From Fertilized Egg to Advanced Therapies: The Power of Stem Cells

In this post, we will explore the world of stem cells, starting with what they are and their potential in medical science. We will then look into the different stem cells types – including embryonic, adult, and induced pluripotent stem cells – and see where they come from.

What are stem cells?

Stem cells are unique cells in the body for two key capabilities: the ability to continually renew themselves in a process called self-renewal and the ability to transform into specific cell types, a process called differentiation (Figure 1).

Figure 1. Definition of stem cells

The process of self-renewal is one of their standout features. Stem cells can divide repeatedly, making more stem cells over long periods. This ability ensures a stable supply of stem cells in the body, crucial for their overall function.

Then, there's differentiation – the process where undifferentiated stem cells develop into differentiated cells with unique roles, like epithelial cells, blood cells, or muscle cells. This ability is vital for the body's growth, repair, and tissue maintenance.

The Promise of Stem Cells

Stem cells hold immense promise in medicine. They offer the potential for replacing or repairing damaged tissues and organs in the body, potentially leading to treatments for diseases that are currently considered incurable. In drug discovery, stem cells provide a platform for testing the efficacy and safety of new drugs, thereby accelerating the development of new therapies (1). But where do they originate from? To answer this question, let's begin by exploring the very start of human life.

The Fertilized Egg to the Blastocyst

The beginning of human embryogenesis is marked by the fertilization of an egg by a sperm cell. After fertilization, the egg undergoes multiple cell divisions, lineage segregations, and morphogenic rearrangements. After around five to six days after fertilization this complex process leads to the formation of a blastocyst containing a inner cell mass and a surrounding layer (trophectoderm) of cells called trophoblasts (Figure 2A) (2, 3).

Figure 2. Development of a fertilized egg to a human

Pluripotent Stem Cells in the Blastocyst

The inner cell mass of the blastocyst are the cells that develop into the baby during pregnancy meaning that these cells have the potential to develop into all the cell types in the body (Figure 2B). They are therefore called pluripotent stem cells.

If the inner cell mass are taken out from the blastocyst and cultured in the laboratory they are termed embryonic stem cells (Figure 3). Interestingly, the embryonic stem cells keep their pluripotency in the laboratory dish and can still develop into any cell type in our body. This has opened up vast possibilities in regenerative medicine and research (4).

Figure 3. Derivation of embryonic stem cells

Blastocyst Implantation and Adult Stem Cells

Following the formation of the blastocyst, it implants itself in the uterus, leading to the development of the embryo and eventually the fetus. As the organism develops, adult stem cells emerge in various organs (Figure 4). Unlike pluripotent stem cells, adult stem cells are multipotent, meaning they can develop into a limited range of cell types, usually those found in the tissue or organ where they reside. For example, a skin stem cell can differentiate into various cell types found in the skin, while a blood stem cell can develop into different types of blood cells (5).

Figure 4. Adult stem cells (same as somatic stem cells) in various organs. Image credit: Learn.Genetics - Genetic Science Learning Center

Autologous vs. Non-Autologous Stem Cells

An important distinction in stem cell therapy is between autologous and non-autologous cells. Embryonic stem cells are not autologous, meaning they are not derived from the patient's own body. This can lead to immune reactions and rejection. In contrast, autologous adult stem cells, harvested from the patient’s own body, significantly reduce the risk of immune rejection. This makes them a safer option for transplantation therapies (6).

Ethical concerns

Creation of embryonic stem cells involves taking the inner cell mass out from the blastocyst. This means that the blastocyst is destroyed. This can raise ethical concerns as the blastocyst has the potential to develop into a baby if implanted in a uterus. However, it's crucial to understand that embryonic stem cells are only collected from extra fertilized eggs that were created during IVF (In Vitro Fertilization) treatments and would otherwise be discarded. Researchers do not create fertilized eggs specifically to obtain embryonic stem cells; they use only those that are surplus to the needs of IVF procedures. The use of adult stem cells do not raise these concerns as they do not involve a fertilized egg.

Induced Pluripotent Stem Cells: Avoiding rejection and ethical concerns

A groundbreaking development in stem cell research is the creation of induced pluripotent stem cells (iPSCs). Scientists can reprogram adult somatic cells (our body cells such as a skin cell or a blood cell) to a pluripotent state, essentially 'rewinding' their developmental clock. This process allows for the generation of patient-specific cells that can become all the cell types in our body without the risk of immune reactions and transplant rejection (1). Another aspect of iPSCs is that their creation does not involve fertilized eggs, thereby avoiding the ethical concerns typically associated with embryonic stem cells. See this blog post for more information about iPSCs.

Summary

As research progresses, the field of stem cell science steadily advances, presenting possibilities for new treatments and remedies for various illnesses. Each discovery contributes to a better understanding of stem cells and their capabilities. The path of stem cell research is ongoing, and the future holds further developments that may be beneficial in medical science.

References and further reading

1. Human inducible pluripotent stem cells: Realization of initial promise - Cell Stem Cell

2. Generation of human blastocyst-like structures from pluripotent stem cells - Cell Discovery

3. Blastocyst: Definition, Stage & Implantation - Cleveland Clinic

4. First complete model of the human embryo - Nature.

5. Stem Cells - Biology LibreTexts - Biology LibreTexts

6. Types of Stem Cell and Bone Marrow Transplants - American Cancer Society