Types of Connective Tissue Proper PDF

# Types of Connective Tissue Proper According to the density of matrix and to the main cells and fibers found, connective tissue proper is classified into: ## I. Loose Connective Tissue 1. Loose areolar C.T. - All the structural elements of connective tissue: cells, fibers, and amorphous su...

# Types of Connective Tissue Proper According to the density of matrix and to the main cells and fibers found, connective tissue proper is classified into: ## I. Loose Connective Tissue 1. Loose areolar C.T. - All the structural elements of connective tissue: cells, fibers, and amorphous substance, are present in the loose areolar connective tissue. - The most common cell types are fibroblast and macrophages. - White collagenous fibers are most prominent. - Functions and sites: Loose areolar connective tissue is the packing and anchoring material and the embedding medium of many structures, including nerves and blood and lymphatic vessels. It binds other tissues, organ components, and organs together. It is found in almost every part of the body. 2. Adipose C.T. - If the loose connective tissue becomes dominated by fat cells it is called adipose tissue. - There are two types of adipose connective tissue according to the vascularity and the function: white and brown. ### A. White Adipose Connective Tissue - White adipose connective tissue is widely distributed in the body. Fat cells contain one large fat droplet which occupies most of the cytoplasm. - It appears yellow as it contains carotene pigment. - Function: It represents the primary site of fat metabolism and storage in the body. ### B. Brown Adipose Connective Tissue - Brown adipose connective tissue is present in small amounts in adults. It is present in large amounts in fetuses and neonates and then it is gradually replaced by white fat. - Fat cells store fat in multiple droplets, not as a single large one liked white fat. - Its brown color is caused by the high vascularity of this tissue. - Function: The main role of brown fat is to provide heat, critical for newborn babies. 3. Reticular C.T. - This is a primitive type of connective tissue that is characterized by the presence of a network of reticular fibers associated with primitive reticular cells (mesenchymal-like cells). - Reticular connective tissue forms the framework of solid organs e.g. lymphoid organs, bone marrow, and liver. - Network of reticular fibers in a typical loose ground substance; reticular cells lie on the network. - Location: Lymphoid organs (lymph nodes, bone marrow, and spleen) 4. Embryonic mesenchymal C.T. - It is an unspecialized connective tissue of the early weeks of embryonic life. - It is formed of undifferentiated mesenchymal cells, ground substance, and fine reticular fibers. - Function: Gives rise to all other connective tissue types. - Location: In Embryo. 5. Mucous C.T. - Mucous connective tissue is formed of stellate (branched like a star) fibroblasts, abundant ground substance (gives a mucin reaction) and fine collagenous fibers. - It is found in the umbilical cord. - In cross-section umbilical cord contains gelatinous mass of mucous connective tissue called Wharton's jelly, including abundant collagenous fibers that enclose the three umbilical blood vessels. - They are two arteries with an outer circular coat of smooth muscle and a thicker longitudinal layer. These arteries contain no internal elastic lamina, and the adventitia is replaced by mucous connective tissue. - The single vein has a larger lumen, yet resembles the arteries because it has a thick muscular wall. - The umbilical vein has no valves or vasa vasorum. ## II. Dense Connective Tissue - Dense connective tissue is characterized by the close packing of their fibers, few cells (fibroblasts and fibrocytes) and only a small amount of amorphous intercellular material. - According to the type of fibers, it is classified into dense white collagenous and dense yellow elastic connective tissues. ### 1. Dense White Collagen Connective Tissue - When collagen fibers are predominant in a dense connective tissue, it is called "dense white collagen connective tissue" because in fresh state it is white in color. - According to the arrangement of the fibers, two types are found: irregular and regular dense white collagen connective tissues. #### A. Irregular Dense White Collagen C.T. - The collagen fibers are arranged in an irregular pattern. There is little ground substance and the cells are mainly fibroblasts and fibrocytes. - Function and sites: Irregular dense white collagenous connective tissue can withstand stretch from several different directions. It is found in areas where tensions are exerted in multiple directions e.g. in deep fascia, dermis of the skin, fibrous capsules of organs (liver, lymph nodes, etc.), fibrous sheath of cartilage (perichondrium), and bone (periosteum). #### B. Regular Dense White Collagen C.T. - The collagen fibers are arranged in a regular parallel pattern. The cells found are fibrocytes with minimal matrix. - Function and sites: Regular dense white collagenous connective tissue can withstand strength in one direction. This group includes tendons, aponeuroses (wide, flat tendons expanded into a fibrous sheet), and ligaments. The histological sections in tendons, aponeuroses and ligaments are almost identical. The fibrocytes of tendons are called "tendon cells." They are also found in the fibrous coat of the eye (cornea and sclera). ### 2. Dense Yellow Elastic Connective Tissue - When elastic fibers are predominant in a dense connective tissue, it is called dense yellow elastic connective tissue. It is yellow in color in fresh state. - It is formed of regular parallel elastic fibers bound together by a small amount of delicate fibers with many fibroblasts and fibrocytes. - Function and sites: These elastic ligaments have the ability to return to their original length after stretching. This type is located in yellow elastic ligaments, ligamentum nuchae (at the back of the neck) and ligamentum flavum (between adjacent vertebrae). # Cartilage - Cartilage and bone are connective tissues in which the amorphous intercellular substance is hardened to provide rigidity, support, and attachment for tissues. - The cartilage is a firm, flexible, and strong connective tissue. - The cartilage, like other connective tissues, is formed of cells, fibers and intercellular substance (matrix). ## I. Cells ### 1. Chondroblasts - At LM level: Chondroblasts are oval or spindle in shape with basophilic cytoplasm. They are located in the inner aspect of perichondrium. - At EM level: Chondroblasts show all the ultrastructural features associated with protein synthesis (abundant RER, ribosomes, prominent Golgi apparatus, and many mitochondria). The nucleus contains euchromatin (pale). - Function: Chondroblasts develop from embryonic mesenchymal cells. They are cartilage forming cells. They continue to synthesize the protein of the matrix and its fibers (type II collagen fibers). When chondroblasts are mature and imprisoned inside lacunae they are called chondrocytes. ### 2. Chondrocytes - At LM level: Chondrocytes appear branched (irregular cell membrane), because they shrink during histological preparation of the sections. Each cell has a central rounded nucleus and basophilic vacuolated cytoplasm. Chondrocytes are embedded in the matrix inside spaces called lacunae. Young chondrocytes can divide inside lacunae once or twice forming what is so-called cell nest. Mature chondrocytes do not divide. The lacunae are surrounded by a deeply basophilic condensed matrix. - At EM level: Chondrocytes have most of the usual organelles associated with protein synthesis. The nucleus contains heterochromatin (dark). - Function: Chondrocytes, like chondroblasts, are responsible for the formation and secretion of collagen fibers (type II) and the protein of cartilage matrix. ## II. Fibers - Both collagen and elastic fibers could be found in the matrix of cartilage. - Collagen fibers type II are produced by chondroblasts and chondrocytes. - The appearance of fibers in the matrix depends on the refractive index of the and its contents. ## III. Ground Substance (Matrix) - Proteins of the matrix (intercellular substance) are produced by cartilage cells (chondroblasts and chondrocytes). They consist of: - Chondo-mucoprotein. (protein polymer). - Chondroitin sulfate (polysaccharide-glycosaminoglycan), which causes the basophilia of the matrix. # Types of Cartilage According to the type of fibers embedded in the matrix, cartilage is classified into: hyaline cartilage, collagenous fibrocartilage, and elastic fibrocartilage. ## 1. Hyaline Cartilage - Perichondrium: Hyaline cartilage is covered by perichondrium (Gr. peri-around), except on the articulating ends of joints. It has two layers, an outer white collagen fibrous vascular layer and an inner cellular layer of chondroblasts. - Matrix: The matrix contains both collagen and elastic fibers. It appears clear basophilic (Gr. hyalos = glass) because matrix has the same refractive index as collagen and elastic fibers, so fibers cannot be seen at LM level. The matrix is condensed around the lacunae that contain chondrocytes (cell nest). - Cells: Chondrocytes are located inside lacunae and they are embedded in the matrix forming cell nests. Young chondrocytes can divide once or twice, so each cell nest may contain one, two, or four chondrocytes. Mature chondrocytes do not divide. ## Functions and Sites 1. Development of Bones - Hyaline cartilage is useful as fetal skeleton owing to its ability to provide support and grow rapidly. 2. Growth of Bones - In children, it is found in epiphyseal plates. Chondrocytes grow and are replaced by bone, so epiphyseal plates enable a bone to grow to its adult length and then they disappear. 3. Articulation of Bones - In adult, it is found in the articular surface of most synovial joints. Hyaline cartilage provides the smooth gliding surfaces needed for the articulation of bones, and so they are termed articular cartilages. 4. Maintains Airways in Respiratory Passages - Hyaline cartilages are found in the nose, larynx, trachea, and bronchi of the respiratory system. It is also found in costal cartilages. ## 2. Elastic Cartilage - Perichondrium: Elastic cartilage is covered by perichondrium. It has two layers, an outer white collagen fibrous vascular layer and an inner cellular layer of chondroblasts. - Matrix: In elastic cartilage, the matrix has many elastic fibers and few collagen type II fibers embedded in a small amount of intercellular matrix. - Cells: Chondrocytes are located inside lacunae and form cell nests. - Functions and Sites: It is a very flexible and semi-rigid type of cartilage. It can return to original shape after being deformed. Elastic cartilage is found in the external ear (ear pinna), auditory (eustachian) tubes, epiglottis, and some cartilages of larynx (corniculate and cuneiform cartilages) where elastic recoil is needed. ## 3. Fibrocartilage - Perichondrium: There is no perichondrium in collagenous fibrocartilage. - Matrix: The intercellular matrix is full of parallel dense collagen fibers (type I). They appear as acidophilic regular bundles in H&E sections. - Cells: Chondrocytes inside lacunae are seen as rows between collagen fibers. - Functions and Sites: Fibrocartilage is strong enough to resist stretching, even under extreme tension. It attaches bone to bone and provides restricted mobility under great mechanical stress. It is found in intervertebral disks and pubic symphysis. - Structure of intervertebral Disc: It is found between the bodies of two adjacent vertebrae. It is formed of an outer ring-like collar of collagenous fibrocartilage known as the annulus fibrosus (collagen type I) and an inner soft jelly-like area called nucleus pulposus of collagen fibers type II. ## CLINICAL NOTE In a condition of prolapsed intervertebral disc, the soft nucleus pulposus is herniated through the annulus fibrosus. This causes inflammation that is very painful and can compress the roots of the spinal nerves at the level involved. # Nutrition of Cartilage - Cartilage, like epithelial tissue, lacks a capillary blood supply of its own, i.e. it is avascular connective tissue. - Nutrients and oxygen reach the cells from perichondrium (an outer fibrous vascular layer that surrounds cartilage) by diffusion through the matrix. # Growth of Cartilage - **Appositional Growth**: It is growth of the cartilage by adding new layers from "outside." It is due to mitosis of chondroblasts in perichondrium. They secrete new matrix to the surface and form chondrocytes inside lacunae. - **Interstitial Growth**: It is growth of the cartilage from "inside." It occurs as the result of division of young chondrocytes within the cartilage. Chondrocytes secrete more matrix and form cell nests. # Bone - Bone is a strong, hard, and rigid specialized form of connective tissue. It is formed of matrix (ground substance and fibers) and cells. In most areas bone is covered by periosteum and lined by endosteum. ## I. Bone Matrix - The matrix consists of closely packed layers or lamellae of calcified collagen fibers type I embedded in amorphous intercellular substance rich in proteins and carbohydrates. Bone matrix consists of organic components, or osteoid (35% of bone dry weight), and inorganic components, or bone mineral (65% of bone dry weight). Bone matrix is formed and produced by osteoblasts and maintained by osteocytes. ### 1. Organic Part of Matrix (Osteoid) - Fibers - Collagen fibers type I forms 90% of the organic portion of bone matrix. That is why bone matrix is highly acidophilic in decalcified sections. - Ground substance - It is mainly composed of proteins and carbohydrates (proteoglycans and glycosaminoglycans). ### 2. Inorganic Part of Matrix - Inorganic part of bone or bone mineral is formed mainly of calcium phosphate, which is the cause of hardness of bone. ## II. Cells of Bone ### 1. Osteogenic Cells - At LM level: Osteogenic or osteoprogenitor cells are mesenchymal stem cells found in periosteum and endosteum. They are small oval or elongated in shape with pale cytoplasm and oval nuclei. - At EM level: Osteogenic cells show all the EM features of metabolically active cells. - **Periosteum**: It is a vascular connective tissue capsule, that covers the whole bone except for its articular surface. Periosteum has an outer vascular fibrous layer of collagen fibers and fibroblasts. The inner layer is formed of osteogenic cells and osteoblasts. - **Endosteum**: It lines the bone marrow cavities. Endosteum has only a cellular forming layer of osteogenic cells and no fibrous one. - Functions: Osteogenic cells are most active during the growth of bones but are reactivated in adult life in the repair of bone fractures. These cells are the only cell of the bone that can divide. They have a capacity for mitosis and further differentiation. If the region is highly vascular, they give rise to osteoblasts. If the region is avascular, they give rise to chondroblasts. ### 2. Osteoblasts - At LM level: Osteoblasts are large rounded branched cells. The cytoplasm is deeply basophilic with eccentric nucleus. They are seen in periosteum and endosteum. - At EM level: Osteoblast shows all the ultrastructural features of actively secreting cells. i.e. many ribosomes, abundant RER, prominent Golgi apparatus, secretory vesicles and mitochondria. - Functions: Osteoblasts are the bone-forming cells found in growing surfaces of bone. They are specialized dividing cells that synthesize organic components of bone matrix (osteoid); i.e. collagen fibers type I and glycoprotein. In vascular regions osteoblasts secrete alkaline phosphatase which stimulates deposition of calcium salts in the matrix and around the cells and their processes. When osteoblasts are trapped inside lacunae and their processes are in narrow tubes called canaliculi, they are called osteocytes. ### 3. Osteocytes - At LM level: When osteoblasts imprisoned (embedded) in hard matrix, they are called osteocytes. The cells are smaller than osteoblasts, and the cytoplasm is less basophilic. Each osteocyte occupies a small cavity called lacuna and its processes (branches) extend into canaliculi in the hard matrix. Osteocytes are connected with each other by their processes through the canaliculi. Osteocytes do not divide, so there is only a single cell in each lacuna (no cell nests as in cartilage). - At EM level: The cytoplasm is rich in mitochondria, ribosomes, RER, and Golgi apparatus. The nucleus is large, dark, and central. - Functions: Osteocytes are the principal cell of mature bone. They maintain bone matrix by sharing in the formation of the organic part of bone matrix, i.e. collagen fibers and proteoglycan. The branches of osteocytes are connected in the canaliculi for the exchange of nutrients and of waste products between the blood stream and the osteocytes. ### 4. Osteoclasts - At LM level: Osteoclasts are large (up to 150 µm in diameter) cells with acidophilic cytoplasm, and containing as many as 50 nuclei. They are located in bony surfaces in shallow depressions termed Howship's lacunae near bone marrow cavities. The surface adjacent to the bone shows a radial striations which was called brush border. - At EM level: The cytoplasm has many nuclei and is rich in lysosomes, mitochondria, and well-developed Golgi complex. The surface facing bone matrix has numerous irregular infoldings and it is called ruffled border, which is a better term than brush border. This ruffled border forms many isolated compartments between the cell and the bone surface. - Functions: Osteoclasts release acid, collagenase, and other lytic enzymes into the compartments between the ruffled border and bone matrix. These materials break down (erode or resorb) bone matrix and release minerals, a process called bone resorption. Osteoclasts are responsible for the remodeling when the shape of the bone is changing as it grows. So, they are called bone eating cells. Osteoclasts are derived from the fusion of blood monocytes and are considered one type of macrophages.