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Chapter 7 Bone Tissue

1 Tissues and organs of the skeletal system

1 Skeletal system

1 Bones

2 Cartilage

The precursor of most bones in embryonic and childhood development. Covers many joint surfaces in mature bones.

3 Ligaments

Hold bones together in joints.

4 Tendons

Structurally similar to ligaments but attach muscle to bone.

2 Functions of the skeletal system

1 Support

2 Protection

For instance, enclosing and protecting the brain.

3 Movement

4 Electrolyte balance

Storage of calcium and phosphate ions and release of them when required.

5 Acid-base balance

Alkaline salts are absorbed and released to maintain pH of blood.

6 Blood formation

Red bone marrow is the major producer of blood cells.

3 Bones and osseous tissue

Bone is connective tissue in which the matrix (the extracellular material) is mineralized with calcium phosphate and other minerals and becomes hardened.

Other tissues in bone include blood, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue.

Bone can refer to simple, calcified tissue or to the entire complex organ with the other tissue types.

1 Osseus tissue

Another term for bone.

2 Mineralization or calcification

The term for the hardening of the bone matrix with calcium phosphate etc.

4 The shapes of bones

1 Long bones

Longer than wider. E.g. humerus

2 Short bones

About the same length as width. E.g. carpal

3 Flat bones

Enclose and protect soft organs and provide broad surfaces for muscle attachment. E.g. cranial bones.

4 Irregular bones

Everything else. Include vertebrae.

5 General features of bones

The description below is for a typical long bone. Most of these features apply to all bones, in varying degrees.

1 Compact bone

For a long bone, the cylinder of dense material that makes up most of its length is composed of compact bone.

2 Medullary or marrow cavity

The compact bone surrounds the marrow cavity

1 Bone marrow

The contents of the marrow cavity. Red and yellow marrow will be described later.

3 Spongy bone

At the ends of the bone, the central space is composed of a loosely organized osseous tissue called spongy bone. Found in the ends of long bones and the middle of most other bones. Spongy bone is always covered by protective compact bone.

4 Diaphysis

The shaft of a long bone.

5 Epiphysis

The head of a long bone.

6 Articular cartilage

The joint ends of bones are covered with a layer of hyaline cartilage.

7 Nutrient foramina

Blood vessels penetrate through the bone through holes called nutrient foramina.

8 Periosteum

The external covering of bones. The periosteum does not cover the articulating surfaces.

1 Fibrous layer

Tough outer layer. The collagen fibers can be continuous with the tendon.

2 Osteogenic layer

Inner layer of bone-forming cells.

9 Endosteum

The internal surface of a bone is covered by endosteum, which also contains bone-forming cells.

10 Epiphyseal plate

In children and adolescents, a plate of hyaline cartilage separates the marrow spaces of the diaphysis and the epiphysis. This is the zone of elongation in bone growth. The plate becomes ossified in adults and is still evident in X-ray images as an epiphyseal line.

2 Histology of osseous tissue

1 Bone cells

1 Osteogenic cells

Stem cells that develop from fibroblasts and give rise to the other bone cell types. The osteogenic cells are found in the periosteum, endosteum and in the central canals described later.

Osteogenic cells can undergo mitosis - can divide and produce more osteogenic cells.

Osteogenic cells go on to produce osteoblasts.

2 Osteoblasts

Osteoblasts are bone-forming cells. They synthesize the soft organic components of the bone matrix, which then hardens by mineral deposition.

3 Osteocytes

If osteoblasts become trapped in mineralized bone matrix, they become osteocytes.

Osteocytes can deposit and resorb bone matrix, thus play an important role in bone and mineral homeostasis.

Osteocytes also sense bone strain, which results in stimulation of formation of new bone tissue.

1 Lacunae

Osteocytes live in small cavities within the bone matrix called lacunae.

2 Canaliculi

These are tiny channels that connect the lacunae to one another. Projections of the osteocytes reach into the canaliculi and connect with neighboring osteocytes. They can pass nutrients, chemical signals, and wastes this way.

4 Osteoclasts

These are bone-dissolving cells found on the bone surface. They have a similar cell lineage to macrophages and other white blood cells.

2 The matrix

The combination of protein fibers in the mineral substances result in great strength and flexibility.

1 Organic matter

About one third of the dry weight of bone, the organic matter is synthesized by the osteoblasts.

Includes collagen and other protein-carbohydrate complexes.

2 Inorganic matter

About two-thirds of the dry weight of bone. Calcium phosphate salts and other minerals.

3 Compact bone

1 Osteon

The osteon is the basic structural unit of compact bone and is composed of two main parts.

1 Concentric lamellae

These are the circular-shaped, concentric layers that are formed by the osteocytes, which inhabit the lacunae in the lamellae.

2 Central canal

There is a central space in each osteon, which is the passageway for blood vessels and nerves.

4 Spongy bone

1 Trabeculae

The rods, plates and spines that make up spongy bone. Spongy bone is filled with spaces, which in turn are filled with bone marrow.

5 Bone marrow

1 Red bone marrow

Produces blood cells. Found in the skull, vertebrae, ribs, sternum and pelvis.

2 Yellow bone marrow

Primarily fat, but can turn into red bone marrow and produce blood cells. Found in the long bones of the limbs.

3 Bone development

1 Intramembranous ossification

This type of bone formation develop within a fibrous sheet similar to the dermis of the skin.

1 Type of bones

Flat bones of the skull and most of the clavicle.

2 Stages of intramembranous ossification

1 Condensation of mesenchyme into soft sheet permeated with blood vessels

2 Deposition of osteoid tissue by osteoblasts on mesenchyme surface; entrapment of first osteocytes; formation of periosteum

3 Bony trabeculae formed by continued mineral deposition; creation of spongy bone

4 Surface bone filled in by bone deposition, converting spongy bone to compact bone. Spongy bone remains in middle layer.

2 Endochondral ossification

This process of bone development starts with a preexisting model of hyaline cartilage.

1 Type of bones

Most bones of the body are produced by endochondral ossification.

2 Stages of endochondral ossification

1 Mesenchyme develops into hyaline cartilage structure in location of future bone.

2 Formation of primary ossification center, bony collar, and periosteum.

3 Vascular invasion, formation of primary marrow cavity, and appearance of secondary ossification center.

4 Bone at birth, with enlarged primary marrow cavity and appearance of secondary marrow cavity in one epiphysis.

5 Bone of child, with epiphyseal plate at distal end.

6 Adult bone, with a single marrow cavity and closed epiphyseal plate.

3 Bone growth

Ossification begins in embryonic development and continues throughout life with bone growth and remodeling. There are two general process: bone elongation and bone thickening.

1 Bone elongation

Bone elongation proceeds through activity of the epiphyseal plates. Remember that the epiphyseal plates are present from infancy through adolescence at one or both ends fo a long bone, at the junction of the diaphysis and epiphysis.

The epiphyseal plate is a region of transition between cartilage and bone and functions as the zone of bone growth as it elongates and corresponds with an individual's increase in height.

1 Metaphysis

The transitional zone between hyaline cartilage and bone that faces the marrow cavity is called the metaphysis.

2 Epiphyseal line

Once growth has stopped, the epiphyseal plate becomes filled with spongy bone, which is evident on X-rays as a denser line, and is referred to as the epiphyseal line.

2 Bone widening and thickening

1 Appositional growth

The deposition of new tissue at the bone's surface, which results in widening of the bone, is called appositional growth.

Appositional growth is similar to the process of intramembranous ossification: osteoblasts in the inner layer of the periosteum ossify the surface of the bone and create layers of calcified tissue with osteocytes trapped within it.

4 Bone remodeling

Bones are constantly remodeling through the absorption of old bone and deposition of new. About 10% of bone tissue is replaced per year.

Through this process, minor fractures are repaired, minerals are released into the blood (for use in other metabolic processes), and the bone is reshaped in response to use and disuse.

The architecture of the bone is determined by the mechanical stresses placed upon it and the bone adapts to those stresses (Wolff's Law of bone...no, you don't need to know the name of the law).

Bone remodeling is a collaboration between the activities of osteocytes and osteoblasts (lay down minerals and organic components; also some dissolving of bone) and also osteoclasts (dissolve bone).

A heavily used bone will induce the activity of osteoblasts, which thicken the bone and deposit new osseus tissue.

4 Physiology of osseous tissue

Bones remain metabolically active throughout life, not only in the growth, maintenance and remodeling of the bone itself, but also through the exchange of minerals with other body fluids.

The nervous and muscular systems are especially dependent upon the appropriate metabolisms of minerals and this is described below.

1 Mineral deposition

Mineral deposition is the crystallization of calcium, phosphate and other minerals, which are taken as ions out of the blood and formed into osseous tissue.

1 Osteoblasts

Osteoblasts begin the mineral deposition process by laying down organic compounds - collagen fibers - in helical patterns along the length of an osteon. The fibers become encrusted with minerals.

Osteoblasts are able to precipitate calcium and phosphate ions in bone: there are inhibitors of this activity in other tissues to prevent the precipitation of minerals in tissues that would be damaged by the formation of solids (most other tissues). The formation of osseus tissue outside of bones is considered ectopic ossification.

The crystallized minerals act as a "nucleus" for other minerals to be precipitated and so a little crystallization induces more crystallization.

2 Mineral resorption

Mineral resorption is the process of dissolving bone. Minerals are released into the blood and are available for other processes.

1 Osteoclasts

Osteoclasts are responsible for mineral resorption. They dissolve bone by producing hydrochloric acid and also by secreted the enzyme acid phosphatase, which digests the collagen of bone matrix.

3 Calcium and phosphate homeostasis

1 Calcitriol

Calcitriol is a form of vitamin D that results from the activities of UV light on the skin, the liver, and kidney.

The principal function of calcitriol is to increase the blood calcium concentration. It does this by:

1 Increasing calcium absorption by small intestine

2 Increases calcium resorption from the bones

Induces the production and activity of osteoclasts.

3 Promotes the reabsorption of calcium by the kidneys

Less calcium is lost in the urine.

2 Calcitonin

Produced by the "C" cells of the thyroid gland. It is secreted when the blood calcium level is too high. It lowers blood calcium by:

1 Inhibiting osteoclast activity

2 Stimulates osteoblast activity

3 Parathyroid hormone

Secreted by the parathyroid gland, PTH is released when blood calcium is low. PTH increases blood calcium by:

1 Stimulates osteoclast production and activity

2 Promotes calcium reabsorption by the kidney

3 Promotes the final step of calcitriol synthesis

4 Inhibits collagen synthesis by osteoblasts

4 Other factors affecting bone

For your information

5 Bone disorders

For your information.

1 Fractures and their repair

1 The healing of fractures

2 The treatment of fractures

2 Other bone disorders

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