How cells learn to 'count'

One of the wonders of cell biology is its symmetry. Mammalian cells have one nucleus and one cell membrane, and most humans have 23 pairs of chromosomes. Trillions of mammalian cells achieve this uniformity—but some consistently break this mold to fulfill unique functions. Now, a team of Johns Hopkins Medicine researchers have found how these outliers take shape.

In experiments with genetically engineered mice, a research team has ruled out a mechanism that scientists have long believed controls the number of hairlike structures, called cilia, protruding on the outside of each mammalian cell. They concluded that control of the cilia count might rely instead on a process more commonly seen in non-mammalian species.

The experiments, described Dec. 2 in Nature Cell Biology and led by Andrew Holland, Ph.D., associate professor of molecular biology and genetics at the Johns Hopkins University School of Medicine, may eventually help scientists learn more about human diseases related to cilia function, such as respiratory infections, infertility and hydrocephaly.

Cilia are ancient structures that first appeared on single-celled organisms as small hairlike "fingers" that act as motors to move the cell or antennae to sense the environment. Nearly all human cells have at least one cilium that senses physical or chemical cues. However, some specialized cell types in humans, such as those lining the respiratory and reproductive tracts, have hundreds of cilia on their surface that beat in waves to move fluids through the system.

"Our main question was how these multicilliated cells become so dramatically different than the rest of the cells in our body," says Holland. "Most cells make exactly one cilium per cell, but these highly specialized cells give up on this tight numerical control and make hundreds of cilia."

In an effort to answer the question, Holland and his team took a closer look at the base of cilia, the place where the organelles attach and grow from the surface of the cell. This base is a microscopic, cylinder-shaped structure called a centriole.

In single-ciliated cells, Holland says, centrioles are created before a cell divides. A cell contains two-parent centrioles that each duplicate so that both new cells gets one pair of centrioles—the oldest of these two centrioles then goes on to form the base of the cilium. However, multicilliated cells create unique structures, called deuterosomes, that act as a copy machine to enable the production of tens to hundreds of centrioles, allowing these cells to create many cilia.

One of the wonders of cell biology is its symmetry. Mammalian cells have one nucleus and one cell membrane, and most humans have 23 pairs of chromosomes. Trillions of mammalian cells achieve this uniformity—but some consistently break this mold to fulfill unique functions. Now, a team of Johns Hopkins Medicine researchers have found how these outliers take shape.

In experiments with genetically engineered mice, a research team has ruled out a mechanism that scientists have long believed controls the number of hairlike structures, called cilia, protruding on the outside of each mammalian cell. They concluded that control of the cilia count might rely instead on a process more commonly seen in non-mammalian species.

The experiments, described Dec. 2 in Nature Cell Biology and led by Andrew Holland, Ph.D., associate professor of molecular biology and genetics at the Johns Hopkins University School of Medicine, may eventually help scientists learn more about human diseases related to cilia function, such as respiratory infections, infertility and hydrocephaly.

Cilia are ancient structures that first appeared on single-celled organisms as small hairlike "fingers" that act as motors to move the cell or antennae to sense the environment. Nearly all human cells have at least one cilium that senses physical or chemical cues. However, some specialized cell types in humans, such as those lining the respiratory and reproductive tracts, have hundreds of cilia on their surface that beat in waves to move fluids through the system.

"Our main question was how these multicilliated cells become so dramatically different than the rest of the cells in our body," says Holland. "Most cells make exactly one cilium per cell, but these highly specialized cells give up on this tight numerical control and make hundreds of cilia."

In an effort to answer the question, Holland and his team took a closer look at the base of cilia, the place where the organelles attach and grow from the surface of the cell. This base is a microscopic, cylinder-shaped structure called a centriole.

In single-ciliated cells, Holland says, centrioles are created before a cell divides. A cell contains two-parent centrioles that each duplicate so that both new cells gets one pair of centrioles—the oldest of these two centrioles then goes on to form the base of the cilium. However, multicilliated cells create unique structures, called deuterosomes, that act as a copy machine to enable the production of tens to hundreds of centrioles, allowing these cells to create many cilia.