Bone function and structure

Hard due to calcium and phosphorus salts
– Resilient due to strands of the elastic protein collagen
Function
– Support for soft tissues
– Provides a place of attachment for muscles
– Protects internal organs
– Stores minerals and fat
– Certain bones produce blood cells in the red marrow

Tissue of the Skeletal System

Bone is a dynamic tissue – it is always
remodeling (building up and breaking down).
•The skeletal system is specialized connective
tissue.

Osteogenic cells

Undifferentiated
- Found in periosteum & endosteum
- Develop into osteoblasts

Osteoblasts

bone building cells.
- synthesize and secrete collagen fibers and other organic
components
- Develop into Osteocyte

Osteocytes

mature osteoblasts
- Maintain bone tissue
- No longer secrete matrix

Osteoclasts

bone crushing cells.
- Originate from fused monocytes (type of WBCs)

The diaphysis

the shaft or body of a
long bone

epiphysis

form the distal and
proximal ends of a long bone

metaphysis

the areas where the
epiphysis and diaphysis join (slide 6 for pic)

epiphyseal plate

the end of active growth in the
epiphysis of the long bones
which contains hyaline cartilage
- The growth plate is always
actively dividing and causing The bone
to elongate from each
end.
- In adults, the epiphyseal cartilage is no
longer present and elongation of bones
has stopped.

Compact bone

Dense outer layer with few internal spaces
– Forms most of the shaft of long bones (arms, legs)
– Solid, hard, external layer of bone
– Resists stresses of weight & movement

periosteum (Dense CT)

Contains blood vessels, nerves, and cells involved in bone growth and repair
• Injury to the periosteum (bruise, fracture) generates pain
• It contains osteoblasts help the bone grow in thickness, but not in length.

osteon

The structural unit of compact bones -Consists of;
osteocytes (living bone cells) arranged in concentric rings (Lamellae) around a central canal .
Each osteocyte lies within a lacuna (small space within the hard matrix). Canaliculi Tiny canals
connect nearby lacunae and the central canal Nutrients, oxygen, and wastes pass from cell to
cell, traveling to and from the blood vessels in the central canal

Spongy bone

Latticework of bone
– Found in small, flat bones (skull) and in the head and near the ends of the shafts of long
bones
– In adults
• The spaces of some spongy bones are filled with red marrow where blood cells
production (Hematopoiesis) take place.
• The cavity in the shaft of long bones is filled with yellow marrow,
a fatty tissue for energy storage

medullary cavity

space within the
diaphysis of long bones that contains fatty
yellow bone marrow in adults.
• The endosteum is a membrane that lines the
medullary cavity .
• The endosteum is composed of osteoclasts,
osteoblasts, and connective tissue.

Bone is

25% water, 25% organic proteins, 50%
mineral salts Organic constituents
• Collagen fibers provide flexibility and tensile
strength.
• Mineral salts:
– Calcium Phosphate (Ca3PO4)2
– Calcium Carbonate (CaCO3 - marble)
– Other trace elements: magnesium, fluoride,
sulfate

The transformation from cartilage to a long
bone (slide 14)

egins when osteoblasts form a collar of bone
around the shaft of the cartilage model
– Osteoblasts then migrate to the bone cavity to form
spongy bone
– Cartilage cells within the growth plate divide, forcing
the end of the bone farther away from the shaft
– As bone replaces the newly formed cartilage in the
region closer to the shaft, the bone lengthens
– The bone diameter also enlarges as the bone
lengthens
• Two regions of cartilage remain at each end of
the long bone
– The cap that covers the surfaces that rub against
other bones
– The growth plate, also called the epiphyseal plate

Parathyroid hormone (PTH)

released from the parathyroid glands increases blood
calcium levels by
- release of calcium from bone.
-increase calcium reabsorption from kidney.
-stimulate kidney to produce active form of vit D which is important for
absorption of dietary calcium.
- increase calcium absorption from intestine

Calcitonin

released from the thyroid gland decreases blood calcium levels

Growth hormone

stimulates bone growth during childhood

Thyroid hormones

ensure that the skeleton grows with the proper proportions

Sex hormones

(testosterone and estrogen)
prompt and stop the growth spurt of puberty
By the end of the teenage years, sex hormones initiate less frequent cell division
– Growth plate thins
– Ends of the bone fuse with the shaft
– Bone can no longer increase in length

Bone fractures

When a bone breaks, bleeding occurs and a clot
forms
• Fibroblasts invade the clot and secrete collagen
fibers that form a callus linking the two parts of
the bone
• Osteoblasts transform this cartilage into bone

Bone remodeling

Bone remodeling repairs tiny cracks and regulates blood calcium levels
• Two antagonistic hormones are involved
– Calcitonin released from the thyroid gland decreases blood calcium levels
• Parathyroid hormone (PTH) released from the parathyroid glands increases blood calcium
levels
• In women, estrogen also plays a role in bone remodeling
– Enhances the absorption of calcium from the digestive system
– Stimulates the formation of bone
– Inhibits the breakdown of bone
• Osteoporosis results when bone is broken down faster than it is deposited

fontanel

Before and shortly after birth,
the bones of the cranium are
connected by membranous
areas called fontanels - gone by 2

Muscle Characteristics

Excitable (they respond to stimuli)
– Contractile (they can shorten)
– Extensible (they can stretch)
– Elastic (they can return to their original length after being shortened or stretched

Muscle Functions

Moving our body
– Maintaining posture
– Pushing against veins and lymphatic vessels to move blood and lymph along
– Generating heat

the names of the internal structures of the muscle fiber

Sarcolemma
• Sarcoplasm
• Myofibril
• T-tubules
• Sarcoplasmic reticulum
(Calcium storage)
• Sarcomere

Muscle fiber=

muscle cell

Sarcoplasmic reticulum

a form of smooth endoplasmic
reticulum found in muscle cells which
stores calcium ions.

Transverse tubules (T tubules)

Pockets in the plasma membrane of a
muscle cell
– Carry signals from motor neurons
deep into the muscle cell to every
sarcomere
Skeletal Muscle

Epimysium

Surrounding the entire muscle
• Outer surface grades into the fascia
• Inner surface sends projections between fascicles to form
perimysium

Perimysium

Wrap each fascicle
• carry larger nerves and blood vessels, and
stretch receptors

Endomysium

Loose
connectiv
e tissue
surroundi
ng each
muscle
fiber

Sarcomere

Actin filaments slide past
myosin filaments,
shortening the sarcomere. It
is covered by troponin-
tropomyosin complex.
– Myosin molecules are
shaped like two-headed golf
clubs. The club-shaped
myosin heads are key to
moving actin filaments

Muscle Contraction

When a muscle is
relaxed, the troponin-
tropomyosin complex
covers the actin-myosin
binding sites
• Muscle contraction
occurs when calcium
ions bind to troponin,
causing it to change
shape
• This change in shape
moves tropomyosin,
exposing the actin-
myosin binding sites

Synapse:

A junction between two nerve cells, consisting of a minute gap across
which impulses pass by diffusion of a neurotransmitter

ATP sources for muscle contraction

typically
used in sequence
– ATP stored in muscle cells
– Creatine phosphate stored in muscle cells
– Anaerobic metabolic pathways
– Aerobic respiration

Aerobic exercise

Enough oxygen is delivered to the muscles to keep them going for long periods
– Increases endurance and coordination
– Promotes development of new blood vessels
– Increases the number of mitochondria
– Typically does not increase size of muscles
– Examples: walking, jogging, swimmin

Resistance exercise

Builds strength
– Muscles increase in size when they are repeatedly made to exert more than
75% of their maximum force
– Increases in muscle size reflect increases in the diameter of existing muscle
cells
– Example: weight lifting