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Bone Biology: Why It Matters

Histotechnologists work on all types of tissue depending on the type of lab they work in and it is not all soft organ tissue. Many histologists also work with bone, the hard connective tissue made up of calcium ions, phosphate ions, hydroxyl ions and other minerals. Different types of bones have different functions. The long bones in our limbs are essential to the support of our muscles, while flat bones such as the ribs serve as protection for vital organs. Bones also assist in the production of blood cells, they help regulate the body’s pH and release minerals into the bloodstream as needed.


Structurally, there are two types of bone, cortical and cancellous. Cortical bone is compact and dense. It is the strongest type of bone and it makes up the outer layer of bone. Cancellous bone is on the inside of the bone and can also be found near joints at the end of long bones. It is referred to as spongy bone because it is less dense and more flexible than cortical bone with a lattice like structure.


Microscopically, the two types of bone are woven and lamellar. Woven bone is immature bone, so embryonic bone or bone in fracture healing. It is also the type of bone seen in bone tumors. Cell distribution and collagen fibers in woven bone are random, but once woven bone is replaced by lamellar bone, the collagen fibers are remodeled into a parallel layer structure.


Bones are made up of three major types of cells, osteoblasts, osteocytes, and osteoclasts. Osteoblasts are bone forming cells. They make osteoid, a gelatinous substance made primarily of collagen, protein, and what is basically an organic glue material. Osteoid is unmineralized when produced but hydroxyapatite, a calcium phosphate, is soon deposited to form mature bone. Osteoblast cells are either in a resting or an active state. As you may have guessed, active osteoblast cells are actively engaged in bone formation. They have a large easily recognized nucleus and are cuboidal in shape. Resting osteoblasts on the other hand, have a flattened, elongated shape and do not have an easily recognizable nucleus.


Some of the osteoblasts get trapped on the inside of this mineralized matrix and become osteocytes. They remain inside of the mature bone tissue and can live as long as the bone they occupy. They have a star-shaped morphology.


Osteoclasts are bone destroying cells, varying in size and shape, which have more than one nucleus. Like osteoblasts, they can either be active or inactive. Bone destroying may sound bad, but bone is actually constantly remodeling. Osteoclasts are formed from osteoclast precursors that meet up at a site where bone resorption is going to take place, and they fuse together to create the multi-nucleus osteoclast. Bone resorption, the osteoclast’s bone destroying process is essential as it releases minerals like calcium into the blood stream.


You can recognize types of osteoblasts, osteocytes, and osteoclasts histologically by morphological presentation, but you can also use immunohistochemical markers. For example, CD73 will tag active osteoblasts and Calcitonin is involved in osteoblast formation. CD44 is a general osteocyte marker while Cathepsin K is a general osteoclast marker.


So, why do we, as histotechs, care about what makes up bone? What you’re investigating will determine whether or not you need decalcified or un-decalcified bone sections. Depending on what is being investigated, it may be necessary, such as when looking at metabolic bone diseases like osteoporosis or rickets, to differentiate mineralized bone from osteoid, which would require an undecalcified section. If you’re looking at bone marrow or tumors however, you will want decalcified sections. The amount of cortical and cancellous bone you have in your specimen are going to impact the time required for decalcification and processing. Cortical bone is going to take longer to decalcify (remember it’s the dense one). Good fixation is always important, but particularly if you need to preserve the structure of the bone marrow, which may mean using something other than buffered formalin, such as a zinc formalin.


Want to learn more about bone histology? Check out Bone Biology 101: A Primer for the Histotechnologist, by Damien Laudier, one of the sessions from the 2020 Virtual Convention now available on NSH’s online learning center.


References:

https://www.histology.leeds.ac.uk/bone/bone.php

https://www.britannica.com/science/bone-formation#ref945369

https://www.britannica.com/science/osteocyte

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2856465/#:~:text=Osteoclasts%20are%20the%20cells%20that,formation%20in%20the%20bone%20marrow.

https://www.leicabiosystems.com/knowledge-pathway/an-introduction-to-decalcification/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2919071/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5467150/

https://www.ncbi.nlm.nih.gov/books/NBK10056/

https://www.britannica.com/science/metabolic-bone-disease#ref1118962

https://www.ncbi.nlm.nih.gov/books/NBK279149/

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