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The distal appendages (DAPs) of centrioles have been proposed to anchor cilia to the plasma membrane, but their molecular composition, assembly, and exact function in ciliogenesis remain poorly understood. Using quantitative centrosome proteomics and superresolution microscopy, we identified five DAP components, including one previously described (CEP164), one partially characterized (CEP89 [ccdc123]), and three novel (CEP83 [ccdc41], SCLT1, and FBF1) DAP proteins. Analyses of DAP assembly revealed a hierarchy. CEP83 recruits both SCLT1 and CEP89 to centrioles. Subsequent recruitment of FBF1 and CEP164 is independent of CEP89 but mediated by SCLT1. All five DAP components are essential for ciliogenesis; loss of CEP83 specifically blocks centriole-to-membrane docking. Undocked centrioles fail to recruit TTBK2 or release CP110, the two earliest modifications found on centrioles prior to cilia assembly, revealing centriole-to-membrane docking as a temporal and spatial cue promoting cilia initiation.


In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including antennae, mouthparts (including mandibles, maxillae and maxillipeds), gills, locomotor legs (pereiopods for walking, and pleopods for swimming), sexual organs (gonopods), and parts of the tail (uropods). Typically, each body segment carries one pair of appendages. An appendage which is modified to assist in feeding is known as a maxilliped or gnathopod.[citation needed]

Appendages may become uniramous, as in insects and centipedes, where each appendage comprises a single series of segments, or it may be biramous, as in many crustaceans, where each appendage branches into two sections. Triramous (branching into three) appendages are also possible.[1]

All arthropod appendages are variations of the same basic structure (homologous), and which structure is produced is controlled by "homeobox" genes. Changes to these genes have allowed scientists to produce animals (chiefly Drosophila melanogaster) with modified appendages, such as legs instead of antennae.[2]

The centrosome is a highly conserved structure composed of two centrioles surrounded by pericentriolar material. The mother, and inherently older, centriole has distal and subdistal appendages, whereas the daughter centriole is devoid of these appendage structures. Both appendages have been primarily linked to functions in cilia formation. However, subdistal appendages present with a variety of potential functions that include spindle placement, chromosome alignment, the final stage of cell division (abscission) and potentially cell differentiation. Subdistal appendages are particularly interesting in that they do not always display a conserved ninefold symmetry in appendage organization on the mother centriole across eukaryotic species, unlike distal appendages. In this review, we aim to differentiate both the morphology and role of the distal and subdistal appendages, with a particular focus on subdistal appendages.

Primitive insects develop appendages directly from embryonic limb buds that grow as external projections, while more derived insect species with complete metamorphosis, such as Drosophila, develop their appendages from imaginal precursors. The external adult body of Drosophila is formed by two different sets of cells, the imaginal discs and the histoblasts. The imaginal discs are specialized epithelial sheets specified in the embryo that grow and become patterned inside the larva. During the pupal stage, imaginal discs evert and differentiate to form the adult structures. The histoblasts are the precursors of the fly abdomen that, in contrast to the imaginal discs, only proliferate during pupal development. There are 19 imaginal discs in the larvae, 9 appearing in pairs, and the genitalia. The wing and haltere discs form the corresponding appendages and also the dorsal thorax. In a similar manner, the leg discs develop the appendage proper and the ventral pleura of the thorax. While thoracic imaginal precursors (wing, haltere, and legs) originate from a single embryonic segment [8,9], the genital disc primordia is a sexually dimorphic compound primordia derived from three abdominal segments (A8, A9, and A10) (reviewed in [10]). In a similar fashion to the genital primordia, the cells from different cephalic segmental identities coalesce together to form the eye-antennal disc [11]. The eye-antennal disc is also a compound structure that gives rise to the olfactory (antenna and maxillary palps) and visual (compound eyes and ocelli) organs plus the head epidermis [12]. We can group the appendages in dorsal or ventral depending on their relative positions within the body and their homology. Therefore, ventral appendages include the legs, antenna, and genitalia, while the wings and halteres are dorsal. In this review, we will focus on the patterning of the thoracic appendages and the antenna.

Specification and patterning of the Drosophila antenna. (A) Model of progressive specification of the eye and antenna territories and P-D subdivision of the antenna disc. Schematic representation of three representative stages of EAD development is shown. In each disc the expression and interactions of genes and signaling pathways required for eye and antenna field specification are schematized. (B) The antenna and leg appendages are homologous structures. Arrows indicate the correspondence between the antenna and leg domains (Postlethwait and Schneiderman, 1971). The expression of the P-D genes Dll, dac, and hth and their overlapping domains are represented by a color code. Compare the relative expression of Dll, dac, and hth in the antenna and the leg imaginal discs. Note that hth and Dll coexpress in a large domain in the antenna while in the leg these genes are expressed in almost exclusive domains.

The skin appendages are epidermal and dermal-derived components of the skin that include hair, nails, sweat glands, and sebaceous glands. Each component has a unique structure, function, and histology. This article describes the unique characteristics of each of these components and provides insight into tissue preparation for microscopic evaluation and the clinical significance of these structures.[1][2][3]

There are many inflammatory, immune-mediated, autoimmune, infectious, neoplastic, and traumatic causes of alterations to the normal function of the skin appendages, some of which include the following:

The skin is the largest organ of the body, covering an area of approximately 2 m2. The skin is composed of the cutis (including the dermis and epidermis), subcutaneous tissue, and skin appendages. The epidermis, which is derived from ectoderm, is the outermost layer of the skin and is mainly composed of keratinocytes. The dermis, which is derived from mesoderm, is located underneath the epidermis and is mainly composed of elastic fibers, type I collagen, and connective tissue. It is formed by the papillary dermis and the reticular dermis. The subcutaneous tissue, which is derived from the mesoderm, is the innermost layer of the skin and is mainly composed of fat and connective tissue. Skin appendages are derived from the skin and include hair, nails, and glands. The main functions of the skin are protection (barrier against ultraviolet radiation, microorganisms, and water loss), the synthesis of vitamin D, detection of sensation (e.g., touch, temperature, pain), and the regulation of body temperature.

Countless aquatic animals rotate appendages through the water, yet fluid forces are typically modeled with translational motion. To elucidate the hydrodynamics of rotation, we analyzed the raptorial appendages of mantis shrimp (Stomatopoda) using a combination of flume experiments, mathematical modeling and phylogenetic comparative analyses. We found that computationally efficient blade-element models offered an accurate first-order approximation of drag, when compared with a more elaborate computational fluid-dynamic model. Taking advantage of this efficiency, we compared the hydrodynamics of the raptorial appendage in different species, including a newly measured spearing species, Coronis scolopendra The ultrafast appendages of a smasher species (Odontodactylus scyllarus) were an order of magnitude smaller, yet experienced values of drag-induced torque similar to those of a spearing species (Lysiosquillina maculata). The dactyl, a stabbing segment that can be opened at the distal end of the appendage, generated substantial additional drag in the smasher, but not in the spearer, which uses the segment to capture evasive prey. Phylogenetic comparative analyses revealed that larger mantis shrimp species strike more slowly, regardless of whether they smash or spear their prey. In summary, drag was minimally affected by shape, whereas size, speed and dactyl orientation dominated and differentiated the hydrodynamic forces across species and sizes. This study demonstrates the utility of simple mathematical modeling for comparative analyses and illustrates the multi-faceted consequences of drag during the evolutionary diversification of rotating appendages.


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