Tardigrades - embryonic and egg development (I)

As microscope amateurs we should be modest and aware of our limitations: most of us work with technical equipment that was available in a similar manner already in the 19th century. And, as a rule an amateur doesn't have a big chemical laboratory.
But we have seen already that many investigations like e.g. into tardigrade anatomy, social behaviour, hatching etc. are possible with an ordinary school microscope.
When attempting to study the embryonic development things get more complicated. Some processes can be seen only during a short period of time, the decisive details might be small and suffer from poor contrast.
So we will have to be patient. Do not expect that those cell divisions jump out of your microscope within a few seconds ...

As a rule the embryonic development can be studied more clearly at those tardigrade eggs that have smooth cell walls and no so-called projections. Projections might look very attractive but at the same time tend to blur our overall visual impression of the egg:

Macrobiotus sp. egg, freely deposited.
Detail of the surface area with the egg projections that look like reversed egg cups in this particular case.

A true artwork by nature but definitely not the appropriate object for embryonic studies.

Bright field illumination, oil immersion objective, high magnification.

A little bit more transparent but still not the best 'insight choice': an egg by Adorybiotus coronifer (Richters)   with 'chillies type' projections and a strong yellow colour:

Yellow egg by the tardigrade Adorybiotus coronifer
from the Bavarian Alps.
Maximum diameter ca. 150 µm.

In some cases the eggs can be seen already in the living animal:

At closer inspection the egg cells might show tiny details indicating whether the egg will become smooth or will have projections. In the image below egg projections are definitely present but still flattened due to the early state of development:

Egg cells in the body of a tardigrade.
The red arrows mark the projections.
Image with ca. 0.1 mm.

The number of egg cells forms may vary within a wide range, from one or two until to 30 or even more. The image below indicates the percentage of the mother body volume dedicated to the tardigrade kids.

Tardigrade (eutardigrade)
with ca. 15 egg cells.
Due to the dark field illumination the egg cells appear white.

When looking closer we might even see the cell nuclei within those egg cells:

Tardigrade Ramazzottius oberhaueseri, detail with 6 egg cells. Raking light illumination for contrast enhancement.
Image width ca. 200 µm.

Already in 1895 Raphael von Erlanger published a long article featuring the embryonic development of Macrobiotus macronyx Dujardin illustrated with 26 (!) colour lithos. As far as the methodology is concerned he notes: "The eggs were immersed in glycerine under a cover glass supported by two glass threads. By this means it was possible to move the eggs and to study them in all orientations."


Come in again next month if you like :-)

Literature

Erlanger, Raphael v.: Beiträge zur Morphologie der Tardigraden.
Morphologisches Jahrbuch, XXII (1895) p. 491 - 513, plus 2 tables.

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