2 . M E T H O D S
35
that they may account for at least some of the activity of embryo
juice.
The interest in embryo juice as a nutrient and stimulating fluid,
and at one time almost the only one of its kind, is now waning in the
light of the positive achievements in the direction of maintaining cells
in culture in media of defined chemical composition (see Chapter 3).
Moreover the recent observations of Frederic and Corin (1962) that
heterologous embryo extracts may cause abnormal chromosome
patterns in cells cultured in their presence suggests that such heterologous extracts should be used with great caution, or not at all.
2. Solid Medium
It has already been emphasized that the movement and outgrowth
of cells from a tissue in a fluid medium are generally limited to the
surfaces of the medium. However, if the tissue is embedded in a drop
of plasma, which is then allowed to clot, cells emerge from the entire
surface of the explant and invade the plasma clot; even in this medium
however, they mostly make their way, if they can, either to the airplasma interface or to the plasma-glass surface where they can again
migrate most easily. While in the plasma the cells often follow each
other along tracks, as though suitable tunnels and surfaces were being
utilized. The plasma which is most usually used is fowl plasma, since
this does not readily clot spontaneously if proper precautions are taken;
it clots, however, as soon as tissues or tissue extracts are added. If
mammalian plasma is used, premature clotting must be prevented by
the addition of a suitable quantity of heparin. The fact that the plasma
is not from the same species as the tissue to be cultured makes surprisingly little noticeable difference to the cellular activity displayed, but in
view of the observations on the effects of heterologous tissue extracts,
perhaps some caution should be exercised here also. The probability is,
however, that the effective constituents of the tissue extracts are the
nucleic acids (Frederic and Corin-Frederic, 1962), and these are not
normally present to any extent in plasma.
The plasma clot has primarily a supporting action, consisting as it
does of a gel of innumerable fine and interlacing fibrin threads. It is
digestible by enzymes liberated by cells of many types, and this property
facilitates its invasion and colonization by cells. Like serum, it probably
also contributes to the nutrition of cells but, by itself, it is incapable of
maintaining the prolonged growth of mechanocytes by cell multiplication. These cells may, on the other hand, survive for long periods
and differentiate extensively in a medium of coagulated plasma alone,
provided that the clot is frequently washed with heparin-plasma
or serum, and is patched with fresh plasma when necessary.
35
that they may account for at least some of the activity of embryo
juice.
The interest in embryo juice as a nutrient and stimulating fluid,
and at one time almost the only one of its kind, is now waning in the
light of the positive achievements in the direction of maintaining cells
in culture in media of defined chemical composition (see Chapter 3).
Moreover the recent observations of Frederic and Corin (1962) that
heterologous embryo extracts may cause abnormal chromosome
patterns in cells cultured in their presence suggests that such heterologous extracts should be used with great caution, or not at all.
2. Solid Medium
It has already been emphasized that the movement and outgrowth
of cells from a tissue in a fluid medium are generally limited to the
surfaces of the medium. However, if the tissue is embedded in a drop
of plasma, which is then allowed to clot, cells emerge from the entire
surface of the explant and invade the plasma clot; even in this medium
however, they mostly make their way, if they can, either to the airplasma interface or to the plasma-glass surface where they can again
migrate most easily. While in the plasma the cells often follow each
other along tracks, as though suitable tunnels and surfaces were being
utilized. The plasma which is most usually used is fowl plasma, since
this does not readily clot spontaneously if proper precautions are taken;
it clots, however, as soon as tissues or tissue extracts are added. If
mammalian plasma is used, premature clotting must be prevented by
the addition of a suitable quantity of heparin. The fact that the plasma
is not from the same species as the tissue to be cultured makes surprisingly little noticeable difference to the cellular activity displayed, but in
view of the observations on the effects of heterologous tissue extracts,
perhaps some caution should be exercised here also. The probability is,
however, that the effective constituents of the tissue extracts are the
nucleic acids (Frederic and Corin-Frederic, 1962), and these are not
normally present to any extent in plasma.
The plasma clot has primarily a supporting action, consisting as it
does of a gel of innumerable fine and interlacing fibrin threads. It is
digestible by enzymes liberated by cells of many types, and this property
facilitates its invasion and colonization by cells. Like serum, it probably
also contributes to the nutrition of cells but, by itself, it is incapable of
maintaining the prolonged growth of mechanocytes by cell multiplication. These cells may, on the other hand, survive for long periods
and differentiate extensively in a medium of coagulated plasma alone,
provided that the clot is frequently washed with heparin-plasma
or serum, and is patched with fresh plasma when necessary.