Feinberg, a bioprinting expert at Carnegie Mellon University, reviews decellularization, one idea to get around the problem of human immune systems rejecting transplanted tissue.  Decellularization involves removing the native cells from an organ but keeping its collagen, which forms the organ’s structure and doesn’t contain live cells. Recellularization is difficult because it requires repopulating the organ with new cells. Intact cells cannot reliably traverse capillary walls or reach correct niches. 

Feinberg introduces FRESH, his 3D-printing-based project, which strives to remedy this issue by reconstructing the structures of the collagen and cell tissues in tandem. A graduate student demonstrates by printing a collagen artery into a petri dish full of gel. A postdoctoral researcher shows printing runs that combine collagen and live liver tumor cells.

Roach watches heart cells beat in unison and learns why alignment and vascularization limit tissue production scale. A researcher prints millimeter-thick cardiac constructs that contract in a coordinated manner; alignment matters to prevent arrhythmias. Another reports that printed ventricles can survive in mice but currently lack valves; the lab has printed collagen valves and plans to integrate them. 

Feinberg recounts that early lab-grown bladders likely lacked true neuromuscular function, and he places whole-organ printing at an early stage of development. He estimates that full development is at least a decade away and will cost six figures for a heart. He notes that smaller, simpler targets will come first, highlighting the near-term focus on structures like hair follicles.

The chapter opens with the history of hair transplantation. In the 1950s, Norman Orentreich showed that transplanted follicles retain their original programming, a concept known as donor dominance. If they are harvested from a site on the body with thick hair growth, they will continue to grow even when implanted on balding scalps. Decades earlier, Shoji Okuda, a Japanese physician, was the first to document successful follicle transplants. He experimented widely with various donor sites, establishing the fact that follicles can be harvested and transplanted to and from literally anywhere on the body. 

This background frames Roach’s visit to San Diego surgeon Dr. Richard Chaffoo to donate follicles for research. Chaffoo describes the artistry of building a natural hairline and warns that repeated procedures can deplete donors. Guided by a nurse assistant, Chaffoo uses a motorized punch to harvest 12 follicular units from the back of Roach’s scalp for delivery to Stemson Therapeutics. At her request, they also transplant one two-hair unit to her left calf to see if it will take. An associate from Stemson collects the remaining follicles for lab dissection. 

Communication from the company then lapses, and nothing comes of the calf graft. Nearly a year later, Stemson invites the narrator to return, signaling a shift in the research approach.

The narrator returns to Stemson Therapeutics and meets Kevin D’Amour, the chief scientific officer, who outlines the lab's current focus: pluripotent stem cells. These are cells that three- or four-day-old embryos are made of, and their strength is that they are not yet programmed. In other words, they have the potential to become any type of human cell. 

Shinya Yamanaka, a researcher in Japan, had a breakthrough when he found a way to revert existing human cells to this pluripotent state. This was groundbreaking: It opened possibilities for treating and curing various diseases, as a patient can have their own cells harvested and converted to perform any function they want. Yamanaka won a Nobel Peace Prize for this discovery in 2012. 

Stemson applies this science to hair. They attempt to turn stem cells from donors like Roach into hair-producing cells. Lisa McDonnell shows Roach around the lab while she explains. Upon the author’s request, McDonnell pulls a vial of Roach’s cells to check on their progress. They turn out to have “petered out,” and there are far too few for robust experiments.

Roach asks to see human hair growing from the skin of mice—this is one of Stemson’s main research areas. D’Amour explains how the team adopted a new strategy since the last time Roach was there. To solve the issue of newly implanted hairs growing in all different directions, the new protocol involves threading a fine suture through droplets of dermal papilla cells and keratinocytes, which self-organize into a primitive follicle before implantation. 

In a procedure room, a biologist examines skin patches on immunodeficient nude mice, which receive a “slurry” of cells and then the follicle-on-a-thread implants; microscopy distinguishes incidental mouse hairs from organized human hairs from iPS-derived follicles. D’Amour projects clinical trials in roughly two years and flags a key unknown: whether new follicles inherit androgen sensitivity. This is a vital question, and the human trials needed to find an answer will be very expensive and time-consuming, as each patient’s individual cells first have to be harvested and converted to stem cells. 

One solution would be to use the already-prepared cells of someone else. These hypothetical cells, called “stealth” cells, would have to evade the host body’s immune system. This issue is the most pressing area of research in regenerative medicine. D’Amour contrasts this regulated path with the unregulated marketplace of “stem cell” fat injections that repurpose liposuction fractions without defined cell populations, causing injury and harm to consumers.