Veterinary Histology and Embryology PDF

Veterinary Histology and Embryology Cat embryo. https://www.vetmeduni.ac.at/en/histology-and-embryology DVM. Leonardo Martín. PhD Muscle – Introduction Contractivity is one of the fundamental properties of protoplasm and is exhibi...

Veterinary Histology and Embryology Cat embryo. https://www.vetmeduni.ac.at/en/histology-and-embryology DVM. Leonardo Martín. PhD Muscle – Introduction Contractivity is one of the fundamental properties of protoplasm and is exhibited in varying degree by nearly all cell types. In the cells of muscle, the ability to convert chemical energy into mechanical work has become highly developed. Locomotion of multicellular animals, beating of their hearts, and movement of their internal organs depends on muscles of different types. Cardiac muscle Skeletal muscle Smooth muscle Objectives Identify smooth, skeletal, and cardiac muscle on route histological preparations Explain the morphological basis for the different functions of these three types of muscle Distinguish between the modes of excitation of these three types of muscle  Muscle Smooth Muscle Light Microscopic Structure Fine Structure Contraction Myogenesis, Hypertrophy, and Regeneration Skeletal Muscle Light Microscopic Structure Fine Structure Contraction Classification of Skeletal Muscle Fibers Myogenesis, Hypertrophy, Atrophy, and Regeneration Cardiac Muscle Light Microscopic Structure Fine Structure Cardiac Nodes and Impulse Conduction Fibers Contraction Myogenesis, Hypertrophy, and Regeneration  Muscle Skeletal muscle Heart Intestine, blood vessels, uterus, esophagus (wall of hollow organs) Muscle - Histological identification Skeletal muscle – very long cylindrical Dilator muscle of iris striated muscle cells with multiple peripheral nuclei Myoepithelial cells Cardiac muscle – short branching striated muscle cells with centrally located nuclei Smooth muscle – closely packed spindle-shaped cells with a single centrally placed nucleus and cytoplasm that appears homogeneous by light microscopy  Muscle Distribution: Skeletal – striated muscles mostly associated with the skeleton  Muscle Distribution: Cardiac – striated muscles associated large artery with the heart of lung Muscle Distribution: Smooth – fusiform cells associated with the viscera, respiratory tract, blood vessels, uterus, etc. Smooth muscle Ureter Ductus deferens Muscle  Smooth Muscle Light Microscopic Structure Smooth muscle. A. Cross section. B. Longitudinal section. The central myocyte nuclei (solid arrows) are absent in several cross sections due to sectional geometry. The tip of a spindle-shaped cell is visible at the dotted arrow. Fibroblast nuclei (open arrow) are dark and smaller than smooth muscle nuclei. Hematoxylin and eosin (x490).  Smooth Muscle Light Microscopic Structure Smooth muscle cells are elongated, spindle-shaped cells. Each cell contains a single, centrally located nucleus. The cells range from 5 to 20 μm in diameter and from 20 μm to 1 mm or more in length. The cytoplasm of smooth myocytes is acidophilic (so they stain pink – red). Within a tissue section, the cross-sectional size of cells is highly variable due to the tapered shape of the cells. Many cross sections of the cell lack nuclear profiles because of the extent of the cell beyond the central nuclear region.  Smooth Muscle Light Microscopic Structure Individual myocytes are surrounded by a fine network of reticular fibers, blood vessels, and nerves. Unlike the skeletal muscle fibers, smooth muscle cells lacks the striated appearance. Smooth Skeletal (striated) muscle muscle  Smooth Muscle Contraction The contractile apparatus of smooth muscle is capable of greater shortening in length and more sustained contractions than that of striated muscle. Contraction is governed by the phosphorylation of the myosin-II molecule in contrast to striated muscle, which is regulated by a troponin–tropomyosin complex. The contraction sequence begins with an increase of calcium in the smooth muscle cell cytoplasm. Calcium increases by entering the cell through voltage-dependent calcium channels in the cell membrane or by inositol 1,4,5-triphosphate (IP3)-induced release of calcium from the smooth endoplasmic reticulum.  Smooth Muscle Contraction mechanism The rise in cytosolic calcium leads to subsequent binding of the calcium to calmodulin. The calcium–calmodulin complex then interacts with myosin light-chain kinase, which initiates phosphorylation of myosin-II and interaction between the actin and myosin-II myofilaments. The overall process leading to actin–myosin interaction is longer when compared with other muscle types, which results in the relatively slow contraction of smooth muscle. Smooth Muscle  Smooth Muscle Contraction modulation Hormones that increase cAMP concentration stimulate contraction. (e.g., Estrogens, oxytocin) cAMP activates myosin light-chain kinase, leading to phosphorylation of myosin and cell contraction. Hormones that decrease cAMP concentration reduce muscle contraction (e.g., progesterone)  Smooth Muscle Endocrine agonist of smooth muscle contraction Effector action Estrogen/testosterone contraction Norepinephrine/epinephrine contraction Acetylcholine contraction Angiotensin contraction Vasopressin contraction Oxytocin contraction Histamine contraction Serotonin contraction Progesterone relaxation  Smooth Muscle Contraction modulation Contraction of smooth muscle is involuntary and respond to the action of the sympathetic and/or parasympathetic nervous systems Unitary smooth muscle, found in the wall of visceral organs, behaves as a syncytium that contracts in a networked fashion (peristalsis). Cells of this arrangement of smooth muscle are extensively connected by gap junctions but sparsely innervated. In contrast, multiunit smooth muscle, found in the iris of the eye, is capable of precise contractions due to individual innervation of each myocyte. The multiunit myocytes lack gap junctions, resulting in reduced coordinated communication between cells.  Smooth Muscle Myogenesis, Hypertrophy, and Regeneration. Smooth muscle tissue increases in size by both hypertrophy (increase in size) and hyperplasia (increase in number) of myocytes. New smooth muscle cells can form through mitosis or by derivation from pericytes. Formation of new myocytes is limited, so healing of smooth muscle is mainly through connective tissue (scar formation).  Skeletal Muscle Light Microscopic Structure Skeletal muscle myocytes are elongated cells that range from 10 to 110 μm in diameter and can reach up to 50 cm in length. These fibers are derived from the prenatal fusion of many individual mononuclear myoblasts. As a result of the fusion, a single myocyte contains multiple oval nuclei, which are peripherally located within the cell. When viewed in longitudinal section, transverse striations are present as alternating light and dark bands. In transverse section, the myocyte has an angular outline and a stippled cytoplasm.  Skeletal Muscle Light Microscopic Structure Peripheral nuclei may be absent in some planes of the cross section of the myocyte. The surrounding cell membrane is visible at higher magnification. Each muscle cell contains myofibrils, which form the dots in cross sections of the fiber at the light microscopic level. The myofibrils are cylindrical and 1 to 2 μm in diameter. Individual myofibrils are composed of thick and thin myofilaments, which are responsible for contraction. The myofibrils align in a longitudinal direction to create the light and dark banding pattern (striated) of the myocyte.  Skeletal Muscle Light Microscopic Structure Thick and thin myofilaments overlap in the darker A band (anisotropic), whereas only thin myofilaments are present in the lighter I band (isotropic). Satellite cells are spindle-shaped cells located adjacent to the cell membrane of the myocyte and within its basement membrane. They are thought to represent a population of inactive myoblasts, which can be activated upon injury to initiate regeneration of muscle fibers. Striated Muscle (Skeletal) Repeating A and I bands alone the cell’s length creates repeating sarcomeres A I A I A I A I  Skeletal Muscle Light Microscopic Structure Individual myocytes are bound together into primary bundles or fascicles (Fig. 5-7). Within a fascicle, an individual myocyte is surrounded by reticular fibers, which form the endomysium. Nerve fibers and an extensive network of continuous capillaries are also present in the endomysium. Each fascicle is surrounded by dense irregular connective tissue, termed the perimysium. Supplying blood vessels and nerves plus muscle stretch receptors (muscle spindles) are located in the perimysium.  Connective tissue layers of skeletal muscle Epimysium - coarse CT Perimysium - less coarse CT Endomysium - delicate CT Perimysium Epimysium Endomysium Connective Tissue Layers of Skeletal Muscle Endomysium Individual cell Striated Muscle Skeletal Cardiac A I “A” Band = dark band Anisotropic = does alter polarized light A (Birefringent) I “I” Band = light band Isotropic = does not alter polarized light Striated Muscle (skeletal) A I Sarcomeres are organized for rapid and highly controlled contraction  Striated Muscle (Skeletal) Sarcomere = structural unit and functional unit of striated muscle  Skeletal Muscle Light Microscopic Structure Most muscles are surrounded on the outer surface by a dense irregular connective tissue layer, the epimysium. The connective tissues of skeletal muscle are interconnected and provide a means by which contractile forces are transmitted to other tissues.  Skeletal Muscle Light Microscopic Structure transversal section Longitudinal section Skeletal muscle, cross section. The Skeletal muscle, longitudinal section. nuclei in the sparse endomysium Notice the cross-striations and the (arrows) belong to either fibroblasts nuclei located in the periphery of or satellite cells. Hematoxylin and the myocytes. Hematoxylin and eosin. eosin.  Skeletal Muscle Light Microscopic Structure The myofibrils of skeletal muscle are comprised of myofilaments. Smooth endoplasmic reticulum surrounds each myofibril and forms terminal cisternae near the T tubule. T tubules extend into the cytoplasm from the cell membrane and surround the myofibrils at the A–I junction. A T tubule plus two terminal cisternae form a triad structure. Peripheral nuclei of the skeletal muscle myofiber are not shown in this illustration.  Skeletal Muscle Myofilament's structure myofilaments of skeletal muscle cells are primarily actin or myosin-II myofilaments contain other proteins involved in either binding the primary filaments together (e.g., actinins, M-line proteins) or regulating the actin and myosin-II interaction (e.g., tropomyosin, troponin). The tropomyosin covers the myosin-II binding sites on the actin. When calcium increases and binds to TnC, tropomyosin moves off the actin binding site and allows myosin-II to interact with actin.  Skeletal Muscle Myofilament's structure Thick myofilaments are composed of myosin-II The myofilaments are arranged to form the light and dark banding pattern visible in a longitudinal section of the myofibril Adjacent thick myofilaments and overlapping thin myofilaments form the A band. Thin myofilaments do not extend to the center of the A band, leaving a more lucent region known as the H band. The thick myofilaments are interconnected down the center of the H band by an M line.  Skeletal Muscle Myofilament's structure Light micrograph (A) and electron micrograph (B) of longitudinally oriented skeletal muscle and schematic representation (C) of a sarcomere. In A, transverse striations consisting of alternating light bands (I bands) and dark bands (A bands) are present. Each I band is bisected by a Z line (arrowheads). In B, the transverse striations can be further resolved into Z lines (Z) that define a sarcomere and bisect the light I band (not labeled). The A band is electron-dense and is bisected by the M line (M, arrowhead), which connects adjacent thick myofilaments. On either side of the M line, an electron-lucent area represents the H band (H), where there is no overlap of thick and thin myofilaments. In C, the arrangement of myofilaments is shown in relation to the electron micrograph.  Skeletal Muscle Contraction An action potential travels down the axon and causes release of acetylcholine from the motor end plate into the synaptic cleft adjacent to the muscle fiber. Acetylcholine binds to receptors on the cell membrane and opens receptor-gated sodium channels into the myocyte. Sodium influxes into the muscle fiber and initiates a wave of depolarization that spreads across the cell membrane. In the resting state before depolarization of the cell membrane, the tropomyosin–troponin complex covers the myosin-II binding sites on the actin filament.  Skeletal Muscle Contraction Myosin-II heads are bound to ATP. As depolarization begins, an action potential spreads across the cell membrane and extends into the T tubules. The depolarization causes the adjacent terminal cisternae to release stored calcium into the cytoplasm around the myofibrils. The calcium binds to troponin (TnC) on the thin myofilaments, causing the troponin to undergo a conformation change. The change in troponin results in the movement of tropomyosin to expose the myosin-II binding sites.  Skeletal Muscle Contraction Actin and myosin-II interact, allowing the increased hydrolysis of ATP. Energy from the ATP hydrolysis is used to bend the head of the myosin-II complex. The movement of the head pulls the attached actin toward the center of the sarcomere, thus shortening the sarcomere and contracting the myocyte overall. The myosin-II head binds to a new ATP and then detaches from the actin filament and the cycle repeats.  Skeletal Muscle Skeletal muscle contraction https://www.youtube.com/watch?v=GrHsiHazpsw  Cardiac Muscle Light Microscopic Structure The striated myocytes of cardiac muscle branch and anastomose Cardiomyocytes have a single central nuclei and the cytoplasm is acidophilic Each cardiac muscle fiber is surrounded by endomysium, similarly to the skeletal muscle Cardiomyocytes are subdivided into groups by connective tissue, analogous to the perimysium of skeletal muscle Cardiomyocytes are surrounded by a extense capillary network  Cardiac Muscle is Striated Muscle Differences From Skeletal Muscle – Mononucleated vs. Multinucleated – Central vs. Peripheral Nuclei – Diad vs. Triad  Cardiac Muscle Myofilament's structure myofibrils are similar to skeletal muscle. The same banding pattern of myofilaments is present. T tubules, located at the Z line, are larger than in skeletal muscle. The mitochondria of cardiac myocytes are larger and more numerous than in skeletal muscle, indicating the degree of aerobic metabolism that occurs in this tissue gap junctions and desmosomes allow transfer of chemical signals between adjacent cells and attachment between cells  Cardiac Muscle Cardiac Muscle: observe the presence of a single nuclei centrally located, the striated appearance, acidophilic cytoplasm and anastomosis between cardiomyocytes  Cardiac Muscle Cardiac Nodes and Impulse Conduction Fibers cardiac nodes and impulse conduction fibers (Purkinje fibers are modified cardiac muscle cells. The cells of the sinoatrial and atrioventricular nodes are clustered together and have more cytoplasm and fewer myofibrils than cardiac myocytes, accounting for their light-staining cytoplasm. The cells stain positively for acetylcholinesterase, which relates to their conductive function. At the ultrastructural level, the cells have mitochondria and sarcoplasmic reticulum but lack T tubules.  Cardiac Muscle Cardiac Nodes and Impulse Conduction Fibers The atrioventricular bundle, composed of impulse conduction fibers similar to nodal cells, originates from the atrioventricular node and supplies both ventricles. As the fibers course toward the apex of the heart, they become larger than adjacent cardiac myocytes and are a prominent feature of the subendocardium.  Cardiac Muscle Contraction Cardiac muscle is stimulated to contract by a mechanism similar to skeletal muscle. As there is less sER in cardiac muscle, an action potential triggers the release of calcium from both the sER and T tubules. Contraction is activated through the interaction of actin and myosin myofilaments. Sequential contraction of heart chambers is stimulated by the orderly spread of the action potentials via gap junctions in the intercalated disks. The number, size, and distribution of the gap junctions plus the type of connexin (the structural protein of the gap junctions) influence the rate of impulse conduction.  Cardiac Muscle Myogenesis, Hypertrophy, and Regeneration The fibers arise by differentiation and growth of single cells. As the cells grow, new myofilaments form. The ability of cardiac muscle cells to divide is lost soon after birth. Enlargement of the heart wall during exercise or cardiac insufficiencies is primarily through hypertrophy rather than hyperplasia. Damage to a section of the heart wall, with the resulting death of that section, is repaired primarily by proliferation of connective tissue rather than by regeneration of any significant number of new cardiac myocytes. Stem cells are under investigation as a possible source of replacement cells for damaged cardiac muscle cells.  Next Lecture Nervous Tissue Dellmann’s Textbook of Veterinary Histology, 6th Ed. Pages 91 – 116