School of Life Sciences

The Sussex Centre for Advanced Microscopy

Interpretation of Cellular Ultrastructure in the TEM

(Alternative Version for the Younger Reader)

N.B. The text contains links to images of (and potted notes on) the various organelles (via highlighted text). There are also some links within this page to Jim Smith's website at McGill University, California. Many thanks to him - I strongly recommend that you have a look at his site which was put together for a Cell Biology course. Check it out HERE!

Very simplified representation of an animal cell and its major organelles

The TEM examination of ultrathin sections allows a complete elucidation of the ultrastructure, or fine structure, of the cell. All the cellular organelles may be seen in detail. The most important thing to remember when attempting to interpret the observed ultrastructure is that it is only a 2-dimensional representation of something which is of course very 3-dimensional in nature. Organelles may be sectioned in many different planes and can have a quite different appearance as a result. Additionally, there is the fourth dimension of time. Cells are dynamic entities; organelles may be forming, maturing , moving about or dividing within the cytoplasm. There is also a continuing process of membrane turnover.

In other words, trying to interpret a single TEM image (micrograph) is akin to examining a single frame from a movie. You could not interpret from this single frame either the plot of the film or the characters involved or what they are up to. In fact, in all probability you may not see some of the characters at all! Therefore, sections need to be examined in some detail, and with experience, may be interpreted correctly.

The PLANT CELL differs from the animal cell in that it possesses a CELL WALL external to its outer cellular (plasma) membrane. Cytoplasmic continuity is maintained between neighbouring cells via plasmodesmata (small channels through the cell wall). The VACUOLE often occupies a large volume within the cell. A plant is dependent for its turgidity upon the effect (if the plant is adequately watered) of the turgor pressure of the vacuole pushing out the cytoplasm against the rigid cell wall. Photosynthetic parts of a plant will also contain CHLOROPLASTS; these possess a double membrane enclosing the stroma in which stacks of thylakoids form grana (these contain the photosynthetic apparatus of the organelle).

N.B. ALSO SEE IMAGE GALLERY AND THE 'DIFFERENCES BETWEEN AN ANIMAL CELL AND A PLANT CELL' (C/O SCIENCENET DATABASE).

Otherwise, at the ultrastructural level, plant and animal cells appear quite similar. Each cell is surrounded by the outer cellular or PLASMA MEMBRANE . The NUCLEUS is surrounded by a double membrane (the nuclear envelope) which is perforated in places by nuclear pores. Within the nucleus is the chromatin which may appear densely- (heterochromatin) or lightly- (euchromatin) stained. Nucleoli (the sites of ribosomal RNA synthesis) are visible usually as spherical regions which are granular and fibrillar in nature.

The rest of the cell is occupied by cytoplasm which contains a range of organelles. The latter are visibly-identifiable structures which have different functions within the cell.The outer nuclear membrane may be continuous with the ROUGH ENDOPLASMIC RETICULUM (RER) in places. It is termed 'rough' because it has ribosomes (often in spiral aggregates called polyribosomes or polysomes) on the outer surface of its membrane. Ribosomes (and polysomes) may also be 'free' within the cytoplasm. Smooth ER has no ribosomes associated with it and is involved in lipid and sterol synthesis. The GOLGI BODY (or Apparatus) within a cell is thought to be an interconnected system, but in thin sections appear as discrete units (termed dictyosomes in plant cells). These units may be compared to a stack of saucers, with each saucer being one sac or cisterna (membrane-enclosed space) of the Golgi. At the periphery of these cisternae secretory vesicles bud off via tubular extensions. The MITOCHONDRIA have two membranes, the inner one of which invaginates to form the cristae; within this inner membrane is the matrix of the mitochondrion.

All the above organelles of the cell are surrounded by the 'CYTOSKELETON', an extremely fine network of proteinaceous strands (microfilaments and microtubules) which, as well as forming a structural 'skeleton' for the cell, are involved in morphogenesis and motility (e.g. targeting of secretory vesicles to the cell membrane).

In general terms the organelles exist to compartmentalise away discrete biochemical functions within the cell. For example, within the confines of the mitochondria conditions are maintained which are optimal for the enzymes responsible for oxidative phosphorylation and ATP synthesis. Organelles also provide a structural framework for the specific biochemical processes which occur within them (e.g. enzyme attachment to organised membranes in the mitochondria and chloroplast).

The organelles should in no sense, however, be imagined to operate in isolation as there is a continuous exchange of metabolites, etc. between those which are functionally-related. Indeed, some of the organelles (such as the ER and Golgi) are very closely functionally and physically-related. Returning to the idea of the dynamic nature of the cell, there is what is termed the 'endomembrane concept' of membrane 'flow' or turnover. The ER is generally considered to be the major site of membrane biogenesis (formation) and from here (and the outer nuclear membrane, to which the ER contacts in places) there is a functional continuum involving the physical transfer of membrane to the Golgi and thence to Golgi-derived secretory vesicles and to the plasma membrane (and/or the tonoplast [vacuolar membrane] in plants). During this process, for example, specific proteins may be assembled in the (rough) ER and then various enzyme activities through the ER and Golgi will result in the required end-product being packaged ultimately into SECRETORY VESICLES for export.