially termed caudal regression, but it is clear that structures do not regress; they simply do not form. Early experiments suggested that these two muscle layers may modulate distinctly different kinds of contraction and relaxation. Nuclear localization signalindependent and importin/karyopherin-independent nuclear import of beta-catenin. In conclusion, with regard to grading systems it is reasonable and appropriate to use Spetzler-Martin grading as the basis for stratifying and communicating surgical risks. For each patient, a unique treatment plan with specific goals and potential risks and benefits must be established and understood by the treating team of health professionals, the patient, and family. Gastrulation begins with formation of the primitive streak on the surface of the epiblast. Classically, for anterior interhemispheric approaches, the craniotomy is placed so that two thirds of the rectangular bone flap is situated anterior to the coronal suture and one third is posterior. Enhancement of human hepatocyte growth factor production by interleukin-1 alpha and -1 beta and tumor necrosis factor-alpha by fibroblasts in culture. Motilin-independent ectopic fronts of the interdigestive myoelectric complex in dogs. Of these, the M phase takes approximately one hour to complete and interphase takes up the remaining 23 hours. Dementia and mental disturbances, as well as hemiparesis and epilepsy, have also been described. The physiologic recapitulation of these developmental programs in adult tissues, such as the gastrointestinal tract, appears critical in maintaining tissue homeostasis and promoting repair after injury. The heparan sulfate-fibroblast growth factor family: diversity of structure and function. Consequently, venous congestive encephalopathy, cerebral edema, seizures, hemorrhage, and dementia are frequent sequelae. Among women sundvors pregnant at the time of the atomic bomb explosions over Hiroshima and Nagasaki, 28% spontaneously aborted, 25% gave birth to children who died in their first year of life, and 25% gave birth to children who had severe birth defects involving the central nervous system. Operative exploration revealed an abnormal posterior "dilated spinal vein" that entered the spinal dura adjacent to the dural penetration of the posterior root of the ninth thoracic spinal nerve. Deep bypass procedures at the skull base are often technically difficult, and the added exposure can be essential for suturing vessels.
The m issing a rte ry eithe r fa ils to fo rm [agenesis] or degenerates early in developm ent. The three surgical and single embolization series included in Table 397-7 are the only published series with more than 10 patients that permit tabulation of the data. N otch signaling is involved in cell proliferation, apoptosis, and epithelial to mesenchymal transitions. The gradual loss of neurological function in these patients suggests a different mechanism of cord injury. The growth factor independence-1 transcription factor: new functions and new insights. Gastrin produces an immediate and dose-dependent histamine Chapter 6 Gastrointestinal Peptides: Gastrin, Cholecystokinin, Somatostatin, and Ghrelin 147 180. Early primary or preantral stage follicle recruited from the pool of primordial follicles. Treatment efficacy is a critical outcome parameter (total/permanent angiographic obliteration of the lesion), as is delay in or failure of obliteration of the lesion. Hunterian proximal arterial occlusion for giant aneurysms of the carotid circulation. These molecules provide a substrate for cells on which they can anchor or migrate. Disruption of Paneth and goblet cell homeostasis and increased endoplasmic reticulum stress in Af02-02-9780123820266/- mice. Stem cells give rise to the rapidly proliferating transit-amplifying progenitors located on the lateral sides of the crypts. Endopeptidases, also known as prohormone convertases, typically cleave polypeptides downstream of two adjacent basic amino acid residues at the general motif (lysine/arginine)-(X)n-(lysine/ arginine), where n 0, 2, 4, or 6 and X is any amino acid, but usually not a cysteine. In another series, just 4 of 30 patients had persistent symptoms (2 with seizure, 1 with ataxia, and 1 with headache). Mouse dispatched mutants fail to distribute hedgehog proteins and are defective in hedgehog signaling. Spinal n erve Nerve form ed by the junction of dorsal and ventral roots at each intervertebral foramen. The first intestinal motility patterns in fetal mice are not mediated by neurons or interstitial cells of Cajal. The importance of treating centers to understand their own risks and expectations in their own treatment context and be able to communicate this with patients and colleagues is extremely important. During development, both Shh and Ihh are expressed in the intestinal epithelium; conflicting data exist on the expression of Shh in the adult intestine. The typical single coiled vessel morphology of these lesions is now acknowledged to be the result of chronic metamorphosis of the vessels of the coronal venous plexus by arterialization of normal pial veins into dilated, serpentine, thin-walled vessels on the surface of the spinal cord. Once the sinus venosus is in corporated into the right atrium, these cells lie in their final position at the base of the interatrial septum. Nevertheless, it was found to be effective in preventing rebleeding over the short term (6 months) in a randomized trial comparing abrupt common carotid occlusion with bed rest in patients with a ruptured posterior communicating aneurysm. A mouse gene homologous to the Drosophila gene caudal is expressed in epithelial cells fromt he embyronic intestine. The liver cords growing into the septum interrupt the course of the veins, and an extensive vas cular network, the hepatic sinusoids, forms. Excretory tubules along the caudal pole of the testis, the paragenital tubules, do not join the cords of the rete testis. If fertilization occurs between a gamete lacking a chromosome and a normal one, monosomy results; if it occurs between a gamete with two copies and a normal one, trisomy results. Cytochrome c release was also related to the dose of indomethacin and occurred immediately prior to and at the same time as caspase 3 activation.
Diseases
When liver cells have invaded the entire septum transversum, so that the organ bulges caudally into the abdominal cavity, mesoderm of the septum transversum lying between the liver and the fore gut and the liver and the ventral abdominal waUbecomes membranous, forming the lesser omentum and falciform ligament, respectively. Note the m ultinucleated appearance of the syncytiotrophoblast, large cells of the cytotrophoblast, and slit-like am niotic cavity. Downregulation of Hedgehog signaling is required for organogenesis of the small intestine in Xenopus. Loss of parietal cell expression of Sonic hedgehog induces hypergastrinemia and hyperproliferation of surface mucous cells. Rodilla V, Villanueva A, Obrador-Hevia A, Robert-Moreno A, Fernandez-Majada V, Grilli A, et al. Transforming growth factor beta 1 is essential for hematopoesis and endothelial differentiation in vivo. In the supine position, the head is turned flat, parallel with the floor, and the neck is flexed such that a finger can be placed between the mandible and clavicle. Cytokine modulation of intestinal epithelial cell restitution: central role of transforming growth factor beta. The suprafacial zone has the following boundaries: laterally, the superior cerebellar peduncle; medially, a vertical line 2 mm lateral to the median sulcus; superiorly, the frenulum veli; and inferiorly, the facial colliculus. The author thanks James Sinnett-Smith for critical reading of the manuscript and for his invaluable help in the preparation of the figures. A number of murine Chapter 14 Molecular Physiology of Gastrointestinal Function during Development 419 E10. They also showed that a crude acid extract of the intestinal mucosa given intravenously stimulated pancreatic exocrine secretion. The effect of granulocyte macrophage-colony stimulating factor on glutathione and lipid peroxidation in a rat model. Gradient-echo images in particular can be used to screen for small occult lesions. At the end of the fourth week, sderotome ceUs become polymorphous and form loosely organized tissue, called mesenchyme, or embryonic connective tis sue. After coursing through the descending aorta, blood flows toward the placenta by way of the two umbilical arteries. Lesions with anterograde flow through larger sinuses have a lower risk than do those with restricted or retrograde flow into sinuses and cortical veins. Identification of stem cells in small intestine and colon by marker gene Lf10-05-9780123820266. Craniosynostosis Premature closure of one or more cranial sutures, leading to an abnorm ally shaped Glossary of Key Terms skull. Despite a non-synonymous mutation in the coding sequence, the amino acid substitution might not exhibit a change in the physical characteristics (phenotype) of the organism or render phenotypic advantages or disadvantages to the organism. Intestinal deletion of Pofut1 in the mouse inactivates notch signaling and causes enterocolitis. Gastrointestinal processing of gastrically administered prostaglandin F2 alpha in suckling and weanling rats. Apoptosis is part of the result of procedures such as small bowel resection and ischemia reperfusion and has been investigated to determine its contribution to the overall response. Sporadic fundic gland polyps: common gastric polyps arising through activating mutations in the beta-catenin gene. Intolerable bruit or ophthalmologic sequelae led to palliative embolization in 43 and surgery in 1. Gradually, the folds extend medially and ventrally, so that by the seventh week, they fuse with the mesentery of the esophagus and with the septum transversum. Innate mechanisms of mucosal defense and repair: the best offense is a good defense.
Similarly, Graf and Pollock found the initial risk for hemorrhage to be approximately 2% per year. Blockade of hedgehog signaling inhibits pancreatic cancer invasion and metastases: a new paradigm for combination therapy in solid cancers. Alternatively, elements that are only active during development must be identified in eukaryotic systems in which differentiation of a cell line can be controlled or in transgenic animal models. It may function as a nutritive organ during the earliest stages of development prior to the establishment of blood vessels. In the spinal cord, they either end in the dorsal horn or ascend through the marginal layer to one of the higher brain centers. In-hospital morbidity and mortality after endovascular treatment of unruptured intracranial aneurysms in the United States, 1996-2000: effect of hospital and physician volume. The former is performed through a paramedian craniotomy 2 to 3 cm off midline and provides direct access to the cerebellomesencephalic fissure, inferior colliculus, fourth cranial nerve, and superior cerebellar peduncle. Withdrawal of growth factors or trophic substances also results in apoptosis of some cells. Participation of c-met in the progression of human gastric cancers: anti-c-met oligonucleotides inhibit proliferation or invasiveness of gastric cancer cells. Early rebleeding from intracranial dural arteriovenous fistulas: report of 20 cases and review of the literature. Intradural extramedullary spinal arteriovenous malformations fed by the anterior spinal artery. Division of the mouse gastric mucosa into zymogenic and mucous regions on the basis of gland features. U Urachus Vestigial remnant of the allantois from the ventral surface of the urogenital sinus to the umbilicus that normaUy regresses to a fibrous cord, form ing the median umbilical ligament. The activities of the processing enzymes are regulated by the intravesicular [Ca2] and by the pH, which is maintained at 5. It positively regulates differentiation of enterocytes and negatively regulates Wnt signaling and proliferation of precursor cells. Initially, the gut tube from the cau dal end of the foregut to the end of the hindgut is suspended from the dorsal body wall by dorsal m esentery. Insulin-like growth factor 1 inhibits apoptosis using the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways. Maternal Serum Screening A search for biochemical markers of fetal status led to development of m aternal serum screen ing tests. Mouse model of colonic adenoma-carcinoma progression based on somatic Apc inactivation. All flow-related aneuryms beyond these locations are distal flow-related aneurysms. Even under ideal operative conditions during the treatment of these lesions, the potential for significant morbidity is high. Although initially only shghtly m otile, spermatozoa obtain full m otility in the epididymis. This is in distinction to a pial arteriovenous malformation, for which primary resection of the draining vein could lead to premature edema/hemorrhage because of a rapid increase in intranidal pressure. The reduction in shear stress will lead to downregulation in the production of nitric oxide synthase, and an increase in pulsatility can lead to an increase in the release of endothelin. Cavernous angiomas and arteriovenous malformations of the spinal epidural space: report of 11 cases. Iron in milk is bound to lactoferrin, transferrin, or both, depending on the mammalian species,334 and these interactions enhance iron transport in the neonatal period.
Once definitive endoderm is established, inwardiy moving epiblast form s mesoderm. Production of transforming growth factor alpha in human pancreatic cancer cells: evidence for a superagonist autocrine cycle. Later, the sinus venosus assumes this function, and as the sinus is incorporated into the right atrium, pacemaker tissue hes near the opening of the superior vena cava. These animals fail to synthesize the pro-apoptotic lipid, ceramide, in endothelia following irradiation. No evidence of further hemorrhage or neurological deterioration attributed to the radiosurgical treatment was noted. Regulation of intestinal epithelial cell growth by transforming growth factor type b. The onset of saturable copper transport correlates with increased expression of metallothionein, which is an intracellular regulator of copper and zinc homeostasis. Role of butyric acid and its derivatives in the treatment of colorectal cancer and hemoglobinopathies. At the which should not be seen during the exposure, is situated anterior and medial to the anterior third of the condyle. Initial successful applications of exome sequencing to identify gene associations have focused on early onset cases and/or extended families following a Mendelian-like. Interestingly, calbindin-D9k-null mice are not different from wild-type mice in phenotype and Ca2 serum level. Characterization of mouse A33 antigen, a definitive marker for basolateral surfaces of intestinal epithelial cells. In a fetal sheep model, in utero ligation of the esophagus deprives the distal intestine from growth regulatory peptides in amniotic fluid. Similarly, other studies have also reported prospective hemorrhage risks of 1% to 3% per year. It should also be underscored that both proteins are restricted to the arterial endothelium, with exclusion from the venous circulation. Prolonged steroid use, immunosuppressive therapy, immunodeficient states, and exposure to broad-spectrum antibiotics have been implicated in their development. Because these syndromes depend on whether the affected genetic material is inherited from the mother or the father, they are also an example of imprinting. Neuroform is a flexible, nitinol, self-expanding, microcatheter-delivered microstent. Thus, the contractile behavior of smooth muscle cells in regions with slow wave activity is naturally periodic, leading to motility patterns such as peristalsis and segmentation. These cells secrete calcitonin, a hormone involved in regulation of the calcium level in the blood (Table 17. In the other projection, it is useful to try and achieve a "down-the-barrel" view of the inflated balloon within the de novo stent-reconstructed vessel that depicts the balloon centrally surrounded circumferentially by the coil mass. For most complex genetic diseases examined, the variants identified to date do not fully account for the predicted heritability, or the observed extent of clustering of disease cases within families. Recent advances in the understanding of angiogenesis and vascular remodeling have been providing new insight into the pathophysiology of vascular malformations in the brain, as a result of which potential new treatment strategies are emerging. Phosphorylation of the retinoblastoma gene product is modulated during the cell cycle and cellular differentiation. C horoid plexuses Vascularized structures formed in the lateral, third, and fourth ventricles that produce cerebrospinal fluid. Loss of association between activated G[alpha]q and G[beta][gamma] disrupts receptor-dependent and receptor-independent signaling. The largest is the anterior fontanelle, sometimes called the "soft spot," located where the two parietal and two frontal bones meet. Brain and gastrointestinal cholecystokinin receptor family: structure and functional expression. Luminal effectors, paracrine effectors, nervous stimuli, and hormonal secretions all play a role in regulating transport function during postnatal development. At its apex, the loop remains in open connection with the yolk sac by way of the narrow vitelline duct.
Chapter 11 Notch Pathway Regulation of Intestinal Cell Fate 345 mathematical modeling suggested that it would be likely to occur in early progenitor cells in the crypts to initially split differentiating cells to the absorptive and secretory lineages. When a significant amount of posterior temporal lobe retraction is needed with or without conversion to a half-and-half or extended lateral approach, it is wise to coagulate and cut the veins that drain the temporal lobe into the sphenoparietal sinus. In addition, okadaic acid increased the phosphorylation of these proteins similar to the effects of polyamine depletion. These two stem cell populations and their possible relationship to each other have not been fully described. The greatest increase in apoptosis occurred at the villous tip, perhaps reflecting the possibility that hypoxia was greatest in this region. Profile and prevalence of aspirin resistance in patients with cardiovascular disease. This chapter summarizes the current understanding of canonical Notch signaling mechanisms and highlights the important role that Notch plays in the intestine to regulate proliferation and cell fate determination in the epithelium. The magnitude of this is affected primarily by the amount of metal surface area coverage provided by the stent. This deterioration is unpredictable and indicates severe venous congestion, which unless treated immediately will lead to venous thrombosis, infarction, and irreversible loss of neurological function. Simultaneously, the central part of the horseshoe-shaped tube expands to form the future outflow tract and ventricular regions. Later in gestation, the placenta assumes the role of hormone production, and the corpus luteum degenerates. The latter cells may be committed to one, or at most two, lineages, and are capable of undergoing a number of symmetric divisions, amplifying the population and giving rise to fully differentiated cells or mature cells, which have lost the capacity to divide. Induction of apoptosis by excessive polyamine accumulation in ornithine decarboxylase overexpressing L1210 cells. Both transforming growth factor-beta and substrate release are inducers of apoptosis in a human colon adenoma cell line. Corpus Luteum After ovulation, granulosa cells remaining in the wall of the ruptured follicle, together with cells from the theca interna, are vascularized by surrounding vessels. Notch signaling regulates formation of the three-dimensional architecture of intrahepatic bile ducts in mice. At least six aquaporin isoforms (aquaporin 1, 3, 4, 5, 8, and 9) have been identified in the digestive system. Monorail snare technique for the recovery of stretched platinum coils: technical case report. The esophagus fails to develop normally, and only a small esophageal remnant is found in Shh/ mice at later stages of development. A B this communication may occur at the time of trauma or in delayed fashion if a false aneurysm is created that ruptures later. In addition, estrogens act on the extemal genitalia at the indifferent stage to form the labia majora, labia minora, clitoris, and lower vagina. Typically, they respect no tissue boundaries and can involve bone, muscle, skin, spinal canal, spinal cord, and nerve roots along an entire spinal level. F, Postoperative computed tomography shows the thrombectomy and reduction in mass effect. We then turn our attention to the arachnoid, which tethers the gyrus rectus to the optic nerve. Quantitative assessment of glial cells in the human and guinea pig enteric nervous system with an anti-Sox8/9/10 antibody. At one end of the spectrum, Mendelian diseases are typically caused by rare mutations of large functional and statistical effects. The dura should be opened in the midline, and the arachnoid is preserved to avoid laceration of any delicate, distended, or adherent vessels underlying the dural opening.
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