Stereocilia

Stereocilia (or stereovilli) are non-motile apical modifications of the cell, which are distinct from cilia and microvilli, but closely related to the latter.

In structure, they are longer than typical microvilli, and have more of the characteristics of the cellular membrane proper. Like microvilli, they contain actin filaments, distinguishing them from microtubule-containing cilia.

They are found in three regions of the body:

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Stereocilia (inner ear)

In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli. The hair cells turn the fluid pressure and other mechanical stimuli into electric stimuli via the many microvilli that make up stereocilia rods. Stereocilia exist in the auditory and vestibular systems.

Morphology

Resembling hair-like projections, the stereocilia are arranged in bundles of 30-300. Within the bundles the stereocilia are often lined up in several rows of increasing height, similar to a staircase. At the core of these hair-like stereocilia are rigid cross-linked actin filaments, which can renew every 48 hours. These actin filaments face their positive ends at the tips of the stereocilia and their negative ends at the base and can be up to 120 micrometres in length. Filamentous structures, called tip links, connect the tips of stereocilia in adjacent rows in the bundles. The tip links are made up of nearly vertical fine filaments that run upward from the top end of a shorter stereocilia to its taller neighbor. Tip links are analogous to tiny springs, which, when stretched, open cation selective channels thus allowing ions to flow across the cell membrane into the hair cells. They also are involved in the force transmission across the bundle and the maintenance of the hair bundle structure.

Epididymis

The epididymis (/ɛpᵻˈdɪdᵻmɪs/; plural: epididymides /ɛpᵻdᵻˈdɪmədiːz/ or /ɛpᵻˈdɪdəmɪdiːz/) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is present in all male reptiles, birds, and mammals. It is a single, narrow, tightly-coiled tube (in adult humans, six to seven meters in length) connecting the efferent ducts from the rear of each testicle to its vas deferens.

Structure

The epididymis can be divided into three main regions:

In reptiles, there is an additional canal between the testis and the head of the epididymis and which receives the various efferent ducts. This is, however, absent in all birds and mammals.