For certain amphibians and reptiles, regrowing body parts is a piece of cake. But for us humans, it’s just not something that’s built into our physiology. Although individual cells in our body are constantly being replaced as they wear out, regenerating entire organs and body parts has been a long-standing dream of modern medicine. But now, thanks to advancements in stem cell research, scientists have successfully cultivated a range of human body structures that have been successfully tested in animals, and small-scale human organs known as “organoids,” which have been used to study human organ function at an unprecedented level of detail. Here are some of the lab-made body parts scientists are hoping will soon revolutionize transplant surgery.

12. Ear

For anyone missing an ear, the process of rebuilding it can be difficult and distressful, involving the painful removal of cartilage from the patient’s ribs. However, a new technique developed by physicians at Cornell University is promising to make things easier. By 3D printing synthetic ears and coating them with living cells that grow around the frame, the researchers can then inject it with a cocktail of living ear cells and collagen from cows. In the past, fabricated ears have been implanted onto rats so scientists could evaluate changes in size and shape as the organs grow over time. It’s believed the technique could one day be used to help people with missing or deformed outer ears.

11. Trachea

A lab-grown trachea made from stem cells was the first tissue-engineered whole organ to have been successfully transplanted into a patient. To make the new airway, stem cells were taken from the patient’s bone marrow and transferred onto a scaffold that was created by stripping the cells from a trachea donated from a person who had recently died. The recipient, who had suffered damage to her own trachea following a rare form of tuberculosis, received the three-inch-long windpipe section without complications and was returned to perfect health.

With the technology still in its early stages, scientists believe that replacing relatively simple structures like the trachea remains the most promising form of regenerative medicine.

10. Bladder

Among the first organs to be grown in the laboratory and successfully transplanted into patients, the bladder is essentially a hollow balloon that collects the urine excreted by the kidneys. Scientists at Wake Forest University in North Carolina grew new bladders using bladder cells taken from patients. Those cells were then transferred onto a scaffold and developed for seven weeks before being surgically attached to replace the old bladders of seven patients in 2006.

More recent research conducted in 2010 revealed that it’s possible to grow new bladders using stem cells taken from the bone marrow of patients.

9. Bones

Historically, if you were born with bone problems or lost bone to disease, the only solution was to receive healthy bone from a donor. Next to blood, bone is the most widely transplanted human tissue in the world, resulting in over two million procedures a year at a cost of more than $5 billion. Unfortunately, the human body is quite fickle and many complications, such as the increased risk of infection, can arise when the immune system rejects what it sees as a foreign intruder.

The good news is that a number of research groups around the world are currently in the process of developing techniques to grow human bones from stem cells in the laboratory. Bonus BioGroup is an Israeli biotechnology company that’s using three dimensional scans to build a gel-like scaffold in the shape of a specific bone. The scaffold is then seeded with stem cells taken from fat and developed into solid bone that can be implanted in the recipient. And since the bone built from the stem cells of the recipient, there’s no chance of the body rejecting it.

8. Liver

The liver is a vital organ responsible for a wide range of functions, including detoxification, protein synthesis and the production of digestive biochemicals. When it fails, it can often result in death if the patient doesn’t receive a timely transplant. However, miniature functioning livers have been grown in the lab by Japanese researchers. Recently, they announced that these tiny organs, referred to as liver buds, were able to connect to the blood supply and function when transplanted into mice. The scientists believe that by transplanting thousands of these liver buds into human patients it could help restore the function of a failing liver. The researchers are warning people to be cautiously optimistic about the development though, as it could still be a while before stem cell-grown liver buds make their way into human patients.

7. Kidney

In 2013, Australian researchers grew the first mini-kidney in a petri dish. However, while these mini-kidneys are used for drug screening, disease modelling and cell therapy, they have proven to be less effective when transplanted into live rats. Similar research is being conducted at Massachusetts General Hospital with the ultimate goal of producing functional kidneys that can be used for transplants. Using a decellularization process, the scientists stripped an old rat kidney of its cells to leave a tangled web of proteins and blood vessels as a scaffold. They then pumped blood vessel cells and kidney cells through the scaffold and incubated it. Once transplanted into a rat, the kidneys were able to filter blood and produce urine much like a normal kidney. 

6. Breast Tissue

Last year, in an effort to study the development of breast cancer, scientists from the Helmholtz Center for Health and Environmental Research in Germany grew mammary glands in a lab. The glands were grown from cultured breast epithelial cells, giving them the opportunity to look at how cells generate the hollow ducts that form a network of grape-like structures in breast tissue.

Researchers used healthy breast tissue taken from women undergoing aesthetic breast reduction and developed a gel that facilitates cell division and dissemination, in a manner similar to the development of mammary glands during puberty. It was reported that the rigidity of the gel increased the spreading of the cells, an invasive form of growth they said is a normal response during breast development. But it’s this normal process that is exploited during tumor progression that the researchers hope to gain a better understanding of by growing healthy breast tissue. By discovering how the process works, they hope to learn more about how things operate at the molecular level, which should provide the basis for developing new therapeutic strategies to fight breast cancer.

5. Pituitary Gland

Located at the base of the brain, the pituitary gland is a pea-sized hormone secreting organ that, among other things, helps control growth, blood pressure and metabolism. Using stem cells taken from a mouse embryo, scientists in Japan were able to grow small pituitary glands in the laboratory. When the tissue was transplanted into mice with defective pituitary glands, it raised levels of the missing hormones in their bodies.

Researchers are now working on creating pituitary tissue using human stem cells. According to Dr. Yoshiki Sasai, the lead scientist working on the study at the RIKEN Centre for Developmental Biology in Kobe, Japan, it is difficult to guess how long it will take to make pituitary glands that can be used in a human transplant, but he’s confident that it’s an achievable goal.

4. Eyes

Since the eyes are among the most complex organs in the human body, trying to make them in the lab has proven to be an incredibly complicated and difficult undertaking.

However, Yoshiki Sasai, Mototsugu Eiraku and their colleagues from the RIKEN Center for Developmental Biology have demonstrated that they can entice mouse and human embryonic stem cells to form retinal precursors. And, after growing the precursors for a few weeks in culture, these cells would spontaneously self-assemble into three-dimensional retinal tissue including multiple layers of neural retinal cells with both rod and cone photoreceptors. The hope is that the methods developed by the research team could one day yield culture tissue that can be transplanted into a damaged or defective human retina to restore vision.

3. Stomach

Researchers experimenting with stem cells at the Cincinnati Children’s Hospital Medical Center were the first to create a three-dimensional stomach in the lab. The synthetic stomach can be used to identify the factors necessary to form a healthy human stomach and also assist scientists in the analysis of certain bacterium that causes gastric disease.

After studying how a stomach naturally forms inside a mother’s womb, researchers placed stem cells inside a petri dish and watched it over the course of a month into an inch-long stomach. Next, they introduced H. pylori bacteria (a leading cause of stomach cancer and peptic ulcer disease) to the petri dish and, within 24 hours, the organ was showing early signs of gastric disease rapidly spreading throughout the tiny stomach.

In the United States alone, 70 million people are affected by digestive diseases. If scientists can scale up their lab-grown stomachs to make them usable in transplants, it would have the potential to dramatically change the lives of not only people with digestive diseases, but also people dealing with eating disorders like anorexia and obesity who have stomachs that are trained to release imbalanced levels of hunger-related hormones like grehlin and leptin.

2. Heart

Earlier this month, scientists at the University of Pittsburgh announced they had created a beating mouse heart after rebuilding it with human stem cells. Researchers prompted stem cells to develop into heart muscle and connective tissue, and then organize into tiny chambers and “beat.” They believe they could use a similar technique to create human heart tissue that can be used in drug testing or for grafts to repair damaged areas. Admittedly, there are still many challenges to overcome before they can get to the stage where they can replace an entire human heart, but many researchers see more value in using beating human heart tissue in the laboratory as a tool for studying heart disease and as a way of safely testing new treatments. Experiments involving the injection of human stem cells into the damaged hearts of mice, rats and guinea pigs have already shown the potential for treating heart disease and a recent study demonstrated successful heart tissue regeneration in a macaque monkey.

1. Miniature Brain

Scientists from the Institute of Molecular Biotechnology in Vienna, Austria, successfully created a miniature brain in the lab in 2013. Using stem cells, they grew a model of a developing brain that was roughly the size of a nine-week-old embryonic human brain. Though the mini-brain didn’t quite look exactly like a real brain, it had active neurons and a very comparable organizational structure.

However, since our functioning brains are the product of years of learning and development during which countless neurological connections are formed from our life experiences, a brain grown newly in a lab wouldn’t initially carry any memories or necessarily have the same ability to control the body. Consequently, it’s very unlikely that anyone would actually want to undergo a full brain transplant in the future unless these very important aspects could be addressed. As such, the main use for these organs will be to study neurological diseases, like Alzheimer’s, and test the effects of new drugs on the brain.