HISTO LEC 203 CAPILLARIES - PDF

(003) CAPILLARIES DR. PEEBLE AGDAMAG | 01/11/21 with the blood sh...

(003) CAPILLARIES DR. PEEBLE AGDAMAG | 01/11/21 with the blood shunted away by contracted sphincters. At any OUTLINE given moment, most sphincters are at least partially closed I. I. CAPILLARIES and blood enters the capillary bed in a pulsatile manner for II. TYPES OF CAPILLARIES maximally efficient exchange of nutrients, wastes, O2, and A. CONTINUOUS CAPILLARIES CO2 across the endothelium. except in the pulmonary B. FENESTRATED CAPILLARIES circulation, blood enters the microvasculature well C. DISCONTINUOUS CAPILLARIES oxygenated and leaves poorly oxygenated. III. PERICYTES IV. CLINICAL APPLICATION  single layer of endothelial cells rolled up as a tube A. DIABETIC MICROANGIOPATHY  make up > 90% of body’s vasculature, V. VENULES  The average diameter of capillaries varies from 4 to 10 μm, which allows transit of blood cells only one at a time, and their individual length is usually not more than 50 μm. I. CAPILLARIES  These minute vessels make up over 90% of the body’s  permit and regulate metabolic exchange between blood vasculature with a total length of > 100,000 km and total and surrounding tissues surface area of= 5000 m2  always function in groups called Capillary beds  cyclical opening and closing of sphincters, → - size and overall shape conforms to structure applied essentially empty capillaries at any given time, - richness of capillary network related to metabolic  with only about 5% (~300 mL in an adult) of the total blood activity of tissues volume moving through these structures.  High metabolic rates (kidney, liver, and  thin walls, extensive surface area, and slow, pulsatile cardiac muscle) blood flow optimize capillaries for exchange of water  Low metabolic rates (smooth muscle and andsolutes between blood and tissues. dense connective tissue)  capillary cells have many features specialized for  Supplied preferentially by one or more terminal arteries molecular transfer by mechanisms ranging from simple branches called Metarterioles (encircled by scattered diffusion to transcytosis. smooth cells)  basal lamina helps determine which macromolecules - Continuous with thoroughfare channels (lack interact with endothelial cells. muscle) connected with post-capillary venules.  Ave. thickness only 0.25 μm  True capillaries branch from metarterioles  The cytoplasm contains mitochondria and most other - At beginning of each true capillary, muscle fibers act organelles, along with a typically large population of as precapillary sphincters that contract or relax to membrane vesicles. control blood entry.  distinctive feature is often nucleus curved to - Contract 5 to 10 cycles per minute, causing blood to accommodate the very small tubular structure pass through capillaries in a pulsatile manner  Along with basal lamina, junctional complexes between - When sphincters closed, blood flows directly from the cells maintain the tubular structure, with variable the metarterioles and thoroughfare channels into numbers of tight junctions having an important role in post capillary venules. capillary permeability.  Major structural variations in capillaries occur in organs with various functions that permit very different levels of metabolic exchange. Capillaries are generally grouped into three histologic types, depending on the continuity of the endothelial cells and the external lamina  Each capillary consists of a thin endothelium, an underlying basal lamina, and a few randomly scattered pericytes. These cells surround the capillaries with branching cytoplasm and are enclosed by a basal lamina that also encloses the capillary endothelium. Figure 1. Part a shows a well-perfused capillary bed with all the sphincters relaxed and open; part b shows a capillary bed Page 1 of 5 CMED 1EF  (003) CAPILLARIES DR. PEEBLE AGDAMAG | 01/11/21  basal lamina continuous and covers fenestrations.in II. TYPES OF CAPILLARIES organs with rapid interchange of substances between tissues and the blood A. Continuous capillaries  kidneys  intestine  choroid plexus  endocrine glands  Some fenestrations are covered by very thin diaphragms of proteoglycans; others may represent membrane invaginations during transcytosis that temporarily involve both sides of the very thin cells C. Discontinuous capillaries Figure 2. Continuous  tight, well-developed occluding junctions between slightly overlapping endothelial cells → continuity along endothelium  well-regulated metabolic exchange across cells  most common  Examples:  Muscle  connective tissue  lungs  exocrine glands Figure 4. Discontinuous  nervous tissue  Ultrastructurally numerous vesicles → transcytosis of  commonly called sinusoids macromolecules in both directions  permit maximal exchange of macromolecules  allow easier movement of cells between tissues and B. Fenestrated capillaries blood  Individual endothelial cells large perforations without diaphragms;  collectively form a discontinuous layer, with wide, irregular spaces between the cell  differ from other capillaries by having highly discontinuous basal laminae and much larger diameters ,often 30 to 40 μm, which slows blood flow  found in the  liver  spleen  some endocrine organs  bone marrow  It is at various locations along continuous capillaries and postcapillary venules are mesenchymal cells called Figure 3. Fenestrated pericytes  have a sieve- like III. PERICYTES  also have tight junctions, but perforations (fenestration) through the endothelial cells allow greater exchange  Mesenchymal cells along continuous capillaries and across endothelium. postcapillary venules  endothelial cells penetrated by numerous small circular  Long cytoplasmic processes partly surrounding endothelial openings or fenestrations ∼ 80 nm diameter layer  Secrete many ECM components Page 2 of 5 CMED 1EF  (003) CAPILLARIES DR. PEEBLE AGDAMAG | 01/11/21  Produce own basal lamina, which may fuse with endothelial  Postcapillary venules converge into larger collecting venules cells that have more distinct contractile cells. With increasing size  With well-developed cytoskeletal networks of myosin, actin,  venules become surrounded by a recognizable tunica media and tropomyosin which indicate that pericytes also dilate or with two or three smooth muscle layers and are called constrict capillaries, helping to regulate blood flow in some muscular venules. A characteristic feature of all venules is organs the large diameter of the lumen compared to the overall  Contractile function to facilitate flow of blood cells. thinness of the wall  Within the CNS, pericytes are important for maintaining the  Functions endothelial blood-brain barrier  Collect blood from capillaries  After tissue injuries, pericytes proliferate and differentiate to  Respond to vasoactive agents (e.g., histamine, serotonin) by altering permeability form smooth muscle and other cells in new vessels as the  Site of exchange of materials between tissue fluid and microvasculature is re-established blood  Pericyte population also includes mesenchymal stem cells  Site of exit of WBCs from blood into tissue important for regeneration of other tissues Figure 5. Capillaries consist only of an endothelium rolled as a tube, across which molecular exchange occurs between Figure 6 blood and tissue fluid. (a) Capillaries are normally. associated with perivascular contractile cells called pericytes (P) that have a variety of functions. The more flattened nuclei belong to endothelial cells. X400. H&E of a spread mesentery preparation IV. CLINICAL APPLICATION V. Diabetic microangiopathy Hyperglycemia (excessive blood sugar that occurs with diabetes) → diabetic microangiopathy  Diffuse thickening of capillary basal laminae  Concomitant decrease in metabolic exchange at these blood vessels, particularly in kidneys, retina, skeletal muscle, and skin. Figure 7 V. VENULES VI. (a) Compared to arterioles (A), postcapillary venules (V) have large  The transition from capillaries to venules occurs gradually. lumens and an intima of simple endothelial cells, with occasional Postcapillary venules (Figure 11–21a) are similar to pericytes (P). (X400; Toluidine blue [TB]) capillaries with pericytes but larger, size varies from 10 microns (post-capillary venules) to 1 mm (muscular venules)  Larger diameter than capillaries; consist of endothelium surrounded by pericytes  Venules are the primary site at which white blood cells adhere to endothelium and leave the circulation at sites of infection or tissue damage Page 3 of 5 CMED 1EF  (003) CAPILLARIES DR. PEEBLE AGDAMAG | 01/11/21 VI. TEST YOUR KNOWLEDGE 1. Which description is true of continuous capillaries? a. Unusually wide lumens b. Most common in both brain and muscle c. Abundant fenestrations d. Lack a complete basement membrane e. Phagocytic cells often seen inserted in the intercellular clefts 2. Which of the following is true of pericytes? a. Are associated with the basal lamina of capillary endothelial cells b. Have similar histological features as contractile cells of the myocardium Figure 8 c. Form a layer of cells joined by gap junctions (b) Larger collecting venules (V) have much greater diameters d. Are terminally differentiated than arterioles (A), but the wall is still very thin, consisting of an e. Capable of forming multinucleated muscle fibers endo- thelium with more numerous pericytes or smooth muscle cells. (X200; H&E) 3. During light microscopic examination of a tissue, you note a vessel that has no smooth muscle but a large amount of connective tissue at its periphery. Which of the following vessels are you examining? a. Arteriole b. Venule c. Elastic artery d. Capillary e. Large vein 4 – 7. Matching Type. Choose the best answer a. Continuous capillaries b. Fenestrated capillaries Figure 10 c. Sinusoids (c) The muscular venule cut lengthwise here has a better defined d. Venules tunica media, with as many as three layers of smooth muscle (M) in some areas, a very thin intima (I) of endothelial cells (E), and a more 4. Site of adhesion to endothelium, and exit of WBC from distinct adventitia (Ad). Part of an arteriole (A) shows a thicker wall blood to tissue than the venule. (X200; Masson trichrome) 5. Transcytosis of macromolecules in both directions Postcapillary venules are important as the site in the vasculature where these cells leave the circulation to become functional in the 6. Individual endothelial cells large perforations without interstitial space of surrounding tissues when such tissues are inflamed or infected. diaphragms 7. Important for rapid interchange of substances between tissues and the blood 8-10. Identification. 8. Lacks muscle connected with post-capillary venules 9. Precapillary sphincters contract how many cycles per minute to cause blood to pass through capillaries in a pulsatile manner 10. Capillaries permit and regulate metabolic exchange Figure 9 between blood and surrounding tissues always function (d) Postcapillary venule (V) from an infected small intestine shows in groups called ______ several leukocytes adhering to and migrating across the intima. (X200; H&E) 11. What supplies a capillary? Page 4 of 5 CMED 1EF  (003) CAPILLARIES DR. PEEBLE AGDAMAG | 01/11/21 12. It is the most common type of capillary 11METATERIOLES; 12CONTINUOUS; CHANNELS; 9 (5-10); 10CAPILLARY BED; Answer: 1B; 2A; 3B; 4D; 5A; 6C; 7B; 8THOROUGHFARE VII. REFERENCES 1. Junqueiras, Basic Histology Text & Atlas 13th Edition 2. Victor P. Eroschenko, Atlas of Histology with Functional Correlations, 13th Edition Page 5 of 5 CMED 1EF