Pre-embryo

Pre-embryo

Diagram of early embryogenesis

The term pre-embryo refers to the early stages of embryogenesis in humans, specifically the period from fertilization until the formation of the blastocyst and its implantation into the uterine wall. This stage is crucial for the establishment of pregnancy and involves several key processes, including fertilization, cleavage, and the formation of the morula and blastocyst.

Fertilization

Fertilization is the process by which a sperm cell from the male merges with an ovum from the female to form a zygote. This occurs in the fallopian tube and marks the beginning of the pre-embryonic stage. The zygote is a single cell that contains the genetic material from both parents.

Cleavage

Following fertilization, the zygote undergoes a series of rapid mitotic divisions known as cleavage. These divisions increase the number of cells without increasing the overall size of the embryo. The resulting cells, called blastomeres, are totipotent, meaning they have the potential to develop into any cell type in the body.

Morula

As cleavage continues, the embryo becomes a solid ball of cells known as the morula. The morula stage occurs approximately 3-4 days after fertilization. The cells of the morula are tightly packed together and begin to differentiate into two distinct cell types: the inner cell mass and the trophoblast.

Blastocyst Formation

Stages of blastocyst development

Around 5-6 days after fertilization, the morula develops into a blastocyst. The blastocyst is characterized by a fluid-filled cavity called the blastocoel, an inner cell mass that will eventually form the embryo, and an outer layer of cells called the trophoblast, which will contribute to the formation of the placenta.

Implantation

Implantation is the process by which the blastocyst attaches to and penetrates the uterine wall. This occurs approximately 6-10 days after fertilization. Successful implantation is essential for the continuation of pregnancy, as it allows the developing embryo to receive nutrients and oxygen from the mother.

Related Pages

• Embryogenesis
• Fertilization
• Blastocyst
• Implantation (human embryo)
• Human reproduction