Creating controlled 3D microtissues

Scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory can now control how cells in vitro connect with one another and assemble themselves into three-dimensional, multicellular microtissues.

The findings could lead to more successful applications of artificial tissues in medicine, such as skin grafts and bone marrow transplants.

The researchers demonstrated their method by constructing a tailor-made artificial cell-signalling system, analogous to natural cell systems that communicate via growth factors.

Artificial tissues are used in medicine for applications such as skin grafts, bone marrow transplants or blood substitutes, as well as in basic medical and biological research. Tissue engineers try to improve upon or repair natural tissues by manipulating living cells from one or more donors, sometimes in combination with synthetic materials. Unfortunately, in this 'top down' approach, the cells assemble themselves randomly, losing the 3D organisation vital to many tissue functions.

“Our method allows the assembly of multicellular structures from the bottom up,” said Professor Carolyn Bertozzi, principal investigator in the research, who directs the Molecular Foundry nanoscience research facility at Berkeley Laboratory.

“In other words, we can control the neighbours of each individual cell in a mixed population. By this method, it may be possible to assemble tissues with more sophisticated properties.”

Bertozzi and her colleague Zev Gartner, assistant professor of pharmaceutical chemistry at the US University of California, enabled cells to react with other cells in a controlled way by coating the cell surface in single-strand DNA chains only 20 bases long.

When the cell met another cell coated with the complementary DNA strand, the single strands would bond and form double-stranded DNA, binding the cells together.

By controlling variables such as strand length and sugar availability, the researchers can use the process to synthesise large, complex microtissues in much the same way a synthetic organic chemist assembles a complex molecule.

Bertozzi and Gartner applied these methods to maintain the survival and replication of hematopoietic progenitor cells, which depend on the presence of the growth factor interleukin-3, by combining them in microtissues with CHO cells (Chinese hamster ovary cells) that were engineered to secrete interleukin-3.

“Since DNA has essentially an unlimited capacity for information storage, there is no theoretical limit on the number of different cell types we can assemble in a structure,” said Bertozzi.

She said the key is to give each cell type its own unique DNA 'bar code', enabling its programmed interaction with any other specified cell type.

“In practice, I think structures with three or four cell types are quite feasible. Such structures would be relevant to many biological organs.”

Practical challenges remain, such as scaling up the production of tailored microtissues to quantities needed for biomedical applications. Beyond that, Bertozzi hopes to refine the present method of modifying cell-surface DNA.

The study is published in the Proceedings of the National Academy of Sciences, available in the online version on 2 March 2009.