What is the Difference Between Kinocilia and Stereocilia

The main difference between kinocilia and stereocilia is that kinocilia are true cilia, having a 9+2 microtubule arrangement, whereas stereocilia do not have microtubules.

Kinocilia and stereocilia are two cilia that occur in the vertebrate inner ear. They are responsible for the auditory and vestibular pathways.   

Key Areas Covered

1. What are Kinocilia
– Definition, Structure, Function
2. What are Stereocilia
– Definition, Structure, Function
3. Similarities Between Kinocilia and Stereocilia
– Outline of Common Features
4. Difference Between Kinocilia and Stereocilia
– Comparison of Key Differences

Key Terms 

Kinocilia, Stereocilia

What are Kinocilia 

Kinocilia are a specialized type of cilia in the sensory epithelium of the vertebrate inner ear. They occur on the apical surface of hair cells that are the sensory receptors of both the auditory and vestibular systems. They involve in the morphogenesis and the mechanotransduction of the hair bundle. The displacement of the hair bundle due to vibrations results in hair cell depolarization and hyperpolarization. Depolarization of the hair cell results in signal transduction through the release of neurotransmitters. Microtubular kinocilium occurs per hair cell. It occurs in the middle of the apical surface of the hair cell, surrounding 20-300 microvilli.

Figure 1: Hair Cell

Furthermore, kinocilia occur in the crista ampullaris of the utricle’s and saccule’s semicircular ducts and sensory maculae. One kinocilium occurs as the longest cilium on the hair cell, along with 40-70 stereocilia. Hair cell depolarizes due to the body’s movement when the stereocilia move towards the kinocilium. It releases neurotransmitters firing the cranial nerve VIII. 

What is Stereocilia

Stereocilia are the mechanosensing organelles of hair cells of the inner ear. They respond to the fluid motion of various functions of the animals in hearing and balance. The length of the stereocilia is about 10-50 micrometers. Hair cells convert mechanical stimuli, such as fluid pressure, into electrical stimuli. Therefore, stereocilia occur in auditory and vestibular systems. There, stereocilia occur in bundles of 30-300 cilia. They form staircase-like structures by lining up in several rows. Also, the core of the stereocilia contains actin filaments that renew every 48 hours. Positive ends of the actin filaments occur at the tips of the stereocilia, while the negative ends occur at the base. The length of the stereocilia is 120 micrometers.

Figure 2: Stereocilia

Moreover, in the auditory system, stereocilia occurs in lines in the organ of Corti in the cochlea of the inner ear. In the hair cell, stereocilia can convert mechanical signals into electrical impulses. In the vestibular system, stereocilia occur in the otolithic organs and semicircular canals. 

Similarities Between Kinocilia and Stereocilia

• Kinocilia and stereocilia are two cilia types that occur in the vertebrate inner ear.
• They are responsible for the auditory and vestibular pathways. 

Difference Between Kinocilia and Stereocilia

Definition

Kinocilia refers to specialized primary cilia present in auditory hair cells (HCs) in the inner ear, while stereocilia refers to actin-based protrusions on auditory and vestibular sensory cells required for hearing and balance.

Structure

Kinocilia are true cilia, having a 9+2 microtubule arrangement, while stereocilia do not have microtubules; instead, they have actin cores.

Shape

Kinocilia have a long, cylindrical shape, while stereocilia have a conical shape. 

Motility

Kinocilia is motile while stereocilia are non-motile. 

Contractility

Kinocilia are contractile, while stereocilia are non-contractile.

Function

Kinocilia helps in locomotion and the movement of particles and mucus in a specific direction, while stereocilia increase the surface area. 

Occurrence

Kinocilia occurs in the inner surface of hollow organs such as bronchioles, Fallopian tubes, trachea, ependymal epithelium, and uterus, while stereocilia occur in the epididymis, Crista, Vas Deferens, and macula of the internal ear. 

Conclusion

In brief, kinocilia and stereocilia are two cilia types that occur in the vertebrate inner ear. Kinocilia are true cilia that contain a 9+2 microtubule structure. They are long, cylindrical. They are motile and contractile. Also, they help in locomotion and the movement of particles in specific directions. Kinocilia also occurs in bronchioles, Fallopian tubes, trachea, uterus, ependymal epithelium, and uterus. In comparison, stereocilia contain actin cores, and they are conical in shape. They are non-motile and non-contractile. The primary function of stereocilia is to increase surface area. Stereocilia occurs in the internal ear’s epididymis, Crista, Vas Deferens, and macula. The main difference between kinocilia and stereocilia is their structure and function. 

References:

1. Wikimedia Foundation. (2023e, February 11). Kinocilium. Wikipedia. 
2. Wikimedia Foundation. (2023l, May 2). Stereocilia (Inner Ear). Wikipedia. 

Image Courtesy:

1. “Haircell frog sacculus” By A. James Hudspeth – Own work (CC-BY SA 3.0) via Commons Wikimedia
2. “Stereocilia of frog inner ear” By Bechara Kachar – Own Work (Public Domain) via Commons Wikimedia