General Histology PDF

# General Histology ## Contents | No. | Chapter | Page | |---|---|---| | 1 | General Histology | 1 | | 2 | Histology of Epithelial Tissue | 1 | | 3 | Histology of Connective Tissue | 15 | | 4 | Histology of Bone and Cartilage | 23 | | 5 | Histology of Muscular Tissue | 30 | | 6 | Histology of Bloo...

# General Histology ## Contents | No. | Chapter | Page | |---|---|---| | 1 | General Histology | 1 | | 2 | Histology of Epithelial Tissue | 1 | | 3 | Histology of Connective Tissue | 15 | | 4 | Histology of Bone and Cartilage | 23 | | 5 | Histology of Muscular Tissue | 30 | | 6 | Histology of Blood Vessel | 35 | | 7 | Histology of Nervous Tissue | 43 | ## General Histology **Body is composed of cells, intercellular substances and tissue fluid. Body develops from 3 germ layers-Ectoderm, Mesoderm and Endoderm. These 3 germ layers formed the 4 primary tissues, namely** 1. Epithelial tissue, 2. Connective tissue, 3. Muscular tissue and 4. Nervous tissue. ## Histology of Epithelial Tissue **Epithelial Tissue or Epithelium** It has a continuous layer or layers of cells with very little or no intercellular substance. The cells are arranged in sheets or in masses. The epithelial cells rest on basement membrane and they are avascular and alymphatic. 1. **Epithelial tissue that develops from Ectoderm** * Epithelium of skin, mouth and anus etc. 2. **Epithelial tissue that develops from Endoderm** * Lining of gastrointestinal tract, respiratory tract, glands etc. 3. **Epithelial tissue that develops from Mesoderm** * Endothelium of blood vessels and lymphatic vessels, mesothelial lining of coelomic cavity such as pleura etc. **Classification of Epithelium** 1. Covering and lining epithelium 2. Glandular epithelium **Turnover of Epithelium:** Epithelia have the highest turnover rate out of the four basic tissue types. The turnover rate varies depending on location and function; skin epithelium turns over every 30 days and colonic mucosal epithelium turns over every week. With the high turnover rate, susceptibility for acquiring mutations and developing neoplasm is also the highest out of the four tissue types. ## Covering and Lining Epithelium In this type, the cells are arranged in layers or sheets and the lowest layer rests on the basement membrane. It is again divided into 3 main types, according to the number of cell layers. 1. Simple Epithelium 2. Stratified Epithelium 3. Pseudo-Stratified Epithelium ### Simple Epithelium It consists of only one layer of cells and all the cells touch the basement membrane. According to the shape of cells, it is divided into 3 sub-types namely, * Simple squamous epithelium * Simple cuboidal epithelium * Simple columnar epithelium #### Simple Squamous Epithelium It is composed of one layer of flat or squamous cells, resting on the basement membrane. **Superior view** * Cells are irregular in outline, like platform plates, hence the name pavement epithelium. Nucleus is single, round and central in position. **Lateral view** * Cells are flat. Nucleus is single, flat and central in position. Cells look like spindles with tapering ends on both sides of the nucleus. **Functions** * It has dialyzing and filtering function. * The thinness of the epithelium also permits diffusion and bidirectional movement of gases, fluids, and nutrients from the free surface to underlying tissues. **Sites** * It can be found in Bowman's capsule of kidney, lining of body cavities, alveolar walls of lung, lining of vascular and lymphatic systems. * It has specific names according to locations: ___ - **Endothelium** - Lining of heart, blood vessels and lymphatic vessels. - **Mesothelium** - Lining of body cavities i.e. pleura, pericardial and peritoneal cavities. - **Mesenchymal Cells** - Lining of sub-dural and arachnoid spaces, anterior chamber of eye and perilymphatic spaces of inner ear. ___ #### Simple Cuboidal Epithelium It is composed of single layer of cubic or square shaped cells, resting on basement membrane. **Superior view** * It appears as hexagonal cells. Nucleus is single, round and central in position. **Lateral view** * It appears as cubic or square shaped cells. Nucleus is single, round and central. **Function** * This epithelium provides protection, forms conduits for gland ducts and may be specialized for active secretion and absorption. **Sites** * It can be found lining of thyroid follicles, collecting tubules of kidney, adult ovary and choroid plexus. #### Simple Columnar Epithelium It is composed of single layer of tall cells, resting on the basement membrane. **Superior view** * It appears as hexagonal cells. Nucleus is single, round and central in position. **Lateral view** * It appears as tall or column-like cells. Nucleus is single, oval and basal in position. **Function** * This epithelium provides absorption, lining and secretion, Protection. **Sites** * It can be found lining of gastrointestinal tract from stomach to upper half of anal canal, lining of excretory ducts of kidney, lining of uterine tube etc. **Simple Columnar Ciliated** * e.g. Ependyma, uterine tubes, oviducts, small bronchioles **Simple Columnar Non-Ciliated** * e.g. stomach, small and large intestines ### Stratified Epithelium This type consists of more than one layer of cells, and only the lowest layer touches the basement membrane. Its main function is for protection, because it can withstand more trauma than the simple type. It is divided into 4 types: * Stratified squamous epithelium * Stratified cuboidal epithelium * Stratified columnar epithelium and * Transitional epithelium #### Stratified Squamous Epithelium There are 3 types of cells in this epithelium * **Basal Layer** * It is the single layer of columnar cells, resting on the basement membrane. * **Middle Layer** * They are few layers of polyhedral cells, gradually flattened into short, spindle shaped forms. Each nucleus is single, spherical or oval and central in position. * **Superficial Layer** * They consist of few layers of flat or squamous cells. Each nucleus is single, flat and central in position. This type of epithelium is sub-divided into 2 types by the presence of non-living material called keratin. * Stratified squamous keratinized e.g. Skin * Stratified squamous non-keratinized epithelium e.g. tongue, esophagus, vagina and palatine tonsil. **Function - wear and tear / protection** #### Stratified Cuboidal Epithelium The general arrangement of cells is the same as above; the only difference is that the superficial layer of cells is cuboidal in shape. **Function** * protection and secretion * lining of ducts of sweat glands, multilayered primary ovarian follicle #### Stratified Columnar Epithelium The general arrangement of cells is the same as above; the only difference is that the superficial layer of cells is columnar. **Function - protection** **Sites** * conjunctiva, cavernous portion of male urethra. #### Transitional Epithelium It lines the hollow organs that are subjected to tension (e.g. urinary bladder). Its appearance varies with degree of distension. There is no obvious basement membrane. **There are groups of cells.** * **Superficial Group of Cells** * It appears as umbrella-shaped cells in many layers. The nucleus is single, flat and central in position. * **Middle Group of Cells** * There are many layers of pear-shaped cells with the broad ends towards the free surface or apex pointing downwards. * **Basal Group of Cells** * There are many layers of polyhedral cells. Each cell has single, round and centrally placed nucleus. **This type of arrangement of cells is seen when the organ is relaxed or urinary bladder is empty.** When the urinary bladder is filled with urine or stretched, these cells will become irregular and flat and they dip into each other so that the whole epithelium has 2-3 layers only. **It will transform into its original stage i.e. it will regain the different cell shapes, after urination.** Therefore, it is called transitional epithelium. **The characteristic features of transitional epithelium are:** * It lines organs which are subjected to tension. * It allows stretching without breaking the cells apart from one another. * There is no leakage of urine during the stretched stage. * There is no basement membrane seen under light microscope. **Sites** * urinary bladder, ureter, 1st part of urethra near the bladder, and covering of calyces and pelvis of kidney. ### Pseudo-Stratified Epithelium There is only one layer of cells but it looks as if there is more than one layer. Nuclei are located at different levels. All the cells touch the basement membrane, but not all the cells reach the cells reach the surface. **This type of epithelium has single layer of cells of different heights, therefore nuclei are at different levels. Hence, the whole epithelium looks like stratified epithelium.** There are 2 types- * Pseudo-stratified columnar ciliated epithelium * e.g. trachea, bronchi * Pseudo-stratified columnar non-ciliated epithelium * e.g. penile urethra ## Histology of Glandular Epithelium Glandular tissue is the epithelial tissue, specialized to perform a secretory function. Glands of glandular tissue are of two kinds. 1. Exocrine gland 2. Endocrine gland **Exocrine glands** are glands secreting out and possessing ducts. **Endocrine glands** are glands secreting into the blood and which have no ducts. ### Classification of Exocrine Glands 1. **According to the number of cells** * A gland may consist of single cell (unicellular gland). e.g. goblet cell. * Most glands are composed of more than one cell (multicellular glands). e.g. Salivary gland. 2. **According to the nature of their secretion** * Mucous glands * Serous glands * Mixed glands #### Mucous Glands These glands secrete a thick, viscid material called mucin. **Histologically** * Secretory acini of these glands are larger in size than those of serous glands. * The lumen of these acini are also large and surrounded by mucous - secreting cells, which are cuboidal or low columnar in shape. * The nuclei of these cells are flattened and are situated at the base of the cells. * The apical part of cytoplasm appear vacuolated (an empty) appearance and basophilic. This is due to the presence of numerous mucigen, the precursor substance of mucin. The basal part of cytoplasm is not deeply basophilic as in serous secreting cell. * e.g. Sublingual salivary gland. #### Serous Glands These glands secrete a clear, watery fluid which contains enzymes. **Histologically** * Secretory acini are small in size with a small rounded lumen. * The serous secreting cells are triangular in shape with apices directed towards the lumen. * Each cell contains a round nucleus situated near the base of cell. * The basal part of cytoplasm is deeply basophilic due to the presence of abundant RNA. * The apical part of cytoplasm appears granular due to the presence of numerous acidophilic zymogen granules. e.g. Parotid gland. #### Mixed Glands These glands produce both mucous and serous type of secretions. **Histologically** * These glands have both serous and mucous secretory units. * The serous secreting units appear as semi-lunar or crescent-shaped structures which capped over the mucous acini called “serous demilunes". e.g. Submandibular salivary gland. ### Classification of Exocrine Glands According to Mode of Secretion 1. Merocrine (eccrine) gland 2. Apocrine gland 3. Holocrine gland #### Merocrine Gland These are glands which discharge their secretory material without loss of any part of the secretory cells. e.g. Sweat glands. #### Apocrine Gland When part of the secretory cell is discharged along with the secretory material, the secretory gland is called an apocrine gland. e.g. Apocrine sweat gland of axilla, mammary gland. #### Holocrine Gland The secretory cells in these glands will first collect secretory material within their cytoplasm. This material is then released by total death and disintegration (breaking up) of the whole cell. These types of glands are called holocrine gland. e.g. sebaceous glands of the skin. ### Classification of Exocrine Glands According to the Shape of the Secretory Unit * Tubular * Hollow cylinder or tubular in shape. * Alveolar (or acinar) * Globular to pear-shaped ending with a small lumen * Tubulo-alveolar * Tubular with alveoli at the end, sides or both * Presence of separate tubular and alveoli. ### Classification of Exocrine Glands According to Duct System **Simple Glands** the excretory duct when present, is single and unbranched * Simple straight tubular (e.g. intestinal crypt) * Simple branched tubular (e.g. stomach) * Simple coiled tubular (e.g. sweat gland) * Simple alveolar (e.g. small mucous gland of urethra) * Simple branched alveolar (e.g. sebaceous gland of the skin) **Compound Glands** If the duct part divides into many branches, the gland formed is a compound gland. Such a gland consists of many component lobules. The several lobular ducts join and finally drain into one main excretory duct. * Compound tubular (e.g. kidney) * Compound alveolar (e.g. mammary gland) * Compound tubulo-alveolar (e.g. large salivary gland, pancreas) ## The Endocrine Glands This group has no ducts of any kind; secretion is released directly into the blood stream. Hence these glands are designated as ductless or endocrine. Most of the endocrine glands are separate entities, recognizable as distinct organs. e.g. Thyroid gland, suprarenal gland, parathyroid gland, pituitary gland, some endocrine tissue occurs as scattered masses within an exocrine gland (e.g. Pancreatic islets). **The cells of glands are arranged in cords (or) plates.** * (e.g. Suprarenal, Parathyroid gland) **A few glands consist of typical epithelial sac or follicles.** * (e.g. thyroid follicles, ovarian follicles) **The secretory product of endocrine glands is named hormones.** All endocrine glands store their secretion inside the gland. The stored secretion may lie within the cytoplasm which is called intra-cellular storage. (e.g. islet cells of pancreas) **The stored secretory material may lie outside the cells which is called extracellular storage.** In this case the secretory material is surrounded by the secretory cells of the gland forming a structure which is called follicle. (e.g. follicles of thyroid gland) ## Histology of Connective Tissue It is one of the 4 primary tissues of the body. It has two characteristics 1. they all developed from embryonic mesenchyme which is itself derived from primitive mesoderm. 2. they all possess a relatively large amount of intercellular substance. ## Functions of Connective Tissue 1. **Mechanical Function** * They provide supporting and connecting substances of the body (i.e. they serves to connect other tissue to one another and to the bony skeleton of the body and also give structural support to soft organs of the bone). 2. **Nutritional Function** * The intercellular substance is responsible for nutritional exchanges as in fluids of the blood and lymph and in the fluid matrix of loose connective tissues. 3. **Defensive Function** * Connective tissue cells (e.g. blood leukocytes) defend the body against bacteria and other foreign bodies. **Some connective tissue (e.g. blood and lymph) has liquid form of intercellular substance and others have intercellular substances varying from a soft jelly to rigid mass and usually contain fibres.** ## Classification of Connective Tissue: * **embryonic connective tissue** * **adult connective tissue** ### Embryonic Connective Tissue * Mesenchyme type * Mucous type #### Mesenchyme Type * arises from Mesoderm except - neuroglia which arises from neural ectoderm, reticulum of thymus which * arise from pharyngeal endoderm and chromatophores which arise from neural crest. **Cells** * can be found in all supporting tissues of body differentiate from these cells. * Lies widely separated from one another * Have amoeboid movement * Cytoplasm is a narrow rim with long fine processes. * Nucleus is Large. #### Mucous Type * It is the connective tissue of umbilical cord. (e.g. Wharton's Jelly) ### Adult Connective Tissue * Ordinary Connective Tissue * Special Connective Tissue #### Loose Connective Tissue (or) areolar Connective Tissue * Widely distributed in body * It forms superficial and deep fascia, part of the network of organs, surrounds blood vessels and fills any space in the body. **Cells are** * Undifferentiated mesenchymal cells (stem cells) * Fibroblast * Flatten, irregular, polyhedral cell with long tapering cytoplasmic processes. * Cytoplasm is sparse and pale. * Nucleus is large, ovoid and pale. * It has 1 or 2 nucleolus and fine, granular chromatin arrangement. * Function is formation of fibres and ground substance. * Fibrocytes are inactive cells. They are much smaller and spindle shaped with acidophilic cytoplasm. * Macrophages or Histocytes * Cells have irregular outline; cell processes are unusually short and blunt. * Cytoplasm is large, Nucleus is smaller, darker and ovaly short and blunt enifo in shaped) with coarse chromatin granules Nucleoli are not obvious. * Plasma cells * They are few in normal tissue but numerous in pathological condition (e.g. chronic inflammation). * Cells are egg shaped. * Cytoplasm is deeply basophilic and rich in endoplasmic reticulum. * Nucleus is large and spheroidal and eccentric in position. * The clumped chromatin is arranged like spokes of a wheel (Cart - wheel appearance). * Its function is protection by production of Antibodies. * Mast cells * Large cells but have relatively small, pale, spheroidal nuclei. Cytoplasm contains numerous coarse, deeply basophilic secretory granules. * They produce Heparin (an anticoagulant) and Histamine (a vasodilator). * They are present along the small blood vessels. * Fat cells * They are normal components of areolar tissue. * They occur singly or in clusters between collagen fibres. * They are large cells with a thin rim of cytoplasm and inconspicuous nucleus displaced to one side. Adult fat cells are usually bloated with a huge amount of fat, but during preparation, fat is dissolved away leaving a clear space known as Signet ring appearance. * Other Types of Cells * E.g. Leukocytes, Pigment cells **Functions of Adult Connective Tissue** * Nutrition - by storage of fat * Lessen motion and act as soft cushion e.g. palm of hand and sole of foot * Supportive function - act as lubricant allowing smooth movement, e.g. fat tissue around eye ball * Conservation of heat - help to maintain a constant body temperature. ## Intercellular Substances * Formed Intercellular Substances * Collagen, reticular and elastic fibres * Amorphous Intercellular Substances * soft ground substance and firm cement substance #### **Soft ground substance** * which is viscid and composed of hyaluronic acid * e.g. Synovial fluid of joints, humour of eye. #### **Firm cement substance** * more viscid and composed of chondroitin sulphuric acid ## Dense Connective Tissue * mainly composed of collagen fibres. * **2 types** * Regular type * Irregular type #### Regular type * the collagen fibres are regularly arranged running mostly in same plane and same direction. * It has great tensile strength and can withstand pulls and stretch in every direction. e.g. tendon, ligaments. #### Irregular type * Formed type contains collagen, elastic and reticular fibres ## Special Connective Tissue * Haemopoietic connective tissue * Supporting connective tissue ### Fibers of connective tissue | Fiber | Collagen Fibres | Elastic Fibres | Reticular Fibres | |---|---|---|---| | 1. Appearance | White fibres | Yellow fibres | Argyrophilic fibres | | 2. Structure | Coarse, long Straight unbranched fibres, usually seen in bundles. Fibres run in same plane and same direction. | Fine fibres which branch and anastomose thus forming a network. Fibres run in all directions. | Fine, delicate short fibres which branch and anastomose freely forming a fine network or reticulum. | | 3.Distribution | Found whenever great strength is required, combined with flexibility and resistance to a stretching force, e.g. tendons, joint capsule and ligaments | Found in places where strength is required combined with ability to be stretched and to return to its original form e.g. blood vessels, particularly aorta, lung, elastic ligaments. | Supportive frame work e.g. lymphatic system, particularly spleen | | 4. Elasticity | Flexible but inelastic | Considerable | Little | | 5. Refractive index | Low | High (elastin) | Low | | 6. Chemical composition | Protein -on boiling yields gelatin | Protein (elastin), Resistant to boiling, dilute acids and alkali and gastric juice. | Protein (recticulin) resistant to weak acid and alkali. | | 7. Staining reaction H & E stain | Pink | Yellow | Black | | | Van Gieson's stain Red | | | | | Silver stain Brown | | | **- Bone and reticular fibres / 30** ## Histology of Bone and Cartilage ### Histology of Bone Bone is the specialized and supporting type of connective tissue. It is also called osseous tissue. **Bone consists of cells, fibers and ground or intercellular matrix.** #### Types of bone cells 1. **Osteoblast** - immature cells 1. **Osteocyte** - bone forming cells, reside in lacunae, lie (singly, posses many tiny long cytoplasmic process run in canaliculi. 1. **Osteoclast** - bone destroying cells, multinucleated, lie in howship's lacunae. (bore remodeling) #### **The bone matrix consists of an inorganic (inorganic component about 65% and organic component 35%).** * **Inorganic component** is calcium compound and phosphate salts. * **Organic compound** is collagenous fibrils which are united in bundles by a special cementing substance. The cementing substance is the ground substance of the matrix, composed of mucopolysaccharide containing chondroitin sulfate. **Woven bone & hapzard organization of collagen in pre-natal bone** * **Lamellar bone in post-natal bone** * **Lamella** on our und **Ground Bone (Grind, ground, ground) (Gyan 6, / - / - /- / - **Calcified bone which was cut as thin as possible, with a fine saw and then grind down to greater thinness on a stone is termed ground bone section.** ### Histology of Cartilage These are specialized type of supporting tissue. There are 3 types of cartilage in the body according to the presence of fibres in the matrix. These are: 1. Hyaline cartilage 2. Elastic cartilage 3. Fibrocarilage #### Hyaline Cartilage It is commonly found in the body. It is glass-like in appearance - hence the name, which is derived from the Greek word (Hyalos) meaning glass. It has a pearl white color and translucent appearance due to its intercellular substance. **Microscopic Appearance** * Hyaline cartilage consists of cells and intercellular substance. * These cells are called chondrocytes and these lie in spaces called lacunae in the intercellular substance. * The cells may lie single or in groups (cell nests). * Chondrocyte possesses a round basophilic nucleus with one or more nucleoli. * The intercellular substance is composed of both formed and amorphous types. * The formed type is mainly collagen fibers while the amorphous type is a sulfated mucopolysaccharide called chondroitin sulfuric acid. * The amorphous intercellular substance has the same refractive index as collagen fibers. * Thus the collagen fibers cannot be seen. * The lining layer of lacuna is high concentration of chondroitin sulfuric acid. * It is most basophilic part and deep staining. * This layer is called the capsule of the chondrocytes. * This cartilage is covered by connective tissue membrane called perichondrium. * This perichondrium is composed of an outer layer of densely arranged collagen fibers and an inner layer of cells. * These cells can be converted into cartilage cells during the growth period of the cartilage. * Most hyaline cartilages are converted into bones. * Some will persist uncalcified state throughout life. e.g. articular cartilage, costal cartilage, tracheal and bronchial cartilages and some of the laryngeal cartilages. #### Elastic Cartilage This type of cartilage contains numerous elastic fibers in its intercellular substance in addition to collagen fibers and chondroitin sulfates. * It is found in places where stiffness together with elasticity is required. (eg. external ear, epiglottis and some laryngeal cartilages) * It is yellow and more opaque than hyaline cartilage * It is more flexible and elastic. * Ensheathing perichondrium is present. **Microscopic Structure** * Microscopic Structure is fundamentally like the hyaline cartilage. * The cells lie singly and in groups and their capsules are similar. * The general matrix also contains ground substance and collagen fibres. * But in addition, it contains a meshwork of branching elastic fibers. #### Fibrocartilage This type occurs in location where a tough support or tensile strength is desirable e.g. intervertebral disc, articular disc, pubic symphysis. * This tissue has opaque appearance and a firm, fibrous texture. * Perichondrium is lacking. In this type of cartilage, the collagen fibres are in bundles which run parallel to the plane of pull exerted. * The chondrocytes are found in rows between the collagen bundles. * The amorphous intercellular substance (chondroitin sulfate) lies between the cells and fiber bundles. * All chondrocytes and lacunae are about the same size. ## Histology of Muscular Tissue There are three types of muscle tissues in the body: skeletal muscle, cardiac muscle, and smooth muscle. These muscles can be identified by their structure and function, with each muscle type showing morphologic and functional similarities as well as differences. **All muscle tissues consist of elongated cells called fibers.** Each muscle fiber is composed of subunits called myofibrils, which contain two types of contractile protein filaments, actin and myosin. **The cytoplasm of muscle cells is called sarcoplasm, and the surrounding cell membrane or plasmalemma is called sarcolemma.** ## Classification of Muscular Tissue Muscle is classified on both functional and structural basis. * **Functionally** it is either involuntary or voluntary. * **Structurally** it is either striated or non-striated. **The two classifications can be combined as follows:** * Smooth, involuntary muscle i.e. smooth muscle * Striated, involuntary muscle i.e. cardiac muscle * Striated, voluntary muscle i.e. skeletal muscle ### Smooth Muscle This type is also called plain, non-striated and involuntary. 1. **Shape** * The individual fibres are elongated tapering spindles * **Length - 50-200μm** * **Diameter - 0.2 - 10 µm** 2. **Structure** * **Nucleus - elongated (ovoid to cigar - shape)** * Pale nucleus lies midway of its fibres, where the fibre is broadest * It is almost centrally located and extends in the long axis of the fibre * **Cytoplasm (Sarcoplasm)** * The plasma membrane is thin * The cytoplasm appears homogenous. E.M reveals myofilaments * Various organelles are grouped inconspicuously near the nucleus 3. **Distribution** * Wall of hollow viscerae (except heart) * Alimentary, Respiratory and Urogenital tracts * Blood vessels and larger lymphatics 4. **Action** * Contraction is slow, sustained and resistant fatigue. It may maintain a constant muscular tension (i.e. tone) in an organ. ### Skeletal Muscle It is a voluntary, cross-striated type of muscle. 1. **Shape** * Cylindrical shape 2. **Size** * **Length - many cm long** * **Diameter - 10-100 µm** * Increases with age, exercise and male hormone influence * Decreases in period of emaciation or disuse 3. **Structure** * **Nuclei** * Numerous, ovoid and peripheral in position, close beneath the sarcolemma. * **Myofibrils** * The fibres are often seen in bundles, called Koelliker's column. * Such bundles are separated from each other by abundant sarcoplasm. * **A Koelliker's column in cross section is called a Cohnheim's area.** * **E.M demonstrates that each myofibril contains fundamental units, the myofilaments of two kinds.** * **The thicker filaments (myosin) are restricted to the extent of dark band (A band)** * **the thin filaments extend throughout the light band. (I band)** #### **Myofibrillar Banding** Each fibril bears alternate dark and light bands about 300 to each millimeter. **Dark band (Anisotrophic Band)** * Myosin of thick, coarse myofilaments are restricted to the extent of the A band and attached to the disc like zone represented by M line. * Middle zone of the A band at time appear paler than the rest. * It is seen only in a relaxed fibre and named the H-band. * It represents region into which actin filaments do not extend. **Light Band (Isotrophic Band) (actin)** * Only the thin myofilaments occur in this band. * They insert on the Z line. * Their free ends extend into the A region for a variable distance. * Z line bisects each I band transversely. #### **Sarcomere : [Z-line - Z-line ] * ** * It is a structural unit of a muscle fibre. * It is included between successive Z bands. * It contains all of A band and the half of each I band that borders A. #### **Sarcoplasm** * This comprises the less specialized cytoplasm between myofibrils and nuclei. * It contains mitochondria, Golgi network, sacroplasmic reticulum, pigment and non-living inclusions (glycogen and lipid droplets). #### Distribution of Skeletal Muscle * Locomotory muscle, sheets of muscle of abdominal wall, diaphragm, middle ear muscle. #### Action of Skeletal Muscle * Contraction is faster that smooth muscle, but it fatigues easier and its action is less sustain. ### Cardiac Muscle * Involuntary, cross-striated muscle is peculiar to the myocardium of the heart. * It also extends into the roots of the large vessels joining the heart. * Cardiac muscle is much like skeletal muscle in structure. * Cardiac muscle consists of short columns, united into a close meshwork. * The meshwork simulates a syncytium because cell junctions are not easily seen. 1. **Shape** * Cylindrical shape 2. **Size** * **Length - 50-100 µm** * **Diameter -10-20 µm** 3. **Structure** * **Nuclei** * Ovoid, central in position * **Myofibrils** * Are distinct but less robust than those of skeletal muscle * Fibrillar striping is like that in skeletal muscle but closer and fainter * **Intercalated Disks (or) Discs** * Highly distinguishing and characteristic features of cardiac muscle fibers are the intercalated disks. * These dark-staining structures are found in the cardiac muscle at irregular intervals and represent the specialized junctional complexes between cardiac muscle fibers. * **Sarcoplasm** * This is rather abundant and sarcoplasm embeds the nucleus myofibrils, Golgi complex, abundant mitochondria, endoplasmic reticulum and some pigments. 4. **Branching** * Branchings are present in cardiac muscle. 5. **Actions** * Rhythmic contraction and relaxation * There is effective spread of contraction throughout the organ. ## Histology of Blood Vessel ### Classification of Blood Vessels 1. **The arterial type (The pre-capillary blood vessels)** * **Elastic (or) conducting arteries** * **Muscular (or) distributing arteries** * **Arterioles** 2. **The venous type (The post-capillary blood vessels)** * **Great vein (or) large vein** * **Median sized vein** * **Venule** 3. **Capillaries** 4. **Sinusoids** ### Arteries #### Histology of Elastic Arteries (conducting Arteries) * **E.g. Aorta, Common carotid artery and pulmonary artery** * **Large sized arteries having elastic fibres.** * **Conduct the blood from heart into muscular arteries.** **Tunica Intima (Innermost Layer)** * **Endothelial Lining** * The endothelial cells are polygonal in shape and line the lumen of the arteries. * **Subendothelial Layer** * It is thick, with many elastic fibres, collagen fibres and some smooth muscles. * **Internal Elastic Lamina** * This is not an outstanding feature of elastic arteries. * The collagen and elastic fibres form a lattice-pattern. * Fibres of this membrane are continuous with the elastic tissue of the tunica media. * Fenestrations are often present in the internal elastic membrane. **Tunica Media (Middle Layer)** * This is very thick coat and may present 80% of the total thickness of aortic wall. * It consists of a series of thick fenestrated elastic laminae. * In the aorta there are 40-60 bands of elastic tissue which arranged concentrically. * In the spaces between the laminae, there are collagen fibres, fibrocytes and muscle cells. **Tunica Adventitia (Outermost Layer)** * This layer is relatively thin and composed chiefly of bundles of collagen fibres which become continuous with the surrounding areolar tissue. * There is no distinctive external elastic lamina or membrane. * Vasa vasorum are present. #### Histology of Muscular Arteries (Distributing or Median Sized Arteries) * **E.g. Brachial, Radial, Ulnar, Femoral and Popliteal artery** * **Walls are thicker in relation with lumen.** **Tunica Intima** * **The endothelium** * It consists of flattened endothelial cells. * **The subendothelial layer** * It consists of delicate elastic and connective tissue fibres and a few fibroblasts. * The amount of subendothelial tissue is variable, being particular absent in the smallest vessels of this group. * **The internal elastic lamina** * It is prominent. It is a thick band. * Because of muscular contraction the internal elastic membrane is thrown into longitudinal folds in cross section, the membrane usually appears as a wavy band. #### Tunica Media * This is the thickest layer of medium sized arteries. * The tunica media consists of almost entirely of concentric layers of circularly disposed smooth muscle fibres. * As many as 40 layers may be present. A few elastic, collagenous and reticular fibres are present among the muscle cells. #### Tunica Adventitia * This is a thinner layer than the media. * Adjacent to the media, there is a condensation of the elastic fibres of the adventitia to form a non-fenestrated sheet called the external elastic membrane. * The inner portion of the adentitia is composed primarily of elastic fibres. * The outer regions is chiefly collagen with vasa vasorum. #### Histology of Arterioles * These vessels have a thick wall, in relation to the lumen. * Blood vessels with a diameter of 100µ or less are called arterioles. * Possess