Bone Structure and Functions

Endosteum

A thin layer of reticular connective tissue. Covers spongy bone trabeculae in the middle layer.

Osteons (haversian systems)

Deeper in compact bone and is organized in cylindrical units

Medullary cavity

Bone marrow occupies the medullary cavity of the shaft and the spaces in the spongy bone of the head.

Osteoclasts

Bone-dissolving cells that develop from a separate line of bone marrow stem cells. Very large, have multiple nuclei, & lie on the bone surface like osteoblasts. Often lies in pits that they have eroded into bone surface. Develop from macrophage-like cells Degrade bone matrix

Appositional growth

Deposition of new bone/cartilage on the surface of existing cartilage/bone Appositional = Around the outside

Osteoblasts

Develop from osteogenic cells & lie in a single layer on the bone surface. Have an active role in adults and produce bone matrix Synthesize the organic matter of bone and deposit matrix. Has lots of mitochondria and rough endoplasmic reticulum.

Intramembranous Ossification

Direct, within membranes (flat bones of the skull) Produces the flat bones of the skull and most of the clavicle (collarbone). The method of depositing new tissue on the bone surface even past the age where our bones can no longer grown in length.

Yellow bone marrow

Dominates in long limbs bones of adults

Head or epiphysis

Each expanded end of bone Enlarged to strengthen a joint and provide added surface area of the attachment of tendons and ligaments

Step 1 of endochondral Ossification

Early cartilage model Forerunner of future bone is a body of hyaline cartilage that approximate its shape. Covered w/ a fibrous perichondrium.

Shaft or diaphysis

Elongated midsection; Provides leverage

Articular cartilage

Ends where two bones meet are covered w/ a thin layer of hyaline cartilage called the articular cartilage (eases join movement).

Step 6 of endochondral Ossification

Late teens to early twenties → Reserve cartilage in the epiphyseal plate is depleted and the primary & secondary marrow cavities are no longer separated. Only remnant of the original cartilage model is the articular cartilage that covers the joint surfaces of the bone.

Rickets & osteomalacia

Minerals resist compression and mineral-deficient can results in easily deformed bones of childhood rickets and the similar adult disease osteomalacia.

Osteogenic

Occur on bone surface, beneath the fibrous connective tissue membranes that cover a bone. Stem cells that give rise to osteoblasts. Only bone cells capable of dividing and making more bone cells.

Epiphyseal lines

Of an adult bone mark the former growth zones of the child's bone

Ruffled border

On side facing the bone, an osteoclast has an unusual comb like row of infoldings of the plasma membrane. On this surface the cell secretes hydrochloric acid and enzymes to dissolve osseous tissue.

4 Kinds of Bone Cells:

Osteogenic, Osteoblasts, Osteocytes, & Osteoclasts

Spongy bone

Porous lattice honeycombed with spaces. Always found in interior of a bone.

Trabeculae

Porous lattice of slender rods and plates; trabeculae align along planes that give the bone the best strain resistance.

Inorganic matter of bone matric

85% calcium phosphate, w/ lesser amounts of calcium carbonate, magnesium, potassium, and fluoride.

Mineral deposition

A crystallization process in which osteoblasts extract calcium, phosphate, and other ions from the blood and deposit them in the osseous tissue.

Periosteum

A fibrous sheath that covers the bone externally Covers both sides of a flat bone (i.e. skull) Connective tissue that wraps around bone

Mineral reabsorption

A process in which the osteoclasts dissolve bone, releasing minerals into the blood and making them available for other uses.

Wolff's Law

Architecture of bone determined by mechanical stresses placed on it and bones adapt to withstand those stresses

Step 4 of endochondral Ossification

At each end of the primary marrow cavity is a transitional zone called a metaphysis - where cartilage is undergoing replacement by bone (undergo cartilage tissue to bone tissue) Appearance of secondary marrow cavity

Parathyroid hormone (PTH)

Secreted by the parathyroid glands, which adhere to the posterior surface of the thyroid. Respond to drop in blood calcium level by secreting PTH. PTH them stimulates bone resorption by osteoclasts, promotes calcium reabsorption by the kidneys, and promotes calcitriol synthesis.

Calcitonin

Secreted by the thyroid gland in response to elevated blood calcium levels. Promotes deposition of the excess calcium into the skeleton and thereby lowers the blood calcium level. Has little influence on adult bone, but regulates blood calcium in children and may help to reduce bone loss in pregnant women.

Bone Marrow

Soft bloody or fatty material enclosed in the bones.

Compact (dense) bone

Solidly filled with opaque matrix

Step 3 of intramembranous ossification

The bone assumes a foundation of spongy bone

Ossification

The formation of bone; Two types of ossification & both begins w/ a soft embryonic connective tissue called mesenchyme (forerunner of adult bone, muscle, blood, and many other tissues b/w skin & gut)

Osseous tissue

The hard, calcified tissue of a bone

Bone Matrix

The stony matter that surrounds the osteocytes and lacunae. ⅓ Organic & ⅔ inorganic matter by weight.

Metaphysis

Where cartilage is undergoing replacement by bone.

Canaliculi

Where dendrites reside

Red bone marrow

Produce blood cells & platelets Has more limited distribution in adults

Flat bones

Protective armor-like plates covering delicate organs beneath, such as the sternum anterior to the heart and the cranial bones enclosing the brain.

Step 1 of intramembranous ossification

An ossification center occurs within the embryonic tissue where mesenchymal cells cluster together, differentiate into osteoblasts, and secrete bone matrix. Some of the osteoblasts become trapped in the bone matrix and become osteocytes.

Step 2 of intramembranous ossification

Blood vessels grow into the area to supply the cells and matrix with oxygen and nutrients. These blood vessels get trapped within the developing bone.

Step 3 of endochondral Ossification

Blood vessels grow inward from the periosteum and invade the ossification center. Osteoclasts arrive & digest calcified tissue in the shaft, hollowing it out and creating the primary marrow cavity. Also deposit layers of bone lining the cavity, thickening the shaft. Secondary ossification center develops at one end of the bone. vascular invasion: Blood vessels coming in

Interstitial growth

Bone elongation results from cartilage growth Inter = Inside Chondrocytes multiply, enlarge, & secrete new matrix b/w them.

Epiphyseal line

Bone elongation results from cartilage growth When bone can then no longer grow & the epiphyseal plate leaves behind an internal mark

Short & irregular bone

Bones that neither fit the description of neither long nor flat bones (i.e. wrist bone & vertebrae)

Calcium homeostasis is regulated by 2 hormones:

Calcitriol & parathyroid hormones (PTH) - both raises blood calcium levels.

Hypocalcemia

Calcium deficiency → Causes dysfunctions ranging from muscle tremors to tetanus - inability of muscle to relax.

Hypercalcemia

Calcium excess → depresses nervous, muscular, and cardiac function.

Step 5 of endochondral Ossification

Childhood & adolescence - the primary & secondary marrow cavities are separated by a wall called the epiphyseal plate. Consists of a middle layer of hyaline cartilage w/ metaphysis on each side. The plate functions as a growth zone enabling the individual to grow in limb length and height.

Organic matter of bone matric

Collagen & large protein-carbohydrates complexes

Ligaments

Collagenous bands that hold bones together at the joints.

Osseous tissue is made up of:

Compact (dense) & spongy bone

Skeletal system

Composed of bones, cartilages, and ligaments joined tightly to form a strong, flexible framework for the body.

Spongy (cancellous) bone

Consists of trabeculae Permeated by tiny spaces that give it a sponge-like appearance These spaces are filled w/ bone marrow & small blood vessels Trabeculae align along planes that give the bone the best strain resistance. All osteocytes are close to the blood supply in the marrow cavity, so no need for central canals or osteons.

Epiphyses

Covered with articular cartilage, the distal and proximal portions at each end of a typical long bone

Osteogenesis imperfecta (brittle bone disease)

Excessively brittle bones resulting from improperly formed collagen Lack flexibility

Cartilage

Forerunner for most bones in embryonic development; forms a growth zone in the bones of children; and it covers many joint surfaces in the mature skeleton

Step 2 of endochondral Ossification

Formation of Primary ossification center, bony collar, and periosteum Cartilage cells begin the process of dying They enlarge and die, leaving behind open space

Osteocytes

Former osteoblasts that have become trapped in the matrix they deposited. Resides in cavities called lacunae (pocket within matrix), which are connected to each other by slender channels called canaliculi. Pass nutrients, wastes, and chemical signals to each other through gap junctions at the tips of their cytoplasmic processes.

Compact (dense) bone

Forms outer shell that surrounds the spongy bone. Prevents bone marrow from seeping out and provides solid attachment surfaces for muscles, tendons, & ligaments. Look like onion slices - layers called lamellae arranged concentrically around a central (osteonic or haversian) canal. Canal = small blood vessels & nerves. Occupy lacunar b/w the lamellae.

Collagen

Gives bone flexibility and strength.

Endochondral Ossification

Indirect, replacing hyaline cartilage (most of the skeleton below the head) Produces most other bones, including the vertebrae, ribs, scapulae, pelvic bones, bones of the limb, & some parts of the skull. Method in which mesenchyme first transforms into a hyaline cartilage "model" in the approximate shape of the bone to come. Cartilage is then broken down and replaced by osseous tissue.

Long bone

Leverage & movement

Lamellae

Look like onion slices - layers

Primary ossification center

Near the middle of this cartilage, chondrocytes begin to inflate & die, while the thin walls b/w them calcify.

Lacunae

Pocket within matrix

Calcitriol

Release when Ca is low The most active form of vitamin D. Raises blood calcium by promoting calcium absorption from digested food, reducing urinary loss of calcium, & stimulating osteoclasts to release calcium from the bones to the blood. Also necessary for bone deposition because w/o its effects on the kidneys and small intestine, blood calcium levels are too low for normal deposition.

Step 4 of intramembranous ossification

Remodeling creates compact bone and the finalized form of the bone

Bone remodeling

Repairs microfractures Reshapes bones Releases minerals into blood

Tendons

Structure similarly to ligaments but attach muscle to bone.

Functions of the Skeletal System

Support, Protection, Movement, Blood Formation, & Electrolyte Balance