An Epithelial cell undergo modifications in three domains:
- Apical domain
- Basal domain
- Lateral domain
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- The apical surfaces of epithelial cells are modified in certain areas in order to facilitate movement of a substance or to increase the cell surface area. This includes:
- CILIA- cytoplasmic processes containing bundle of microtubules.
- STEREOCILIA- (stereovilli), microvilli of unusual length
- MICROVILLI - cytoplasmic processes containing a core of actin filaments
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- In 1676, Anton van Leeuwenhoek described cilia as “thin feet or little legs”.
- Cilia are long, motile microtubule-based organelles projecting from almost all cell types.
- Types - Primary, Motile and Nodal cilia based on function.
- They are present singly or in massive large numbers, usually 5 -10 micrometer in length.
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- Found in large numbers.
- Motile cilia and their counterpart flagella has 9+2 axonemal organisation.
- Present in trachea, bronchi and oviducts.
- Arranged in orderly manner and it helps in clearing mucus and transport of ova and fluid towards uterus.
- They give a crew cut appearance to the epithelial surface.
- AXONEME and CILIARY TIP - DOUBLET MICROTUBULE in 9+2 arrangement.
- BASAL BODY- TRIPLET MICROTUBULES in 9+0 arrangement.
- A TUBULE- 13 PROTOFILAMENTS.
- PARTIAL B TUBULE- 10 PROTOFILAMENTS.
- TRANSITION ZONE
- DYNEIN ARMS - INNER AND 2 OUTER .
- RADIAL SPOKE - both are attached to A tubule.
- NEXIN - Binds with B tubule.
- INTRAFLAGELLAR TRANSPORT - Bidirectional.
- ANTEGRADE TRANSPORT - KINESIN 2 Motor
- RETROGRADE TRANSPORT - DYNEIN 2 Motor
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- ALAR SHEETS, BASAL FOOT AND STRIATED ROOTLETS.
- Main function - anchoring and coordinating ciliary beat.
- The basal foot - an accessory structure found in the mid region of the basal body.
- All basal feet are oriented in the same direction , so that they function in coordinating ciliary movement.
- They are involved in adjusting basal bodies by rotating them to the desired position.
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- DYNEIN ARMS - A tubule forms temporary cross bridges with B tubule.
- SLIDING MOVEMENT - Hydrolysis of ATP.
- EFFECTIVE STROKE - Cilia becomes rigid and exhibits forward movements.
- RECOVERY STROKE - Nexin and Radial spoke accumulate energy to bring back cilia to straight position.
- The inner dynein arms are responsible for creating the discrete ciliary wave pattern by controlling the size and shape of the ciliary bend.
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- Nodal cilia - Found in the embryo on the bilaminar embryonic disc at the time of gastrulation.
- They are concentrated in the area that surrounds the primitive node, hence their name nodal cilia.
- Perform rotational movement.
- They play an important role in early embryonic development- Mainly axis determination.
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- IMMOTILE solitary projections found on almost all eukaryotic cells.
- The term monocilia implies that only a single cilium per cell is usually present.
- In humans seen in rete testis, epithelial cells lining the biliary tract, epithelial cells of kidney tubules, ependymal cells of ventricles, photoreceptor cells in the retina, and the vestibular hair cells of the ear.
- They function as chemosensors, osmosensors and mechanosensors.
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- In many mammalian cells, signaling through the primary cilia seems to be essential for controlled cell division and subsequent expression.
- Important function is the regulation of key signaling pathways such as Hedgehog, Wingless, and insulin like growth factor 1 (IGF-1)/insulin.
- CILIARY APPARATUS FORMATION - Based on Centriole theory and Acentriole theory.
- Procentrioles - Precursors of centrioles reach the cell surface.
- Then Basal body complex is established with Alar sheets, basal Foots and striated rootlets.
- Polymerization of A and B microtubules helps in elongation of axoneme till the tip of the centrioles – ELONGATION STAGE
- Mature cilium is finally formed.
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- Inhibition of NOTCH signalling – For Motile ciliogenesis.
- Reduction of BMP (Bone morphogenic proteins).
- CEP 63 ( Centrosomal proteins) ,PLK4(polo like kinase)- Centriole Generation.
- FOXJ1 - Responsible for basal bodies docking to the apical surface and Intra Flagellar Transport then helps in growth of motile cilia.
- CILIOPATHIES: Genetic disorder of the cilia– the basal bodies of primary cilia is dysfunctional.
- ALSTROM SYNDROME (progressive loss of vision and hearing, heart disease, obesity, type 2 diabetes).
- MECKE - GRUBER SYNDROME (lethal autosomal disorder- occipital encephalocoele, postaxial polydactyly and B/L dysplastic cystic kidneys).
- NEPHRONOPHTHISIS (inflammation and fibrosis of kidney).
- RESPIRATORY INFECTIONS , ANOSMIA (complete loss of smell).
- MALE INFERTILITY.
- PRIMARY CILIARY DYSKINESIA: Autosomal recessive disorder. Defect in dynein arms of cilia. This condition prevents the clearing of mucus from the lungs, ears and sinuses.
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- TRIAD of features - Situs Inversus, Bronchiectasis and Chronic Sinusitis.
- Due to defect in outer dynein arms it results in Laterality defects during axis determination including a complete mirror image of the normal situs (situs inversus) or random arrangement (situs ambiguous or heterotaxy) of thoracoabdominal organs.
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- Primary cilium is found in endocrine pancreas (a-, b-, and d cells) , and exocrine pancreas , with the exception of acinar cells.
- Several animal models of TYPE 2 DIABETES have shown evidence of ciliary defects, suggesting a link between primary cilium and the pathophysiology of the disease.
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- Small fingerlike cytoplasmic projections.
- Measure about 1-3 micron in length.
- Core is filled with clusters of actin filaments.
- Microvilli and surface glycocalyx combine to form what is referred to as Brush or striated border of absorptive epithelial lining such as in the small intestine and the proximal convoluted tubules in the kidney.
- Function - It serve to increase the surface area of the epithelial lining, thus facilitating absorption.
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- Stereocilia of the genital ducts are extremely long processes.
- Under light microscope - Resemble the hairs of a paint brush.
- They are supported by internal bundles of actin filaments, cross-linked by fimbrin.
- Stereocilia develop from microvilli by the lateral addition of actin filaments to the actin bundle as well as elongation of the actin filaments.
- Ezrin, anchors the actin filaments to the plasma membrane.
- Absence of villin from the tip of the stereocilium.
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- Enabling fluid propelling by motile cilia movement and cargo transport in non - motile cilia and dynein motors all these power the development, survival, and wellbeing of human beings.
- The extraordinary complexity of the primary cilium promises exciting future, such us the potential of cilia-related signaling molecules as therapeutic targets for new treatments fighting diabetes.
- Hence, understanding the structure and function of these cilia has made a great impact on understanding vertebrate development and human ciliopathy diseases.
- Sreekumar, V.; Norris, D.P. Cilia and development. Curr. Opin. Genet. Dev. 2019, 56, 15–21. [CrossRef] [PubMed]
- Satir, P.; Christensen, S.T. Overview of structure and function of mammalian cilia. Annu. Rev. Physiol. 2007, 69, 377–400. [CrossRef]
- Wheway, G.; Nazlamova, L.; Hancock, J.T. Signaling through the Primary Cilium. Front. Cell Dev. Biol. 2018, 6, 8. [CrossRef]
- Praveen K, Davis EE, Katsanis N. Unique Among Ciliopathies: Primary Ciliary Dyskinesia, a Motile Cilia Disorder.