Looptail mice (right) have a thicker skin than normal mice (left) because their skin stem cells are packed together more tightly, causing the cells to be taller and thinner and thus more likely to divide in a perpendicular direction, giving rise to additional epidermal cell layers. (Box et al., 2019)
The body’s organs are formed by specialized stem cells that divide to produce daughter cells that can develop into all the different cell types that make up the tissue. The fate of a particular daughter cell often depends on its precise position within the organ, and so stem cells carefully control the direction in which they divide so that their daughters are produced in just the right place. A new study from the Devenport lab has found that, in embryonic mice, the shape of the developing skin influences the direction in which skin stem cells divide, helping to ensure that the epidermis forms correctly.
The epidermis consists of multiple layers of cells that together form a barrier that protects the body from the outside world. During embryonic development, the epidermis starts off as a single “basal” layer of stem cells that divide in the plane of the cell layer to produce more stem cells that eventually cover the entire growing embryo. Some of the stem cells then begin to divide perpendicularly to the plane so that one of the daughter cells is positioned above the basal layer and can begin to form the additional, suprabasal layers of the epidermis. But the relative number of perpendicular and planar divisions needs to be carefully balanced. “Too many perpendicular divisions can deplete the stem cell pool, while too many planar divisions leads to defective barrier formation,” explains Danelle Devenport, an associate professor of molecular biology.
At later stages of embryonic development, perpendicular cell divisions are initiated by a group of proteins within the stem cells themselves, but these proteins do not appear to be involved at earlier stages when the epidermis first starts to become multilayered. “What regulates division orientations early in epidermal development was unknown,” explains Kimberly Box, who recently completed her graduate studies in the Devenport lab. “Moreover, the cues that direct planar division orientations in the epidermis had not been identified.”
To investigate these questions, Box, Devenport, and postdoc Bradley Joyce took pieces of embryonic mouse skin and observed the tissue’s development for several hours under the microscope, allowing them to monitor the orientation of the stem cells’ divisions and the subsequent fate of their daughter cells.
In particular, Box and colleagues examined embryonic skin from the “Looptail” strain of mice that carry a mutation in a gene called Vangl2. Due to defects in the formation of their spinal cords, the skin of these animals fails to completely cover their backs. The skin that exists on the animals’ flanks is thicker than normal, which the researchers suspected could be caused by excessive perpendicular stem cell divisions. Indeed, following the skin’s development under the microscope confirmed that the stem cells in Looptail mouse skin are more likely to divide perpendicularly, producing more suprabasal daughter cells capable of forming extra epidermal cell layers.
Intriguingly, the researchers noticed that, because the skin wasn’t fully stretched over the Looptail animals’ backs, the basal stem cells at the animal’s flanks were packed together more tightly than normal, making the cells taller and thinner than usual. A 135 year-old observation, known as Hertwig’s rule, suggests that flat cells in a single cell layer will divide along whichever of their two dimensions is longest. Box and colleagues wondered whether this rule might also apply in three dimensional tissues comprised of multiple cell layers, so that the tall, thin stem cells found in Looptail mouse skin would tend to divide perpendicularly. To test this idea, the researchers stretched the Looptail skins to make the basal stem cells shorter and fatter, and, sure enough, they began to divide in the plane of the basal layer instead of perpendicular to it.
The team found that the same rule also controlled stem cell divisions in normal mouse skin. Loosely packed basal cells are relatively flat and tend to divide in a planar direction, whereas basal cells that are squeezed together tightly are taller than they are wide, and become more likely to divide perpendicularly. Devenport and colleagues think that this helps balance the direction of stem cell divisions during embryonic development. “We propose that, under normal developmental conditions, this mechanism allows the stem cell layer to expand through planar divisions to accommodate embryo growth,” Devenport says. “When the layer has expanded sufficiently and the stem cells reach a particular density, they can switch to perpendicular divisions and generate new epidermal layers.”
The study was published in eLife on June 12. The next step, say the team, is to investigate how Hertwig’s rule works and determine how skin stem cells “know” their shape and orient their divisions accordingly.
K. Box, B.W. Joyce, and D. Devenport. Epithelial geometry regulates spindle orientation and progenitor fate during formation of the mammalian epidermis. eLife 8:e47102 (2019). DOI: 10.7554/eLife.47102.
Stem cells in the basal layer of the epidermis can divide in either a planar or perpendicular direction. Box and colleagues found that this is determined by how tightly the stem cells are packed together and the resulting changes in their shape. (Box et al., 2019)