What Are Human Adult Stem Cells?

Adult stem cells or adult tissue-specific stem cells are non-embryonic stem cells that are found in the tissues and organs of adults.

They have the specific characteristics of self-renewal and generate differentiated and specialized cells that contribute to tissue homeostasis and regeneration following injuries or diseases. They are found in several organs including the brain, liver, bone marrow, eyes, gut, skin, and muscle [1].

1. Origin of Adult Stem Cells

To better understand how and where adult stem cells originated, it is important to discuss early embryonic development. Following the fecundation of an egg by a sperm, the fertilized egg begins a series of cell divisions that lead to the formation of a mass of cells known as the blastula.

This mass contains two types of cell masses, the trophoblast, and the inner cell mass. The trophoblast leads to the formation of the placenta and the inner cell mass to the formation of an embryo [2].

Before the generation of a fully developed embryo, the inner cell mass, composed of embryonic stem cells, will generate three types of tissues known as domains: the endoderm, the mesoderm, and the ectoderm.

At this stage of development, unlike the embryonic stem cells that can generate any type of cells, stem cells that are found in these domains are committed to the generation of tissues that are specific for each domain.

For instance, the endoderm, also known as the internal layer, generates lung cells (alveolar cells), thyroid cells, and digestive cells (pancreatic cells).

The mesoderm will generate cardiac muscle cells, skeletal muscle cells, tubule cells of the kidney, red blood cells, and smooth muscle cells (in the gut). The ectoderm will generate the skin cells of the epidermis, pigment cells, and neurons of the brain [3].

Finally, although most of the cells progress to generate different tissues and organs, few cells conserve stem cell-like characteristics that are committed to the continuous maintenance and repair of tissues and organs throughout the life of an individual. These cells are known as adult stem cells or adult tissue-specific stem cells.

2. Adult Stem Cells of the Bone Marrow

The bone marrow is the niche of hematopoietic stem cells (HSCs) that generate cells of the blood that contribute to the transport of nutrients and oxygen, coagulation, and immunity. HSCs produce two types of progenitor cells: myeloid progenitors and lymphoid progenitors.

The myeloid progenitors generate erythrocytes (red blood cells), platelets for coagulation, and myeloblasts that produce basophil, eosinophil, neutrophil, and monocytes that contribute to immunity. Lymphoid progenitors produce lymphocytes T and lymphocytes B which are also key players in immunity [4].

3. Brain Adult Stem Cells

Adult neural stem cells are found in the hippocampal dentate gyrus (DG) and the brain subventricular zone (SVZ). Hippocampal neural stem cells generate hippocampal neurons named granule neurons of the dentate gyrus (DG) that are essential to memory function [5].

The subventricular adult neural stem cells generate neuroblast precursors of interneurons that migrate to the olfactory bulb, a neural structure involved in receiving sensations of smell and discriminating between different odors [6].

4. Intestinal Adult Stem Cells

In the small intestine and colon, adult stem cells are located at the base of the crypts, which are protrusions of the gut wall that project into the gut lumen and contribute to the transport of absorbed nutrients into the body.

These stem cells contribute to the renewal and function of the gut epithelium through the generation of enterocytes, goblet cells, tuft cells, Paneth cells, and microfold cells [7].

The enterocytes contribute to the gut absorption of nutrients, the goblet cells secrete mucus and hormones, the tuft cells play a role in nutrient sensing, Paneth cells secrete anti-bacterial products, and the microfold cells play a role in mucosal immunity.

5. Adult Stem Cells of the Skin

The skin adult stem cells are found in the epidermis of the skin where they contribute to the maintenance of tissue homeostasis, hair regeneration, and epidermis repair after injury.

They are located at the base of the epidermis within a niche known as the epidermal proliferative unit (EPU). Within the EPU, inner cells differentiate and move outward to continually replace the cells at the surface of the skin [8].

6. Liver Adult Stem Cells

Following hepatectomy, chemical injury, or diseases, the liver can regenerate suggesting the presence of liver stem cells that can produce new liver cells. However, the existence of these cells is controversial as other studies suggested that the generation of new cells is performed by normal liver cells called hepatocytes [9].

It has been shown that hepatocytes are also able to proliferate and generate new liver cells. Nonetheless, other studies demonstrated the presence of liver stem cells within the liver, indicating that both hepatocytes and liver stem cells are involved in liver regeneration [10].

7. Adult Stem Cells of the Eyes

The eye is composed of the cornea, the retina, the lens, the optic nerve, and the retinal pigment epithelium. Studies have shown the presence of stem cells within the retina named Müller cells. These cells play a vital role in maintaining and repairing the eye during physiological conditions [11].

However, other studies have suggested the presence of other types of stem cells such as corneal epithelial cells, and retinal pigment epithelial (RPE) cells [11].

8. Muscle Adult Stem Cells

Within muscles, a population of stem cells known as satellite stem cells, have the potential to regenerate and repair muscles, cartilage, and bones. They are found between the muscle fiber and the basal lamina.

Satellite stem cells can differentiate into myocytes to produce muscle cells, osteoblasts to produce bone tissue, chondroblasts to produce cartilage tissue and adipocytes.

9. Frequently Asked Questions about What Are Human Adult Stem Cells?

What are human adult stem cells?

Human adult stem cells are a type of undifferentiated cell found among differentiated cells in a tissue or organ. They can renew themselves through cell division and can differentiate to yield specialized cell types.

Where are adult stem cells found in the human body?

Adult stem cells are found in various tissues and organs throughout the body, including the bone marrow, brain, skin, liver, skeletal muscles, and blood vessels.

What is the difference between adult stem cells and embryonic stem cells?

Adult stem cells are multipotent, meaning they can differentiate into a limited range of cell types specific to the tissue or organ where they are found. Embryonic stem cells, on the other hand, are pluripotent and have the potential to differentiate into any cell type in the body.

What are the potential applications of adult stem cells in medicine?

Adult stem cells hold promise for regenerative medicine, including treating various diseases and injuries by replacing damaged or diseased cells with healthy ones. They are also studied for their potential in drug discovery and understanding of disease mechanisms.

How are adult stem cells collected for research or medical purposes?

Adult stem cells can be collected from various sources, such as bone marrow aspiration, adipose tissue (fat) extraction, blood draw, and certain tissues during surgery. The collection method depends on the specific type of stem cell and its intended use.

Can adult stem cells be used for treating diseases?

Yes, adult stem cells have been used in clinical trials to treat a range of diseases, including blood disorders, autoimmune diseases, and certain types of cancer. However, further research is needed to fully understand their potential and refine treatment protocols.

Are there any ethical concerns associated with the use of adult stem cells?

Unlike embryonic stem cells, the use of adult stem cells typically does not raise ethical concerns related to embryo destruction.

However, ethical considerations may arise regarding the collection, storage, and use of adult stem cells, especially in the context of informed consent and patient privacy.

How do adult stem cells contribute to tissue repair and regeneration?

Adult stem cells play a crucial role in tissue homeostasis, repair, and regeneration by replenishing damaged or dying cells and promoting tissue renewal.

They can migrate to sites of injury or disease, differentiate into specialized cell types, and release signaling molecules that regulate the healing process.

Conclusion

Adult stem cells are tissue-committed stem cells that play a critical role in tissue homeostasis, repair, and regeneration in physiological and pathological conditions. These cells are also being used in stem cell research and in clinical applications such as in bone marrow or liver transplantation.

References

[1] Regad, T., Sayers, T. and Rees, R., 2015. Principles of stem cell biology and cancer: future applications and therapeutics. John Wiley & Sons.

[2] Moore, K.L., Persaud, T.V.N. and Torchia, M.G., 2018. The developing human-e-book: clinically oriented embryology. Elsevier Health Sciences.

[3] Young, H.E. and Black Jr, A.C., 2004. Adult stem cells. The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology: An Official Publication of the American Association of Anatomists, 276(1), pp.75-102.

[4] Seita, J. and Weissman, I.L., 2010. Hematopoietic stem cell: self‐renewal versus differentiation. Wiley Interdisciplinary Reviews: Systems Biology and Medicine, 2(6), pp.640-653.

[5] GoodSmith, D., Chen, X., Wang, C., Kim, S.H., Song, H., Burgalossi, A., Christian, K.M. and Knierim, J.J., 2017. Spatial representations of granule cells and mossy cells of the dentate gyrus. Neuron, 93(3), pp.677-690.

[6] Alvarez-Buylla, A. and Garcıa-Verdugo, J.M., 2002. Neurogenesis in adult subventricular zone. Journal of Neuroscience, 22(3), pp.629-634.

[7] Clevers, H., 2013. The intestinal crypt, a prototype stem cell compartment. Cell, 154(2), pp.274-284.

[8] Potten, C.S., 1974. The epidermal proliferative unit: the possible role of the central basal cell. Cell Proliferation, 7(1), pp.77-88.

[9] He, L., Pu, W., Liu, X., Zhang, Z., Han, M., Li, Y., Huang, X., Han, X., Li, Y., Liu, K. and Shi, M., 2021. Proliferation tracing reveals regional hepatocyte generation in liver homeostasis and repair. Science, 371(6532).

[10] Herrera, M.B., Bruno, S., Buttiglieri, S., Tetta, C., Gatti, S., Deregibus, M.C., Bussolati, B. and Camussi, G., 2006. Isolation and characterization of a stem cell population from adult human liver. Stem cells, 24(12), pp.2840-2850.

[11] Huang, C., Albon, J., Ljubimov, A.V. and Grant, M.B., 2020. Stem cells in the eye. In Principles of Tissue Engineering (pp. 1115-1133). Academic Press.

[12] Montano, M., 2014. Translational biology in medicine. Elsevier.