Content
- Embryonic development of fish: basic concepts
- Types of eggs according to the organization of the calf inside:
- Types of eggs according to the amount of veal:
- Typical stages of embryonic development
- How fish reproduce: development and temperature
- Embryonic development of fish: stages
- How fish reproduce: zygotic phase
- Fish reproduction: segmentation phase
- Fish reproduction: gastrulation phase
- Fish reproduction: differentiation and organogenesis phase
- ectoderm:
- mesoderm:
- endoderm:
During the embryonic development of any animal, crucial processes are carried out for the formation of new individuals. Any failure or error during this period can cause serious harm to the offspring, including fetal death.
The embryonic development of fish is well known, thanks to the fact that their eggs are transparent and the entire process can be observed from the outside using instruments such as a magnifying glass. In this article by PeritoAnimal we will teach some concepts about embryology and, in particular, about how fish reproduce: embryonic development.
Embryonic development of fish: basic concepts
To address the embryonic development of fish, we first need to know some basic concepts of embryology, such as the types of eggs and the phases that make up early embryonic development.
We can find different types of eggs, according to the way in which the calf (nutritive material present in the egg of animals that contains protein, lectin and cholesterol) is distributed and its quantity. To begin with, let's call the result of the union of an egg and a sperm as an egg, and as a calf, the set of nutrients that is inside the egg and will serve as food for the future embryo.
Types of eggs according to the organization of the calf inside:
- isolated eggs: the calf is found evenly distributed throughout the interior of the egg. Typical of poriferous animals, cnidarians, echinoderms, nemertines and mammals.
- eggs telolect: the yolk is displaced towards an area of the egg, being opposite the place where the embryo will develop. Most animals develop from this type of egg, such as molluscs, fish, amphibians, reptiles, birds, etc.
- Centrolecitos eggs: the yolk is surrounded by the cytoplasm and this, in turn, surrounds the nucleus that will give rise to the embryo. Occurs in arthropods.
Types of eggs according to the amount of veal:
- eggs oligolectics: they are small and have little calf.
- mesolocyte eggs: Medium sized with a moderate amount of veal.
- macrolecite eggs: they are big eggs, with a lot of veal.
Typical stages of embryonic development
- Segmentation: in this phase, a series of cell divisions occurs that increase the number of cells needed for the second phase. It ends up in a state called a blastula.
- Gastrulation: there is a reorganization of the blastula cells, giving rise to the blastoderms (primitive germ layers) which are the ectoderm, the endoderm and, in some animals, the mesoderm.
- Differentiation and organogenesis: the tissues and organs will form from the germ layers, forming the structure of the new individual.
How fish reproduce: development and temperature
Temperature is closely related to the incubation time of eggs in fish and their embryonic development (the same occurs in other animal species). There is usually a optimum temperature range for incubation, which varies by about 8ºC.
Eggs incubated within this range will have a greater chance of developing and hatching. Likewise, eggs incubated for long periods of time at extreme temperatures (outside the optimal range of the species) will have a lower hatch probability and, if they hatch, the individuals born may suffer from serious anomalies.
Embryonic development of fish: stages
Now that you know the basics of embryology, we'll delve into the embryonic development of fish. fish are telolectic, that is, they come from telolecite eggs, those that have the yolk moved to an egg zone.
In the next topics we will explain how is the reproduction of fish.
How fish reproduce: zygotic phase
The newly fertilized egg remains in the zygote state up to the first division. The approximate time this division takes place depends on the species and the temperature of the environment. In zebra fish, Danio rerio (the most used fish in research), the first segmentation occurs around 40 minutes after fertilization. Although it seems that during this period there are no changes, within the egg decisive processes for further development are taking place.
Meet: Fish that breathe out of water
Fish reproduction: segmentation phase
The egg enters the segmentation phase when the first division of the zygote occurs. In fish, the segmentation is meroblastic, because the division does not completely cross the egg, as it is impeded by the yolk, being limited to the area where the embryo is located. The first divisions are vertical and horizontal to the embryo, and are very fast and synchronized. They give rise to a bunch of cells installed on the calf, constituting the discoidal blastula.
Fish reproduction: gastrulation phase
During the gastrulation phase, a rearrangement of the discoidal blastula cells occurs by morphogenetic movements, that is, the information contained in the nuclei of the different cells already formed, is transcribed in a way that forces the cells to obtain a new spatial configuration. In the case of fish, this reorganization is called involution. Likewise, this phase is characterized by a decrease in the rate of cell division and little or no cell growth.
During involution, some cells of the discoblastula or discoidal blastula migrate towards the yolk, forming a layer over it. This layer will be the endoderm. The layer of cells that remains in the heap will form the ectoderm. At the end of the process, the gastrula will be defined or, in the case of fish, the discogastrula, with its two primary germ layers or blastoderms, the ectoderm and the endoderm.
Know more about: saltwater fish
Fish reproduction: differentiation and organogenesis phase
During the differentiation phase in fish, the third embryonic layer appears, located between the endoderm and the ectoderm, called mesoderm.
The endoderm invaginates forming a cavity called archentor. The entry to this cavity will be called blastopore and will result in the fish's anus. From this point, we can distinguish the cephalic vesicle (brain in formation) and, on both sides, the optical vesicles (future eyes). After the cephalic vesicle, the neural tube it forms and, on both sides, the somites, structures that will eventually form the bones of the spine and ribs, muscles and other organs.
During this phase, each germ layer will end up producing several organs or tissues, so that:
ectoderm:
- Epidermis and nervous system;
- Beginning and end of the digestive tract.
mesoderm:
- Dermis;
- Musculature, excretory and reproductive organs;
- Celoma, peritoneum and circulatory system.
endoderm:
- Organs involved in digestion: internal epithelium of the digestive tract and adnexal glands;
- Organs in charge of gas exchange.
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