Fertilization – Embryology

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Chapter 2 –Development of the Cardiovascular System
Chapter 3 – Development of the AtriaThe atria are the two upper chambers of the heart. They act as reservoirs for blood that will fill the ventricles. and Venous Return
Chapter 4 – Development of the Ventricles and Major Arterial Vessels

Fertilization, a fundamental step in human reproduction, occurs in the fallopian tubes, where the egg meets the sperm.

The Fertilization of the Egg

At this stage, the egg is free and unattached to the walls of the reproductive organs, allowing it to move. When a sperm successfully penetrates the egg, their membranes fuse, initiating an extraordinary biological process.

Once this fusion is complete, the egg creates a barrier to prevent other sperm from entering.

The genetic material contained in the nuclei of the egg and sperm combines, resulting in a single unique cell called a zygote. This moment also determines the genetic sex of the embryo, encoded in its chromosomes.

Cellular Multiplication

From the moment of fertilization, a rapid process of cellular multiplication begins. The zygote, initially a single cell, divides into two, then four, sixteen, and so on, during the first three days following conception.

Vitellins duct

The newly formed cluster of cells starts organizing into complex structures. A cavity appears at the lower pole of this cluster, known as the vitellins duct.

This cavity plays a crucial role in embryonic development, as it will become the foundation for the intestines, mouth, and pharynx, forming the initial framework of the digestive system.

Embryonic disc

On the upper side of the cell cluster, a structure known as the embryonic disc begins to form.

This part is responsible for anchoring the embryo to the endometrium, the inner lining of the uterus.

The Ovum Attaches and Penetrates the Endometrium

This process, called implantation, occurs approximately six days after fertilization. The embryonic disc gradually penetrates the endometrium, establishing an essential connection for the embryo’s development.

Once this attachment is secured, the endometrium completely envelops the embryo, providing it with a protective environment and the nutrients necessary for its growth.

The Amniotic Cavity Forms

As the embryo nests into the endometrium, it continues to organize itself. A new cavity forms above the embryo: the amniotic cavity. This structure will develop into the amniotic sac, a fluid-filled environment where the fetus will grow throughout pregnancy.

The amniotic fluid within this sac plays a crucial role by protecting the fetus from external shocks, maintaining a stable temperature, and enabling free movement. This is also the sac referred to when it is said that a pregnant woman “has had her water break” as labor approaches.

After One Week

The embryo, now just over a week old, takes the form of a flat disc positioned between two distinct cavities: the yolk sac below and the amniotic cavity above. This developing embryo is encased within a spherical structure, firmly anchored into the endometrium.

At this stage, a critical milestone is approaching: the development of the circulatory system, one of the very first systems to form. This nascent network will play a vital role in supplying the nutrients and oxygen essential for the embryo’s rapid growth.