Amniotic stem cells are the mixture of

stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

s that can be obtained from the

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...

as well as the amniotic membrane. They can develop into various tissue types including skin, cartilage, cardiac tissue, nerves, muscle, and bone. The cells also have potential medical applications, especially in organ regeneration. The stem cells are usually extracted from the

amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shor ...

amniocentesis Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is ne ...

which occurs without harming the embryos. The use of amniotic fluid stem cells is therefore generally considered to lack the ethical problems associated with the use of cells from embryos. In 2009, the first US amniotic stem cell bank was opened in

Medford, MA Medford is a city northwest of downtown Boston on the Mystic River in Middlesex County, Massachusetts, United States. At the time of the 2020 U.S. Census, Medford's population was 59,659. It is home to Tufts University, which has its campus alo ...

, by

Biocell Center Biocell Center is an international company specializing in the cryopreservation and private banking of amniotic fluid stem cells. The company is headquartered in Italy with several international locations and is involved with numerous partnerships ...

, an international company specializing in the cryopreservation and private banking of amniotic fluid stem cells.

History

The presence of embryonic and foetal cells from all

germ layers A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three pri ...

in the amniotic fluid was gradually determined since the 1980s. Haematopoietic progenitor cells were first reported to be present in the amniotic fluid in 1993, specifically up to the 12th week of pregnancy. It was suggested that these originated from the

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...

. In 1996, a study indicated that non-haematopoietic progenitor cells were also present in the amniotic fluid. This was later confirmed as

mesenchymal stem cells Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage ce ...

were obtained. In addition, evidence indicated that

embryonic stem cells Embryonic stem cells (ESCs) are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre- implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they co ...

are part of the fluid, although in very small quantities. At around the same time, it was determined that stem cells from the amniotic membrane also have

multipotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...

potential. AS their differentiation into

neural In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...

and

glial cells Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...

as well as

hepatocyte A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: * Protein synthesis * Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, ...

precursors was observed.

Properties

The majority of stem cells present in the amniotic fluid share many characteristics, which suggests they may have a common origin. In 2007, it was confirmed that the amniotic fluid contains a heterogeneous mixture of multipotent cells after it was demonstrated that they were able to differentiate into cells from all three germ layers but they could not form

teratoma A teratoma is a tumor made up of several different types of tissue, such as hair, muscle, teeth, or bone. Teratomata typically form in the ovary, testicle, or coccyx. Symptoms Symptoms may be minimal if the tumor is small. A testicular terato ...

s following implantation into

immunodeficient Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...

mice. This characteristic differentiates them from embryonic stem cells but indicates similarities with

adult stem cells Adult stem cells are undifferentiated cells, found throughout the body after development, that multiply by cell division to replenish dying cells and regenerate damaged tissues. Also known as somatic stem cells (from Greek σωματικóς, m ...

. However, foetal stem cells attained from the amniotic fluid are more stable and more plastic than their adult counterparts making it easier for them to be reprogrammed to a pluripotent state. A variety of techniques has been developed for the isolation and culturing of amniotic stem cells. One of the more common isolation methods involves the removal of amniotic fluid by amniocentesis. The cells are then extracted from the fluid based on the presence of

c-Kit Proto-oncogene c-KIT is the gene encoding the receptor tyrosine kinase protein known as tyrosine-protein kinase KIT, CD117 (cluster of differentiation 117) or mast/stem cell growth factor receptor (SCFR). Multiple transcript variants encoding diff ...

. Several variations of this method exist. There is some debate whether c-Kit is a suitable marker to distinguish amniotic stem cells from other cell types because cells lacking c-Kit also display differentiation potential. Culture conditions may also be adjusted to promote the growth of a particular cell type.

Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are highly abundant in the amniotic fluid and several techniques have been described for their isolation. They usually involve the removal of amniotic fluid by amniocentesis and their distinction from other cells may be based on their

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

or other characteristics.

Human leukocyte antigen The human leukocyte antigen (HLA) system or complex is a complex of genes on chromosome 6 in humans which encode cell-surface proteins responsible for the regulation of the immune system. The HLA system is also known as the human version of th ...

testing has been utilised to confirm that the MSCs stem from the fetus and not from the mother. Originally it was proposed that the MSCs were discarded from the embryo at the end of their life cycle but since the cells remained viable in the amniotic fluid and were able to proliferate in culture this hypothesis was overturned. However, it remains unclear whether the cells originate from the fetus itself, the placenta or possibly the

inner cell mass The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of ...

of the

blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...

. Comparison of amniotic fluid-derived MSCs to bone-marrow-derived ones showed that the former has a higher expansion potential in culture. However, the cultured amniotic fluid-derived MSCs have a similar phenotype to both adult bone-marrow-derived MSCs and MSCs originating from second trimester fetal tissue. In animals, the MSCs seem to have a unique immunological profile which was observed after their isolation and ''in vitro'' culturing.

Embryonic-like stem cells

As opposed to mesenchymal stem cells, embryonic-like stem cells are not abundant in the amniotic fluid, making up less than 1% of amniocentesis samples. Embryonic-like stem cells were originally identified using markers common to embryonic stem cells such as nuclear

Oct4 Oct-4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), is a protein that in humans is encoded by the ''POU5F1'' gene. Oct-4 is a homeodomain transcription factor of the POU family. I ...

CD34 CD34 is a transmembrane phosphoglycoprotein protein encoded by the CD34 gene in humans, mice, rats and other species. CD34 derives its name from the cluster of differentiation protocol that identifies cell surface antigens. CD34 was first descri ...

vimentin Vimentin is a structural protein that in humans is encoded by the ''VIM'' gene. Its name comes from the Latin ''vimentum'' which refers to an array of flexible rods. Vimentin is a type III intermediate filament (IF) protein that is expressed ...

alkaline phosphatase The enzyme alkaline phosphatase (EC 3.1.3.1, alkaline phosphomonoesterase; phosphomonoesterase; glycerophosphatase; alkaline phosphohydrolase; alkaline phenyl phosphatase; orthophosphoric-monoester phosphohydrolase (alkaline optimum), systematic ...

stem cell factor Stem cell factor (also known as SCF, KIT-ligand, KL, or steel factor) is a cytokine that binds to the c-KIT receptor (CD117). SCF can exist both as a transmembrane protein and a soluble protein. This cytokine plays an important role in hematopoie ...

and c-Kit. However, these markers were not necessarily concomitantly expressed. In addition, all of these markers can occur on their own or in some combination in other types of cells. The

pluripotency Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...

of these embryonic-like stem cells remains to be fully established. Although those cells which expressed the markers were able to differentiate into

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

, adipogenic,

osteogenic Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in the ...

, nephrogenic,

and

endothelial cells The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...

, this did not necessarily occur from a homogenous population of undifferentiated cells. Evidence in favour of their embryonic stem cell nature is the cells’ ability to produce clones.

Clinical applications

The use of amniotic stem cells instead of embryonic stem cells may be advantageous in some cases for various reasons including that the former do not form teratomas. Their enhanced stability and plasticity compared to adult stem cells may also be an advantage. Stem cells from both the amniotic fluid and membrane are utilised for therapeutic approaches.

Foetal tissue engineering

Possible applications include the use of amniotic stem cells for foetal

tissue engineering Tissue engineering is a biomedical engineering discipline that uses a combination of Cell (biology), cells, engineering, Materials science, materials methods, and suitable biochemistry, biochemical and physicochemical factors to restore, maintai ...

to reconstruct birth defects in

infant An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...

s. This would circumvent the complications that are often associated with harvesting stem cells from foetal tissue. A small amount of amniotic fluid provides a large enough quantity of cells for the tissue engineering process and could help correct a number of defects including

diaphragmatic hernia Diaphragmatic hernia is a defect or hole in the diaphragm that allows the abdominal contents to move into the chest cavity. Treatment is usually surgical. Types * Congenital diaphragmatic hernia ** Morgagni's hernia ** Bochdalek hernia * Hiatal h ...

and possibly repair premature membrane rupture during pregnancy. If frozen and banked, the cells may also be used for similar purpose later in life.

Cardiovascular tissue engineering

Several studies have been carried out to investigate the potential of amniotic stem cells to differentiate into cardiac cells. Although c-Kit sorted cells express some genes common in cardiac cells, success in this area is still limited. Co-culturing, i.e. mixing cells and plating them together, of human amniotic stem cells with neonatal rat ventricular

myocyte A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...

s (NRVM) caused the cells to form functional

gap junction Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...

s with each other, an indicator for cardiac-like cells. However, these results may be due to the specific features of the NRVM or fusion of the cells rather than the amniotic stem cell’s own potential to differentiate into cardiac cells. In general, these types of techniques are considered to be potentially significant but further investigations are required. Another area of interest is the use of these cells for improvement of cardiac tissue following a

myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may ...

. Several strategies have been tested in rats including the injection of dissociated amniotic stem cells into the infarct region, which yielded conflicting results from several research groups. In contrast, injection of amniotic stem cell aggregates seems to improve the function of the tissue significantly by reducing the size of the infarct area and improving the function of the left ventricle. In addition,

vasculature The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

density has been shown to increase. Injection of cells immediately following the infarct is particularly beneficial as the cells protect the cardiac tissue from further damage. Moreover, other findings have brought the proof of concept that

secretome The secretome is the set of proteins expressed by an organism and secreted into the extracellular space. In humans, this subset of the proteome encompasses 13-20% of all proteins, including cytokines, growth factors, extracellular matrix proteins a ...

of amniotic stem cell could act as an effective paracrine agent against Doxorubicin induced cardiotoxicity, confirming the potential importance of this cellular population in the field of cardiological research.

Kidney injury repair

Following the discovery that amniotic stem cells are able to differentiate into renal cells, this was further explored in several studies. These showed that ''in vitro'' the cells were able to contribute to early

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...

structures as well as being able to integrate into early kidney structures ''

ex vivo ''Ex vivo'' (Latin: "out of the living") literally means that which takes place outside an organism. In science, ''ex vivo'' refers to experimentation or measurements done in or on tissue from an organism in an external environment with minimal ...

'' and continue their development into mature

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ca ...

s. Results obtained for the use of amniotic stem cells in the

postnatal The postpartum (or postnatal) period begins after childbirth and is typically considered to end within 6 weeks as the mother's body, including hormone levels and uterus size, returns to a non-pregnant state. The terms puerperium, puerperal perio ...

kidney were far less encouraging as the cell’s contribution to the tissue was very small. However, the cells were able to exert a protective effect on tubular cells in mice with

acute tubular necrosis Acute tubular necrosis (ATN) is a medical condition involving the death of tubular epithelial cells that form the renal tubules of the kidneys. Because necrosis is often not present, the term acute tubular injury (ATI) is preferred by pathologists ...

. Amniotic stem cells can also be used to treat chronic damage. This was shown in mouse models for

Alport syndrome Alport syndrome is a genetic disorder affecting around 1 in 5,000-10,000 children, characterized by glomerulonephritis, end-stage kidney disease, and hearing loss. Alport syndrome can also affect the eyes, though the changes do not usually affect v ...

, where the cells prolonged survival of the animals by slowing down the progression of the disease. The same effect was observed in mouse models where human amniotic stem cells were used to treat uretral obstruction.

Ethical Considerations

The use of foetal cells has been highly controversial because the tissue is usually obtained from the foetus following

induced abortion Abortion is the termination of a pregnancy by removal or expulsion of an embryo or fetus. An abortion that occurs without intervention is known as a miscarriage or "spontaneous abortion"; these occur in approximately 30% to 40% of pregnan ...

. In contrast, foetal stem cells in the amniotic fluid can be obtained through routine

prenatal testing Prenatal testing consists of prenatal screening and prenatal diagnosis, which are aspects of prenatal care that focus on detecting problems with the pregnancy as early as possible. These may be anatomic and physiologic problems with the health of ...

without the need for abortion or foetal

biopsy A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a diseas ...

References

{{DEFAULTSORT:Amniotic Stem Cells Stem cells