Biologists often encounter mind-bending expansion in complexity the closer they look into the details. Notable examples include the length of DNA strands in each human cell (2-3 meters per cell, and about 20 billion kilometers of DNA in an adult), the surface area of the lung in humans (50-100 square meters, the size of a tennis court), or the complex network of the human brain which is responsible for all thought and action (about 86 billion neuronal connections in an average 1.2 liter brain). Recently Dr. Natalie Sims and Dr. Pascal Buenzli found that one type of cell within the skeleton forms a network of cellular junctions nearly as complex as the human neuronal network with an estimated 42 billion connections!
Bone tissue contains many types of cells that are responsible for maintaining the structural integrity of the skeleton, healing fractures and daily microdamage, and even creating new blood cells. Osteoblasts are bone-forming cells that secrete proteins that bind mineral and become new bone. Osteoblasts eventually lay down enough protein to entomb themselves within the skeleton where they reside long-term. These entombed osteoblasts are referred to as osteocytes and were previously thought to have no active function.
As imaging and optics technology advanced, bone researchers later discovered that these osteocytes are actually surrounded by a dense network of incredibly small branched dendritic tubes (see figure above comparing neural and osteocyte branches). Scientists assumed that these processes were responsible for sensing loads within the skeleton by turning mechanical stimuli into chemical signals to other bone cells. This mechanosensing ability drives skeletal adaptation to load, referred to as Wolff’s Law, and is responsible for decreased bone mass in cosmonauts who undergo long periods of weightlessness and increased bone mass in ‘big boned’ individuals and athletes.
In addition to the massive number of cell to cell connections, Dr. Sims and Dr. Buenzli also estimated the surface area of the walls of the canaliculi to be 215 square meters, larger than the combined surface area of the lungs, intestines, and skin. This enormous area suggests that osteocytes may be a significant endocrine organ. This has led researchers to believe that osteocytes may also be responsible for retention of calcium and phosphate in the kidney, control of blood cell production within bone marrow, glucose metabolism, and lactation. The osteocyte is not inactive at all!
Peer edited by Gar Yeung