CHAPTER
VII
LIMB
MORPHOGENESIS
EDGAR ZWILLING
I. Introduction
301
A.
Descriptive
301
Β.
Experimental Background
303
II. An Hypothesis of Limb Development
305
A.
Evidence for the Hypothesis
307
B.
Mechanisms
319
III. Proximo-distal Sequence in Limb Development
321
IV. Mapping Presumptive Limb Areas
323
V. Regulatory Phenomena
324
VI. Cell Death in Limb Morphogenesis
327
VIL Tissue Mass Phenomena in Limb Development
327
References
328
I. Introduction
This chapter will be devoted to an account of the development of
limbs of tetrapod vertebrates. In it I would like to trace the elaboration
of a limb from its earliest known inception and include in this synthesis
as much information as possible about the mechanisms which are
important in the morphogenetic processes involved. At the same time
I should like to point out some of the problems which are still to be
resolved. Much of the early material will be omitted in order that recent
information may be discussed more fully.
A. Descriptive
Whether in an embryo whose limbs begin to form early or one in
which limbs form relatively late (amphibia) the first indication of an
appendage is a thickening of the somatopleure lateral to the somites and
frequently in fairly close association with them. In some forms there
is an indication that the paired limb thickenings are the persisting
remnants of a ridge (crest of Wolff) which extends the length of the
embryo and which has been related to the fin fold hypothesis of limb
origin of Gegenbaur (1865) and Braus (1906). In many embryos a crest
of Wolff cannot be discerned. The proximity of the initial limb swelling
to the somites has led many authors to conclude that somitic tissue
301
VII
LIMB
MORPHOGENESIS
EDGAR ZWILLING
I. Introduction
301
A.
Descriptive
301
Β.
Experimental Background
303
II. An Hypothesis of Limb Development
305
A.
Evidence for the Hypothesis
307
B.
Mechanisms
319
III. Proximo-distal Sequence in Limb Development
321
IV. Mapping Presumptive Limb Areas
323
V. Regulatory Phenomena
324
VI. Cell Death in Limb Morphogenesis
327
VIL Tissue Mass Phenomena in Limb Development
327
References
328
I. Introduction
This chapter will be devoted to an account of the development of
limbs of tetrapod vertebrates. In it I would like to trace the elaboration
of a limb from its earliest known inception and include in this synthesis
as much information as possible about the mechanisms which are
important in the morphogenetic processes involved. At the same time
I should like to point out some of the problems which are still to be
resolved. Much of the early material will be omitted in order that recent
information may be discussed more fully.
A. Descriptive
Whether in an embryo whose limbs begin to form early or one in
which limbs form relatively late (amphibia) the first indication of an
appendage is a thickening of the somatopleure lateral to the somites and
frequently in fairly close association with them. In some forms there
is an indication that the paired limb thickenings are the persisting
remnants of a ridge (crest of Wolff) which extends the length of the
embryo and which has been related to the fin fold hypothesis of limb
origin of Gegenbaur (1865) and Braus (1906). In many embryos a crest
of Wolff cannot be discerned. The proximity of the initial limb swelling
to the somites has led many authors to conclude that somitic tissue
301