The human body is made up of billions of cells working together to maintain the normal function of life. These cells are formed or divided from a fertilized egg. The egg splits into two cells and further divides into four cells and so on. The process continues until all the cells in the body are generated. Specialized cells group together and form tissues to complete a unique task, such as nerves and muscles. Cells are divided in a controlled manner to maintain their normal function. When cells are damaged or cell death occurs, the surrounding cells help the replacement by further cell division. However, when the damage cannot be repaired, diseases or medical problems may arise. Disruption of the normal process of embryo development by radiation, chemicals or viruses may lead to birth defects. For example, fetal alcohol spectrum disorder is the brain cell death of an embryo that has been exposed to the toxic effects of alcohol during the developmental stages. This damage is irreversible even after the removal of the alcohol, resulting in growth deficiencies and retarded brain development.
To replace cells or to develop a human body, stem cells act as an internal repair system. They start working in the body during early development and growth. Stem cells are unspecialized cells, which can divide continuously and maintain an undifferentiated state. Moreover, the stem cell products — their daughter cells — can differentiate into different types of cells for replacement and transform into specialized functional cells for the body, such as a muscle cell or a brain cell. Recently, scientists have distinguished two primary kinds of stem cells based on their characteristics: embryonic stem cells, and non-embryonic or adult stem cells. In the next section, we will discuss the types of stem cells in further detail. For now, complete the following activity to further your understanding of stem cell differentiation.
Let's start with the basics.
During human embryo development, the cells are differentiated to produce special functions of the body. A blastocyst is formed at the beginning stage to make any type of body cell, and can give rise to the cells of a given germ layer. The cells become more restricted when further developed, to form an eye or organ.
Watch the video animation to understand this more.
Then answer the following questions:
- What is a blastocyst?
- How many days does it take for a blastocyst to form?
- At the beginning of embryo development, there are three germ layers. Please give the names of the germ layers.
Activity 1 feedback
- A blastocyst is a pre-implantation embryo of about 150 cells produced by cell division following fertilization. The blastocyst is a sphere made up of an outer layer of cells, a fluid-filled cavity, and a cluster of cells on the interior (the inner cell mass). The outer cell mass will be developed into a placenta and the inner cell mass will form a disk and change the geographic position together with embryo growth. Finally, it will form a germ layer and will make the entire animal from these cells.
- About 5 days, usually 4–6 days
- Ectoderm, mesoderm and endoderm