10 Week Histology PDF

Tongue (39, HE) Skeletal muscle (12, iron-H) Cardiac muscle (13, HE) Cardiac muscle (13A, PTAH) Colon (53n, HE) The text and graphics of this handout are the property of the University of Debrecen and are protected by exclusive use rights! Tongue (39, HE) This specimen is a sagittal secti...

Tongue (39, HE) Skeletal muscle (12, iron-H) Cardiac muscle (13, HE) Cardiac muscle (13A, PTAH) Colon (53n, HE) The text and graphics of this handout are the property of the University of Debrecen and are protected by exclusive use rights! Tongue (39, HE) This specimen is a sagittal section of the tip of the tongue. The tongue is ideally suited for the demonstration of muscle tissue since it comprises visceral striated muscle fibres running in every direction, the morphology of which is identical to that of skeletal muscle fibres. These muscles, as opposed to cardiac and smooth muscle, do not contain individual muscle cells. Instead of them, muscle fibres are the basic unit of skeletal muscle, which is a morphological and functional syncytium. Since muscle fibres run in every direction, the specimen displays them in cross, longitudinal and oblique sections. Have a closer look at the longitudinal section of muscle fibres and notice that the elongated, stick-like nuclei are always at the periphery of the multinucleated muscle fibres. The nuclei are euchromatic and have rounded edges. Based on these features, it is easy to distinguish them from fibrocyte nuclei. The heterochromatic fibrocyte nuclei with pointed ends are present in the narrow connective tissue spaces between muscle fibres. Muscle fibres are surrounded by a plasma membrane (in other words, sarcolemma in its modern, narrower sense), lamina externa (a structure similar in composition to basal lamina) and a fine reticular fibre-rich sheath, the endomysium. These structures are not identifiable separately under a light microscope. Satellite cells involved in muscle regeneration are situated between the muscle fibre plasma membrane and the lamina externa, which cannot be distinguished from muscle fibre nuclei. On a longitudinal section of a muscle fibre, observe the eosinophilic, linear structures running parallel with the long axis of muscle fibres within the sarcoplasm; these are the myofibrils. Along these myofibrils, perpendicular to their long axis, an alternating pattern of darker and lighter stripes is visible, which is responsible for the cross striation – note that these can only be seen at higher quality, sharper regions of this virtual specimen. The darker stripes are the A bands, and the lighter ones are the I bands. Cross striation is based on the highly organised, periodic arrangement of thin and thick myofilaments within the myofibrils, which creates an alternating sequence of areas with stronger and weaker light absorbance following histological staining. The thinner bands (Z, H and M lines) present in the sarcomeres, the contractile (functional) units of skeletal muscle fibres, are not visible in this specimen (but they are clearly identifiable in the electron microscopic images). The nuclei are obviously also located on the periphery of muscle fibres cut in cross section. Cross striations are not identifiable on cross sections. The minute eosinophilic points within the muscle fibres correspond to the cross sections of myofibrils. Here and there, the fine reticular connective tissue of the endomysium, and little blood vessels (capillaries) within that are visible. On the periphery of fascicles (bundles of muscle fibres) the vascularized connective tissue is named perimysium. Observe also the (mostly) stratified squamous non-keratinized epithelium on the natural surface of the tongue. The connective tissue layer below the epithelium forms connective tissue papillae. Identify mucous and serous acini of small salivary glands, and their associated excretory ducts, at the inferior part of the tongue (which in this case lie close to the artificial, cut surface). In addition, the organ also contains white adipose tissue with univacuolar adipocytes, as well as cross and longitudinal sections of peripheral nerves. Skeletal muscle (12, iron-H) This specimen features skeletal muscle fibres, primarily in longitudinal section, but – especially towards the periphery – cross sections of muscle fibres are also visible. Iron-haematoxylin staining (a variant of conventional haematoxylin) makes cross striations stand out much better. In addition to the dark A and the light I bands, this specimen also features the lighter H zone at the centre of the A band and the darker Z line (forming the border of sarcomere) at the centre of the I band. Here and there, adipocytes are visible between muscle fibres. For the peculiarities of muscle tissue, please refer to the previous section.  Figure 11.10a Detailed structure of sarcomere with the distinguishable bands X35,000. Figure 11.5 Electron micrograph of skeletal muscle fiber. This low-magnification electron micrograph shows the general organization of skeletal muscle fibers. Small portions of three muscle fibers in longitudinal profile are included in this micrograph. The muscle fiber on the right reveals a nucleus at its periphery. Two fibers—one in the middle and another on the left —exhibit regular profiles of myofibrils separated by a thin layer of surrounding sarcoplasm (Sr). Each repeating part of the myofibril between adjacent Z lines is a sarcomere (S). The cross-banded pattern visible on this micrograph reflects the arrangement, in register, of the individual myofibrils (M); a similar pattern found in the myofibril reflects the arrangement of myofilaments. The detailed features of a sarcomere are shown at higher magnification in Figure 11.10a. The presence of the connective tissue in the extracellular space between the fibers constitutes the endomysium of the muscle. ×6,500. Images and text for Fig. 11.5 are from Wojciech Pawlina: Histology, a text and atlas (8th ed.) Wolters Kluwer, thePoint. Cardiac muscle (13, HE) This specimen features cardiac muscle tissue, which belongs to the group of striated muscles. Cardiac muscle consists of individual muscle cells termed cardiomyocytes. Cross striation is identifiable on the longitudinally cut cardiomyocytes, where the darker A and the lighter I bands are distinguishable. The euchromatic nucleus usually also harbours a nucleolus. The round or oval shaped nuclei are typically located at the centre of the cells. Close to the nucleus, cardiomyocytes often display a yellowish discoloration, which corresponds to lipofuscin, an ageing pigment which accumulates during the extremely long life span of the cells (cardiomyocytes practically do not renew). Cardiomyocytes are often branched and are X or Y shaped. The borders of adjacent cardiac muscle cells are called intercalated discs or lines of Eberth. These borders appear as dark transverse lines thicker than the A bands. Along the cell borders, cell junctions only visible at the electron microscopic level (see EM image) are present. The barely visible eosinophilic bands running parallel to one another are the myofibrils. The sarcoplasm displays lighter areas corresponding to glycogen stores, which have been lost during conventional tissue processing. In cross sectional images of cardiomyocytes, nuclei are still found at the centre of the cells, and the eosinophilic dots sometimes visible in the sarcoplasm are cross sections of myofibrils. The fine connective tissue (endomysium) between cardiomyocytes contains the elongated, heterochromatic nuclei of fibrocytes, blood vessels, and adipocytes. Cardiac muscle (13A, PTAH) This specimen was stained using phosphotungstic acid–haematoxylin, abbreviated as PTAH. This staining method enables a better visibility for cross striation, A and I bands, and the lines of Eberth (darker and thicken than A bands) on longitudinal sections of cardiomyocytes. The specimen also features cardiac muscle cells in cross section; however, at that plane, neither cross striations nor lines of Eberth are visible. At the outer edge of the specimen, the external layer of the heart, the epicardium is visible, which is lined by mesothelium. Below that, identify the subepicardial connective tissue, with the blood vessels of the heart embedded in adipose tissue. For any other particulars of cardiac muscle, please refer to the description for the HE stained specimen above.  FIGURE 11.21 Structure of cardiac muscle. This electron micrograph reveals portions of two cardiac muscle cells joined by an intercalated disc. The line of junction between the two cells takes an irregular, step-like course, making a number of nearly right-angle turns. In its course, different parts of the intercalated disc are evident. These include the transverse components (fascia adherens and maculae adherentes) and lateral components (gap junctions and maculae adherentes). The macula adherens (MA) is enlarged in inset 1 (×62,000). The fascia adherens (FA) is more elongated than the macula adherens. In contrast to the macula adherens, the fascia adherens occupies a much larger area along the irregular outline of the transverse component of the intercalated disc. The gap junction (GJ) is enlarged in inset 2 (×62,000). The fascia adherens (FA) is enlarged in inset 3 (×62,000). The fascia adherens of the intercalated disc corresponds to the zonula adherens of epithelial tissues. Other features typical of cardiac muscle are also present: mitochondria (Mi), sarcoplasmic reticulum (SR), and components of the sarcomere, including Z lines (Z), M line (M), and myofilaments. This particular specimen is in a highly contracted state, and consequently, the I band is practically obscured. ×30,000. (Part a reprinted with permission from Zhang L , Ina K , Kitamura H , et al. The intercalated discs of monkey myocardial cells and Purkinje fibers as revealed by Image and text are from Wojciech Pawlina: Histology, a text and atlas (8th ed.) Wolters Kluwer, thePoint. scanning electron microscopy. Arch Histol Cytol 1996 ; 59 : 453 – 465.) Colon (53n, HE) This specimen has been sectioned parallel to the longitudinal axis of the colon. The wall of the large intestine comprises a considerably thick smooth muscle layer, which is involved in the passage of intestinal contents and its local mixture. That thick smooth muscle layer consists of an inner (closer to the lumen) circular and an outer longitudinal layer. Smooth muscle cells in the inner circular layer are oriented along concentric circles, whose planes are perpendicular to the long axis of the gut; cells in the longitudinal layer are oriented parallel with the long axis of the gut. Smooth muscle cells (myocytes) in the circular layer are therefore visible in cross section, and those in the longitudinal layer are in longitudinal section in this specimen. The longitudinal section of smooth muscle cells displays a spindle-shaped euchromatic nucleus with rounded ends, which is centrally located. The cytoplasm surrounding the nucleus may display seemingly empty regions, which correspond to accumulated glycogen lost during routine tissue processing. The nucleus is sometimes a bit wavier, which indicates that the myocyte was contracted at the time of fixation. Smooth muscle cells do not display cross striation, since their myofilaments are not oriented parallel with each other, they instead form a network. That arrangement does not allow for a periodic striation to develop. Note the apparently lighter eosinophilic staining of smooth muscle cells relative to collagen fibres found in the surrounding connective tissue. In a cross section of a myocyte, the central nucleus – if it is present at the plane of sectioning – appears round (it is, in fact, elongated, as evident in a longitudinal section). In most of the cases, however, the nucleus does not show in myocyte cross sections, since the nucleus is much shorter than the cell itself, and the thickness of the slide is too small for the difference in length between nucleus and cell. Therefore, the plane of sectioning usually does not contain the nucleus. In between the inner circular and outer longitudinal smooth muscle layers, somewhat more lightly stained nervous tissue is identifiable, which is responsible for the innervation of the smooth muscle layers. The strongly heterochromatic, elongated nuclei visible in smooth muscle tissue here and there actually belong to fibrocytes (if that nucleus has sharp tips and is located farther away from blood vessels) or endothelial cells (muscle tissue is vascular). Remarks on Histology hand-outs and Histology structure lists Structure lists Every effort has been made to include all identifiable structures in our slides that are required from students to be recognized during written tests or oral examinations. However, as the bulletin states on the requirements, the basis of all histology exams is the material found in official textbooks, delivered during lectures and discussed during practicals/seminars. Therefore, even if a structure is missing from these structure lists but mentioned in official textbooks or presented on lectures or shown during practicals/seminars and recognizable in our compulsory slides then the identification of such structure may be required during written tests and oral exams. On the other hand, students are encouraged to let the examiner and the Department know if such a histological structure is missing from our lists. Students are also expected to be aware that final exam in Anatomy, Histology and Embryology includes all material taught in the 1 st year even if these first-year structures are not included in these lists. Hand-outs These are not to replace the official textbooks and notes expected to be taken by students during lectures and practicals/seminars. The purpose of these hand-outs is to ease and assist the first step of your learning curve. Therefore, knowledge of material presented in hand-outs is definitely not a guarantee for a successful written test or oral exam. Please, do not forget that during written tests and oral exams demonstration of theoretical background of histological sections is also required and this theoretical background is not included in our hand-outs.