Histology of Connective Tissues PDF

Histology Connective tissues  Types of tissues Epithelial tissue Connective tissue Muscular tissue Nervous tissue  Connective tissues The connective tissues are classified according to the nature of cellular components and intercellular substances i...

Histology Connective tissues  Types of tissues Epithelial tissue Connective tissue Muscular tissue Nervous tissue  Connective tissues The connective tissues are classified according to the nature of cellular components and intercellular substances into the following major groups: - Connective tissue proper (Fibrous Tissue) - Skeletal or supporting tissues (Cartilage and Bone) - Fluid or vascular tissues (Blood and Lymph)  Skeletal or supporting tissues Skeletal tissues build up the endoskeleton of the vertebrate body, giving it support, and hence they are called supporting tissues. They also serve for the attachment of the muscles to bones, and hence their name. In addition, they protect delicate structures as the brain and spinal cord. There are two types of skeletal tissues: cartilage and bone  Cartilage Cartilage is a type of connective tissue in which the solidification of its matrix renders it resistant to either pressure or tension. The matrix of cartilage is avescular. For the purpose of nutrition, it is surrounded by a highly vascular layer of dense fibroelastic tissue, the perichondrium, from which the tissue obtains the nutritive substances by diffusion. Cartilage also lacks lymphatic vessels and nerves. Perichondrium The perichondrium is a sheath of dense connective tissue that surrounds cartilage in most places It forms an interface between the cartilage and the tissues supported by the cartilage It harbors the cartilage’s vascular supply, as well as nerves and lymphatic vessels  Cartilage Cartilage is composed of: - cells called chondriocytes (chondros : cartilage + kytos : cell) Chondrocytes synthesize and maintain ECM components and are located in matrix cavities called lacunae. - And intercellular matrix secreted by these cells, according to the nature of matrix, cartilage is distinguished into three types:  Hyaline  Elastic  Fibrous  Hyaline cartilage Is the most common type of cartilage. It possess a pale blue or pearly translucent matrix. In the matrix there are collagenous fibers which are not visible, since the matrix and fibers have the same refractive index. In the matrix are spaces called lacunae which contain the cartilage cells (chondroblasts or chondriocytes) arranged singly or in clusters usually consisting of two or four fully differentiated cells. The area of matrix surrounding each lacuna is denser than the rest of the matrix and thus forms a capsule around the lacuna. Is found in the costal cartilage that bind the bony portions of the ribs to the sternum, and in cartilaginous rings of the trachea and the bronchi.  Hyaline cartilage Smooth tissue, homogeneous and semitransparent It has a perichondrium (Except in the articular cartilage of joints) The chondrocytes are arranged in small groups (isogenous group) Matrix is composed of collagen fibers (most type II) embedded in a firm, hydrated gel of proteoglycans and structural glycoproteins. In adults its found in the articular surfaces of movable joints : - Joints - Ribs : in the ventral ends - Walls of respiratory passages (nose, larynx, trachea, bronchi) - Embryonic skeleton : forms the temporary skeleton that is gradually replaced by bone  Hyaline cartilage The Chondrocytes : - Cells occupy relatively little of the hyaline cartilage mass - At the periphery of the cartilage, young chondrocytes (or chondroblasts) have an elliptic shape - Deeper in the cartilage, they are round and may appear in groups of up to eight cells called “isogenous aggregates” - As the chondrocytes become more active in secreting collagens and other ECM components, the aggregated cells are pushed apart and occupy separate lacunae  Hyaline cartilage The perichondrium - Except in the articular cartilage of joints, all hyaline cartilage is covered by the perichondrium (essential for the growth and maintenance of cartilage) - Consists largely of collagen type I fibers and fibroblasts - In the inner layer of the perichondrium are progenitor cells for chondroblasts that divide and differentiate into chondrocytes  Elastic cartilage It has a matrix that contains, in addition to the collagenous fibers present in hyaline cartilage, a network of elastic fibers which gives the tissues certain elasticity. Elastic cartilage occurs in the epiglottis of the larynx, in the Eustachian tube wall, and in the ear pinna.  Elastic cartilage Similar to hyaline cartilage except that it contains an abundant network of elastic fibers in addition to collagen type II This flexible tissue possesses a yellowish color Chondrocytes lying between the fibers as a isogenous group It has a perichondrium Found in : - in the auricle of the ear - The walls of the external auditory canals - The auditory (eustachian) tubes - The epiglottis  Fibrous cartilage It is composed of numerous parallel bundles of dense collagenous fibers, between which are seen one or several lacunae containing chondrocytes. Fibrous cartilage is functionally adaptable to those situations in the animal body where a combination of tensile strength and flexibility is required, e.g. intervertebral discs. Chemically, the matrix is composed of chondromucoid, chondriotin sulphuric acid and albuminoid.  Fibrous cartilage Consists of dense masses of white collagen fibres in a matrix similar to that of hyaline cartilage Chondrocytes are widely dispersed and occur singly and in aligned isogenous aggregates and produce matrix containing type I collagen Regions with chondrocytes and matrix are separated by other regions containing bundles of type I collagen The relative scarcity of proteoglycans makes the matrix of fibrocartilage more acidophilic than that of hyaline or elastic cartilage Found in intervertebral discs : act as lubricated cushions and shock absorbers preventing adjacent vertebrae from being damaged by abrasive forces or impacts  Bone Bone tissue is the main constituent of the adult skeleton. Bones provides solid support for the body. They protects vital organs such as those in the cranial and thoracic cavities, and harbors cavities containing bone marrow where blood cells are formed. Bone tissue also serves as a reservoir of calcium, phosphate, and other ions that can be released or stored in a controlled way. Bones gives attachment to muscles and tendons. They allow movement of the body as a whole and of parts of the body, by forming joints that are moved by muscles  Bone tissue Bone is a strong and durable type of specialized connective tissue composed of calcified ECM called “ the bone matrix”, and three major cell types. Its major constituent (65%) is a mixture of calcium salts, mainly calcium phosphate  it gives bone great hardness. The remaining third is organic material, called osteoid, which is composed mainly of collagen, proteoglycans and several glycoproteins.  Bone Bone is fundamentally a connective tissue consisting of cells, collagenous fibers, and a matrix impregnated with calcium salts, mainly phosphate and carbonate, and an organic material called Ossein. Types of bones: There are two types of mammalian bone: - Dense (compact) bone - Spongy bone Both types are present in practically every bone, for they are merely different arrangements of the same histologic elements.  Bone cells The cellular component of bone contributes less than 2% of bone mass  Osteoblasts ( osteon: bone + blastos: germ)  Osteocytes (osteon: bone + kytos: cell) : found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes extending into small canaliculi between lamellae  Osteoclasts (osteon: bone + klastos: broken) : multinucleated, giant cells involved in the resorption and remodeling of bone tissue  Bone cells Osteoblasts are responsible for the synthesis of the organic components of bone matrix. They are exclusively located at the surface of bone tissue, side by side, in a way that resembles simple epithelium. When they are actively engaged in matrix synthesis, osteoblasts have a cuboidal to columnar shape and basophilic cytoplasm. When their synthesizing activity declines, they flatten and basophilia is reduced.  Bone cells Osteocytes, which derive from osteoblasts, lie in the lacunae situated between lamellae of matrix. The thin, cylindrical matrix canaliculi house cytoplasmic processes of osteocytes. Processes of adjacent cells make contact, and molecules are passed via these structures from cell to cell.  Bone cells Osteocytes Many osteoblasts are gradually surrounded by the material they secreted and differentiate into osteocytes enclosed within the lacunae. Osteocytes are flat (almond shaped), less RER, smaller Golgi complexes, and more condensed nuclear chromatin comparing to osteoblasts. During the transition from osteoblasts to osteocytes, the cells extend many long dendritic processes which also become surrounded by calcifying matrix. Osteocytic processes come to occupy the many canaliculi that radiate from each lacuna. They receive nutrients from microvasculature in the central canals of the osteons via very small channels called canaliculi that interconnect the lacunae. Osteocytes maintain the bony matrix, and their death is followed by rapid matrix resorption.  Bone cells Osteoclasts Osteoclasts are very large, motile cells with multiple nuclei (fusion of bone marrow–derived cells). They play a major role in matrix resorption during bone growth and remodeling. In active osteoclasts, the surface against the bone matrix is folded into irregular projections, forming a ruffled border. This adhesion zone creates a microenvironment between the osteoclast and the matrix in which bone resorption occurs  Periosteum & Endosteum External and internal surfaces of bone are covered by 2 tissue layers: - The periosteum - The endosteum  The periosteum is organized much like the perichondrium. Its composed of two layers - The outer layer : dense connective tissue, with small blood vessels, collagen bundles, and fibroblasts - The inner layer :is cellular containing bone lining cells, osteoblasts as well as the osteoprogenitor cells and osteoclasts - The principal functions of periosteum are to nourish the bone tissue and provide a continuous supply of new osteoblasts for appositional bone growth or repair  Periosteum & Endosteum  The endosteum : - Very thin and covers small trabeculae of spongy bone - Thinner than the periosteum, endosteum also contains osteoprogenitor cells, osteoblasts, and bone lining cells  Bone anatomy : compact bone Compact bone makes up about 80% of the body bone mass Dense and looks smooth and homogeneous The hardest and strongest structure of the bone It is made up of a large number of parallel tube shaped units called osteons (Haversian systems) Each osteons is made up of a central canal surrounded by a series of expanding rings (lamella)  Bone anatomy : compact bone Canal is linked with neighbouring canals by the perforating canals (volkman canal) Between the adjacent lamellae are cavities called lacunae, in each of which sits an osteocyte Osteocytes communicate with each other through the canaliculi  Bone anatomy: Spongy (cancellous, trabecular) bone Macroscopic : similar to a sponge with lot of pores. Microscopic : a framework formed from trabeculae, which consist of a few lamellae and osteocytes interconnected by canaliculi. The spaces between the trabeculae contain red bone marrow. Osteocytes are nourished by interstitial fluid diffusing into the bone through the tiny canaliculi. Spongy bone is lighter than compact bone, reducing the weight of the skeleton  Bone matrix Inorganic material represents about 65% of the dry weight of bone matrix. Calcium, phosphate, bicarbonate, citrate, magnesium, potassium, and sodium ions are also found. The organic matter embedded in the calcified matrix includes type I collagen, proteoglycan aggregates, and multi-adhesive glycoproteins. The association of minerals with collagen fibers during calcification is responsible for the hardness and resistance of bone tissue. After a bone is decalcified, its shape is preserved, but it becomes as flexible as a tendon  Structure of a limb bone A typical long bone, e.g. femur, consists of a long shaft, the diaphysis, and two swollen ends each are called epiphysis, the shaft is hollow and contains a soft fatty tissue, the bone marrow. In certain regions, the bone marrow (red) contains highly vascular sinuses in which red and white blood cells are produced. The whole bone is surrounded by a fibrous membrane, periosteum, and the cavity is lined by a more delicate membrane, endosteum. The shaft and epiphysis are formed of Dense bone, but towards the end of shaft and below epiphysis, bone is spongy. In the spongy bone, there is an interconnecting system of bony bars with the spaces between them in continuity with the marrow.