# Vitamin D and Primary Ciliary Dyskinesia: A Topic to Be Further Explored
## 1 Introduction  
Flagella and motile cilia show a conserved axonemal structure. They consist of a ring of nine microtubular doublets associated with a central pair of microtubules [1; 2; 3; 4]. This architecture gives the classical 9 + 2 microtubular arrangement often searched for in patients harboring a cilia's potential defect (Figure 1). In examining the ciliary ultrastructure, the diagram of a transverse section of a normal cilium depicts the internal ultrastructure and the ciliary axis used to identify cilia orientation concerning each other. A ciliary membrane typically surrounds the axoneme (central core). It has nine peripheral microtubule doublets and two single central microtubules (Figure 1). The ultrastructural analysis is, however, challenging and needs to be carried out in several sections. Often some parts may be in focus. In contrast, other details of the ultrastructural investigation may show the limitations of this technique on getting focused images in all fields (Figure 2). Figure 1: Ciliary Ultrastruct-re. A transverse section of a normal cilium is depicted in this diagram showing the internal ultrastructure and the ciliary axis used to identify cilia orientation with respect to each other. The central core or axoneme is surrounded by a ciliary membrane and has nine peripheral microtubule dou-blets associated with two single central microtubules. Figure 2: Cilia of a healthy individual showing a classic arrangement (transmission electron microscopy, see the bar for details regarding magnification). Some of these tangential sections were cut midway between the cell surface and the top of the waves. Ciliary cross-sections represent cilia in wave crests. Some cross-sections are not in focus because of the specific cut occurring at the time of the ciliary stroke. This is due to the technical difficulty in having all cross-sections with the same planar cut section. are carried by the A-type tubule and develop toward the B-microtubule of the adjacent doublet. IDA and ODA form part of a specific protein complex of 96 nm repeat unit. It includes four identical ODAs, six single-headed IDMs, one two-headed IDA, the three Rs, and a single nexin-dynein regulatory complex (N-DRC). The activity of the dynein arms is regulated and coordinated by the N-DRC. In the sperm flagellum, which has an ultrastructurally similar axonemal structure, accessory structures are also recognized. They include the mitochondrial sheath (MS), fibrous sheath (FS), and outer dense fibers (ODFs) [4]. This anatomy is specific to the sperm tail. They are needed for fertile sperm production and critical for the movement of the sperm in the female reproductive tract. Their central doublet must orientate neighboring cilia to beat in a coordinated way in a parallel fashion. Such ciliary beating coordination may be assessed by measuring the standard deviation of the angles formed between the central microtubules of neighboring cilia with the x-axis [5,6]. Motile cilia malfunction causes primary ciliary dyskinesia or PCD. It is a genetic disease, which affects nearly 1:10,000 people worldwide with slightly geographic variation [7]. The symptomatology begins in the newborn period. The spectrum includes a chronic nasal discharge associated with a wet cough. In childhood, it progresses to recurrent upper and lower respiratory infections. It may eventually evolve in pulmonary scarring with the formation of bronchiectasis. It is well known that a dysfunction of the airways' motile cilia is the basis of the symptoms. The airway motile cilia are specifically accountable for mucus clearance in the airways. In addition, motile cilia are crucial for the life of an organism. They are present in the brain, fallopian tube, eustachian tube, and middle ear. They function in fluid movement and play a crucial role in the embryonic node during development. _Situs inversus_, opposite of the _situs solitus_ with a typical arrangement, is a congenital condition in which the major visceral organs are mirrored or reversed from their standard locations [8, 9, 10].