Bones I (FALL 2024) PDF

Bones & Skeletal Tissue Intro to Bones Bone acts as a replacement tissue, using cartilage as a model to build itself. During fetal development, a flexible framework of cartilage is created to guide where bones will grow. As the fetus develops and during childhood, bone starts to form on this cart...

Bones & Skeletal Tissue Intro to Bones Bone acts as a replacement tissue, using cartilage as a model to build itself. During fetal development, a flexible framework of cartilage is created to guide where bones will grow. As the fetus develops and during childhood, bone starts to form on this cartilage. By the time a baby is born, most of the cartilage has been turned into bone. Some cartilage continues to be replaced as a child grows, and a small amount of cartilage remains in adults. Human Skeleton Human skeleton consists initially made up of cartilages and fibrous membranes → soon replaced by bone Skeletal Cartilage Sculpted to fit location Primarily composed of H2O Spring back after compression NO nerves/blood vessels Surrounded by dense perichondrium Provides extra reinforcement Provides blood vessels that nourish cartilage Growth of Cartilage Unlike bone, cartilage has a flexible matrix Cartilage grows in two ways: Appositional growth Cartilage-forming cells in perichondrium secrete matrix against external face of existing cartilage New matrix laid down on surface of cartilage Interstitial growth Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within New matrix made within cartilage Bone Bone is mineralized connective tissue Exhibits four types of cells: Osteoblasts Bone lining cells Osteocytes Osteoclasts Despite its inert appearance, bone is a highly dynamic organ that is continuously resorbed by osteoclasts and reformed by osteoblasts.  1. Support For body and soft organs 2. Protection Protect brain, vertebrae around spinal cord, and vital organs Seven 3. Anchorage for Movement Levers for muscle action 4. Mineral and growth factor storage Important Calcium and phosphorus, and growth factors reservoir Functions 5. Blood cell formation Hematopoiesis occurs in red marrow cavities of certain bones of Bone 6. Triglyceride (fat) storage Fat, used for an energy source, is stored in bone cavities 7. Hormone production Osteocalcin secreted by bones helps to regulate insulin secretion, glucose levels, and metabolism Bones are classified by their location and shape 206 named bones in human skeleton Divided into two groups based on location Axial skeleton Long axis of body Skull, vertebral column, rib cage Appendicular skeleton Bones of upper and lower limbs Girdles attaching limbs to axial skeleton Classification of Bones 1. Long bones Longer than they are wide Limb bones 2. Short bones Cube-shaped bones (in wrist and ankle) Sesamoid bones form within tendons (example: patella) Vary in size and number in different individuals 3. Flat bones Thin, flat, slightly curved Sternum, scapulae, ribs, most skull bones 4. Irregular bones Complicated shapes Vertebrae and hip bones Bone Structure Bones are organs because they contain different types of tissues Bone (osseous) tissue predominates, but a bone also has: nervous tissue Cartilage fibrous connective tissue, muscle cells epithelial cells in its blood vessels Three levels of structure Gross Microscopic Chemical Gross Anatomy of Bone Every bone has a dense outer layer that looks smooth and solid to the naked eye External layer is compact bone Internal layer is spongy bone Also called trabecular bone made up of a honeycomb of small, needle-like or flat pieces of bone called trabeculae Open spaces between trabeculae are filled with red or yellow bone marrow Flat Bones Structure of short, irregular, and flat bones Consist of thin plates of spongy bone covered by compact bone Compact bone sandwiched between two connective tissue membranes Periosteum covers outside of compact bone Endosteum covers inside portion of compact bone Bone marrow is scattered throughout spongy bone; no defined marrow cavity Hyaline cartilage covers area of bone that is part of a movable joint Long Bones: Structure With few exceptions: all long bones have same general structure: Diaphysis (shaft) tubular shaft that forms long axis of bone Consists of compact bone surrounding central medullary cavity that is filled with yellow marrow in adults Epiphyses (bone ends) ends of long bones that consist of compact bone externally and spongy bone internally Articular cartilage covers articular (joint) surfaces Between diaphysis and epiphysis is epiphyseal line Remnant of childhood epiphyseal plate where bone growth occurs Two types of Membranes Periosteum: (peri– = “around” or “surrounding”). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. white, double-layered membrane that covers external surfaces except joints Anchoring points for tendons and ligaments Endosteum (end– = “inside”; oste– = “bone”), where bone growth, repair, and remodeling occur Delicate connective tissue membrane covering internal bone surface Covers trabeculae of spongy bone Like periosteum, contains osteogenic cells that can differentiate into other bone cells Blood Vessels and Nerves Bones are very well vascularized! (unlike cartilage) Nutrient artery supplies inner spongy bone and bone marrow Branches further to supply the compact bone Several epiphyseal arteries and veins serve the epiphysis in the same way Nerves accompany blood vessels through the nutrient foramen openings Hematopoietic Tissue in Bone Also called Red marrow Found within trabecular cavities of spongy bone and diploë of flat bones, such as sternum In newborns, medullary cavities and all spongy bone contain red marrow In adults, red marrow is located in heads of femur and humerus, but most active areas of hematopoiesis are flat bone diploë (spongy bone) and some irregular bones (such as the pelvis) Yellow marrow can convert to red, if person becomes anemic Bone Markings & Features Bones are rarely smooth or featureless Distinct bone markings tell a story of function! Maybe muscles or ligaments attach, nerves pass through? Three types of markings: Projection: outward bulge of bone May be exaggerated due to increased stress from muscle pull or is a modification for joints Depression: bowl- or groove-like cut-out that can serve as passageways for vessels and nerves, or plays a role in joints Opening: hole or canal in bone that serves as passageways for blood vessels and nerves Microscopic Anatomy of Bone 5 major Cells of Bone Tissue Osteoprogenitor Cells Osteoblasts Osteocytes Bone lining Cells Osteoclasts *It is the presence of all above cells that make bone a dynamic living tissue – continuously resorb (break down) and deposit bone (build up) in a process called remodeling* Bone remodeling Bone remodeling is a highly complex process by which old bone is replaced by new bone, in a cycle comprised of three phases: Initiation of bone resorption by osteoclasts The transition (or reversal period) from resorption to new bone formation Bone formation by osteoblasts This process occurs due to coordinated actions of osteoclasts, osteoblasts, osteocytes, and bone lining cells Osteogenic Cells Also called osteoprogenitor cells These cells are undifferentiated with high mitotic activity Immature osteogenic cells are found in the deep layers of the periosteum and the marrow Mitotically active stem cells in periosteum and endosteum When stimulated, they differentiate into osteoblasts or bone-lining cells Some remain as osteogenic stem cells Osteoblasts Bone-forming cells that secrete unmineralized bone matrix called osteoid Osteoid is made up of collagen and calcium-binding proteins Collagen makes up 90% of bone protein Osteoblasts are actively mitotic 3. Osteocytes Mature bone cells in lacunae that no longer divide Maintain bone matrix and act as stress or strain sensors Respond to mechanical stimuli such as increased force on bone or weightlessness Communicate information to osteoblasts and osteoclasts (cells that destroy bone) so bone remodeling can occur 4. Bone-lining cells Flat cells on bone surfaces believed to also help maintain matrix (along with osteocytes) 5. Osteoclasts Derived from same hematopoietic stem cells that become macrophages Giant, multinucleated cells function in bone resorption (breakdown of bone) When active, cells are located in depressions called resorption bays Cells have ruffled borders that serve to increase surface area for enzyme degradation of bone Also helps seal off area from surrounding matrix Review Slide Recap: Dynamic Nature of Bone Despite its inert appearance, bone is a highly dynamic organ that is continuously resorbed by osteoclasts and reformed by osteoblasts There is evidence that osteocytes act as mechanosensors and orchestrators of this bone remodeling process The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. Review Slide Table 2. Bone Cells Cell type Function Location Deep layers of the periosteum Osteogenic cells Develop into osteoblasts and the marrow Growing portions of bone, Osteoblasts Bone formation including periosteum and endosteum Maintain mineral Osteocytes Entrapped in matrix concentration of matrix Bone surfaces and at sites of Osteoclasts Bone resorption old, injured, or unneeded bone Microscopic Anatomy of Compact Bone Although it appears solid, a close look reveals tiny passageways for nerves and blood vessels Compact bone Also called lamellar bone Consists of: Osteon (Haversian system) Canals and canaliculi Interstitial and circumferential lamellae Osteon (Haversian system) An osteon is the structural unit of compact bone Consists of an elongated cylinder that runs parallel to long axis of bone Acts as tiny weight-bearing pillars An osteon cylinder consists of several rings of bone matrix called lamellae Lamellae contain collagen fibers that run in different directions in adjacent rings Withstands stress and resist twisting A Single Osteon Bone salts are found between collagen fibers  Canals and canaliculi Central (Haversian) canal runs through core of osteon Contains blood vessels and nerve fibers Perforating canals: canals lined with endosteum that occur at right angles to central canal Connect blood vessels and nerves of periosteum, medullary cavity, and central canal  Canals and canaliculi (cont.) Lacunae: small cavities that contain osteocytes Canaliculi: hairlike canals that connect lacunae to each other and to central canal Osteoblasts that secrete bone matrix maintain contact with each other and osteocytes via cell projections with gap junctions When matrix hardens and cells are trapped the canaliculi form Allow communication between all osteocytes of osteon and permit nutrients and wastes to be relayed from one cell to another  Interstitial lamellae Lamellae that are not part of osteon Some fill gaps between forming osteons; others are remnants of osteons cut by bone remodeling Circumferential lamellae Just deep to periosteum, but superficial to endosteum, these layers of lamellae extend around entire surface of diaphysis Help long bone to resist twisting Microscopic Anatomy of Bone Spongy bone Appears poorly organized but is actually organized along lines of stress to help bone resist any stress Trabeculae, like cables on a suspension bridge, confer strength to bone No osteons are present, but trabeculae do contain irregularly arranged lamellae and osteocytes interconnected by canaliculi Capillaries in endosteum supply nutrients Chemical Composition of Bone: Inorganic & Organic components Organic components Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid Osteoid, which makes up one-third of organic bone matrix, is secreted by osteoblasts Consists of ground substance and collagen fibers, which contribute to high tensile strength and flexibility of bone Resilience of bone is due to bonds in or between collagen molecules that stretch and break to dissipate energy and prevent fractures If no additional trauma, bonds re-form Inorganic components Hydroxyapatites (mineral salts) Makeup 65% of bone by mass Consist mainly of tiny calcium phosphate crystals in and around collagen fibers Responsible for hardness and resistance to compression Bone is half as strong as steel in resisting compression and as strong as steel in resisting tension Lasts long after death because of mineral composition Can reveal information about ancient people Review Slide Compact bone is the denser, stronger of the two types of bone tissue It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection. The microscopic structural unit of compact bone is called an osteon, or Haversian system. Bones Recap Spongy bone, also known as cancellous bone, contains osteocytes housed in lacunae They are not arranged in concentric circles, Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.