Specializations of the Cell Surface (Cell Surface Modifications)

This set covers vocabulary related to apical, lateral, and basal cell surface specializations, including the structure and function of microvilli, cilia, flagella, and associated clinical conditions.

Cell Polarity

Variations in cell form, structure, and function characterized by distinct apical, lateral, and basal surfaces.

Microvilli

Finger-like surface extensions ($0.1\,\mu\text{m}$ width and $1-3\,\mu\text{m}$ long) that increase surface area for absorption and diffusion.

Brush border

A dense fringe of microvilli seen at the apical surface of absorptive epithelia in the intestinal tract and kidneys.

Fimbrin and Villin

Actin bundling proteins that cross-link actin filaments into closely packed bundles within the structure of microvilli.

Cilia

Hairlike projections of the plasma membrane that are microtubule-based and involved in sensing signals or generating movement.

Flagella

A single, longer motile projection that propels cells, such as mammalian sperm, via an undulating wave type of movement.

Stereocilia

Long, non-motile apical modifications that contain actin but lack an axoneme; found in the epididymis and sensory cells of the inner ear.

Primary Cilia

Non-motile cilia with a $9+0$ axoneme arrangement that function as sensors for extracellular signals like light, odorants, and liquid flow.

Motile Cilia

Cilia with a $9+2$ axoneme arrangement responsible for moving mucus over epithelial surfaces or transporting the ovum toward the uterus.

Axoneme

The structural core of cilia and flagella consisting of a ring of microtubule scaffolding, usually in a $9+0$ or $9+2$ arrangement.

Basal body

A structure derived from a centriole that contains nine triplets of microtubules and anchors both motile and non-motile cilia.

Dynein

A motor protein with ATPase activity arranged along microtubules that generates the force required for ciliary and flagellar movement.

Nexin

An elastic interdoublet link protein responsible for maintaining the nine-fold configuration and facilitating the recovery stroke in cilia.

Intraflagellar Transport (IFT)

The process by which molecules are transported up or down the microtubules of the axoneme via motor proteins.

Kartagener Syndrome

A motile ciliopathy (Immotile cilia syndrome) caused by a congenital defect in dynein synthesis, leading to infertility and respiratory issues.

Situs inversus

A complete right-to-left reversal of thoracic and abdominal organs, where the heart is on the right and the liver is on the left.

Mammalian Sperm Flagellum Structure

An internal structure characterized by a $9+9+2$ arrangement, including 9 additional dense fibers around the axoneme for protection.

Occluding Junctions

Lateral domain specializations, specifically tight junctions, that seal cells together to prevent leakage.

Basal Lamina

A layer of the basement membrane that separates and supports the epithelium from the underlying connective tissue.

Mechanotransduction

A physical function of microvilli or stereocilia where mechanical stimuli are converted into cellular signals, such as in hearing and balance.