Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 29

called the cytosol and it contains many enzymes, ribosomes, and proteins, as well
as the cytoskeleton. The main organelles are the nucleus, the Golgi apparatus, the
endoplasmic reticulum, and the mitochondria. DNA replication and messenger RNA
synthesis occur in the nucleus, the place where the continuity of the cell's different
functions is ensured. Ribosomes, which are either bound to the endoplasmic retic-
ulum or not, carry out the replication of proteins. The Golgi apparatus is the site
of protein-lipid or protein-sugar coupling and acts as a filter for proteins to direct
them toward their final destination. Lysosomes specialize in intracellular digestion.
Vesicles ensure the transport of products (either synthesized or degraded) within the
cell or to its periphery so that they can be secreted to the cell's exterior. Cellular
compartments are connected to each other and two-way exchanges occur.
The plant cell presents a microstructure similar to that of the animal cell, but is
distinguished by two essential differences: the presence of a cellulose wall lining the
extracellular membrane and the presence of chloroplasts in the aerial parts. The cel-
lulose wall thickens and stiffens the phospholipid membrane. The plant cell presents
different hardnesses and fragilities. Furthermore, the wall isolates each cell from its
neighbors or from its surroundings. Because of this, each cell can have a different
ionic or molar composition than its neighbor, which is not the case with animal cells.
The thick walls of plants are true barriers, limiting exchanges with the outside. This
poses a problem in that it is difficult for the fixing agent or the inclusion resins to
penetrate the cell wall. The chloroplast is the site of photosynthesis.
The plant cell has heterogeneous areas in terms of hardness that are sometimes
very impermeable and therefore difficult to embed correctly.
Starting from a simple animal cell, we find increasing complexities of the
collections of cells composing tissues.
Epithelial tissues and nerve tissues are homogeneous and may be considered as
multilayered, often having a polarity (top to bottom).
Muscle tissue is a fibrous and textured tissue.
Connective tissue is a support tissue. It is heterogeneous and more or less textured
by the presence of collagen fibers. It ranges from slightly textured to mineralized. It
makes up cartilage, bone, dentine, shells, etc.
In plants, the organism's rigidity and support come from lignin, which reinforces
the cell wall, particularly in the vessels found in wood. These materials have a poorly
organized texture compared to crystalline materials, for example.
Often these different tissues are combined to constitute organs, and the result is
a mixed-composite material (bone/cells, epidermis/dermis, parenchyma, and con-
ductive vessel in a plant, for example). Some structures are intended to ensure the
continuity of the species and are particularly resistant (e.g., cysts in animals and
spores or nodules in plants).
Yeasts are single-cell eukaryotes with a highly individualized nucleus and
organelles. Bacteria are prokaryotes in which there is no well-defined nucleus or
organelles. Yeasts and bacteria have an external membrane that is reinforced by a
wall (usually a glycoprotein wall) that enables them to resist harsh external condi-
tions, particularly desiccation. This wall differs in nature depending on the species;
it is often difficult to penetrate, making sample preparation for these organisms