Skeletal Tissue

The skeleton includes various types of connective tissues, primarily cartilage and bone. During embryological life, the skeleton is primarily cartilage but this is replaced by bone with minor amounts of cartilage persisting in adult life.

 

I. Structure and Location of Cartilage

A. Basic structure

1. Primarily water

2. Non-vascular

3. No nervous tissue

4. Perichondrium—dense connective tissue surrounding cartilage

5. Cellular components

            a. Chondrocytes

                        i. Secrete extracellular matrix

B. Types of cartilage

1. Hyaline—most abundant

            a. Articular cartilage

            b. Costal cartilage

            c. Laryngeal cartilage

            d. Tracheal and bronchial cartilage

            e. Nasal cartilage

2. Elastic—more elastic fibers

            a. Ears and epiglottis

3. Fibrocartialge—compressible with tensile strength

            a. Alternating parallel rows of chondrocytes and collagen

            b. Sites of heavy pressure and stretch

                        i. Vertebral discs

                        ii. Knee

C. Growth

1. Appositional

            a. Cartilage forming cells embedded in perichondrium layer

2. Interstitial

            a. Chondrocytes within lacunae in center of cartilage

 

*Although cartilage can be calcified, calcified cartilage is not bone. Bone is a separate type of connective tissue.

 

II. Bone

A. Function

1. Support

2. Protection

3. Movement

4. Mineral storage

5. Hematopoiesis

 

III. Classification of Bone

A. Type

1. Compact—external

2. Spongy—internal

B. Shape

 

1. Long bone

            a. Longer than wide

            b. Shaft with two ends

            c. Mostly compact

            d. Bones of limbs

2. Short bone

            a. Cube-like

            b. Mostly spongy

            c. Sesamoid—bones embedded in tendon

                        i. Patella

3. Flat bone

            a. Spongy bone embedded within parallel layers of thin compact bone

4. Irregular bone

            a. Vertebrae and hip bones

            b. Complicated shapes

            c. Mostly spongy with a thin covering of compact bone

 

IV. Bone Structure

A. Structural levels

1. Gross anatomy

2. Microscopic anatomy

3. Chemical composition

B. Gross anatomy of long bones

 

1. Diaphysis: shaft; long axis

            a. Constructed of a collar of thick compact bone

            b. Central medullary cavity

                        i. Contains fat—yellow marrow

2. Epiphysis: bone ends

            a. Exterior is compact bone

            b. Interior is spongy bone

            c. Articular cartilage covers joint surface

                        i. Absorbs stress

            d. Epiphyseal line

                        i. Remnant of epiphyseal plate

ii. Region of hyaline cartilage that grows during development

3. Membranes

 

            a. Periosteum

                        i. Double layer

                        ii. Fibrous outer layer; dense irregular CT

iii. Osteogenic—bone forming cells (osteoblasts); destroying cells (osteoclasts)

iv. Vascular, includes NT and lymph

v. Sharpey’s fibers: connect periosteum to bone; system of collagen fibers penetrating bone; densest at muscle and tendon attachment points

            b. Endosteum: covers trabeculae of spongy bone in marrow cavites

                        i. Contain osteoblasts and osteoclasts

 

C. Gross anatomy of short bones

            a. Layer of spongy bone sandwiched between parallel layers of compact bone

            b. Periosteum covers compact bone

            c. Endosteum covers spongy bone

                        i. Spongy bone layer is referred to as diploe

            d. Marrow not confined to a cavity

D. Hematopoietic tissue: Red marrow

            a. Red marrow cavities

                        i. Spongy bone of long bones

                        ii. Diploe of short bones

            b. In adults, fat containing medullary cavity extends into epiphysis

                        i. Little red marrow

            c. RBC’s produced primarily in diploe

 

V. Microscopic Structure of Compact Bone

A. Structural unit: Haversian System; Osteon

 

1. Elongated cylinders parallel to bone long axis

            a. Concentric rings: lamella

                        i. Unidirectional collagen fibers along long axis

                        ii. Adjacent lamella have collagen in opposite directions

2. Central (Haversian) canal: core of osteon

            a. Blood vessels and NT

3. Perpendicular canals (perforating or Volkmanns)

            a. Connect periosteum to central and medullary cavities

                        i. Blood supply and NT innervation

4. Lacunae: cavities containing osteocytes

5. Canaliculi: connect lacunae to each other and central canal

 

VI. Microscopic Structure Spongy Bone

 

A. Trabeculae: needle-like (flat) pieces

B. Trabeculae appear less organized than structures of compact bone

            a. No osteon

            b. Organization is based on lines of stress

            c. Lamella and osteocytes are irregularly organized;

 

VII. Chemical Composition of Bone

A. Organic

1. Cells

            a. Osteocytes

            b. Osteoblasts

            c. Osteoclasts

2. Osteoid: organic part of matrix; made by osteoblasts

            a. Proteoglycans

            b. Glycoproteins

            c. Collagen fibers

B. Inorganic

1. Hydroxyapatites (mineral salts)

            a. Calcium crystals in and around extracellular matrix

                        i. Make bones hard

 

VIII. Bone Marking

A. Sites of muscle and ligament attachment

1. Tuberosity

2. Crest

3. Trochanter

4. Line

5. Tubercle

6. Epicondyle

7. Spine

B. Projections that contribute to joint formation

1. Head

2. Facet

3. Condyte

4. Ramus

C. Depressions and opening permitting blood vessels and nerves to enter bone

1. Meatus

2. Sinus

3. Fossa

4. Groove

5. Fissure

6. Foramen

 

IX. Formation of Bone

A. Intramembranous ossification

 

 

1. Skull, clavicles, flat bones

2. Process:

            a. Ossification center forms in fibrous connective tissue membrane

                        i. Mesenchymal cells differentiate into osteoblasts

            b. Bone matrix is secreted into membrane

                        i. Osteoblast secrete osteoid

            c. Woven bone and periosteum is formed

                        i. Network of trabeculae encloses local blood vessels

                        ii. Exterior mesenchyme differentiates into periosteum

            d. Trabeculae thicken and form bone collar

                        i. Replaced by lamellar bone

                        ii. Spongy bone persists to form red marrow

B. Endochondral ossification

 

1. Cartilage bone are used as a pattern for bone construction

            a. Primary ossification center at the center of the hyaline cartilage

2. Hyaline cartilage is broken down during ossification

3. Process prior to ossification

 

            a. Perichondrium becomes infiltrated by blood vessels

                        i. Becomes periosteum

                        ii. Underlying mesenchymal cells differentiate into osteoblasts

4. Ossification

 

            a. Bone collar forms around hyaline model

                        i. Osteoblasts in periosteum secrete osteoid against hyaline cartilage

            b. Cartilage in center of the diaphysis calcifies

                        i. Chondrocytes hypertrophy

                        ii. Signal matrix to calcify

                        iii. Shaft is impermeable and chondrocytes die

            c. Periosteal bud invades internal cavities

                        i. Bud brings blood vessels, NT, lymph tissue, osteoblasts and osteoclasts

                        ii. Osteoblasts secrete osteoid around remaining hylaline cartilage

            d. Medullary cavity forms

                        i. Proximal and distal growth of ossification center

                        ii. Osteoclasts break down spongy bone and open medullary cavity

iii. Hyaline cartilage model continues to elongate and is chased by forming ossifying shaft

            e. Epiphyses ossify

i. At birth, diaphysis surround spongy done, medullary cavity is widening, and epiphyses are cartilaginous

ii. Secondary ossification centers form in epiphyses prior to birth and follow same course as that described above for primary ossification center

C. Postnatal Bone Growth

1. Length of long bones

 

            a. Parallel events of endochondral ossification

                        i. Zone 1: Epiphyseal side; cartilage is added to top; bone lengthens

                        ii. Zone 2: Chondrocytes close to shaft; cartilage matrix ossifies

iii. Zone 4: Epiphyseal/diaphysis junction; cartilage spicules covered with bone matrix to form spongy bone

iv. Spongy bone is digested by osteoclasts allowing medullary cavity to grow

2. Bone remodeling: maintain proper proportion by selective resorption and appositional growth

3. Bone thickness: appositional growth

 

X. Bone Homeostasis

A. Remodeling

 

1. Balance of bone formation and resorption at perosteal and endosteal surfaces

a. Processes are balanced to maintain constant bone mass

b. Remodeling units

                        i. Packets of osteoblasts and osteoclasts

2. Differential

            a. Bones and parts of bones remodel at different rates

3. Control of remodeling

 

            a. Hormonal mechanism: not related to strength; associated with mineral balance

                        i. Parathyroid hormone (parathyroid gland)

                        ii. Calcitonin (thyroid gland)

                        iii. PTH released in response to low ionic calcium in blood

iv. Osteoclasts are activated to digest bone matrix and release calcium into blood

v. Calcitonin is released in response to high calcium in blood

vi. Calcium salts are deposited into bone

            b. Mechanical stress: bone respond; mechanism unknown

B. Repair of fractures

 

1. Phases of simple fracture repair

            a. Hematoma: clot of damaged vascular tissue

            b. Fibrocartilaginous callus formation: soft callus

                        i. New vascular tissue

                        ii. Osteoblasts migrate and begin reconstruction

                        iii. Fibroblasts bring gap with collagen

                        iv. Osteoblast begin to form spongy bone

            c. Bony callus formation: formation of woven bone

            d. Remodeling