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

• The skeletal system provides structural support for the entire body.

• Individual bones or groups of bones provide a framework for the attachment of soft tissues and organs.

Storage of minerals and lipids

• Calcium is the most abundant mineral in the human body.

• Calcium salts of bone are a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in the body fluids.

• Also acts as a mineral reserve, the bones of the skeleton store energy reserves as lipids in areas filled with yellow marrow

Yellow Bone Marrow

Blood Cell Production

• Red blood cell, white blood cells, and other blood components are produced in red marrow which fills the internal cavities of many bones.

Protection

• Many soft tissues and organs are surrounded by skeletal elements.

• The ribs protect the heart and lungs, skull encloses the brain, the vertebrae shields the spinal cord, and the pelvis cradles delicate digestive and reproductive organs.

Leverage

• Many bones function as levers that can change the magnitude and direction of the forces generated by the skeletal muscles.

• The movements produced range from dainty motion of the fingertips to changes in the position of the entire body.

Cellular structures of Bones

• The process of bone formation is osteogenesis. This is done with:– Osteoblast- immature bone cells; mononucleate bone-

forming cells, that also manufacture hormones, and eventually become entrapped in the bone matrix to become osteocytes.

– Osteocytes-the mature bone cell, they occupy are known as lacunae; functions include: formation of bone, matrix maintenance and calcium homeostasis. Also its been shown to act as mechano-sensory receptors — regulating the bone's response to stress and mechanical load.

Cellular structures of Bones

• Osteolysis is the breaking down of bones– Osteoclast- are the cells responsible for bone

resorption, they break down bone; the cells are large, and multinucleated they are located on bone surfaces equipped with phagocytic-like mechanisms that activate enzymes to breakdown bone

Bone Tissue• a specialized form of connective tissue and it is

the main element of the skeletal tissues • It is composed of cells and an extracellular

matrix in which fibers are embedded. • Bone tissue is unlike other connective tissues in

that the extracellular matrix becomes calcified.

Bone Shapes

• Every adult skeleton contains 206 major bones, which can be divided into six broad categories according to their individual shapes.

• Long bones• Flat bones• Sutural bones• Irregular bones• Short bones • Sesamoid bones

Long Bones

• Long bones are relatively long and slender.

• Long bones are located in the arm, forearm, thigh, leg, palms, soles, fingers and toes.

• The femur, the long bone of the thigh, is the largest and heaviest bone in the body.

Flat Bones• Flat bones have thin,

roughly parallel surfaces. Flat bones form the roof of the skull, the sternum, the ribs, and scapula.

• They provide protection for underlying soft tissue, and offer extensive surface area for the attachment of skeletal muscles.

Sutural Bones• Sutural bones or Wormian

bones, are small, flat, irregularly shaped bones between the flat bones of the skull.

• There are individual variations in the number, shape and position of the sutural bones. Their borders are like pieces of a jigsaw puzzle, and they range in size from a grain of sand to a quarter.

Irregular Bones• Irregular bones have

complex shapes with short, flat notched, or ridge surfaces.

• The spinal vertebrae, the bones of the pelvis, and several skull bones are irregular bones.

Short Bones

• Short bones are small and boxy.

• Examples of short bones include the carpal bones (wrists) and tarsal bones (tarsal)

Sesamoid Bones• Sesamoid bones are

generally small, flat, and shaped somewhat like a sesame seed.

• They develop inside tendons and are most commonly located near joints at the knees, the hands, and feet.

• Sesamoid bones may form in at least 26 locations.

Bone MarkingsSurface Features

• Surface features can yield an abundant amount of anatomical information.

• Anthropologists, criminologist, and pathologist can often determine the size, age, sex and general appearance of an individual on the basis of incomplete skeletal remains.

Surface Features

• Elevations and projections(general):

• Process: any projection or bump

• Ramus: an extension of a bone making an angle with the rest of the structure.

Surface Features• Processes formed where

tendons and ligaments attach.• Trochanter: large, rough

projection.• Tuberosity: a smaller, rough

projection.• Tubercle: a small, rough

projection• Crest: a prominent ridge• Line: a low ridge• Spine: a pointed process

Surface Features

• Processes formed for articulation with adjacent bones.

• Head: expanded articular end of an epiphysis, separated from the shaft by a neck.

• Neck: narrow connection between the epiphysis and diaphysis

Surface Features

• Condyle: a smooth, rounded articular process

• Trochlea: a smooth, grooved articular process shaped like a pulley.

• Facet: a small, flat articular surface

Surface Features

• Depressions• Fossa: a shallow

depression.• Sulcus: a narrow groove

Surface Features• Openings• Foreman: a rounded

passageway for blood vessels or nerves.

• Canal: a passageway through the substance of a bone.

• Fissure: an elongated cleft.• Sinus or antrum: a

chamber within a bone, normally filled with air

Bone Structure• Diaphysis: shaft of a long bone• Epiphysis: the head if a long bone• Metaphysis: the region of the long

bone between the epiphysis and diaphysis, corresponding to the location of the epiphyseal cartilage of the developing .

• Marrow cavity: or medullary cavity the space within in the bone that contains the marrow.

• Cortex: Spongy bone that consists of an open network of struts and plates with a thin covering of compact bone.

Bone HistologyImportant Vocabulary

• 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.

Important Vocabulary• 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.

Structure of Compact Bone• 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.

The Structure of Spongy Bone• 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• 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

• Endosteum: an incomplete layer that lines the marrow cavity.

• This is the layer which is active during bone growth, repair, and remodeling, covers

Bone Growth Formation

• 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: hyaline cartilage 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 epiphyseal plates.

• The articular 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.

Endochondral Ossification• 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.

Intramembranous Ossification• 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

The Blood and Nerve Supplies• 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.

Dynamic Nature of Bones• 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.

• Bone is continually remodeled, recycled, and replaced.

• 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.

Effects of Exercise on the bone• 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.

Hormonal and Nutritional Effects on the Bone

• 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.

Hormones Involved in the regulation of Bone Growth and Maintenance

• 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.

Skeleton as a Calcium Reserve

• 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.

Hormones and Calcium Balance• 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)

Factors That Decrease Blood Calcium

Factors that Increase Blood Calcium

• 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.

Fracture Repair

• 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.

Types of fractures

Aging and the Skeletal System• 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.