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The Skeletal System (Chp 6).

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1 The Skeletal System (Chp 6)

2 Skeletal System Overview
Parts of the skeletal system: Bones (skeleton) Joints Cartilages Ligaments (bone to bone) Tendon* (bone to muscle) Two divisions: Axial skeleton Appendicular skeleton – limbs and girdle

3 Skeletal Cartilage Our skeleton is initially made up of cartilage, but most of it is replaced by bone. The cartilages that remain in adults are found mainly in region where more flexible skeletal tissues are needed. Skeletal cartilage is made of cartilage tissue that consists primarily of water. The high water content accounts for its resilience. Cartilage contains no blood vessels or nerves and is surrounded by a layer of dense, irregular connective tissue called the perichondrium The perichondrium helps resist outward expansion during compression an is the source of blood vessels from which nutrients diffuse

4 Skeletal Cartilage Locations
Hyaline: support with flexibility and resilience; articular cartilage at the ends of bones at most movable joints, costal cartilages which connect ribs to the sternum, respiratory cartilage & nasal cartilages Elastic: flexible & able to withstand repeated bending; supports external ear and forms the epiglottis Fibrocartilage: highly compressible, great tensile strength (intermediate); at sights of both heavy pressure and stretch such as the menisci of the knee & the discs between vertebrate

5 The bones and cartilages of the human skeleton.
Epiglottis Larynx Thyroid cartilage Cartilage in external ear Cartilages in nose Trachea Cricoid cartilage Lung Articular Cartilage of a joint Cartilage in Intervertebral disc Costal cartilage Respiratory tube cartilages in neck and thorax Pubic symphysis Bones of skeleton Axial skeleton Meniscus (padlike cartilage in knee joint) Appendicular skeleton Cartilages Articular cartilage of a joint Hyaline cartilages Elastic cartilages Fibrocartilages

6 Growth of Cartilage Cartilage consists of cells called chondrocytes encased in small cavities (lacunae) within an extracellular matrix containing jellylike ground substance and fibers. Appositional Growth: cartilage cells in the surrounding perichondrium secrete new matrix against the external face of existing cartilage Interstitial Growth: the lacunae bound chondrocytes inside the cartilage divide and secrete new matrix expanding the cartilage within During normal bone growth and aging, calcium salts may be deposited in cartilage. Calcified cartilage is not bone

7 Functions of Bones Support of the body Protection of soft organs
Movement due to attached skeletal muscles Storage of minerals and fats Blood cell formation

8 Bones of the Human Body The adult human skeleton has 206 bones
Two basic types of bone tissue Compact bone Homogeneous Spongy bone Small needle-like pieces of bone Many open spaces

9 Microscopic Anatomy of Bone
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

10 Diagram of a Single Osteon
Artery with capillaries Structures in the Central canal Vein Nerve fiber Lamellae Collagen fibers run in different directions Twisting force

11 Classification of Bones Based on Shape

12 Long Bones Typically longer than wide
Diaphysis – central shaft, composed of compact bone Epiphysis – Ends of the bone, composed mostly of spongy bone Ex) Femur, humerus

13 Structures of a Long Bone
Periosteum Outside covering of the diaphysis Fibrous connective tissue membrane Sharpey’s fibers Secure periosteum to underlying bone Arteries Supply bone cells with nutrients


15 Short Bones, Flat Bones & Irregular Bones
Generally cube-shape, contain mostly spongy bone Examples: Carpals, tarsals Flat bones Thin and flattened, usually curved Thin layers of compact bone around a layer of spongy bone Examples: Skull, ribs, sternum Irregular bones Irregular shape, do not fit into other bone classification categories Example: Vertebrae and hip

16 Flat bones consist of a layer of spongy bone sandwiched between two thin layers of compact bone.

17 Changes in the Human Skeleton
In embryos, the skeleton is primarily hyaline cartilage During development, much of this cartilage is replaced by bone Cartilage remains in isolated areas Bridge of the nose Parts of ribs Joints

18 Types of Bone Cells Osteogenic Cell: stem cell in the inner layer of the periosteum; that gives rise to osteoblasts Osteoblasts: Bone-forming cells Osteocytes: Mature bone cells Osteoclasts: Bone-destroying cells; Break down bone matrix for remodeling and release of calcium Bone remodeling is a process executed by both osteoblasts and osteoclasts (a) Osteogenic cell (b) Osteoblast (c) Osteocyte (d) Osteoclast Stem cell Matrix-synthesizing cell responsible for bone growth Mature bone cell that maintains the bone matrix Bone-resorbing cell

19 Bone Development Osteogensis & ossification both refer to the process of bone tissue formation In embryos this leads to the formation of bony skeleton Bone growth is also a form of ossification that occurs from birth into early adulthood Although bones are capable of thickening throughout our lives, most ossification in adults is for remodeling and repair of bones.

20 Formation of the Bony Skeleton
Before week 8 – fibrous membranes & hyaline cartilage When bone develops from a fibrous membrane it is called intramembranous ossification (membrane bone forms) Bone formation from the replacement of hyaline cartilage is called endochondral ossification (cartilage or endochondral bone forms)

21 Intramembranous Ossification I
Mesenchymal cell Collagen fiber Ossification center Osteoid Osteoblast Ossification centers appear in the fibrous connective tissue membrane. • Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. 1

22 Intramembranous Ossification II
Osteoblast Osteoid Osteocyte Newly calcified bone matrix Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies. • Osteoblasts begin to secrete osteoid, which is calcified within a few days. • Trapped osteoblasts become osteocytes. 2

23 Intramembranous Ossification III
Mesenchyme condensing to form the periosteum Trabeculae of woven bone Blood vessel 3 Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae called woven bone. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum.

24 Intramembranous Ossification IV
Fibrous periosteum Osteoblast Plate of compact bone Spongy Bone cavities contain red marrow 4 Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. • Trabeculae just deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates. • Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.

25 Endochondral Ossification in Long Bone
Week 9 Month 3 Birth Childhood to adolescence Articular cartilage Secondary ossification center Spongy bone Epiphyseal blood vessel Area of deteriorating cartilage matrix Epiphyseal plate cartilage Hyaline cartilage Spongy bone formation Medullary cavity Bone collar Blood vessel of periosteal bud Primary ossification center Bone collar forms around hyaline cartilage model. 1 Cartilage in the center of the diaphysis calcifies and then develops cavities. 2 The periosteal bud invades the internal cavities and spongy bone begins to form. 3 The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. 4 The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. 5

26 Post-Natal Bone Growth

27 Bone Growth & Remodeling
Epiphyseal plates allow for growth of long bone during childhood Endochondral ossification occurs at the articular cartilages and epiphyseal plates as the bone grows in length. Remodeling occurs to maintain proper bone proportions.

28 Homeostatic Controls of Ca2+ Levels in Blood

29 Importance of Ionic Calcium
Necessary for numerous physiological processes: The human body contains g of calcium, 99% as bone minerals and the balance mostly within cells; only ~ 1.5g in blood. Concentration of calcium in blood ~ 9-11mg/100mL Calcium is absorbed from the intestine under the control of vitamin D metabolites

30 Bone Response to Mechanical Stress I
Wolff’s Law states that a bone grows or remodels in repsone to the forces or demands placed on it A bone’s anatomy reflects the stresses that are placed upon it Bones are stressed when weight bears down on them or muscles pull on them. This loading is usually off center causing the bone to bend This results in compression on one side and tension on the other

31 Controls of Bone Remodeling
The mechanism by which bones respond to mechanical stimulation is unclear Deforming a bone causes an electrical current, it is hypothesized that electrical signals may direct remodeling It is also believed that the hormonal loop determines whether & when remodeling will occur in response to blood calcium levels Mechanical stress determines where remodeling occurs

32 Bone Markings I

33 Bone Markings II

34 Bone Fractures A break in a bone Types of bone fractures
Closed (simple) fracture – break that does not penetrate the skin Open (compound) fracture – broken bone penetrates through the skin Bone fractures are treated by reduction and immobilization Realignment of the bone

35 Some Common Types of Fractures

36 Fractures – Closer Look

37 Fractures – Closer Look

38 Fractures – Closer Look

39 Stages in the Healing of a Bone Fracture

40 Osteomalacia & Rickets
Osteomalacia: “soft bones”; bones are not mineralized sufficiently; often due to lack of or inability to process vitamin D Osteoid is produced, but calcium salts are not deposited Bones soften & weaken Pain is felt when weight is put on the affected bones Rickets: analogous disease in children; much more severe than osteomalacia because bones are in a period of rapid growth Often results in bowed legs and deformities of the pelvis, skull and ribcage Epiphyseal plates cannot be calcified so they continue to enlarge causing the ends of the bones to become visibly enlarged and abnormally long

41 Osteoporosis Group of diseases in which bone reabsorption occurs faster than bone deposit. Matrix composition remains normal, but bone mass is reduced and the bones become porous and lighter. Most common in elderly; as sex hormone production declines; insufficient exercise, diet, abnormal vitamin D receptors, smoking & hormonal conditions also contribute. Usually treated with calcium & vitamin D supplements, weight-bearing exercises & hormone replacement therapy (HRT) & other drugs.

42 Paget’s Disease Excessive bone formation & breakdown (usually localized) Rarely occurs before age 40, mostly in elderly. The newly formed bone is made rapidly; it has a high spongy to compact bone ratio & reduced mineralization leading to spot weakening Eventually osteoclast activity wanes, but osteoblasts stay active and bones thicken irregularly or marrow fills There are drug therapies.

43 Developmental Aspects
Most bones begin ossifying by 8 weeks, by 12 weeks, primary ossification centers are obvious At birth, most long bones have ossified diaphyses After birth secondary ossification centers form and the epiphyses start to ossify Epiphyseal plates remain and allow for long bone growth in childhood and adolescence In childhood and adolesence, bone formation occurs more than reabsorption, in young adults the processes are in balance and in old age, reabsorption dominates formation

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