WHAT IS BLASTULATION? STAGES AND CHARACTERISTICS - BIOLOGY - 2025

The blastula stage is a stage of embryo development in which a series of cellular rearrangements that later allow of the embryo proper and correct formation implantation originate.

Immediately after an egg is fertilized by the sperm, a zygote will form that will go through successive cell and mitotic divisions to form an individual. These stages are summarized in five transformations that the zygote will undergo: fertilization, segmentation, blastulation, gastrulation and finally organogenesis.

Segmentation and blastulation are vitally important processes where the zygote will divide into two blastomeres to form the morula and finally the blastula.

The formation of the blastula is what will allow the embryo, through gastrulation, to develop three germ layers (ectoderm, mesoderm and endoderm) that will gradually endow the embryo with its own human characteristics, such as skin, organs and the different fabrics.

Stages prior to blastulation

Fertilization

The first step in the formation of an embryo is fertilization. Through it, the union of two heterogametes, an egg and a sperm, is generated. This union will give rise to a cell known as a zygote.

Through fertilization, the sperm penetrates the protective coatings of the ovum, introducing the spermatic nucleus into the ovular cytoplasm. The fusion of these gametes, a female and a male, is a process that involves two steps:

1. The penetration, by the sperm, of the external structure that surrounds the ovum, called the zona pellucida of the ovum.
2. And, finally, the fusion of the membranes of both gametes to form a single zygote.

Segmentation

As its name implies, it consists of the segmentation of the zygote within the uterine tube. In this stage, the zygotic cell division occurs through mitosis.

24 hours after fertilization, mitosis allows the zygote to divide into two identical daughter cells made up of subunits called blastomeres.

Blastomeres are each of the cells into which a zygote divides to give rise to the first embryonic stages. Three days later, a mitotic multiplication process begins that will increase the number of blastomeres until the morula is formed.

The morula is named for its structure similar to a small blackberry, as can be seen in the following image.

Blastulation stage

In this stage the compaction of the morula begins. The different blastomeres that make it up establish tight cell junctions.

In blastulation, an inner cell mass is left within the morula that will be completely isolated from the environment of the uterine tube. This cell mass will be called a blastocyst, blastula or embryo.

A blastocyst is a 5- to 6-week-old embryo that is made up of more than approximately 200 cells. The development of the blastocyst constitutes the stage prior to the implantation of the embryo in the maternal uterus.

Each blastocyst is made up of:

-Trophoblast

Also called blastoderm, it is the outer cell layer of the blastocyst that contains the various cells necessary for the formation of the future placenta that will allow the fetus to feed.

It is a thin layer made up of the different cells that will provide nutrients to the developing embryo. The purpose of this layer is to help the embryo adhere to the walls of the uterus.

By means of the trophoblast the embryo is implanted in the uterine endometrium.

-Embryoblast

They constitute the cells that form the trophoblast and that are responsible for producing a liquid known as a blastocele.

-Blastocele

It is the internal cavity that makes up the blastocyst and is filled with fluid. In it are the deepest cells that will constitute the embryonic disc from which the fetus will be formed.

The blastocele in turn is divided into two thin layers: the epiblast, composed of rounded cells; and the hypoblast, made up of cubic cells.

Stages after blastulation

The next stage, called gastrulation, consists of the formation of the gastrula.

The gastrula, also called tridermal gastrula, gives the embryo three fundamental layers that will be the precursors of its tissues. These layers are called: ectoderm, mesoderm and endoderm.

During gastrulation the primary tissues begin to develop.

And, finally, the last stage of the embryonic process is reached, which is organogenesis, when the three embryonic layers begin to form in the embryo, beginning to provide it with a small human form.

These three layers are:

Ectoderm

Develops in the embryo the epidermis, mucous membranes of the body's natural openings (oral cavity, nostrils), the central nervous system, lining and glandular epithelium of the digestive tract, liver, bile and respiratory tract, pancreas, gallbladder, urethra, prostate, thyroid, parathyroid, thymus, and oocyte and sperm germ line cells.

Endoderm

It develops the digestive tract, and the inner lining of some organs such as the lungs. In addition, it provides the embryo with nervous tissue, the epidermis and its derivatives such as: nails and hair.

Mesoderm

It constitutes the dermal layer of the skin. It develops in the embryo the circulatory, excretory system and the gonads. The skeleton, musculature, connective tissues and the renal system are formed.

References

1. Blastula. Retrieved on August 22, 2017 from ecured.cu
2. Center for embryonic medicine. Retrieved on August 22, 2017 from pgdcem.com
3. Cleavage, the Blastula stage, and gastrulation. Retrieved on August 22, 2017 from boundless.com
4. Embryonic development. Retrieved on August 22, 2017 from duiops.net
5. Stages of embryonic development. Retrieved on August 22, 2017 from wordpress.com
6. Stages of embryonic development. Retrieved on August 22, 2017 from um.es
7. Stages of embryonic development. Retrieved on August 22, 2017 from blogspot.com
8. Gil, D. Embryology. Retrieved on August 22, 2017 from files.wordpress.com
9. Instituto Santo Tomás. General embryology. Retrieved on August 22, 2017 from files.wordpress.com
10. Mandal, A. Formation of Blastula. Retrieved on August 22, 2017 from news-medical.net
11. What is a blastocyst? Retrieved on August 22, 2017 from institutobernabeu.com.