Histology:Blood and Muscle Tissue Olaleye O.O. 2B10
BloodIntroductionBlood (about 5.5 L in a man) consists of the cells and fluid that flow in a regularunidirectional movement within the closed circulatory system.Blood is propelled mainly by the rhythmic contractions of the heart and is made up oftwo parts:• formed elements, or blood cells, and• plasma, the liquid in which the formed elements are suspended.
The formed elements are;• erythrocytes (red blood cells),• platelets, and• leukocytes (white blood cells).
If blood is removed from the circulatory system, it will clot.This clot contains formed elements and a clear yellow liquid called SERUM,which separates from the coagulum.Blood that is collected and kept from coagulating by the addition ofanticoagulants (eg, heparin, citrate) separates, when centrifuged, into layers thatreflect its heterogeneity.The hematocrit is an estimate of the volume of packed erythrocytes per unitvolume of blood.The normal value is 40–50% in men and 35–45% in women.
Plasma • Liquid plasma-imparts fluid properties to blood• Plasma components -water(90%) -proteins (albumin, globulins and fibrinogen) -electrolytes -waste material -nutrients (glucose, lipids and amino acids) -blood gases (oxygen, carbon dioxide and nitrogen) -regulatory substances (hormones and enzymes)
Blood cells• Erythrocytes (RBCs) • Leukocytes (WBCs) • Thrombocytes (platelets) The modiﬁed Romanovsky-‐type stain methylene blue (basic dye) related azures (basic dye) eosin (acidic dye)
Erythrocytes• Anucleate cells, devoid of organelles• 7-8 mm, biconcave disk, maximized cell surface area• Contain hemoglobin (transport of oxygen and carbon dioxide)• Develop from proerythroblasts
Leukocytes Possession of specific and non specific (azurophilic granules) • Granulocytes • Agranulocytes Neutrophils Lymphocytes Eosinophils Monocytes Basophils
Neutrophils• Most numerous• 10-12mm• Multilobed nucleus (polymorphonuclear neutrophils)• Heterochromatin and euchromatin• Tertiary granules• Motile cells• Active phagocytes at the site of inflammation
Eosinophils• Similar size• Bilobed nucleus (heterochromatin and euchromatin)• Large, eosinophilic, refractile granules• Azurophilic granules-lysosomes• Function-allergic reactions, parasitic infections and chronic inflammation
Basophils• Least numerous • Lobed nucleus obscured by the basophilic granules • HeterochromaDn/euchromaDn • Speciﬁc granules (heparin, histamine, heparan sulfate and leukotrienes) • Azurophilic granules (lysosomes) • FuncDon closely related to the mast cells-‐ hypersensiDvity and anaphylaxis (bind Ab secreted by plasma cells)
Lymphocytes• The main functional cells of the immune system• 30% total blood leukocytes• Recirculating immunocompetent cells• Small, medium, large (6-30mm)• Slightly indented, spherical nucleus• Thin cytoplasm (small lymphocytes) T lymphocytes – cell-mediated immunity B lymphocytes – production of circulating Ab Natural killer cells (NK cells) – programmed to kill virus-infected cells and some tumor cells
Monocytes• Precursors of the cells of the mononuclear phagocytotic system• Largest leukocytes - 18 mm• Travel from the bone marrow to the body tissues where they differentiate• Indented nucleus• Function- antigen-presenting cells in the immune system and phagocytosis
Thrombocytes• Small, membrane bounded anucleate cytoplasmatic fragments, derived from megakarioblasts (bone marrow)• 2-3mm• Function -surveillance of blood vessels -blood clot formation -repair of injured tissue
• This granulocyte has very tiny light staining granules (the granules are very difficult to see). • The nucleus is frequently multi-lobed with lobes connected by thin strands of nuclear material. These cells are capable of phagocytising foreign cells, toxins, and viruses. • When taking a Differential WBC Count of normal blood, this type of cell would be the most numerous. • Normally, neutrophils account for 50-70% of all leukocytes. If the count exceeds this amount, the cause is usually due to an acute infection such asNeutrophil appendicitis, smallpox or rheumatic fever. If the count is considerably less, it may be due to a viral infection such as influenza, hepatitis, or rubella.
Muscle TissueIntroduction:Muscle tissue is composed of differentiated cells containing contractileproteins.The structural biology of these proteins generates the forces necessary forcellular contraction, which drives movement within certain organs and thebody as a whole.Most muscle cells are of mesodermal origin, and they are differentiated mainlyby a gradual process of lengthening, with simultaneous synthesis ofmyofibrillar proteins.
Three structurally and functionally distinct types of muscle are found invertebrates: • smooth muscle, • skeletal muscle and • cardiac muscle.
Skeletal muscle• is composed of bundles of very long, cylindrical, multinucleated cells that showcross-striations.• Their contraction is quick, forceful, and usually under voluntary control. It iscaused by the interaction of thin actin filaments and thick myosin filamentswhose molecular configuration allows them to slide upon one another.• The forces necessary for sliding are generated by weak interactions in thebridges that bind actin to myosin.
Smooth MuscleSmooth muscle consists of spindle shaped cells of variable size.The largest smooth muscle cells occur in the uterus during pregnancy(12x600 µm).The smallest are found around small arterioles (1x10 µm).Smooth muscle cells contain one centrally placed nucleus.The chromatin is finely granular and the nucleus contains 2-5 nucleoli.The innervation of smooth muscle is provided by the autonomic nervoussystem.Smooth muscle makes up the visceral or involuntary muscle.
Types of smooth muscleTwo broad types of smooth muscle can be distinguished on the basis of the typeof stimulus which results in contraction and the specificity with which individualsmooth muscle cells react to the stimulus:The multiunit type represents functionally independent smooth muscle cellswhich are often innervated by a single nerve terminal and which never contractspontaneously (e.g. smooth muscle in the walls of blood vessels).The visceral type represents bundles of smooth muscle cells connected by GAPjunctions, which contract spontaneously if stretched beyond a certain limit (e.g.smooth muscle in the walls of the intestines).
Skeletal MuscleSkeletal muscle consists of very long tubular cells, which are also called muscle fibres.The average length of skeletal muscle cells in humans is about 3 cm (sartorius muscle up to 30cm, stapedius muscle only about 1 mm).Their diameters vary from 10 to 100 µm.Their fibres contain many peripherally placed nuclei.Up to several hundred rather small nuclei with 1 or 2 nucleoli are located just beneath theplasma membrane.Skeletal muscle fibres show in many preparations characteristic cross-striations. It is thereforealso called striated muscle.Skeletal muscle is innervated by the somatic nervous system.Skeletal muscle makes up the voluntary muscle.
Structure of skeletal muscleMuscle fibres in skeletal muscle occur in bundles, fascicles, which make upthe muscle.The muscle is surrounded by a layer of connective tissue, the epimysium,which is continuous with the muscle fascia.Connective tissue from the epimysium extends into the muscle to surroundindividual fascicles perimysium.A delicate network of loose connective tissue composed of fine collagenousand reticular fibres endomysium is found between the muscle fibres of afascicle.Finally, each muscle fibre is surrounded by a basement membrane.
The Contractile Apparatus of Skeletal MuscleThe spatial relation between the filaments that make up the myofibrils withinskeletal muscle fibres is highly regular.This regular organisation of the myofilaments gives rise to the cross-striation,which characterises skeletal and cardiac muscle.Sets of individual "stria" correspond to the smallest contractile units of skeletalmuscle, the sarcomeres.Rows of sarcomeres form the myofibrils, which extend throughout the length ofthe skeletal muscle fibre.
Depending on the distribution and interconnection ofmyofilaments a number of "bands" and "lines" canbe distinguished in the sarcomeres.• I-band - actin filaments,• A-band - myosin filaments which may overlap withactin filaments,• H-band - zone of myosin filaments only (no overlapwith actin filaments) within the A-band,• Z-line - zone of apposition of actin filamentsbelonging to two neighbouring sarcomeres (mediatedby a protein called alpha-actinin),• M-line - band of connections between myosinfilaments (mediated by proteins, e.g. myomesin, M-protein).
The average length of a sarcomere is about 2.5 µm (contracted ~1.5 µm,stretched ~3 µm).The protein titin extends from the Z-line to the M-line. It is attached to the Z-line and the myosin filaments.Titin has an elastic part which is located between the Z-line and the borderbetween the I- and A-bands.Titin contributes to keeping the filaments of the contractile apparatus inalignment and to the passive stretch resistance of muscle fibres.Other cytoskeletal proteins interconnect the Z-lines of neighbouring myofibrils.Because of this connection, the A- and I-bands of neighbouring myofibrils lieside-by-side in the muscle fibre.These cytoskeletal proteins also connect the Z-lines of the peripheralmyofibrils to the sarcolemma.
Types of Skeletal MuscleSkeletal muscle cells respond to stimulation with a brief maximal contraction - they are of the twitch type.Individual muscles fibres cannot maintain their contraction over longer periods, therefore the sustainedcontraction of a muscle depends on the "averaged" activity of often many muscle fibres, whichindividually only contract for a brief period of time.The force generated by the muscle fibre does depend on its state of contraction at the time ofexcitation.Excitation frequency and the mechanical summation of the force generated is one way to graduate theforce generated by the entire muscle.Another way is the regulation of the number of muscle fibres which contract in the muscle.Additional motor units, i.e. groups of muscle fibres innervated by one motor neurone and its branches, arerecruited if their force is required.The functional properties of the muscle can be "fine-tuned" further to the tasks the muscle performs byblending functionally different types of muscle fibres:
Type I fibres (red fibres)Red muscles contain predominantly (but not exclusively) red muscle cells.Red muscle fibres are comparatively thin and contain large amounts of myoglobin andmitochondria.Red fibres contain an isoform of myosin with low ATPase activity, i.e. the speed with whichmyosin is able to use up ATP. Contraction is therefore slow.Red muscles are used when sustained production of force is necessary, e.g. in the controlof posture.
Type II fibresWhite muscle cellsPredominantly found in white muscles,They are thicker and contain less myoglobin.ATPase activity of the myosin isoform in white fibres is high, and contraction is fast.Type IIA fibres (red) contain many mitochondria and are available for both sustained activityand short-lasting, intense contractions.Type IIB/IIX fibres (white) contain only few mitochondria. They are recruited in the case ofrapid accelerations and short lasting maximal contraction. Type IIB/IIX fibres rely onanaerobic glycolysis to generate the ATP needed for contraction.
NB: Skeletal muscle fibresdo not contractspontaneously. Skeletalmuscle fibresare not interconnected viaGAP junctions but dependon nervous stimulation forcontraction.All muscle fibres of a motorunit are of the same type.
Cardiac MuscleCardiac muscle, the myocardium, consists of muscle cells, cardiomyocytes, with onecentrally placed nucleus.Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µmwide.Cardiac muscle cells exhibit cross-striations.Cardiac muscle cells excitation is mediated by rythmically active modified cardiac musclecells.Cardiac muscle is innervated by the autonomic nervous system, which adjusts the forcegenerated by the muscle cells and the frequency of the heart beat.Cardiac muscle is for these reasons also called involuntary striated muscle.
Structure of cardiac muscleThe cross-striations in cardiac muscle are less distinct, in part because rows ofmitochondria and many lipid and glycogen droplets are found between myofibrils.In contrast to skeletal muscle cells, cardiac muscle cells often branch at acute angles andare connected to each other by specialisations of the cell membrane in the region ofthe intercalated discs.Intercalated discs invariably occur at the ends of cardiac muscle cells in a regioncorresponding to the Z-line of the myofibrils (the last Z-line of the myofibril within the cell is"replaced" by the intercalated disk of the cell membrane).In the longitudinal part of the cell membrane, between the "steps" typically formed by theintercalated disk, we find extensive GAP junctions.
T-tubules are typically wider than in skeletal muscle, but there is only one T-tubuleset for each sarcomere, which is located close to the Z-line. The associatedsarcoplasmatic reticulum is organised somewhat simpler than in skeletal muscle.It does not form continuous cisternae but instead an irregular tubular networkaround the sarcomere with only small isolated dilations in association with the T-tubules.NB: Cardiac muscle does not contain cells equivalent to the satellite cells ofskeletal muscle. Therefore cardiac muscle cannot regenerate.