more than meets the eye

The Scientist | When Antonie van Leeuwenhoek looked down at a prokaryote through his simple microscope made of a single mounted lens back in the 1660s, he discovered the first organelle. Captivated by the fluttering “legs” that would later be called the cell’s eyelashes, or cilia in Latin, he might have wondered about the origin of their movement.

Researchers had known that properly moving cilia were not only important to protozoans swimming in their dish, but also for the proper function of the ciliated cells that comprised organs: from the trachea to the lining of the brain and the female reproductive tract, and there was more to come.1 We soon realized that the flashier variety of cilia—the kind that moved—was only a small part of the story. Cilia with no ability to move studded cells of sensory organs like the eye or the insect ear. When I returned from Copenhagen, researchers began to point out that many ordinary tissue cells also bore non-moving single cilia.

Only recently did we discover that those cilia were actually crucial as signaling appendices, acting as cells’ antennae. They have since opened up many more questions regarding the evolution and function of this intriguing organelle.

As the first organelle ever observed by scientists, it’s interesting to reflect on how the cilium became a necessary component of most cells in the body. Many people now conclude that the complexity of the nucleated cell arose by a series of invasive/symbiotic events. The major organelles—mitochondria, chloroplast, perhaps even the nucleus itself—are the results of such invasions or engulfments within a basic bacterial cytoplasm. My colleagues and I have proposed that the sensory 9+0 cilium could have originated in this way, when a large enveloped RNA-containing virus whose core was the primitive centriole failed to exocytose completely after the invasion of the cytoplasm, leaving a bud. When the protein transport mechanism permitted the bud to grow and to accumulate specific membrane proteins, the sensory cilium was born. Later, motility and an efficient coupling between sensory information and motile response evolved.