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EDGAR
ZWILLING
contributes to the formation of the limb. Despite experimental evidence
that limbs may develop without such tissue, this view persists and
receives support from many descriptions which reveal that the ventral
border of adjacent somites actually sends a tongue of tissue into the
limb bud (Milaire, 1957). Be this as it may, the mesodermal thickening
at the limb site increases in size and becomes recognizable as a limb
bud. Shortly after the mesodermal thickening appears, the overlying
ectoderm, initially indistinguishable from the adjacent regions, becomes
thickened. At first this thickening is due (in the chick) to the cuboidal
cells of the epithelial covering becoming more columnar. Subsequently
the portion of the epidermis which covers the free edge of the bud becomes thickened. In most vertebrate limb buds this thickening takes
the form of a raised crest or ridge, the so-called apical ectodermal ridge.
In the chick embryo (Saunders, 1948) and in reptiles (Milaire, 1957)
the cells of this ridge (in cross section) take the form of a nipple-like
extension of the epidermis and appear to be a pseudo-stratified columnar
epithelium. In other forms there may be a thickened cap on the conical
outgrowth (most amphibia) or an apparently multi-layered ridge which
conforms more to the outline of the mesoblast than does the chick
ridge. Shortly after the formation of a limb bud a number of distinctive
asymmetries become apparent. The dorsal surface is more rounded and
the apical ridge is located more ventrad, giving the ventral surface a
flatter and shorter appearance. The ridge itself is higher (thicker) in the
mid portion of the bud than at either end.
The next prominent event is the elongation of the bud. This grows out
of the body of the embryo and forms, initially, an elongated core of
mesoderm with an epidermal covering. A prominent ectodermal ridge,
which is restricted to the distal end and tapers down to the thin pre- and
post-axial ectoderm, is present at this stage. Eccentricities in the shape
of the limb with associated asymmetries of the apical ridge of ectoderm
soon become evident. Shortly after the initial elongation, the limb paddle
is formed at the distal end of the appendage. This region is characteristically flattened and broad and from it will form the digits. At this stage
the apical ectodermal ridge is still obvious and is restricted to the region
of the paddle.
Histological observations reveal that the mesoderm in the limb bud
(e.g. chick) consists initially of a compact, poorly vascularized outer
zone which encloses an inner, sinus riddled, spongy core. Even while
distal elongation is still taking place the character of the more proximal
regions begins to change: the central core of mesoderm becomes more
compact and the distinction between the two zones becomes less obvious.
This marks the initiation of the mesenchymal condensation which leads
to the formation of cartilage. The condensation of the limb cartilages
EDGAR
ZWILLING
contributes to the formation of the limb. Despite experimental evidence
that limbs may develop without such tissue, this view persists and
receives support from many descriptions which reveal that the ventral
border of adjacent somites actually sends a tongue of tissue into the
limb bud (Milaire, 1957). Be this as it may, the mesodermal thickening
at the limb site increases in size and becomes recognizable as a limb
bud. Shortly after the mesodermal thickening appears, the overlying
ectoderm, initially indistinguishable from the adjacent regions, becomes
thickened. At first this thickening is due (in the chick) to the cuboidal
cells of the epithelial covering becoming more columnar. Subsequently
the portion of the epidermis which covers the free edge of the bud becomes thickened. In most vertebrate limb buds this thickening takes
the form of a raised crest or ridge, the so-called apical ectodermal ridge.
In the chick embryo (Saunders, 1948) and in reptiles (Milaire, 1957)
the cells of this ridge (in cross section) take the form of a nipple-like
extension of the epidermis and appear to be a pseudo-stratified columnar
epithelium. In other forms there may be a thickened cap on the conical
outgrowth (most amphibia) or an apparently multi-layered ridge which
conforms more to the outline of the mesoblast than does the chick
ridge. Shortly after the formation of a limb bud a number of distinctive
asymmetries become apparent. The dorsal surface is more rounded and
the apical ridge is located more ventrad, giving the ventral surface a
flatter and shorter appearance. The ridge itself is higher (thicker) in the
mid portion of the bud than at either end.
The next prominent event is the elongation of the bud. This grows out
of the body of the embryo and forms, initially, an elongated core of
mesoderm with an epidermal covering. A prominent ectodermal ridge,
which is restricted to the distal end and tapers down to the thin pre- and
post-axial ectoderm, is present at this stage. Eccentricities in the shape
of the limb with associated asymmetries of the apical ridge of ectoderm
soon become evident. Shortly after the initial elongation, the limb paddle
is formed at the distal end of the appendage. This region is characteristically flattened and broad and from it will form the digits. At this stage
the apical ectodermal ridge is still obvious and is restricted to the region
of the paddle.
Histological observations reveal that the mesoderm in the limb bud
(e.g. chick) consists initially of a compact, poorly vascularized outer
zone which encloses an inner, sinus riddled, spongy core. Even while
distal elongation is still taking place the character of the more proximal
regions begins to change: the central core of mesoderm becomes more
compact and the distinction between the two zones becomes less obvious.
This marks the initiation of the mesenchymal condensation which leads
to the formation of cartilage. The condensation of the limb cartilages