Cartilage: Histology

Overview

Definition

Cartilage is a type of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology that forms structural components of the human skeleton and provides support to various organs.

Composition

Chondrocytes:

• The major cell type of the cartilage tissue
• Synthesize extracellular matrix Extracellular matrix A meshwork-like substance found within the extracellular space and in association with the basement membrane of the cell surface. It promotes cellular proliferation and provides a supporting structure to which cells or cell lysates in culture dishes adhere. Hypertrophic and Keloid Scars (ECM) components and become embedded in them ( lacunae Lacunae Bones: Development and Ossification)
• Young chondrocytes retain the ability to divide; daughter cells secrete new ECM and become surrounded by the new lacunae Lacunae Bones: Development and Ossification.
• Have low oxygen needs; nutrition is provided by perichondrium since cartilage is lacking blood vessels and innervation
• Stimulated by somatotropins from the pituitary Pituitary A small, unpaired gland situated in the sella turcica. It is connected to the hypothalamus by a short stalk which is called the infundibulum. Hormones: Overview and Types, inducing synthesis Synthesis Polymerase Chain Reaction (PCR) of sulfated glycosaminoglycans (GAGs) and secretion Secretion Coagulation Studies of proteoglycans Proteoglycans Glycoproteins which have a very high polysaccharide content. Basics of Carbohydrates

Chondroblasts:

• Perichondrial cells that are located at the periphery of the cartilage and develop into chondrocytes 
• Conversion of chondroblasts to chondrocytes occurs when chondroblasts become surrounded by the new matrix they produce.

Extracellular matrix Extracellular matrix A meshwork-like substance found within the extracellular space and in association with the basement membrane of the cell surface. It promotes cellular proliferation and provides a supporting structure to which cells or cell lysates in culture dishes adhere. Hypertrophic and Keloid Scars:

• Produced by chondrocytes 
• Abundant, usually of firm consistency Consistency Dermatologic Examination and resistant to compression Compression Blunt Chest Trauma
• Composed of collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology (type Ⅱ most prevalent), proteoglycans Proteoglycans Glycoproteins which have a very high polysaccharide content. Basics of Carbohydrates, and glycoproteins Glycoproteins Conjugated protein-carbohydrate compounds including mucins, mucoid, and amyloid glycoproteins. Basics of Carbohydrates
• Chondroitin sulfate Chondroitin sulfate Derivatives of chondroitin which have a sulfate moiety esterified to the galactosamine moiety of chondroitin. Chondroitin sulfate a, or chondroitin 4-sulfate, and chondroitin sulfate c, or chondroitin 6-sulfate, have the sulfate esterified in the 4- and 6-positions, respectively. Chondroitin sulfate B is a misnomer and this compound is not a true chondroitin sulfate. Connective Tissue: Histology (aggrecan) is the most abundant proteoglycan in the matrix of hyaline cartilage

Chondrogenesis

Embryonic development

• Cartilage develops from embryonic mesenchyme.
• Rounding up mesenchymal osteoprogenitor cells indicates the beginning of the process.
• Chondroblasts are formed and begin secreting ECM consisting mostly of aggrecan and type Ⅱ collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology.
• Chondroblasts are converted to chondrocytes.
• Both cells are rich with rough endoplasmic reticulum Endoplasmic reticulum A system of cisternae in the cytoplasm of many cells. In places the endoplasmic reticulum is continuous with the plasma membrane (cell membrane) or outer membrane of the nuclear envelope. If the outer surfaces of the endoplasmic reticulum membranes are coated with ribosomes, the endoplasmic reticulum is said to be rough-surfaced; otherwise it is said to be smooth-surfaced. The Cell: Organelles (RER) for collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology synthesis Synthesis Polymerase Chain Reaction (PCR).
• Cartilage growth:
  • Interstitial growth: mitotic division of pre-existing chondrocytes → ECM formation
  • Appositional growth: formation of chondroblasts from progenitor cells in the perichondrium
  • Isogenous cells are formed by the mitotic division of chondroblasts.

Skeletal growth during childhood

• Embryonic skeleton is composed primarily of cartilage.
• Cartilage gradually calcifies and is replaced by bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types as chondrocytes are replaced by osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification.
• Epiphyseal plate is a thin layer of cartilage that persists at the ends of long bones Long bones Length greater than width. Bones: Structure and Types after birth and allows for longitudinal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types growth.
• Once the epiphyseal plate is completely ossified, no further longitudinal growth occurs.

Regeneration Regeneration The physiological renewal, repair, or replacement of tissue. Wound Healing and repair

• Since perichondrium loses the ability to form new cells, the regeneration Regeneration The physiological renewal, repair, or replacement of tissue. Wound Healing of cartilage is almost nonexistent.
• Perichondrial cells will produce mostly dense connective tissue Dense connective tissue Connective Tissue: Histology.
• Repair is also limited since chondrocytes are locked up in lacunae Lacunae Bones: Development and Ossification and cannot travel to damaged areas.

Types of Cartilage

There are 3 main types of cartilage tissue:

• Hyaline cartilage (most abundant)
• Elastic Elastic Connective Tissue: Histology cartilage
• Fibrocartilage

Clinical Relevance

• Osteoarthritis Osteoarthritis Osteoarthritis (OA) is the most common form of arthritis, and is due to cartilage destruction and changes of the subchondral bone. The risk of developing this disorder increases with age, obesity, and repetitive joint use or trauma. Patients develop gradual joint pain, stiffness lasting < 30 minutes, and decreased range of motion. Osteoarthritis: a disease caused by degeneration of articular (hyaline) cartilage. The process of degradation and faulty repair is mediated by chondrocytes. Risk factors are age, obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity, female gender Gender Gender Dysphoria, and joint trauma. 
• Osteochondritis Osteochondritis Inflammation of a bone and its overlaying cartilage. Osteochondritis Dissecans dissecans: a joint disorder characterized by focal aseptic necrosis Focal Aseptic Necrosis Osteochondritis Dissecans of the articular cartilage and subchondral bone Subchondral Bone Osteochondritis Dissecans. This condition is usually associated with the detachment of a bone-cartilage fragment, which becomes displaced in the joint space. Causes pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways and swelling Swelling Inflammation of the affected joint, which catches and locks during movement. 
• Chondrosarcoma Chondrosarcoma Chondrosarcoma is a malignant bone tumor characterized by the production of a cartilaginous matrix. This bone tumor most commonly affects adults over the age of 50. Chondrosarcoma usually presents with a slowly increasing mass (or swelling) with a dull achy pain. Chondrosarcoma: malignant bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types tumor Tumor Inflammation of chondrocytes that most commonly affects pelvic and long bones Long bones Length greater than width. Bones: Structure and Types. Most common in older adults. Usually presents as a slow-growing mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast associated with dull achy pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways. Diagnosed with imaging and biopsy Biopsy Removal and pathologic examination of specimens from the living body. Ewing Sarcoma, and treatment involves wide surgical excision.
• Enchondroma: a benign Benign Fibroadenoma bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types tumor Tumor Inflammation originating in cartilage. This condition rarely causes pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways or other symptoms, and is most commonly picked up incidentally on imaging. Usually small in size (< 5 cm) and does not require treatment.