Objectives
- Identify the 3 types of cartilage
- Identify the microscopic anatomical features of bone and cartilage
- Identify features of developing bone
Cartilage & Bone Overview
Cartilage and bone are specialized skeletal connective tissues (CT). Because of special properties of their extracellular matrix, they can serve as semi-rigid (cartilage) or rigid (bone) structural elements that give shape to the body and enable gross movements of parts of the body, powered by contraction of skeletal muscles. Cartilage and bone are living, metabolically active supportive structures. Bone in particular is constantly being remodeled and is capable of extensive repair (following fracture). One of bone’s most remarkable characteristics is that it is constantly being destroyed and rebuilt anew, all the while maintaining its basic morphology and structural integrity. The first kind of cartilage we will study, hyaline cartilage, not only provides flexible support for the trachea and bronchi, but also covers the articular surfaces of many long bones, and forms the skeleton of the embryo. Embryonic hyaline cartilages are the immediate precursors of most the bones of the axial (base of skull, vertebrae) and appendicular (upper limbs, shoulder girdle, lower limps, pelvic girdle) skeleton.
Cartilage:
Cartilage is an avascular (no blood vessels) specialized CT and consists of chondrocytes and substantial extracellular matrix (ECM). To maintain viability of this avascular CT, the ECM has a high ratio of glycosaminoglycans to type II collagen fibers which allows for diffusion of necessary substances from surrounding vascularized CTs. The 3 types of cartilage differ in appearance, mechanical properties, and make-up of their ECM:
Bone:
Bone, like other types of CT, consists of cells and ECM. The distinguishing feature is the mineralization of the matrix which builds hard tissue that is able to provide support and protection of the body. Bone also functions as storage of calcium and phosphate. Mature bone can be compact (arranged in cylindrical units called osteons or Haversian systems) or spongy (arranged as trabeculae or spicules). Immature bone is found in the developing fetus. Bone matrix is mainly composed of type I collagen, some type V collagen, and much smaller amounts of other types of collagen. Ground substance made up of noncollagenous proteins is the other major component of bone ECM. Bone cells are found within spaces of the matrix known as lacunae. Adjacent lacunae are connected by small tunnels known as canaliculi.
Bone Cell types:
- Osteocytes– maintain viability of bone; cell processes of osteocytes reach through the lacunae for communication through gap junctions
- Osteoblasts– secrete ECM; developmental precursors of osteocytes; once surrounded by ECM, osteoblasts become known as osteocytes
- Osteoprogenitor cells– another developmental precursor of osteocytes found in the periosteum of growing bones
- Osteoclasts– phagocytic cells that resorb bone; present on bone surfaces where bone is being remodeled or damaged
- Bone-lining cells– found on bone surfaces where there is no active growth; derived from osteoblasts which remain after bone deposition ends
>Cartilage & Bone Slides
The slide contains 2 different slices of trachea specimen, longitudinal on the left and cross-section on the right. The trachea is the windpipe, transferring air to the lungs. Even in this screen shot of low magnification of the slide, you can see the darker-stained regions of cartilage (3 on the left and 2 1/2 on the right). The trachea is made up of hyaline cartilage (HC) providing structure for the trachea and keep it open for the passage of air while allowing for pliability to move as food passes through the esophagus which is located just posterior in the neck. As you look closely at the cartilage, notice the dense CT capsule surrounding the HC known as perichondrium. Examine the center portion of the HC and find the chondroblasts in lacunae surrounded by ECM. Also notice what you don’t see, blood vessels. CT is avascular.
The epiglottis is found in the oropharynx, just posterior to the tongue. Most of the time, the trachea is open, allowing for air to pass but the epiglottis folds over it during swallowing to prevent food and liquid from entering the airway. The epiglottis contains elastic cartilage (EC) running through the center. HC and EC look fairly similar with an H&E stain but an elastin stain demonstrates a significant increase in elastic fibers in EC compared to HC. You may also notice elastic fibers found in the CT of the blood vessels found in the epiglottis.
This slide depicts 2 vertebral bodies of the spine and an intervertebral disc (IVD). The IVD consists of the central nucleus pulposus (NP) surrounded by the anulus fibrosus (AF). The AF is made up of fibrocartilage which is a mixture of dense regular CT and hyaline cartilage. Look for the chondrocytes by identifying lacunae surrounded by the wavy collagen fibers. Another distinction of fibrocartilage is the absence of perichondrium. Vertebral bodies consist mostly of spongy bone surrounded by a thin layer of compact bone.
This is a cross-section of a rib bone. You can see the outer ring is made of compact bone while the center of the rib is filled with spongy bone. The very dark pink vertical lines along the bottom of the slide are artifact from the thin slice of tissue folding. The outer edge of the compact bone is covered by periosteum where fibroblasts (flat) can be found but as you move deeper into the compact bone, you will see osteoblasts (round). Osteocytes can be found in the lacunae. The spongy bone is formed from projections of compact bone into the center of the specimen. The inner surface of the compact bone along with the trabeculae are lined with endosteum, a discontinuous layer of osteoblasts. The marrow compartment of the spongy bone is filled with blood vessels where you will find different stages of red and white blood cells.
This is a section of the fibula after a dried ground preparation. The organic components are removed through drying, leaving the mineral. Slices are at least 2.3mm thick while other tissues are around 4um thick. There is no staining. This preparation is useful to view the osteons, or Haversian systems of compact bone. You will be able to identify lamella of the bone, osteocytes within lacunae, and canaliculi.
Each of these specimen slices is a different part of the hand. Specimen A is perpendicular to the long axis of the hand at the level of the carpal (wrist) bones. Specimen B is perpendicular to the long axis of the hand at the level of the metacarpal (hand) bones. Specimen C is parallel to the long axis of a digit (finger). The different zones of bone formation are seen in specimen C.
>More Practice
This is a section of the auricle, the pinna, of the outer ear. It is stained with orcein, a type of elastic stain. Use this slide to identify skin, CT proper, and cartilage.
This is a section of the pubic symphysis where the two hip bones meet anteriorly and are joined by cartilage. This cartilaginous disc rests between 2 articular surfaces of bone. Overall, the pubic symphysis is made up of 2 types of cartilage, bone, and ligaments. The pink strands radiating from the bone are muscle fibers. Use this image to identify features of bone and cartilage.
This specimen is a fetal rat which allows us to see different developing tissues. Use this slide to review features of cartilage and bone and reviewing other tissues.
>More Practice Answers
Slide 35, Auricle
- Stratified squamous epithelium (the most apical layer is squamous and there are multiple layers of cells).
- Dense irregular CT (high volume of collagen fibers (dark pink), irregular pattern).
- Elastic cartilage (The black fibers are elastic fibers. Notice the cells, chondrocytes, are found in lacunae indicating a type of cartilage).
Slide VCU 18, Pubic Symphysis:
- A) compact bone; B) osteocytes; C) Haversian canal; D) osteoblasts
- A) spongy bone; B) trabeculae or spicule; C) endosteum, osteoblasts
- periosteum
- A) fibrocartilage (notice the dense fibers in the ECM); B) hyaline cartilage lining the articular surface of the bone
Slide UVA BC5_16 Fetal Rat:
- Intramembranous ossification- notice the presence of trabeculae instead of cartilage
- A) osteoblasts; B) osteocytes; C) osteoid
- A) Hyaline cartilage
- stratified squamous- notice the top layer of cells is flat (squamous) and there are multiple layers of cells (stratified), this will become skin
