BLASTOGENESIS AND MORPHOGENESIS
169
The following conclusions can be drawn:
1. There is no "embryonic reserve" to assure blastogenesis.
2. The blastogenic cells are derived from various tissues of the parent
so that no organogenic specificity may be attributed to any one embryonic layer.
3. The blastogenic cells are cells which have been occupied with
bodily structure and function, but which, in the blastogenic zone, become
freed from this to acquire new organogenic competence, sometimes entirely different from that which they were demonstrating in the parent.
4. At the blastogenic site the somatic cells regain an embryonic condition—they dedifferentiate.
The problem of blastogenic determination is the same as the problem
of gametogenic potency. It will not be resolved by looking for an embryonic reserve or a blastogenic line which does not exist. Even the origin
of the blastogenic cells is of less significance than the physiological
factors which, in the blastogenic area, allow some somatic cells to regain
their autonomy and their ontogenic ability.
Blastogenesis provides evidence of the ability of somatic cells to dedifferentiate (Brien, 1926). However, it was accepted with difficulty.
Biologists were citing experiments with culture in vitro in opposition to
it. "It is one of the most important results of the culture of tissues that
it has shown the irreversible character of cellular
differentiation"
(Bounoure, 1940). Today we are less categorical. Cellular dedifferentiation is recognized experimentally. In budding, it is a fact (Fig. 12).
Dedifferentiation may be only partial or superficial for, as soon as the
cells have returned from ontogenesis they resume their original histological orientation, as in the ectoblastic cells of the tunicate bud or in
scissiparity in annelids. But dedifferentiation is often much deeper and
even complete, since the blastogenic cells form tissues and organs very
different from those from which they originated, e.g., the mesenchymal
cells from the budding vesicle of Clavelina are capable of constructing all
the organs of blastozooids and the cells of the epicardium of Polyclinidae
strobilae acquire the same power.
This dedifferentiation is demonstrated morphologically by an increase
in height of the blastogenic cells as well as by a biochemical change. The
dedifferentiated, resume a strong basophilia and great power for proliferation. The
ectoderm, originally a single layer, becomes stratified in this area exclusively,
reforming embryonic ectoblastic cells, basal in position, similar to interstitial cells.
Endoderm also dedifferentiates into endoblast. The stratified ectoblast and the
endoblast thus dedifferentiated will form a young polyp or a medusa (Brien, 1941).
169
The following conclusions can be drawn:
1. There is no "embryonic reserve" to assure blastogenesis.
2. The blastogenic cells are derived from various tissues of the parent
so that no organogenic specificity may be attributed to any one embryonic layer.
3. The blastogenic cells are cells which have been occupied with
bodily structure and function, but which, in the blastogenic zone, become
freed from this to acquire new organogenic competence, sometimes entirely different from that which they were demonstrating in the parent.
4. At the blastogenic site the somatic cells regain an embryonic condition—they dedifferentiate.
The problem of blastogenic determination is the same as the problem
of gametogenic potency. It will not be resolved by looking for an embryonic reserve or a blastogenic line which does not exist. Even the origin
of the blastogenic cells is of less significance than the physiological
factors which, in the blastogenic area, allow some somatic cells to regain
their autonomy and their ontogenic ability.
Blastogenesis provides evidence of the ability of somatic cells to dedifferentiate (Brien, 1926). However, it was accepted with difficulty.
Biologists were citing experiments with culture in vitro in opposition to
it. "It is one of the most important results of the culture of tissues that
it has shown the irreversible character of cellular
differentiation"
(Bounoure, 1940). Today we are less categorical. Cellular dedifferentiation is recognized experimentally. In budding, it is a fact (Fig. 12).
Dedifferentiation may be only partial or superficial for, as soon as the
cells have returned from ontogenesis they resume their original histological orientation, as in the ectoblastic cells of the tunicate bud or in
scissiparity in annelids. But dedifferentiation is often much deeper and
even complete, since the blastogenic cells form tissues and organs very
different from those from which they originated, e.g., the mesenchymal
cells from the budding vesicle of Clavelina are capable of constructing all
the organs of blastozooids and the cells of the epicardium of Polyclinidae
strobilae acquire the same power.
This dedifferentiation is demonstrated morphologically by an increase
in height of the blastogenic cells as well as by a biochemical change. The
dedifferentiated, resume a strong basophilia and great power for proliferation. The
ectoderm, originally a single layer, becomes stratified in this area exclusively,
reforming embryonic ectoblastic cells, basal in position, similar to interstitial cells.
Endoderm also dedifferentiates into endoblast. The stratified ectoblast and the
endoblast thus dedifferentiated will form a young polyp or a medusa (Brien, 1941).