SKELETAL SYSTEM INTRODUCTION

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SKELETAL SYSTEM INTRODUCTION

• Bone Structure, Development, Growth and
  Classification
• Anatomy and Physiology

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Parts of a Bone

• Bones are fairly complex organs that contain many
  types of tissues
• Bone tissue
• Cartilage (to cushion bones at joints, etc.)
• Connective tissue
• Blood
• Nervous tissue

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Bone Classification

• There are five main types of bones, classified
  according to their shapes
• Long bones (e.g. thigh and forearm bones)
• Short bones shorter and cubelike (e.g. finger
  and toe bones)
• Flat bones platelike, with broad surfaces (e.g.
  ribs, scapulae)
• Irregular bones contain many different shapes
  (e.g. vertebrae, facial bones)
• Sesamoid/round bones usually found near joints
  embedded in tendons (e.g. patella/kneecap)

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Parts of a Bone

• Epiphysis end of the bone articulates with
  another bone coated with a protective layer of
  articular cartilage
• Diaphysis shaft of the bone, between the two
  epiphyses.

A human femur, the largest bone in the body
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Bone Composition

• Diaphysis
• Walls are mostly composed of tightly packed
  compact bone with no gaps between cells
• Center of bone forms a tube-shaped medullary
  cavity which contains soft connective tissue
  called marrow

• Epiphyses
• Mostly composed of spongy bone, with thin outer
  layer of compact bone
• Spongy bone contains plates (trabeculae) that
  create spaces for cells and make bone lighter

• Entire bone covered by a tough covering called
  the periosteum
• endosteum lines the medullary cavity.
• Bone contains projections called processes and
  other structures
• that allow attachment of ligaments, tendons,
  muscles, nerves, etc.

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Here Today, Gone to Marrow

• Red marrow Very important in hematopoiesis, or
  formation of blood cells
• Red blood cells (RBCs, erythrocytes)
• White blood cells (WBCs, leukocytes)
• Platelets (important for clotting)
• Yellow marrow does not take part in blood cell
  formation stores fat
• NOTE As we age, red marrow turns increasingly to
  yellow marrow. The reverse can occur in
  situations when the supply of blood cells is
  deficient.

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Bone Development and Growth

• Intramembranous bones
• Broad, flat bones in skull are intramembranous
  bones
• Originate between layers of primitive connective
  tissue whose cells differentiate into osteoblasts
• Osteoblasts are bone-forming cells that deposit
  bone matrix around them to form spongy bone. This
  may become compact bone later. (When osteoblasts
  are surrounded by bone matrix, they are called
  osteocytes.)
• Periosteum forms from osteoblasts outside
  developing bone compact bone in periosteum
  protects internal contents

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Endochondral Bones

• Most bones in the body are endochondral bones,
  which develop from similarly-shaped masses of
  cartilage. The process
• Cartilage models grow rapidly, then begin to
  differentiate.
• Cartilage cells begin to die periosteum begins
  to form as cartilage begins to calcify
• Osteoblasts form and produce spongy bone with
  time, compact bone forms around spongy bone.

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Growth of Long Bones

• In long bones, bone tissue first replaces
  cartilage in the diaphysis at the primary
  ossification center.
• Bone grows from primary ossification center
  toward the ends of the cartilaginous model
  epiphyses remain as cartilage and keep growing.
• Later, secondary ossification centers form at
  epiphyses and deposit spongy bone a band of
  cartilage called the epiphyseal plate forms
  between the ossification centers and remains
  until the bone stops growing (23-25 years old)

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The Epiphyseal Plate

• This is the site where bone lengthening occurs
  growth occurs in a layer that contains cells
  undergoing mitosis rapidly and allowing cartilage
  to grow.
• As we age, osteoclasts break down calcified
  cartilage
• Acid for inorganic portion (w/o carbon)
• Enzymes f/lysosomes for organic portion

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The Epiphyseal Plate (cont.)

• As osteoclasts break down calcified cartilage,
  osteoblasts form bone tissue to replace it. Once
  this occurs, the bone is ossified (and more or
  less permanent).
• This whole process is called endochondral
  ossification.
• At about 23 years (females) or 25 years (males),
  the ossification centers meet and the epiphyseal
  plates ossify--this is the end of bone growth.

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Bone Homeostasis

• Two opposing processes help maintain nearly
  constant bone mass
• Resorption osteoclasts break down bone tissue
• Deposition osteoblasts deposit new bone matrix
  to replace resorbed tissue.

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What Do Bones Need?

• Nutrition
• Calcium absorbed into inorganic portion of bone
  matrix maintains bone shape/strength
• Vitamin D required for small intestine to
  properly absorb calcium from food not common in
  food, so must be obtained from fortified milk or
  synthesized by the body (with the help of UV
  light from the sun)
• - Vit. D deficiency leads to rickets (children)
  or osteomalacia (adults), marked by bone
  deformation

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• Vitamin A Required for proper functioning of
  osteoblasts and osteoclasts deficiency retards
  bone development
• Vitamin C Required for synthesis of collagen,
  protein that strengthens and thickens bone tissue

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Hormones

• Proper bone growth controlled by hormones from
  pituitary, thyroid, and parathyroid glands, as
  well as ovaries/testes.
• Growth hormone (pituitary) stimulates division
  of cartilage cells at epiphyseal plates.
  Deficiency results in pituitary dwarfism excess
  results in pituitary gigantism or acromegaly.

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• Thyroid hormone secreted to promote ossification
  at epiphyseal plates
• Excess leads to premature ossification that can
  halt growth, but deficiency also reduces amount
  of growth hormone secreted.
• Parathyroid hormone stimulates an increase in
  osteoclast number and activity (excess can break
  down bones prematurely/excessively)

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• Male sex hormones (androgens) and female sex
  hormones (estrogens) stimulate and control timing
  of bone growth and ossification. Estrogens have
  a greater effect than androgens, so female bones
  tend to ossify and reach their maximum length
  before male bones.

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Brain Teaser

• Bones of athletes are generally stronger and
  heavier than those of non-athletes. What are
  some reasons that this might occur?