How is a tissue built?

Tissues change in many ways in the period that they are part of a living or ganism. They are created in fairly repeatable structural patterns, and we k now that the patterns are due to both the genes and the (mechanical) enviro nment, but we do not know exactly what part or percentage of a particular p attern to consider the genes, or the environment, responsible for. We do no t know much about the beginning of tissue construction (morphogenesis) and we do not know the methods of tissue construction. When the tissue structur e is altered to accommodate a new loading, we do not know how the decision is made for the structural reconstruction. We do know that tissues grow or reconstruct themselves without ceasing to continue with their structural fu nction, but we do not understand the processes that permit them to accompli sh this. Tissues change their structures to altered mechanical environments , but we are not sure how. Tissues heal them selves and we understand littl e of the structural mechanics of the process. With the objective of describ ing the interesting unsolved mechanics problems associated with these biolo gical processes, some aspects of the formation, growth, and adaptation of l iving tissues are reviewed. The emphasis is on ideas and models. Beyond the objective is the hope that the work will stimulate new ideas and new obser vations in the mechanical and chemical aspects of developmental biology. [S 0148-0731 (00)00106-0].