Cells in tissues

Cells in tissues

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Tissue (biology): In biology, tissue is a cellular organizational level intermediate between cells and a complete organ. A tissue is an ensemble of similar cells from the same origin that together carry out a specific function.(Wikipedia) Are all the cells of a tissue of a kind?

In the definition Tissue what is from the same origin?

What you say is true that most of the cells in a tissue are of the same kind. However, as with most rules in biology, there is almost always an exception to these rules. There are only four types of tissues (can be split up into further subdivisions): muscle, epithelial, connective and nervous. Also, a muscle tissue is made up of an ensemble of myocytes (muscle cells). Therefore, the answer to your question would be yes, the cells in a tissue are (if not all, then) almost all of the same kind.

Many different types/kinds of cells comprise a single tissue. For instance, the epithelial tissue of mammalian skin can contain cells that produce hair, excrete sweat, or do neither of these things but simply serve as a barrier. These cells are all epithelial tissue. Here is another example of the diversity of cell types in a single tissue: the cells of the brain are part of 'nervous tissue', but there are thousands of different kinds of neuronal cells (the precise number is growing actually; see this website, where this image was grabbed).

Blood is technically a tissue, and yet there are many, many different kinds of blood cells.

In reference to your second question, the cells share a common origin in a tissue because they come from a small number of precursor cells, like the depiction in the image above for blood cells.

AP Biology : Types of Cells and Tissues

Specialized cells line the insides of our blood vessels. These cells help control vasoconstriction and vasodilation and play an important role in the permeability of blood vessels. These cells can be calssified as which of the following?

Endothelial cells line the insides of blood vessels and lymphatic vessels and have many important functions, including but not limited to those described in the question. One additional function of endothelial cells is involvement in blood clotting.

Endothelium generally lines fully internal pathways (such as the vascular system), while epithelium generally lines pathways that are open to the external environment (such as the respiratory and digestive systems). Nerve cells are specialized for signaling, and red blood cells are specialized for oxygen transport.

Example Question #1 : Understanding Epithelial And Endothelial Cells

Which of the following is not composed of epithelial cells?

Lining of the chambers of the heart

Lining of intestinal tracts

Lining of the chambers of the heart

Epithelial cells compose the outside of the body, namely skin and the lining of systems that connect to the outside of the body, such as the respiratory, excretory, and digestive tracts. The stomach is part of the digestive tract, as are the intestines. Knowing that the digestive tract is lined with epithelial cells allows us to eliminate the lining of the intestine from the answer choices. Similarly, the alveoli are a part of the respiratory system and nephrons are part of the excretory system.

The lining of the heart's chambers is part of the vascular lining, which is made of endothelial cells and is not exposed to the outside environment.

Example Question #3 : Understanding Epithelial And Endothelial Cells

Which of these is not a classification of epithelial cell?

Epithilial tissue is usually classified in two ways: by the shape of the cells and by their organization. Cell shape can be flat and polygonal (squamous), elongated and rectangular (columnar), or short and rectangular (cuboidal). Cells can be found in a single layer (simple) or multiple layers (stratified). Pseudostratified epithelium is usually columnar in shape and consists of a single cell layer that has the appearance of multiple layers.

Sprilli is a classification of spiral-shaped bacteria.

Example Question #4 : Understanding Epithelial And Endothelial Cells

Which of the following structures of the vertebrate eye controls the amount of light entering the pupil?

Light enters the eye through the pupil and is focused at the back of the eye to form an image on the retina. The retina contains rods and cones that can convert the image to nerual signals for the brain to interpret.

The iris is the muscle around the pupil that allows it to dilate or constrict. Changing the size of the pupil will alter the amount of light entering the eye.

The ciliary muscles attach the lens to the scelera (the outer white part of the eye). Contracting or relaxing these muscles will change the focal point of the lens, allowing the eye to properly focus the image on the retina. The ciliary muscles and lens do not affect the amount of light to enter the eye.

The ciliary body is adjacent to the ciliary muscles and produces aqueous humor, a liquid that fills the space between the cornea and the lens.

Example Question #5 : Understanding Epithelial And Endothelial Cells

What is the function of epithelial cuboidal cells in the human body?

Epithelial cuboidal cells are involved in secretion and absorption in the exocrine system in the lining of glands.

Example Question #6 : Understanding Epithelial And Endothelial Cells

What is the specialized function of stratified epithelia?

Protection from mechanical and chemical forces

Absorption and filtration

Structural support for tissues

Protection from mechanical and chemical forces

Stratified epithelia are composed of two or more layers of epithelial cells. The increased number of cells associated with stratified epithelia creates a more complex function stratified epithelia are involved in protection from mechanical and chemical forces.

Example Question #1 : Understanding Epithelial And Endothelial Cells

Which of the following best describes the role of epithelial tissue in the human body?

Support and connect different types of tissues and organs

Contract to create movement

Transmit chemical and electrical signals

Act as a regulatory barrier between two locations in the body

Act as a regulatory barrier between two locations in the body

Epithelial tissue lines the blood vessels, organs, and cavities in the human body. The primary function of epithelial tissue is to regulate secretion, absorption, and transport across surfaces. Epithelial tissue additionally plays a role in protection and detection of signals therefore, the role of the epithelial tissue is to act as a regulatory barrier.

Example Question #8 : Understanding Epithelial And Endothelial Cells

Epithelial cells can be which of the following morphologies?

Epithelial cells can be categorized by morphology, or shape. Epithelial cells exist in the following morphologies: squamous, columnar, or cuboidal.

Example Question #9 : Understanding Epithelial And Endothelial Cells

Which of the following is not a function of epithelial tissue?

Epithelial tissue acts as a barrier in the human body. The functions of epithelial tissue are broad and include selective absorption, secretion, transport, and protection.

Example Question #1 : Understanding Epithelial And Endothelial Cells

Which of the following best describes the morphology of squamous cells in epithelial tissue?

They are wider than they are tall

They are taller than they are wide

Their height and width are equal

They are wider than they are tall

Epithelial cells have three distinct morphologies: squamous, cuboidal, and columnar. Squamous cells are wider than they are tall. In other words, they are flat.

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Some cells act as individual cells and are not attached to one another. Red blood cells are a good example. Their main function is to transport oxygen to other cells throughout the body, so they must be able to move freely through the circulatory system. Many other cells, in contrast, act together with other similar cells as part of the same tissue, so they are attached to one another and cannot move freely. For example, epithelial cells lining the respiratory tract are attached to each other to form a continuous surface that protects the respiratory system from particles and other hazards in the air.

Many cells can divide readily and form new cells. Skin cells are constantly dying and being shed from the body and replaced by new skin cells, and bone cells can divide to form new bone for growth or repair. On the other hand, some other cells — like certain nerve cells — can divide and form new cells only under exceptional circumstances. Nervous system injuries (such as a severed spinal cord) generally cannot heal by the production of new cells, which results in a permanent loss of function.

Many human cells have the primary job of producing and secreting a particular substance, such as a hormone or an enzyme. For example, special cells in the pancreas produce and secrete the hormone insulin, which regulates the level of glucose in the blood. Some of the epithelial cells that line the bronchial passages produce mucus, a sticky substance that helps trap particles in the air before it passes into the lungs.

Table of Contents

Contributors to Volume 3
Foreword to Volume 3
Foreword to Volume 1
Foreword to Volume 2
Contents of Volume 1
Contents of Volume 2
1. The Molecular Organization of Cells and Tissues in Culture
I. Introduction
II. Methods
III. Cellular Organization at the Molecular Level
IV. Fibrogenesis in Tissue Culture
2. Tissue Culture in Radiobiology
I. Historical Introduction
II. The Effects of Radiation on Cell Proliferation
III. Effects of Radiation on the Growth of Tissue and Cell Cultures
IV. Effect of Radiation on the Growth of Isolated Cells
V. Irradiation of Selected Parts of the Cell
VI. Some Fallacious Methods of Growth Measurement
VII. Non-mitotic Cell Death
VIII. Degenerative Changes in the Cytoplasm
IX. Radiation and Cell Differentiation
X. The Radiosensitivity of Malignant Cells
XI. Chemical Factors Influencing Radiosensitivity
XII. Effects of Radiation on the Culture Medium
XIII. Comparison of in vitro with in vivo Sensitivity
XIV. Comparison of Different Sorts of Ionizing Radiation
XV. Effects of Ultraviolet Radiation
XVI. Conclusion
3. Effects of Invading Organisms on Cells and Tissues in Culture
I. Introduction
II. Protozoa
III. Bacteria
IV. Mycoplasma (PPLO)
V. Miyagawanella
VI. Poxviruses
VII. Herpes Viruses
VIII. Adenoviruses
IX. Papova Viruses
X. Myxoviruses
XL Avian Sarcomas and Lymphomas
XII. Picorna Viruses
XIII. Arboviruses
XIV. Miscellaneous Viruses
XV. Apologia and Epilogue
4. Cell, Tissue and Organ Cultures in Virus Research
I. Introduction
II. Isolation of Viruses in Tissue Culture
III. Identification of Viruses
IV. Titration of Viruses
V. Replication of Viruses
VI. Viral Genetics
VII. Production of Virus Vaccines
VIII. Transformation of Cells by Viruses
IX. Virus Studies in Organ Cultures
5. Antibody Production in Tissue Culture
I. Introduction
II. Early Attempts at in vitro Studies
III. Recent in vitro Studies
IV. Combined in vitro-in vivo Studies
V. Conclusions
6. Tissue Culture in Pharmacology
I. Introduction
II. Methods of Assessment of Drug Activity
III. Tissue Culture in Screening Systems in Cancer Chemotherapy
IV. Tissue Culture in the Study of Pharmacologically Active Agents
7. Invertebrate Tissue and Organ Culture in Cell Research
I. Introduction
II. Design of Culture Media
III. Preparation of Materials and Techniques
IV. Sources of Cells and Tissues
V. Growth by Cell Division in Tissue Cultures
VI. Aggregation of Dissociated Cells
VII. Culture of Organs and Embryos in Developmental Studies
VIII. Invertebrate Tissue Culture in Pathology
8. Introduction and Methods Employed in Plant Tissue Culture
I. Introduction
II. Techniques of Organ, Tissue and Free Cell Culture
9. The Nutrition and Metabolism of Plant Tissue and Organ Cultures
I. Introduction
II. Aspects of Inorganic Nutrition
III. Aspects of Nitrogen Nutrition and Metabolism
IV. Carbohydrate Requirements and Physiological Effects of Sugars and Sugar Alcohols
V. Vitamin Nutrition
VI. Requirements for Growth-regulating Substances
VII. Pathways of Biosynthesis, "Biochemical Genetics" and Adaptation in Organ and Tissue Cultures
VIII. The Release of Metabolites into the Culture Medium
IX. The Resistance to Growth in Culture Encountered with some Organs, Tissues and Cells
10. Growth, Differentiation and Organogenesis in Plant Tissue and Organ Cultures
I. Introduction
II. Growth and Differentiation in Cultured Roots
III. Histogenesis in Callus Cultures
IV. Growth and Differentiation in Suspension Cultures
V. General Conclusions
11. The Use of Tissue Culture in Phytopathology
I. Introduction
II. The Crown-gall Disease
III. Genetic Tumours
IV. Virus Tumours
V. Spruce Tumours
VI. Fern Tumours
VII. Insect Galls
VIII. Nematodes
IX. Bacterial Diseases
X. Fungal Diseases
XI. Virus Diseases
Author Index
Taxonomic Index
Subject Index

Forces in cell biology

Mechanobiology — the study of how physical forces control the behaviour of cells and tissues — is a rapidly growing field. In this issue, we launch a Series of specially commissioned Review articles that discuss exciting recent developments in this area.

Cell behaviour is guided not only by chemical signals, but by the mechanical properties of the cells and their environment. Cells are able to sense and transduce external mechanical inputs into biochemical and electrical signals that influence processes such as cell proliferation, adhesion, migration and fate. Such mechanotransduction is important in development and homeostasis and, importantly, affects the progression of diseases including muscular dystrophies, cardiomyopathies, fibrosis and cancer. Although our understanding of the specific mechanisms of force sensing and transduction is still limited to the more tractable biological systems, technological advances have contributed to the rapid growth of this multidisciplinary field. Mechanobiology combines in vitro, cell-based work with ex vivo and in vivo experiments at the tissue and organismal level, and brings together classic biology, engineering and physics, to address fundamentally cell biological questions about how mechanics affect cellular processes. As such it has emerged as one of the most exciting areas within the scope of Nature Cell Biology in the last decade.

In light of its significant growth and the high interest of this field to our broad cell biology audience, we are delighted to launch a Series of specially commissioned Review articles discussing recent advances, with a Review by Carl-Philipp Heisenberg, accompanied by Research Highlights on recent mechanobiology studies published elsewhere. The Series can be found on a dedicated page of our website ( where readers can also access an online library comprised of related articles published in Nature Cell Biology and other Nature journals.

In this issue, Heisenberg and co-authors review our current understanding of how mechanonsensing and mechanotransduction occur in the context of tissue organization during embryogenic development. For many developmental processes, a mechanical understanding is still lacking, but some morphogenetic processes have been studied in more detail. The authors provide an overview of concepts that have emerged from studying such systems, including germband extension and the shaping of imaginal discs in Drosophila as well as Xenopus gastrulation and zebrafish epiboly. This Review also serves as an excellent introduction to how cells perceive and transduce force.

The mechanobiology field has been driven forward largely thanks to the development of sophisticated methods to probe and quantify the response to forces by cells and tissues, ranging from microscopy-based tools to molecular force sensors. Simultaneously, the use of microfabricated devices has made possible the manipulation of cellular constraints. Next month, Pere Roca-Cusachs and Xavier Trepat will discuss the methods used to measure the forces generated by cells, and their applicability in the laboratory.

The site of a cell's adhesion to the underlying matrix or to other cells is central for coupling the extracellular matrix or neighbouring cells to cell-intrinsic mechanosensing structures, to generate mechanical feedback and/or translate forces into biochemical signals. Future Reviews in the Series will discuss such events and how they relate to cell migration, cell shape and growth, and nuclear events such as chromatin organization and transcription, in development, homeostasis and disease.

Much remains to be discovered in this stimulating field at the crossroads of biology, biophysics, bioengineering, and we look forward to Nature Cell Biology continuing to be a key outlet for this flourishing area of research. We thank our authors and referees for their contributions, and hope that this Series serves as both information and inspiration for our readers.

Basics on Cell Biology | Cell Tissues & Functions | Biology

Cell biology deals with the smallest unit of life. This course contains explanatory videos on cells as wells as tissues. At the end of every video you can revise your concepts with fun filled exercises.

This course consists of video sessions with basic animation for better visualization and understanding.

At the end of every session you can answer the fun filled exercises which will help you to revise your concepts well.

On completion of this course, you will be able to answer all the questions from the chapters Cells and also tissues.

You will also get a clear understanding of all the functions of both cells and tissues.

Cell biology is the study of cell structure and function, and it revolves around the concept that the cell is the fundamental unit of life.

Focusing on the cell permits a detailed understanding of the tissues and organisms that cells compose. Some organisms have only one cell, while others are organized into cooperative groups with huge numbers of cells.

On the whole, cell biology focuses on the structure and function of a cell, from the most general properties shared by all cells, to the unique, highly intricate functions particular to specialized cells.

On understanding this, you will be able to take your learning in biology further and study the various organ systems.

Key control mechanism allows cells to form tissues and anatomical structures in the developing embryo

Under a microscope, the first few hours of every multicellular organism's life seem incongruously chaotic. After fertilization, a once tranquil single-celled egg divides again and again, quickly becoming a visually tumultuous mosh pit of cells jockeying for position inside the rapidly growing embryo.

Yet, amid this apparent pandemonium, cells begin to self-organize. Soon, spatial patterns emerge, serving as the foundation for the construction of tissues, organs and elaborate anatomical structures from brains to toes and everything in between. For decades, scientists have intensively studied this process, called morphogenesis, but it remains in many ways enigmatic.

Now, researchers at Harvard Medical School and the Institute of Science and Technology (IST) Austria have discovered a key control mechanism that cells use to self-organize in early embryonic development. The findings, published in Science on Oct. 2, shed light on a process fundamental to multicellular life and open new avenues for improved tissue and organ engineering strategies.

Studying spinal cord formation in zebrafish embryos, a team co-led by Sean Megason, professor of systems biology in the Blavatnik Institute at HMS, revealed that different cell types express unique combinations of adhesion molecules in order to self-sort during morphogenesis. These "adhesion codes" determine which cells prefer to stay connected, and how strongly they do so, even as widespread cellular rearrangements occur in the developing embryo.

The researchers found that adhesion codes are regulated by morphogens, master signaling molecules long known to govern cell fate and pattern formation in development. The results suggest that the interplay of morphogens and adhesion properties allows cells to organize with the precision and consistency required to construct an organism.

"My lab's goal is to understand the basic design principles of biological form," said Megason, co-corresponding author on the study. "Our findings represent a new way of approaching the question of morphogenesis, which is one of the oldest and most important in embryology. We see this as the tip of the iceberg for such efforts."

Insights into how cells self-organize in early development could also aid efforts to engineer tissues and organs for clinical uses such as transplantation, the authors said.

"Constructing artificial tissues for research or medical applications is a critically important goal, but currently one of the biggest problems is inconsistency," said lead study author Tony Tsai, research fellow in systems biology in the Blavatnik Institute. "There is a clear lesson to learn from understanding and reverse engineering how cells in a developing embryo are able to build the components of an organism in such a robust and reproducible way."

A micropipette assay measures adhesion force between two cells. Credit: Tony Tsai/Sean Megason/Harvard Medical School

Spearheaded by Tsai and in collaboration with Carl-Philipp Heisenberg and colleagues at IST Austria, the research team first looked at one of the most well-established frameworks for morphogenesis, the French flag model.

In this model, morphogens are released from localized sources in the embryo, exposing nearby cells to higher levels of the signaling molecule than cells farther away. The amount of morphogen a cell is exposed to activates different cellular programs, particularly those that determine cell fate. Concentration gradients of morphogens therefore "paint" patterns onto groups of cells, evocative of the distinct color bands of the French flag.

This model has limitations, however. Previous studies from the Megason lab used live-cell imaging and single-cell tracking in whole zebrafish embryos to show that morphogen signals can be noisy and imprecise, particularly at the boundaries of the "flag." In addition, cells in a developing embryo are constantly dividing and in motion, which can scramble the morphogen signal. This results in an initial mixed patterning of cell types.

Nevertheless, cells self-sort into precise patterns, even with a noisy start, and in the current study, the team set out to understand how. They focused on a hypothesis proposed over 50 years ago, known as differential adhesion. This model suggests that cells adhere to certain other cell types, self-sorting in a way similar to how oil and vinegar separates over time. But there was little evidence that this plays a role in patterning.

To investigate, Megason, Tsai and colleagues developed a method to measure the force by which cells adhere to one another. They placed two individual cells together and then pulled on each cell with precisely controlled suction pressure from two micropipettes. This allowed the researchers to measure the precise amount of force needed to pull the cells apart. By analyzing three cells at once, they could also establish adhesion preferences.

The team used this technique to study the patterning of three different types of neural progenitor cells involved in building the nascent spinal cord in zebrafish embryos.

The experiments revealed that cells of a similar type strongly and preferentially adhered to one another. To identify the relevant adhesion molecule-encoding genes, the researchers analyzed the gene expression profile of each cell type using single-cell RNA sequencing. They then used CRISPR-Cas9 to block the expression of candidate genes, one at a time. If pattern formation became disrupted, they applied the pulling assay to see how much the molecule contributed to adhesion.

Live-cell imaging shows the dynamic environment and extent of cell movement that occurs as the nascent spinal cord is formed during early development. Credit: Tony Tsai/Sean Megason/Harvard Medical School

Three genes—N-cadherin, cadherin 11 and protocadherin 19—emerged as essential for normal patterning. The expression of different combinations and different levels of these genes was responsible for differences in adhesion preference, representing what the team dubbed an adhesion code. This code was unique to each of the cell types and determined which other cells each cell type stays connected to during morphogenesis.

"All three adhesion molecules we looked at are expressed in different amounts in each cell type," Tsai said. "Cells use this code to preferentially adhere to cells of their own type, which is what allows different cell types to separate during pattern formation. But cells also maintain some level of adhesion with other cell types since they have to collaborate to form tissues. By piecing together these local interaction rules, we can illuminate the global picture."

Because the adhesion code is cell-type specific, the researchers hypothesized that it is likely controlled by the same processes that determine cell fate—namely, morphogen signaling. They looked at how perturbations to one the most well-known morphogens, Sonic hedgehog (Shh), affected cell type and corresponding adhesion-molecule gene expression.

The analyses revealed that both cell type and adhesion-molecule gene expression were highly correlated, both in level and spatial position. This held true across the entire nascent spinal cord, where patterns of gene expression for cell type and adhesion molecule changed together in response to differences in Shh activity.

"What we found is that this morphogen not only controls cell fate, it controls cell adhesion," Megason said. "The French flag model gives a rough sketch, and differential adhesion then forms the precise pattern. Combining these different strategies appears to be how cells build patterns in 3-D space and time as the embryo is forming."

The researchers are now further investigating the interplay between morphogen signaling and adhesion in developing embryos. The current study looked at only three different cell types, and there are many other adhesion-molecule candidates and morphogens that remain to be analyzed, the authors said. In addition, the details of how morphogens control both cell type and adhesion molecule expression remain unclear.

Better understanding these processes could help scientists uncover and reverse engineer the fundamental mechanisms by which a single-celled egg constructs a whole organism, the authors said. This could have profound implications in biotechnology, particularly for efforts to build artificial tissues and organs for transplantation or for testing new drug candidates.

"The issue with tissue engineering right now is that we just don't know what the underlying science is," Megason said. "If you want to build a little bridge over a stream, maybe you could do that without understanding physics. But if you wanted to build a big suspension bridge, you need to know a lot about the underlying physics. Our goal is to figure out what those rules are for the embryo."

Extracellular Matrix and Cell Adhesion Molecules

Quick look:Recent research shows that ECM and associated CAMs are critical for the functioning of most cells. The integrity of tissues is also dependent on the adhesion, by CAMs, of cells to cells and cells to the Extracellular Matrix

Extracellular matrix (ECM)
All cells in solid tissue are surrounded by extracellular matrix.
Both plants and animals have ECM. The cell wall of plant cells is a type of extracellular matrix. In animals, the ECM can surround cells as fibrils that contact the cells on all sides, or as a sheet called the basement membrane that cells ‘sit on’. Cells in animals are also linked directly to each other by cell adhesion molecules (CAMs) at the cell surface.

ECM is composed of proteins and polysaccharides. Connective tissue is largely ECM together with a few cells.

  • For cells ECM provides:
    • mechanical support
    • a biochemical barrier
    • a medium for:
      1. extracellular communication that is assisted by CAMs
      2. the stable positioning of cells in tissues through cell matrix adhesion
      3. the repositioning of cells by cell migration during cell development and wound repair
    • tensile strength for tendons
    • compressive strength for cartilage
    • hydraulic protection for many types of cells
    • elasticity to the walls of blood vessels
    • bones and teeth
    • the cell wall of bacteria
    • the shells of molluscs and
    • chitinised to form the exoskeleton of insects

    Cell adhesion molecules (CAMs)

    • Cell adhesion molecules belong mainly to a family of chemicals called glycoproteins. They are located at the cell surface and form different types of complexes and junctions to join:
      • cells to cells
      • cells to ECM
      • ECM to the cell cytoskeleton
      • The adhesion of cells to one another to provide organised tissue structure
      • the transmission of extracellular cues and signals across the cell membrane
      • the migration of cells through the regulation of CAM assisted adhesions

      Definitions offer “the substance between cells” and the “material in the intercellular space”, but ECM is much more important than these words suggest. Recent research shows that the functioning of cells is very influenced by cell extracellular matrix.

      Extracellular matrix (ECM) – a house model
      Consider a house one that has been colour washed on the outside. The house, rather like a cell, has different rooms for different functions a dinning room for dining, the kitchen for cooking and so on. But houses, like cells, do not stop at the outermost wall. Each house connects to the outside through wiring for telephone and electricity and through pipes for water, sewerage and probably gas.

      Around every house there is also a space. Apart from any garden there is always a space immediately beyond the outermost wall. Usually at the front of the house there is an area where milk bottles are left, where post and papers stick out of the letterbox, where plants are grown in window boxes or hanging baskets. Nearer the roof there might be a security camera with sensors and lights and higher still a satellite t/v dish and other aerials. In other words there is a space around the house that is very important to it. Some objects necessary to life in the house are located outside, and activities and information about the weather e.g. from a weather forecast, within the house, can also change what is placed outside, such as garden chairs and a hosepipe. Similarly, a cell can change the ECM molecules it secretes or the adhesion receptors that are found on its surface. What is very clear is that items placed outside the house, or cell, greatly influence what goes on inside, and what goes on inside influences what is placed around the outside.

      And in the cell…
      And so it is with a cell and the extracellular matrix and cell adhesion molecules around it. Many properties of the cell surface and internal functions of the cell are dependent on proteins that extend from the cell surface into the ECM or to the surface of other cells. These proteins, rather like the satellite aerial, security camera and sensors of our house model receive messages about the immediate environment and exercise a surveillance function.

      In addition many of the proteins on the cell surface carry complex carbohydrate modifications . For this reason this area outside the cell has been called the glycocalyx (from the Greek ‘glycos’ meaning sweet, and the latin ‘calyx’ meaning cup). Like the protein components, the sugars are involved in adhesions between cells and, like the colour wash on a house, they also have a protective function.

      Extracellular Matrix – What is it?
      A general form is found widely distributed in animals. The two main groups of biochemicals that make up the basic ECM are complex chains of sugar molecules (polysaccharides) and polysaccharides joined to protein (glycoproteins such as fibronectin, laminin and thrombospondin) and include the very viscous substance proteoglycans. Embedded in this can be various types and amounts of structural and insoluble collagen fibres and flexible elastic fibres that give resilience to tissues.

      Modified forms appear in the form of bone, the exoskeleton of an insect, animal shells and the cell wall of plants.

      ECM – where does it come from?
      All cells can make extracellular matrix but certain specialist cells produce a specific type of ECM:
      Fibroblast cells secrete connective tissue ECM
      Osteoblast cells secrete bone-forming ECM and
      Chondroblast cells secrete cartlilage-forming ECM.
      Fibroblasts and epitheal cells together make basement membrane ECM

      ECM – what does it do?
      This depends on where it is and how specialised the ECM is. Different forms in different locations have different properties

      Specialised types of ECM in animals
      ECM can be modified, mainly by calcification to produce bones, teeth and shells or chitinisation to form the chitin exoskeleton of insects. These types of ECM clearly provide mechanical facility and protection.
      A less rigid type of ECM forms tendons and cartilage and a soft transparent gel form is found for example in the cornea of the eye where it provides hydraulic protection.

      Specialised ECM in plants
      The ECM in plants is mainly cellulose and surrounds each cell. Along with water it contributes to the total rigidity of the plant. The ability of a tree to grow to a great height and retain its rigidity is partly due to the cellulose ECM of the cell walls together with other biochemicals including lignin and extensins.

      A less easily observed form of ECM is found in vertebrates in three main forms

      1. Connective tissue – This contains lots of ECM and only a few cells.
      2. Basal lamina – This can be considered as the ECM of epithelial cells but formed into a tough layer containing a great many collagen fibres and laminin and upon which the cells of the epithelia ‘sit’. Very little ECM surrounds each individual cell and they are joined to each other in different ways.
      3. Pericellular matrix – With a few exceptions all cells are surrounded by cell extracellular matrix to some degree. It is this material that not only gives mechanical support by binding cells together but with the glycocalyx provides a biochemical barrier around the cell, a docking facility for imports and exports to and from the cell, and a medium through which chemical signalling can take place. Recent work indicates that ECM sugar molecules may have an important role to play in cancer biology.

      Cell Adhesion Molecules (CAMs)
      Very few cells exist and work in isolation. Most cells exist as a system or society. CAMs help to keep the society intact by providing different degrees and types of adhesion. Research work is indicating that CAMs, like ECM, is involved in cell signalling. CAMs are well suited to do this job since some of them traverse the plasma membrane and provide a route into the cell. The adhesive nature of the molecules also provides a ‘sticky surface’ and some of these inadvertently ‘capture’ RNA viruses such as those that cause common colds.

      Cell ‘Do It Yourself’ (DIY) – adhesives and junctions
      As with some ‘Do It Yourself’ (DIY) adhesives, CAMs are better at sticking some materials but they can also be used for joining others.
      There are four main families of CAMs (types of adhesive) and these are used in different situations:

      1. Those involved in Cell to Cell junctions are mainly molecules in the family called Cadherins and depend on the presence of Calcium ions to function (think of Ca-adhesion). These molecules are transmembrane glycoproteins and link the cytoskeleton of one cell to the cytoskeleton of another.
      2. Those involved in Cell to Matrix junctions belong to a large family of CAMs called integrins (think of integrins helping cells perform integration).
        Integrins are also found as ‘anchor’ plates in focal adhesion and hemidesmosome type junctions.
        Transmembrane proteoglycans are also involved in adhesion to ECM and the linkage to the cytoskeleton.
      3. The Immunoglobulin super family include special adhesion molecules used in the nervous system.
      4. The selectins are special CAMs that bind to cell-surface carbohydrate and are involved with inflammation response mechanisms.

      Junctions for adhesives
      Just as there are different types of cell adhesive molecules, there are different types of links or junctions. There are two main ones:

      1) Tight junctions – these do not allow molecules to pass from cell to cell but they pull the walls of the two cells very close together.

      2) Gap junctions – these join two cells together with a cluster of fine tubes. Gap junctions allow small molecules, up to a molecular weight of 1200, to pass from one cell to another. In this way cells pass chemicals to a neighbouring cell in need. An example of ‘The Society of Cells’ at work.

      Image of human epithelial cells with cadherin stained green and nucleus blue. The green staining cadherin is very widely distributed between these cells. This is why it appears that the plasma membrane is stained green.

      (courtesy of Louise Cramer, Laboratory for Molecular Cell Biology & Cell Biology Unit, University College London, UK and Vania Braga, Imperial College London, UK)

      CAMs and Cancer – a real life application
      ‘Cut’ and ‘Paste’ are critical commands in some cancers

      ECM and CAMs are involved in many disorders. In certain types of cancer CAMs may be involved in the spreading of cancer cells from a primary site to a secondary one. At the primary site cell to cell adhesion is lost. Cells are ‘cut’ free and transported away to a second site. Here cell adhesion is increased and the cell is ‘pasted’ into its new location. The cell divides and with better adhesion stays put and a secondary cancer develops. (This is a simple description but the principle is correct).
      Clearly the ability to understand and control ‘cut’ and ‘paste’ commands in the cancer growth ‘programme’ could help our understanding of how secondary cancers develop.

      Tissue and Cell

      Tissue and Cell is devoted to original research on the organization of cells, subcellular and extracellular components at all levels, including the grouping and interrelations of cells in tissues and organs. The journal encourages submission of ultrastructural studies that provide novel insights into.

      Tissue and Cell is devoted to original research on the organization of cells, subcellular and extracellular components at all levels, including the grouping and interrelations of cells in tissues and organs. The journal encourages submission of ultrastructural studies that provide novel insights into structure, function and physiology of cells and tissues, in health and disease. Bioengineering and stem cells studies focused on the description of morphological and/or histological data are also welcomed.

      Studies investigating the effect of compounds and/or substances on structure of cells and tissues are generally outside the scope of this journal. For consideration, studies should contain a clear rationale on the use of (a) given substance(s), have a compelling morphological and structural focus and present novel incremental findings from previous literature.

      Entropy plays an important role in how living cells form tissues

      The process that causes living cells to club together to create tissues is driven by both biochemistry and thermodynamics, according to a new study by an international team of scientists. The group’s experiments and computer simulations could help scientists improve technologies for creating artificial tissues and organs.

      Multicellular organisms from simple worms to complex mammals comprise tissues and organs that form via the organization of many single cells. This alignment of cells is driven by several processes, some that are biochemical and others that are related to cell-to-cell contact and other interactions with cell exteriors. While cellular alignment processes often have miniscule effects on individual cells, collectively they play a crucial role in the formation and health of tissues.

      Alignment often occurs in response to the anisotropy of the cells’ environment, and this results in the migration of cells along a specific direction. This is called “contact guidance” and plays important roles in both tissue growth and tissue homeostasis – the latter being the process by which tissue is maintained in a steady state. While scientists know that contact guidance is important, the underlying mechanism has been poorly understood until very recently.

      Biochemistry versus entropy

      Now, researchers in the UK, Netherlands, Iran, and Italy have shown that contact guidance can be driven by both biochemical and entropy related processes, depending on the degree to which the cells are confined in an anisotropic environment. Led by Vikram Deshpande at the University of Cambridge, the team placed human muscle cells (myofibroblasts) on substrates containing micropatterned channels made of fibronectin. This is a large glycoprotein that makes up the extracellular matrix of tissues. As well as mediating cell interactions, it also plays roles in cell adhesion, growth and migration.

      The cells were placed on the substrates at low densities so that cell-to-cell contact was avoided. The cells measure about 160 micron across and the team observed their behaviour in channels of three different widths – 50, 160 and 390 micron.

      The team found that cells in the narrower channels were aligned more than those in wider strips. In the narrower strips, the team concluded that contact guidance occurred because the cells must change their shapes and energy to adjust to the narrower environment — processes that are driven by the biochemical processes within the cells.

      What is happening is a little bit counterintuitive

      Vikram Deshpande

      What surprised the scientists, however, is that contact guidance also occurred in channels much wider than the size of the muscle cells. In this case, the researchers say that the process is driven by an increase in entropy – the thermodynamic tendency of the system to move towards disorder.

      “What is happening is a little bit counterintuitive,” explains Deshpande, “You can think that in an aligned system is not maximally disordered, but actually in this case, the maximally aligned system is the most disordered one”.

      He says that the phenomenon can be understood by imagining a few matches in a matchbox. If you shake the matchbox, instead of taking a random orientation, the matches would align themselves along the edges of the box. Analogously, cells aligned along the anisotropy of their environment represent a system with a higher entropy.

      “There are certain factors that you can experimentally measure, such as the traction, or investigate the shapes to look at the size of their cytoskeletal arrangements. But there are certain features in understanding cellular behavior that are not directly measurable,” added Deshpande. “This is why we also simulated the Gibbs free energy of the cells, to go beyond the experiments.”

      Critical width

      The team combined the analysis of the cells’ shapes with a statistical analysis of their fluctuations not related to temperature. The resulting model also predicted, that upon increasing the channel width above a certain critical value, the cell orientation would not be driven by its internal biochemistry, but rather by entropy.

      The results could have important implications for healthcare, medicine and tissue engineering – which could be achieved by manipulating the shape and organization of cells by changing the geometry of their environment. A better understanding of contact guidance could also help doctors predict the spread of diseases such as metastasizing cancer.

      Human immunity is perhaps a touchy-feely process

      While the experiment was done on a flat 2D surface, the team is already working on expanding their research to encompass more life-like conditions. “In lots of cases inside the body, surfaces are not flat and the cells are not growing on a flat surface either,” says Deshpande. “We are really interested in understanding how effectively curvature is a driving cue for guiding cells and why do different kinds of cells respond differently to various surfaces and curvatures.”

      According to Patrick McGarry from the National University of Ireland Galway, this study “provides a ground-breaking insight into the thermodynamics of biological cells”. McGarry, who was not involved in the research adds, “The seminal finding that entropy is a key driver of cell alignment is fundamental to the assembly and function of living tissue and has highly important implications for the field of regenerative medicine”. He adds, “The work provides a new paradigm for the fusion of thermodynamics, biology, and computational mechanics, leading to a new understanding of the active response of living matter to the surrounding physical environment”.

      The results are reported in the Biophysical Journal.

      Martina Ribar Hestericová is a science writer based in Switzerland

      Watch the video: Intro til cellen (October 2022).