Histology MSK

Histology - MSK I- Cartilage • Cartilage are modified type of connective tissue in which the extracellular matrix is hardened to provide rigidity, support and attachment for soft tissues. • Characteristic Features Of Cartilage: 1. The extra cellular matrix is firm. 2. It is avascular, no lymphatic vessels or nerves. 3. It is usually covered by perichondrium. 4. Cells are isolated in small cavities in the ground substance called lacunae. Perichondrium • Site: Cartilage is usually covered by a membrane called perichondrium except inside joint cavities. • Structure: It consists of two layers: 1. An outer fibrous layer: dense CT, rich in type І collagen & few in blood vessels. 2. An inner vascular & cellular layer (chondrogenic layer): contain chondroblasts (forming new cartilage). • Functions: 1. Growth of cartilage at its periphery. 2. Nourishment as it contains blood vessels. N.B: Inside joint cavities: these functions are committed by synovial fluid Components Of Cartilage: 1. Extracellular matrix including: • Ground substance. • Fibers: Collagen & elastic fibers. 2. Cartilage cells: chondroblasts & chondrocytes. 1 Histology - MSK I- Extracellular matrix: • It is responsible for the firmness of the cartilage. It is produced by cartilage cells. • Structure: 1- Ground substance: A- Water: 60 - 80% of cartilage weight. - Function: Diffusion of metabolites to the cartilage cells. B- Proteoglycans: Firm gel, consists of glycosaminoglycans as: - Chondroitin sulfate - Keratan sulfate > Bound to hyaluronic acid C- Glycoproteins: such as chondronectin (anchors chondrocytes to the matrix). 2- Fibers: mainly type II collagen fibrils, in addition to type I collagen and elastic fibers according to type of cartilage. II- Cartilage cells: 1- Chondroblasts: • Origin: from undifferentiated mesenchymal cells. • Site: in the inner cellular layer of perichondrium. • Function: a. They secrete the extracellular matrix. b. They become imprisoned inside lacunae forming chondrocytes. • Structure: - Shape: Oval. - Nucleus: Oval. - Cytoplasm: basophilic. 2- Chondrocytes: • Origin: From the chondroblasts after secreting the matrix, trapped in lacunae. • Site and shape: a. Young chondrocytes: - Site: underneath the perichondrium. - Shape: spindle. 2 Histology - MSK b. Mature chondrocytes: - Site: deeply situated. - Shape: rounded. • The matrix is condensed around the lacunae forming the capsule rich in chondroitin sulphates • Function: They maintain the extracellular matrix. Chondrocytes can divide by mitosis Types Of Cartilage: 1- Hyaline cartilage: • Fresh state: a. Smooth & firm. b. Bluish in color. • Sites: a. Skeleton of the embryo. b. Epiphyseal plates in growing age. c. Costal cartilages. d. Nose, larynx, trachea and bronchi in the respiratory system. e. Articular surfaces of synovial joints. • Characteristic features: - Perichondrium: It is usually covered with perichondrium except inside joint cavity. - Nutrition: from perichondrium, but if it is present inside the joint cavity, it is nourished by diffusion from the synovial fluid. - Matrix: abundant, glassy, pale basophilic & no apparent fibers (WHY). - Fibers: Type II collagen fibrils. - Chondrocytes: widely scattered, number of chondrocytes/capsule is 1-8 forming a cell nest. 3 Histology - MSK 2- Elastic cartilage: • Fresh state: a. Flexible, can bear mechanical stress without permanent deformation. b. Yellowish in color. • Sites: a. The auricle of the ear (ear pinna). b. External auditory meatus. c. Epiglottis. • Characteristic features: - Perichondrium: covered always with perichondrium. - Nutrition: Perichondrium. - Fibers: large number of elastic fibers (stained by VVG & orcein), in addition to collagen type II. - Chondrocytes: Numerous, number of chondrocytes/capsule is 1-3. 3- White fibro cartilage: • Fresh state: a. It has a great strength & flexibility. b. Whitish in color. • Sites: a. The intervertebral disc. b. The symphysis pubis. c. The temperomandibular joint d. The sternoclavicular joint. e. The menisci of knee joint. • Characteristic features: - Perichondrium: Never covered with perichondrium. - Nutrition: from the surrounding connective tissue of joint capsule and ligaments. - Fibers: large number of type I collagen fibers, in addition to type II collagen. - Chondrocytes: Few, number of chondrocytes/capsule is 1-2. 4 Histology - MSK Hyaline cartilage Elastic cartilage Smooth, firm, bluish in Flexible, yellow in fresh state. fresh state. Covered with Always covered with perichondrium. Perichondrium perichondrium, EXCEPT inside joint cavity. 1. Perichondrium. Perichondrium. 2. Inside joint cavity Nutrition by synovial fluid. Character Extracellular matrix • Type II collagen • Large number of fibrils (not appear) elastic fibers + type • It is pale basophilic, II collagen fibrils. glassy appearance. Widely scattered. Chondrocytes Cell (1-8 chondrocytes/ lacuna) 1. Costal cartilages. 2. Nose , larynx , trachea , bronchi Sites 3. Articular surfaces of joints. 4. Skeleton of embryo. 5. Epiphyseal plate. Numerous. 1-3 chondrocytes/lacuna 1. Auricle of the ear. 2. External auditory canal. 3. Epiglottis. 5 White fibrocartilage Strong , flexible , white in fresh state. Never covered with perichondrium. From the surrounding connective tissue of joint capsule and ligaments. • Type I collagen fibers + type II collagen. Few . 1-2 chondrocytes/lacuna. 1. The intervertebral disc. 2. The symphysis pubis. 3. Tempromandibular j. 4. Sternoclavicular j. 5. Menisci of knee j. Histology - MSK Growth Of Cartilage: Growth of cartilage is very limited in adults. • There are 2 types of growth: 1- Appositional growth: • It is growth of cartilage at its periphery by adding new layers from perichondrium. • It is caused by mitosis of chondroblasts in the perichondrium. • They secrete new matrix on the surface, then transform to chondrocytes. 2- Interstitial growth: • It is growth of cartilage from inside. • It occurs as a result of mitosis of chondrocytes within the cartilage. • They form cell nests in the lacunae & secrete more matrix. Types Definition Events Appositional growth Interstitial growth It is growth of cartilage at its periphery It is growth of cartilage from inside by adding new layers from outside • Mitosis of chondroblasts in the perichondrium. • They secrete new matrix on the surface, then transform to chondrocytes. • Mitosis of chondrocytes within the cartilage. • They form cell nests in the lacunae and secrete more matrix. Healing of Cartilage: • Cartilage has a limited ability to repair (WHY), if extensively damaged, the cartilage tear will be repaired with fibrous tissue. 6