Basic multicellular units

Under normal circumstances, the skeleton undergoes a dynamic process of bone remodeling. Bone tissue responds to stress and injury through continuous replacement and repair. This process is completed by the basic multicellular unit, which includes both osteoblasts and osteoclasts. Osteoclasts are involved with resorption or breakdown of bone and continuously create microscopic cavities in bone tissue. Osteoblasts are involved in bone formation and continuously mineralize new bone in the cavities created by osteoclasts. Until peak bone mass is achieved between the ages of 25 and 35, bone formation exceeds bone resorption for an overall increase in bone mass. Trabecular bone is more susceptible to bone remodeling in part owing to its larger surface area. 

Jilka RL (2003) Biology of the basic multicellular unit and the pathophysiology of osteoporosis. Med Pediatr Oncol 41 182-185... 

The osteoblast and osteoclast can be considered to be the basic multicellular units of bone. The osteoblast plays an important role in mediating local osteoclast activity through the release of chemical messengers. The principal factors responsible for stimulation of bone resorption, such as parathyroid hormone, interleukin-1 (Il-l), and IL-6, have minimal effects on osteoclasts, but osteoblasts have receptors for these substances. 

The basic unit of life is the cell and the simplest living sytems are single cells that possess all the above capabilities, drawing their building materials from simple chemical substances in their, generally aqueous, environment. More complex life forms, for example. Homo sapiens, are multicellular organisms in which every cell does not possess all of the seven pillars of life and life is possible only because the cells form a society whose health is dependent on the integrated activities of the different cell types within the system. In the human body there are some 10 such cells. 

Cells are the fundamental units of life. They are functional entities, each of which is enclosed in a semipermeable membrane that varies in composition and function both over a single cell surface and between different cell types. There are two basic forms of cell prokaryotic and eukaryotic. Prokaryotes are most noted for their small sizes and relatively simple structures. Presumably because of these traits, in addition to their remarkably rapid reproduction rates and biochemical diversity, various prokaryotic species occupy virtually every ecological niche in the biosphere. In contrast, the most conspicuous feature of the eukaryotes is their extraordinarily complex internal structure. Because eukaryotes carry out their various metabolic functions in a variety of membrane-bound organelles, they are capable of a more sophisticated intracellular metabolism. The diverse metabolic regulatory mechanisms made possible by this complexity promote two important lifestyle features required by multicellular organisms cell specialization and intercellular cooperation. Consequently, it is not surprising that the majority of eukaryotes are multicellular organisms composed of numerous types of specialized cells. 

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