Three-dimensional reconstruction of two zebrafish hair cells; these are filled with small mitochondria (in white), some of which form a large network (in gold) at the base of the cell, near the structures (in purple) that ensure communication with neurons. Image credit: McQuate et al. (CC BY 4.0)
Our ability to perceive sounds relies on tiny cells deep inside our ears which can convert vibrations into the electrical signals that our brain is able to decode. These ‘hair cells’ sport a small tuft of short fibers on one of their ends that can move in response to pressure waves. The large amount of energy required for this activity is provided by the cells’ mitochondria, the small internal compartments that act as cellular powerhouses. In fact, reducing mitochondrial function in hair cells can lead to hearing disorders.
Mitochondria are often depicted as being bean-like, but they can actually adopt different shapes based on the level of energy they need to produce. Despite this link between morphology and function, little is known about what mitochondria look like in hair cells. Filling this knowledge gap is necessary to understand how these structures support hair cells and healthy hearing.
To address this question, McQuate et al. turned to zebrafish, as these animals detect vibrations in water through easily accessible hair cells on their skin that work just like the ones in the mammalian ear. Obtaining and analysing series of 3D images from a high-resolution microscope revealed that hair cells are more densely populated with mitochondria than other cell types. Mitochondrial organisation was also strikingly different. The side of the cell that carries the hair-like structures featured many small mitochondria; however, on the opposite side, which is in contact with neurons, the mitochondria formed a single large network. The co-existence of different types of mitochondria within one cell is a novel concept.
Further experiments investigated how these mitochondrial characteristics were connected to hair cell activity. They showed that this organisation was established gradually as the cells aged, with cellular activity shaping the architecture (but not the total volume) of the mitochondria.
Overall, the work by McQuate et al. provides important information necessary to develop therapeutics for hearing disorders linked to mitochondrial dysfunction. However, by showing that various kind of mitochondria can be present within one cell, it should also inform studies beyond those that focus on hearing.
