Bone Connective Tissue

BIOL 2304

Dr. Aglaia Chandler

Bone

• Two types: Compact and Spongy

• Compact bone consists of osteons (Haversian systems) osteocytes in lacunae and lamellae.

• Spongy bone is characterized by trabeculae filled with red bone marrow

trabeculae

Compact Bone

Compact Bone Terms

Osteon/Haversian System: structural unit of compact bone. Oriented parallel to shaft and forming a group of hollow tubes through which an artery, vein and nerve pass into and through bone.

Lacunae: small cavities (halo’s) containing osteocyte

Osteocyte: true bone cell, spider shaped and found in lacunae at the junctions of the lamellae

Lamellae: layers of the collagen fiber matrix with each layer going in opposite direction to the adjacent layer.

Compact Bone TermsLamellae: may be concentric (forming rings like a tree),

circumferential (encircling the entire bone structure), or interstitial (interspersed in matrix of lamellae).

Canaliculi: Hair like canals that connect each lacunae

and in turn connect to the Central canal. Remove wastes

and bring nutrients into osteocytes

Volkman’s canal/perforating canal: Canals running

perpendicular to the Haversian canals, but connecting to

them. They bring in the artery, vein and nerves to the

bone structure.

Compact Bone Structure

Compact Bone Structure

Spongy Bone

• composes the inner portion of the bone lining the marrow cavity with a honeycomb (laced) appearance of trabeculaeand spicules. Although it looks poorly organized it is designed to withstand the specific stresses put on each bone because of their trabeculae.

• Trabeculae are tiny bone struts or plates that form very strong support structure for the spongy bones. They are irregularly arranged lamellae and osteocytes, but contain no osteons per se as they receive nutrients from the marrow tissue.

Spongy Bone Histology

Spongy Bone Histology

Spongy bone is the tissue that makes up the interior of bones; compact bone is

the tissue that forms the surface of bones. In long bones, spongy bone forms

the interior of the epiphyses; the diaphysis (shaft) consists of compact bone

surrounding the central marrow cavity.

Flat bone structure

Bone Structure

• Articular cartilage: Consists of Hyaline cartilage covering the end of the bone surface where it articulates with another bone,

(e.g. femur and tibia, humerusand scapula). Fibrocartilagemakes up the menisci of the knee joints.

• Epiphyses: The end of the bone. One at each end of long bones.

• Epiphyseal line: Remnant of the cartilaginous “growth plate” or epiphyseal plate.

Bone structurePeriosteum: Tough outer

connective tissue covering on bone. Consists of 2 layers; outside is dense irregular CT and deeper layer lined with osteoblast and osteoclast cells.It is richly supplied by blood vessels and nerves and secured to bone by Sharpey’s fibers.

Endosteum: connective tissue covering on inside of bone cavities. Is osteogenic in that it contains osteoblasts and osteoclasts.

Diaphysis: The shaft of the bone between the two epiphyses. Contains the medullarycavity and is filled with yellow marrow in adults.

Sharpey’s fibers: Bundles of collagenous fibers that tightly attach the periosteum to bony matrix.

Bone structure

Bone Blood Supply• Bone is highly vascular and well nourished by blood.

Bone marrow

• Yellow marrow is found in medullary cavity of long bones and is not hematopoietic in adults. Yellow marrow replaces red marrow as we mature and is made up mainly of fat.

• Red marrow is found in the axial skeleton and girdles and in the epiphyses of the femur and humerus and is very active hematopoietically.

Bone Tissue

• Two types: a) Compact b) Spongy

Bone Cells

1.. Osteoclasts: Bone destroying cells (C = chewing)

2. Osteoblasts: Bone generating cells (B=building)

3. Osteocytes: Mature bone cells, spider shaped; maintain bone tissue

Bone Cells

Bone Cells

Chemical Composition of Bone

• In addition to bone cells, the majority of compact and spongy bone is composed of inorganic molecules (Calcium and phosphate).

• These molecules are called hydroxyapatite and are made of calcium and phosphate --naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH).

• The combination of these form a cement like material that gives bone its hardness and strength. In combination with collagen fibers that form the matrix of bones and allows for elasticity and flexibility.

Bone Matrix

• The matrix of bone is made up of organic and inorganic matter.

• The organic portion is of collagen fibers and various proteoglycans, glycosaminoglycans and glycoproteins.

• The inorganic portion is calcium phosphate salts “hydroxyapatite” and calcium carbonate

• The combination of these makes for a bone that is very strong and yet flexible.

Bone Growth and Development

Bone Formation

• Bone is derived from hyaline cartilage by either interstitial ossification or appositional growth.

• Interstitial ossification is growth within cartilage.

• Appositional growth is growth from the periphery or outside edge.

Bone Development

• Ossification or osteogenesis

- is the process of forming new bone

• Two methods of ossification:

1. Endochondral ossification

2. Intramembranous ossification

Endochondral vs. Intramembranous Ossification

• Endochondral ossification (cartilage model replaced by bone) is involved in: embryonic bone formation, growth in length of long bones, and fracture healing

• Intramembranous ossification – forms FLAT BONES (skull, sternum, mandible, and clavicle). Multipotent mesenchymal cells differentiate directly into osteoblasts and bone is formed. Growth and modeling occur by bone formation on the convex surface and bone resorption on the concave surface.

• Appositional growth: adding layers outside, like rings on a tree. Interstitial growth: growth from within.

– The formation of LONG BONES takes place via Endochondral ossification as well as Intramembranous ossification. Both appositional (bone width) and Interstitial(bone length) growth take place in each

Endochondral vs. Intramembranous Ossification

• Bone growth begins long before

birth. The basic shape of a long bone,

such as an arm bone is first formed as

CARTILAGE.

• MANY BONES IN A NEWBORN BABY

ARE COMPOSED ALMOST ENTIRELY

OF CARTILAGE. "SOFT SPOT" OF

BABY’S HEAD!

• Later the cartilage cells will be replaced

by cells that form the bones. THE

CARTILAGE IS REPLACED DURING

OSSIFICATION, OR THE PROCESS OF

BONE FORMATION.

• Ossification begins to take place up to

seven months before birth as MINERAL

(CALCIUM AND PHOSPHORUS)

DEPOSITS ARE LAID DOWN NEAR THE

CENTER OF THE BONE (CENTER OF

OSSIFICATION) IN EACH BONE.

Endochondral Ossification

• The process by which bone is formed from hyaline cartilage

• Most bones in the body are formed by this method (including the vertebrae, pelvic bones and limb bones).

• Consists of 3 sites of ossification.

1. The primary ossification center

2. The metaphysis

3. The secondary ossification center

Endochondral Ossification

The formation of long bones begins (~8wks gestation) with the proliferation and aggregation of mesenchymal cells at the site of future bone. These cells differentiate into chondroblasts and produce cartilage. The cartilage (mainly type II and proteoglycans) assumes the shape of the future bone. The area surrounding the cartilage in the diaphysis (the perichondrium) starts to give rise to bone forming cells (osteoblasts) and this layer is now known as the periosteum. As a result a thin layer of bone is formed around the cartilage model. The chondrocytes within the center of the model become hypertrophic and begin to synthesize alkaline phosphatase. Subsequently the matrix surrounding undergoes calcification.

Endochondral Ossification

The calcified matrix inhibits the diffusion of nutrients causing the death to the chondrocytes. This leaves large holes where blood vessels can grow into once they have pierced the thin layer of bone on the diaphysis. Cells from the periosteum enter with the invading blood vessels and become osteoprogenitor cells in the cavity left by the dying chondrocytes. When the invading osteoblasts come in contact with chondrocytes in the cavity they begin to lay down osteoid (bone that lacks minerals) on the remaining calcified cartilagenous spicules. Additional bone is formed via appositional growth.

Endochondral Ossification

Initially the perichondrium becomes the periosteum and starts laying down bone on the diaphysis. Then after vessel invasion, the growth plate forms at the primary ossification center. Later when vessels invade the proximal end of the bone, a secondary center is formed.The secondary ossification center expands until it fills the epiphysis except for the articular cartilage on the joint surface and the physis itself.

The physis (growth plate), with the articular cartilage, is the site of endochondral ossification. It is covered peripherally with perichondrium. It grows transversely by appositional growth, longitudinally by interstitial growth. It shrivels ~14yrs due to estrogen

The Secondary Ossification Center

• Begins at the time of birth.

• Forms in the epiphysis and develops similarly to the primary ossification center.

• The bone formed in the secondary ossification site persists as spongy bone and growth occurs beneath the outer covering of hyaline cartilage which persists as articular cartilage within the joint cavity on each end of the epiphysis.

Bone Growth

• Once the cartilage models of embryonic development are replaced by bone, they must continue to grow through infancy, childhood and adolescence.

– Increased length: bones continue to lengthen because hyaline cartilage remaining in the epiphysealplates continues to grow. As adulthood approaches, this cartilage becomes less active and is eventually replaced by bone.

Bone Growth

Appositional growth - Increased Width: bones continue to widen as osteoblasts form more layers of bone around the outside and osteoclasts break down some of the bony matrix inside.

• Why would bone need to be broken down inside as it grows outside?

Appositional Growth

• adding layers outside, like rings on a tree

Appositional Growth

Control of Bone Growth

• Bones increase in length and width because of the influence of minerals, vitamins, and hormones in the body.

• Calcium and phosphate are necessary for calcification.

• Vitamins A, C, and D promote bone growth.

• The specific hormones which affect growth are: growth hormone (GH), thyroid hormone (T3 and T4), and the sex steroids (estrogen and testosterone).

Hormones & Vitamins

Bone Growth: Increase in Length

• BONE TISSUE FORMS AS OSTEOCYTES SECRET MINERAL DEPOSITS

THAT REPLACE THE CARTILAGE. OR A BONE MATRIX GRADUALLY

REPLACES THE ORIGINAL CARTILAGE.

• The long bones develop and grow THROUGH OUT CHILDHOOD at centers

of ossification in their EPIPHYSIS (ENDS).

• Growth occurs in the EPIPHYSEAL DISC or PLATE (GROWTH PLATES)

AT THE JUNCTION OF THE DIAPHYSIS WITH EACH EPIPHYSIS (AT

EACH END OF THE BONE)

• AN EPIPHYSEAL DISC IS STILL CARTILAGE, AND THE BONE GROWS

IN LENGTH AS MORE CARTILAGE IS PRODUCED ON THE EPIPHYSIS

SIDE.

• ON THE DIAPHYSIS SIDE, OSTEOBLAST (CELLS THAT PRODUCE

BONE MATRIX, (A BLAST CELL IS A "PRODUCING" CELL, AND "OSTEO"

MEANS BONE) TO REPLACE CARTILAGE.

• Between the ages of 16 and 25 years, ALL of the cartilage of the

EPIPHYSEAL DISC IS REPLACED BY BONE. THIS IS CALLED CLOSURE

OF THE EPIPHYSEAL DISC, AND THE BONE LENGTHENING PROCESS

STOPS (Epiphyseal Line).

• In adults, cartilage is found in those parts of the body where FLEXIBILITY IS

NEEDED.

• SUCH PLACES INCLUDE THE TIP OF THE NOSE, THE EXTERNAL EAR,

THE VOICE BOX (LARYNX), AND THE ENDS OF BONES WHERE JOINTS

ARE FORMED. CARTILAGE IS ALSO FOUND WHERE THE RIBS ARE

ATTACHED TO THE BREASTBONE (STERNUM), THUS ALLOWING THE

RIB CAGE TO MOVE DURING BREATHING.

• CARTILAGE PROVIDES AN IMPORTANT COMBINATION OF STRENGTH

AND FLEXIBILITY.

Epiphyseal Plate Growth Zones

• Consists of five distinct zones:

1. Zone of resting cartilage

2. Zone of cartilage proliferation

3. Zone of cell hypertrophic cartilage

4. Zone of calcified cartilage

5. Zone of ossification

Epiphyseal Plate Growth Zones

Intramembranous Ossification

• Bone formed by replacing a fibrous membrane and not from cartilage. Ex. Skull and clavicle

Basic Overview of Process– during the first 8 wks of embryonic development, fibrous

membranes (CT) form in the areas of future flat bones

– beginning around 8 wks, an “ossification center” forms in the membrane. This center is composed of osteoblasts.

– the osteoblasts begin to secrete hydroxyapatite

– the internal spongy bone forms

– the external compact bone forms

Intramembranous Ossification

Intramembranous Ossification

Bone Remodeling

• Even though the bones in an adult do not continue to grow as described above, they are constantlybeing remodeled. This means that bone is always being broken down by osteoclasts and reformed by osteoblasts .

• Each week we turn over about 5% of our bone mass.

Bone Remodeling

• A. Wolfe's Law - shape of bone is altered by mechanical forces

• B. Bone Remodelling

• 1. Osteoclasts - reabsorb = turned on by bone disuse have ruffled border &

hydrolytic enzmes, therefore, lots of lysosomes2. Osteoblasts - secrete new bone with disuse of bone3. Activity of blasts and clasts regulated by hormonal control:

• a. parathyroid hormone - increases calcium absorption (increase blood

concentrations of calcium--maximizes tubular reabsorption of calcium within the kidney )

b. calcitonin - decreases calcium absorption (reduces blood calcium levels)

Osteoporosis

• imbalance between bone resorption and bone formation

In normal bone up to 10% of all bone mass

may be undergoing remodeling at any point

in time.

Bone is resorbed by osteoclasts after which

new bone is deposited by osteoblast cells

Osteoporosis

• Trabecular bone is more active, more subject

to bone turnover, to remodeling.

• Common osteoporotic fracture sites, the

wrist, the hip and the spine, have a

relatively high trabecular bone to cortical

bone ratio.

Bone Markings and Landmarks

Bone Markings and Landmarks