Embryonic development of fish

To get into the embryonic development of fish, we must first 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 how the yolk is distributed and how much it contains.To begin with, we will call the cell resulting from the union of an egg and a sperm and, vitellum, the set of nutritional elements found in inside the egg and will serve as food for the future embryo.

Types of eggs according to the organization of the vitello inside:

• Isolecite eggs : the vitello is uniformly distributed throughout the interior of the egg.Typical of poriferous animals, cnidarians, echinoderms, nemertines and mammals.
• Telolecitos eggs : the vitello is displaced towards an area of ​​the egg, being opposite to The place where the embryo will develop.Most animals develop from these types of eggs, for example, mollusks, fish, amphibians, reptiles, birds, etc.
• Little center eggs : the vitello is surrounded by cytoplasm and, in turn, envelops the nucleus that will give rise to the embryo.It occurs in arthropods.

Types of eggs according to the amount of vitelo:

• Oligolecitos eggs : they are small and have little vitello.
• Mesolecyte eggs : medium size with a moderate amount of vitello.
• Macrolecitos eggs : they are large eggs with a large amount of vitello.

Typical phases of embryonic development

• Segmentation : in this phase a series of cell divisions take place that increase the number of cells needed for the second phase.It ends in a state called blastula.
• Gastrulacion : there is a reorganization of the blastula cells giving rise to blastoderms (primitive germ layers) that are ectoderm, endoderm and, in some animals, mesoderm.
• Differentiation and organogenesis : the tissues and organs will form from the germ layers, the structure of the new individual being fixed.

The temperature is closely related to the incubation time of eggs in fish and their embryonic development (the same occurs in other animal species).Generally there is an optimal temperature range for incubation, which varies by about 8ºC.

Eggs incubated within this range will have a higher probability of developing and hatching.Also, those eggs incubated for long periods of time at extreme temperatures ( out of the last range of the pecie) will have a lower probability of hatching and, if they do, born individuals may suffer serious abnormalities .

Now that you know the basic concepts of Embryology, we will enter the embryonic development of the fish.The fish are telolecitic, that is, they come from telolecyte eggs, those that had the vitellas displaced to an area of ​​the egg.

Zygotic phase

The newly fertilized egg remains in zygote status until the first division, the approximate time at which this split occurs depends on the species and the temperature of the medium.In the zebrafish, Danio rerio (fish most used in research), the first segmentation occurs around 40 minutes after fertilization.Although it seems that there are no changes in this period, In the interior of the egg, decisive processes are taking place for further development.

Phase of s egmentation

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 pass through the egg since it is prevented from vitelus, but rather It is limited to the area where the embryo is located.The first divisions are vertical and horizontal to the embryo, they are very fast and synchronous.They give rise to a mound of cells seated on the vitellus, constituting the discoid blastula.

Phase of gastrulation

During the gastrulation phase there is a rearrangement of the cells of the discoid blastula by morphogenetic movements, that is, the information contained in the nuclei of the different cells already formed, is transcribed so that forces the cells to obtain a new spatial configuration.In the case of fish, this reorganization is called involvement.Also, this stage is characterized by a decrease in the rate of cell division and little or no cell growth.

During involvement, some cells of the discoblastula or discoid blastula migrate to the vitellum, forming a layer on it.This layer will be the endoderm.The layer of cells that remain on the mound will form the ectoderm.At the end of the process , the gastrula will be defined or, in the case of fish, discogastrula with its two primary germ layers or blastoderms, the ectoderm and the endoderm.

Phase of differentiation and organogenesis

DuringIn the phase of differentiation, in fish, the third embryonic layer appears, located between the endoderm and the ectoderm, called the mesoderm.

The endoderm is invaded forming a cavity called archaenteron.The entrance to this cavity will be called blastoporo and will give rise to the anus of the fish.From this point, the cephalic vesicle (encephalon in formation) can be distinguished and, on both sides, the vesiculasopticas (future eyes).Next the cephalic vesicle will form the neural tube and, on either side, the Somites, structures that will eventually form the bones of the spine and ribs, muscles and other organs.

Throughout this phase, each germ layer will eventually produce 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 attached glands.
• Organs responsible for gas exchange.