LIMB MORPHOGENESIS IN VERTEBRATES
185
limb differentiate in a proximo-distal sequence in the apical zone of the
limb bud under the inductive influence of the a.e.r. However, the precise origin of the new cellular material successively differentiated distally still remains obscure. It is generally agreed, although there is no
supporting experimental evidence, that it comes from the adjacent
mesodermal area which was determined in a previous step.
(b) The role of the a.e.r. in limb development has been elucidated by
a great deal of experimental work (Saunders, 1948; Zwilling, 1955,
1956b; Tschumi, 1956; Hampe, 1957; Saunders et ah, 1957a.)
The function of that structure is to promote the growth of the distal
mesoderm and to assure its regional determination. This activity is not
specific, for the a.e.r. is unable to modify the original properties of
the mesoderm (Zwilling, 1955). The a.e.r. is actually considered as the
only part of the limb bud ectoderm possessing morphogenetic properties
acting on the development of the limb. However, this view is still controversial (Amprino and Camosso, 1955; Bell, Kaighn and Fessenden,
1959).
(c) More recent investigations have revealed the existence of a primary
and dominant morphogenetic activity asymmetrically distributed in the
postaxial part of the mesoderm of the chick embryo limb bud (Zwilling,
1956a, b, c; Zwilling and Hansborough, 1956; Saunders et al., 1958;
Amprino and Camosso, 1959). It spreads from proximal to more distal
regions and maintains the morphological and the functional integrity of
the a.e.r. during the first stages of limb morphogenesis. Its absence in
some abnormal conditions is followed by the regression of the ectodermal
thickening and subsequently by a defective limb development (Zwilling,
1956c). According to Amprino and Camosso, who do not acknowledge
the role of the a.e.r., the mesodermal factor acts directly on the more
distal parts of the mesoderm giving them regional specificity.
III. Histochemical and Morphological Data
The distribution of ribonucleic acid (RNA), glycogen, mucopolysaccharides, alkaline and acid phosphatases has been studied in serial
sections at the most important embryonic stages of limb development.
These substances are too few to enable us to understand the metabolic
mechanisms in which they intervene. At present we simply regard histochemical analysis as a means of detecting, with high topographical
specificity, the functional areas of embryonic structures. In some cases,
these activities may be unequally distributed in a cellular field which
seemed to be completely homogeneous when studied with ordinary
histological methods. In other cases, there is a clear relation between the
distribution of some substances and the local vascular pattern; these
185
limb differentiate in a proximo-distal sequence in the apical zone of the
limb bud under the inductive influence of the a.e.r. However, the precise origin of the new cellular material successively differentiated distally still remains obscure. It is generally agreed, although there is no
supporting experimental evidence, that it comes from the adjacent
mesodermal area which was determined in a previous step.
(b) The role of the a.e.r. in limb development has been elucidated by
a great deal of experimental work (Saunders, 1948; Zwilling, 1955,
1956b; Tschumi, 1956; Hampe, 1957; Saunders et ah, 1957a.)
The function of that structure is to promote the growth of the distal
mesoderm and to assure its regional determination. This activity is not
specific, for the a.e.r. is unable to modify the original properties of
the mesoderm (Zwilling, 1955). The a.e.r. is actually considered as the
only part of the limb bud ectoderm possessing morphogenetic properties
acting on the development of the limb. However, this view is still controversial (Amprino and Camosso, 1955; Bell, Kaighn and Fessenden,
1959).
(c) More recent investigations have revealed the existence of a primary
and dominant morphogenetic activity asymmetrically distributed in the
postaxial part of the mesoderm of the chick embryo limb bud (Zwilling,
1956a, b, c; Zwilling and Hansborough, 1956; Saunders et al., 1958;
Amprino and Camosso, 1959). It spreads from proximal to more distal
regions and maintains the morphological and the functional integrity of
the a.e.r. during the first stages of limb morphogenesis. Its absence in
some abnormal conditions is followed by the regression of the ectodermal
thickening and subsequently by a defective limb development (Zwilling,
1956c). According to Amprino and Camosso, who do not acknowledge
the role of the a.e.r., the mesodermal factor acts directly on the more
distal parts of the mesoderm giving them regional specificity.
III. Histochemical and Morphological Data
The distribution of ribonucleic acid (RNA), glycogen, mucopolysaccharides, alkaline and acid phosphatases has been studied in serial
sections at the most important embryonic stages of limb development.
These substances are too few to enable us to understand the metabolic
mechanisms in which they intervene. At present we simply regard histochemical analysis as a means of detecting, with high topographical
specificity, the functional areas of embryonic structures. In some cases,
these activities may be unequally distributed in a cellular field which
seemed to be completely homogeneous when studied with ordinary
histological methods. In other cases, there is a clear relation between the
distribution of some substances and the local vascular pattern; these