International Journal of Marine Science, 2024, Vol.14, No.4, 256-265 http://www.aquapublisher.com/index.php/ijms 257 2 Overview of Cilia Biology 2.1 Structure and types of cilia Cilia are microtubule-based structures that extend from the surface of many eukaryotic cells. They are broadly classified into two types: motile and non-motile (primary) cilia. Motile cilia are typically found in large numbers on the cell surface and are responsible for generating fluid flow across epithelial surfaces, such as in the respiratory tract and the brain's ventricles (Bearce and Grimes, 2020; Thouvenin et al., 2020), Non-motile cilia, on the other hand, usually occur singly and function primarily as sensory organelles, detecting mechanical and chemical signals from the environment. The structure of cilia includes a core axoneme composed of microtubules arranged in a "9+2" pattern in motile cilia and a "9+0" pattern in primary cilia (Lee et al., 2015). 2.2 Cilia function in cellular processes Cilia play crucial roles in various cellular processes, including fluid movement, signal transduction, and cellular orientation. In the central nervous system, motile cilia generate cerebrospinal fluid (CSF) flow (Figure 1), which is essential for brain development and the maintenance of the body axis (Cantaut-Belarif et al., 2018; Thouvenin et al., 2020), Cilia-driven CSF flow also facilitates the transport of signaling molecules, such as urotensin neuropeptides, which are critical for the straightening of the vertebrate body axis (Zhang et al., 2018), Additionally, cilia are involved in the establishment of planar cell polarity (PCP) in epithelial tissues, aligning cellular structures along a common axis (Chien et al., 2015; Chien et al., 2018), This alignment is crucial for the proper functioning of multiciliated cells in tissues such as the respiratory epithelium and the skin. The study by Thouvenin et al. (2020) demonstrated the bidirectional velocity distribution of cerebrospinal fluid (CSF) flow in zebrafish embryos at 30 hours post-fertilization. This figure, through experimental measurements and numerical simulations, reveals the bidirectional flow pattern of CSF within the central canal, highlighting the influence of ciliary motion on fluid dynamics. This illustration provides important insights for studying how cilia regulate CSF flow during embryonic development. 2.3 Genetic regulation of cilia formation The formation and maintenance of cilia are tightly regulated by a set of conserved genes and molecular pathways. Intraflagellar transport (IFT) proteins, such as IFT46, are essential for the assembly and function of cilia. Mutations in these genes can lead to various ciliopathies, characterized by defects in cilia structure and function (Lee et al., 2015), The V-ATPase accessory protein Atp6ap1b has also been shown to play a critical role in the development of ciliated organs by regulating proton flux and cytoplasmic pH, which are necessary for the proliferation of precursor cells that form cilia (Gokey et al., 2015), Furthermore, the planar cell polarity pathway and mechanical strain during embryonic development are key factors in determining cilia length, motility, and planar positioning, ensuring the proper function of cilia in processes such as left-right axis patterning (Chien et al., 2015; Chien et al., 2018; Xu, 2024). Cilia are versatile organelles with diverse structural types and functions, ranging from fluid movement to signal transduction. Their formation and function are governed by a complex network of genetic and molecular mechanisms, highlighting their importance in cellular and developmental processes. 3 Molecular Mechanisms of Axis Development 3.1 Signaling pathways involved in axis formation Axis development in vertebrates is orchestrated by a complex interplay of signaling pathways, including Wnt, TGF-β, and FGF pathways. These pathways are crucial for the establishment of the dorso-ventral (DV) and antero-posterior (AP) axes. The Wnt/β-catenin signaling pathway, activated by maternal determinants post-fertilization, plays a pivotal role in the formation of the Nieuwkoop center and subsequently the Spemann organizer, which are essential for DV and AP patterning (Carron and Shi, 2016), The TGF-β family, particularly Nodal and BMPs, are integral in DV patterning, with BMPs establishing gradients that define cellular fates along the DV axis (Bier and Robertis, 2015; Takebayashi-Suzuki and Suzuki, 2020), Additionally, the FGF pathway, along with retinoic acid (RA) and Wnt, contributes to the AP axis formation by creating gradients that pattern the trunk and posterior regions (Carron and Shi, 2016; Durston et al., 2019), The integration and fine-tuning of these signaling pathways ensure the correct establishment of the body plan (Roet al., 2015).
RkJQdWJsaXNoZXIy MjQ4ODYzNA==