Stem Cells—A New Era of Tissue Replacement Takes Shape

Many firms are conducting product development and research in the areas of skin replacement, vascular tissue replacement and bone grafting or regeneration. Stem cells, as well as transgenic organs harvested from pigs, are under study for use in humans. At its highest and most promising level, regenerative medicine may eventually utilize human stem cells to create virtually any type of replacement organ or tissue.

In one recent, exciting experiment, doctors took stem cells from bone marrow and injected them into the hearts of patients undergoing bypass surgery. The study showed that the bypass patients who received the stem cells were pumping blood 24% better than patients who had not received them.

In another experiment, conducted by Dr. Mark Keating at Harvard, the first evidence was shown that stem cells may be used for regenerating lost limbs and organs. The regenerative abilities of amphibians have long been known, but exactly how they do it, or how it could be applied to mammals, has been little understood. Much of the regenerative challenge lies in differentiation, or the development of stem cells into different types of adult tissue such as muscle and bone. Creatures such as amphibians have the ability to turn their complex cells back into stem cells in order to regenerate lost parts. In the experiment, Dr. Keating made a serum from the regenerating nub (stem cells) of a newt’s leg and applied it to adult mouse cells in a petri dish. He observed the mouse cells “de-differentiate,” or turn into stem cells. In a later experiment, de-differentiated cells were turned back into muscle, bone and fat. These experiments could be the first steps to true human regeneration. Keating is continuing to make exciting breakthroughs in regenerative research.

As of mid-2023, a small number of biotech startups were planning human trials of induced pluripotent state cells (IPSC) which behave like embryonic cells. These human stem cells are produced without human cloning or the use of human embryos or eggs. Adult cells are drawn from a skin biopsy and treated with four reprogramming factors (called Yamanaka factors), rendering cells that can produce all human cell types and grow indefinitely. This may have the potential to be used in cellular reprogramming as a part of personalized medicine aimed at reversing disease or in therapies aimed at reversing cellular aging. The process could be used to regenerate cells in patients suffering from Parkinson’s Disease, macular degeneration or heart disease. A pioneer in this research is Shinya Yamanaka of Kyoto University, a Nobel Prize winner for the creation of IPSC.

The potential of the relatively young science of tissue engineering appears to be unlimited. Transgenics (the use of organs and tissues grown in laboratory animals for transplantation to humans) is considered by many to have great future potential, and improvements in immune system suppression will eventually make it easier for the human body to tolerate foreign tissue instead of rejecting it. There is also increasing theoretical evidence that malfunctioning or defective organs such as livers, bladders and kidneys could be replaced with perfectly functioning “neo-organs” (like spare parts) grown in the laboratory from the patient’s own stem cells, with minimal risk of rejection.

The ability of most human tissue to repair itself is a result of the activity of these cells. The potential that cultured stem cells have for transplant medicine and basic developmental biology is enormous.

Diabetics who are forced to cope with daily insulin injection treatments could also benefit from engineered tissues. If they could receive a fully functioning replacement pancreas, diabetics might be able to throw away their hypodermic needles once and for all.

Elsewhere, the harvesting of replacement cartilage, which does not require the growth of new blood vessels, is being used to repair damaged joints and treat urological disorders. Genzyme Corp. won FDA approval for its replacement cartilage product Carticel, the first biologic cell therapy to become licensed. Genzyme’s process involves harvesting the patient’s own cartilage-forming cells, and, from those cells, re-growing new cartilage in the laboratory. The physician then injects the new cartilage into the damaged area. Full regeneration of the replacement cartilage is expected to take up to 18 months.

Another cartilage therapy has been developed by Vericel Corp. (www.vcel.com). Its matrix-induced autologous chondrocyte implantation (MACI) process was approved by the FDA in 2016. With this process, a small amount of health cartilage is removed from a patient’s knee. Cells called chondrocytes are removed from the sample and seeded in a petri dish onto a scaffold made of biodegradable collagen, creating a living mesh which is inserted back into the damaged area of the patient’s knee. This procedure is far less invasive than a full knee replacement.

The next big thing in tissue replacement is three-dimensional (3D) printers to fabricate a variety of shapes made of living tissue, including tubes suitable for blood vessels, cartilage for use in human joints and patches of skin and muscle. The process takes stem cells harvested from a patient and treats them in the lab to stimulate multiplication, creating cell aggregates. The resulting “bioink” is loaded in cartridges shaped like syringes with extrusion nozzles. Software directs the printer to extrude the bioink in a precise pattern of layers interspersed with hydrogel (a gelatinous water-based substance) used to mold the cells into the desired shape. The printed tissue is then allowed to grow into mature cells suitable for research.

Although in its infancy, one San Diego, California firm called Organovo, Inc. (organovo.com) already produces commercial 3D bioprinters for use in research. In early 2014, Organovo launched its first product, slivers of human liver tissue for use in laboratories to test drug toxicity. Physicians and researchers can find out how a patient’s liver will respond to different treatments before going to the expense of clinical trials. In addition to liver tissue, the firm is focused on kidney tissue and intestinal tissue for the treatment of inflammatory bowel disease (IBD). Ultimately, it is hoped that 3D bioprinting will be able to produce viable replacement organs.

Revivicor (www.revivicor.com), a division of United Therapeutics, is working to breed pigs with human genes. Organs from these animals may be transplanted into humans with fewer immune system rejection problems. Researchers at the National Heart, Lung and Blood Institute in Bethesda, Maryland have been testing the specialized pig organs in baboons. Revivicor ultimately plans to breed up to 1,000 pigs per year and ship organs rapidly by helicopter.

Companies to Watch in Replacement Tissues, including 3-D Printing of Tissues:

ViaCord (formerly ViaCell, Inc. before its acquisition by PerkinElmer), in Boston, Massachusetts (www.viacord.com), develops therapies using umbilical cord stems. Also, their ViaCord product enables families to preserve their baby’s umbilical cord at the time of birth for possible future use in treating over 40 diseases and genetic disorders.

Cytori Therapeutics, in San Diego, California (www.cytori.com), is focused on the use of adult Adipose-Derived Regenerative Cells (ADRCs). Its goal is to apply these cells as therapies for chronic heart failure, burn care, soft tissue injury and sports medicine.

Allevi (formerly BioBots), in Philadelphia, Pennsylvania (allevi3d.com), offers desktop 3-D bioprinters capable of printing hydrogels such as collagen, hyper-elastic bone and conductive tissues.

Aspect Biosystems Ltd. in Vancouver, British Columbia (www.aspectbiosystems.com) is developing a portfolio of 3-D bio-printed human tissues used in predictive pre-clinical models and implantable tissue therapies.

Materialise NV, in Leuven, Belgium (www.materialise.com) offers 3-D printing technology for many industries including health care, for which it designs implants, surgical guides and other medical devices.

Internet Research Tip:

For an excellent primer on genetics and basic biotechnology techniques, see:

National Center for Biotechnology Information

www.ncbi.nlm.nih.gov

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