Overview and histology of Skeletal system

Lecture Notes

 
I. Functions of Skeletal System

A. Protection of internal organs

B. Movement of skeleton

C. Support of body parts

D.  Storage of fat (in yellow bone marrow) and minerals (calcium phosphate & calcium carbonate)

E. Site of formation (hematopoesis) of all blood cell types in red bone marrow.
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Describe the functions of the skeletal system.
II. Histology of skeletal system
The skeletal system is primarily constructed of connective tissues.

A. Cartilage tissues-
avascular so does not heal as fast as bone
all cartilage tissue provide a more resilient surface to promote cushioning.
chondroblasts secrete matrix of protein gel and fibers while chondrocytes maintain matrix. These cells live in lacunae (open pockets in matrix). 1. Hyaline cartilage- (no fibers)
 most of fetal skeleton, epiphyseal plates (growth centers of long bone ends) and articular surfaces on long bones within joints, between ribs and sternum

2. Elastic cartilage (elastin fibers) -
ear and epiglottis, not really skeletal element

3. Fibrocartilage (collagen fibers)
- intervertebral disks, pubic symphysis
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B. Dense regular connective tissue
fibroblasts secrete fibrous matrix (collagen fibers)
-tendons (attach muscle to bone) andl ligaments (connect bone to bone)

C. Bone tissues-
vascular so rapid healing
osteoblasts secrete matrix of calcium and phosphate crystals/collagen fibers while osteocytes maintain matrix. These cells live in lacunae (open pockets in matrix)

1. Spongy or cancellous bone tissue
bone matrix is formed into trabeculae (open construction of needle-like structures )
found in the interior of most bones as well as inside epiphysis of long bones
lightens bone and holds red bone marrow (hematopoietic tissue)

2. Compact bone tissue
matrix formed in osteon (dense tube-like structure) which surrounds central canal through which blood vessels pass so that water, gasses, nutrients and wastes pass to osteocytes.
found in layer on outside of bones
Provides most of strength of bone as well as prevents breakage

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Compare and contrast bone tissues concerning structure, function and location.

3. Components of bone matrix

a) Cells
 osteoblasts iniatially secrete bony matrix (deposition of matrix)
 osteocytes maintain bony matrix
osteoclasts dissolve bony matrix (resorption of matrix)

b) Collagen fibers-
-provide flexibility in bone

c). phosphate and calcium salts (calcium carbonate) or crystals
provide strength and rigidity of bone
about 65% of bone by weight

4. Bone matrix formation
Osteoblast cells secrete collagen fibers
then lay down salt crystals to form matrix around themselves keeping open space open (lacunae)
first bone tissue to be made is spongy bone tissue.
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Describe the components of bone matrix, what function each component provides and how matrix is formed

III. Bone structure

A. General bone structure
 periosteum – covering bone
compact bone tissue – outside tissue
spongy bone tissue – interior tissue
compact bone tissue – outside tissue
periosteum – covering bone

B. Bone types 1. Flat bones, e.g., sternum, skull, ribs

2. Short bones, e.g., wrists and ankles

3. Irregular, e.g., face, vertebrae

4. Long bones (longer than wide = elongated) e.g., bones of limbs (arms, legs, fingers, toes)

a. Diaphysis
shaft of bone made of compact tissue that contains yellow (fat storage) marrow in medullary cavity- storage of fat and lightening bone

b. Epiphyses (2)
 knobby bone ends (articulation) with spongy tissue and red marrow (energy storage and blood cell formation) inside-articular site covered with hyaline cartilage

c. Periosteum (outside membrane of living bone)
 contains osteoblasts and osteoclasts-
active area of bone remodeling and bone widening,
tendon attachment site

d. Perichondrium (outside membrane of living cartilage) covers hyaling cartilage on artcular surfaces of epiphyses-
maintains cartilage surfaces

e. Endosteum (inside membrane lines medullary cavity)
active area of bone remodeling

d. Epiphyseal line (adult) or plate (pre-adult)
epiphyseal plate is bone lengthening growth center of bone (hyaline cartilage)
epiphyseal line is fusion of bone tissue after growth finished
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Diagram a cross section of a long bone and describe functions of each structure.

C. Bone surface features (contours) 1. Openings for passage of vessels/nerves, (foramen, meatus, fissure)

2. Projections for muscle attachment (trochanter, tubercle, spine)

3. Projections for joint formation, (head, condyle)

IV. Bone physiology A. Bone ossification
 secretion of new bone matrix into pre-existing fetal structure or tissue

1. Intramembranous ossification forms flat and some irregular bones
Bone replaces fetal fibrous membrane a. Osteoblasts invade and secrete collagen and encourage calcium salt deposition into open fibrous membrane to form trabeculae of spongy bone. Osteoblasts get trapped in open pockets of matrix (lacunae), receive less nutrients, then slow matrix deposition (and then are known as osteocytes)

b. Periosteum forms around spongy bone.

c. Eventually outer layer of spongy bone is remodeled (changed) into compact bone that surrounds spongy bone interior of flat bone.

 2. Endochondral ossification - forms most bones (long and short)
Bone replaces growing fetal hyaline cartilage model a. Osteoblasts invade surface (= primary ossification site) and begin to secrete collar of spongy bone matrix around cartilage model.. Periosteum begins to form on outside of bone.

b. Cartilage on inside is deprived of nutrients and begins to die thereby forming cavity in cartilage matrix.

c. Osteoblasts invade cartilage matrix cavity and begins to fill with spongy bone outward. Bone matrix forming outward meets bone forming inward from periosteum.

 B. Bone growth (Increase in bone size)
bone matrix is added to growing structure

1. Widening or thickening
occurs in all bones
 osteoblasts in periosteum secrete bony matrix onto external bone surface
 accompanied by bone resorption by osteoclasts in endosteum of medullary cavity so bone gets wider and cavity gets larger.

2. Lengthening
occurs only at epiphyseal plates of long bones through teen years
osteoblasts invade cartilaginous cartilaginous epiphyses (=secondary ossification site) to form bony epiphyses. Continuous replacement of growing hyaline cartilage with bone at epiphyseal plate elongates bone.

Process of bone lenghthening in detail (epiphyseal plate)

a) Hyaline cartilaginous epiphysis is a reserve of chondrocytes

b) Chondrocytes undergo mitosis (proliferate) to increase number and amount of matrix

c) Chondrocytes enlarge (hypertrophy) matrix

d) Chondrocytes die and pockets in cartilaginous matrix is filled with bony (ossifiy or calcifiy) matrix by osteoblast activity (form spongy bone tissue.)

 Eventually spongy bone is remodeled (replaced by compact bone tissue) or removed to form medullary cavity in adult bone.

Compare and contrast: the types of bone ossification; the types of bone growth; and where each process occurs.
C. Bone remodeling
Osteoclasts remove collagen and allow calcium crystals to dissolve, osteoblasts may then lay down different tissue or none at all.
 bone is dynamic--thickness and type of tissue vary depending on age, gender and activity level
 remodeling is a balance of bone deposition and destruction! osteoblasts vs. osteoclasts activity

D. Bone repair of fracture
1. Inflammation brings in blood and clot forms around site (hematoma). More blood brings in phagocytes to remove dead debris, more water to flush out bacteria, nutrients to build, antibiotics to inhibit bacterial growth. This stage may last a couple weeks.

2. Invasion of fibroblasts invade to form collagen fibers and chondroblasts invade to form fibrocartilage where there is no bone. this fibrocartilage block lasts about 3 weeks.

3. Osteoblasts invade the cartilage and ossify to form spongy bone. This stage lasts about 12-16 weeks.

4. Remodeling occurs with steoclasts remove dying bone tissue and compact bone replaces spongy bone
Compare and contrast the processes of bone remodeling and repair.

 E. Bone growth regulation
 primary hormone controlling skeleton size is HGH , increasing HGH targets epiphyseal plate cartilage cells to increase rate of growth.
1. As juvenile - human growth hormone (HGH) controls growth by level of its secretion from pituitary gland

2. Disorders

a. Juvenile
 Hyposecretion = pituitary dwarfism
 Hypersecretion = pituitary giantism

b. Adult - hypersecretion = acromegaly

F. Homeostasis of blood calcium (example of hormonal negative feedback loops)

Loops provide a constant source of calcium in blood for other systems (i.e., muscle contractions, nerve conductions, blood coagulation, etc.) as well as storage of calcium so bones are hard enough.
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1. CT (Calcitonin)- produced by parafollicular cells of thyroid gland
a. production stimulated by high levels of blood calcium

b. Acts to:
inhibit osteoclast cell activity (reduce bone matrix (mass)resorption) so

c. overall response is lowered blood calcium level

2. Parathyroid hormone (PTH)- produced in parathyroid gland near thyroid

a. production stimulated by low levels of blood calcium

b. Acts to:
Increase osteoclast cell number and activity thereby increasing bone matrix (mass) resorption
increase Ca resorption from urine by nephron (kidney) cells
Increase formation of Calcitriol by kidney cells

c. overall response being an increase in blood calcium
 

3. Calcitriol (Vitamin D)-precursor chemical produced by kidney and then modified in skin

a. production stimulated by higher levels of PTH (low levels of blood calcium)

b. Acts to increase resorption of Ca from diet by intestinal cells

c. response is to aid PTH in raising blood calcium levels.

Construct the negative feedback hormonal cycles that regulate blood calcium. CLICK TO VIEW AND PRINT THE COMPLETE HORMONE CYCLE TEMPLATE..
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Professor Thomas M. Lancraft
Human Anatomy and Physiology Courses
at St. Petersburg College
St. Petersburg/Gibbs Campus

6/2006