4. CELL T Y P E S
163
have seen not confined to amoebocytes, is very extensively practised
by them and is one of their most characteristic features.
The cells which in vitro are classed as mechanocytes are mostly
derived from tissues in the body which have considerable amounts of
intercellular matrix, i.e. of collagen and mucoproteins in various forms,
and the production of these materials can also go on under their
influence in vitro. Amoebocytes, in general, do not produce collagen
and they tend to accumulate mucopolysaccharides within their own
cytoplasm rather than release it continuously into the surroundings
They may, of course, release it at times by liberating their stored
granules, as do goblet cells and mast cells. It would appear likely then
that the production of collagen and/or intercellular mucopolysaccharides are diagnostic features of mechanocytes and help to
emphasize the similarity of all the cells which fall into this class.
Mechanocytes derived from muscular tissues generally produce much
less in the way of intercellular materials than mechanocytes derived from
the connective tissues, and there is some justification for dividing
mechanocytes into two sub-groups, myxoblasts or mucin-producers and
myoblasts or muscle-producers (Willmer, 1960). It is significant that
reticulin and collagen fibres appear very quickly in the medium of
tissue cultures when mechanocytes are growing freely in them and they
seldom appear in the absence of these cells, though occasionally epithelial cells, or some of them, seem to be able to produce collagen. It is
interesting, also, to notice that collagen contains large quantities of
hydroxyproline and that this substance is produced during the unorganized growth of cells from the carrot, where it is normally built into
a structural protein (Steward and Pollard, 1958). If it is not so removed,
or is added to the medium in which the carrot cells are being cultured,
growth is inhibited. This would seem to suggest the idea that hydroxyproline is in some way a by-product of active growth of certain cell
types even in the plant kingdom. It is interesting also to notice that
glutamic acid, glycine, proline and hydroxyproline, which are all
extensively present in collagen, are amino acids which are not required
by bone cells. Presumably such cells produce these acids during the
course of their normal metabolism (see p. 299, and Vol. 2, Chapter 4).
T o return to the properties of the surfaces of the cells of various types,
it has been mentioned that epitheliocytes normally cohere in the form
of sheets so that the cells make almost as much contact with each other
as they can. It seems probable that desmosomes and terminal bars are
structures which are specially characteristic of epithelial cells, although
somewhat similar structures appear at the synapses between certain
nerve cells, and they also occur between endothelial andmesothelialcells.
Nerve cells are of course direct derivatives of epithelial cells.
163
have seen not confined to amoebocytes, is very extensively practised
by them and is one of their most characteristic features.
The cells which in vitro are classed as mechanocytes are mostly
derived from tissues in the body which have considerable amounts of
intercellular matrix, i.e. of collagen and mucoproteins in various forms,
and the production of these materials can also go on under their
influence in vitro. Amoebocytes, in general, do not produce collagen
and they tend to accumulate mucopolysaccharides within their own
cytoplasm rather than release it continuously into the surroundings
They may, of course, release it at times by liberating their stored
granules, as do goblet cells and mast cells. It would appear likely then
that the production of collagen and/or intercellular mucopolysaccharides are diagnostic features of mechanocytes and help to
emphasize the similarity of all the cells which fall into this class.
Mechanocytes derived from muscular tissues generally produce much
less in the way of intercellular materials than mechanocytes derived from
the connective tissues, and there is some justification for dividing
mechanocytes into two sub-groups, myxoblasts or mucin-producers and
myoblasts or muscle-producers (Willmer, 1960). It is significant that
reticulin and collagen fibres appear very quickly in the medium of
tissue cultures when mechanocytes are growing freely in them and they
seldom appear in the absence of these cells, though occasionally epithelial cells, or some of them, seem to be able to produce collagen. It is
interesting, also, to notice that collagen contains large quantities of
hydroxyproline and that this substance is produced during the unorganized growth of cells from the carrot, where it is normally built into
a structural protein (Steward and Pollard, 1958). If it is not so removed,
or is added to the medium in which the carrot cells are being cultured,
growth is inhibited. This would seem to suggest the idea that hydroxyproline is in some way a by-product of active growth of certain cell
types even in the plant kingdom. It is interesting also to notice that
glutamic acid, glycine, proline and hydroxyproline, which are all
extensively present in collagen, are amino acids which are not required
by bone cells. Presumably such cells produce these acids during the
course of their normal metabolism (see p. 299, and Vol. 2, Chapter 4).
T o return to the properties of the surfaces of the cells of various types,
it has been mentioned that epitheliocytes normally cohere in the form
of sheets so that the cells make almost as much contact with each other
as they can. It seems probable that desmosomes and terminal bars are
structures which are specially characteristic of epithelial cells, although
somewhat similar structures appear at the synapses between certain
nerve cells, and they also occur between endothelial andmesothelialcells.
Nerve cells are of course direct derivatives of epithelial cells.