Functions of the Skeletal System

1. Support• The bones of the legs, pelvic girdle, and vertebral

column support the weight of the erect body.• The mandible (jawbone) supports the teeth.• Other bones support various organs and tissues.

2. Protection • The bones of the skull protect the brain.• Ribs and sternum (breastbone) protect the lungs

and heart.• Vertebrae protect the spinal cord.

3. Movement• Skeletal muscles use the bones as levers to move

the body.4. Reservoir for minerals and adipose tissue

• 99% of the body’s calcium is stored in bone.• 85% of the body’s phosphorous is stored in bone.• Adipose tissue is found in the marrow of certain

bones5. Hematopoiesis

• A.k.a. blood cell formation.• All blood cells are made in the marrow of certain

bones.

Bone Classification

1. Long Bones • Much longer than they are wide.• All bones of the limbs (including fingers and

toes) except for the patella (kneecap), and the bones of the wrist and ankle.

• Consists of a shaft plus 2 expanded ends.

2. Short Bones• Roughly cube shaped.• Bones of the wrist and the ankle.

3. Flat Bones• Thin, flattened, and usually a bit

curved.• Scapulae (shoulder blades), sternum,

ribs and most bones of the skull.4. Irregular Bones

• Have weird shapes that fit none of the 3 previous classes.

• Vertebrae, hip bones, 2 skull bones - ( sphenoid and the ethmoid bones).

Bones are composed of multiple tissue types

•Bone tissue (a.k.a. osseous tissue)

•Fibrous and loose connective tissue

•Cartilage

•Vascular tissue

•Lymphatic tissue

•Adipose tissue

•Nervous tissue

•Epithelium

•Myeloid tissue

Bone Cells1. Osteoblasts

• Bone-building cells• Synthesize and secrete collagen fibers and other organic components of

bone matrix (osteoid) framework for deposition• Initiate the process of calcification• Found in both the periosteum and the endosteum• Formed by osteogenic stem cells that line Haversian Canals and go

through mitosis in the inner layer of the periosteum

2. Osteocytes

• Mature bone cells

• Osteoblasts that have become trapped by the secretion of matrix

• No longer secrete matrix

• Responsible for maintaining the bone tissue

3. Osteoclasts

• Huge multi-nucleated cells derived from the fusion of as many as 50 WBC

• Cells that digest bone matrix – this process is called bone reabsorption and is part of normal bone growth, development, maintenance, and repair

• Concentrated in the endosteum

• Secrete digestive enzymes and lots of mitochondria and lysosomes

Compact BoneCompact Bone

-dense, solid outer layer of all bones

Bone is an extremely dynamic tissue!!!!

– Cancellous Bone (spongy) • flat, honeycomb inner layer with

needle-like projections (trabeculae)

The composition of spongy bone (cells and matrix) is the same as that of compact bone. In spongy bone, however, the lamellae of collagen are not arranged concentrically around a central canal, but run parallel to one another. Osteocytes sit in lacunae between lamellae.

Compact Bone

• Consists of multiple cylindrical structural units known as osteons or haversian systems.

• Each osteon consists of a single central canal, known as a haversian canal, surrounded by concentric layers of calcified bone matrix.

– Haversian canals allow the passage of blood vessels, lymphatic vessels, and nerve fibers.

– Each of the concentric matrix “tubes” that surrounds a haversian canal is known as a lamella.

– All the collagen fibers in a particular lamella run in a single direction, while collagen fibers in adjacent lamellae will run in the opposite direction. This allows bone to better withstand twisting forces.

Anatomy of Compact Bone

Haversion canal

Volkmann’s canal

• Spider-shaped osteocytes occupy small cavities known as lacunae at the junctions of the lamellae. Hairlike canals called canaliculi connect the lacunae to each other and to the central canal.

• Canaliculi allow the osteocytes to exchange nutrients, wastes, and chemical signals to each other via intercellular connections known as gap junctions.

Anatomy of Compact Bone cont.

Anatomy of a Long Bones

Anatomy of Short, Flat, & Irregular

Bones

• Thin plates of – periosteum-covered

compact bone on the outside

– endosteum-covered spongy bone within.

• no diaphysis or epiphysis

• Contain bone marrow between their trabeculae, but no marrow cavity.

Bone Marrow (myeloid tissue)• soft connective tissue in the medullary cavity of a long bone, the spaces of spongy bone, and

the larger haversian canals.• There are 2 main types: red & yellow.• Red bone marrow = blood cell forming tissue = hematopoietic tissue

• Red bone marrow looks like blood but with a thicker consistency.

• Children - the medullary cavity of nearly every bone is filled with red bone marrow.

• Young to Middle-aged adults - the shafts of the long bones are filled with fatty yellow bone marrow.

– Yellow marrow no longer produces blood– in the event of severe or chronic anemia, it can transform back into red marrow

• Adults - red marrow is limited primarily to the axial skeleton, plus the pectoral girdle, pelvic girdle, and proximal heads of the humerus and the femur.

Bone Marrow Transplant

•bone marrow in the breast bone, skull, hips, ribs and spine contains stem cells that produce the body's blood cells

•patients with leukemia, anemia, and some immune deficiency diseases•the stem cells in the bone marrow malfunction, producing an excessive number of defective or immature blood cells or low blood cell counts

•The immature or defective blood cells interfere with the production of normal blood cells, accumulate in the bloodstream and may invade other tissues.

•Large doses of chemotherapy and/or radiation are required to destroy the abnormal stem cells and abnormal blood cells. Also, destroys healthy bone cells and stem cells

•A bone marrow transplant enables physicians to treat these diseases with aggressive chemotherapy and/or radiation by allowing replacement of the diseased or damaged bone marrow after the chemotherapy/radiation treatment.

•While bone marrow transplants do not provide 100 percent assurance that the disease will not recur, a transplant can increase the likelihood of a cure or at least prolong the period of disease-free survival for many patients.

Bone Matrix• Consists of organic and inorganic components• 1/3 organic and 2/3 inorganic by weight.

• Organic component consists of several materials that are secreted by the osteoblasts:

– Collagen fibers and other organic materials

» These (particularly the collagen) provide the bone with resilience and the ability to resist stretching and twisting.

• Inorganic component of bone matrix– Consists mainly of 2 salts: calcium phosphate and calcium hydroxide.

These 2 salts interact to form a compound called hydroxyapatite.– Bone also contains smaller amounts of magnesium, fluoride, and sodium.– These minerals give bone its characteristic hardness and the ability to resist

compression.

This bone has been demineralized.

Bone Matrix

• Bones store 98% of bodies calcium reserves and therefore responsible for maintaining blood-calcium levels.

• Calcium is important in the body for blood clotting, transmission of nerve impulses, and skeletal and cardiac muscle contraction.

Osteogenesis

• A hormone released by the parathyroid (PTH) stimulates the osteoclasts to breakdown bone when blood calcium levels are LOW.

• The hormone calcitonin from the thyroid senses HIGH calcium levels in the blood and stimulates the osteoblasts to start bone deposition.

• Before week 8, the human embryonic skeleton is made of fibrous membranes and hyaline cartilage.

• After week 8, bone tissue begins to replace the fibrous membranes and hyaline cartilage.– The development of bone from

a fibrous membrane is called intramembranous ossification (skull, jaw, collarbone)

– The replacement of hyaline cartilage with bone is known as endochondral ossification.

Formation of the Bony SkeletonOsteogenesis/Ossification

Intramembranous Ossification

1. Mesenchymal Cells within fibrous (collagen) connective tissue membrane (skull, jaw, collarbone) form osteoblasts (centers of ossification)

2. Osteoblasts (golgi apparatus) secrete matrix (complex carbs and collagen fibers) called mucopolysaccharides or “ground substance”

3. Ground substance solidifies as calcium salts are deposited within it.

4. Trabeculae (needle-like bony spicules that contain osteoblasts) appear (spongy bone) and are covered by compact bone.

5. Flat bones continue to grow as additional osseous layers are added to outside.

Intramembranous Ossification

Intramembranous Ossification

Endochondral Ossification• Hyaline cartilage is broken down to become bone• Most bones in the body develop via this model.

Endochondral Ossification

1. An epithelial membrane with blood vessels and osteoblasts (periosteum) forms around the center of the cartilage.

2. A collar of bone is formed by the periosteum from osteoblasts formed within it and cartilage begins to calcify as blood vessels enter at the midpoint of the developing diaphysis and bring in calcium for calcification.

3. This area of initial ossification is called the primary ossification center.

4. Bone calcifies in length toward both epiphyses and “secondary ossification” occurs as spongy bone is calcified in the center of each epiphysis

Growth in Length

•At puberty, bone length is increased dramatically:1. growth hormone2. thyroid hormone3. sex hormones

• Osteoblasts begin producing bone faster than the rate of epiphyseal cartilage expansion 1. Bone grows while the epiphyseal plate gets narrower ultimately disappears 2. A remnant (epiphyseal line) is visible on X-rays

Growth in Bone Thickness

• Osteoblasts beneath the periosteum secrete bone matrix on the external surface of the bone. This obviously makes the bone thicker.

• At the same time, osteoclasts on the endosteum break down bone and thus widen the medullary cavity.

• This results in an increase in shaft diameter even though the actual amount of bone in the shaft is relatively unchanged.

Fractures

Epiphyseal Fracture

Fracture Fracture Hematoma

Bony CallusFormation

Bone Remodeling Complete

Fracture Repair

Bone Healing Process

1. Fracture hematoma (blood clot) forms and osteoclasts “clean up” (reactive stage of bone healing triggered by periosteal damage, blood vessel damage, and bone cell death)

Reparative Stage:1. The periosteum cells form chondroblasts

(which form new hyaline cartilage), fibroblasts (which form new colagen fibers) and osteoblasts, all combining with the blood clot to produce a tissue called a fracture callus.

2. The fracture callus is eventually replaced with bone through endochondral ossification