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HENRIETTE HERLANT-MEEW1S
coelomocytes during histolysis and phagocytosis of necrosed tissues and
the discharge of nutritive substances have also been studied. It is
probable that each of these factors plays some part, but more experiments are needed to support the various hypotheses. The blastocytes are
characterized at this stage by enlargement of the cell as well as by an
increase in the volume of its nucleus and nucleolus, and the cytoplasm
becomes richer in RNA and thiol-proteins. These changes affect all
regenerative elements in the same way, whether they are de-differentiating epithelial cells or basal replacement cells at the site of injury, or
mesodermal neoblasts in neighbouring segments. We should add that in
the last case, the changes seem to occur before migration and are not
affected by X-irradiation. It is essential for all layers to be activated; if
activation is limited to one layer only, let us say the ectoderm, healing
is not followed by regeneration.
3. The third condition is the multiplication of regenerative elements
which guarantee blastema growth : this phenomenon relates to blastocytes located in the cicatricial region only ; when neoblasts are involved,
it occurs only when the cells have arrived in this zone and is blocked by
X-irradiation. Cell proliferation is fostered by the presence of nutritive
substances whose source is most likely to be in the regenerating nervous
system, but they may also be carried by some amoebocytes (such as
eleocytes) which accumulate more particularly in the healing zone.
These cells are loaded with reserve granules, but the mechanism of their
release must be further investigated.
4. The fourth condition is that of cell differentiation and arrangement
in the bud. Here, we have to deal with the problem of accurate separation of the layers, each layer reconstructing its own elements, or of the
totipotency of certain cells which can participate in the repair of several
organs of different ontogenetic origin. As a rule, it seems that to some
extent each tissue retains its individuality in the blastema and is repaired from its own elements, provided that the conditions described
above are met. This does not exclude the possibility that cells which
have taken on embryonic characteristics can participate in the elaboration of any tissue. The variability in tissues from species to species can
bring about either of these processes.
5. The fifth condition concerns the caudal or cephalic nature of the
bud. It is linked with the continuance or disappearance of a growth
zone characterized by the presence of cells of embryonic type in all
tissues. In the first case, this zone remains localized near the hinder end
of the worm which continues to grow while new segments differentiate
cephalad: pygidial regeneration can thus go on until total or partial
repair of the amputated fragment is accomplished. In the second case,
blastocytes of embryonic type rapidly disappear, growth stops and, in
HENRIETTE HERLANT-MEEW1S
coelomocytes during histolysis and phagocytosis of necrosed tissues and
the discharge of nutritive substances have also been studied. It is
probable that each of these factors plays some part, but more experiments are needed to support the various hypotheses. The blastocytes are
characterized at this stage by enlargement of the cell as well as by an
increase in the volume of its nucleus and nucleolus, and the cytoplasm
becomes richer in RNA and thiol-proteins. These changes affect all
regenerative elements in the same way, whether they are de-differentiating epithelial cells or basal replacement cells at the site of injury, or
mesodermal neoblasts in neighbouring segments. We should add that in
the last case, the changes seem to occur before migration and are not
affected by X-irradiation. It is essential for all layers to be activated; if
activation is limited to one layer only, let us say the ectoderm, healing
is not followed by regeneration.
3. The third condition is the multiplication of regenerative elements
which guarantee blastema growth : this phenomenon relates to blastocytes located in the cicatricial region only ; when neoblasts are involved,
it occurs only when the cells have arrived in this zone and is blocked by
X-irradiation. Cell proliferation is fostered by the presence of nutritive
substances whose source is most likely to be in the regenerating nervous
system, but they may also be carried by some amoebocytes (such as
eleocytes) which accumulate more particularly in the healing zone.
These cells are loaded with reserve granules, but the mechanism of their
release must be further investigated.
4. The fourth condition is that of cell differentiation and arrangement
in the bud. Here, we have to deal with the problem of accurate separation of the layers, each layer reconstructing its own elements, or of the
totipotency of certain cells which can participate in the repair of several
organs of different ontogenetic origin. As a rule, it seems that to some
extent each tissue retains its individuality in the blastema and is repaired from its own elements, provided that the conditions described
above are met. This does not exclude the possibility that cells which
have taken on embryonic characteristics can participate in the elaboration of any tissue. The variability in tissues from species to species can
bring about either of these processes.
5. The fifth condition concerns the caudal or cephalic nature of the
bud. It is linked with the continuance or disappearance of a growth
zone characterized by the presence of cells of embryonic type in all
tissues. In the first case, this zone remains localized near the hinder end
of the worm which continues to grow while new segments differentiate
cephalad: pygidial regeneration can thus go on until total or partial
repair of the amputated fragment is accomplished. In the second case,
blastocytes of embryonic type rapidly disappear, growth stops and, in