BIOL 2401  A & P                  Lecture Notes Skeletal System                     Dr. Weis           

Skeletal System

Function :  support and protection

       storage of calcium and phosphorus

       production of blood cells in the marrow

        movement

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Bone and cartilage are supportive connective tissue, that contain cells, fibers, ground substance.

(matrix = fibers + ground substance)

 

I. Cartilage

Cells ---> chondrocytes

ground substance ---> proteoglycans

fibers ---> determine the three types of cartilage

II. BONE

A. Gross

1. Structure

  Cells ---> osteocytes, osteoblast, osteoclasts

  BONE CELLS ::

osteoprogenitor cells : are stem cells
differentiate into osteoblasts
found on inner surface of periosteum

Osteoblast : associated with bone formation
synthesize organic components of matrix
(collagen & glycoproteins called osteoid)
 associated with calcification

Osteoclasts : multinucleated giant cells
found at bone surfaces along two linings (periosteum, endosteum)
cause resorption of bone by dissolving the matrix by enzymes to release minerals

 

Collagenous fibers

Inorganic salts (hydroxiapatites) provide hardness and rigidity consist of calcium carbonate (CaCO4)& calcium phosphate (CaPO4 )

                        Two types of bone matrices laid down by osteoblasts (named for gross appearance)

Both present in varying degrees in most bones

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  Histology :

I. Spongy bone

Spongy bone show slender irregular bars to form a meshwork, whose spaces are filled with bone marrow

Trabeculae will align to stress changes and form an irregular lamellae (layers)

Osteocytes interconnect by canalicui to provide nutrients by diffusion from marrow spaces

 

II. Compact Bone

Arranged in OSTEONS  (* = components of osteon)

* lamella (layers)

Compact bone have lamellae regularly arranged determined by blood vessels

* central canal : longitudinal channels (Haversian) for blood supply

* osteocytes in lacunae (small spaces)

* canaliculi        (channels that interconnect lacunae)

* perforating canals (Volkmann)

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A specialized connective tissue membrane lines internal and external bone surfaces.

1. Endosteum : lines the internal bone surfaces of compact and spongy bone
lines the canals of compact bone and contains osteoblasts and osteoclasts

2. Periosteum : double fold of C.T.

Surrounds the outer bone except at the articular surfaces where there is hyaline cartilage.

Contains blood vessels, lymphatics, nerves

Provides an insertion point for muscles

Extensions of periosteum will form ligaments

Anchored to bone by collagenous bundles known as Sharpy's fibers

    B. Classification of Bones 

1. Long bones.... longer than they are wide

primarily compact bone with spongy at the epiphysis

                                                 i.e. humerus, femur, metacarpals, metatarsals

 

2. Short bones... boxy, cubelike.   Spongy bone covered with a thin layer of compact bone

                                                 i.e.  wrist (carpals) & ankles (tarsals)

 

3. Flat bones ... 2 layers of compact bone with spongy in between creating thin and flat surfaces

i.e. skull, ribs, scapula

the flat bones offer a large surface area for skeletal muscle attachment

 

4. Irregular...complicated shapes.  Spongy bone covered with a thin layer of compact bone

                                                 i.e. spinal vertebrae

 

5. Round.... circular/round

i.e. sesmoids, patella

 

Most Bones will have spongy bone on the inside with compact bone on the outside. 

i.e. (Short, flat, irregular, & round)

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Long Bone anatomy :

a. Shaft or diaphysis...compact bone with medullary cavity

b. ends or epiphysis... spongy bone primarily with a layer of compact bone covering

c. epiphyseal line (plate)...growth area, cartilage present in young bone

d. periosteum...double membrane -->

e. endosteum... C.T. membrane lining inner bone surfaces
and trabeculae of spongy bone.  Contains osteoblasts and osteoclasts

f. articular cartilage... hyaline cartilage remains along epiphyseal surface where long bone articulate

Prominent features used to identify bone and create skeletal terminology

Skeletal terms --->

elevations..........processes

projections for attachments........trochanter, tuberosity, crest, spine, tubercle

articulations.......condyle, trochlea, head

depressions.........fossa

openings............foramen, fissure, meatus

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III. OSSIFICATION :

A. Osteogenesis

B. Bone Growth

A. Osteogenesis :  Bone development --> Two types

1. Intramembranous

Defined as : bone development on or within a membrane
Starts as a connective tissue sheet, primarily collagen fibers and mesenchymal cells that line up and differentiate into osteogenic cells (blasts)

osteoblasts deposits collagenous matrix around themselves, spread out and
create a bone plate that extend and fuse together to form a network.  Once the osteoblasts are trapped, they become osteocytes
The periosteum will then forms, and in this region, the trabeculae thicken and eventually are replaced to form compact (lamellar) bone
Intramembranous examples include --> flat bone of the skulls, mandible, clavicle

 

2. Endochondrial

replacement of cartilage by bone
starts with a band shaped area surrounding the center of the diaphysis and deposits a ring of bone.

At the center of the diaphysis the cartilage cells that grow larger, secrete calcium in the matrix  and shut themselves off from nutrition.
These cells die, leaving large spaces in the cartilage matrix

Developing blood vessels called  periosteal buds will penetrate the boney ring and grow into these large spaces and open up cavities. 
These formed cavities will be the primary marrow spaces. The periosteal buds (containing artery, vein, nerves and lymphatics) will carry osteoblasts and osteoclasts that will line up on the surface of the cartilage matrix and continue the process as seen in INTRAMEMBRANOUS growth. 
This deposition of bone in the center of the diaphysis constitutes a primary ossification center.

This process extends toward both ends of cartilage, and the same type process almost occurs in the epiphyseal region and is known as the second center of ossification. 
In this region, spongy bone is retained (not replaced by compact bone) and no medullary cavity forms.

Cartilage will be replaced by bone except

 

B. BONE GROWTH

occurs    Longitudinally (length) @ epiphyseal plates

  Appositionally (width) at endosteal/periosteal surfaces

The process called LONGITUDINAL BONE GROWTH causes an increase in length.

When growth ceases, the plate will be replaced by bone.  The plate line can still be seen in adult bones.

            To increase the bone diameter, APPOSITIONAL GROWTH occurs by deposition of new periosteal bone that forms intramembranously.

Examples of bone growth are seen in the long bones.

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IV. Histophysiology :: The importance of vitamins and hormones

A. VITAMIN D

            1. Deficiency of vitamin D results in a faulty resorption of calcium and therefore a decrease in phosphate.

The cartilage matrix fails to ossify and the epiphyseal plate becomes thick and irregular.

in children the deficiency is called Rickets in adults it is known as OSTEOMALACIA (bone softening due to the decrease in calcium content)

 

B. VITAMIN C

deficiency is called scurvy and causes a decrease production of bone matrix and destruction of osteocallagenous fibers making bones weak and brittle.

 

C. VITAMIN A

a deficiency causes osteoblasts to not synthesize the bone matrix normally, and causes a decrease in the rate of growth and can interfere with remodeling.

 

D. VITAMIN B12       (Cobalamin)

coenzyme in the synthesis of nucleic acids (and therefore proteins, some for bone)

E. HORMONES

1. Growth hormone (GH)

2. Thyroid Hormones (T3, T4)

works with GH and promotes osteoblast activity

 

3. Parathormone ---> Parathyroid glands

4. Calcitonin ---> from C-cells of thyroid gland

5. Sex hormones ---> (estrogen, testosterone)

production cause the closure of the secondary centers of ossification.

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V. Remodeling, Reconstruction, and Repair

The bone architecture is NOT static, areas are locally destroyed and reformed.

Remodeling results from resorption in certain areas and deposition of new bone elsewhere.

 Resorption is associated with osteoclasts causing enzymatic lysis known as OSTEOLYSIS.

Fracture :

Classification based on

body surface exposure (open or closed)

patterns created by different forces ( see next page)

Repair after a fracture ::

VII. Aging and Bone diseases

A. Aging due to inadequate ossification --> thin, weak, called osteopenia

B. Diseases :

 

Fracture Patterns :