Human Embryonic Development & Stem Cell Classification

Stages of Human Embryonic Development

1. If a sperm finds an egg in a fallopian tube, it tries to get through its membrane. The egg then produces a hard coating that prevents the penetration of a second spermatozoon. Fertilization ends when the nuclei of the sperm and egg fuse. From this moment, embryonic development begins.
2. The zygote, formed by a single cell, continues to move down the tube on its way to the uterus and begins to divide.
3. The cells continue to divide until they form an embryo of 32 cells, called a morula.
4. A cavity forms inside the morula, which takes the form of a hollow ball, and is called an early blastocyst.
5. A condensed group of cells inside the blastocyst forms a compact mass at one end, ready to become the late blastocyst. The inner mass cells of the blastocyst originate the embryo, while the outer cover cells produce structures outside the embryo, such as the placenta.
6. The embryo nests in the wall of the uterus, which has been prepared for this event and is covered with a layer called the endometrium. This process continues until the 14th day after fertilization.

Key Milestones in Embryonic Development

• Implantation: A key point is the embryo's implantation in the endometrium. Almost 70% of zygotes and early embryos are naturally removed prior to implantation in the endometrium.
• Formation of the Nervous System: Once implanted, the embryo continues to grow and develop. Around 14 days, a group of cells begins to form what will become the nervous system.
• Organ Function Begins: By two months of embryonic development, the period now called fetal begins. The embryo, now a fetus, has more than 90% of human body structures.

Understanding Stem Cell Types

• Totipotent Cells: These cells can form a complete individual. Besides the zygote, the first eight cells resulting from its division are still totipotent.
• Pluripotent Cells: They cannot form a complete individual but retain the ability to generate each and every one of the cell types that comprise it.
• Multipotent Cells: These cells retain some capacity to form adult stem cells, somatic stem cells, or tissue-specific cells.