Influence of Implanted Micromeres on Reduction
Gradients and Mitochondrial Distribution in
developing Sea-urchin Eggs
by SVEN HORSTADIUS1
From the Zoological Institute, Uppsala
(1936) has described reduction gradients in different stages of developing sea-urchin and starfish larvae. His results concerning the gradients in seaurchin mesenchyme blastulae and early gastrulae have been confirmed with
Janus green. There exists a stronger vegetal (acropetal) reduction gradient and
a weaker animal one, appearing later, in the normal whole larva. Isolated
vegetal halves show a similar vegetal reduction gradient, but in most cases no
animal gradient. In isolated animal halves the reduction starts in the thick plate
of the enlarged apical tuft and no vegetal gradient appears. Micromeres implanted laterally in the animal part of a whole egg induce there a reduction
centre. Four micromeres implanted into an isolated animal half induce a new
vegetal reduction gradient and restrict the animal gradient, so that the reduction
gradient system now resembles that of a normal egg. A comparison of isolated
animal and vegetal halves shows that the reduction seems to start at the same
time and to proceed simultaneously in both kinds of halves. We can, therefore,
state that the metabolism in isolated animal halves is not identical with that of
the same material when forming the animal part of a normal larva. A comparison
of the reduction gradients in these different cases shows a remarkable conformity
with the animal and vegetal morphogenetic gradients known from the analysis
by fragmentation and transplantation experiments. (These results have been
published in full, Horstadius, 1952.)
Lindahl (1936) has suggested that the animal principle in the sea-urchin egg
is connected with carbohydrate metabolism, whereas the vegetal one in later
stages might be characterized by a breakdown of proteins. Gustafson (1952) has
contributed evidence that mitochondria contain a series of enzymes involved
in protein synthesis. The animal metabolic type favours the development of
mitochondria whereas the vegetal produces inhibitors for mitochondrial development. In a normal late blastula or early gastrula the mitochondria thus
decrease in number from the animal towards the vegetal pole. In corresponding
CHILD
1
Author's address: Zoologiska Institutionen, Uppsala, Sweden.
[J. Embryol. exp. Morph. Vol. 1, Part 3, pp. 257-259, September 1953]
258
S. H O R S T A D I U S — G R A D I E N T S I N S E A - U R C H I N E G G S
stages of eggs animalized by iodosobenzoic acid the distribution is more uniform, due to the weakening of the vegetal metabolism. In Li-treated larvae the
gradient is depressed, particularly in the more vegetal regions (Gustafson &
Lenicque, 1952).
80
60
40
an.
\\an.
+4
^--.... \ \.an.+8
*"" ^ ^""**** Control
20
o
An., pole
Veg. pole
FIG. 1. Curves showing the mitochondrial distribution along the
animal-vegetal axis in late blastulae-early gastrulae of seaurchins (Control), in isolated animal (an.) and vegetal halves
(veg.) and in animal halves with 4 (an. + 4) or 8 implanted micromeres (an.+ 8). R.M.D., relative mitochondrial density. From
Lenicque, Horstadius, & Gustafson, Exp. Cell Res. (in press).
Dr. Gustafson, Mr. Lenicque, and I have made experiments to investigate
whether, in analogy with the induction and inhibition of reduction gradients,
differences in the distribution of mitochondria also occur when comparing
whole eggs, isolated animal halves, and vegetal halves as well as animal halves
with implanted micromeres.
The results are shown in Fig. 1. The mitochondrial curve of animal halves
(an.) not only lies at a higher level but also differs in shape from that of a whole
egg at a corresponding stage {Control). The curve of isolated vegetal halves
{veg.) is found at a still lower level than that of control eggs and it is less complicated in form, nearly a straight line. The depressing power of the vegetal
principle on the mitochondrial gradient is clearly demonstrated by implanting
S. HORSTADIUS—GRADIENTS IN SEA-URCHIN EGGS
259
micromeres into isolated animal halves. The addition of four micromeres (an.
+ 4) results in a curve lying between those of an animal half and of a normal
egg. If eight micromeres have been implanted (an. + 8) the curve lies lower than
that of the controls and resembles more that of vegetal halves.
REFERENCES
CHILD, C. M. (1936). Roux Arch. EntwMech. Organ. 135, 426.
GUSTAFSON, T. (1952). Nitrogen Metabolism, Enzymic Activity, and Mitochondrial Distribution in relation to Differentiation in the Sea-Urchin Egg. Uppsala: Almqvist & Wiksells
Boktryckeri AB.
& LENICQUE, P. (1952). Exp. Cell Res. 3, 251.
HORSTADIUS, S. (1952). /. exp. Zool. 120, 421.
LINDAHL, P. E. (1936). Acta Zool., Stockh. 17, 179.