Research turns stem cells to generate bone, heart muscle within days

Researchers at the Stanford University School of Medicine are able to make pure populations of any of 12 cell types, including those needed to generate bone, heart muscle and cartilage, within days rather than the weeks or months previously required.

The ability, hailed as a key step toward clinically useful regenerative medicine, namely repairing damage after a heart attack or creating cartilage or bone to reinvigorate creaky joints, is acquired by mapping out the sets of biological and chemical signals necessary to direct human embryonic stem cells to make pure populations of tissue cells.

"Regenerative medicine relies on the ability to turn pluripotent human stem cells into specialized tissue stem cells that can engraft and function in patients," said Irving Weissman, the director of Stanford' s Institute for Stem Cell Biology and Regenerative Medicine, and a senior author of a study published in the recent issue of the journal Cell.

"Here we used our knowledge of the developmental biology of many other animal models to provide the positive and negative signaling factors to guide the developmental choices of these tissue and organ stem cells."

The result, as Weissman put it: "within five to nine days we can generate virtually all the pure cell populations that we need."

Embryonic stem cells are pluripotent, meaning they can become any type of cell in the body by responding to a variety of time- and location-specific cues within the developing embryo that direct them to become specific cell types.

Unlike many other animals, human embryonic development is a mysterious process, particularly in the first weeks after conception. This is because cultivating a human embryo for longer than 14 days is banned by many countries and scientific societies. But researchers do know that, like other animals, the human embryo in its earliest stages consists of three main components known as germ layers: the ectoderm, the endoderm and the mesoderm.

Each of these germ layers is responsible for generating certain cell types as the embryo develops. The mesoderm, for example, gives rise to key cell types, including cardiac and skeletal muscle, connective tissue, bone, blood vessels, blood cells, cartilage and portions of the kidneys and skin.

"The ability to generate pure populations of these cell types is very important for any kind of clinically important regenerative medicine, as well as to develop a basic road map of human embryonic development," said Stanford graduate student Kyle Loh, a lead author of the study. "Previously, making these cell types took weeks to months, primarily because it wasn' t possible to accurately control cell fate. As a result, researchers would end up with a hodgepodge of cell types."

To know what signals drive the formation of each of the mesodermally derived cell types, the researchers started with a human embryonic stem cell line, which they chemically nudged to become cells that form what' s known as the primitive streak on the hollow ball of cells of the early embryo; and knowing that often the cells progressed down the developmental path through a series of consecutive choices between two possible options, they then experimented with varying combinations of well-known signaling molecules as a way to coax these cells to become ever-more-specialized precursor cells.

The quickest, most efficient way to micromanage the cells' developmental decisions was to apply a simultaneous combination of factors that both encouraged the differentiation into one lineage while actively blocking the cells from a different fate - a kind of "yes" and "no" strategy.

For example, according to a news release from Stanford Medicine News Center, cells in the primitive streak can become either endoderm or one of two types of mesoderm. Inhibiting the activity of a signaling molecule called TGF beta drives the cells to a mesodermal fate. Adding a signaling molecule called WNT, while blocking the activity of another molecule known as BMP, promotes differentiation into one kind of mesoderm; conversely, adding BMP while blocking WNT drives the cells to instead become the other type of mesoderm.

"We learned... it is equally important to understand how unwanted cell types develop and find a way to block that process while encouraging the developmental path we do want," Loh was quoted as saying. By guiding the cells' choices at each fork in the road, the researchers were able to generate bone cell precursors that formed human bone when transplanted into laboratory mice and beating heart muscle cells, as well as 10 other mesodermal-derived cell lineages.

At each developmental stage, the researchers conducted single-cell ribonucleic acid (RNA) sequencing to identify unique gene expression patterns and assess the purity of individual cell populations. By looking at the gene expression profile in single cells, they were able to identify previously unknown transient states that typified the progression from precursor to more-specialized cells, in particular for the first time a transient pulse of gene expression that precedes the segmentation of the human embryo into discrete parts that will become the head, trunk and limbs of the body.

"The segmentation of the embryo is a fundamental step in human development," said Loh. Understanding when and how segmentation and other key developmental steps occur could provide important clues as to how congenital birth defects arise when these steps go awry. Endit