VII.
LIMB
MORPHOGENESIS
307
A. Evidence for the Hypothesis
I should like, now, to present evidence, both for and against the
various portions of the hypothesis outlined above. A chronological
sequence will not be adhered to in this presentation.
1. Initiation of Limb Development
Unlike many other major structures, little or nothing is known, even
at the biological level, of the tissue interactions responsible for the
normal initiation of limb development. There is virtually no causal
analysis of the events leading to the formation of the initial thickening
of the somatopleure which becomes the limb bud. Balinsky's (1956,
1957) hypothesis, based on re-investigation of a phenomenon originally
described by him in 1925, represents one of the few attempts in this
direction. His earlier research revealed that grafts of ear vesicles or nasal
placodes to the flank of early urodeles 'induced' the formation of limbs
at this site. Electron micrographs of serially sacrificed embryos in
various stages in the induction of limbs by such grafts revealed that the
basement membrane between the graft and the overlying flank epidermis
is absent or extremely thin. The mesodermal thickening from which the
supernumerary limb develops forms in close proximity to the region of
deficient basement membrane. According to Balinsky the area deficient
in the basement membrane serves to trap the presumptive limb mesoderm and provides conditions which favour establishment of contact
between this tissue and the epidermis. Such contact is required for the
proper epidermal-mesodermal interaction which leads to limb formation.
Thus far, Balinsky's hypothesis applies only to the artificially induced
limbs. No studies of the condition of the basement membrane in the
normal limb-forming areas have been made to date, so that the relevancy
of Balinsky's hypothesis to normal limb development is still to be
established.
2. Importance of the Ectodermal Ridge
While there was some evidence that the ectoderm played an active
part in amphibian limb morphogenesis (Steiner, Filatow, Balinsky,
cited above) the prevailing opinion for a long time was that the ectoderm
had only a passive role. In 1948 J. W. Saunders, Jr., published evidence
which was largely responsible for a change in opinion about the role of
the ectoderm and which, in fact, revived interest in limb morphogenesis
and led to a number of new experimental approaches. Saunders found
that deletion of the ectodermal ridge from limb buds of chick embryos
resulted in the cessation of formation of distal limb parts. The extent of
distal deficiency depended on the stage from which the ridge was
L
A.M. I
LIMB
MORPHOGENESIS
307
A. Evidence for the Hypothesis
I should like, now, to present evidence, both for and against the
various portions of the hypothesis outlined above. A chronological
sequence will not be adhered to in this presentation.
1. Initiation of Limb Development
Unlike many other major structures, little or nothing is known, even
at the biological level, of the tissue interactions responsible for the
normal initiation of limb development. There is virtually no causal
analysis of the events leading to the formation of the initial thickening
of the somatopleure which becomes the limb bud. Balinsky's (1956,
1957) hypothesis, based on re-investigation of a phenomenon originally
described by him in 1925, represents one of the few attempts in this
direction. His earlier research revealed that grafts of ear vesicles or nasal
placodes to the flank of early urodeles 'induced' the formation of limbs
at this site. Electron micrographs of serially sacrificed embryos in
various stages in the induction of limbs by such grafts revealed that the
basement membrane between the graft and the overlying flank epidermis
is absent or extremely thin. The mesodermal thickening from which the
supernumerary limb develops forms in close proximity to the region of
deficient basement membrane. According to Balinsky the area deficient
in the basement membrane serves to trap the presumptive limb mesoderm and provides conditions which favour establishment of contact
between this tissue and the epidermis. Such contact is required for the
proper epidermal-mesodermal interaction which leads to limb formation.
Thus far, Balinsky's hypothesis applies only to the artificially induced
limbs. No studies of the condition of the basement membrane in the
normal limb-forming areas have been made to date, so that the relevancy
of Balinsky's hypothesis to normal limb development is still to be
established.
2. Importance of the Ectodermal Ridge
While there was some evidence that the ectoderm played an active
part in amphibian limb morphogenesis (Steiner, Filatow, Balinsky,
cited above) the prevailing opinion for a long time was that the ectoderm
had only a passive role. In 1948 J. W. Saunders, Jr., published evidence
which was largely responsible for a change in opinion about the role of
the ectoderm and which, in fact, revived interest in limb morphogenesis
and led to a number of new experimental approaches. Saunders found
that deletion of the ectodermal ridge from limb buds of chick embryos
resulted in the cessation of formation of distal limb parts. The extent of
distal deficiency depended on the stage from which the ridge was
L
A.M. I