Unit 2 Skeletal System

Unit 2: Skeletal System Objectives ● Compare and contrast osteoblasts and osteocytes ● Identify major long, short, flat, and irregular bones in the body ● Describe the process of Ca regulation in the body Bird’s Eye View ● Bones are the scaffolding of the body with many functions ○ ○ ○ ○ Providing shape Allowing for motion Protection of organs Cell production in bone marrow ● In total an adult has about 206 bones Gross Bone Anatomy Basic Bone Anatomy (1 of 2) ●Periosteum is a tough and fibrous connective tissue that covers bone. ○ Contains blood vessels that transport blood and nutrients to nurture bone cells ○ Contains lymph vessels and nerves ○ Acts as an anchor point for ligaments and tendons Basic Bone Anatomy (2 of 2) ●Epiphysis: end of a long bone ●Diaphysis: region running between the two epiphyses. ○ Medullary cavity: hollow within the diaphysis that stores yellow marrow ○ Yellow marrow has a high fat content and can convert to red marrow, which makes red blood cells, in an emergency. Figure 6–2 Bone Tissue (1 of 3) ●Compact bone ○ Dense, hard, tightly packed tissue ○ Found in shafts of long bones and outer layer of other bones ○ Contains units called osteons (haversian systems) ■Contain mature bone cells (osteocytes) that form concentric circles around a central (haversian) canal, containing blood vessels Bone Tissue (2 of 3) ●Osteons run parallel to each other with perforating canals (blood vessel passages) literally connecting with them to ensure sufficient oxygen and nutrients for the bone cells. Bone Tissue (3 of 3) ●Spongy (cancellous) bone ○ Arranged in bars and plates called trabeculae ○ Irregular holes between trabeculae make the bone lighter and provide space for red bone marrow, which produces red blood cells Bone Cells ● ● ● ● ● Osteoprogenitor Cells Osteoblasts Osteocytes Osteoclasts Bone Lining Cells https://app.jove.com/embed/player?id=14011&t=1&s=1&fpv=1 Osteoprogenitor Cells • Osteoprogenitor cells – Non-specialized ▪ Can turn into other cell types as needed – Found in periosteum, endosteum, and central canal of compact bones Osteoblasts ● Responsible for the construction of osteoids ○ Mature from mesenchymal stem cells ■ Regulation from parathyroid hormone and vitamin D ■ Mature osteoblasts are responsible for protein synthesis ● Q: What do we expect there to be a large amount of? ○ Osteoid: found in areas of new bone formation ■ Converts to mature mineralized bones Osteocytes ● Most abundant bone cell ● Connect via tunnels called canaliculi ○ Allows for communication ○ Canaliculi serves for movement of nutrients and waste disposal ● Responsible for mechanosensing and responding to strain Osteoclasts ● Responsible for reabsorbing mineralized bone ○ Pumps carbonic acid and enzymes to degrade the bone ○ Activated by osteoblast activity ■ This coupling is important for calcium regulation Bone Lining Cells ● Old osteocytes that no longer play a role in synthesis ○ Flat and thin with little activity ● When the bone lining cell peels back osteoclasts are stimulated for reabsorption Bone Matrix ● Osteoid ○ Immature bone matrix (not yet mineralized) ● Bone Collagens (type of protein) ○ Bone is mostly Type 1 collagen ○ Gives bone its rigid structure ● Extracellular Matrix ○ Mostly made of mineral ■ Calcium and phosphate ○ Gives bone mechanical properties Bone Types ● ● ● ● Long Short Bones Flat Bones Irregular Bones Long Bones ● Long shaft with two bulky ends ○ Predominantly compact bone however have a large amount of spongy bone at the ends ● Three distinct zones: ○ ○ ○ ○ Epiphysis ■ Region at end of bone ■ Allows attachment site at joints Metaphysis ■ Transitional area ■ Attachment site of most tendons Diaphysis ■ Center: primarily a structural function In development epiphyseal plate is a fourth zone Epiphyseal Plate ● Responsible for longitudinal growth ● Cells replicate in the zone of proliferation ● Active during stages of growth ○ Stops somewhere between 18-21 Short Bones ● Formed in similar manner to long bones ● Vary in shape and size ● Cube shape and primarily spongy bone Flat Bones ● Cortical shell and stiff interior ● Broad and flat ○ Major focus is on providing protection or flat surface for attachments Irregular Bones ● Primarily spongy bone ● Bones of the spine Compact and Spongy Bone ● Compact: ○ Denser and stronger tissue ○ Makes up the outer cortex ○ Made up of Osteons (Haversian System) ● Spongy (Cancellous) ○ Trabeculae in place of Haversian System ○ Lined with endosteum ○ Makes bone lighter and gives space for red bone marrow Gross Bone Anatomy ● Diaphysis: ○ Medullary cavity filled with bone marrow ○ Outer wall composed of dense hard compact bone ● Epiphysis: ○ Filled with spongy bone ○ Covered in hyaline cartilage Bone Anatomy ● Periosteum ○ Tough and fibrous tissue covering bone ○ Provides anchor points for ligaments ● Bone Marrow ○ Red: makes blood cells ○ Yellow: High in fat Blood and Nerve Supply ● Arteries enter through nutrient foramen ○ Serves the spongy bone and medullary cavity ● Nerve follows similar pathways Calcium Homeostasis ● Regulated by PTH, Vit. D and Calcitonin ● Low Calcium: ○ PTH: role is to increase serum levels of calcium ■ Increases kidney’s ability to convert Vit. D to active form ■ Stimulates osteoblasts resulting in upregulation of osteoclasts ● High Calcium ○ PTH is down regulated and Calcitonin is released to down regulate osteoclast Coupling and Remodeling ● Role is to keep bone strong and heal damage ● Dual Regulation ○ Local ■ Mechanical Strain, variety of proteins ○ Systemic ■ Vit. D, PTH, calcitonin ● Uncoupling: ○ Goal is for absorption and formation to reach homeostasis ■ Need for serum Ca outweighs the need for coupling Coupling Signals ● Matrix Derived Factors ○ Bone matrix stores a variety of growth factors including platelet derived growth factor and insulin like growth factors ■ Factors are deposited by osteoblasts during matrix production and released by osteoclasts ● Main influence: stimulation of osteoblast progenitors ● Osteoclast Secreted Factors ○ Contain anti-resorptive inhibitors: may reduce bone resorption without impacting bone formation ● Osteoclast Membrane Bound Factors ○ Osteoclasts interact directly with mature osteoblasts to promote their activity Coupling Mechanisms There are different ways that bone breakdown (resorption) and bone formation can be linked, even though they don’t happen at the same time. These are possible mechanisms that help coordinate the process: 1. 2. 3. Osteoclast signals: Osteoclasts (cells that break down bone) release signals that trigger early bone-forming cells (osteoblasts) to start developing. Other signals from nearby cells in the bone unit help fine-tune the number and activity of these bone-forming cells. Support from other cells: Some cells help pass along these signals to both developing and mature osteoblasts. These include: ○ (B) Cells that are already in the osteoblast family and are located in the protective layer over the remodeling bone. ○ (C) Reversal cells, which are found on the bone surface and help prepare for new bone formation. ○ (D) Osteocytes, which are mature bone cells embedded in the bone that help communicate about bone changes. Physical changes: The physical changes made by osteoclasts, such as the pits they create in the bone or mechanical stress sensed by osteocytes, send additional signals to make sure the right amount of new bone is made by mature osteoblasts. The timing for each of these steps has been observed in bone samples from adult humans. Growth and Repair (Osteogenesis) ● Bone modeling starts at about 8 weeks after conception ○ Bone will start to replace cartilage ○ Primary Ossification: ■ Happens in utero ○ Secondary Ossification: ■ Starts around birth ■ Growth will continue as long as growth plates are present Wolff’s Law ● Bone’s model and remodel in response to mechanical environment ○ Osteocytes sense mechanical signals and will mediate osteoclasts and osteoblasts in their vicinity