Hair Cell Heterogeneity

The earlier literature on ultrastructure of vertebrate sensory hair cells suggested that there were several different types of cells in amniotes (reptiles, birds, mammals) but only a single type (called Type 2) in anamniotes (fishes and amphibians). However, a number of studies in our lab over the past 10 years have shown that this is not the case. Indeed, we have demonstrated that fishes, as other vertebrates, have a number of different types of sensory hair cells. The functional significance of having different hair cell types is not yet understood, but there is reason to believe that the different cell types may respond to different frequency ranges. This work is reviewed most recently in a paper by Lanford, Platt, and Popper (2000) and in Popper (2000), while the initial description of multiple hair cell types in fishes was first provided in a paper by Chang, Popper, and Saidel (1992).

The transmission electron micrograph below is from two different types of hair cells from the oscar, Astronotus ocellatus. The image on the left shows the large subnuclear ER and large mitochondria found in hair cells of hte striolar region of the utricle while the image at the right is from a hair cell in the extrastriolar region. The striolar hair cells, which we refer to at "type-I like" is very similar to the amniote type I hair cell, while the cell to the right is typical of amniote type II hair cells.

One of the major defining characteristics of type I hair cells in amniotes is the presence of a calysx surrounding the cell. The calyx is an enlargement of the afferent nerve terminal. While not common in fish, Lanford and Popper have found in fish (Lanford and Popper, 1996). The TEM below (left) shows a single afferent nerve terminal innervating multiple hair cells in the crista ampullaris of a goldfish. The drawing on the right compares a hair cell from a goldfish to that of an amniote. This finding strongly supports the argument that major hair cell structures arose very early in the evolution of fishes and that hair cell patterns found in amniotes are most likely derived from the basic hair cell pattern layed down over 200 million hears ago at the origin of vertebrates. These results also strongly support the argument that diversity in hair cell structure is probably related to the diversity of function found in even the earliest representations of the vertebrate ear See Lanford, Platt, and Popper (2000) and Fay and Popper (2000) for a discussion of the evolution of the ear and hair cells.