Previous morphological studies of cell sorting in Hydra vulgaris have considered only clusters of cells. Here, we present ultrastructural observations on the adherent region of cell pairs brought into contact (following dissociation) using a three-dimensional laser manipulator. There was a much larger area of close membrane contact between endodermal cell pairs in comparison with ectodermal cell pairs. Separation distances between membranes were categorized into three classes: closest distance (<4 nm); medium distance (5–25 nm); and cleavage (>25 nm). The sum of distances in the closest and medium categories as a proportion of total contact length was significantly greater (P<0.01) for endodermal cells (49.0+/−6.5 %) than for ectodermal cells (26.7+/−4. 4 %). In intact Hydra, this sum of distances was also significantly greater for endodermal cells, indicating that newly adherent cells, even after adhesion for only 10 min, display similar morphological characteristics to cells in intact Hydra. This suggests that close membrane contacts contribute to differential cell adhesion, which may form the basis of the cell sorting process.

REFERENCES

Berking
S.
(
1997
).
Pattern formation in Hydrozoa
.
Naturwissenschaften
84
,
381
–.
Gierer
A.
,
Berking
S.
,
Bode
H.
,
David
C. N.
,
Flick
K. M.
,
Hansmann
G.
,
Schalle
H.
,
Trenkner
E.
(
1972
).
Regeneration of Hydra from reaggregated cells
.
Nature New Biol
239
,
98
–.
Kishimoto
Y.
,
Murate
M.
,
Sugiyama
T.
(
1996
).
Hydra regeneration from recombined ectodermal and endodermal tissue. I. Epibolic ectodermal spreading is driven by cell intercalation
.
J. Cell Sci
109
,
763
–.
Overton
J.
(
1977
).
Formation of junctions and cell sorting in aggregates of chick and mouse cells
.
Dev. Biol
55
,
103
–.
Sarras
M. P.
Jr.
,
Huff
J. K.
,
Palmiter-Thomas
P.
(
1992
).
High molecular weight secretory products of the human pancreatic ductal epithelium and the effect of secretin on their discharge
.
Pancreas
2
,
132
–.
Sarras
M. P.
Jr.
,
Yan
L.
,
Grens
A.
,
Zhang
X.
,
Agbas
A.
,
Huff
J.
,
St John
P. L.
,
Abrahamson
D. R.
(
1994
).
Cloning and biological function of laminin in Hydra vulgaris
.
Dev. Biol
164
,
312
–.
Sato-Maeda
M.
,
Uchida
M.
,
Graner
F.
,
Tashiro
H.
(
1994
).
Quantitative evaluation of tissue-specific cell adhesion at the level of a single cell pair
.
Dev. Biol
162
,
77
–.
Shimizu
H.
,
Sawada
Y.
,
Sugiyama
T.
(
1993
).
Minimum tissue size required for hydra regeneration
.
Dev. Biol
155
,
287
–.
Steinberg
M. S.
(
1970
).
Does differential adhesion govern self-assembly processes in histogenesis? Equilibrium configurations and the emergence of a hierarchy among populations of embryonic cells
.
J. Exp. Zool
173
,
395
–.
Sugiyama
T.
,
Fujisawa
T.
(
1977
).
Genetic analysis of developmental mechanisms in hydra. I. Sexual reproduction of Hydra magnipapillata and isolation of mutants
.
Dev. Growth Differ
19
,
187
–.
Technau
U.
,
Holstein
T. W.
(
1992
).
Cell sorting during the regeneration of Hydra from reaggregated cells
.
Dev. Biol
151
,
117
–.
West
D. L.
(
1978
).
The epitheliomuscular cell of Hydra: its fine structure, three-dimensional architecture and relation to morphogenesis
.
Tissue & Cell
10
,
629
–.
Wood
R. L.
(
1977
).
The cell junctions of Hydra as viewed by freeze-fracture replication
.
J. Ultrastruct. Res
58
,
299
–.
Wood
R. L.
,
Kuda
A. M.
(
1980
).
Formation of junctions in regenerating Hydra: septate junctions
.
J. Ultrastruct. Res
70
,
104
–.
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