by Lorne Taichman
I am looking down the barrel of a microscope at living, human, skin cells. It is 7:30 a.m. on a morning in April 1990. I am in my lab. I am growing these cells for my research on gene therapy. The filters on the microscope lend an eerie yellow-grey hue to the field, but the cells are clearly recognizable by their cuboidal shapes, which remind me of the cobblestones outside the Metropolitan Museum of Art on Fifth Avenue.
The cells are clustered on the surface of the petri dish. A rough guess at the number – about eighty in a cluster. Yesterday there were about forty. They have doubled in number over the past twenty-four hours. Vigorous, healthy, skin cells. That doubling was accomplished by cell division, an essential activity of all living cells. Those cells may appear static and unmoving, but time lapse photography would show them sliding about, exploring the confines of the petri dish. In a few days, though, with continued cell growth, the individual clusters would merge into a confluent lawn. There will be no space for continued growth laterally, but cell division would continue, and remarkably, the cells would begin to pile up and form a multilayered tissue, much like the epidermis of the original skin. I am always amazed at this innate ability. These cells are true to their origins even when cast adrift far from the intact skin of their origin.
We get the skin from the newborn nursery in the hospital across the road. When a non-religious circumcision is performed, the nurses place the foreskin tissue, which would normally be discarded, into an unmarked container. We are allowed to use this tissue without getting consent from the parents, because the identity of the little boy is not disclosed to us. Occasionally, cells from a foreskin grow more sluggishly than usual. I don’t like to think what poor growth might portend. Even if I did know something about that boy’s future, I do not know his identity and, therefore, cannot communicate with his parents. He is safe from any unrequested intrusion.
Cells usually grow for a limited period in culture and then, in the parlance of the profession, they poop out. Human skin cells are no exception. Like us, who are limited by aging and senescence, our cells also age when freed from the confines of our bodies. When we first cultured these cells, they would do well for the first several weeks but then they would lose their vigor and die. As a result, we were dependent on a continual supply of foreskins. With time, we learned how to achieve better cell growth and longer culture life.
One stumbling block was getting skin cells to attach to the surface of a petri dish where they can grow. Skin cells do not readily adhere, and if they fail to attach, they die. This problem was overcome with the use of what we call helper cells.
In the early 1980s, two scientists at Harvard, Jim Rheinwald and Howard Green, discovered that cells from a mouse embryo could greatly enhance the ability of skin cells to attach to the dish surface. When these mouse cells are added to the culture, they secrete chemicals that coat the dish surface and facilitate skin cell attachment. The mouse cells were appropriately named helper cells. The use of helper cells represented a giant leap in our ability to efficiently farm these skin cells in the laboratory.
The cultures are bathed continually in a nutrient broth that contains a diet of sugars, minerals, amino acids, and vitamins. In this medium, the cells grow sluggishly. They certainly would not double in number overnight. The key to outstanding cell performance is fetal calf serum. Fetal calf serum is a misnomer. It is not from an unborn calf. It is extracted from the vein of a living pregnant cow. That bovine mother, in all her tenderness and caring, has produced chemicals that circulate in her blood and nourish her developing fetus and, quite incidentally, allow our skin cells to flourish. With fetal calf serum in the mix, we can grow cells for many doublings. When I first started to work with these cells, I needed a continuous supply of foreskins. My daughters still tease me about the foreskin tissue I retrieved from the local hospital and stored in our home refrigerator until it could be brought to the laboratory. With the use of helper cells, fetal calf serum, and a few other special ingredients, a single foreskin can keep my lab amply supplied with cells for a year or so and, even longer, if we make use of cell freezing.
In culture, skin cells must grow. If they don’t, they eventually die. Cell freezing stops the clock. Cell freezing is a bit of an art. Cells are damaged by the act of freezing, especially the thawing portion. You may not believe me, but adding hefty amounts of table sugar to the freezing media avoids that damage by allowing the water crystals in the frozen cell interior to slide over one another during thawing, rather than fracturing. We have cells frozen away from a good number of newborn baby boys. I wonder if, years hence, should a little boy become ill, might his own frozen cells save his life? Alas, anonymity. We will never know.
One final note. The recipe for skin cell growth in culture has been so refined and so successful that some have even used these cultures as living dressings to treat burn victims with their own cells. Skin cells are not the only cell types we can grow in culture to be transplanted back to the donor: others include bone marrow cells, cartilage cells, muscle cells, blood vessel cells. The list grows.
Now that I am at an advanced age, I wonder if there might not be a way to harvest brain cells when we are young and won’t miss them, and then return them when we really need them, like today. Unfortunately, it is too late for me.