Medical Physiology/Cellular Physiology/Cell junctions and Tissues

Introduction

Cells are organized to form tissues, and tissues are organised to form organs. Tissues are formed by an aggregation of like cells carrying out a like function. Cells are linked together by cell junctions and are supported by a matrix which they themselves secrete.

The cell junctions are basically of three types, tight, desmosomal, and gap. We will look at these in more detail in the next section.

Histologists classify the tissues according to their physical features, physiologists tend to classify tissues acording to their function. There are four major types of tissue : epithelial; connective; neural; and muscular. The following table summarizes the difference between these tissues.

**Summary of Tissue Properties**
	Features	Matrix	Other Characeristics
Epithelial	Organized in sheets. Provides covering: Lines body surface, hollow organs, cavities and tubes. Provides tissues for secretory glands.	Matrix is minimal in ammount, forming the basement membrane of the sheets of epithelial cells.	Cells are polarized with a 'surface' and 'basement' side. The surface side may have special characteristics such as cilia or microvilli. The membrane functions also often differ.
Connective	Characterised by a varied and extensive matrix. Cells are usually scattered irregularly in this matrix. Cell shape is irregular to round.	Matrix is varied consisting of numerous protein fibers in ground substance. Consistency varies from liquid (blood) to gelatinous; from firm (fibrous tissue,cartilage) to solid (bone).	Found throughout body, forms the support and structure for organs and the body itself.
Nerve	Cells can generate electrical signals. Highly branched.	Neural tissue has no matrix. Support is provided by other specialized neural cells.	Found throughout body with concentrations in the brain, the spinal cord and the enteric nervous system of the gut.
Muscle	Cells can generate electrical signals, which result in contraction. 'Voluntary' muscle makes up skeletal muscles; Cardiac muscle powers the heart; smooth muscle surrounds hollow organs and tubes.	Muscular tissue does not secrete its own matrix. Skeletal muscle is supported by fibous tissue, and is attached to tendons.	Intestinal Smooth muscle and Cardiac muscle both have cell to cell communication.

Cell Junctions

Transmission electron microscope image of a thin section cut through the developing brain tissue (telencephalic hemisphere) of an 11.5 day mouse embryo. This higher magnification image of "Embryonic brain 80415", shows an area of the luminal surface of the telencephalon, which has a junctional complex and pinocytotic vesicles. The junctional complex is divided into three types of junctions: 1) the most apical is the tight junction, which controls and/or restricts the movement of molecules across epithelial layers and helps maintain polarity, 2) the zonula adherens and 3) the desmosome, which is a spot junction. The pinocytotic vesicles are formed from coated pits in the plasma membrane and are involved in endocytosis.

It is the junctions that allow individual cells to be organized into tissues. Numerous proteins called cell adhesion molecules (CAMs) have been identified, and the illustrations shown really represent a generqalization. These proteins are being intensively studied because of their importance in cancer and the embryonic development organization of the nervous system. According to Ganong the proteins can be divided into four large groups, integrins, IgG superfamily, cadhedrins and selectins. A Biochemistry text should be consulted of more information on these proteins.

The tight junctions can be found at the apexes of many lumen epithelial cells, and forms a 'water tight' occlusion that prevents leaking of water, substrates and ions from the extra cellular fluid to the lumen. That having been said, there is evidence that some tight junctions are more 'leaky' than others, and that this 'leakiness' is under hormonal control. Desmosomes and Zona Adherens junctions hold cells together.The Zona Adherens is a continuous band, whereas Desmosomes are more in the nature of a 'spot weld'. They allow passage of some substances between the cells. Gap junction give direct communication from cell to cell. They are found in many epithelial cells, but most famously they exist in Cardiac muscle and intestinal smooth muscle

Electron Micrograph of negatively stained en:Proximal convoluted tubule of Rat en:Kidney tissue at a magnification of ~55,000x and 80KV. This is a close-up of the en:Zonula occludens (en:Tight junction) out of a larger image of a Kidney Tissue with Tight junction.

Tight (Zona Occludans) Junctions

Zona Adherens Junctions

Desmosome Spot Junctions

Gap Junctions

Epithelia

Epithelia from Colon

Normal epidermis and Dermis

Ciliated columnar epithelium

Microvilli-Duodenum

Histologists classify epithelia cells by appearence. Single layer epithelias is called 'simple'; multilayer is called stratified. The diagram below shows a typical histological classification. Plain light microscope appearances appear in the thumbnails to the right.

Histological classification of Epithelia

Illustration showing layers of the skin

Illustration showing an enterocyte, a small intestine epithelial cell. These cells are bound by tight junctions, depicted by the yellow ovals in the illustration. Note the microvilli which increase the absorptive area about a hundredfold.

As physiologists we find it more useful to classify epithelia by function:

Exchange Epithelia
Transport Epithelia
Protective Epithelia
Ciliated epithelia
Secretion epithelia

A brief description is given below. More detail is given in the sections dealing with different orga types.

Exchange Epithelia

Thin flattened cells that allow the easy exchange of gasses. These are found in he capillaries and the lungs.

Transport Epithelia

These epithelia are found lining the tubes of the intestine and the kidney.

The cells are usually columnar and regulate the exchange of nutrients an ions.
The apical surface usually has microvilli, fingerlike projections that can increase the absorptive area up to 100X.
These cells are usually bound together with tight junctions
These cells usually have a high metabolism (numerous mitochondria)

Protective Epithelia

Prevent exchange between the internal and external environment, examples include:

skin
linings of upper respiratory system
mouth
oesaphagus
urethra
vagina

Ciliated epithelia

Line parts of the respiratory system - where they propel mucous laden with particulate matter out of the lung and the fallopian tube - where they waft the ovum to the uterus.

Secretion epithelia

Form either exocrine or endocrine glands

Connective Tissue

Connective tissue can be classified into the following groups:

Loose Connective Tissue
Dense Irregular Connective Tissue
Dense Regular Connective Tissue
Adipose Tissue
Blood
Cartilage
Bone

Detailed histology is really outside the scope of this book. Physiologists need to know enough to under stand function. Brief descriptions are given below, fuller descriptions will be given in the appropriate sections on systems. A good histology book is recommended for more a detailed description.

Loose Connective Tissue

Loose Connective Tissue has a gel like ground substance with more ground substance than fibers or gel. The main cell type are fibroblasts, and it is foundaround blood vessels and organs and under the epithelia. What anatomists call 'loose fasia' is mainly Loose Connective Tissue.

Dense Irregular Connective Tissue

Dense Irregular Connective Tissue has more fibers than ground substance. The fiber type is mainly collagen, and the dominant cells are fibroblasts. Dense Irregular Connective Tissue forms the sheaths of nerves and muscles. Anatomists 'organized fascia' is of this type.

Dense Regular Connective Tissue

Parallel fibers of collagen. This connective tissue makes up ligaments and tendons

Adipose Tissue

Fat cells are found distributed throughout the body. There is very little ground substance

Blood

This will be reviewed in detail in the sections on Blood and immunology

Cartilage

Hyaline cartilage

Cartilage in polarized light

There are really two kinds of cartilage:

Hyaline Cartilage
Fibro cartilage.

Hyaline Cartilage is found lining the joints and is mainly firm hyaluronic acid. Fibro cartilage is found in the ear, the septum of the nose, and tracheal rings. A characteristic of cartilage is that it has no blood supply.