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 damage to the progeny, even the death of the unborn.
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 on our site we will teach you some concepts about embryology and, specifically, how is the embryonic development of fish
Embryology Basics
To delve 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, depending on how the yolk is distributed and how much it contains. To begin with, we will call the egg the cell resulting from the union of an ovule and a spermatozoon and, yolk, the set of nutritional elements that is found inside the egg and will serve as food for the future embryo.
Types of eggs according to the organization of the yolk inside:
- Isolecytic Eggs: The yolk is evenly distributed throughout the interior of the egg. Typical of poriferous animals, cnidarians, echinoderms, nemertines and mammals.
- Telolecytic eggs: the yolk is displaced towards an area of the egg, being opposite the place where the embryo will develop. Most animals develop from these types of eggs, for example, mollusks, fish, amphibians, reptiles, birds, etc.
- Centrolecyte eggs: the yolk is surrounded by cytoplasm and this, in turn, surrounds the nucleus that will give rise to the embryo. It occurs in arthropods.
Types of eggs according to the amount of yolk:
- Oligocyte eggs: they are small and have little yolk.
- Mesolecito Eggs: Medium in size with a moderate amount of yolk.
- Macrolecithus eggs: these are large eggs with a large amount of yolk.
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 up in a state called a blastula.
- Gastrulation: a reorganization of the blastula cells occurs, giving rise to the blastoderms (primitive germ layers) that are the ectoderm, endoderm, and, in some animals, the mesoderm.
- Differentiation and organogenesis: tissues and organs will form from the germ layers, establishing the structure of the new individual.
Relationship between development and temperature
Temperature is closely related to the incubation time of fish eggs and their embryonic development (the same thing happens in other animal species). There is generally an optimal temperature range for incubation, which varies by about 8ºC.
Eggs incubated within this range will have a higher chance of developing and hatching. Likewise, those eggs incubated for long periods of time at extreme temperatures (outside the optimal range of the species) will have a lower probability of hatching and, if they do, born individuals can suffer from serious anomalies
Fish embryonic development stages
Now that you know the basic concepts of embryology, we will delve into the embryonic development of fish. The fish are telolecithal, that is, they come from telolecithal eggs, those that had the yolk displaced to an area of the egg.
Zygotic phase
The newly fertilized egg remains in the zygote state until the first division, the approximate time at which this cleavage occurs depends on the species and the temperature of the medium. In the zebrafish, Danio rerio (the fish most used in research), the first cleavage occurs around 40 minutes after fertilization. Although it seems that no changes take place during this period, decisive processes for later development are taking place inside the egg.
Segmentation phase
The egg enters the cleavage phase when the first division of the zygote occurs. In fish, the segmentation is meroblastic, because the division does not go completely through the egg since it is prevented by the yolk, but is limited to the area where it is found the embryo The first divisions are vertical and horizontal to the embryo, they are very fast and synchronous. They give rise to a mound of cells sitting on the yolk, constituting the discoidal blastula
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, it 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 stage is characterized by a decrease in the rate of cell division and little or no growth cell phone.
During involution, some cells of the discoblastula or discoid blastula migrate into the yolk, forming a layer on it. This layer will be the endoderm The layer of cells left over the mound will form the ectoderm Al 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
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 archenteronThe entrance to this cavity will be renamed blastopore and will lead to the anus of the fish. From this point, you can distinguish the cephalic vesicle (brain in formation) and, on both sides, the optic vesicles (future eyes). Following the cephalic vesicle, the neural tube will form and, on either side, the somites, structures that will eventually form the bones of the spinal column and ribs, muscles and other organs.
Throughout this phase, each germ layer will eventually produce several organs or tissues, so:
Ectoderm
- Epidermis and nervous system
- Beginning and end of the digestive tract
Mesoderm
- Dermis
- Musculature, excretory and reproductive organs
- Coelom, peritoneum and circulatory system
Endoderm
- Organs involved in digestion: inner epithelium of the digestive tract and associated glands.
- Organs responsible for gas exchange.
