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An organization's structure divides it into smaller units that allow the units to specialize in something and produce the desired product or service. The structure should specify how units reports to other units or people, so there can be overall management and leadership. Structure thus breaks down and it brings it back together so that the mission and programs can be accomplished.
cells are units of structure
Cells.
Vertebrae
database configuration monitoring, DBMS hardware configuration (a DBMS and related database may span computers, networks, and storage units) and related database mapping
Neurons
Vertebrae
neurons
While you will not be responsible for all units, it is important that you work in conjunction with your neighbors to make sure that you do not spread infestations back and forth among yourselves.
Density does not have a structure, it is just a measurement of grams per cubic centimeter (or whatever units you prefer).
Tissues
No, they do not. They are basic units of structure such as in muscles.