Embryonic development (fertilization, segmentation, gastrulation, organogenesis)

Embryonic development

Embryonic development is the process by which the embryo is formed and develops. It lasts until 9 – 12 weeks and ends with the formation of the fetus.


Fertilization is defined as a process in which gametes (an egg and a sperm) come together to form a complete zygote. Both sperm and egg contain a single set of 23 chromosomes, which come together to form 46 in the final zygote. To make sure that the resulting embryo ends with only one set of 46 chromosomes, only one sperm must join with a single egg.

In the context of mammals, the egg cell is preserved by an extracellular matrix, consisting mainly of a group of glycoproteins called ‘zona pellucida’. Once a sperm is able to bind to the layer of the zona pellucida, a cascade of biochemical events occurs: acrosomal reactions.

In the context of placental mammals, the acrosome consists of digestive enzymes that are used to initiate the breakdown of the extracellular matrix surrounding the egg, thus allowing the sperm cell membrane to fuse with the egg. The union of these two cell membranes forms an opening in which the sperm nucleus can be transferred to the center of the egg, where the membranes of the nucleus of both sperm and egg cells begin to degrade, with the two haploid genomes coming together to form a unique diploid genome.

The phase of neckline and blastula

The initial stages of growth of multicellular organisms begin with a zygote cell, which then undergoes rapid cell division to form the initial cell group, or blastula. This rapid division of cells is known as the ‘splitting’ process. Once the splitting process has produced well over 100 cells, the developing embryo is called a blastula.

This blastula is typically a spherical layer of cells, otherwise known as a blastoderm, surrounding a fluid-filled cavity: the blastocele. Mammalian embryos form a new structure called a blastocyst at this stage, which may be characterized by a mass within the cells that is obviously distinct from the outer region of the blastula.

During the rapid splitting process, it was discovered that cells divide without increasing total mass, meaning that this process is simply a large single-celled zygote that divides into several smaller cells. Each of these cells within the blastula can be called a blastomere.

The specialization phase of the demerger

The type of cleavage that occurs within the developing embryo depends on the volume of yolk; which is formed inside the egg itself. In placental mammals, which include humans, where nourishment is provided solely by the body and the mother’s blood supply, the eggs have only a very small volume of yolk, and therefore undergo a type of cleavage called holoblastic cleavage.

Other species of animals undergo a process called meroblastic cleavage, such as birds, which have more yolk inside the egg sac that is used to feed the developing embryo.

In mammalian embryos, the blastula develops further to form a blastocyst at the next stage of its early development. At this stage, the cells within the blastula begin to arrange themselves in two separate layers: an outer layer called the trophoblast and an inner cell mass.

The inner cell mass can also be known as the embryoblast, with this mass of cells continuing to develop and form the complete embryo. At this stage of the development process, it was discovered that the internal cell mass consists of embryonic stem cells that will soon differentiate into the different cell types necessary for the body to function.

The phases of gastrulation and somitogenesis

The next stage of embryo development is gastrulation. In this phase the cells within the structure of the blastula rearrange to eventually form three distinct layers of cells in a process known as ‘gastrulation;.

During this phase, the blastula proceeds to fold in on itself to form those three distinct layers of embryonic cells. Each of these new layers is called the germ layer, with each germ layer differentiating into a different organ system within the body.

The three germ layers are called mesoderm, endoderm and ectoderm. The ectoderm initiates the formation of the nervous system and skin layers. The germ layer of the mesoderm activates the formation of muscle cells and connective tissue within the body. Finally, the endoderm layer initiates the formation of columnar cells that are located within the digestive system, as well as many other internal organs.


Once these three layers have fully established and begin to develop separately, the body begins to form and the embryo becomes a fetus. There are many structural genes used in this process of specialization, where if there are mutations, parts of the body can grow in the wrong place (for example, organs that grow outside the body instead of under the skin), or it can cause spontaneous abortion of the embryo.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.

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