4. CELL T Y P E S
159
of phospholipins by Luzatti and Husson (1962) and by Stoeckenius
(1962).
Among the cells of the connective tissues (mechanocytes) the number
of types is again limited, though the different intercellular matrices
perhaps impose greater differences in morphology (e.g. bone cells,
cartilage cells etc.). Among the amoebocytes (other than the somewhat
ephemeral leucocytes), the monocytes, macrophages, microglia,
osteoclasts and perhaps one or two other groups are less easily assigned
to their own groups on morphological grounds alone, perhaps because
they show more variable forms within each group.
In short, most of the viable "dedifferentiated" cells in the early stages
of culture can be classed as either epitheliocytes, mechanocytes or
amoebocytes. The majority of muscle cultures, after a time, are indistinguishable from those of mechanocytes or amoebocytes. Neuroglia
cells maintain something of their own character, but tend to approach
the form of either mechanocytes (the oligodendroglia) or amoebocytes
(the astrocytes), as also do the Schwann cells to a lesser degree, and it is
only the neurones which remain at least partially differentiated when
they are cultured but, as remarked earlier, they are a law unto themselves in that they hardly migrate at all in culture, as compared with
other types of cells (see p. 147, and Vol. 2, Chapter 9.).
V . T H E POSSIBLE E V O L U T I O N OF T H E C E L L T Y P E S
Probably the most fundamental approach to the problem of the
meaning of these cell types is that which follows evolutionary and developmental lines. How did these families of cells arise and what is their
significance? Perhaps a more profitable question would be: how did
the original differences arise and what purposes were served by the
original differentiations? This topic has been recently and extensively
discussed elsewhere (Willmer, 1960) and need not be further treated
in detail here. The essential idea which was then suggested as a working
hypothesis was that a hollow blastula-like organism was one of the
early evolutionary stages through which most of the metazoa must have
passed in the evolution of colonial organization. In order to preserve
the ionic equilibrium of the bias tula and to prevent its cavity from
becoming a cess-pit, it was postulated that the cells composing the
spherical shell were of two opposing types with reference to the passage
of water, ions or other metabolites, so that excessive activity on the part
of the cells of one type could be balanced by corresponding activity
of an opposite kind by the others. All the cells of the hollow bias tula were
necessarily epitheliocytes at first, in the sense that they cohered closely
159
of phospholipins by Luzatti and Husson (1962) and by Stoeckenius
(1962).
Among the cells of the connective tissues (mechanocytes) the number
of types is again limited, though the different intercellular matrices
perhaps impose greater differences in morphology (e.g. bone cells,
cartilage cells etc.). Among the amoebocytes (other than the somewhat
ephemeral leucocytes), the monocytes, macrophages, microglia,
osteoclasts and perhaps one or two other groups are less easily assigned
to their own groups on morphological grounds alone, perhaps because
they show more variable forms within each group.
In short, most of the viable "dedifferentiated" cells in the early stages
of culture can be classed as either epitheliocytes, mechanocytes or
amoebocytes. The majority of muscle cultures, after a time, are indistinguishable from those of mechanocytes or amoebocytes. Neuroglia
cells maintain something of their own character, but tend to approach
the form of either mechanocytes (the oligodendroglia) or amoebocytes
(the astrocytes), as also do the Schwann cells to a lesser degree, and it is
only the neurones which remain at least partially differentiated when
they are cultured but, as remarked earlier, they are a law unto themselves in that they hardly migrate at all in culture, as compared with
other types of cells (see p. 147, and Vol. 2, Chapter 9.).
V . T H E POSSIBLE E V O L U T I O N OF T H E C E L L T Y P E S
Probably the most fundamental approach to the problem of the
meaning of these cell types is that which follows evolutionary and developmental lines. How did these families of cells arise and what is their
significance? Perhaps a more profitable question would be: how did
the original differences arise and what purposes were served by the
original differentiations? This topic has been recently and extensively
discussed elsewhere (Willmer, 1960) and need not be further treated
in detail here. The essential idea which was then suggested as a working
hypothesis was that a hollow blastula-like organism was one of the
early evolutionary stages through which most of the metazoa must have
passed in the evolution of colonial organization. In order to preserve
the ionic equilibrium of the bias tula and to prevent its cavity from
becoming a cess-pit, it was postulated that the cells composing the
spherical shell were of two opposing types with reference to the passage
of water, ions or other metabolites, so that excessive activity on the part
of the cells of one type could be balanced by corresponding activity
of an opposite kind by the others. All the cells of the hollow bias tula were
necessarily epitheliocytes at first, in the sense that they cohered closely