BODY CELLS
A good deal has been said thus far about living cells without anything
at all having been said to tell what they look like, or how they are
made up, beyond the statement that they consist of living protoplasm,
which is of a jellylike consistency. To look at living cells through a
microscope would almost surely be a disappointment at first, for
protoplasm is so transparent that not much of its form can be seen on
direct inspection. Fortunately for our knowledge of how cells are made
up, protoplasm that has been properly killed and preserved takes stain
very well, and different chemical substances in the protoplasm stain
differently. Thus features that could not be made out at all in the
living cells become clearly visible after killing and staining. The
first thing that attracts the attention when cells thus prepared are
studied is that every cell has somewhere within it, and usually near its
middle, a spot which is more deeply stained than any other part of the
cell. This indicates the presence of a substance or substances that take
stain more readily than the mass of the protoplasm. This peculiarity led
to the naming of the deeply staining portion of the protoplasm
_chromatin_, referring to the ease of staining. The part of the cell
which contains chromatin is called the _nucleus_. In many kinds of cells
the nucleus can be made out by an expert observer without resorting to
stains, although the details of structure cannot be seen in that way.
[Illustration: NUCLEUS OF A CELL, SHOWING CHROMATIN NETWORK
(After Martin’s “Human Body”)]
We now know that the nucleus, or rather the chromatin that it contains,
plays a remarkable and interesting rôle in the life of the cell. To this
we shall return presently. The remainder of the protoplasm, outside of
the nucleus, shows the greatest possible variety of form, according to
the kind of cell at which we happen to be looking. In some of the
simpler types this part of the protoplasm seems to be merely a nearly
uniform mass, perhaps with tiny particles scattered through it. In other
types the protoplasm is drawn out into long slender threads, and these
threads may have many branches; or the protoplasm may be distorted into
a thin shell inclosing a mass of fat; or it may be subdivided into dense
and thin portions with sharp lines of division between them. These
various forms are related to the special functions which the cells have,
and we shall learn more about them as we take up the different functions
in order. On the whole, study of cell structure shows clearly that the
protoplasm outside the nucleus carries on the greater part of the
metabolism or power development, and is correspondingly important as the
seat of the special functions shown by the cell. If it is a muscle
cell, this is the part that does the moving; if a gland cell, this is
the part that secretes. Nevertheless, the nucleus is a vital part of the
cell. It has been definitely proven that a cell from which the nucleus
is lost cannot survive more than a brief time. To gain some idea of the
actual part played by the nucleus, we shall have to return to it in some
detail.
[Illustration: DIAGRAM SHOWING CELL SUBDIVISION
_A_, a cell; _B_ to _F_, successive stages in its subdivision; _a_,
cell-sac; _b_, cell contents; _c_, nucleus. (From Martin’s “Human
Body”)]
Before undertaking a further description of the nucleus itself, we shall
be helped to an understanding of its function if we trace briefly the
history of the cells which make up our body. At the beginning, as we
probably all know, we start life as a single cell. This cell, after a
series of events which will be described in a later chapter, begins the
process known as development. Development consists of a series of
subdivisions of cell material. At first the single cell divides into
two; each of these then divides, giving four. At the next stage eight
are formed, then sixteen and so on, until finally the millions of cells
that make up the body are produced, all derived from the original single
cell. We know that in the adult body there are very many different kinds
of cells. Since they are all derived from a single cell, these
differences must have put in their appearance during the course of the
various cell divisions. In fact, this happens all along; at definite
points in the process the two cells that come from the subdivision of
some particular one will not be alike. The special kinds of cells that
are thus produced become the starting points for whole masses of similar
cells in the fully developed body. In human beings, and probably in most
other kinds of animals, the very first subdivision does not result in
any difference between the cells. The proof of this is that sometimes,
in fact fairly often, the two cells become separated. When this happens
twinning results, and the twins are exactly alike, being known as
“identical twins.” Not only are they alike in all other respects, but
they are always of the same sex, a fact that has escaped the attention
of some writers of fiction, who have made twins, identical in all other
features, brother and sister, instead of both boys or both girls. Twins
that are not identical come from different original cells that happened
to start developing together. Such twins need have no more resemblance
than any members of the same family, and may or may not be of the same
sex.
In every cell division the first step consists in a division of the
chromatin of the nucleus, which is followed by a division of the rest of
the protoplasm. The process by which the chromatin is subdivided is so
curious as to be worth a brief description. The
[Illustration: DIAGRAM ILLUSTRATING DIVISION CHANGES IN A CELL NUCLEUS
WITH FOUR CHROMOSOMES
(From Martin’s “Human Body”)]
chromatin material is not a simple lump in the nucleus. It looks rather
like a tiny string of beads thrown down carelessly, so as to become all
mixed together. Each bead is a single bit of chromatin, and these bits
are strung on a tiny thread. In an ordinary cell the beads are so mixed
together that no order can be distinguished among them, but if a cell
that is about to begin dividing is looked at it is found that the string
has straightened itself out, and also that it has broken into pieces.
The individual pieces are called _chromosomes_ and their number is
always the same for any one kind of animal or plant. There is a
parasitic worm whose cells have only four chromosomes, and the number
ranges from this up to as many as forty-eight in human beings. It may be
that other species have even more, but they become so hard to count when
there are as many as forty-eight that the number cannot be stated with
certainty. So far as can be judged, the number of chromosomes has little
to do with the complexity of the animal or plant, for some complex forms
have few chromosomes, and some simple forms many.
At the same time as the chromatin is breaking up into chromosomes two
tiny spots put in their appearance in the protoplasm of the cell on
opposite sides of the nucleus, and tiny threads extend from one spot to
the other through the nucleus. There are as many threads as there are
chromosomes, the whole group making up a spindle-shaped figure. The
chromosomes now become arranged at the middle of the spindle, and
apparently each chromosome becomes fastened to a thread. Next each
chromosome splits lengthwise through the middle and by what looks like a
shortening of the threads the split halves are pulled apart and drawn
to opposite tips of the spindle. The purpose of this elaborate scheme
seems to be to insure an exactly equal division of the chromosomes
between the cells, and the necessity of such an equal division will
become clear when we learn something of what the chromatin is for.
Meanwhile the description of cell division can be finished by saying
that after the halves of the chromosomes are pulled apart the whole mass
of protoplasm divides through the middle. As we stated above, sometimes
the cells thus produced are alike and sometimes they are different,
according to whether they are destined to become parts of similar or of
different structures. In either case the chromatin material that goes
into the two cells is exactly alike, so that if the cells themselves
become different there must have developed a difference in the
protoplasm at the two ends of the cell from which they came. Our bodies
are made up of millions of cells, of a great many different kinds, but
however different they may be the chromatin of each exactly duplicates
that of every other one, or did when the cells were first formed; there
is reason to believe that the chromatin may become changed during the
lifetime of the cells, at least in some cases.
We may be interested in inquiring how long this process of cell division
keeps up. Many children do not get through growing until they are twenty
years old or more. Does cell division keep on during all this time? More
than that; are there any cases of cell division that continue after full
growth is reached? The answer to both these questions can be given in a
brief paragraph. There are some tissues, particularly the outer layer of
the skin, the connective tissues, the blood-corpuscle-forming tissues,
and the reproductive tissues, in which cell division continues during
all or most of life. The others finish at birth or shortly thereafter.
We are born with the precise number of muscle cells with which we shall
die, unless accident deprives us of some meanwhile; and if this happens
no new ones will be formed to replace those that are lost. The same is
true of gland cells. The last cell divisions among nerve cells are
believed to occur within a few months after birth. As most of us have
observed in our own cases, bodily injuries, if at all severe, are
followed by the formation of scars. This means that connective tissue
has grown in to fill the place of the cells destroyed by the injury,
which cannot be replaced by cells of their own sort, since they have
lost the power of cell division.
We have tried, in the above paragraphs, to get some idea of what living
cells are like, and how they are derived, but have not attempted any
detailed picture of particular kinds of cells. That will have to wait
till we reach the story of the different kinds of bodily activity, when
the cells that carry on each kind will have to be described more
exactly. Something has also been told of the chromosomes, but the full
account of them and their meaning is to be taken up in a later chapter,
devoted to the matter of heredity and reproduction. In what remains of
the present chapter we wish to talk about the conditions in which cells
live so that we shall easily picture how they carry on their metabolism.
As an introduction to this topic a word may be said about the wide
differences of complexity that are found in animals. They range from the
simplest imaginable, a single cell with its nucleus and with protoplasm
that appears almost uniform throughout, to a highly organized body like
that of man, composed of millions of cells of many different kinds.
Between these extremes almost every possible form is seen. The
one-celled animals themselves show a wide range of complexity, and as
soon as animals begin to be formed of numbers of cells grouped together
the possibilities of complexity increase in proportion. One important
difference between one-celled and many-celled animals needs to be
emphasized; that is the matter of size. There are definite limits to the
size that a single cell may attain; these limits are just over the
boundary of naked eye vision. If animals are to attain larger sizes,
they must necessarily be composed of many cells. The life of a
single-celled animal presents no special problem, since it has only to
take in through its outer layer from the surrounding water the various
food materials and the oxygen which its metabolism requires, and to
discharge into the same water any chemical products that may result from
that same metabolism, and the question of whether it will live or die
depends only on whether the water in which it happens to be contains
sufficient materials and is otherwise suitable as a place to live. A
many-celled animal, whose cells are arranged in not more than two
layers, is in practically the same situation, for every cell has a
frontage on the water and so can carry on interchanges of material
directly; but the moment complexity reaches a stage where any cells are
buried beneath other cells some special arrangement must be provided so
that the buried cells can obtain the needed substances for their
metabolism. The arrangement consists, in general, of furnishing what
may be called an internal water frontage for the buried cells. In other
words, complex animals have spaces all through their bodies, and these
spaces are filled with fluid. There are no living tissues so dense that
the cells of which they are composed are completely cut off from contact
with body fluid. In thinking of our own bodies we should realize that
this same arrangement applies; every one of our millions of living cells
has contact with the fluid with which all the spaces of our bodies are
filled, and it is from this fluid that the cells obtain the materials
for their metabolism, and into this same fluid they discharge whatever
substances their metabolism may produce.
The total amount of body fluid is not large, for the spaces among the
cells are in most cases extremely tiny; it follows that with all the
millions of cells absorbing food materials and oxygen from this fluid
and discharging waste materials into it the time will soon come when no
more food or oxygen will be left to be absorbed and there will be no
more capacity for holding waste substances. If this state of affairs
were actually to happen, metabolism would come to an end and death would
be the result; evidently there must be some means of keeping the body
fluids constantly renewed in respect to the things which the cells need
for their metabolism, and constantly drained of the waste substances
which the cells pour out. The way in which this renewal is accomplished
is simple; part of the body fluid is separated off from the rest in a
system of pipes, known to us as the blood vessels, and this part is kept
in motion; at intervals along the system are stations at which the
moving fluid can exchange substances with the fluid which actually comes
in
[Illustration: DIAGRAM SHOWING HOW THE MOVING BODY FLUID RENEWS THE
STATIONARY, AND IS ITSELF RENEWED IN LUNGS, DIGESTIVE TRACT, AND
KIDNEYS]
contact with the cells; thus the stationary fluid can obtain from the
moving fluid the materials which the cells, in turn, are constantly
withdrawing from it, and can pass on to the moving fluid the products
with which the cells are continuously charging it. All that is necessary
to complete the successful operation of the system is to have additional
stations at which the moving fluid can obtain supplies of food materials
and of oxygen, and stations where it can get rid of the wastes which it
accumulates from the stationary fluid, and there must be a pump by which
the moving fluid is kept in motion. We are familiar with the moving
fluid under the name of blood; the system of pipes in which it moves are
the blood vessels; the pump which keeps it in motion is the heart; the
various supply stations include the digestive organs, the lungs, and the
kidneys. In later chapters the operation of all these stations will be
described in detail. The present outline has been given to show in a
general way how the problem of metabolism is handled in highly organized
bodies in which the individual cells have no direct access to food or
oxygen supplies.