Plan of the lecture1. General concepts about skeleton2. Bone as an organ3. Functions of the skeleton4. Classification of bones5. Types of bone ossification6. Development of bones
The Skeletal System!Our First System Cells (Osteocytes) Tissues (Osseous Tissue) Organs (Bones) Systems (Skeletal)
THE LOCOMOTOR APPARATUS – ITSCOMPONENTS 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 The appendicular skeleton is composed of 126 bones of the upper and lower limbs and theskeleton 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 THEPERIOSTEUM 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 tissueb) 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 cellb) osteoclasts resorb or break down the bonec) osteocytes are mature bone cells.An equilibrium between osteoblasts and osteoclasts maintains the bone tissue.
OSSIFICATION OSTEOBLASTS FORM NEW BONE MATRIX OSTEOCLASTSCOLLAGEN MAINTAIN SHAPESTERNGTH CALCIUM DEPOSITION OSTEOCYTES MAINTAIN BONE
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 periosteumExternally bone is covered byperiosteum (except articularsurfaces). The periosteumadheres to the surface of thebones.It consists of two layers closelyunited together:a) The outer layer fibrous layerb) The inner layer or bone- forming layer (cambial)
Structure ofthe 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.
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 skeletona) Haemopoiesisb) 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 functionThe bone marrow provideshaemopoiesis function and biologicalprotection of the organism. It takes NB: Thepart in nutrition, development and bones of thegrowth of the bone. The red marrow embryo andconcerned with haemopoiesis and bone new-bornformation, has an active role in the contain onlyhealing of fractures. Red marrowpredominates in infants and in red marrow.children, with growth of child the redmarrow is gradually replaced byyellow 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 skeletona) Supportb) Protectionc) Body movement
Support (weight bearing) The skeleton forms a rigid framework to which are attached the soft tissues and organs of the body.
Protectionfunction 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 shape Tubular bonesa) Long tubular bones b) Short tubular bones humerus, metacarpal, radius, ulna, metatarsal bones and phalanges femur, tibia, fibula
Classification of bonesSpongy bonesa) Long spongy bones sternum, ribs, etcb) Short spongy bones carpal and tarsal bonesc) Sesamoid bones knee-cap pisiform bone, etc.
Classification of bonesFlat bonesSkull bones Bones of the vault of the skullGirdle 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 ossification1. 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 cells2. 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
Intramembranous Ossification: Step 2 Blood vessels grow into the area: to supply the osteoblasts Spicules connect: trapping blood vessels inside bone
Intramembranous Ossification: Step 3 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
Endochondral Ossification: Step 2 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
Endochondral Ossification: Step 3 Blood vessels enter the cartilage: bringing fibroblasts that become osteoblasts spongy bone develops at the primary ossification center
Endochondral Ossification: Step 4 Remodeling creates a marrow cavity: bone replaces cartilage at the metaphyses
Endochondral Ossification: Step 5 Capillaries and osteoblasts enter the epiphyses: creating secondary ossification centers
Endochondral Ossification: Step 6 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 accessorypoints 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
Fracture Repair: Step 1 Bleeding: produces a clot (fracture hematoma) establishes a fibrous network Bone cells in the area die
Fracture Repair: Step 2 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
Fracture Repair: Step 3 Osteoblasts: replace central cartilage of external callus with spongy bone
Fracture Repair: Step 4 Osteoblasts and osteocytes remodel the fracture for up to a year: reducing bone calluses
Osteoporosis Bone reabsorption>bone Risk Factors production Body build – short Osteopenia begins between women have less bone ages 30 and 40 mass Women lose 8% of bone mass Weight – thinner at per decade, men 3% greater risk Decrease in bone Smoking – decreases massincrease fracture risk estrogen levels Decreased levels of estrogen primarily Lack of dietary calcium Most important cause of fracture Exercise – decrease rate in women>50 of absorption 35% of bone mass may be gone Drugs – alcohol, by age 70 cortisone, tetracycline Vertebrae & femur neck are Premature menopause most affected
OsteopeniaFigure 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