Chorion (egg cell)
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The egg shell of fish and insects is called chorion ( Greek: chorion = "skin") . The chorion has a multilayer structure and is excreted by the follicle cells during oogenesis . It mainly serves to protect the embryo mechanically and to regulate the water for the environment inside the egg.
For the layer surrounding the embryo of the amniotes, see the article Chorion .
construction
In the structure of the chorion, a distinction is made between the mostly two-layer exochorion on the outside and the finer endochorion on the inside of the egg. The exochorion consists mainly of lipoproteins ( chorionin ) and forms an inner, soft and an outer, hard layer. The structure of the endochorion is similar to the hard cuticle of insects and is composed of several fine layers that differ in their structure and, in addition to lipoproteins, also contain polyphenols and sclerotized proteins.
Corresponding to the cell boundaries of the follicle cells, the chorion usually has a honeycomb-like pattern to which other structures such as cusps, appendages or ridges can be added. To protect against dehydration, the chorion can be surrounded by a water-repellent ( hygroscopic ) secretion from the appendix glands. In addition, there is often a thin, water-impermeable layer of wax below the endochorion .
At the front end of the egg , the chorion has a single or several fine pores that serve as micropyle for the sperm as an entrance opening for the fertilization of the egg cell . In the orthoptera , the pores are on the ice sides or even on the rear pole.
function
The main purpose of the chorion is the mechanical protection of the embryo and the regulation of water for the environment inside the egg. Depending on the environment in which the egg is laid, it either protects against water loss or serves to actively absorb water from the environment. In particular with eggs that are placed in a moist substrate or directly in the water, this can lead to a strong increase in the volume of the egg and a burst of the chorion, which is then only surrounded by the yolk membrane.
The gas exchange also takes place via the chorion. In the case of very thin egg shells, this can take place over the entire surface, but usually it takes place via fine channels in the egg shell that connect the interior of the egg with the environment.