FUNCTION AND ROLE OF THE LATERAL LINE SYSTEM

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CHAPTER 1

OVERVIEW

The amazing ability of various blind or deep-sea fish to navigate through waters where little or no light penetrates can be attributed to the presence of a sensory system found in fish and amphibia, which is responsible for spatial navigation, balance and even social behaviours like schooling and mate selection, known as the lateral line.

The lateral line system, present in fish and amphibians, is a mechanosensory system that detects water flow and current. It located on the flanks of the fish (lateral sides), hence the appropriate name for the system. There are two main types of lateral line:

  1. The Anterior Lateral Line (ALL)
  2. The Posterior Lateral Line (PLL)

The lateral line itself is formed of tiny clusters of cells called neuromasts. Each neuromast houses about 20 or so cells which function as mechanoreceptors. The neuromasts are the functional units of the lateral line.

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Neuromasts sense mechanical current pressure and transduce these impulses into sensory information which is then passed to the sensory (afferent) neurons just underneath the neuromast surface and innervating it. The size and shape of the lateral line varies between different species of fish.

The lateral line system is an extremely essential sensory system, playing a vital role in schooling behaviour, location of prey, escaping from predators, especially balance and navigation.

STRUCTURE AND TYPES:

The lateral line is of 2 types: The anterior and the posterior lateral line. The ALL contains the cranial neuromasts and the PLL contains the trunk neuromasts. 4 day old zebrafish larvae show 8 discrete lateral lines with differing average neuromast numbers (Rabile and Kruse, 2000), (fig. A and B).

  1. Supraorbital (SO) (3 neuromasts)
  2. Infraorbital (IO) (4 neuromasts)
  3. Mandibular (M) (2 neuromasts)
  4. Opercular (OP) (1 neuromast)
  5. Otic (O) (Superior and inferior rami: 2 neuromasts)
  6. Middle (MI) ( Superior and inferior rami: 2 neuromasts)
  7. Occipital (OC) (1 neuromast)
  8. Posterior (P) ( Dorsal and ventral rami: Around 11 neuromasts)F1.large.jpg

ANATOMY OF THE NEUROMAST

The lateral line neuromasts differ in patterning in various species, but the fundamental structure of the neuromast is the same across species.

F1.large.jpg

Neuromasts consist of modified epithelial cells, which show structural and functional similarity to the hair cells found in higher organisms (reptiles, birds and mammals) in the auditory and vestibular systems. There are 2 main types of neuromasts:

  1. Canal neuromasts- located in subdermal canals along the lateral line (found in elasmobranchs and many telosts)
  2. Superficial Neuromasts- located on the body surface of the fish.

Each neuromast is arranged in a rosette-formation. The hair cells of the neuromasts are extremely similar to the hair cells found in the inner-ear. They possess a long kinocilium (sensory cell) and many supporting cells (stereocillia), which function as the receptors and transducers of sensory impulses.

This group of hair cells is covered by a protective and flexible layer of cupula above. The hair cell bodies lie within the epithelial tissue. (Fig C). These hair cells are modified epithelial cells, which contain 40-50 bundles of microvilli that act as mechanoreceptors. These bundles are arranged in increasing orders of length, hence taking a ‘staircase’-like appearance.

They possess innervations from both, afferent and efferent neurons. As and when a stimulus (like a mechanical force or vibration) is received, the hair cells transduce these stimuli via rate coding. The cells will produce a constant firing for as long as the stimulus is in fuelling the receptors.

During mechanical or vibrational stimulation, the pressure causes the cupula to bend in the direction of the pressure. The intensity with which the cupula bends will depend on the magnitude of the water pressure or mechanical force being exerted around the fish. As the hairs in the neuromasts bend due to the force, there will be a change in the ionic permeability of the cells. It is seen that if the deflection of the hairs is towards the longer hair, the hair cells will be depolarized, causing a net excitatory impulse by causing a depolarization and the release of neurotransmitters as the impulse moves up afferent lateral neurons. Deflection towards the shorter hairs causes hyperpolarization, and the effect seen is completely opposite, (Flock, A. (1967)).

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Both, superficial and canal neuromasts use this method of electrical signal transduction. However, the difference between their organization on the epidermis provides them with differential capacities related to mechanoreception.

Since the superficial neuromasts are located more outwardly, they come in direct contact with the external environment of the fish. Superficial neuromasts have the same basic ‘staircase’ hair cell organization. But by and large, the organization of the said hair cells is random and disorganised, wherein in there is no accurate gradient as such of microvilli by length or size.

This might hint at the superficial neuromasts’ ability to have a broad detection range (Peach, M. B., & Rouse, G. W. (2000)), to detect a wide array of mechanical deflections. Conversely, canal neuromasts allow for a more refined mechanoreception like detecting pressure gradients instead of just detecting the presence or absence of mechanical or pressure perturbances. (Peach, M. B., & Rouse, G. W. (2000)) The canals on the lateral line of the fish can act as indicators for pressure differentials. As differing water pressure would move into the canals of the lateral line, the canal fluid will flow in the direction of the pressure applied. This will result in a directional deflection of the cupulae in the neuromasts, analogous to the route of the mechanical pressure in the environment of the fish.

FUNCTION AND ROLE OF THE LATERAL LINE

Coombs, S., Braun, C. B., & Donovan, B. (2001) established the role of the lateral line, particularly in predator fish. Predatory fish (mottled sculpin) would orient themselves towards a source causing vibrations or disturbances in the water. They observed that these vibrations were picked up by the fish and it would then orient itself according to the directional flow before trying to capture the prey. The fish showed the same behaviour even when the vibrations were made to be produced by a metal sphere instead of the prey, or if the fish were blinded. To confirm these results, they inhibited signal transduction from the fish’s lateral lines by treating them with CoCl2, which is known to disrupt ionic transport, and the fish showed a weakened response to the same stimulus given to them.

Furthermore, they used gentamicin to disrupt the canal receptors and mechanical scraping of the neuromasts of the lateral line to check whether both the lines were used equally for orienting to prey. They found that the disruption to canal neuromasts affected the fish’s predatory behaviour the most. On the other hand, superficial neuromasts are more responsible for rheotactic behaviour.

EVOLUTION OF THE LATERAL LINE

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.

The amphibian lateral line system develops from a series of lateral line placodes. The different phases of development from early induction, to pattern formation, differentiation, morphogenesis, and metamorphic fate were summarized in the first part of this review (Schlosser, 2002a). Here, a survey of the diversity of lateral line systems in amphibians is presented indicating that most phases of lateral line development have been subject to evolutionary changes. Several trends suggest important roles for both adaptive changes and internal constraints in amphibian lateral line evolution. Many of these trends involved the coordinated modification of different derivatives of lateral line placodes suggesting that these placodes are not only autonomous developmental modules, but also units of evolutionary variation that tend to be modified in a coherent and largely context-independent fashion.

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The amphibian lateral line system develops from a series of lateral line placodes. The different phases of development from early induction, to pattern formation, differentiation, morphogenesis, and metamorphic fate were summarized in the first part of this review (Schlosser, 2002a). Here, a survey of the diversity of lateral line systems in amphibians is presented indicating that most phases of lateral line development have been subject to evolutionary changes. Several trends suggest important roles for both adaptive changes and internal constraints in amphibian lateral line evolution. Many of these trends involved the coordinated modification of different derivatives of lateral line placodes suggesting that these placodes are not only autonomous developmental modules, but also units of evolutionary variation that tend to be modified in a coherent and largely context-independent fashion.