Extraembryonic membranes – how do they develop?

The eggs of Tribolium beetles give rise to small larvae, which have legs, can crawl around and will most of all eat. But after the fertilization of the egg, there is still a long way to go before a fully formed larva will hatch. A very important part of the development of these eggs is the formation of two extraembryonic membranes. One of these membranes (the serosa) is currently the focus of my research so here I will explain how these membranes are formed during development.

A newly laid egg is not much more than a big sac full of fat and protein with only one nucleus (half of the DNA from the mother, half from the father). In insects, the first step in development is to multiply this DNA many times and form a single cell layer on the outside of the egg. This stage with the single cell layer is called the blastoderm stage.

Forming the blastoderm

The starting point of insect eggs is an egg with one single nucleus in the egg yolk (Figure below, step 1). This single nucleus will replicate many times until many nuclei are in the yolk (step 2). These nuclei will move towards the periphery of the egg (step 3), and once there, new cell membranes will be formed around each nucleus (step 4). Once the cell membranes have formed, the egg is covered by a single cell layer and this stage is called the blastoderm stage. Not all cells from this layer will become part of the embryo. From this single layer 3 different cell types originate; cells which will form the embryo (Figure below, green), cells which will form an extraembryonic membrane called the amnion (blue) and cells which will form an extraembryonic membrane called the serosa (red).



For further development, the embryo will move from the surface of the egg to the inside of the egg. This is part of the process called gastrulation and also involves the movements of the extraembryonic membranes. Within the embryonic tissue the cells are reorganized too but I will not discuss that here.
The movements of the extraembryonic membranes and the embryo will lead to the embryo being inside the egg and the extraembryonic serosa completely surrounding both the embryo and the yolk (see Figure below). The extraembryonic amnion will be on the bottom of the embryo.


I have illustrated these movements of the extraembryonic membranes and the embryo in a movie:

We can also see this same process in the live embryo. Tania Vazquez Faci made movies of living Tribolium embryos during gastrulation. In these eggs the cell nucleus are stained green. What you can see is that the egg is in the blastoderm stage and the extraembryonic serosa will move around the embryo. The right side of the egg (where the embryo will form) will first become more densely populated with cells (more green) and shortly after that the serosa will fold around it.

Further development and dorsal closure

The embryo will continue developing inside the egg. Although I will not go into too much detail, there is one process that I would like to discuss. This process involves the extraembryonic membranes, the amnion and the serosa. So far, the embryo is still much like a multi-layered sheet of cells, but for development to continue this sheet of cells needs to form a sphere in which the organs of the larva can develop further. They do this by connecting the two sides of the sheet of cells at the top of the egg (dorsal side). This process is called dorsal closure (see Figure below). This process starts with the fusion of the amnion and the serosa (1 in the Figure below). Once fused, the serosa starts to contract towards the dorsal (top) side, pulling the amnion along with it (2 in the Figure below). There the contracted serosa will degenerate as the dorsal organ (d.o.). The amnion will than connect the two sides of the embryonic sheet forming a hollow structure in which the organs can form.

dorsal closure

After the completion of dorsal closure, the embryo will complete its development. Once completed, the larva will hatch. The development of these extraembryonic membranes is crucial for the embryo. On the one hand these membranes are involved in the movements of the embryo in the egg, on the other hand these membranes (especially the serosa) serve as a protective barrier against outside threats. Very soon I will write about the protective functions of the serosa during development.

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