\section{Epithelial Tissue in multicellular development}


\subsection{Features of epithelia}
 

\subsection{Classifications of epithelial shapes}
Based on the shape of their cells, epithelial tissues can be broadly classified into squamous, cuboidal, columnar, pseudo-stratified, and stratified epithelium. Their shapes and structures are crucial in their functional diversification. 
\par Simple squamous epithelia are composed of thin, flat cells which help facilitate diffusion and filtration. 
\par Cuboidal epithelial are typically made up of cube-shaped cells, optimized for absorption and secretion. \par Columnar epithelial tissues, on the other hand, are made up of tall, rectangular cells, which often are enhanced with specialized structures like cilia or microvilli to promote and perform absorption and secretion. These types are characterised by a single layer of cells but epithelial tissues can also be made up of these types of cells stratified over each other. \par Stratified epithelia, characterized by multiple layers of cells over each other, are most often important for protection

\subsection{Developing epithelia}
Epithelial tissues being 
These features allow epithelial tissues to form crucial protective structures in organisms, enabling a single cell—the fertilized embryo—to undergo morphogenesis. Polarity and mechanosensitive signalling arising from coordinates cytoskeletal mechanisms in epithelia. Acting through coordinated mechanisms, such as spatially controlled cell proliferation, cell division orientation, movement, and cell death, epithelial cells can drive global alterations in tissue morphology \cite{heisenberg2013forces, hopyan2011budding}. During development, the specification of epithelia often proceeds the specification of other tissue types as cells on the exterior undergo specialization and polarization. Additionally, through their barrier structures, they defend against external environmental challenges while mediating and orchestrating intricate cellular interactions essential for embryonic growth and organogenesis \cite{fristrom1988}.  Developmental morphogenesis of an organism involves numerous challenges as the embryo takes shape \cite{thomson1917growth, shahbazi2020mechanisms, kimmel1995stages}. This change in form and function is influenced by physical and chemical factors, including the environment and the organism's interactions. Embryonic epithelia are critical during this morphogenesis because they function as a barrier and regulatory tissue to maintain control over these factors \cite{fristrom1988, quintin2008epithelial}.




\section{Roles of the Intermediate filaments in Development}

\section{Interplay of Cytoskeletal elements}

\section{Epithelial morphogenesis in Teleost}
 In this section we focus on the development of the zebrafish embryo to understand the development of the earliest epithelial layer called the enveloping layer (EVL) and contrast it to the developmental and structure of the EVL in medaka. 
\par 
 Other keratins such as keratin 4 do not seem to be expressed at the same time.  This elevation of keratin 18 specifically is also associated with the activation of mTOR signaling (marked by phospho-S6). This growth is dependent on lateral inhibition of TAZ nuclear localization due to the compression by the larger keratin18-containing MPC.
\par  Right after this is established, zygotic genome activation and activation of mechanosensitive cell divisions take place\cite{xiong2014interplay,wragg2016transcriptional}. 


3. Intermediate Filaments in Development (Keratin Focus)
3.1 Introduction to Intermediate Filaments
Intermediate filaments are a family of cytoskeletal filaments that, despite their myriad functions in mechanical support and signaling in cells, have been understudied in developmental contexts. Intermediate filaments have a surprisingly conserved structure with a central alpha-helical rod domain flanked by end domains that can be organized according to various sizes, shapes, and functions.
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The functional diversification of these end domains has led to the formation of subclasses of intermediate filament proteins, such as: a complex group of proteins in epithelial keratins (40–70 kDa), single protein vimentin (53 kDa) of mesenchymal origin, glial fibrillary acidic protein (GFAP, 50 kDa) in astroglial cells, and three different neurofilament proteins—NF-L (65 kDa), NF-M (105 kDa), NF-H (135 kDa)—as well as nuclear lamins (A and B).

3.2 Keratin’s Role in Development
During development, flexible yet robust proteins such as intermediate filaments, particularly keratins, may play crucial roles as tissues undergo dynamic changes.

On a single-cell level, loss of keratins leads to softer cells (40\%) over the nucleus and in the cell body as measured by atomic force microscopy (AFM) and magnetic tweezers\cite{ramms2013keratins}. Reintroduction of basal keratins Keratins 14-5 leads to recovery of the mechanical properties of the cell and a stiffer cell. In more complex tissues, keratins already show more intricate distinct functions. 
4. Role of Actomyosin in Development
(Section not yet written.)

5. Interplay of Actin and Keratin
(Section not yet written.)

6. Transition to Research Focus
(Section not yet written.)


Multicellular evolution from unicellular cells was led by specialization of function, with the earliest evidence pointing towards epithelial specialization \cite{leys2012epithelia}. Epithelia are most often identified by critical features in their structure, such as apicobasal cell polarity, adhesive junctions between their apical and lateral domains between cells, and a basement membrane\cite{belahbib2018new}. These features allow epithelial tissues to form crucial protective structures in organisms, enabling a single cell—the fertilized embryo—to undergo morphogenesis. \newline
\textbf{[Find more refs]}. Acting through coordinated mechanisms such as spatially controlled cell proliferation, cell division orientation, movement, and cell death of individual cells can give rise to global alterations in tissue morphology \cite{heisenberg2013forces,hopyan2011budding}. 

\subsection{ Background IFs}
Intermediate filaments are a family of cytoskeletal filaments that, despite their myriad functions in mechanical support and signalling in cells, have been understudied in developmental contexts. Intermediate filaments have a surprisingly conserved structure with a central alpha-helical rod domain flanked by end domains that can be organized according to various sizes, shapes and functions. The functional diversification of these end domains has led to the formation of subclasses of IF proteins, such as: a complex group of proteins in epithelial keratins ( 40 -70 kDa), single protein vimentin ( 53 kDa) of mesenchymal origin, glial fibrillar acidic protein (GFAP, 50kDa) protein in astroglial cells, and three different neurofilament proteins NF-L (65 kDa), NF-M (105 kDa), NF-H (135 kDa) and nuclear lamins (A and B).  

During development, flexible and robust rigid proteins such as intermediate filaments could play very interesting roles with changes occurring in tissues. 

. Intermediate filament diversity
. Intermediate filament structure
. Intermediate filament dunctions
. Keratin structure and mechanics
. Keratin function diversity



Discussion copy
In cells, different keratin pairs show distinct expressions and functions depending on the type of cell \cite{yoon2019Keratinintermediatefilaments}. Different keratin pairs confer cells with different properties in functions such as cell migration, division, and wound closure. Basal keratins K5/K14 are associated to cell division and stratification and are expressed specifically in the leader cells in migrating carcinoma cells \cite{cheung2013collective}. 
\par
The role of keratins in cell movements is nuance due to the diversity of keratin types and the different functional interactions of each of these keratins in affecting cellular properties in cell migration \cite{chung2013Networkinggaloreintermediate}. Keratins can affect both the adhesion between cells and cell-ECM as well as the interacting with the migratory cytoskeleton both of which affect cell migration. In migrating cells, desmosomes are regulated by the migration related cues and are less adhesive in motile cells. Strong keratin-desmosomal interactions promote epithelial integrity and hinder cell motility stablizing the cell. Consequently during physiological migration or due to mutations the interaction of desmosomes and keratin is limited \cite{roberts2011Desmosomedynamicsmigrating}.But cell-cell attachments are part of the picture in cell motility as connections mediated through integrins and hemidesmosomes also affect the mechanics of movement. Mechanisms that attach keratins to the ECM are conserved even among IFs in C elegans with primitive hemidesmosomal proteins attaching to primitiv cytoplasmic intermediate filaments \cite{zhang2011tensioninducedmechanotransductionpathway}. 
\par
Discuss keratin dynamics in epithelial spreading and migration \cite{sonavane2017Mechanicalsignalingroles}
\par
Discuss keratin dynamics in wound models and EMT \cite{mendezVimentinInducesChanges2010}
\par 
Discuss what is known about other intermediate filaments in migration \cite{de2018intermediate}
\par
IN conclusion

Keratins Type I and Type II represent two of the four cytoplasmic IF types in cells. Keratin filaments are semi-flexible with persistence lengths much shorter than actin and microtubules. These properties are largely due to molecular assembly and their architecture, which fundamentally differ from other cytoskeletal elements. Each keratin monomer forms via parallel attachments of type I and type II filaments, with each type having specific interactions usually. These heterodimers, unlike other cytoskeletal proteins, form more complex monomers, which first involve these heterodimers associating via a knob-pocket mechanism in an antiparallel and staggered fashion. Four to eight of these assemble laterally into unit-length filaments, which intercalate longitudinally, eventually generating mature KFs with the characteristic 8–12 nm diameter \cite{eldirany2019human}. These ULFs bundle together with electrostatic and hydrogen bond interactions to form filaments and further bundles. Inside cells, these filaments most often occur as bundles rather than individual filaments. This complex association gives credence to mechanisms that could explain the dynamics and regulation of keratin bundles in vivo.

Keratins are a vertebrate epithelial invention with multiple pieces of evidence that suggest a dynamic and interactive role in complex tissues interacting with complex biomechanical and biochemical networks \cite{sanghvi-shahIntermediateFilamentsJunction2017b,yoon2019Keratinintermediatefilaments}. In this thesis, we studied the mechanical and cytoskeletal crosstalk of keratin early in development, describing the roles of keratins unknown so far in early development. Our results confirmed some evidence about keratin functions observed in other systems and provided insights into hitherto unknown interactions of the keratin cytoskeleton. The early zebrafish is a remarkably robust and dynamic system that can be used to study the dynamics of the keratin cytoskeleton organized in an epithelium. The myriad functions that keratins play in epithelial spreading have been discussed in more detail in the next section.

\section{Functions of keratin in epithelial spreading} 
During epiboly, actomyosin contraction and flow within the YSL generate the forces pulling the margin of the EVL towards the vegetal  pole of the gastrula. These pulling forces not only stress the EVL but also the YSL to which the EVL is mechanically coupled at its margin. This stress and the forces generating it lead to the movements of complex, interconnected tissues, including the forming epiblast. In the EVL, buildup of stress triggers keratin network maturation, which again increases EVL tissue viscosity, thereby ensuring that the tissue remains intact when stress rises during epiboly. In the YSL, keratins are required for stress-dependent actin network accumulation, and efficient contraction is needed for pulling the EVL over the yolk cell. This dual mechanosensitive function of keratin, coupling EVL viscosity to YSL contractility, secures robust and efficient EVL spreading during epiboly. \par



\section{Crosstalk between keratin and other cytoskeletal systems}
A crucial factor in mediating these mechanical functions is the ability of  Keratin and other intermediate filaments to act as mechanical coordinators across the cell. The organization of these filaments in the cells allows the integration of signals from the cell periphery, where they attach to desmosomes, to the cell interior, where they interact with different mechanosensitive components of the cell, including the nucleus and nuclear lamina. Interactions with desmosomes are the sites at which the mechanical functions of intermediate filaments are most prominent. The interaction is mediated intracellularly by plaque proteins such as plakoglobin, plakophilin, and desmoplakin. Inside the cell keratins and other intermediate filaments can bind to cell-cyle regulators and cytoskeletal proteins either directly or with linkers. On the nucleus they attach with nuclear lamina proteins and affect properties of the lamin cytoskeleton directly or indirectly. 


Interactions of intermediate filaments with other cytoskeletons are quite evident in vitro. 
\par
Direct interactions aside the crosstalk between intermediate filaments and other parts of the cytoskeleton is mediated by linker proteins that can specifically interact between different IFs and parts of the cytoskeleton. 

Although some cross-reactivity between different keratin isoforms has been observed in vitro, the precise binding kinetics, specificity, and regulatory mechanisms governing these interactions remain incompletely understood.

Evidence suggests mechanical sensitivity of intermediate filaments upon   In alveolar epithelial cells, keratin 8 and keratin 18 are expressed. Upon shear stress, the amount of filamentous keratins extracted from these cells but not the soluble degraded keratin was reduced, especially at higher levels of shear stress. This reduction of filamentous keratin8 is not seen upon stretching. This is mediated through serine phosphorylation mediated by PKC at the 73rd position. 