Bone Structure and Skeletal System. Chapters 6, 7, and 8. Function of the Skeletal System. The skeletal system has five functions. Support Storage of minerals and lipids Blood cell production Protection Leverage. Support.
6, 7, and 8
The matrix of bone is very dense and contains deposits of calcium salts.
Osteocytes: A bone cell responsible for the maintenance and turnover of the mineral content of the surrounding bone. They account for most of the cell population
Lacuna: small pit or cavity
Canaliculi: Microscopic passage way between cells: in bone canaliculi permit the diffusion of nutrients and waste to and from the osteocytes.
Periosteum : the layer surrounding the bone, consisting of an outer fibrous region and an inner cellular region.
Lamellae: concentric layers; the concentric layers of bone within an osteon.
Osteoblast: a cell that produces the fibers and matrix of the bone. In a process called osteogenesis.
Osteoid: organic matrix of the bone before calcium salt are added.
Osteoclast: a cell that dissolves the fiber and matrix of the bone.
Osteogenic layer: the inner, cellular, layer of periosteum that participates in bone growth and repair.
Osteolysis: the breakdown of the mineral matrix of the bone.
Osteon: the basic histological unit of compact bone, consisting of osteocytes organized around a central canal and separated by concentric lamellae.
The basic functional unit of mature compact bone is the osteon, or Haversian System. In an osteon, the osteocytes are arranged in concentric layers around a central canal, or Haversian canal.This canal contains one or more blood vessels that carry blood to and from the osteon. Central canals generally run parallel to the surface of the bone.
Other passageways, known as perforating canals or canal of Volkmann, extend roughly perpendicular to the surface. Blood vessels in these canals supply to osteons deeper in the bone to tissue of the marrow cavity.
In spongy bone the lamellae are not arranged in osteons. The matrix in spongy bone forms struts and plates called trabeculae.
The thin trabeculae branch, creating an open network. There are blood vessels in the matrix of the spongy bone. Nutrients reach the osteocytes by diffusion along the canaliculi that open onto the surface of trabeculae. Red marrow is found between the trabeculae of spongy bone, and blood vessels within the tissue delivers nutrients to the trabeculae and remove waste generated by the osteocytes
Spongy bone is located where bones are not heavily stress or where stresses arrive from many directions.
Spongy bone is much lighter than compact bone.
Finally, the framework of trabeculae supports and protects the cells of the bone marrow.
Red marrow is responsible for blood cell formation and yellow bone marrow- adipose tissue is important for energy reserves.
Periosteum: The layer that surrounds a bone, consisting of a an outer fibrous region and inner cellular region.
1.periosteum isolates the bone from surrounding tissue.
2. provides a route for circulatory and nervous supply.
3. actively participates in bone growth and repair.
Near the joints the periosteum becomes continuous with the connective tissues that lock the bones together.
The fibers of the periosteum are interwoven with those of tendons attached to the bone.
Endosteum: an incomplete layer that lines the marrow cavity.
This is the layer which is active during bone growth, repair, and remodeling, covers
The boney Skelton begins to form about 6 weeks after fertilization, when the embryo is approximately 12 mm long. At this stage the existing skeleton elements are nothing more than cartilaginous.
The process of replacing tissues with bone is called ossification. This term refers specially to the formation of bone.
The process of calcification - the deposition of calcium salts – occurs during ossification.
Most simply the process of bone formation or ossification involves two major phases.
1st step: hyalinecartilage model is completely covered with bone matrix (bone collar) by bone forming cells called osteoblasts.
By birth or shortly after the hyaline cartilage models have been converted to bone except for two regions: articular cartilages(that cover the bone ends) and the epiphysealplates.
Thearticular cartilages persist for life, reducing friction at the joint surfaces.
The epiphyseal plates provide for longitudinal growth of the long bones during childhood.
During development, most bones originate as hyaline cartilage that are miniature models of the corresponding bones of the adult skeleton.
Cartilage is replaced by bone
Examples: most bones of the body
Involves a 6-step process:
1. A cartilage model forms
2. Growth occurs by interstitial & appositional mechanisms
3. Primary ossification centers develop
4. A medullary cavity develops
5 Secondary ossification centers develop at epiphyses
6. Hyaline cartilage is replaced by articular cartilage at the
ends, and between the diaphysis and the epiphyses by the epiphyseal (bony) plate.
Simpler process than endochondral ossification.
Examples: flat bones of the skull
Involves a 4-step process:
1. An ossification center develops
2. Calcification occurs due to mineral deposition
3. Trabeculae are formed in the interior
4. Mesenchyme is replaced with periosteum and
a thin layer of compact bone
Osseous tissue is highly vascular, and the bones of the skeleton have an extensive blood supply. In a typical bone such as the humerus, three major sets of blood vessels develop.
1. Nutrient Artery and Vein: the blood vessels that supply the diaphysis form by invading the cartilage model as endochondral ossification begins. The vessels enter the bone through one or more passageways called nutrient foramina in the diaphysis
Metaphseal Vessels: supply blood to the inner (diaphyseal) surface of each epiphyseal cartilage is being replaced by bone.
Periosteal Vessels: Blood vessels from the periosteum provide blood to the superficial osteons of the shaft. During endochondral formation, branches of periosteal vessels enter the epiphyses, providing blood to the secondary ossification centers.
The organic and mineral components of the bone matrix are continuously being recycled and renewal through the process of remolding.
Bone remolding goes on throughout life, as part of normal bone maintenance.
Remolding can replace the matrix but leave but leave the bone as a whole unchanged, or it may shape, internal architecture, or mineral content of the bone.
The rate of turnover varies from bone to bone and from moment to moment.
When deposition exceeds removal, bone gets stronger; when removal exceeds deposition, bones get weaker.
Turnover and recycling of minerals give each bone the ability to adapt to new stresses. The sensitivity of osteoblasts to electrical events theorized as the mechanism that controls the internal organization and structure of bone.
When bone is stressed, the mineral crystals generate minute electrical fields. Osteblasts are apparently attracted to these electrical fields and, once in the area, begin to produce bone.
This finding has led to the successful use of small electrical fields in stimulating the repair of severe fractures.
When you don’t use you lose. The stresses applied to the bones during physical activity are essential to maintaining bone strength and bone mass.
Normal growth and development can not occur without a constant dietary source of calcium and phosphate salts. Lesser amounts of minerals such as, magnesium, fluoride, iron, manganese, are also required.
Calcitriol: primary source: kidneys. It promotes calcium and phosphate ion absorption along the digestive tract.
Growth Hormone: Pituitary gland: Stimulates osteoblast activity and the synthesis of bone matrix.
Thyroxine: thyroid gland (follicle cells) With growth hormone, stimulates osteoblast activity and the synthesis of bone matrix.
Sex Hormones: Ovaries(estrogen), Testis (androgens),stimulates osteblast activity and synthesis of bone matrix.
Parathyroid Hormone (PTH): parathyroid gland Stimulates osteoclasts [and osteoblasts] activity, elevates calcium ion concentration in body fluids.
Calcitonin: Thyroid gland (C cell):Inhibits osteoclasts activity: promotes calcium loss at kidneys; reduces calcium ion concentration in body fluids.
Calcium is the most abundant mineral in the human body.
A typical human body contains 1 – 2 Kg (2.2 – 4.4 lb) of calcium, with roughly 99 percent of it deposited in the skeleton.
Calcium ions play a role in a variety if physiological processes, so the body must tightly control calcium ion concentrations in order to prevent damage to essential physiological systems.
Even small variation from normal affect cellular operations: larger can cause a clinical crisis.
Calcium ions are important in both the membrane and the intracellular activities of neurons and muscle cells, especially cardiac muscle cells.
If concentration of calcium increase by 30%, neurons and muscle cells become relatively unresponsive.
If levels decrease by 35%, neurons become so excitable that convulsions can occur.
A 50% reduction in calcium concentration generally causes death.
A fluctuation in calcium levels is closely monitor and variations are less than 10% daily.
Calcium homeostasis is maintained by a pair of hormones with opposing effects.
These hormones, parathyroid hormone and calcitonin, coordinate the storage, absorption and excretion of calcium ions.
Three target sites are involved: bones (storage), the digestive tract (absorption), and the kidneys (excretion)
Calcium Ions concentration in the blood falls below normal, cells of the parathyroid glands, releases parathyroid hormone (PTH) into the bloodstream. Parathyroid hormone has three major effects.
1. Stimulating osteoclast activity and enhancing the recycling of minerals by osteocytes.
2. Increasing the rate of intestinal absorption of calcium ions by enhancing the action of calcitriol.
Under normal circumstances, calcitriol is always present, and parathyroid hormone controls its effect on the intestinal epithelium.
3. Decreasing the rate of excretion of calcium ions at the kidneys.
Under these conditions more calcium ions enter the body fluids and losses are restricted. The calcium concentration increases to normal levels and homeostasis is restored.
If the calcium ion concentration of the instead rises above normal, special cells (parafollicular cells or C cells) in the thyroid gland secrete calcitonin. This hormone has two major functions, which together act to decrease calcium concentrations in body fluids.
1. Inhibiting osteoclast activity
2. Increasing the rate of excretion of calcium ions at the kidneys
Under these conditions, less calcium enters the body fluids because osteoclasts leave the mineral matrix alone.
More calcium leaves body fluids because osteoblasts continue to produce new bone matrix while calcium ion excretion at the kidneys accelerates.
The net result is a decline in the calcium ion concentration of body fluids, restoring homeostasis.
By providing a calcium reserve, the skeleton plays a primary role in the homeostatic maintenance of calcium ion concentration in body fluids.
This can have a direct affect on the strength and shape of the bones in the skeleton.
When large numbers of calcium ions are on the move in the body fluids the weaker the bones and when calcium salts are deposited, the bones become denser and stronger.
Despite its strength, bones can crack or even break if subjected to extreme loads, sudden impact, or stresses from unusual directions. Damage is called a “fracture”.
Most fractures heal even after severe damage as long as the following are present:
The blood supply is present
The cellular components of the endosteum and periosteum survive the injury.
Skeletal bones become thinner and weaker as a result of the aging process.
Osteopenia is inadequte ossification and all of us become slightly osteopenic as we age.
The reduction bone mass begins between 30 and 40. It is at this time that a decrease in osteoblastic activity begins and osteoclastic activity continues.
Once reduction begins, women lose roughly 8% of their skeletal mass every decade , whereas, men will lose 3% of their skeletal mass every decade.
Not all parts of the skeleton are equally effected. Epiphyses, vertebrae, and jaw lose more mass than other sites.
Results: fragile limbs.,reduction in height and loss of teeth.
When reduction in bone mass is sufficient to compromise normal function, the condition is known as osteoporosis.
Sex hormones are important in maintaining normal rates of bone deposition.
Over the age of 45, estimated 29 % of women and 18 % of men have osteoporosis.
Osteoporosis can also result as a secondary effect of cancer. These cancers release a chemical called osteoclast –activating factor (OAF) which can produce severe osteoporosis.