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Page 1: Bones and skeletal tissues 2014

Bones and Skeletal TissuesBIO 351 Human Anatomy & Physiology IDr. Barbara T. Wizer

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Skeletal Cartilages Skeletal cartilages are made from cartilage,

surrounded by a layer of dense irregular connective tissue called the perichondrium. Hyaline cartilage is the most abundant skeletal cartilage,

and includes the articular, costal, respiratory, and nasal cartilages.

Elastic cartilage is more flexible than hyaline, and is located only in the external ear, the epiglottis of the larynx, and the eustachian (auditory) tube.

Fibrocartilage is located in areas that must withstand a great deal of pressure or stretch, such as the cartilages of the knee (menisci), and the intervertebral discs

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Fibrocartilage in the menisci of the knee

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Hyaline cartilage covers the articular surfaces of bones.

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The costal cartilages are made up of hyaline cartilage.

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Classification of Bones, 1 There are two main divisions of the bones of

the skeleton: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, vertebral column, and rib cage

The appendicular skeleton consists of the bones of the upper and lower limbs, and the girdles that attach them to the axial skeleton

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Structural Classification of Bones5 Types: Long bones are longer than they are wide, have a definite

shaft and two ends, and consist of all limb bones except the patellas, carpals, and tarsals.

Short bones are somewhat cube-shaped and include the carpals and tarsals.

Flat bones are thin, flattened, often curved bones that include most skull bones, the sternum, scapulae, and ribs.

Irregular bones have complicated shapes that do not fit in any other class, such as the vertebrae and coxae.

Sesamoid bones – “extra” bones that develop in tendons or near joints, esp. in feet + hands. Include the patellas, which are the only sesamoid bones consistently present in skeleton

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Functions of Bones Bones support the body and cradle the soft

organs, protect vital organs, allow movement, store minerals such as calcium and phosphate, and house hematopoietic tissue (which forms

blood cells) in specific marrow cavities.

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Gross Anatomy Bone markings are projections, depressions,

and openings found on the surface of bones that function as sites of muscle, ligament, and tendon attachment, as joint surfaces, and as openings for the passage of blood vessels and nerves.

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Compact and Spongy Bone All bone has a dense outer layer consisting of

compact bone that appears smooth and solid. Internal to compact bone is spongy bone,

which consists of honeycomb, needle-like, or flat pieces, called trabeculae.

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Structure of a Typical Long Bone Long bones have a tubular bone shaft, consisting of a bony

collar surrounding a hollow medullary cavity, which is filled with yellow bone marrow in adults.

Epiphyses are at the ends of the bone, and consist of internal spongy bone covered by an outer layer of compact bone.

The epiphyseal line (or scar) is located between the epiphyses and diaphysis, and is a remnant of the epiphyseal plate (= growth plate).

The external surface of the bone is covered by a connective tissue membrane called the periosteum.

The internal surface of the bone is lined by a connective tissue membrane called the endosteum.

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Location of Hematopoietic Tissue Hematopoietic tissue of bones, red bone

marrow, is located within the trabecular cavities of the spongy bone in flat bones, and in the epiphyses of long bones.

Red bone marrow is found in all flat bones, epiphyses, and medullary cavities of infants, but in adults, distribution is restricted to flat bones and the proximal epiphyses of the humerus and femur.

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Microscopic Anatomy of Bone,1 The structural unit of compact bone is the osteon, or

Haversian system, which consists of concentric tubes of bone matrix (the lamellae) surrounding a central Haversian canal that serves as a passageway for blood vessels and nerves.

Perforating, or Volkmann’s, canals lie at right angles to the long axis of the bone, and connect the blood and nerve supply of the periosteum to that of the central canals and medullary cavity.

Osteocytes occupy lacunae at the junctions of the lamellae, and are connected to each other and the central canal via a series of hair-like channels, canaliculi.

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Microscopic Anatomy of Bone, 2 Circumferential lamellae are located just

beneath the periosteum, extending around the entire circumference of the bone, while interstitial lamellae lie between intact osteons, filling the spaces in between.

Spongy bone lacks osteons but has trabeculae that align along lines of stress, which contain irregular lamellae.

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The microscopic image of bone tissue show circumferential lamellae.

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Note the trabeculae in spongy bone.

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Chemical Composition of Bone Organic components of bone include cells

(osteoblasts, osteocytes, and osteoclasts) and osteoid (ground substance and collagen fibers), which contribute to the flexibility and tensile strength of bone.

Inorganic components make up 65% of bone by mass, and consist of hydroxyapatite, a mineral salt that is largely calcium phosphate, which accounts for the hardness and compression resistance of bone.

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Hydroxyapatite, the inorganic component of bone

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Formation of the Bony SkeletonBone develops in one of 2 ways: Intramembranous ossification - bones form

within fibrous connective tissue membranes. The cranial bones and clavicles develop in this way.

In endochondral ossification hyaline cartilage models are laid down first, then replaced by calcified bony matrix, forming all bones below the skull except for the clavicles.

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Fetal skull showing the fontanel between the frontal and parietal bones—what is the function of the fontanels?

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The epiphyseal plates are made up of hyaline cartilage and are the places where long bones grow in length.

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The epiphyseal plates are replaced with epiphyseal lines when growth in height is completed.

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Postnatal Bone Growth Growth in length of long bones occurs at the

osteogenic zone through the rapid division of the upper cells in the columns of chondrocytes, calcification and deterioration of cartilage at the bottom of the columns, and subsequent replacement by bone tissue — endochondral growth.

Growth in width, or thickness, occurs through appositional growth due to deposition of bone matrix by osteoblasts beneath the periosteum.

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Hormonal Regulation During infancy and childhood, the most

important stimulus of epiphyseal plate activity is growth hormone from the anterior pituitary, whose effects are modulated by thyroid hormone.

At puberty, testosterone and estrogen promote a growth spurt, but ultimately induce the closure of the epiphyseal plate.

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Bone Remodeling In adult skeletons, bone remodeling is a balance of

bone deposit and removal; bone deposit occurs at a greater rate when bone is injured, and bone resorption allows minerals of degraded bone matrix to move into the blood.

Osteoblasts are the bone forming cells and osteoclasts are the cells that break down bone.

In response to mechanical stress and gravity, bone grows or remodels in ways that allow it to withstand the stresses it experiences.

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Osteoclast

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Osteoporosis A group of disorders in which the rate of bone resorption

exceeds the rate of formation. Bones have normal bone matrix, but bone mass is reduced

and the bones become more porous and lighter-- increasing the likelihood of fractures.

Older women are especially vulnerable to osteoporosis, due to the decline in estrogen after menopause.

Other factors that contribute to osteoporosis include a petite body form, insufficient exercise or immobility, a diet poor in calcium and vitamin D, abnormal vitamin D receptors, smoking, and certain hormone-related conditions.

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Normal boneOsteoporotic bone

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Bone Diseases Osteoporosis Bone loss - results in increased risk of

fractures – esp. vertebrae, hip (proximal femur), wrist

Rickets (children)/ Osteomalacia (adults) demineralization of bone due to Vit. D

deficiency Osteomyelitis – infection of bone Osteosarcoma – malignant tumor

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Regulation of Blood Calcium LevelAn example of Homeostasis Hormones1 – parathyroid hormone (PTH) produced by the parathyroid glands in the neck,

secreted when blood Ca level is low, causes bone breakdown and release of Ca from the bone into the blood

2 – calcitonin produced by the thyroid gland in the neck, secreted

when blood Ca level is high, causes decreased breakdown of bone matrix so blood Ca level drops

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Causes of Hypercalcemia Hyperparathyroidism Malignancy – cancer with metastasis to bone,

certain solid tumors, certain hematologic malignancies

Renal failure Certain meds Hypervitaminosis D


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