http://www.collembola.org/publicat/integum/microtub.htm
Last updated on
2012.12.06
by Frans Janssens
Frans Janssens,
Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium
Jean-Auguste Barra,
Laboratoire de Zoologie, Université Louis Pasteur, Strasbourg, 67000, France
Luc De Bruyn,
Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium
In construction.
Abstract
We propose a new model for
the formation of the secondary square headed microtubercles
and a new generic hypothesis for the formation
of the epicuticular microtubercles of Collembola.
Preliminary note
The terminology 'primary' and 'secondary' granules or tubercles of the cuticula
is confusing.
In general 'primary tubercles' refers to the epicuticular microtubercles with
a triangular shape. The triangular shape is assumed to be the basic form.
Traditionally, 'secondary tubercles' are microtubercles with a different
shape that may be derived from the primary microtubercles
with a triangular shape.
However, Nickerl & al (2012:4) use the term 'secondary granules'
for the tubercular constructs of the exocuticula/epidermis on which the
primary microtubercular pattern is superimposed.
In this paper, we prefer to use the following terminology
1. primary microtubercles : the epicuticular triangular microtubercles,
2. secondary microtubercles : the epicuticular square and/or polygonal
microtubercles,
3. tubercles : the exocuticular tubercles on which the
primary microtubercular pattern is superimposed.
Introduction
Fig.1. Formation of square headed microtubercle
After Lawrence & Massoud (1973)
 |
Fig.1bis. Simulation of square microtubercles derived from fused triangular microtubercles
Modified after Nickerl & al. 2012 Fig.3E
 |
Lawrence & Massoud (1973), cited from Palissa (2000:4,fig.6),
model the secondary square microtubercles as a straightforward fusion of
two primary triangular microtubercles (fig.1).
Given each triangular microtubercle carries an orthogonal wax channel in the
center of the microtubercle, a square microtubercle, when formed by two
triangular microtubercles, then should have 2 such parallel channels
(see simulation fig.1bis).
However, this is in contradiction with what is observed (fig.Iv).
Therefore we propose an alternative hypothesis.
We have found some evidence that the (secondary) square microtubercles
are formed directly by a combination of four epicuticular ridges.
Observations
Fig.Ct. Coecobrya tenebricosa (bar=1µm)
Triangular microtubercles
After Nickerl & al. 2012 Fig.3E
 |
Fig.Iv. Isotoma viridis (bar=500nm)
Square microtubercles
After Nickerl & al. 2012 Fig.2B''
 |
Fig.Fq. Folsomia quadrioculata
Triangular, square and pentagonal microtubercles
After Nickerl & al. 2012 Fig.4C
 |
The shape of the microtubercle is clearly related to the number of ridges
that interconnect to the microtubercle.
In Coecobrya tenebricosa the epicuticular ultrastructure is formed by
a raster of interconnecting ridges that connect per three ridges forming as
such triangular microtubercles (fig.Ct).
In Isotoma viridis the epicuticular ultrastructure is formed by
a raster of interconnecting ridges that connect per four ridges forming as
such square microtubercles (fig.Iv).
In Folsomia quadrioculata the epicuticular ultrastructure is formed by
a more complex raster of interconnecting ridges that connect per three,
per four, or per five ridges forming as such respectively triangular,
square and pentagonal microtubercles (fig.Fq).
More complex polygonal microtubercles may be formed when even more ridges get
interconnected (see fig.Fq).
Note all microtubercles have one central orthogonal channel.
Fig.2. TEM of tangential section of epicuticula of
Isotomurus palustris
Barra (1973 unpublished)
 |
A tangential ultra-thin section of the epicuticular square microtuberculae of
Isotomurus palustris (Barra 1973 unpublished) (fig.2)
shows three types of transparancy, an indication of three different
types of epicuticular material.
This section is taken at a slight cuticular depression: the upper left
corner and the lower right corner section occurs at the microtubercular basis
and trough the interconnecting ridges,
while in the center the section occurs at a level above the ridges.
Checking a tangential microtubercular section that is taken sufficiently
high above the epicuticular 'ground' surface (fig.2: A)
one can easily distinguish:
1. a very light, electron translucent material a at the sides
of the square head; this material continues centrally away from the
walls of the square, forming a kind of inwards triangular section.
This material has the same level of opacity as the material of the ridges
(e.g., see fig.2 upper left corner). So, we can assume it is the same material.
2. a very dark, electron dense material b at the corners
of the square head.
3. at the core of the microtubercle,
a less dark, less electron dense material c in the form of a cross.
Discussion
The square microtubercle is formed by a
combination of four interconnecting ridges.
These four ridges do not actually 'fuse' together:
they can still be recognised as isolated triangles in the
tangential sections of the square microtubercles.
Put otherwise, the microtubercle remains hollow, an orthogonal channel
remains available.
We consider this mechanism of combining ridges as a generic
mechanism for the formation of any shape of microtubercle.
Note that in fig. 2. there are a few aberrant triangular microtubercles present
among the square microtubercles.
Note that the secondary triangular microtubercle (fig.2: B)
is formed by three ridges.
Fig.4. Section of square microtubercle
Model
 |
Fig.3. Section of square microtubercle
Barra (1973 unpublished)
 |
We will now define a new model for the secondary square microtubercle
based on one of the tubercles in fig.2. Tubercle A is enlarged in fig.3.
The model of the square microtubercle is based on the following generic model
of an interconnecting ridge. The main body of the ridge is modelled as
a cylinder. At each end of the cylinder, a torus is placed transversally.
The radius of the torus is proportional to the radius of the cylinder.
The torus simulates the ridge end (= the wall of the
microtubercular pillar that is penetrated by the ridge). As each ridge forms
a bridge between two microtubercles, in our model we consider the
interconnected pillar walls as being part of the ridge model.
To model the internal structure of the microtubercle, the ridge ends are
interconnected by 'internal ridges' that are strongly reduced in size.
By using such a universal ridge model,
simulating microtubercle shapes boils down to placing properly dimensioned
ridge models in their appropriate arrangement.
The simulation of the square microtubercle (of fig.3) can be seen in fig.4,
in which the
tangential section is simulated by clipping the model with a horizontally
displaced plane.
The four 'internal' ridge ends are obviously recognised and compare well
with the TEM micrograph (fig.3).
The dark patches at the square corners and in the core of the square
microtubercle appear to correspond, according to the model,
with the hollow body of the microtubercle and the central one corresponds
with the transmicrotubercular (wax) channel.
Conclusion
The formation of the shape of the head of the epicuticular microtubercle
is the result of a generic process of combining interconnecting
ridges. Three ridges form a triangular microtubercle.
Four ridges form a square microtubercle.
More ridges form a polygonal microtubercle.
A correlation can be made between the shape of the microtubercle and the type
of microtubercular arrangement:
Table I. Relation between microtubercular shape and arrangement
| Shape of microtubercle | Arrangement |
|---|
| Triangular | Hexagonal |
| Square | Rhombic |
| Polygonal | Irregular |
History
- 2001.12.31: bare bones release.
- 2002.08.20: Lawrence & Massoud 1973, cited from Palissa, 2000.
- 2012.12.06: Nickerl & al, 2012.
References