Bone Tissue Chapter 6

Bone and Calcium Homeostasis:1. Parathyroid Hormone

causes calcium release from bone, increasing blood calcium levels

four layers of cartilage cells at the epiphyseal plate: • Zone of Proliferating Cartilage

cells undergoing active mitosis

Exercise:

Exercise promotes bone formation by causing mechanical stress. This leads to healthy, strong bones. Lack of stress leads to demineralization and collagen reduction, ultimately weakening the bone.

Matrix Composition: Inorganic Components

Hydorxyapatite (calcium phosphate salt), calcium carbonate, magnesium, sodium, potassium, fluoride, sulfate, carbonate, and hydroxide.

Short Bone

equal in length and width, mostly spongy bone with a surface layer of compact. Found in ankles and wrist.

Bone Cells-2. Osteoblast

formed from osteogenic cells, these are the bone formers. They are responsible for producing the boney matrix (hence they are the fibroblast of bone tissue). Once an osteoblast has surrounded itself in boney matrix it matures into an osteocyte

Two types of bone formation:2. Endocondral

from a cartilage model-------1. Mesenchyme tissue is first replaced with hyaline cartilage. This cartilage takes on the general shape of the intended bone. The cartilage model grows rapidly for a while. Periosteum forms from the surrounding connective tissue. 2. Eventually a dense supply of blood vessels invades the diaphysis, bringing in osteogenic cells. This becomes the primary ossification center and eventually the cartilage here is broken down and osteoblast activity replaces it with spongy bone. 3. Secondary ossification centers then arise in the epiphyses. The cartilage here is broken down and replaced with spongy bone. 4. As in intramembranous bone formation, spongy bone is always laid first and then remodeling results in compact bone filling in. Thus when you look at the diaphysis it is primarily compact bone whereas, the epiphysis is covered in a thin layer of compact bone but is mainly spongy bone. 5. At the end of this process the only remaining cartilage is found in an area located between the epiphyses and the diaphysis called the epiphyseal plate. At this located lengthwise growth of the bone will continue until the end of puberty at which point it will completely ossify.

Two types of bone formation:1. Intramembranous-

from a membranous sheet of connective tissue----bone formation begins within an embryonic connective tissue called mesenchyme, which condenses and becomes enriched with a dense blood supply. The cells here then enlarge and become osteogenic (committed to becoming bone cells). Parts of the membrane begin taking on trabecular shape. Then osteoblasts develop and move into these areas to lay down osteoid tissue (similar to bone but lacking minerals). Eventually minerals get deposited and permanent spongy bone forms. Continued remodeling and reforming of bone by the osteoclasts and osteoblasts results in replacement of surface areas of spongy bone with compact.

four layers of cartilage cells at the epiphyseal plate: • Zone of hypertrophic Cartilage

the old cells left behind after new cells form, osteoblasts invade here and start depositing calcified salts

four layers of cartilage cells at the epiphyseal plate: • Zone of Calcified Cartilage

thin layer of dead cartilage cells which become calcified • Bone- Osteoclasts then invade, degrading the calcified matrix and allowing osteoblasts to come in and deposit bone; eventually the epiphyseal plate ossifies and growth ceases.

Periosteum

tough outer membrane covering the surface of bone excluding areas with articular cartilage

Flat Bone

typically two plates of compact bone encasing a layer of spongy bone. Provide protection and large surfaces for muscle attachment. Examples include cranial bones, sternum, ribs, and shoulder blade.

Endosteum

inner membrane that lines the inside of the medullary cavity, rich in stem cells

Articular Cartilage-

layer of hyaline cartilage covering the epiphyses, cushions the joints

Yellow Marrow

red marrow that has become fatty in young to middle aged adults. Serves as an energy reserve. Found in the medullary cavity of the long bones

Metaphysis

region where diaphysis joins epiphyses

Open Reduction-

setting a bone during surgery

Closed Reduction

setting a bone without performing surgery

Mineral Storage and Release

- Ex. calcium salts - When blood calcium is low osteoclasts break down bone matrix to release calcium - When blood calcium levels are high bone growth is stimulated as Calcium is deposited in the bone matrix

Irregular Bone

- complex shape, vary in the amount of compact vs. spongy bone. Examples include vertebrae and facial bones

• Vitamin C

- important in collagen synthesis. W/o bones become thin and fragile

Diaphysis

- shaft of long bone, made of mostly compact bone

2. Formation of Granulation Tissue

- soft fibrous tissue produced at site of injury as cells start invading

Osteon Parts: Central Canal-

Also known as the harversian canals, these channels contain blood vessels and nerves. Thus providing a direct blood supply to this type of bone tissue. • Central canals are interconnected via transverse canals called perforating canals or Volkmann's Canals. These canals allow blood flow and communication between neighboring osteons, the periosteum, and medullary cavity containing the endosteum and the central canals.

Types of Bone Formation:

Bone is constantly being broken down and reformed. This is one of the reasons that broken bones can heal within a matter of weeks.

Bone Homeostasis:

Bone is constantly changing. Even in grown adults old worn and damage bone is continually being broken down and replaced with new bone. Osteoclasts are in charge of bone resorption (break down) and osteoblasts are in charge of laying new bone. In order for this to happen your body needs to keep an ongoing supply of minerals, vitamins and certain hormones. Ex. Calcium is one of the minerals necessary; however, without the presence of vitamin D humans are unable to absorb calcium in the digestive tract. A great example of how prevalent bone remodeling is can be seen in the rate of turnover at the distal end of a femur. This area of bone is completely replaced every four months.

Bone and Calcium Homeostasis

Bone is the location of our major storage reserve of calcium. In this way it can act as a buffering system for blood calcium levels. When calcium levels are two high it can be deposited in bone. When calcium levels are too low osteoclast activity can release calcium by breaking down bone. Two hormones involved in this process:

Histology of Bone Tissue:

Keeping in mind that bone is a connective tissue there are several cell types that comprise it. The matrix which supports these cells is rich in fibers such as collagen and inorganic compounds such as hydroxyapatite, a type of calcium based salt, which helps give the tissue its' strength and form.

Interstitial growth

Length-wise growth, occurs at the epiphyseal plate

Structure of Compact Bone

Mature bone cells (osteocytes) are located in areas of boney matrix which make concentric patterns around a central canal where blood vessels are located. As bone is laid in concentric rings around the central canal the osteon forms. As osteons abut neighboring osteons compact bone is formed.

Structure of Spongy Bone

Osteocytes lie in boney plates called trabeculae. The osteocytes here get their nutrients by diffusion into this area from nearby blood supplies. These plates of bone (trabeculae) are separated by large voids or pores thus giving the bone a spongy appearance.

Bone Marrow:

The soft tissue which occupies the medullary cavity, a centrally located hollow area of long bones (like your femur) and the trabecular spaces of spongy bone

Bone Growth at the Epiphyseal Plate:

This thin line of cartilage remaining in the epiphysis of the long bones provides a place where the bone can continue to grow in length. Growth is most active here during the maturation process (up through the end of puberty). After puberty, in the adult the epiphyseal plate ossifies typically becoming non-existent. A close look at the plate reveals several layers of cartilage cells (resting cells, young dividing cells, older enlarging cells and dying cells which are being replaced with boney tissue) which contribute to the growth process.

appositional growth.

Width-wise growth; This is performed throughout life as osteoblasts and osteoclasts remodel the bone. Osteoclasts do bone resorption (breakdown) and osteoblasts do bone deposition.

four layers of cartilage cells at the epiphyseal plate: •

Zone of Resting Cartilage- Zone of Proliferating Cartilage Zone of hypertrophic Cartilage Zone of Calcified Cartilage

• Vitamin D

allows absorption of calcium. W/o this bones weaken and deform (Rickets or Osteomalacia)

Fracture-

any break in a bone

Medullary (marrow) Cavity

area of long bone, located in the center of the diaphysis, in which yellow marrow is found, or spaces of spongy bone, in which red marrow is found

Hematopoiesis-

blood cell formation

1. Fracture Hematoma

blood clotting at the site of injured blood vessels contained within the bone.

Bone Cells-4. Osteoclast

bone degrader; responsible for bone resorption/breakdown

four layers of cartilage cells at the epiphyseal plate: • Zone of Resting Cartilage

closest to the epiphysis, these cells are resting and have a role only on anchoring the epiphyseal plate to the bone of the epiphyses

Matrix Composition: Organic Components

collagen and protein-carbohydrate complexes

3. Callus Formation

collagen is deposited as well as cartilage and bone to splint and repair the injury. Takes four to six weeks.

Osteon Parts-Lamella

concentric rings of boney matrix which form the osteon. Osteoblasts produce the bone the forms these rings. Once osteoblasts become isolated (surrounded by the bone they have produced) they turn into osteocytes. • Osteocytes reside within small spaces termed lacunae. • In order to communicate with one another osteocytes send their arms out through thin channels called canaliculi. As they make contact with each other, neighboring osteocytes are able to communicate with each other via gap junctions. In this manner nutrients and wastes are passed along

Epiphyses

ends of bone, mainly spongy bone with a thin covering of compact

Bone Tissue-Spongy

functions in blood cell formation (location of red marrow) and some support Structural Unit- trabeculae; interconnected plates of boney tissue. Spongy bone has large spaces in between neighboring trabeculae where the red marrow is found.

Bone Tissue-Compact

functions in support and protection. Structural Unit of Compact Bone- osteon Osteons are circular arrangements of boney tissue (comparable to the growth rings of a tree). There is little to no space between neighboring osteons

Long Bone

greater in length then width; slightly curved for added strength, consist of a diaphysis and two ends termed epiphyses. Made mostly of compact bone with spongy bone being found in the epiphyses. Found in the legs, arms, fingers and toes

4. Remodeling

hard callus persist for 3-4 months during which spongy bone is being laid to bridge the gap caused by the break. Next, spongy bone will be replaced by compact.

Growth Hormone-

helps trigger growth at the epiphyseal plate Male and Female sex hormones, Testosterone and Estrogen, promote bone formation and ossification of epiphyseal plate

Red Marrow

hemopoietic tissue; makes blood cells. Red marrow is found in the trabecular spaces of spongey bone.

Red marrow-

makes RBC, WBC and platelets

Bone Cells-3. Osteocyte

mature bone cell; resides in little spaces (air pockets in the boney matrix) called lacunae. Lacunae are interconnected by thin channels termed canaliculi. .

• Vitamin A

needed for normal osteoblast/osteoclast activity. W/o bone development is retarded

Bone Cells-1. Osteoprogenitor Cells

stem cells which have committed to the bone lineage. Found in the endosteum, inner surface of the periosteum and blood vessel canals in the bone. These cells will give rise to bone forming cells called osteoblasts

Bone and Calcium Homeostasis:2. Calcitonin-

stimulates calcium deposition into bone, decreasing blood calcium levels

• Growth Hormone

stimulates division of cartilage cells. W/o you get pituitary dwarfism. Too much will result in pituitary gigantism

Yellow marrow-

stores fat

Aging Bone:

• As we age calcium is lost from the bone resulting in weakening of the bone or a condition termed osteoporosis. Additionally, other parts of the boney matrix are not being produced in the same quantity which can lead to further weakening possibly resulting in fracture of the bone.

Roles of Bones

• Give structure, support and protect • Aid in body movement by acting as lever systems