Essentials of Anatomy and Physiology

some number is maintained in the cells of the new
individual.

AGING AND CELLS

Multicellular organisms, including people, age and
eventually die; our cells do not have infinite life spans.
It has been proposed that some cells capable of mito-
sis are limited to a certain number of divisions; that is,
every division is sort of a tick-tock off a biological
clock. We do not yet know exactly what this cellular
biological clock is. There is evidence that the ends of
chromosomes, called telomeres, may be an aspect of it.
With each cell division, part of the telomeres is lost
(rather like a piece of rope fraying at both ends), and
eventually the telomeres are gone. With the next divi-
sion, the ends of the chromosomes, actual genes,
begin to be lost. This may be one signal that a cell’s life
span has come to an end (there are probably many dif-
ferent kinds of signals).
Cellular aging also involves the inevitable deterio-
ration of membranes and cell organelles. Just as the
parts of a car break down in time, so too will cells.
Unlike cars or machines, however, cells can often
repair themselves, but they do have limits. As cells
age, structural proteins break down and are not
replaced, or necessary enzymes are not synthesized.
Proteins called chaperones, which are responsible

for the proper folding of many other proteins and

for the repair or disposal of damaged proteins, no

longer function as well as cells age. Without chaper-

ones, damaged proteins accumulate within cells and

disrupt normal cellular processes. Clinical manifesta-

tions of impaired chaperones include cataracts (see

Box 9–1) and neurodegenerative diseases such as

Alzheimer’s disease (see Box 8–6), Parkinson’s dis-

ease (see Box 8–7), and Huntington’s disease (see

Table 21–4).

Much about the chemistry of the aging process

remains a mystery, though we can describe what hap-

pens to organs and to the body as a whole. Each of the

following chapters on body systems provides a brief

discussion of how aging affects the system. Keep in

mind that a system is the sum of its cells, in tissues and

organs, and that all aging is ultimately at the cellular

level.

SUMMARY

As mentioned at the beginning of this chapter, human

cells work closely together and function interdepend-

ently. Each type of human cell makes a contribution to

the body as a whole. Usually, however, cells do not

function as individuals, but rather in groups. Groups

of cells with similar structure and function form a tis-

sue, which is the next level of organization.

Cells 63

Table 3–4 STAGES OF MITOSIS

Stage Events

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

1. The chromosomes coil up and become visible as short rods. Each chromosome is really two
   chromatids (original DNA plus its copy) still attached at a region called the centromere.


2. The nuclear membrane disappears.


3. The centrioles move toward opposite poles of the cell and organize the spindle fibers,
   which extend across the equator of the cell.


4. The pairs of chromatids line up along the equator of the cell. The centromere of each pair
   is attached to a spindle fiber.


5. The centromeres now divide.


6. Each chromatid is now considered a separate chromosome; there are two complete and
   separate sets.


7. The spindle fibers contract and pull the chromosomes, one set toward each pole of the cell.


8. The sets of chromosomes reach the poles of the cell and become indistinct as their DNA
   uncoils to form chromatin.


9. A nuclear membrane re-forms around each set of chromosomes.


10. The cytoplasm divides; new cell membrane is formed.