Anatomy and Physiology: Skeletal System

• The skeleton is an organ system of bones, cartilage, ligaments, and connective tissues.
• The primary organs of the skeletal system are the bones.
• Bones form the framework of the body.
• Bone marrow, either red or yellow, is contained in the interior cavity of the bone.
• Compact bone, also known as dense or cortical bone, is relatively dense, white, smooth, and solid.
• Compact bone makes up 80% of total bone mass.
• Spongy bone, also known as cancellous or trabecular bone, is internal to compact bone.
• Spongy bone appears porous and makes up the remaining 20% of total bone mass.

Cartilage and Connective Tissue

• Cartilage is a semi-rigid connective tissue that is more flexible than bone.
• Hyaline cartilage connects the ribs to the sternum (costal).
• It covers the ends of most bones at moveable joints (articular).
• Hyaline cartilage supports the external nose (nasal).
• Hyaline cartilage is found with growth plates (epiphyseal).
• Hyaline cartilage provides the model for the formation of the bones during development.
• Fibrocartilage is located at sites of heavy pressure and stretching.
• Examples include intervertebral discs, the pubic symphysis, and menisci in the knee joints.
• Ligaments are dense regular connective tissue and anchor bone to bone.
• Tendons are dense regular connective tissue that anchors muscle to bone.

Bone Function

• Bones support the body and protect tissues and organs from trauma.
• Bones serve as attachment sites for muscles.
• Bones act as a system of levers to modulate direction and magnitude of force.
• Hematopoiesis is the process of blood cell production that occurs in red bone marrow.
• Bones store minerals such as calcium and phosphate and energy reserves in lipids stored in yellow bone marrow.

Bone Classifications

• Flat bones are flat, have thin surfaces, may be slightly curved.
• Flat bones protect underlying soft tissue and serve as a large surface area for muscle attachment.
• Cranial vault of the skull, scapulae, sternum, and ribs are examples of flat bones.
• Long bones tend to be longer than they are wide
• Long bones have an elongated cylindrical shaft (diaphysis) and two ends (epiphysis).
• Examples of long bones are found in the upper and lower limbs
• Short bones are approximately the same length and width.
• Examples: carpal and tarsal (wrist and ankle) bones, and sesamoid bones like the patella.
• Sesamoid bones are found within tendons.
• Sesamoid bones tend to be small, flat, and oval shaped, increase muscle leverage, and the patella is an example.
• Irregular bones have elaborate, complex shapes.
• They do not fit into other categories.
• Examples include vertebrae, bones of the skull, and the hip bones.

Structures of a Long Bone

• The diaphysis is a long, hollow cylindrical shaft that provides leverage and weight support.
• The diaphysis has an external, relatively thick layer of compact bone and an internal thin layer of spongy bone surrounds the central medullary cavity.
• In children, the medullary cavity is filled with red marrow.
• In adults, the medullary cavity is filled with yellow marrow.
• Knobby ends of the long bone are called the epiphysis
• The epiphysis consists of an external, thin layer of compact bone and an internal core filled with spongy bone.
• Spongy bone resists stresses in all directions.
• At the moveable joints, articular cartilage covers spongy bone
• The metaphysis is the region of mature bone between the diaphysis and epiphysis
• The metaphysis contains the epiphyseal (growth) plate.

Macroscopic Features of Bones

• The periosteum covers the external surface of bone.
• The periosteum consists of: an outer fibrous layer of dense irregular CT that protects the bone, anchors blood vessels and nerves, providing attachment sites for tendons and ligaments.
• The periosteum consists of an inner cellular layer made of osteoprogenitor cells, osteoblasts & osteoclasts
• The periosteum is secured to the underlying bone by perforating fibers.
• The endosteum covers the internal surface of bone within the medullary cavity.
• The endosteum is an incomplete layer of cells containing osteoprogenitor cells, osteoblasts & osteoclasts.

Unique Structures of Other Bones

• Short, flat and irregular bones differ from long bones in structure.
• The external surface is compact bone.
• The interior is composed of spongy bone.
• The spongy bone is also called diploë in the flat bone of the skull.
• Short, flat and irregular bones lack a medullary cavity.

Red vs. Yellow Marrow

• Marrow is the soft connective tissue of the bone.
• Red marrow is hematopoietic tissue.
• In adults, red marrow is located within: spongy bone of the long bones and diploë of flat bones
• Red marrow is located within medullary cavities of long bones is children
• Yellow marrow replaces red marrow in medullary cavities of adults
• Yellow marrow is primarily adipocytes.
• Yellow marrow may revert back to red marrow under extreme anemic conditions.

Four Major Bone Cell Types

• The four major bone cell types are osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts
• Osteoprogenitor cells are mitotic stem cells that can rise to the osteoblasts
• Osteoprogenitor cells are found in the periosteum and endosteum
• Osteoblasts produce new bone matrix (osteoid).
• Osteoblasts are located in the periosteum and endosteum where the produce most of the new matrix on the exterior of the bone
• They become trapped within calcified osteoid and differentiate into osteocytes.
• Osteocytes are mature bone cells that cannot make new bone
• Osteocytes reside within the bone matrix in lacunae and are connected by canniculi
• They are connected to other nearby osteocytes and help maintain the bone matrix and detect mechanical stress in the bone matrix.
• Osteoclasts are large, multinuclear phagocytic cells
• Osteoclasts are derived from hematopoietic progenitor cells
• Osteoclasts are located within depressions on the bone surface called resorption lacuna.
• Osteoclasts break down the bone matrix in the process called resorption.

Composition of Bone Matrix

• The organic component of the bone matrix makes up ~1/3rd of the matrix.
• It consists of collagen fibers and a semi-solid ground substance (proteoglycans and glycoproteins).
• The organic matrix gives bone tensile strength and resists stretching and twisting.
• The inorganic matrix consists of mineral salts (~2/3rd of the matrix).
• The mineral salts are primarily Calcium Phosphate
• Calcium Phosphate combines with other salts to form Hydroxyapatite crystals that deposit along the long axis of collagen.
• The inorganic components harden the matrix.
• Mineral salts accounts for rigidity and inflexibility and gives bone compression strength.

Bone Matrix: Formation & Resorption

• Osteoblasts secrete bone matrix, osteoid.
• Osteoid calcifies as a result of high plasma Calcium levels or when bone needs to respond to greater stress.
• Osteoclasts destroy bone matrix to release calcium into blood plasma.

The Osteon of Compact Bone

• The structural unit of compact bone is called an osteon
• Osteons run parallel with the long axis and function as weight-bearing pillars
• Osteons resist torsion stress and have the appearance of a bullseye when viewed in cross-section.

Components of the Osteon

• Central canals contain small blood vessels and nerve fibers that serve the osteon's cells.
• Central canals run parallel to the long axis.
• The osteon is composed of concentric lamellae, which are collagen fibers running in alternating directions in each lamella
• This gives bones strength and resilience.
• Osteocytes are mature bone cells that maintain the bone matrix.
• Osteocytes are isolated in lacunae
• Lacunae are found at the junctions of the lamellae
• Canaliculi
• Tiny canals between lacunae
• Osteocytes connect with one another via gap junctions to pass nutrients & waste and to communicate with one another.

Structures of Compact Bone

• Perforating (Volkmann) canals connect and supply small blood vessels and nerves to the central canals & medullary cavity.
• Perforating (Volkmann) canals lie at right angles to the long axis of the bone
• Circumferential lamellae are rings of bone that run the entire circumference of the bone, protecting it against torsion.
• External circumferential lamellae are found internal to the periosteum.
• Internal circumferential lamellae are found internal to the endosteum.
• Interstitial lamellae are located between intact osteons, are incomplete, have no central canal, and are remnants of resorbed osteons or fill-ins.

Structures of Spongy Bone

• Structure appears more haphazard than compact bone.
• Spongy bone is an open lattice of trabeculae.
• Trabeculae are marrow plates and rods of bone
• They align along lines of stress and distribute stress throughout the bone.
• Spongy bone lacks osteons
• Spongy bone does possess parallel lamellae.
• Spongy bone lacks blood vessels
• Some canaliculi open to the surface to allow the diffusion of nutrients and waste.

Hyaline Cartilage

• Hyaline cartilage is resilient and flexible and is covered by dense irregular connective tissue called the perichondrium.
• The extracellular matrix of Hyaline cartilage consists primarily of collagen and a ground substance is similar to that of bone but lacks the inorganic salts.
• The ground substance of hyaline cartilage contains between 60-70% water, it is highly compressible, and a good shock absorber.
• Cells of the cartilage are called chondroblasts
  • They are derived from mesenchyma
• Chondrocytes are located in matrix
• Mature cartilage cells found in lacunae
• Hyaline cartilage lacks blood vessels and nerves and nutrients are supplied to cells by diffusion.

Cartilaginous Growth

• Cartilaginous Growth begins during embryonic development and gives rise to bone growth.
• Interstitial and appositional growth occur simultaneously
• Interstitial growth (length) occurs within internal regions of cartilage.
• There are four steps of interstitial growth:
  1. Chondrocytes in lacunae undergo mitotic division
  2. Daughter cells (in same lacunae) are called chondroblasts
  3. As chondroblasts secrete new matrix, they are forced to separate. Once in own lacuna referred to as chondrocytes (again)
  4. Cartilage grows as chondrocytes produce more matrix
• Interstitial growth declines rapidly as the cartilage becomes semi-rigid and can no longer expand.
• Appositional growth (width)
  • Undifferentiated stem cells divide
  • New undifferentiated stem cells and committed cells are derived from
  • Chondroblasts that secrete new matrix
  • Matrix surrounds cells and they become chrondrocytes pushing apart and adding to periphery

Five Cartilaginous Zones

• Five cartilaginous zones are involved in interstitial bone growth
• Zone 1, the Zone of resting cartilage, is furthest from medullary cavity of diaphysis.
• It resembles healthy cartilage and secures the epiphysis to epiphyseal plate.
• Zone 2 is The Zone of Proliferating cartilage
• Cells undergo rapid mitotic division and the cells are aligned in columns.
• Zone 3 is the Zone of hypertrophic cartilage, here the cells no longer divide.
• The cells become hypertrophic and resorb matrix. Zone 4 is the Zone of calcified cartilage
• Mineral deposition occurs in the matrix and chrondrocytes die.
• Lastly, is the Zone of ossification
• Vascularization and bone deposition occurs here.

Bone Formation

• Ossification or osteogenesis is how bones form
• Bone formation begins in the embryo through childhood and adolescence.
• There are two types of bone formation resulting in different kinds of bone formations.
• Intramembraneous yields flat bones
• Endochondral yields most bones of the skeleton.
• Some examples of these bones are Long bones, Bones of the pelvis, vertebrae, & the clavicle

Intramembranous Ossification

• Intramembranous ossification is bone growth within a membrane.
• Fibrous connective tissue is used as a supportive structure on which ossification occurs.
• Intramembranous ossification creates flat bones
• Intramembranous ossification operates in four steps
  • One: mesenchymal cells become osteoblasts and secrete osteoid.
  • Two: Calcification of the osteoid
  • Three: Formation of woven bone and the periosteum
  • Four: Replacement of woven bone by lamellar bone.

Endochondral ossification

• Endochondral ossification is Bone development that replaces cartilage
• Fetal hyaline cartilage model develops (week 8 -12 development).
• Cartilage calcification and formation of the periosteal collar
• Primary ossification center forms in the diaphysis.
• Secondary ossification center forms in the epiphysis
• Bone replaces cartilage in the long bone structure

Interstitial and Appositional bone growth,

• Interstitial growth involves Bones growing in length.
• Epiphyseal plates undergo cartilage growth on edge and on diaphysis side; chondrocytes die and remodeling takes place
• Epiphyseal plates close, and epiphysis and diaphysis fuse
• Appositional bone growth involves Bones growing in width.
• Deposition of bone on outside and osteoclasts resorb on indside
• The Rate of bone addition is greater than rate of resorption
• This causes for thicker, stronger bones withstanding trauma.

Bone Mass

• Bone continues to renew and reshape
• Bone recycles ~5-7% of our bone mass every week.
• Bone replacement occurs at the periosteal and endosteal surfaces, slower in compact bone than in spongy bone
• Two processes mediate this dynamic process: bone deposition and bone resorption
• Remodeling is regulated by hormones, particularly the thyroid and sex hormones, and mechanical stress on the bones.
• Bones grow or remodel in resposne to demans put upon them (Wolff's Law)

###Hormones

• Growth hormone (GH) released from the anterior pituitary stimulates liver to produce somatomedins to stimulate chondrocytes
• Thyroid hormones regulate metabolic rate of cells and are permissive hormones for this process
• Sex hormones are produced in gonads and include estrogen/androgens
• Increase secretions at puberty
• Glucocorticoids (Cortisol) produced in adrenal gland regulate blood glucose and interfere with epiphyseal growth
• In high levels
• Serotonin in neurotransmitters and bone regulators

Calcitrol

• Produced in skin with exposure to sunlight using cholesterol/diet
• Converted to enzymatick form in liver and kidney
• Mediating absorbtion of calcium levels in intestinal tract

PTH and Calcitonin

• Parathyroid hormone released from parathyroid in response to calcium levels
• Regulate kidney to trigger calcium reabsorbtion and decreases PO4 absorbtion
• Enhances calcitrol production in intestines
• Calcitonin functions in high levels of calcium
• Secreated in response to Calcium levels
• Bone inhibits activity in osteoclast

Bone Fractures

• Avulsion
• Colles
• Comminuted
• Complete
• Compound (Open)
• Compression
• Depressed
• Displaced
• Epiphyseal
• Greenstick
• Hairline
• Impacted
• Incomplete
• Linear
• Oblique
• Pathologic
• Pott
• Simple(Closed)
• Spiral
• Stress
• Transverse

Repair of the simple fracture

• A fracture hematoma forms due to torn blood vessels.
• A Fibrocartilaginous callus forms as fibrous tissue invades
• Bone forms to heal the break
• The broken section is replaced

The skeletal system is an organ system comprised of bones, cartilage, ligaments, and connective tissues. Bones form the framework of the body. This lesson covers compact and spongy bone differences and hyaline cartilage.