An Overview of Human Cells for Light Microscopists
A 3D modelling article
by Mol Smith 2010
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  Generic Human Cell  Neurons  Synapses    Resources and external links
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The Human Cell: the miracle of life
Since there are quite a few 3D models in existence to help illustrate the internal structure of individual human cells, I thought an article about cells would be of interest to light microscopists - especially as we only get to see them as stained, flat, one-dimensional objects. I think that an in-depth discussion of each cell, is far too specialised an area for me to cover here, but hopefully I can point out a few things on interest. Possibly, seeing the cells as 3D objects close-up, will provide greater insight into what can only be described as the miracle of life.


A Generic Human Cell
Each cell in the human body (and animals) is specialised but most will contain structures and processes common to all. Fig. 1 below, which has been derived from an inexpensive 3D model, clearly shows some of the more important processes. A brief explanation of the function of each of these follow Fig 1.

Plasmic Membrane
This is the cell surface membrane, completely encasing all internal processes. Made of two layers of lipid (bilayer) sandwiched between two protein layers, it forms a partially permeable barrier - controlling exchanges (gas) between the cell and the exterior environment. Numerous proteins are also present in the membrane acting as sensors for taste and hormones, as are tiny pores to control the essential entry and exit of irons, e.g. chloride, sodium, potassium, and calcium.

Endoplasmic Reticulum
There are two types of Endoplasmic Reticulum: rough and smooth - the former, so called because of the presence of ribosomes on its surface, displaying a 'pebbled' surface in a scanning electron microscope image. Both types are flattened membrane-bounded sacs called cisternae. The 'smooth' type is the site of lipid and steroid synthesis. The 'rough' ER (ribosomes on surface) transports ribosomes through the cisternae.

Ribosomes
These small organelles, consisting of a large and a small submit, are made of RNA and proteins in approximate equal parts. They are the site of protein synthesis, holding in place various interacting molecules. Long protein chains are formed at the the intersection of the large and small submit.

Mitochondria
The fuel-cell of living cells, mitochondria combine sugar and Oxygen to provide ATP (Adenosine triphosphate *wiki) - the power source for living entities. They are about the size of an average-sized bacterium.

Lysosomes
These are simple spherical sacs containing digestive enzymes, and they are concerned with breaking down structures of molecules.

Golgi
Commonly called the Golgi Apparatus, this stack of flattened membrane sacs (cisternae), continuously forms at one end of the stack and buds off as vesicles at the other. Their function is the processing and transport of many cell materials including enzymes.

Centrioles
These are small hollow cylinders (0.3m - 0.5m long) which occur in pairs within the cell cytoplasm (originating in an area called the centrosome). Each tube is constructed of 9 triplets of microtubules, and is thought that adjacent triplets may be attached to each other by fibrils. The centrioles replicate themselves at the beginning of nuclear division, and the two new pairs migrate to opposite ends of the spindle: the structure on which the chromosomes become aligned.

The diagram below shows a generic human cell cut-away to reveal the internal processes described above. this diagram should help you relate the cell components to my 3d model image above.

Please note: the diagram was authored by MesserWoland and Szczepan1990 15 October 2006(2006-10-15), created with Inkscape, based on the graphics from en wiki. http://en.wikipedia.org/wiki/File:Biological_cell.svg
Permission: (Reusing this file) copyright Multi-license with GFDL and Creative Commons CC-BY-SA-2.5 and older versions (2.0 and 1.0)



Diagram of a typical animal cell. Organelles are labelled as follows:

1 -Nucleolus *wiki
2 -Nucleus
*wiki
3 -Ribosome
*wiki
4 -Vesicle
*wiki
5 -Rough endoplasmic reticulum
*wiki
6 -Golgi apparatus (or "Golgi body")
*wiki
7 -Cytoskeleton
*wiki
8 -Smooth endoplasmic reticulum
*wiki
9 -Mitochondrion
*wiki
10 -Vacuole
*wiki
11 -Cytosol
*wiki
12 -Lysosome
*wiki
13 -Centriole
*wiki

The Cell Nucleus
I have omitted the cell nucleus from my model image above for reasons of clarity. The image below shows a cut-away model representation of the Cell Nucleus. Note: I have made the nucleus membranes semi-transparrent so you can peer inside the nucleus itself!

The Nucleus is the largest organelle within the human cell. it is enclosed by an envelope of two membranes which are perforated by nuclear pores (not visible in the model above). It contains chromatin - an extended form of chromosomes during interphase, and a nucleolus, which manufactures RNA.

Life Span of Human Cells
It is interesting to note that with the exception of most (not all) cells of the brain, your entire body, and what constitutes it, is completely replaced a number of times throughout your life-time. Cells throughout the body continue to renew themselves by a process of replication. This is done at a different rate depending on the life-span of the cell type. The brain is the only organ where this process of replication does not occur (except for the cells - neurons - in the hippocampus). Fortunately, although the number of neurons within the brain begin to decline due to cell death from around the age of 28 years, it is hardly noticed until our very late years due to the presence of around 100 billion Neurons at birth.

Here is a list of some of the cell life-spans of the human body

Sperm cells     2-3 days

Epithelia of small intestine   1 week or less

Skin epidermal cells    2 - 4 weeks

Lymphocytes    2 months - a year (highly variable)

Red blood cells    4 months


For a more comprehensive list, please visit: http://vitanetonline.com/forums/1/Thread/1001


Here is a tiny animated movie I made showing the generic human cell spinning.





We have taken a fair look at a generic huuman cell hand can gain a good appreciation of it through the diagrams and 3D models used. Now we can take a similar look at a few of the specialised cells in the human body using similar techniques. Let's
take a closer look here at a Neuron!

     
     


Comments to the author
Mol Smith are welcomed.

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