VII.
LIMB MORPHOGENESIS
327
VI. Cell Death in Limb Morphogenesis
Cell death as a normal feature of many developmental processes is
now recognized and has been the subject of an extensive review
(Glücksmann, 1951). Fell and Canti (1934) have described a focus of
degenerating cells (which appears in the living limb bud as an 'opaque
patch') which is present, in a five day limb bud, at the site of the future
knee joint. The precise role of degeneration in the elaboration of a joint
is not clear. Degenerating cells are seen in the ectodermal ridge, especially
at the pre- and post-axial ends which are becoming flat; apparently
degeneration of cells is one feature of ridge regression which occurs in
regions where there has been a cessation of outgrowth (where there is no
or little maintenance factor). Saunders (1953) has initiated a study
which reveals that cell degeneration is involved in shaping a number of
regions (shoulder, etc.) of the limb. Apparently a good deal of tissue
which is capable of forming cartilage but which is not actively involved
in the formation of a cartilaginous element degenerates a;nd thereby
helps to mould the form of the limb. Degenerative phenomena accompany
the regression of interdigital material and thus play a part in shaping
the hand (foot). Saunders (unpublished work) has done experiments in
which a region of the shoulder which ordinarily degenerates has been
transplanted to a foreign site. If this is done prior to stage 23 the tissues
do not undergo degeneration but such tissue will degenerate regardless
of site if it is transplanted subsequent to stage 23. Studies of the
mechanism which mediate this sort of morphogenetic death as well as
those which keep the degeneration within normal confines should prove
to be both fascinating and important.
VII. Tissue Mass Phenomena in Limb Development
Several aspects of limb development are either influenced or conditioned by requirements for minimal amounts of tissues for which,
under some circumstances, there seems to be an active competition.
Polydactyly may be accompanied by deficiencies in the bones proximal
to the accessory digits. In the so-called souris luxée (Hovelacque and
Noel, 1923) and luxate mice (Carter, 1954) the distal end of the tibia
is represented by ligamentous tissue rather than cartilage and bone.
Presumably the condensation of precartilaginous tissue at the end of the
tibia has not been sufficient to satisfy the requirement for differentiation
of cartilage. There is some likelihood that the tissue which ordinarily
would have contributed to the tibia was involved in the formation of
accessory digits.
A more striking tissue mass dependence has been shown by Bretscher
LIMB MORPHOGENESIS
327
VI. Cell Death in Limb Morphogenesis
Cell death as a normal feature of many developmental processes is
now recognized and has been the subject of an extensive review
(Glücksmann, 1951). Fell and Canti (1934) have described a focus of
degenerating cells (which appears in the living limb bud as an 'opaque
patch') which is present, in a five day limb bud, at the site of the future
knee joint. The precise role of degeneration in the elaboration of a joint
is not clear. Degenerating cells are seen in the ectodermal ridge, especially
at the pre- and post-axial ends which are becoming flat; apparently
degeneration of cells is one feature of ridge regression which occurs in
regions where there has been a cessation of outgrowth (where there is no
or little maintenance factor). Saunders (1953) has initiated a study
which reveals that cell degeneration is involved in shaping a number of
regions (shoulder, etc.) of the limb. Apparently a good deal of tissue
which is capable of forming cartilage but which is not actively involved
in the formation of a cartilaginous element degenerates a;nd thereby
helps to mould the form of the limb. Degenerative phenomena accompany
the regression of interdigital material and thus play a part in shaping
the hand (foot). Saunders (unpublished work) has done experiments in
which a region of the shoulder which ordinarily degenerates has been
transplanted to a foreign site. If this is done prior to stage 23 the tissues
do not undergo degeneration but such tissue will degenerate regardless
of site if it is transplanted subsequent to stage 23. Studies of the
mechanism which mediate this sort of morphogenetic death as well as
those which keep the degeneration within normal confines should prove
to be both fascinating and important.
VII. Tissue Mass Phenomena in Limb Development
Several aspects of limb development are either influenced or conditioned by requirements for minimal amounts of tissues for which,
under some circumstances, there seems to be an active competition.
Polydactyly may be accompanied by deficiencies in the bones proximal
to the accessory digits. In the so-called souris luxée (Hovelacque and
Noel, 1923) and luxate mice (Carter, 1954) the distal end of the tibia
is represented by ligamentous tissue rather than cartilage and bone.
Presumably the condensation of precartilaginous tissue at the end of the
tibia has not been sufficient to satisfy the requirement for differentiation
of cartilage. There is some likelihood that the tissue which ordinarily
would have contributed to the tibia was involved in the formation of
accessory digits.
A more striking tissue mass dependence has been shown by Bretscher