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Bones are the main skeletal elements; the skeleton also contains cartilages
Skeletal cartilages: the few cartilages that remain in adults are found mainly in regions where flexible skeletal tissue is needed. A skeletal cartilage is made of some variety of cartilage tissue, which contains primarily water. The high water content accounts for its resilience, that is its ability to spring back to its original shape after being compressed. The cartilage contains no nerves or blood vessels.
It is surrounded by a layer of dense irregular connective tissue, the perichondrium, which acts as a girdle to resist outward expansion when the cartilage is compressed. The perichondrium also contains the blood vessels from which the nutrients diffuse through its matrix to reach the cartilage cells. This mode of nutrient delivery limits the cartilage thickness.
All 3 types of cartilage tissue have the same basic components: cells (condrocytes) encased in small cavities (lacunae) and extracellular matrix formed by jelly-like ground substance and fibers.
Hyaline cartilages provide support with flexibility and resilience; they are the most abundant type of cartilage. Chondrocytes are spherical and they amount for only 1 to 10% of the cartilage volume; they have only collagen fibers in their matrix. They include the following subtypes:
Function: Supports and reinforces; has resilient cushioning properties; resists compressive stress.
a) articular: cover the ends of most bones at movable joints; they absorb compression at joints.
b) costal: they connect the ribs to the sternum.
c) respiratory: form the skeleton of the larynx, and reinforce other respiratory passageways.
d) nasal: support the external nose.
Description: Similar to hyaline cartilage, but more elastic fibers
Function: Maintains the shape of a structure while allowing great flexibility.
Fibrocartilages: they are highly compressible and have great tensile strength. They are the intermediate between hyaline and elastic cartilages. They consist of roughly parallel rows of chondrocytes alternating with thick collagen fibers. They are present in zones of the body subjected to both heavy pressure and stretch as the menisci of the knee and the discs between vertebrae (resilent cushions between the bony vertebrae).
Description: Matrix similar to but less firm than that in hyaline cartilage; thick collagen fibers predominate.
Function: Tensile strength with the ability to absorb compressive shock.
Location: Intervertebral discs; pubic symphysis; discs of knee joint.
Because they are formed by various types of tissues, bones are organs. Osseus (bone) tissue dominates, and they also contain nervous tissue (their nerves), cartilage tissue in their articular cartilages, fibrous connective tissue lining their cavities, and muscle and epithelial tissues in their blood vessels.
Gross anatomy: each bone has bone markings that serve as sites of muscle, ligament, and tendon attachments, as joint surfaces, or as conduits for blood vessels and nerves. Bones are classified by their shape in
- long; - short; - flat; - irregular
Bone textures: every bone has a dense outer layer that looks smooth and solid, which has been named compact bone. Internal to this part is spongy bone, a honeycomb of flat pieces called trabeculae.
Diaphysis: forms the long axis of the bone, its shaft, and is formed by compact bone that surrounds a central medullary cavity, which in adults contains fat tissue (also known as yellow marrow).
Epiphysis: are the bone ends, which are usually more expanded than the diaphysis. Their interior contains spongy bone. The joint surface of each epiphysis is covered with a thin layer of hyaline cartilage, which cushions the opposing bone ends during joint movement and absorbs mechanical stress.
It is secured to the underlying bone by perforating (Sharpey’s) fibers. The periosteum also provides anchoring points for tendons and ligaments. Internal bone surfaces are covered by delicate connective tissue membranes called endosteum, which covers the trabeculae of spongy bone and lines the canals that pass through the compact bone. The endosteum contains both osteoblasts and osteoclasts.
Structure of the rest of bone types they all consist of thin plates of periosteum-covered compact bone on their outside, and endosteum-covered spongy bone within. They are not cylindrical, they do not have a shaft nor they present epiphyses. Also there is no significant medullary (marrow) cavity present.
In bone tissue there are several cell types surrounded by extracellular matrix of their making
- osteogenic cells (actively mitotic stem cell in the periosteum and the endosteum).
- osteoblasts (bone forming cells)
- osteoclast (cells that destroy bone tissue)
Osteogenic cell: Stem cell
Osteblast: Matrix-synthesizing cell responsible for bone growth
Osteocyte: Mature bone cell that maintains the bone matrix
Osteoclast: one-resorbing cell
Running through the core of each osteon is the central canal containing small blood vessels and nerves (Fig. 6.7a p 181). Perforating canals lie at right angles to the long axis of the bone and connect the blood and nerve supply of the periosteum to those of the central canals and the medullary cavity. All these canals are lined with endosteum.
- maintain the bone matrix; if they die the surrounding matrix is resorpted;
- act as stress or strain sensors in cases of bone deformation or other damaging stimuli. They communicate with osteoblasts and osteoclasts which produce bone remodeling.
Not all the lamellae in compact bone are part of osteons. Lying between intact osteons are interstitial lamellae. They either fill the gaps between forming osteons or are remnants of osteons that have been cut through by bone remodeling.
Circumferential lamellae are located just deep to the periosteum and just superficial to the endosteum. They extend around the entire circumference of the diaphysis and collaborate to effectively resist twisting of the long bone.
It looks like a poorly organized tissue. However, the trabeculae align precisely along lines of stress and help the bone resist stress. Only a few cells thick, trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi. No osteons are present. Nutrients reach the osteocytes by diffusing through the canliculi from capillaries in the endosteum surrounding the trabeculae.
Chemical composition of bone.
Bone has organic and inorganic components. Its organic components include all its cell types, and a substance named osteoid, the organic part of the matrix. Osteoid makes up approximately 1/3 of the matrix, and includes ground substance and collagen fibers, both of which are made and secreted by osteoblasts. Particularly collagen contributes not only to the bone structure but also to the flexibility and great tensile strength that allow the bone to resist stretch and twisting.
Most of bone tissue (65% by mass) consists mainly of calcium phosphates present in the form of tiny, tightly packed crystals in and around the collagen fibers in the extracellular matrix. These crystals account for the most notable characteristic of bone: its exceptional hardness, which allows it to resist compression.
The proper combination of organic and inorganic matrix elements allows bones to be exceedingly durable and strong without been brittle. Healthy bone is half as strong as steel in resisting compression and fully as strong as steel in resisting tension.
This is important because when bone remains in place for long periods the calcium crystallizes and becomes more brittle, condition that facilitates fracture, the most common disorder of bone homeostasis.
Bone remodeling: In the adult skeleton, bone deposit and bone resorption occur both at the surface of the periosteum and of the endosteum. These 2 processes constitute bone remodeling, and they are coupled and coordinated by groups of adjacent osteoblasts and osteoclasts called remodeling units, with help from the stress-sensing osteocytes.
In healthy young adults, total bone mass remains constant, an indication that the rates of bone deposit and resorption are essentially equal. However, remodeling does not occur uniformly. For example, the distal part of the femur is fully replaced every 5-6 months, whereas its shaft is altered much more slowly.
Bone deposit occurs whenever bone is injured or added bone strength is required. For optimal bone deposit, a healthy diet rich in proteins, vitamins C, D, and A, together with Ca, P, Mg, Mn (to name a few minerals) is essential. New osteoid deposits by osteocytes are marked by the presence of an osteoid seam, an unmineralized band of bone matrix 10-12 micrometers wide. Between the osteoid seam and the older mineralized bone there is an abrupt transition called the calcification front.
Bone resorption is accomplished by osteoclasts that are giant multinucleate cells that arise from the same hematopoietic stem cells that differentiate into macrophages. Osteoclasts move along a bone surface, digging grooves as they break down the bone matrix. The part of the osteoclast that touches the bone is highly folded to form a ruffled membrane that binds tightly to the bone, sealing off the area of bone destruction. This ruffled border secretes:
- lysosomal enzymes, which digest the organic matrix;
- hydrochloric acid, that converts the Ca salts into soluble forms that pass easily into aqueous solution.Osteoclasts may also phagocytize the demineralized matrix and dead osteocytes. The digested matrix end products, growth factors, and dissolved minerals are then endocytozed, transported across the osteoclast, and released at the opposite site where they enter first the interstitial fluid and then the blood
The remodeling process that goes continuously in the skeleton is regulated by 2 control mechanisms:
- negative feedback hormonal mechanism that maintains the homeostasis of calcium ion concentration in the blood;
- response to mechanical and gravitational forces acting on the skeleton.
To understand the hormonal mechanism lets mention some of the physiological processes in which Ca ions play a role: - muscle contraction; - synapses; blood coagulation; secretion by glands and nerve cells; cell division (cytokinesis).
Less than 1.5 g is present in the blood, and the hormonal loop normally maintains blood Ca++ levels within the very narrow of 9-11 mg/100 mL of blood. Calcium is absorbed from the intestine under the control of vitamin D metabolites.
As blood Ca rises, the stimulus for PTH release ends, and the blood Ca++ concentration falls. The effects of calcitonin in adults appear to be neglible.Hormonal controls serve to maintain blood Ca++ homeostasis. Thus, bones serve as a storehouse from with Ca ions are drawn as needed
. Bending compresses the bone on one side and subjects it to tension (stretching) on the other .Both forces are minimal toward the center of the bone since they cancel each other because they are of equal magnitude and are in opposite directions. This is why a bone has spongy bone tissue instead of compact bone, gaining lightness, without jeopardy.
The mechanisms by which bone responds to mechanical stimuli are still uncertain, but it is known that deforming a bone produces an electrical current.
Conclusion: the skeleton is continuously subjected to both hormonal influences and mechanical forces. It can be speculated that:
hormonal control mechanism determines whether and when remodeling occurs in response to changing blood Ca concentration ;
- mechanical stress determines where it occurs; this stress determines which osteoclasts are most sensitive to PTH stimulation, so that bone in the least stressed areas (which is temporarily dispensable) is broken down.
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