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fully developed. Still other parts of the body, such as the lens, iris,
retina, central nervous system, and certain visceral organs, though not
appendages, are nevertheless of considerable interest because of their
remarkable powers of regeneration.
3. Reptiles, Birds and Mammals
Apart from the regeneration of jaws, which has been claimed in some
lizards and turtles, and some occasional reports of the limited growth of
legs, the only reptilian structure which regenerates reliably is the lizard
tail. This is often lost by autotomy as a protective measure, whereupon
a new outgrowth develops. This regenerated tail, however, differs from
the original in that its internal skeleton consists of an unsegmented,
tapered cartilaginous tube instead of true vertebrae. In this respect,
therefore, it is of particular interest from the morphogenetic point of
view.
Birds and mammals are unable to regenerate any of the structures
which lower vertebrates can replace. Certain epidermal appendages,
such as feathers, hairs and claws can be regenerated, and indeed are
regularly replaced as a part of the moulting phenomenon. To a limited
extent, teeth also can be replaced. However, the more histologically
complex structures, such as tails and legs, cannot regenerate despite the
fact that their individual component tissues are regeneratively competent. Epidermis, nerve fibres, muscle fibres, and bones all retain
considerable reparative powers, yet they are unable to co-ordinate
sufficiently to produce any organized outgrowth. The only mammalian
appendages possessing the faculty to regenerate are the antlers of
deer.
In view of the unique phenomenon which antler growth represents,
this process probably deserves more serious scientific attention than it
has thus far received. Antlers are developmentally interesting for many
reasons, not the least of which is their remarkably rapid rate of growth,
which in some larger specimens may exceed one half a centimetre per
day. Consisting of bone, nerves, blood vessels and skin, they are, except
for the absence of muscle and articulating segments, as morphologically
complex as limbs or tails. Usually present only in male animals, antlers
are shed in the winter or spring and replaced by new outgrowths during
spring and summer. The growth of antlers is achieved by the production
of apically situated undifferentiated cells, which subsequently become
chondrified and then converted directly into bone. As long as growth
continues the antlers are enveloped in skin, but due to circulatory
restrictions occurring in late summer the skin becomes necrotic and is
rubbed off. This exposes the bare bony antler which is dead tissue.
Ultimately the whole antler will be shed following the weakening of its
R.
J.
GOSS
fully developed. Still other parts of the body, such as the lens, iris,
retina, central nervous system, and certain visceral organs, though not
appendages, are nevertheless of considerable interest because of their
remarkable powers of regeneration.
3. Reptiles, Birds and Mammals
Apart from the regeneration of jaws, which has been claimed in some
lizards and turtles, and some occasional reports of the limited growth of
legs, the only reptilian structure which regenerates reliably is the lizard
tail. This is often lost by autotomy as a protective measure, whereupon
a new outgrowth develops. This regenerated tail, however, differs from
the original in that its internal skeleton consists of an unsegmented,
tapered cartilaginous tube instead of true vertebrae. In this respect,
therefore, it is of particular interest from the morphogenetic point of
view.
Birds and mammals are unable to regenerate any of the structures
which lower vertebrates can replace. Certain epidermal appendages,
such as feathers, hairs and claws can be regenerated, and indeed are
regularly replaced as a part of the moulting phenomenon. To a limited
extent, teeth also can be replaced. However, the more histologically
complex structures, such as tails and legs, cannot regenerate despite the
fact that their individual component tissues are regeneratively competent. Epidermis, nerve fibres, muscle fibres, and bones all retain
considerable reparative powers, yet they are unable to co-ordinate
sufficiently to produce any organized outgrowth. The only mammalian
appendages possessing the faculty to regenerate are the antlers of
deer.
In view of the unique phenomenon which antler growth represents,
this process probably deserves more serious scientific attention than it
has thus far received. Antlers are developmentally interesting for many
reasons, not the least of which is their remarkably rapid rate of growth,
which in some larger specimens may exceed one half a centimetre per
day. Consisting of bone, nerves, blood vessels and skin, they are, except
for the absence of muscle and articulating segments, as morphologically
complex as limbs or tails. Usually present only in male animals, antlers
are shed in the winter or spring and replaced by new outgrowths during
spring and summer. The growth of antlers is achieved by the production
of apically situated undifferentiated cells, which subsequently become
chondrified and then converted directly into bone. As long as growth
continues the antlers are enveloped in skin, but due to circulatory
restrictions occurring in late summer the skin becomes necrotic and is
rubbed off. This exposes the bare bony antler which is dead tissue.
Ultimately the whole antler will be shed following the weakening of its