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& Dixon, 1978a
). Throughout pre- and prometamorphosis, the connective
tissue remains immature and very thin except in the typhlosole. When the
adult stem cells appear at stage 60, the connective tissue suddenly increases in
thickness, total cell number, and proliferative activity. In addition, remark-
able ultrastructural changes occur in the connective tissue close to the epi-
thelium. The thin and continuous basal lamina suddenly becomes thickened
by vigorous folding and then amorphous (
Fig. 11.3
A and B;
Ishizuya-Oka &
Shimozawa, 1987b
). Through such modified basal lamina, subepithelial
fibroblasts that possess well-developed rough endoplasmic reticulum often
make contacts with the adult progenitor/stem cells. Thereafter, as the adult
epithelial cells differentiate, their basal lamina becomes thin again, and the
cell contacts between the two tissues become rare. This modification of the
basal lamina following the fibroblast-epithelial cell contacts can be reproduced
in the tadpole intestine cultured
in vitro
, whenever the adult epithelium suc-
cessfully develops (
Ishizuya-Oka & Shimozawa, 1992
). These observations
A
B
LE
AE
BL
BL
F
rER
D
C
LE
BL
F
F
Figure 11.3 Epithelial
-
connective tissue interfaces in normal X. laevis intestines (A and B)
and heat shock-treated intestines of wild-type (Wt) (C) or ST3 transgenic (Tg) tadpoles
(D). Throughout pre- and prometamorphosis, the basal lamina (BL) underneath the larval
epithelium (LE) is thin and continuous (A). At the start of metamorphic climax, the basal
lamina becomes thickened through vigorous folding (B). Subepithelial fibroblasts
(F) possessing well-developed rough endoplasmic reticulum (rER) often make contact
with the adult progenitor/stemcells (AE) (inset; arrowheads). In premetamorphic (stage 54)
Wt intestine, the basal lamina remains thin after heat shock (C). However, it becomes
amorphous or absent in Tg intestine overexpressing ST3 (D), where subepithelial fibro-
blasts often make contact with the epithelium (arrowhead). Bars: 1
m.
m
