Home to connective tissues!
As the name implies, supportive connective tissues are strong and durable tissues that serve as supportive structures for other types. Most of the supportive elements of your body are bones! The skeleton, in serving as the anchoring points for muscles and tendons, enables not only support, but movement. In addition, bones are protective structures for critical organs like the brain, spinal cord, heart and lungs. Bone as we know, is very strong and durable but relatively inflexible.
Now, think of your ears, nose, trachea, and parts of your thoracic cage where support is important but flexibility is equally so. Consider what would happen if ears were supported by bone. The mere act of bumping into a door frame or catching an ear on an object in our environment would sometimes result in a lost ear! For this reason, cartilage is the supportive tissue used where durability and flexibility are specific needs. Cartilage is also found covering the ends of long bones in the synovial joints as articular cartilages. Here, the smooth and slippery texture of cartilage helps minimize friction in joints. Supportive connective tissues then, include various types of cartilage and bone.
All connective tissues contain protein fibers but in supportive connective tissues these fibers are usually hidden by the semisolid or solid ground substances within the matrix. Therefore, in learning to identify cartilage and bone as connective tissues one has to use other criteria besides the presence of visible fibers. In cartilage and bone, one can always see lacunae!
Here is an example of cartilage with lacunae labelled!
Lacunae occur in cartilage and bone because gelatinous or solid ground substances do not flow around cells like fluids. After the extracellular matrix of cartilage and bone is formed, cells become less active metabolically and probably shrink in size. The spaces remaining around them become visible as the lacunae. In bone, lacunae are inter-connected via minute channels called canaliculi. Osteocytes link to one another via cytoplasmic extensions through canaliculi to provide an avenue for diffusion of nutrients and wastes. Canaliculi of bone are essential for osteocytes because diffusion cannot occur across crystalline solids.
In this view of bone, note the canaliculi linking lacunae!
Connective Tissue Growth
Connective tissues grow in two ways, by interstitial or appositional growth. The term interstitial implies growth takes place from within the tissue as cells divide by mitosis. Appositional growth on the other hand, implies a tissue grows by adding more cells or materials to its surface.
Consider now how crystals enlarge, by deposition of solids on pre-formed crystal complexes. Bone is essentially a crystal so growth involves formation of new layers, called lamellae, over pre-existing layers. Bone enlargement then, is always a product of appositional growth.
Here is a closeup view of lamellae in compact bone!
Bone Formation
Bone always forms within one of two pre-existing connective tissues, either mesenchyme or cartilage. Mesenchyme is the embryonic connective tissue most often viewed in slides of the umbilical cord.
Here is a view of mesenchyme. This looks like a young, undifferentiated tissue! Notice the very fine protein fibers.
In the cranium and a few other locales flat bones form within mesenchyme tissues in a process called intramembranous bone formation. In most other locales where long bones develop, cartilage models of bone are the first structural elements to form. Thereafter, bone replaces this cartilage in a process called endochondral bone formation. Epiphyseal(growth) plates of long bones persist as remnants of the original cartilage and these continue to expand via interstitial growth. However, bone replacement balances this cartilage growth nicely until puberty when hormonal changes occur. This results in a loss of the cartilage in these epiphyseal plates and this terminates longitudinal growth of bones at this time. So, try to remember, bone growth and bone formation are two different but related concepts! Lengthening of long bones is really a product of interstitial growth of cartilage in epiphyseal plates and replacement of this cartilage by endochondral bone formation.
After growth in height ceases, bones can still be strengthened or repaired because mesenchyme cells are found in the periosteal and endosteal membranes. When these mesenchyme cells are stimulated to form osteoblasts, new layers of bone are built on pre-existing layers via appositional growth. Since this bone growth is a product of mesenchyme, it is more accurately intramembranous bone formation. Again, bone growth and bone formation are two slightly different concepts!
Take a look at bone forming within mesenchyme via intramembranous bone formation. Remember, bone has lacunae! Can you see these in the new bone? Also, bone forming cells are usually found along the edges where new layers are formed, can you see these?
Now, compare the intramembranous bone formation to this slide where cartilage is being replaced by bone in a growth plate via endochondral bone formation.
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