Chapter 6 Flashcards
Skeletal System
Includes:
- Bones
- Joints
- Supporting Tissues
Bones
Main Organ
Composed of - More Osseous tissue; Dense Regular, Dense Irregular Collagenous Connective Tissue, and Bone Marrow;
Functions of Bone
- Protection- Certain bones protect underlying organs; (Skull, Sternum, Ribs)
- Mineral Storage & Acid-Base Homeostasis- Storehouse for Calcium, Phosphorus, and Magnesium salts; Minerals present as electrolytes, acids, & bases
- Blood Cell Formation- Bone houses [red bone marrow]-> connective tissue involved in formation of blood cells [Hematopoiesis]
- Fat Storage- Bone also contains [Yellow Bone Marrow]-> made up of fat cells (adipose); Stores triglycerides; Fatty acids from breakdown of triglycerides can be used as fuel
- Movement- Bone serves as sites for attachment of most skeletal muscles; Muscles contract and pull on bones; (Generates Movement)
- Support- Skeleton supports weight of the body, providing its framework
Bone Structure:
[5 classes]
206 bones in human body
Long bones, Short bones, Flat bones, Irregular bones, & Sesamoid bones
Long Bones
- Overall shape, NOT size
- Longer than wide
- Includes most bones in Arms & Legs
Short Bones
- Cube shaped
- Long as they are wide
- Includes bones of Wrist [Carpals] & Ankle [Tarsals]
Flat Bones
- Thin & Broad
- Includes Ribs, Pelvis, Sternum, & Skull bones
Irregular Bones
- Irregular shape, so it doesn’t fit into other classes
- Includes Vertebrae & certain Skull Bones
Sesamoid Bones
- (usually) Small, Flat, & Oval shaped
- Located within Tendons
- Mechanical advantage of Tendons
- Muscles have better leverage
Example: Kneecap [Patella]
Long Bone Structure
- Periosteum: Membrane composed of Dense Irregular Collagenous Connective Tissue; Rich with blood vessels & nerves; Surrounds outer surface of Long Bones
- Perforating Fibers: Made of Collagen; Anchors Periosteum to underlying bone surface by penetrating deep into Bone Matrix - Diaphysis: Shaft of Long Bone, Each end is Epiphysis; Covered with a thin layer of Hyaline Cartilage [Articular Cartilage], found within joints [Articulations] between bones
- Hollow Cavity: [Marrow] within Diaphysis, containing Red or Yellow Bone Marrow; - Compact Bone: One of Two bone textures: Hard or Dense Outer Region; Allows bone to resist linear compression & twisting forces; (1st BONE STRUCTURE)
- Spongy Bone: [Cancellous] inside Cortical Bone with a Honeycomb framework of bony struts; Allows Long Bone to resist forces from many directions, providing cavity for Bone Marrow (2nd BONE STRUCTURE)
- Endosteum: Bony strut of Spongy bone & Inner surfaces covered by thin membrane
- Epiphyseal Lines: Separates Proximal & Distal Epiphysis from Diaphysis; Epiphyseal plates [Growth Plates], Lines of Hyaline Cartilage found in bones of children
Structure of Short, Flat, Irregular, & Sesamoid Bones
- Does Not have Diaphysis, Epiphyses, Medullary Cavities, Epiphyseal Lines, or Epiphyseal plates;
- Covered by Periosteum, Perforating fibers, Blood vessels, and Nerves
- Internal Structure: Two outer layers of thin Compact bone with middle layer of Spongy bone [diploe] & associated Bone Marrow
- Some Flat & Irregular bones of skull contains hollow, air-filled spaces, which reduces bone weight
Blood & Nerve Supply to Bone
- Bones are supplied with blood vessels & sensory nerve fibers
- Blood supply to Short, Flat, Irregular, and Sesamoid bones is provided by vessels in Periosteum that penetrate bone
- Long bones get third of their blood supply from Periosteum; Supplies Compact Bone. The remaining 2/3 is supplied by 1-2 [Nutrient Arteries], Enters the bone through a small hole in Diaphysis [Nutrient Foramen]; Nutrient Arteries bypass Compact bone to supply internal structures of bone.
- Epiphysis receives some blood supply from Nutrient Arteries, but majority comes from small blood vessels
Red Bone Marrow
- Loose Connective Tissue that supports blood-forming hematopoietic cells;
- Amount of Red Marrow decreases with age; In an adult, its located In Pelvis, Proximal Femur & Humerus, Vertebrae, Ribs, Sternum, Clavicles, Scapulae, & some bones of Skull
- Children need (MORE) Red Marrow to assist in growth & development
Yellow Bone Marrow
-Triglycerides, Blood Vessels, & Adipocytes
Microscopic Structure of Bone Tissue
-Bone,[Osseous Tissue] : Primary tissue in the bone; Composed of ECM with scattered cells
-ECM of Bone: Unique
Inorganic Matrix- 65% of bones weight
Organic Matrix- remaining 35%; Consists of collagen fibers and ECM components
Inorganic Matrix
- Calcium salts; Bones store 85% of total calcium salts, as well as Phosphorus
- [CALCIUM & PHOSPHORUS]* -> salts exist as [Hydroxyapatite Crystal Ca10(PO4)6(OH)2]
- Crystal structure makes bone (ONE OF THE ) hardest substances in the body; Strong & Resistant
- Allows bone to be protective and supportive
- Bicarbonate, Potassium, Magnesium, & Sodium
Organic Matrix
- Consists of protein fibers, Proteoglycans, Glycosaminoglycans, Glycoproteins, and bone specific proteins;
- Collagen: protein fiber that forms cross-links and helps bones to resist torsion & tensile forces; Collagen Fibers align with Hydroxyapatite Crystals, which enhances hardens of bone
- Glycosaminoglycans & Proteoglycans create an Osmotic gradient, which draws water into Osteoid and helps tissue to resist compression
- Glycoproteins in Osteoid, binds different components of Osteoid & Inorganic Matrix
Bone Cells
- Dynamic Tissue; Always changing as old bone breaks down for materials to build new bone
- 3 Types: Osteoblasts, Osteocytes, & Osteoclasts
Osteoblasts
- Active bone in Periosteum & Endosteum;
- Osteogenic Cells: Flattened; Disperse into Osteoblasts when stimulated by chemical signals
- Osteoblasts: Bone building cells; Bone deposition*
- Bone Deposition: Osteoblasts secrete organic matrix materials; Assists in formation of Inorganic Matrix
Osteocytes
- Osteoblasts surround themselves with Matrix in Lacunae; In return becomes Osteocytes that no longer synthesize bone matrix
- No longer metabolically active (EXCEPT) for maintaining bone ECM; Appears to have the ability to recruit Osteoblasts to reinforce bone under tension
Osteoclasts
-Responsible for Bone Resorption; Cell secretes hydrogen ions & enzymes
(Break down bone matrix)
-Different overall cell shape than other two-> (Large Multinucleated Cells) Derived from fusion of cells from bone marrow
-Located in shallow depressions on the Internal & External surface of bone
-Hydrogen ions dissolve components of Inorganic Matrix; Enzymes break down Organic Matrix
-Substances from breakdown includes nutrients, minerals, amino acids, & sugars; Absorbed by methods into Osteoclast cytosol
-Substances can be released into blood, might be either reused or excreted from body as waste products
Histology of Bone
-Outermost Compact Bone & Inner Spongy Bone tissue;
Structure : *Stress; Tends to strain or deform objects like bone
* Compact bone resembles a forest of tight packed trees; Each tree represents a unit, [Osteon]
* Ring of each tree is made up of [Lamellae]-> thin layers of bone
Osteon Structure
- Each Osteon contains 4-20 Lamellae arranged in layered structures, [Concentric Lamellae]
- Lamellar arrangement is stress resistant
- Collagen fibers of close Lamellae runs in opposite directions; Resist twisting & bending forces
Central Canal : Endosteum-lined hole in center of Osteon; Blood vessels & nerves supply bone
- Osteocytes in Lacunae: Small cavities between Lamellae, filled with ECF
- Lacunae is connected by [Canalculi]-> Canals
Overall Compact Bone Structure
-Osteons, NOT perminant
-Osteoclasts break down & Osteoblasts rebuild bone matrix, which depends on the bones need or body’s need;
(Leaves Behind) :
Interstitial Lamellae- Fills space between circular osteons, representing remnant of old osteons
Circumferential Lamellae- Outer & Inner layers of Lamellae, Inside Periosteum; Adds strength & is at the boundary with Spongy Bone
Perforating Canals- Comes from blood vessels in Periosteum, traveling perpendicular;* When it travels, it goes into central canals of neighboring Osteons and connecting them together
Structure of Spongy Bone
- Usually not weight bearing like Compact Bone; Less Densely packed
- Network of struts reinforcing compact bone; Resisting force from different directions
- Provides protective structure for Bone Marrow tissue
Structure of Spongy Bone
[Continued]..
Trabeculae- struts or ribs of bone; Covered with Endosteum ;
- Usually not arranged into Osteons
- Composed of Concentric Lamellae with Osteocytes in Lacunae; Communicate through Canalculi
- No central or perforating canals supplying blood to Trabeculae; Obtain blood from vessels in bone marrow
Structure of Spongy Bone
[Continued]..
Osteopetrosis- Marble Bone Disease; Defective Osteoclasts
(Do) (Not) properly degrade bone, causing bone mass to increase and become weak & brittle
-Main Forms:
1. Infantile: Inherited and more severe; Openings of skull & marrow cavities fail to enlarge with growth which traps nerves that cause blindness and deafness. Treated with drugs to stimulate Osteoclasts & Red Marrow
2. Adult: Inherited; Develops during adolescence or later
Symptoms- Bone pain, Recurrent fractures, Nerve trapping, & Joint pain
Ossification [Osteogenesis]
- Begins in Embryonic period and is a process is Bone Formation; Continues through childhood and most bones are complete by 7
- First bone formed is Immature Primary bone; Irregular arranged collagen bundles, Ostocytes, & Sparse Inorganic Matrix
- Primary Bone is broken down (usually) by Osteoclasts and replaced with Mature, Secondary, or Lamellar Bone
2 Ossification Mechanisms
- Intramembraneous- Membrane of Embryonic Connective Tissue
- Endochondral- Hyaline Cartilage
Intramembraneous Ossification
- Forms Flat Bones during fetal development (Skull & Clavicles)
- Primary Bone: within Mesenchymal Membrane that is composed of Embryonic Connective Tissue; Populated by Mesenchymal Cells
- Flat Bone: two outer layers of Compact Bone with Inner layer of Spongy Bone
- Middle layer of Spongy Bone: ossifies before Outer Compact Bone and begins in Primary Ossification Center
- Early Spongy Bone: formed as Osteoblasts continue to lay new bone down from [Trabeculae]
Intramembraneous Ossification
[Continued]…
-Smaller Trabeculae merges from larger structures
-Some Mesenchymal Cells disperse and form Periosteum; Some Vascular Tissue in Spongy Bone turns into Bone Marrow
-Spongy Bone is Deep to Periosteum, becoming heavily Calcified; Structure is rearranged to form Immature Compact Bone
-Larger Bones, more than one Primary Ossification Center; Leads pieces of bone that must fuse to one another
Example: [Fontanels]-> Incompletion of Ossification; Skulls of newborn babies
Events of Intramembraneous Ossification
-Mesenchymal Cells disperse into Osteogenic Cells, then into Osteoblasts at Primary Ossification center
-Osteoblasts secrete Organic Matrix, Calcium Salts, & other Inorganic Matrix components are deposited in Trabeculae [Calcification]->
+Hardens bone (Primary)
+Osteoblasts trapped in Lacunae become Osteocytes
Endochondral Ossification
- Bone development, all bones, (EXCEPT) Clavicles
- Begins in fetal stage for (MOST) bones; Some bones (Wrist & Ankle) Ossify later
- Many bones complete by 7
Endochondral Ossification for Developing Bone
- Hyaline Cartilage Model: Chondrocytes, Collagen & ECM surrounded by Connective Tissue Membrane [Perichondrium] & Immature Cartilage Cells [Chondroblasts]
- Begins at Primary Ossification Center; Primary bone is (First) Synthesized and then replaced with Secondary Bone
- Long Bones contain Secondary Ossification Centers
Model Forms, Epichondral Ossification forms in Steps:
- Chondroblasts in Vessel-filled Perichondrium disperse (First) into Osteogenic Cells, then into Osteoblasts forming Periosteum
- Bone begins where Osteoblasts have built Bone Collar on External surface of Bone
- At the same time, Internal Cartilage begins to Calcify and Chondrocytes die as blood supply connection is severed; Leaving Calcified Cartilage & Tiny Cavities
Endochondral Ossification Steps
- Osteoclasts etch opening in Bone Collar for blood vessels & bone cell entry into Primary Ossification Center
- Osteoblasts replace Calcified Cartilage with early Spongy Bone; Others Enlarge Bone Collar
- Cavities enlarge & combine; Medullary cavity develops
- Secondary Ossification Centers develop in Epiphyses
- Remaining Ossified Cartilage is replaced by Bone
- Medullary cavity is Enlarged by Osteoclasts; Fills with Bone Marrow
- Epiphyses is finished Ossifying
Endochondral Ossification Steps:
[Continued]…
- Cartilage (ONLY REMAINS) in Epiphyseal plates & on Articular surfaces where the bone interacts with joints, [Articular Cartilage]
- Articular Cartilage persists into Adulthood; Epiphyseal plates are replaced with Bone (over time) , once growth in length creases
Osteoporosis
*Most common Bone Disease; Bones become brittle & weak due to Inadequate Inorganic Matrix which increases risk of Fractures
- Causes: Diets, Female gender, Age, Lack of exercise, Genetics
- Diagnosis: Bone Density Measurement
- Prevention: Balanced diet, Weight bearing exercise, & Estrogen replacement
- Treatment: Drug that inhibits Osteoclasts or stimulate Osteoblasts
Growth In Length
- Long Bones lengthen by [Longitudinal Growth]; Involves division of Chondrocytes in Epiphyseal Plate
- Bone takes place at Epiphyses on the side closest to Diaphyses
Epiphyseal Plate
- Composed of Hyaline Cartilage that (DID NOT) Ossify;
- 5 zones of Cells: Reserve Cartilage,Proliferation, Hypertrophy & Maturation, Calcification, & Ossification
Zone of Reserve Cartilage
- Closest to Epiphyses;
- Cells that are (NOT) directly involved in bone growth, but (CAN) be recruited for cell division
Zone of Proliferation
-Actively dividing Chondrocytes in Lacunae
Zone of Hypertrophy & Maturation
- Next region closer to Diaphysis;
- Mature Chondrocytes
Zone of Calcification
-Dead Chondrocytes, some Calcified
Zone of Ossification
- Last Region;
- Calcified Chondrocytes & Osteoblasts
Zones of Epiphyseal Plates
- Actively involved in Longitudinal Growth (EXCEPT) the Zone of Reserve Cartilage;
- Chondrocytes divide in the Zone of Proliferation, forcing cells ahead into the next zones (toward Diaphyses)
- Chondrocytes that reach the Zone of Hypertrophy & Maturation enlarge and stops dividing
Longitudinal Growth Process
- Chondrocytes that reach the Zone of Calcification die, due to being far from blood supply while the Matrix Calcifies;
- Calcified Cartilage is replaced with bone from the Zone of Ossification
- Osteoblasts invade Calcified Cartilage and lay bone on top
- (Eventually) Calcified Cartilage & Primary Bone are resorbed by Osteoclasts and completely replaced with Mature Bone
Longitudinal Growth Process
[Continued]…
- Continues ONLY if Mitosis still continues in the Zone of Proliferation*
- Mitotically Plate (slows) around the age of 12-15 while Ossification continues; Epiphyseal Plate shrinks as the Zone is overtaken by Calcification & Ossification Zone
- Between the age of 18-21, the Zone of Proliferation is completely Ossified; Longitudinal Growth stops and Epiphyseal Plate is closed
- Epiphyseal line is Calcified remnant of Epiphyseal Plate
Bone Growth in Width
- Appositional Growth*
- Osteoblasts, In between Periosteum & Bone surface, lays new bone
- Appositional Growth (DOES NOT) result in Immediate formation of Osteons; Instead, new Circumferential Lamellae are formed
Appositional Growth
- As new Lamellae are added, older deeper Circumferential Lamellae are removed or reconstructed into Osteons;
- Bone Growth in Width can continue after bone growth in length ceases, which depends on factors like hormones, diet, and forces to which bone is subjected
Achondroplasia
- Common cause of Dwarfism; Gene defect is Inherited from a parent or new mutation
- Defective Gene produces abnormal growth receptor on Cartilage, which interferes with Hyaline Cartilage Model used in Endochondral Ossification; Also in Articular & Epiphyseal Cartilage
- Bones form & grow abnormally; Resulting in short limbs, Disproportionally long trunk, and Facial abnormalities
- Long term includes Joint disorders, Respiratory difficulties, & Spinal Cord compression; Can be managed with medication
Factors that play a role in How Much Cell Division occurs & How long the process remains Active in Epiphyseal Plate:
- Main factor is Hormones; Hormones are secreted by Cells of Endocrine Glands
- Growth Hormone: secreted by Anterior Pituitary Gland; Enhances Protein Synthesis & Cell Division in (ALL) Tissues, Including Bone
Longitudinal & Appositional Growth Effect
- Increases cell division rate of Chondrocytes
- Increases Osteogenic cell activity, Including Zone of Ossification
- Stimulates Osteoblasts in Periosteum
Male Sex Hormone [Testosterone] Effect
- Increases Appositional Growth; Bones in Males become thicker with more Calcium Salt Deposition
- Increases rate of Mitosis in Epiphyseal Plate, which leads to “Growth Spurts” in teenage years
- Accelerates closure of Epiphyseal Plate
Female Sex Hormone [Estrogen] Effect
- Increases Longitudinal Bone’s Growth rate; Inhibits Osteoclast activity
- When Estrogen levels spike, “Growth Spurts” occur
- Accelerates closure of Epiphyseal Plate at a (MUCH FASTER) rate than Testosterone; Leads to average height differences between genders
Excess Growth Hormone Produces Two Conditions
- Depends on when it develops *
1. Childhood-Gigantism: Epiphyseal Plates (HAVE NOT) Closed; Therefore, Individuals grow tall due to excessive Longitudinal & Appositional Bone Growth
- Adulthood-Acromegly: Epiphyseal Plates (ARE) Closed; No height increase, but enlargement of Bone, Cartilage, & Soft Tissue
- Skull, Bones of Face, Hands, Feet, & Tongue [Affected]
- [Can] cause Heart & Kidney Malfunction
Bone Remodeling
-Continuous process of Bone Formation & Loss after Length Growth is complete; New Bone is formed, by [Bone Deposition]-> old bone is removed by [Bone Resorption]
Reasons for Bone Remodeling & Repair
- Maintenance of Calcium Ion Homeostasis
- Replacement of Primary Bone with Secondary Bone
- Bone Repair
- Replacement of old brittle bone with newer bone
- Adaptation to tension & stress
Bone Remodeling
[Childhood & Healthy Bone]
- In Healthy Bone: the process of formation & loss occurs simultaneously; Bone broken down by Osteoclasts matches the Bone formation by Osteoblasts
- In Childhood: deposition processes much faster than resorption; Once Epiphyseal Plates close & Longitudinal Growth is complete, deposition & resorption becomes equivalent
Bone Deposition
- Carried out by Osteoblasts
- Found In Periosteum & Endosteum; Making Organic Matrix & Facilitate the formation of Inorganic Matrix
- Secretes Proteoglycans & Glycoproteins that bind to Calcium Ions
- Secretes Vesicles that contain Calcium Ions, ATP, & Enzymes; Binds to Collagen Fibers, in which the Calcium Ions Crystalize rupturing vesicle and begins the Calcification process
Bone Resorption
- Osteoclasts secrete Hydrogen Ions on the Bone’s ECM
- Hydroxyapatite crystals in Inorganic Matrix are pH sensitive; Breaks down in acidic environment created by Osteoclasts
- Calcium ions and other liberated minerals can be used/refused elsewhere in the body
Osteoclasts Secreting Enzymes
- Degrade Organic Matrix, Includes Proteoglycans, Glycosaminoglycans, & Glycoproteins
- Breakdown products of Matrix are taken into Osteoclasts for reuse
Bone Resorption in Response to Tension & Stress
- Compression: Squeezing or Pressing together; Occurs when bones are pressed between body’s weight and ground, which stimulates bone deposition
- Tension: Stretching force; Bone deposition occurs in regions of bone exposed to tension
- Pressure: Downward continuous force; Bone resorption is stimulated in regions of bone exposed to continuous pressure
Additional Factors Influencing Bone Remodeling
- Hormones: Testosterone promotes bone deposition; Estrogen inhibits Osteoclast activity
- Age: Growth Hormone & sex hormones decline as individuals age; Decreases protein synthesis in bone
- Calcium Ion Intake: (diet) must be adequate to support bone deposition
- Vitamin D Intake: (diet) must be adequate to promote Calcium Ion absorption from the Gut and prevent Calcium Ion loss in Urine
- Vitamin C Intake: (diet) adequate for synthesis of Collagen
- Vitamin K Intake: (diet) adequate for synthesis of Calcium Ion binding Glycoproteins secreted by Osteoblasts
- Protein Intake: (diet) adequate for Osteoblasts to synthesize Collagen Fibers in Organic Matrix
Calcium Ion Homeostasis
- Bones store (MOST) of Calcium Ions in body
- Stored Calcium Ions are not only used for bone deposition & remodeling, but is used throughout the body for several critical processes [Muscle Contraction]
- Negative Feedback loop maintains Calcium Ion in Homeostasis in blood
- Calcium Ion levels in blood are closely monitored; Both high & low levels can lead to major homeostatic disruptions [Death]
Negative Feedback Loop
-Stimulus & Receptor: When Calcium Ion levels drop in blood, it is detected by Parathyroid Cells
-Control Center & Effector: Parathyroid cells act as control center; Secrete Parathyroid Hormone, PTH
-Effect/Response: PTH stimulates effects that increase blood calcium ion levels;
Increases Osteoclast activity; breaks down Inorganic Matrix of bone releasing calcium ions from hydroxyapatite crystals
-Increases absorption of Calcium from Gut
-Inhibits Calcium loss in Urine
Homeostasis & Negative Feedback
-As Calcium Ion levels return to normal in the blood, change is detected by Parathyroid Cells; Secretion of PTH is reduced, closing the feedback loop
Calcium Ion Homeostasis
- Increased blood calcium levels trigger different negative feedback loop; First response is drop in PTH secretion by Parathyroid Gland
- Calcitonin is secreted by Thyroid Gland; Opposite effects as PTH; Leads to bone deposition, pulling Calcium Ions out to manufacture Inorganic Bone Matrix. Most active during Bone Growth & less in adulthood
- Vitamin D, important for Calcium Ion homeostasis due to absorption of Ions
Bone Repair
- Most dramatic Bone injury is [Fracture]-> broken bone
- 2 Types: Simple & Compound
Simple Fracture
-Skin & Tissue around fracture remains Intact
Compound Fracture
-Skin & Tissue around the fracture is damaged
Healing Process of Fracture
- Hematoma-(blood clot) that fills in the gap between Bone Fragments:
- Mass of blood cells & proteins forms due to ruptured blood vessels
- Bone Cells in surrounding areas die - Fibroblasts & Chondroblasts- (Periosteum) Infiltrate Hematoma and Forms Soft Callus-> mixture of Hyaline Cartilage & Collagenous Connective Tissue; Bridges gap between Fragments
- Fibroblasts form Dense Irregular Collagenous Connective Tissue
- Osteogenic Cells become Chondroblasts; Secretes Hyaline Cartilage - Osteoblasts build Bone Callus- (Hard Callus) Collar of Primary Bone made by Osteoblasts in Periosteum; Forms a bridge between Fragments
- Bone Callus is Remodeled & Primary Bone is replaced with Secondary Bone- bone regains previous structure & strength after several months
