general osteology

General osteology

DR. JAMIL ANWAR

Plan of the lecture1. General concepts about skeleton

2. Bone as an organ

3. Functions of the skeleton

4. Classification of bones

5. Types of bone ossification

6. Development of bones

The Skeletal System! Our First System

Cells (Osteocytes)

Tissues (Osseous Tissue)

Organs (Bones)

Systems (Skeletal)

THE LOCOMOTOR APPARATUS – ITS COMPONENTS AND FUNCTIONAL ROLE

The skeleton is a complex of hard structures that is of mesenchymal origin and possesses a mechanical significance. The term skeleton comes from a Greek word meaning “dried up”.

NB: All the bones and articulations of the body make up the passive part of the locomotor apparatus.

The skeleton

The science concerned with the study of bones is termed osteology.

The skeletal system of an adult is composed of approximately 206 bones. Each bone is an organ of the skeletal system.

For the convenience of study, the skeleton is divided into axial and appendicular parts.

The axial skeleton The axial skeleton

consists of 80 bones that form the axis of the body and which supports and protects the organs of the head, neck, and trunk.

Skull Auditory ossicles Hyoid bone Vertebral column Thoracic cage

The appendicular skeleton

The appendicular skeleton is composed of 126 bones of the upper and lower limbs and the bony girdles, which anchor the appendages to the axial skeleton.

The shoulder girdle (the scapula and clavicle)

The upper limb (the humerus, ulna, radius and bones of the hand)

The pelvic girdle (the hip bone) The lower limb (the femur, tibia,

fibula and bones of the foot)

BONE AS AN ORGANSTRUCTURE OF A BONE AND STRUCTURE OF THE PERIOSTEUM

Bone (osis) is one of the hardest structures of the body. It possesses also a certain degree of toughness and elasticity. Its color, in a fresh state, is pinkish-white externally, and red within.

Types of bone tissueThere are two types of bone tissue:

a) compact bony tissue

b) spongy bony tissue

The names imply that the two types differ in density, or how tightly the tissue is packed together.

There are three types of cells that contribute to bone homeostasis.

a) osteoblasts are bone-forming cell

b) osteoclasts resorb or break down the bone

c) osteocytes are mature bone cells.

An equilibrium between osteoblasts and osteoclasts maintains the bone tissue.

OSSIFICATION

OSTEOBLASTSFORM NEW BONE

MATRIX

COLLAGENSTERNGTH

CALCIUM DEPOSITION

OSTEOCYTES

MAINTAIN BONE

OSTEOCLASTS

MAINTAIN SHAPE

Structure of bone On examining a cross section of

any bone, it is composed of two kinds of bony tissue:

Compact tissue, it is dense in texture and it is always placed on the exterior of the bone.

Cancellous tissue consists of slender fibers and lamellae, which join to form a reticular structure and it is placed in the interior of the bone

Macromicroscopic structure of bone

The morphofunctional unit of the bone is the osteon, or Haversian system.

The osteon consists of a system of bony lamellae arranged concentrically around a canal, which is called Haversian canal and this canal contains nerves and vessels. The bone lamellae consist of osteocytes, their lacunae, and interconnecting canaliculi and matrix.

The spongy bone tissue Spongy (cancellous) bone is

lighter and less dense than compact bone. Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow. The canaliculi connect to the adjacent cavities, instead of a central haversian canal, to receive their blood supply.

The spongy bone tissue It may appear that the

trabeculae are arranged in a haphazard manner, but they are organized to provide maximum strength similar to braces that are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.

The periosteum

Externally bone is covered by

periosteum (except articular

surfaces). The periosteum

adheres to the surface of the

bones.

It consists of two layers closely

united together:

a) The outer layer fibrous layer

b) The inner layer or bone-forming layer (cambial)

Structure of the periosteum The periosteum is rich in

vessels and nerves, and it contributes to the nutrition and growth of the bone in thickness. Nutrients are conveyed by blood vessels penetrating in great number the outer (cortical) layer of the bone from the periosteum through numerous vascular openings (foramina nutricia).

The interior of each long tubular bone of the limbs presents a cylindrical cavity named marrow cavity and it is lined with the medullary membrane called endosteum.

Chemical Composition of Bone

Functions of the skeletal system

1. Support - framework for the body 2. Protection - skull, vertebrae, ribcage 3. Leverage - bones are levers, joints are

fulcrums 4. Mineral storage – (calcium) 5. Lipid Storage – (yellow marrow) 6. Blood cell formation - hematopoiesis

Functions of the skeleton

Biological functions

Mechanical functions

Biological functions of the skeleton

a) Haemopoiesis

b) Mineral storage.

Bone marrow

The bony compartments contain bony marrow, medulla ossium. Two types of bone marrow can be distinguished:

red bone marrow white bone marrow The white or yellow marrow fills up the medullary

cavities of the shafts of the long tubular bones. The red marrow is located within the cancellous

tissue and extends into the larger bony canals (Haversian canals) that contain blood vessels.

Haemopoiesis function

The bone marrow provides haemopoiesis function and biological protection of the organism. It takes part in nutrition, development and growth of the bone. The red marrow concerned with haemopoiesis and bone formation, has an active role in the healing of fractures. Red marrow predominates in infants and in children, with growth of child the red marrow is gradually replaced by yellow marrow.

NB: The bones of the embryo and new-born contain only red marrow.

Haemopoiesis function

The red bone marrow of an adult produces white blood cells, red blood cells, and platelets.

In an infant, the spleen and liver produce red blood cells, but as the bones mature, the bone marrow performs this task.

It is estimated that an average of 1 million blood cells are produced every second by the bone marrow to replace those that are worn out and destroyed by the liver.

Mineral storage

The inorganic matrix of bone is composed primarily of minerals calcium and phosphorus. These minerals give bone rigidity and account for approximately two-thirds of the weight of bone.

About 95% of the calcium and 90% of the phosphorus, within the body, are stored in the bones and teeth.

In addition to calcium and phosphorus, lesser amounts of magnesium and sodium salts are stored in bones.

Mechanical functions of the skeleton

a) Support

b) Protection

c) Body movement

Support (weight bearing)

The skeleton forms a rigid framework to which are attached the soft tissues and organs of the body.

Protection function

Protection is assured by the property of the bones to form body cavities which protects the vital important organs.

The skull and vertebral column enclose the central nervous system.

The thoracic cage protects the heart, lungs, great vessels, liver and spleen.

The pelvic cavity supports and protects pelvic organs.

Even the site where blood cells are produced is protected within the central portion of certain bones.

Body movement Bones serve as anchoring

attachments for most skeletal muscles. In this capacity, the bones act as levers, with the joints functioning as pivots, when muscles, which are regulated by the nervous system, contract to cause the movement.

Classification of bones by shapeTubular bones

a) Long tubular bones humerus, radius, ulna, femur, tibia, fibula

b) Short tubular bones metacarpal, metatarsal bones and phalanges

Classification of bonesSpongy bones

a) Long spongy bones sternum, ribs, etc

b) Short spongy bones carpal and tarsal bones

c) Sesamoid bones knee-cap pisiform bone, etc.

Classification of bonesFlat bones

Skull bones Bones of the vault of the

skull

Girdle bones The scapula The hip bone, etc.

Classification of bonesMixed bones

The vertebrae are mixed, or irregular bones (their bodies are referred to spongy bones, but their arches and processes are referred to flat bones).

Bone formation (osteogenesis)

Osteogenesis occurs throughout life but in different ways 1. embryo responsible for laying down of bony

skeleton (ossification well started by 8th week) 2. bone growth continues until early adulthood 3. remodeling & repair continues for life• Ossification - The process of replacing other

tissues with bone (endochondral and intramembranous)

Calcification - The process of depositing calcium salts Occurs during bone ossification and in other

tissues

2 types of ossification

1. Intramembranous (dermal ossification) Formation of most of the flat bones of the skull and the

clavicles from a fibrous membrane Fibrous connective tissue membranes are formed by

mesenchymal cells

2. Endochondral Formation of bone in hyaline cartilage

Both lead to the same type of bone Both begin with migration of mesenchymal cells from c.t. to

areas of bone formation No blood supply chondroblasts Blood supplyosteoblasts

Intramembranous Ossification: Step 1

Mesenchymal cells aggregate: differentiate into

osteoblasts begin ossification

at the ossification center

develop projections called spicules


Blood vessels grow into the area: to supply the

osteoblasts Spicules connect:

trapping blood vessels inside bone


Spongy bone develops and is remodeled into: osteons of compact bone periosteum or marrow cavities

Endochondral Ossification

Begins in the second month of development

Uses hyaline cartilage “bones” as models for bone construction

Requires breakdown of hyaline cartilage prior to ossification

Endochondral Ossification: Step 1

Chondrocytes in the center of hyaline cartilage: enlarge form struts and calcify die, leaving cavities in

cartilage


Blood vessels grow around the edges of the cartilage

Cells in the perichondrium change to osteoblasts: producing a layer of

superficial bone around the shaft which will continue to grow and become compact bone .


Blood vessels enter the cartilage: bringing fibroblasts that

become osteoblasts spongy bone develops at

the primary ossification center


Remodeling creates a marrow cavity: bone replaces cartilage

at the metaphyses


Capillaries and osteoblasts enter the epiphyses: creating secondary

ossification centers


Epiphyses fill with spongy bone: cartilage within the

joint cavity is articulation cartilage

cartilage at the metaphysis is epiphyseal cartilage

Primary centers of ossification

In the second month of the intrauterine life, the primary points of ossification appear first, in the shafts, or diaphyses of tubular bones, and in the metaphyses.

They ossify by perichondral and enchondral osteogenesis.

Secondary and accessory points of ossification

The secondary points of ossification appear shortly before birth or during the first years after birth and they develop by encondral osteogenesis.

The accessory points of ossification appear in children, adolescents, and even adults in the appophyses of bones (e.g. tubercles, trochanters, the accessory processes of the lumbar vertebrae).

Blood Supply of Mature Bones 3 sets of blood vessels develop Nutrient artery and vein:

a single pair of large blood vessels enters the diaphysis through the nutrient foramen

Metaphyseal vessels: supply the epiphyseal cartilage

where bone growth occurs Periosteal vessels provide:

blood to superficial osteons secondary ossification centers

Effects of Hormones and Nutrition on Bone1. Growth hormone

Single most important stimulus to the epiphyseal plate (dwarfism/gigantism)

2. Thyroid hormone Moderates growth hormone to insure proper

proportions of growth3. Sex hormones (estrogens & androgens)

A great ‘rush’ at puberty = growth spurt Lead to a breakdown of cartilage that leads

to a closure of plates …steroids!4. Calcitriol

Made in kidneys; synthesis requires cholecalciferol Helps absorb calcium & phosphorus from GI tract

Additional dietary regulators Need adequate calcium, phophorus, magnesium,

flouride, iron, & manganese Calcium is necessary for:

Transmission of nerve impulses Muscle contraction Blood coagulation Secretion by glands and nerve cells Cell division

Vitamin D – absorption of calcium from GI Vitamin C – formation of collagen Vitamin A – stimulates osteoblast activity Vitamins K and B12 - help synthesize bone proteins

Bone Fractures (Breaks)

Bone fractures are classified by: The position of the bone ends after fracture The completeness of the break The orientation of the bone to the long axis Whether or not the bones ends penetrate the skin

Types of Bone Fractures Nondisplaced – bone ends retain their normal position Displaced – bone ends are out of normal alignment Complete – bone is broken all the way through Incomplete – bone is not broken all the way through Linear – the fracture is parallel to the long axis of the bone Transverse – the fracture is perpendicular to the long axis of the bone Compound (open) – bone ends penetrate the skin Simple (closed) – bone ends do not penetrate the skin Comminuted – bone fragments into three or more pieces; common in the

elderly Spiral – ragged break when bone is excessively twisted; common sports

injury Depressed – broken bone portion pressed inward; typical skull fracture Compression – bone is crushed; common in porous bones Epiphyseal – epiphysis separates from diaphysis along epiphyseal line;

occurs where cartilage cells are dying Greenstick – incomplete fracture where one side of the bone breaks and

the other side bends; common in children

The Major Types of Fractures

Fracture Repair: Step 1

Bleeding: produces a clot

(fracture hematoma) establishes a fibrous

network Bone cells in the area

die


Cells of the endosteum and periosteum: Divide and migrate into

fracture zone Calluses stabilize the

break: external callus of

cartilage and bone surrounds break

internal callus develops in marrow cavity


Osteoblasts: replace central

cartilage of external callus

with spongy bone


Osteoblasts and osteocytes remodel the fracture for up to a year: reducing bone

calluses

Osteoporosis Bone reabsorption>bone

production Osteopenia begins between

ages 30 and 40 Women lose 8% of bone mass

per decade, men 3% Decrease in bone

massincrease fracture risk Decreased levels of estrogen

primarily Most important cause of fracture

in women>50 35% of bone mass may be gone

by age 70 Vertebrae & femur neck are

most affected

Risk Factors Body build – short

women have less bone mass

Weight – thinner at greater risk

Smoking – decreases estrogen levels

Lack of dietary calcium Exercise – decrease rate

of absorption Drugs – alcohol,

cortisone, tetracycline Premature menopause

Osteopenia

Figure 6–19 The Effects of Osteoporosis on Spongy Bone.

Homeostatic Imbalances Osteomalacia

Bones are inadequately mineralized causing softened, weakened bones

Main symptom is pain when weight is put on the affected bone

Caused by insufficient calcium in the diet, or by vitamin D deficiency

Rickets Bones of children are inadequately mineralized causing

softened, weakened bones Bowed legs and deformities of the pelvis, skull, and rib cage

are common Caused by insufficient calcium in the diet, or by vitamin D

deficiency

Developmental Aspects of Bones

The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms

At birth, most long bones are well ossified (except for their epiphyses)

By age 25, nearly all bones are completely ossified In old age, bone resorption predominates A single gene that codes for vitamin D docking

determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life

THE END

general osteology

Health & Medicine