The lateral lines of fish run the length of their body, and can be seen as a series of vaguely visible pores. The crux of the lateral line’s capabilities is the neuromast. This is a type of mechanoreceptive organ (i.e. one which can detect mechanical stimulus, such as how our skin can detect touch) which can sense the mechanical changes of the surrounding water. Two types of neuromast are seen in the lateral line. There are superficial neuromasts which are freestanding, and spread over the head, trunk and tail fins of the fish. There are also the canal neuromasts, which can be found embedded in lateral line canals along the body of this fish. The numbers of neuromasts present in a fish can vary greatly, with some fish having only 50 superficial neuromasts on each side of their bodies, while others can have several thousand. Neuromasts consist of receptive hair cells, which are modified epithelial cells. These are covered in bundles of up to 150 stereovilli (also known as stereocilia) hairs, which provide the mechanoreceptive capabilities. In the literature there seems to be some disagreement as to if they could be considered stereovilli, or microvilli. Stereovilli are non-motile (non-moving), and typically longer than microvilli. They also have more characteristics of cellular membrane than microvilli, but are generally closely related, containing similar actin filaments. Actin filaments (also known as microfilaments), are tiny structures found in eukaryotic cells, and form part of the cytoskeleton (used to provide mechanical stability to cells). They are also useful in cytokinesis (movement of the cell). Similar actin filaments containing stereovilli are also found in the human inner ear, which is used in the detection of motion. The bundles of stereovilli are organised in rough “staircases” of height. The stereovilli in both lateral lines and human ears are covered in a flexible jelly-like cupula, for protective purposes.
The bundles of stereovilli found on the tips of neuromasts differ in height, growing longer from one side of the bundle to the other. Each bundle contains a singular kinocilium. The kinocilium is microtubular, and is involved in both the morphogenesis (cellular reconfiguration) of the hair bundle, and the mechanotransduction (conversion of mechanical stimulus in to electrochemical activity) of the cell. In the case of morphogenesis, the kinocilium leads the way. Once a prospective hair cell has differentiated from its precursory phases, the kinocilium migrates from the apical surface (apex) of the cell to an edge. Microvilli on the apical surface then grow in to stereovilli, establishing the height gradient of the hair bundle. Interestingly the numbers, lengths, and positions of the stereovilli in the bundles are very consistent across members of the same species, generally varying by less than 5%. Of the various discovered mutations that can cause deafness in humans, about half of them affect the formation of the hair bundles. In mammals, the kinocilium regresses after morphogenesis has been completed. In fish, it remains as a part of the hair bundle.
The kinocilium functions in mechanotransduction by providing a point of reference for the movements within the stereovilli. When the stereovilli are deflected (such as by sound, as is the case in the human ear), their bases are bent. This bending opens up ion channels located at their tips, and polarises the cell. This transduces the mechanical stimulus in to an electrical impulse. The kinocilium dictates in which way the cell polarises. In the case of the cells in the utricle (part of the balancing apparatus within the human ear, detecting movements in the horizontal plane), if the stereovilli bend towards the kinocilium then the cell polarisation is excitatory (depolarising). If the stereovilli bend away from the kinocilium, the polarisation is inhibitory (hyperpolarising). In the case of the utricle, these movements are actuated by the effect of gravity on tiny calcium deposits on the ends of the stereovilli (known as otolith). This information is combined by the brain with vertical movement information coming from another part of the ear using an identical mechanism to dictate orientation. In the case of the stereovilli on the neuromasts of a fish’s lateral line, the deflections occur mechanically via the movement of water over their surfaces.