Also lacking are the associated components that enable motility including the outer and inner dynein arms, and radial spokes.

[8][9] Most vertebrate cell types possess a single non-motile primary cilium, which functions as a cellular antenna.

Motile cilia are also present in the oviducts (fallopian tubes) of female (therian) mammals, where they function in moving egg cells from the ovary to the uterus.

Transition zone structure and function is conserved across diverse organisms, including vertebrates, Caenorhabditis elegans, Drosophila melanogaster and Chlamydomonas reinhardtii.

For many cilia, the basal body, where the cilium originates, is located within a membrane invagination called the ciliary pocket.

The cilium membrane and the basal body microtubules are connected by distal appendages (also called transition fibers).

[40] Multiciliated cells are found lining the respiratory tract where they function in mucociliary clearance sweeping mucus containing debris away from the lungs.

[12] In the reproductive tract, smooth muscle contractions help the beating of the cilia in moving the egg cell from the ovary to the uterus.

Epithelial sodium channels (ENaCs) are specifically expressed along the entire length of cilia in the respiratory tract, and fallopian tube or oviduct that apparently serve as sensors to regulate the periciliary fluid.

[16] While lacking the central apparatus there are dynein arms present that allow the nodal cilia to move in a spinning fashion.

The movement creates a current flow of the extraembryonic fluid across the nodal surface in a leftward direction that initiates the left-right asymmetry in the developing embryo.

[42] The motile cilia on the central cells rotate to generate the leftward flow of extracellular fluid needed to initiate the left-right asymmetry.

[42] The motile cilia on sperm cells and many protozoans enables swimming through liquids and are traditionally referred to as "flagella".

[43] Ciliates are eukaryotic microorganisms that possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface.

An early step is docking of the basal body to the growing ciliary membrane, after which the transition zone forms.

[45][46] Exceptions include Drosophila sperm and Plasmodium falciparum flagella formation, in which cilia assemble in the cytoplasm.

[50][51] Some primary cilia on epithelial cells in eukaryotes act as cellular antennae, providing chemosensation, thermosensation and mechanosensation of the extracellular environment.

[52][10] These cilia then play a role in mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile.

[31][58] Defects in cilia adversely affect many critical signaling pathways essential to embryonic development and to adult physiology, and thus offer a plausible hypothesis for the often multi-symptom nature of diverse ciliopathies.

[30][52] Genetic mutations compromising the proper functioning of cilia, ciliopathies, can cause chronic disorders such as primary ciliary dyskinesia (PCD), nephronophthisis, and Senior–Løken syndrome.

In addition, a defect of the primary cilium in the renal tubule cells can lead to polycystic kidney disease (PKD).

In another genetic disorder called Bardet–Biedl syndrome (BBS), the mutant gene products are the components in the basal body and cilia.

Several studies already showed impaired glucose tolerance and reduction in the insulin secretion in the ciliopathy models.

[59] Epithelial sodium channels (ENaCs) that are expressed along the length of cilia regulate periciliary fluid level.

[13] In cystic fibrosis that results from mutations in the chloride channel CFTR, ENaC activity is enhanced leading to a severe reduction of the fluid level that causes complications and infections in the respiratory airways.

[41] Since the flagellum of human sperm has the same internal structure of a cilium, ciliary dysfunction can also be responsible for male infertility.

[67] The transportation of the immature egg cell, and the embryo to the uterus for implantation depends on the combination of regulated smooth muscle contractions, and ciliary beating.

Dysfunction in this transportation can result in an ectopic pregnancy where the embryo is implanted (usually) in the fallopian tube before reaching its proper destination of the uterus.

Smoking (causing inflammation), and infection can reduce the numbers of cilia, and the ciliary beat can be affected by hormonal changes.