Anatomy and Physiology Test #5 - Bones

Osteogenic Cells

Unspecialized stem cells found in bone tissue that divide to form new osteoblasts for bone growth and repair.

• Sit in the endosteum (inner lining of bone marrow cavity), periosteum (outer covering of bone), and the central canals within bones.

Osteoblasts

Specialized, cube-shaped cells that form and help mineralize organic matter of matrix. They become trapped in their own secretions to become osteocytes.

Osteocytes

Osteoblasts that have become trapped in the hard, mineralized bone matrix they formed.

• They reside in tiny microscopic cavities known as lacunae and each osteocyte sends out hair like projections through tiny channels called canaliculi. These connect osteocytes to each other and to blood vessels through gap junctions to share nutrients and signals

• Regulate bone remodeling - monitor physical stress and direct when to build or break down tissue

Osteoclasts and their precursors

Specialized, multi-nucleated giant cells that break down and resorb bone tissue. They develop in bone marrow by the fusion of 3-50 hematopoietic stem cells (same cells that give rise to monocytes).

• They form a ruffled border to secrete lysosomal enzymes and acids that break down the bone matrix.

• Reside in pits called resorption bays that they have eaten into the surface of the bone

What is the bone matrix made of? What is the composition of those materials?

• Dry weight is 1/3 organic (mostly collagen)

  • Collagen fibers, ground substance (GAGs, proteoglycans and glycoproteins)

  • Collagen resists tension - provides flexibility and tensile strength (allows the bone not to snap), and prevents brittleness

• 2/3 inorganic matter

  • 85% hydroxyapatite (crystallized calcium phosphate salt)

  • 10% calcium carbonate

  • Other minerals - Fluoride, Sulfate, Potassium, Magnesium

  • Gives bone its hardness, rigidness, and ability to resist compression

Intramembranous Ossification

• Process - Bone develops directly from fibrous connective tissue membranes

  • Mesenchyme condenses into a sheet of soft tissue

  • Osteoblasts gather trabecular to form osteoid tissue (uncalcified bone)

  • Calcium phosphate is deposited in the matrix transforming the osteoblasts into osteocytes

  • Osteoclasts remodel the center to contain marrow and spaces and osteoblasts remodel the surface to form the compact bone

  • Mesenchyme at the surface gives rise to the periosteum

• Bones formed - Flat bones of the skull, mandible, and clavicles

Endochondral Ossification

• Process - Bone replaces a pre-existing hyaline cartilage model

  • Mesenchymal cells differentiate into chondroblasts, creating a hyaline cartilage template shaped like a future bone

  • Blood vessels gradually invade the cartilage template and break it down so bone can take its place

• Bones Formed - Most bones below the skull, long bones

Appositional vs Interstital Growth

• Appositional - deposits new bone tissue/layers at the bone surface under the periosteum. So, bone increases in width/thickness.

  • Osteoblasts lay down matrix in layers parallel to the outer surface and osteoclasts dissolve bone on the inner surface. If one process outpaces the other, bone deformities occur (osteitis deformans).

• Interstital - bone increases in length by growth of epiphyseal plate

Effects of Hypocalcemia

• Hypocalcemia - abnormally low level of calcium ions in the blood plasma

  • Calcium normally binds to cell surface contributing to resting membrane potential. But, with less calcium, sodium channels open more easily, causing hyper-excitability of the nervous and muscular system, leading to muscle spasms, tremors, or tetany (Medical condition).

• Effects:

  • Muscle cramps and spasms

  • Numbness and tingling

  • Reflex changes

  • Seizures

  • Arrhythmias

  • Mental changes

  • Skin and hair issues

Hormones for Calcium Homeostasis - Calcitonin

• Calcitonin

  • secreted by C cells of the thyroid gland when calcium concentration rises too high

  • reduces osteoclast activity by as much as 70% in 15 minutes which stops bone resorption and prevents calcium from flowing into the blood

  • Increases the number and activity of osteoblasts (bone formation)

  • Important role in children, but little effect in adults

Hormones for Calcium Homeostasis - PTH

• PTH (Parathyroid Hormone)

  • secreted by the parathyroid glands which are found on the posterior surface of the thyroid gland

  • Released when calcium blood level is too low (hypocalcemia)

  • Binds to osteoblasts causing them to release osteoclast stimulating factor to stimulate osteoclast activity

  • PTH acts on the distal tubules of the kidneys to stimulate Ca reabsorption and decrease Ca excretion in the urine.

  • inhibits collagen synthesis and bone deposition by osteoblasts

Hormones for Calcium Homeostasis - Calcitriol

• Calcitriol (A form of/Activated Vitamin D)

  • behaves as a hormone (blood-borne messenger)

  • stimulates intestine to absorb calcium, phosphate, and magnesium to maintain calcium homeostasis

  • promotes osteoclast activity to raise blood calcium concentration to the level needed for bone deposition

Parts of bone - Compact bone

• The hard, dense outer layer

• Basic structural unit = Osteon (Haversian system)

• Cylinders of tissue forms from layers (lamellae) of matrix arranged around central canal holding a blood vessel

• Collagen fibers alternate between right and left hands helices from lamella to lamella

• Osteocytes connect to each other and their blood supply by tiny cell processes in canalicul

Parts of a bone - Spongy bone

• Spongelike appliance formed by rods and plates of bone called trabeculae

• Spaces are filled with red bone marrow that makes red and white blood cells

• Structural unit = trabeculae which have few osteons or central canals

  • trabeculae develop along bone’s lines of stress

• Provides strength with little weight

Parts of bone - Bone Marrow

• Soft tissue that occupies the medullary cavity of a long bone or the spaces amid the trabeculae of spongy bone

• Red marrow looks like thick blood, very abundant in early development

• Yellow marrow is fatty marrow of long bones in adults

• With old age, yellow marrow is replaced with reddish jelly

Parts of a bone - Diaphysis

The tubular shaft forming the long axis of the bone; surrounding the central cavity.

Parts of a Bone - Epiphysis

The expanded distal and proximal ends of the bone, composed mainly of spongy bone surrounded by a thin layer of compact bone.

Parts of a bone - Metaphysis

The region between the epiphysis and diaphysis containing hyaline cartilage during growth (epiphyseal plate) or ossified tissue in adults (epiphyseal line).

Parts of bone - Medullary Cavity

Hollow cavity within the diaphysis containing yellow bone marrow (fat) in adults or red marrow in infants.

Parts of bone - Periosteum

Double-layered fibrous membrane surrounding the outer surface of the bone (except at articular surfaces).

• Outer layer - dense irregular connective tissue

• Inner layer - contains osteoblasts and osteogenic cells

Parts of bone - Endosteum

Delicate connective tissue membrane lining internal bone surfaces

• Lines the medullary (marrow) cavity of long bones

• Covers the trabeculae of spongy bone

• Lines the central (Haversian) canals of osteons

Parts of bone - Articular Cartilage

Smooth hyaline cartilage covering epiphyseal surfaces at joint articulations to reduce friction and absorb shock.

Parts of bone - Epiphyseal Plate

A specific layer of hyaline cartilage within the metaphysis that is responsible for lengthening the long bone through interstitial growth. During early adulthood, hormone changes cause the cartilage to stop dividing and completely ossify (calcify). Once fused, it dissappears, leaving behind only the epiphyseal line within the metaphysis.