General Histology PDF

INTRODUCION TO HISTOLOGY TISSUE PROCESSING CELL AND ITS ORGANELLES EPITHELIAL TISSUE Epithelial tissue are one more layers of cells which line the outer surfaces of the body, inner surfaces of the body cavity, tubes and sacks, inner and outer surfaces of the organs of the body. Epithelia cells vary...

INTRODUCION TO HISTOLOGY TISSUE PROCESSING CELL AND ITS ORGANELLES EPITHELIAL TISSUE Epithelial tissue are one more layers of cells which line the outer surfaces of the body, inner surfaces of the body cavity, tubes and sacks, inner and outer surfaces of the organs of the body. Epithelia cells vary in shape, size, function and location. They are normally found on a basement membrane which separates them from the underlying connective tissue. Generally, the functions of epithelia tissues include- 1) Protection – They protect the body from abrasion, microorganisms harsh environmental factors & chemicals etc. 2) They facilitate exchange of materials like exchange of 02 and CO2 in the lunge, absorption of nutrients in the intestine secretion of substances into the food substrates in the G.I.T, reabsorption & secretion of substances in the kidney tubules. 3) They form special barriers like the blood – brain barrier, blood – placenta barrier, blood –thymus barrier etc CLASSIFICATION OF EPITHELIA TISSUE Epithelia tissue are classified based on the following criteria - 1) Shape of the cells, 2) Number of cell layers, 3) Presence of processes on the layers, 4) Presence of physiochemical substances on the cell, Based on shape a) Squamous epithelium – flattened cells which are far wider than they are high. b) Cuboidal epithelium- The cells are as tall as they are wide. c) Columnar epithelium – The cells are taller than they are wide. Based on no of layers a) Simple or unilayer – Has only on layer of cells. b) Stratified/ multilayer– Has more than one layer of cells. c) Pseudostratified epithelium – One layer of cells which appear to be more than one layer Based on presence of process a) Ciliated epithelium - meaning that cilia are present on the epithelium b) With microvilli - microvilli are present on the surface. The presence of the microvilli can give the cell a striated/ brush border appearance on the lumen. c) Flagellated eg sperm cells d) Presence of physiochemical properties (substances) - This is present in association with stratified squamous epithelium. If keratin materials are present it is called keratinized stratified squamous epithelium but if keratin is net present they are called non-keratinsed stratified squamous epithelium. FURTHER CONSIDERATION 1. Simple squamous epithelium cells are flat and appear like flakes. It is one layer of cell on a basement membrane. It is found lining - 1) The bowman’s capsule & the loop of Henle of the kidney tubules 2) The alveoli of the lungs 3) The inner or luminal surfaces of blood vessels and the inner chambers of the heart. In this places they are called endothelia 4) Inner surfaces of the body cavities, outer surfaces of visceral organs. In this case, they are described as mesothelia. 2) Simple cuboidal epithelium- The is a single layer of Cuboidal cells on the basement membrane. The cells appear like squares. They are found lining - 1) The PCT & DCT of the kidney 2) The collecting duct of the kidney tubule 3) The outer surfaces of the ovary where they are called germinal epithelium. 4) The respiratory bronchioles 5) The follicles of the thyroid gland 6) The rete testis 3) Simple columnar epithelium – This is one layer of columnar cell on a basement membrane. The cells are taller than they are wide. They are found in the 1. stomach, 2. Small & large intestines. In these areas they normally bear microvilli especially in the small intestine which increases their surface area for absorption. They equally have secretary functions (some are modified to form glands). 4) Pseudo stratified columnar epithelium – This is type of epithelium which consists of columnar cells with two nuclei giving an impression that there are more than one layer of cell. This type of epithelium is found lining the auditory tube and vas deferens. 5) Simple columnar ciliated epithelium- This is a layer of columnar cells on the basement membrane with cilia present on their apical surfaces. They are found lining the uterus b/4 puberty and the uterian /fallopian tube. In this area, the ciliary movements facilitate movement of substances found on them as they beat like wave. 6) Pseudostratified columnar ciliate epithelium. PSCCE or respiratory epithelium consists of two types of cells. The main cells are columnar and sangwiched inb/w the columnar cells close to the basement membrane are short cells called the basal cells. On the apical surfaces of the columnar cells are present cilia. The ciliary action move substances that have trapped dust particles. They are found lining the respiratory tracts which include the nasal cavity, the larynx nasopharynx, the trachea and the bronchioles. 7) Stratified cuboidal epithelium- Two or more layer of cuboidal cells on the basement membrane. It is found lining some ducts like the lactiferous duct of the mammary gland. 8) Stratified columnar epithelium – Two or more layer of columnar cells on the basement membrane. It is rare and may be found lining the penileuretra. 9) Simple cuboidal ciliated epithelium. A layer of cuboidal cell with cilia on their apexes. It is found in the ependyma cells of the ventricles of the brain. 10) Stratified squamous epithelium – This type of Epithelium consists of several layers of flattened cells. They are found in areas in contact with abrasion. It is subdivided into two types- a) Nonkeratinized stratified S E- In this type keratin materials are about at the apical surfaces. They are found lining organs & surfaces which are in contact with abrasion and at the same time are leapt moist eg the buccal cavity, the oropharynx, largngopharynx, epiglottis and voice box of the largnx the osophagus, the distal end of the anal canal and the vagina. b) Keratinized stratified S.E _ This is the type of stratified S.E with keratin materials present on the surface. It is found in the epithemis of the skin and the filiform papillae. The keratin materials thouglen the skin makes it water proof and resistant to Uv rages of the sun. 11) Transitional epithelium _ These consist of 4-6 Layers of cells on a basement membrane. The basal cells are either columnar or cuboidal in shape. The intermediate cells are oval or polyhedral in shape and subsequently toward surface the cells become progressively larger and the apical cells are umbrella-shaped. The cells alter their shape with a state of distension of the organ. They are found lining the urinary tract and are d/4 called the urothelia. They line the renal pelvis, the ureter, the urinary bladder and part of urethra. GLANDULAR EPITHELIUM (GLANDS) Glands are epithelia cells which are capable of elaborating secretions ie epithelia cells that have secretory functions. If there is only one epithelia cell with secretory function among several non-secretory cells, such a cell is described as a unicellular gland. On the other hand, Multicellular glands are those glands which consist of several secretory cells. Most of the glands in the body are multicellular glands. Eg of unicellular glands include – goblet cell and paneth cell found in the respiratory and digestive systems. Multicellular glands are sub-classified into two types 1) Endocrine glands- These are multicellular glands whose secretions are passed directly into the blood stream or ISF and are conveyed to their target organs through the blood. They normally lack ducts and are described as ductless glands. They can as well be called internally secreting glands eg pituitary gland, pineal gland, thyroid, parathyroid, thymus, pancreatic Islets of langerhans, suprarenal glands etc. 2) Exocrine glands – These are multicellular glands whose secretions are passed directly to the surface of the epithelium or through a duct to their target organ. They can be described as externally secreting glands eg salivary gland, pancreas, kidney, liver etc. Exocrine glands can be classified as tubular or alveolar glands depending on the shape of the secretory unit. it is described as tubular if the secretory unit is shaped like a tube. A tubular gland may be straights, coiled or branched.it is said to be alveolar or acini if the secretory unit is rounded or bottom –flask shaped. Depending on the nature of the ducts, exocrine glands can be classified into simple or compound glands. Simple glands are glands with single unbranched ducts while compound glands are glands with one main excretory duct which gives rise to several generations of ducts ie several secretory units drain into smaller ducts which unite to form one main excretory duct. On that basis the following types of glands can be differentiated; 1) Simple straight tubular gland 2) Simple coiled tubular gland eg sweat gland. 3) Simple branched tubular glands. eg uterine glands & gastric glands. 4) Simple alveolar gland 5) Simple branched acinar (alveolar) eg sebaceous gland 6) Compound tubular glands 7) Compound alveolar (acina) gland 8) Compound tubule-alveolar gland (Rasemose) gland Classification based on the nature of secretion 1. Mucous glands- These are glands whose secretion are sling to touch or viscous. It consists of muco proteins eg the goblet cells. 2. Serous glands –These are glands whose secretions are watery and proteinous – in –nature. The secretions consists of lysozyme which are bactericidal eg the parotid salivary gland. 3. Mixed (seromucous) glands- These are glands which secrete both mucous and serous substances eg the sublingual and submandibular salivary glands. 4. Miscellenous glands –These are glands whose secretion are neither mucous nor serous in nature eg the sebaceous gland which secrets an oily substance called sebum, ceruminous gland of the external ear which secrete a waxy substance and the seminal vesicle which secret an alkaline milky substance. Classification of glands based on mode of secretion 1. Holocrine glands –This is a made of secretion in which the secretion are released by a complete disintegration of the cells ie the entire cell disintegrate and get discharged as secretion eg sebacious gland, ovary & testes. 2. Merocrine/accrine glands-This is a mode of secretion in which the secretion are moved out of the cells by exocytosis after which the cells remains intact, eg salivary glands. 3. Apocrine glands –These are glands that discharged the apical part of the cells as secretions. The cells d/4 become shorter eg sweat & mammary glands.  The secreting cells of a gland constitute its parenchyma while the connective tissue in which it lies is called the stroma. CONNECTIVE TISSUES DENSE ORDINARY CONNECTIVE TISSUE 1. Dense connective tissue is vascular and contains fewer cells than loose connective. 2. Two distinct forms of dense connective tissue are recognized, based on the arrangement of their collagen fibers. They are; a. Regular and b. Irregular dense connective tissue. In the regular form, bundles of parallel fibers extend in the direction of tension and constitute effective arrangements for transmitting unidirectional pull. Examples are tendons, aponeurosis and ligaments. In the irregular form of dense connective tissue, fiber bundles lie oriented in a number of planes, that is, they are not all parallel. Accordingly, they resist stretch from various directions. This sort of tissue is suited for many purposes. It constitutes 1) the fibrous protective covering of bones and cartilages; 2) the innermost part of the skin (reticular layer of dermis); 3) the deep fascia of the body; 4) the tough fibrous wrappings of the heart, brain, spinal cord, nerves, and skeletal muscles; 5) the supporting elements (fibrous capsuIe and septa or trabeculae) of glands and organs; and 5) the fibrous valves that ensure one-way flow in the circulatory system. CARTILAGE Cartilage is a specialized form of connective tissue in which the extracellular matrix has a firm consistency. The matrix endows cartilage with the resilience that allows the tissue to bear mechanical stresses without distortion. Cartilage is a strong but slightly flexible semi-rigid supporting tissue. It is resilient enough to withstand compression forces resulting from locomotion and weight bearing and yet can bend. Much of the cartilage that develops prenatally is subsequently replaced by bone tissue. Thus, cartilage plays a key role in the development and growth of long bones, but the cartilaginous growth plates in these bones disappear when postnatal growth is over. The articulating ends of bones nevertheless remain capped by articular cartilages that provide polished gliding surfaces for unimpeded joint motion. Cartilage is also present; 1. in some joints that are not freely movable, 2) as costal cartilages interconnecting top ten pairs of ribs with the sternum, and 3) in the walls of the major airways and respiratory passages (nose, trachea, larynx, and bronchi), where it provides flexible support and guards against airway collapse as a result of respiratory movements or external compression. Like epithelial tissue, cartilage has no capillary blood supply of its own, so its cells must obtain their oxygen and nutrients by long-range diffusion. Cartilage has 2 main components: the cells called chondrocytes and extracellular matrix. The extracellular matrix consists of fibers and ground substance. Variations in the composition of these matrix components produce three types of cartilage. 1. Hyaline cartilage (the most common form) 2. Fibrocartilage (Tendon insertions and intervertebra1 discs of the vertebral column). 3. Elastic cartilage: Hyaline Cartilage has a pearly white translucent appearance resembling that of frosted glass (Gk. hyalos, glass).Hyaline cartilage is the most common and most studied of the 3 types.fresh hyaline cartilage is bluish-white and transluscent. Embryonic chondroblasts differentiating at sites of developing cartilages begin to secrete macromolecular constituents of cartilage matrix. Cells at the periphery of such sites give rise to a fibrous covering known as the perichondrium. The inner part of this covering is described as chondrogenic because it repeatedly gives rise to new chondroblasts that build up more matrix, adding to that already formed. The cells in the outer part of the perichondrium differentiate into collagen-producing fibroblasts, and as a result the developing cartilage becomes covered with a layer of fibrous perichondrium. This outer part of the perichondrium typically remains in adult life, but there are a few instances where both layers of the perichondrium disappear. Articular cartilages, for example, are devoid of a perichondrium. Chondroblasts buried in cartilage matrix are described as chondrocytes. Their tiny matrix enclosed compartments are termed lacunae. In growing cartilages, chondrocytes divide and a partition of matrix begins to form between the daughter cells, resulting in a so-called cell nest of two or four cells. Each living chondrocyte usually fills its lacuna, but fixation usually results in a shrinkage artifact between the cell border and the lacunar wall. Mature chondrocytes are large secretory cells with a spherical nucleus and prominent nucIeolus. Initially their cytoplasm is basophilic, indicating an extensive rER, but later it contains numerous fat droplets and appears vacuolated. CHONDROCYTES At the periphery of hyaline cartilage chondrocytes have an elliptical shape Hyaline Cartilage Matrix 40% of the dry weight of hyaline cartilage consists of collagen embedded in an amorphous (without definite shape or visible differentiation in structure) intercellular substance. With optimal staining, the matrix of hyaline cartilage has a slightly basophilic appearance in H&E stained sections. Its resilient gel structure has a distinctive macromolecular organization. Fibrocartilage A distinctive feature of fibrocartilage is its conspicuous parallel bundles of type I collagen fibers, which are strong enough to resist stretching under extreme tension. Because these bundles are noticeable, they can mislead beginners into mistaking regular dense connective tissue for fibrocartilage unless care is taken to distinguish &between flat fibrocartilage and larger, more rounded chondrocytes wihin lacunae. Many tendon attachments to cartilages are made of fibrocartilage. The chondrocyte lacunae present in the somewhat basophilic matrix typically lie in rows between the collagen bundles (see Plate 8-3). Fibrocartilage is avascular. In adult life, it lacks a perichondnum. Besides constituting tendon insertions, this extremely strong form of cartilage is found in the public symphysis, intervertebral disks of the vertebral column, and articular surfaces of Synovial Joints. Elastic Cartilage The cartilage of the external ear and epiglottis, structures that are subjected to a great deal of bending, is highly flexible and resilient because it contains elastic fibers as well as collagen. An even more resilient kind of cartilage, known as elastic cartilage, is adapted primarily to withstand repetitive bending. It supports the epiglottis and external ear, which are required to be flexible but capable of springing back when bent. Elastic cartilage resembles hyaline cartilage except that in addition to its content of fine type II collagen fibrils, the ma& contains acidophilic elastic fibers. The chondroblasts that produce the various matrix constituents (including the elastin) become embedded in ma~axs chondrocytes. Sihlated in lacunae, the chondrccytes are in some case arranged as cell nests, as in hyaIine cartilage. Also, the fibrous layer of perichondrium ~rsiscs in elastic cartilages. Connective tissues are those tissues which consist predominantly of extracellular matrix secreted by its cells which are spaced within the matrix. Connective tissues fill the interstices b/w more specialized tissues. They give structural support to specialized tissues. As a result, they can be called supporting tissues. Connective tissues also give protecting to more specialized tissues. Connective tissues originate from mesoderm. BASIC COMPONENTS OF CONNECTIVE TISSUES These are- 1. Cells eg 2) Matrix / ground substance / intercellular or extracellular matrix in which the and fibers are embedded 3) Fibers which include collaginous fibers, Elastic fibers and Reticular fibers. NB Fibers are sometimes regarded as part of the matrix. CLASSIFICATION OF CONNECTIVE TISSUES Connective tissue are broadly classified into two a) General connective tissue and b) Specialized connective tissue Specialized connective include; 1. Lymphoid tissue 2. Cartilage 3. Blood and 4. Bone General C.Ts are classified into two depending on the density of the fibers – 1. Dense/ fibrous C.T - Those with densely packed fibers. 2. Loose C. T - Those with less dense fibers. DENSE C. Ts ARE FURTHER SUB CLASSIFIED INTO a) Regular dense and b) Irregular dense C. T Based ON THE ARRANGEMENT OF THE FIBERS Loose C. Ts are sub classified into a) Embryonic and (b) Adult C. T. The embryonic types are found mainly in the embryo and include -  Mesenchyma C.T and  Mucoid C. T The Adult loose C.Ts are those found in adults and are sub classified into  Loose areolar C.T  Reticular C.T and  Adipose C.T MESENCHYMA C.T Mesenchyma C.T consists of a spongy, fluidy grand substance. The cells are the mesenchyma cells which are pluripotential stem cells. Some collagenous fibers are present. Mesenchyma connective tissues are found in developing embryo where they help to separate the developing organs of the embryo. It is a transcient type b/cos it can be converted to any other type of connective tissue in the adult. MUCOID C.T These are types of loose connective tissue with viscous jelly –like or slimy ground substances. The ground substance is made up of mucour proteins/ substance. There are stray – shaped cells and some rounded cells may be present. Collaginous fibers are scattered within the ground substance. This type of connective tissue is found in form of Wharton’s jelly found in the unbilical cord it can also be found in the vitrous body of the eye. LOOSE AREOLA C. T These are tissues with loosely arranged bundles of collaginous fibers which interlace, interwove and criss-cross to form network. Elastic fibers are also present. B/w the fibers are found spaces called areola hence the name. Numerous connective tissue cells are present. these include fibroblast cells, macrophages etc. They are found in the submucosa of digestive organs, subserosa of viscera organs and blood vessels. They form stroma of most organs in the body and are found b/w blood vessels, nerves and lymphatic. RETICULAR C. T Consist of reticular fibers and reticular cells. Reticular cells are fibroblasts and histocytes. It is found in lymph nodes, spleen, thymus gland, bone marrow etc. ADIPOSE C.T This is simply an aggregation of fat cells which may be surrounded by reticular fibers and rich capillary network and are well cascularised. They are found deep to the skin ie subcutaneous tissue (penicullus adiposus), In the adult female breast and buttocks, in hollow spaces and around some organs like the orbit where they protect the eyeball during its movement and the peripephric fats around the kidney which protects it during its movement. It serves as a store of nutrition It generates heat It serves as insulator It gives protection to some organs DENSE C.T These are connective tissues with densely packed collaginous fibers. Depending on the arrangement of the fibers they are subdivided into two types 1. Regular dense connective tissue which consist of dense packing of collaginous fibers which run parallel to each other and along the same line of force. Elastic fibers may be present. The predominant cells are fibroblast cells. The tissue has high tensile strength. Egs include ligament, tendon and aponeurosis. Irregular dense connective tissue which consist of dense packing of collaginous fibers which interlace, crisscross, and interwove to form network. Elastic fibers may be present but predominantly are fibrolast cells. It is found in the superficial fascia, the capsule of most glands, the perichondron, the periosteum etc. CELLS OF CONNECTIVE TISSUES The C.T cells are of two types- A- Resident cells which are of the following types 1) Fibroblast cells 2) Fat cells 3) Mesenchyma cells 4) Pigment cells B - Migratory cells which are net actually C.T cells but may migrate into connective tissue. These include – 1) Plasma cells 2) Mast cells 3) Eosinophil cells 4) Basophil cells 5) Lymphocyte cells etc Nervous Tissue NEURONS Neurons are composed of three basic parts: the cell body (soma or perikaryon); the dendrites which receive information from other neurons; and a single axon, which conducts electrical impulses away from the cell body. Cell Body The cell body contains a large vesicular nucleus with a single prominent nucleolus, mitochondria, and other organelles. It has abundant RER, reflecting high rates of protein synthesis. At the light microscopic level, the RER stains intensely with basic dyes and is referred to as Nissl substance. Microtubules and neurofilaments contribute to the neuronal cytoskeleton and play important roles in axonal transport. Pigment granules such as lipofuscin ("wear and tear" pigment) and melanin (found in some catecholamine-containing neurons) may be seen in the cytoplasm. Dendrites Dendrites are neuronal processes that receive information and transmit it to the cell body. Extensive dendritic branching serves to increase the receptive area of the neuron. Axons Axons are thin, cylindrical processes typically arising from the perikaryon (or from a proximal dendrite) through a short pyramidal-shaped region called the axon hillock. The cell membrane of the axon is called the axolemma, and the cytoplasm of the axon is called the axoplasm. Axonal transport Axons contain abundant microtubules and neurofilaments. Axon fast transport uses microtubules. It proceeds in both anterograde and retrograde directions. Anterograde transport is powered by kinesins, whereas retrograde transport is powered by dynein. Synaptic boutons Axons terminate in specialized endings known as synaptic boutons, which contain synaptic vesicles full of neurotransmitters. MeClical Myelin Axons may be unmyelinated or myelinated, depending on the type of covering provided by their supporting cells. Unmyelinated Axons Unmyelinated axons in peripheral nerves are surrounded by the cytoplasm of Schwann cells.. These axons have a small diameter and a relatively slow conduction velocity. A single Schwann cell may ensheath several axons. Myelinated Axons Myelinated axons are larger in diameter and are ensheathed in myelin. Schwann cells are the myelin-forming cells of the peripheral nervous system (PNS). Myelination in the PNS begins during the fourth month of development. One Schwann cell will myelinate only one axon in peripheral nerves. Oligodendrocytes are the myelin-forming cells of the central nervous system (CNS). In the CNS, myelination begins during the fourth month of development and continues into the second decade of life. An individual oligodendrocyte is able to myelinate many axons. Node of Ranvier At the junction between two myelin-producing cells, there is a discontinuity in the myelin. This creates a "collar" of naked axon, called a node of Ranvier, which is exposed to the extracellular space. The action potential skips from node to node in a process called salutatory conduction. Myelinated axons conduct action potentials rapidly. Composition Because myelin is of membrane origin, it is rich in phospholipids and cholesterol. Classification of Neurons by Neuronal Processes Unipolar neurons Unipolar neurons have one axon and no dendrites and probably occur only during development. Pseudounipolar neurons Pseudounipolar neurons have a single process close to the perikaryon, which divides into two branches. One branch extends to a peripheral ending, and the other extends to the CNS. Pseudounipolar neurons are found in dorsal root ganglia and most cranial ganglia. Bipolar neurons Bipolar neurons have one axon and one dendrite. Bipolar neurons are found in the cochlear and Vestibular ganglia as well as in the retina and olfactory mucosa. Multipolar neurons Multipolar neurons have one axon and multiple dendrites. Most neurons in the body are multipolar (e.g.,ventralhorn neuronsin the spinalcord). Classification of Neurons by Functional Role Motor neurons Motor neurons control effector organs and muscle fibers. Sensory neurons Sensory neurons receive sensory stimuli from the internal or external environment and relay them to the CNS. Synapses Synapses are specialized membrane junctions designed for the unidirectional communication between neurons or between neurons and effector cells. The pre- and postsynaptic membranes are separated by only 20 nm; this space is called the synaptic cleft. Types of synapses Synapses are either between an axon and a dendrite (axodendritic) or between an axon and a cell body (axosomatic). Synapses between dendrites (dendrodendritic) and between axons (axoaxonic) also occur. Synaptic vesicles Synapses contain synaptic vesicles. They consist of 30- to 50 spherical or ovoid structures in the axoplasm that contain neurotransmitter (e.g., acetylcholine [ACh]). Neurotransmitter is released into the synaptic cleft at the synapse when synaptic vesicles fuse with the presynaptic membrane. Neurotransmitters may either excite (depolarize) or inhibit (hyperpolarize) the postsynaptic membrane, depending on the type of receptor to which it binds. Certain neurotransmitters are inactivated in the synaptic cleft by enzymatic degradation (e.g.,ACh is broken down by acetylcholinesterase [AChE]), whereas others are taken up by the presynaptic cell (e.g., norepinephrine) in a process called reuptake. Neuromuscular Junction The neuromuscular junction occurs at the motor end plate. It is the synapse between neurons and muscle cells. At the neuromuscular junction, the axon forms a number of small branches that fit into grooves on the muscle where the postsynaptic membrane is convoluted into numerous folds, called the subneural clefts. ACh released from the axon depolarizes the sarcolemma via the acetylcholine nicotinic receptors. NEUROGLIA Neuroglia (nerve glue) serve as the connective tissue cells of the nervous system.Although they do not generate or transmit neural impulses, they play an important role in the normal functioning of the nervous system. They form the myelin sheaths ofaxons and provide metabolic support to neurons. Neuroglia of the CNS include microglia, astrocytes, oligodendrocytes, and ependymal cells. In the PNS, neuroglia cells consist of Schwann cells. Astrocytes are the largest of the neuroglial cells.They have centrally located nuclei and numerous long processes with expanded vascular end-feet, or pedicels, which attach to the walls of blood capillaries. Astrocytes are important in controlling the microenvironment of nerve cellsand participate in the maintenance of the blood-brain barrier. Oligodendrocytes Oligoderidrocytes have small nuclei and contain abundant mitochondria, ribosomes, and microtubules. Oligodendrocytes myelin ate axons in the CNS. Microglia Microglia are small, dense, elongated cellswith elongated nuclei. They originate from the mesoderm, unlike other neuroglial cells,which originate from the neuroectoderm. Microglia are phagocytic and are part of the mononuclear phagocyte system. EpendymalCells Ependymal cells line the ventricular cavities of the brain and the central canal of the spinal cord. They are capable of mitosis and can develop long processes that deeply penetrate the neural tissue. Cilia on the ependymal cells help move cerebrospinal fluid through the ventricles. Schwann Cells Schwann cells contain elongated nuclei that lie parallel to the axons of peripheral neurons. Schwann cells myelinate peripheral axons. MUSCLE ISSUE GENERALFEATURES Muscle is classified as skeletal, cardiac, or smooth. Some general features of all three types of muscle are summarized below. SKELETAL MUSCLE General Features Skeletal muscle fibers consist of long cylindrical fibers with multiple ovoid nuclei located peripherally beneath the sarcolemma (plasma membrane) and with striations composed of alternating dark and light bands. The dark bands are called A bands because they are anisotropic (birefringent) in polarized light. In the center of the A band a paler region, the H band, is seen in relaxed muscle. The light bands are called 1bands (isotropic), and a dark transverse line, the Z line, bisects each 1band. These bands and the Z lines are well demonstrated in electron micrographs of skeletal muscle. Myofibrils Skeletal muscle fibers contain 1- to 2-mm myofibrils that lie in the sarcoplasm (cytoplasm) parallel to the long axis of the muscle fiber. Myofibrils are composed of a series of sarcomeres that consist of interdigitating polarized thin filaments and bipolar thick filaments. The sarcomeres are the basic units of contraction of striated muscle. Sarcomere Structure The banding pattern seen in striated muscle is caused by the arrangement of thin and thick myofilaments. Thick filaments occupy the central portions of the sarcomere. Thin filaments attach at one end to the Z line and run parallel to, and between, the thick filaments. 1- bands are composed of thin filaments only. A bands are composed mostly of thick filaments and the thin filaments between them. H bands are composed of thick filaments only. Thin filaments Thin filaments are composed of the proteins actin, tropomyosin, and troponin. Actin is a long fibrous structure (F-actin) composed of two strands of spherical or globular G-actin monomers twisted in a double helix:. The filament is polar and contains myosin-binding sites on the G-actin monomers. Tropomyosin is a polar molecule containing two polypeptide chains in the form of an a-helix. The tropomyosin molecules lie head-to-tail to form filaments that lie in the grooves of the actin helix. Troponin (Tn) is composed of three polypeptides: TnT binds to tropomyosin at intervals along the thin filament, TnC binds calcium ions, and Tnl inhibits actin- myosin interaction. Thick filaments Thick filaments are composed of myosin. Myosin is a molecule that contains a tail and two heads.. The tail fiber is formed from portions of two heavy chains, which are wound in a coil. The heads are globular regions formed by the association of part of one heavy chain with two light chains. Myosin heads function as active sites for ATPase activity and as actin-binding sites. Transverse Tubular System Skeletal muscle fibers contain fingerlike invaginations of the sarcolemma that surround each myofibril. These invaginations constitute the transverse (T) tubule system. Note the following:  Each T tubule lies between the two cisternae of the sarcoplasmic reticulum (SR) to form a triad.  There are two triads in each sarcomere, which are present at the junction between the A and 1bands.  These units serve to couple excitation of muscle cellsto their contraction (excitation-contraction coupling). CARDIAC MUSCLE Cardiac muscle has an arrangement of sarcomeres similar to that in skeletal muscle as well as a T tubule system associated with the SR (near the Z line). However, unlike skeletal muscle fibers, the fibers are electrically coupled through gap junctions. Cardiac muscle fibers are joined together by junctional complexes called intercalated discs. SMOOTH MUSCLE Smooth muscle is found in the walls of blood vessels and hollow viscera. Bands of smooth muscle cells can be found in the erector pili muscles of the skin.Gap junctions electrically couple smooth muscle cells. Filaments Smooth muscles contain actin and myosin filaments, but the filaments are not arranged in orderly arrays as in skeletal muscle.. Bundles of myofilaments course obliquely in the cell, forming a lattice-like arrangement. A sliding filament mechanism of contraction is thought to occur.. Thin filaments insert into dense bodies located within smooth muscle cytoplasm and attach to their membranes. Contraction Smooth muscle contraction may be triggered by various stimuli such as autonomic nerves or hormones. Depolarization of the cell membrane results in an influx of Ca2+from outside the cell.Ca2+is sequestered in either the cell membrane or in the sparse SR.