The hierarchy of organ, tissue, cell in the human body

As another new semester begins, what better place to begin than the hierarchy of structure in the human body — organ systems, organs, tissues and cells. I hope this doesn’t sound old-fashioned, but it’s a strict hierarchy!

It sounds easy enough, but it’s easy to lose your bearings when you hear statements like “bone tissue has a lot of blood vessels”.  Sure, that’s true — and that’s why bone heals faster than, say, cartilage.  But this does not mean that blood vessels are a part of the bone tissue.  Blood vessels, in fact, are made of tissues like muscle, connective tissue, epithelium. So, if a blood vessel were a part of the bone tissue, then you’d have one tissue that’s part of another tissue — bad news for our hierarchy!

Osteoblast settles down and changes her last name

Many people these days are aware that their bones are in a constant state of flux, and that’s just one more reason to get some exercise — the mechanical stresses tell the cells in your bones to add more bone matrix (the nonliving material between the cells — mostly the hard mineral calcium phosphate, but also collagen and other ingredients).

There’s an important transition in the life of your bone cells. The early, “young and restless” cells that travel around and secrete bone matrix, sowing their wild oats, so to speak, and perhaps “having a blast”, are conveniently known as osteoblasts.   Gradually, most of the space gets filled in with hard matrix, and eventually the osteoblasts become trapped in their own secretions.  Once the cell is forced to settle down “on site” in this way, it is (again conveniently) known as an osteocyte.

The metaphor of “Mrs. Osteocyte” having “changed her last name” from “blast” to “cyte” was too perfect to resist, although I must point out that the metaphor relies on gender stereotypes that are rapidly becoming obsolete.

Anyway, Mrs. Osteocyte has plenty of work left to do and is not at all isolated, as it might seem. She maintains a firm grip on all her neighbors, sharing nutrients, wastes and chemical signals with them, directly through gap junctions. Indeed, osteocytes play an important role in detecting the mechanical forces that act on the bone, and secreting signals that direct the activity of osteoblasts. So, in effect the older generation still calls the shots!