310
EDGAR
ZWILLING
deficiencies which result from excision of the ridge (without mesoderm)
are due to degeneration of highly susceptible undetermined distal
mesenchyme which succumbs to the deleterious influence of the amniotic
fluid. By the time that the epidermis heals over the apical zone the
undetermined presumptive digital tissues are too damaged to recover
and continue development. On the other hand, the more proximal mesoderm, not quite so susceptible to exposure, may, after the lateral limb
ectoderm heals over it, undergo regulation and form digital structures,
presumably under the influence of the more proximal tissues.
Bell's view (Bell, Kaighn and Fessenden, 1959) that the ectoderm 'is
not necessary' is based on experiments in which limb buds were briefly
exposed to focused ultra-sound. Following such treatment the ectoderm
may be peeled off readily. Even if it is left in place it tends to come off
spontaneously at some time during the 2-4 hours following irradiation.
19% of the denuded limb-bud mesoblasts, raised as flank grafts, and
11%, raised as intracoelomic grafts, formed distal limb structures.
Similarly, 24 and 12% of 'irradiated/not stripped' buds (which subsequently lose their ectoderm) formed distal structures. The flank grafts
became, during the 24-48 hours following the operation, covered by
flank ectoderm. However, the intracoelomic grafts did not acquire an
ectodermal covering but developed distal structures nevertheless. Bell
et al., suggest that the basement membrane may be an important factor
in this development. The basement membrane was either not present in
ectoderm-free mesoblasts immediately after irradiation or broke down
and was thus lost within 6 hours after implantation of the grafted
mesoblast. The possibility is advanced that an intact basement membrane, without its ectoderm, may inhibit outgrowth while the absence
of an 'intact but static' (presumably 'static' means one without its
ectoderm) may allow outgrowth to proceed.
4. Additional Evidence for a Positive Role for the Ectoderm
Even if we accept Amprino's experiments and those of Bell at face
value and concede that there may be circumstances under which limb
development may occur in the absence of an ectodermal ridge or,
indeed, the entire ectodermal covering of the limb bud, we must consider
all of the evidence when attempting to evaluate the morphogenetic role
of the ectoderm.
There are many experiments which strongly indicate the positive role
of the ectodermal ridge. In the experiments of Saunders, Cairns and
Gasseling (1957) (vide supra) the blocks of presumptive thigh mesoderm
formed digits only when in direct contact with the ectodermal ridge. In
cases where even a thin layer of host (wing) mesoderm grew (or remained)
between the ridge and thigh mesoderm the latter did not form digits.
EDGAR
ZWILLING
deficiencies which result from excision of the ridge (without mesoderm)
are due to degeneration of highly susceptible undetermined distal
mesenchyme which succumbs to the deleterious influence of the amniotic
fluid. By the time that the epidermis heals over the apical zone the
undetermined presumptive digital tissues are too damaged to recover
and continue development. On the other hand, the more proximal mesoderm, not quite so susceptible to exposure, may, after the lateral limb
ectoderm heals over it, undergo regulation and form digital structures,
presumably under the influence of the more proximal tissues.
Bell's view (Bell, Kaighn and Fessenden, 1959) that the ectoderm 'is
not necessary' is based on experiments in which limb buds were briefly
exposed to focused ultra-sound. Following such treatment the ectoderm
may be peeled off readily. Even if it is left in place it tends to come off
spontaneously at some time during the 2-4 hours following irradiation.
19% of the denuded limb-bud mesoblasts, raised as flank grafts, and
11%, raised as intracoelomic grafts, formed distal limb structures.
Similarly, 24 and 12% of 'irradiated/not stripped' buds (which subsequently lose their ectoderm) formed distal structures. The flank grafts
became, during the 24-48 hours following the operation, covered by
flank ectoderm. However, the intracoelomic grafts did not acquire an
ectodermal covering but developed distal structures nevertheless. Bell
et al., suggest that the basement membrane may be an important factor
in this development. The basement membrane was either not present in
ectoderm-free mesoblasts immediately after irradiation or broke down
and was thus lost within 6 hours after implantation of the grafted
mesoblast. The possibility is advanced that an intact basement membrane, without its ectoderm, may inhibit outgrowth while the absence
of an 'intact but static' (presumably 'static' means one without its
ectoderm) may allow outgrowth to proceed.
4. Additional Evidence for a Positive Role for the Ectoderm
Even if we accept Amprino's experiments and those of Bell at face
value and concede that there may be circumstances under which limb
development may occur in the absence of an ectodermal ridge or,
indeed, the entire ectodermal covering of the limb bud, we must consider
all of the evidence when attempting to evaluate the morphogenetic role
of the ectoderm.
There are many experiments which strongly indicate the positive role
of the ectodermal ridge. In the experiments of Saunders, Cairns and
Gasseling (1957) (vide supra) the blocks of presumptive thigh mesoderm
formed digits only when in direct contact with the ectodermal ridge. In
cases where even a thin layer of host (wing) mesoderm grew (or remained)
between the ridge and thigh mesoderm the latter did not form digits.