Cell Recognition and Adhesion

Some organisms, such as bacteria, are unicellular; that is, the entire organism is a single cell. Others, such as plants and animals, are multicellular—composed of many cells. Often these cells exist in specialized blocks of cells with similar functions, called tissues. Your body has about 60 trillion cells, arranged in different kinds of tissues such as muscle, nerve, skin, and so forth. Two processes allow cells to arrange themselves in groups:

► Cell recognition, in which one cell specifically binds to another cell of a certain type

► Cell adhesion, in which the relationship between the two cells is "cemented"

Both processes involve the plasma membrane. They are most easily studied if the cells in a tissue are separated into individual cells, then allowed to adhere to one another again. Simple organisms provide a good model for the complex tissues of larger species.

A living sponge is a multicellular marine animal with a simple body plan (see Chapter 32). The cells of the sponge are stuck together, but they can be disaggregated mechanically by passing the animal several times through a fine wire screen. What was an animal is now hundreds of individual of cells, suspended in seawater. Remarkably, if the cell suspension is shaken for a few hours, the cells bump into one another and stick together in the same shape as a sponge! The cells recognize and adhere to one another.

There are many different types (species) of sponges. If disaggregated cells from two different species of sponge are placed in the same container, the cells of the two species float around and bump into one another. But the cells of each species stick only to other cells of the same species. Two different sponges form, just like the ones at the start of the experiment.

Such tissue-specific and species-specific cell adhesion is essential in the formation and maintenance of tissues and multicellular organisms. Think of your body. What keeps muscle cells bound to muscle cells and skin to skin? This is so obvious a characteristic of complex organisms that it is easy to overlook. You will see many examples of specific cell adhesion throughout this book; here, we describe its general principles. As you will see, cell recognition and adhesion depend on membrane proteins.

Cell recognition and adhesion involve proteins at the cell surface

The molecule responsible for cell recognition and adhesion in sponges is a huge membrane glycoprotein (80% sugar) that is partly embedded in the plasma membrane, with the recognition part sticking out and exposed to the environment (and to other sponge cells). As we saw in Chapter 3, a macro-molecule such as a protein not only has a specific shape, but also has specific chemical groups exposed on its surface

(a) Homotypic binding

(a) Homotypic binding

Sponge Molecule Photo
Tissue from a red sponge contains similar cells bound to each other.

JjJ The sponge tissue can be disaggregated into single cells by passing it through a fine mesh screen.

JjJ The sponge tissue can be disaggregated into single cells by passing it through a fine mesh screen.

Exposed regions of membrane proteins bind to each other...

Recognition Groups Cell Membrane

^ ...causing cells to adhere

Exposed regions of membrane proteins bind to each other...

^ ...causing cells to adhere

Substances Interact With Each Other

where they can interact with other substances, including other proteins. Both of these features allow binding to other specific molecules (Figure 5.5a).

In most cases, the binding of cells in a tissue is homotypic; that is, the same molecule sticks out of both cells, and the exposed surfaces bind to each other. But heterotypic binding (of cells with different proteins) also can occur (Figure 5.5b). For example, when the mammalian sperm meets the egg, different proteins on the two types of cells have complementary binding surfaces. Similarly, some algae form similar-appearing male and female reproductive cells (analogous to sperm and eggs) that have flagella to propel them toward each other. Male and female cells can recognize each other by heterotypic proteins on their flagella. In the majority of plant cells, the plasma membrane is covered with a thick cell wall,

(b) Heterotypic binding

+ Mating type

Pi These gametes from a marine alga look identical but have different cell surface proteins.

+ Mating type

Cell Membrane Recognition
— Mating type
Plasma Cell Membrane

to each other by complementary protein binding.

I The gametes adhere

5.5 Cell Recognition and Adhesion (a) In most cases (including the aggregation of animal cells into tissues), protein binding is homotypic: molecules of the same protein occur on the surfaces of two cells and adhere to each other. (b) Heterotypic binding occurs between two different but complementary proteins.

I The gametes adhere

Complementary Proteins

to each other by complementary protein binding.

but this structure, too, has adhesion proteins that allow cells to bind to one another.

Cell adhesion proteins from many multicellular organisms have been characterized. Some of them do not just bind the two cells together, but initiate the formation of specialized cell junctions. In this case, the functions of cell recognition and cell adhesion reside in different molecules.

that "cement" their relationship. These specialized structures, called cell junctions, are most evident in electron micrographs of epithelial tissues, which are layers of cells that line body cavities or cover body surfaces. We will examine three types of cell junctions that enable cells to make direct physical contact and link with one another: tight junctions, desmo-somes, and gap junctions.

Specialized celljunctions

In a complex multicellular organism, cell recognition proteins allow specific kinds of cells to bind to each other. Often, both cells contribute material to additional membrane structures tight junctions seal tissues and prevent leaks. Tight junctions are specialized structures at the plasma membrane that link adjacent epithelial cells. They result from the mutual binding of strands of specific membrane proteins, which form a series of joints encircling each epithelial cell (Figure 5.6a). They are found in the region surrounding the

Plasma membranes

Intercellular space

Junctional protein

Tight junctions bar the movement of dissolved materials through the space between epithelial cells. There is no intercellular space where there is a tight junction. Long rows of tight-junction proteins form a complex meshwork, seen at the bottom of the freeze-etched image.

Plasma membranes.

Intercellular, space

Lumen of intestine

Plasma membranes.

Intercellular, space

Cytoplasmic plaque

Adhesion protein

Keratin fiber

Desmosomes tightly link adjacent cells but permit materials to move around them in the intercellular space. Anchored in dense plaques, cell adhesion proteins cross the intercellular space, binding adjacent cells together. Keratin fibers extend through the cytoplasm from one plaque to another.

Lumen of intestine

Tight junctions bar the movement of dissolved materials through the space between epithelial cells. There is no intercellular space where there is a tight junction. Long rows of tight-junction proteins form a complex meshwork, seen at the bottom of the freeze-etched image.

Cytoplasmic plaque

Adhesion protein

Keratin fiber

Desmosomes tightly link adjacent cells but permit materials to move around them in the intercellular space. Anchored in dense plaques, cell adhesion proteins cross the intercellular space, binding adjacent cells together. Keratin fibers extend through the cytoplasm from one plaque to another.

Gap Junction And Metastasis

Plasma membranes membranes

Connexons

Gap junctions let adjacent cells communicate. Dissolved molecules and electric signals may pass from one cell to the other through the channel formed by two connexons extending from adjacent cells.

Plasma membranes membranes

5.6 Junctions Link Animal Cells Together (a,b) Tight junctions and desmosomes are abundant in epithelial tissues. (c) Gapjunctions are also found in some muscle and nerve tissues, in which rapid communication between cells is important.

Connexons

Gap junctions let adjacent cells communicate. Dissolved molecules and electric signals may pass from one cell to the other through the channel formed by two connexons extending from adjacent cells.

lumen (cavity) of organs such as the intestine. Tight junctions have two functions:

► They prevent substances from moving through the spaces between cells. Thus, any substance entering the body from the lumen must pass through the epithelial cells.

► They restrict the migration of membrane proteins and phospholipids from one region of the cell to another.

Thus, the proteins and phospholipids in the apical (tip) region of the cell facing the lumen can be different from those in the basolateral regions facing the sides and bottom of the cell (basolateral: basal = bottom; lateral = side).

By forcing materials to enter certain cells, and by allowing different ends of cells to have different membrane proteins with different functions, tight junctions help ensure the directional movement of materials into the body.

desmosomes hold cells together. Desmosomes are also specialized structures associated with the plasma membrane. They hold adjacent cells firmly together, acting like spot welds or rivets (Figure 5.6b). Each desmosome has a dense structure called a plaque on the cytoplasmic surface of the plasma membrane. This plaque is attached to special cell adhesion proteins in the plasma membrane. These proteins stretch from the plaque through the plasma membrane of one cell, across the intercellular space, and through the plasma membrane of the adjacent cell, where they bind to the plaque proteins in that cell.

The plaque is also attached to fibers in the cytoplasm. These fibers, which are intermediate filaments of the cy-toskeleton (see Figure 4.20), are made of a protein called keratin. They stretch from one cytoplasmic plaque across the cell to connect with another plaque on the other side of the cell. Anchored thus on both sides of the cell, these extremely strong keratin fibers provide great mechanical stability to epithelial tissues, which often receive rough wear in protecting the organism's body surface integrity.

gap junctions are a means of communication. Whereas tight junctions and desmosomes have mechanical roles, gap junctions facilitate communication between cells. Each gap junction is made up of specialized protein channels, called connexons, that span the plasma membranes of two adjacent cells and the intercellular space between them (Figure 5.6c). Dissolved molecules and electric signals can pass from cell to cell through these junctions. We will describe their role in more detail, as well as that of plas-modesmata, which perform a similar role in plants, when we discuss cell communication in later chapters, especially in Chapter 15.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • gordon
    Do sponges have cell recognition?
    7 years ago
  • ABDULLAH MEDHANIE
    What is the difference between cell recognition and cell adhesion?
    7 years ago
  • Asfaha
    What is an analogy for plasma membrane spongebob?
    7 years ago
  • KAROLIINA
    What part of the cell membrane is cellular adhesion and recognition?
    7 years ago
  • Adalberta
    What type of junction acts like rivets or "spot welds"?
    7 years ago
  • drew martin
    What structure of plasma membrane is involved in cell to cell regognition?
    7 years ago
  • antti
    What special structures keep two neighboring cells stuck together?
    7 years ago
  • Michele Bowers
    What is cell to cell recognition of membrane proteins?
    7 years ago
  • fnan kiros
    Where are nerve fibers located on a plasma membrane?
    6 years ago
  • garry
    Where the plasma membrane of neighboring cells have been bound together to form an impenetrable?
    6 years ago
  • kaarina
    What does proteogans, protein fibers and cell adhesion proteins have in common?
    6 years ago
  • aaron
    What are recognition substances in the plasma membrane composed of?
    6 years ago
  • Hiwet
    What are exposed to the extracellar surface for cell recognition and adhesion to other cells?
    6 years ago
  • MAKDA BISRAT
    What enables the single cell sponge to adhere to other cells of its kind?
    5 years ago
  • RUBY
    What occurs in cell cell recognition?
    4 years ago
  • Fern
    Which plasma membranes of two cells locked together?
    4 years ago
  • bellisima
    Is simple epithelial tissue stuck together with intercellular substance li9ke cytoplasm?
    3 years ago
  • christopher
    What is the function of Plasma membrane of sponge?
    3 years ago
  • Terttu
    What is responsible for cell to cell recognition?
    3 years ago
  • lena
    What is adesion and recognization?
    2 years ago
  • patricia
    How does celltocell recognition by membrane plasma proteins occur?
    2 years ago
  • uranio
    What is cell to cell recogination?
    2 years ago
  • Severi
    What the function of recognition in plasma membrane?
    2 years ago
  • aloisa conti
    What is the relationship between enzyme and cell recognition?
    2 years ago
  • Amalda
    How does plasma membrane help in recognition of cell?
    2 years ago
  • Ensio Narjus
    Which protein is responsible for cell cell recognition?
    2 years ago
  • P
    How is the function of cell membrane is cell recognition?
    1 year ago
  • LELIO
    How does the membrane functions respond to cell to cell recognition?
    1 year ago
  • orso
    How a plasm membrane allow cell recognition?
    12 months ago
  • Genet
    What is intercellular recognisation?
    11 months ago
  • tellervo
    What is function of cell recoginition?
    10 months ago
  • temesgen
    What is cellcell recognation?
    10 months ago
  • CARAMELLA
    What part of a cell is responsible for cell to cell recognition?
    9 months ago

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