The Questions of Developmental Biology

13. In the regenerating salamander limb, the epidermis forms an apical ectodermal cap. The cells
beneath it dedifferentiate to form a blastema. The differentiated cells lose their adhesions and reenter
the cell cycle. This does not happen in mammals.
14. In hydras, there appear to be head activation gradients, head inhibition gradients, foot
activation gradients, and foot inhibition gradients. Hydra budding occurs where these gradients
are minimal.
15. In mammals, medical researchers are testing whether paracrine factors may permit local
regeneration. Bone and neural cells are being returned to embryonic conditions in the hopes that
they will regrow. Natural inhibitors of neural regeneration have recently been discovered, and
their circumvention may allow spinal cord regeneration.
16. The maximum life span of a species is how long its longest observed member has lived. It is
largely characteristic of a given species. Life expectancy is the time at which approximately 50
percent of the members of a given population of a species still survive.
17. There are several levels at which we can study aging, including cellular, biochemical, and
genetic studies. Reactive oxygen species (ROS) can damage cell membranes, inactivate proteins,
and mutate DNA. Mutations that alter the ability to make or degrade ROS can change the life
span of the mutants.
18. Mitochondria may be a target for proteins that regulate aging.
19. Aging is the time-related deterioration of the physiological functions necessary for survival
and reproduction. The phenotypic changes of senescence (which affect all members of the
species) are not to be confused with diseases of senescence, such as cancer and heart disease
(which affect individuals).
*You can see why the funding of Social Security is problematic in the United States. When it was created in 1935, the
average working citizen died before age 65. Thus, he (and it usually was a he) was not expected to get back what he
had paid into the system. Similarly, marriage "until death do us part" was an easier feat when death occurred in the
third or fourth decade of life. The death rate of young women due to infections associated with childbirth was high
throughout the world before antibiotics.