Compartments in Scyphozoa

Abstract

Polyps of Scyphozoa have a cup-shaped body. At one end is the mouth opening surrounded by tentacles, at the other end is an attachment disc. The body wall consists of two tissue layers, the ectoderm and the endoderm, which are separated by an extracellular matrix, the mesoglea. The polyp's gastric cavity is subdivided by septa running from the apical end to the basal body end. The septa consist of two layers of endoderm and according to biology textbooks the number of septa is four. However, in rare circumstances Aurelia produces polyps with zero, two, six, or eight septa. We found that the number was always even. Therefore we propose that two types of endoderm exist, forming alternating stripes running from the oral body end to the aboral end. The stripes have some properties of developmental compartments. Where cells of different compartments meet, they form a septum. We also propose that the ectoderm is subdivided into compartments. The borders of the ectodermal and endodermal compartments are perpendicular to each other. Tentacles of the polyp and rhopalia (sense organs) of the ephyra (young medusa), respectively, develop at the border between two ectodermal compartments. The number can be even or odd. Rhopalia formation is particularly favored where two ectodermal and two endodermal compartments meet.

A model for tube formation and branching in Cnidaria

“…As a result, the formation of four new compartments is favoured in the very tip ( Figure 5). In rare cases, by chance a different number of compartments forms, but this number was found to be always even and is therefore consistent with the proposed model [3]. With distance from the tip, and thus with decreasing positional value, the width of the endodermal compartments increases.…”

“…There are several ways in which a "lateral activation of locally exclusive states" can be accomplished [8]. In fact, a different set of equations as used here have been used elsewhere to describe compartment formation in the endoderm of polyps of Scyphozoa and Anthozoa [3,4]. This may indicate that it is much too early to discuss details or specific molecules in such a mechanism.…”

“…Our assumption is that a certain tissue property, called "positional value" or "sources" in Cnidaria research, changes gradually from the basal to the apical position of a field of cells. The positional value d affects the system of lateral activation of locally exclusive states (denoted by d in equations 3b and 4b) [3,4]. A simulation of the equations 1a, 2a, 3b and 4b in a field of cells which display a flat gradient of positional values along the y-axis results in stripes of A and B cells parallel to that axis ( Figure 3B).…”

“…Owing to the interactions, two times two populations of A-and B-cells form which develop into four short stripes originating in the tip of the future cone-like protrusion. Parameters as in Figure 3, into two times two compartments, forming longitudinal stripes (Endo A and Endo B) [3]. Epithelial cells of the same compartment stay together, but form a septum where cells of different compartments meet .…”

“…Recently an alternative has been discussed [3,4]. The basic idea is that the epithelium which finally forms a tube is not uniform.…”