The basic principles of histology and cell structure serve as an important background for the study of specific cells, tissues, and organ systems. This laboratory serves as an introduction to the rest of the course – you will use the principles you learn here in every subsequent lab.

Cells are difficult to see by light microscopy. To help us visualize the structure and features of cells, dyes are used to impart a particular color to cells. These dyes react with different chemical features of proteins, nucleic acids and carbohydrates and can be used to highlight certain cellular structures.

Hematoxylin and eosin (H&E) staining is the standard method of staining in histology. Hematoxylin is a basic dye (positively charged) that binds to negatively charged DNA and RNA and is blue in color. Eosin is pink in color and is an acidic dye (negatively charged) that binds to positively charged particles like the mitochondria and many components of the cytoplasm. Positively charged structures are therefore said to be "eosinophilic." While H&E staining is widely used, it is limited in its ability to differentiate between cytoplasmic organelles and many other tissue components.

The periodic acid-Schiff method (PAS) is useful for staining structures rich in polysaccharides (glycogen), mucopolysaccharides (ground substance, basement membrane, mucous), glycoproteins (thyroglobulin), and glycolipids. In this method, periodic acid oxidizes 1,2-glycols and 2,2-amino alcohols to aldehydes, which are then stained reddish purple by the Schiff reagent.

Osmium staining blackens lipids and stains the Golgi apparatus under the light microscope. It is also used as a fixative for electron microscopy.

The cell is the fundamental unit of living organisms. Cells grow, adapt to their environment and reproduce, processes which characterize life. Cells also assemble into groups to form complex structures. Cells and the extracellular material they make comprise the tissues of our bodies. Several different types of tissues then organize to form organs. The cells in an organ communicate and work together to perform the functions of that organ.

The cell is limited by the cell membrane, also known as the plasma membrane. The cell’s content is divided into two main compartments: the nucleus and the cytoplasm that surrounds the nucleus. Cytoplasm is further divided into organelles, cytosol and inclusions. Organelles are assemblies of specific macromolecules organized to carry out complex functions. Many organelles are surrounded by a membrane that separates their internal environment from the cytoplasm. Membrane-bound organelles concentrate enzymes and reactants, increasing biochemical efficiency and isolating harmful proteins and molecules from the rest of the cell. Cytosol is a gel-like substance that contains dissolved macromolecules, organic compounds and ions. In addition, cytosol contains the cytoskeleton (microtubules, actin filaments and intermediate filaments) that organize the organelles and provide mechanical support. Lastly, inclusions are insoluble substances in the cytosol such as glycogen and lipid droplets.

One of the most important skills that you will take away from this course is the ability to understand the dimensions of cells and their subcellular components. When it comes to size, there are three key facts that you must remember:

With the unaided eye, one can only see exceptionally large cells, such as the human ovum, which has a diameter of 100 µm. Therefore, we must use a microscope to visualize cells in a tissue. Microscope images in this course come from the light microscope (magnification up to 400x) and the electron microscope (magnification up to 500000x). The limit of resolution of the light microscope is 0.2 µm, while the practical limit of resolution of the electron microscope is about 1 nanometer (nm). Thus, light microscopes allow one to visualize cells and their larger components such as nuclei, nucleoli, secretory granules, lysosomes, and large mitochondria. The electron microscope is necessary to see smaller organelles like ribosomes, macromolecular assemblies, and macromolecules.

With light microscopy, one cannot visualize directly structures such as cell membranes, ribosomes, filaments, and small granules and vesicles. Using an appropriate staining technique, however, makes aggregates of these smaller structures or the regions they occupy visible by light microscopy. For example, while it is not possible to see the membranes and ribosomes that compose the surface of the rough endoplasmic reticulum, these structures are represented in light microscope slides by clumps of basophilic material in certain regions of the cell. It is important to be able to correlate the appearance of cells at the light microscope level with the structures visible in electron micrographs.

The cell membrane is about 10 nm thick and cannot be resolved by the light microscope. The limits of the cell can be visualized with the light microscope when there is a heavy concentration of glycoproteins or proteoglycans at the cell surface. The presence of large amount of carbohydrate on the cell membrane makes Periodic acid-Schiff (PAS) an effective method to stain the cell membrane.

The nucleus is limited by a nuclear envelope that consists of a two membrane bilayers and nuclear pores that allow passage of material into and out of the cell. Chromatin, complexes of DNA and protein, is the major component of the nucleus and consists of two histological structures. Heterochromatin is condensed chromatin scattered throughout the nucleus or accumulated along the inner surface of the nuclear envelope. Heterochromatin is considered transcriptionally inactive. In contrast, euchromatin in abundant in cells engaged in transcription. Euchromatin is dispersed and not easily stained.

The nucleus often contains one or more nucleoli that are spherical or oval bodies composed chiefly of ribonucleoproteins. Nucleoli are usually stained with basic dyes because of their high RNA content and are prominent in cells that are actively participating in protein synthesis.

The endoplasmic reticulum (ER) is a system of interconnected membranous sacs, channels, or cisternae in the cytoplasm. It has two subtypes: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). The RER is a ribbon-like structure surrounding the nucleus near the base of the cell. Its surface appears rough due to the ribosomes attached to its membrane and it is the first organelle into which membrane-bound or extracellular proteins are inserted. SER lacks ribosomes and participates in lipid synthesis and detoxification.

The Golgi apparatus is a system of membranous cisternae and vesicles arranged in stacks near the nucleus. The Golgi processes and modifies sugar side chains on proteins that are being secreted or destined for the plasma membrane or other membrane-bound organelles like the lysosome. Therefore, the Golgi apparatus is particularly prominent in cells synthesizing large amounts of glycoproteins and proteoglycans, such as goblet cells that produce mucous in the gut epithelium. The Golgi can be stained with osmium or silver stains and appears as a network of black-staining tubules or clusters of granules.

Secretory vesicles or granules usually contain specific substances synthesized by cells that are exported to the extracellular medium. They include zymogen granules, mucous droplets, and mast cell granules.

Mitochondria are organelles that vary greatly in number, size, and shape between different cells. They are unusual in that they contain their own mitochondrial DNA and ribosomes; mitochondrial proteins come from genes in both the nuclear and mitochondrial DNA. These organelles also undergo self-replication. Structurally, two features characterize mitochondria: double bilayer membranes, and cristae, folds that project from the inner membrane into matrix.

Lysosomes also vary in size and shape, but can be recognized as membrane-bound organelles containing granular material. There are more than 40 lysosomal enzymes that are active at acidic pH.

Animal and plant cells undergo a precise type of division called mitosis. Before cell division, the entire genome is copied. This appears at the light microscope level as a duplication of chromosomes. During mitosis, the two sets of chromosomes are precisely separated and each daughter cell receives one complete set. The final result is the production of two daughter cells identical in their genomic content. In the timeline of mitosis, division of the nucleus (karyokinesis) precedes division of the cytoplasm (cytokinesis).

Mitosis involves 4 distinct phases: prophase, metaphase, anaphase, and telophase. Each mitotic division is separated by interphase.

After cytokinesis, chromosomes unravel and reassume the thread-like appearance of chromatin. The nucleoli reappear and the nuclear envelope is fully reconstituted.