الجهاز العظمي عند الحيوان The skeletal system

The skeletal system can be divided into three parts:
1) The axial skeleton which includes the skull, hyoid bones,vertebra, ribs and sternum.
2) The apendicular skeleton which includes the limbs or legs.
3) The heterotopic or visceral which includes the os penis and os
cordis (a small amount of bone found in the bovine heart). Not all
species have visceral skeletons.
Bones are held together by connective tissue bands called ligaments.
Joints or articulations are the area where two bones meet.
The study of bones is termed osteology.
Bone is the second hardest substance in the body. It is living tissue
composed mostly of minerals (approximately 66%), but it also contains
cells, blood vessels, nerves, connective tissue and collagen.
The structural properties of bone are illustrated below.
One bone is soaked in a strong acid (on the right), the other bone is heated to a very high temperature
The bone soaked in acid is now totally flexible and can be "tied in a
knot." The mineral content of the bone has been dissolved in the acid
and only the connective tissue elements of the bone remain.
The bone heated to a very high temperature has turned to a pile of dust.
All that remains is the dry, hard minerals the soft tissue has been
burned away.
Parts of the bone (gross structure of bone)
1) Epiphysis or growth plate: Is an area of growing bone composed of cartilage. The cartilage will eventually ossify to bone.
2) Periosteium: Is a fibrous membrane surrounding the entire bone,
immature bone cells (osteoblasts) are located here. It is important in
growth and healing of bone.
3) Articular cartilage: Articular cartilage covers the articular
surfaces (where two bones meet) of bones, it is smooth and shiny.
Cartilage ( there are three types) is similar to tough connective
tissue. It is more flexible than bone and not as hard.
4) Cancellous spongy bone: Is usually located at the ends of long bones.
It is porous bone lined with red marrow, this sponge like network is
formed from spicules (thin plates of bones). Erythrocyte (red blood
cell) production occurs here.
5) Cortex or diaphysis: Is the dense, compact bone forming the "shaft"
of long bones. It is composed of thousands of microscopic tubules,
inside some of the larger tubules are blood vessels and nerves. This
type of bone "grows" outward as more tubules are laid down to increase
the diameter of the bone.
6) Medullary cavity or marrow: Is the center of the bone. Red blood
cells (erythrocytes) are also produced here. As animals age the "red
marrow" changes to fatty, yellow marrow. Blood cells are produced in the
marrow of long bones until animals reach maturity, after maturity other
parts of the skeletal system, mostly the ribs and sternum, produce
erythrocytes.
Endosteum: Is similar to periosteum but lines the medullary cavity.

A section of bone shaft is enlarged to show the microscopic tubules
which compose the bone shaft.
The above diagram shows the structure of the bone shaft or cortex. The
strength of the shaft comes from concentric layers of tubules running
the length of the shaft, these tubules are made of bone cells. Just
like other tissue there is a turn over in bone cells with new cells
forming and a very slow replacement of old cells.
The tissue and cells described below make up the microscopic structure
of the bone, the type of cell can not be identified from the "rough"
diagram above.
a) periosteum (see above for definition)
b) endosteum (see above for definition)
c) osteoblast: These are precursors to the mature bone cells, the
osteocytes. They secrete an essential enzyme in the mineralization
process, called phosphatase.
d) osteoclast: These are special giant multinucleated bone cells that
break down the mineralized bone by secretion of destructive enzymes.
There are 3 main types of bone cells:
Osteoblast: Osteoblast are young, immature cells that can divide and will become mature bone cells with time.
Osteocytes: Osteocytes are mature bone cells which make up most of the skeletal system.
Osteoclast: Osteoclasts are specialized bone cells that function to lyse
or breakdown osteocytes and release calcium into the body, when calcium
is needed.
Formation of bone
There are several types of bone formation. Below is a simplified
explanation of cartilage or fibrous membrane ossification/formation.
1) The osteoblasts (immature cells that forms bone) secrete osteoid a
"liquid" form of calcium, plus other minerals (the gray droplets). These
minerals are precursors to solid bone.
2) After osteoid is secreted, then phosphatase (an enzyme needed for the
conversion of the liquid to a solid) is also secreted to "mineralize"
the osteoid. Bone is then formed (the dark gray area surrounding the
cell) .
Most of the structure of bone is mineral, about 66% calcium and
phosphorus. The newly formed bone now surrounds a single osteocyte (the
osteoblast has now matured and is termed an osteocyte)
Cytoplasmic extensions (a) connecting mature bone cells (b (osteocytes). The actual bone is gray in color (c).
3) The osteoblasts that secreted osteoid and phosphatase are now
"trapped" between large deposits of solid bone. The cells are connected
to each other by a network of cytoplasmic extensions (a). Cells receive
nutrition through these channels. These osteoblasts will mature to form
osteocytes
Metabolic and hormonal influences on bone growth and homeostasis:
Like all body systems, there is a delicate balance of hormones, vitamins
and minerals needed to maintain homeostasis of the skeletal system.
Below is a brief explanation of the major hormones and nutrients involved in the skeletal system.
Hormones will be covered in depth later during the endocrine system.
Somatotropin (growth hormone): Is secreted from the anterior pituitary.
It is essential for the growth of bones in young animals.
Parathyroid hormone: Is secreted by the parathyroid glands, and
stimulates the reabsorption of bone and release of calcium into the
blood stream.
Vitamin D: This vitamin is needed for absorption of calcium from the diet (digestive system).

Arthrology
The study of joints or articulations is termed arthrology.
The bones of the body are held together at joints or articulations. These range from the tiny "sutures" of the skull bones
(which are actually held together by connective tissue early in life, but ossify in the adult.) To the largest type joint,
the ball and socket which joins the axial and apendicular skeletons.
Types of joints
1) Fibrous: Bones are joined by fibrous connective tissue
2) Cartilaginous: There is no joint cavity, bones are united by cartilage
3) Synovial: The "true joints" which have a joint cavity.
Parts and functions of a complex synovial joint, the stifle.
Stifle joint of dog (termed the knee joint in humans)
) Trochlear groove: The deep groove in the anterior distal femur where the patella and patellar ligaments are located.
2 & 7) Cruciate ligaments: The two tough connective tissue bands
(anterior and posterior) which hold the bones together and cross inside
the joint.
3 & 8) Collateral ligaments: Two connective tissue bands (lateral
and medial) they also hold bones together outside the capsule.
4) Tibial crest: Part of proximal tibia where ligaments attach.
5) Fibula
6) Meniscus: The two small half moon shaped fibrous cartilage plates wedged between the bones which act to cushion the joint.
9) Patella: The "knee cap" which is a small bone embedded in the
patellar ligament (extension of the tendon of the quadriceps muscle of
the upper leg). It moves over the femorotibial joint with flexion of the
stifle.

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