THE CELL

1.1. Overview

1.2. Microscopes and Techniques

1.3. Different Appearances of Cells According to Technique

1.4. Ultrastructure and Function of Cell Membranes

1.5. Intercellular Junctions: Ultrastructure and Function of Tight Junctions

1.6. Intercellular Junctions: Ultrastructure and Function of Anchoring Junctions

1.7. Intercellular Junctions: Ultrastructure and Function of Gap Junctions

1.8. Ultrastructure and Function of the Nucleus and Nucleolus

1.9. Ultrastructure and Function of the Nucleus: Chromatin and Matrix

1.10. Ultrastructure and Function of the Nuclear Envelope

1.11. Ultrastructure and Function of Mitochondria

1.12. Ultrastructure and Function of Mitochondrial Cristae and Matrix

1.13. Ultrastructure and Function of Smooth Endoplasmic Reticulum

1.14. Ultrastructure and Function of Rough Endoplasmic Reticulum

1.15. Ultrastructure and Function of Ribosomes

1.16. Ultrastructure of the Golgi Complex

1.17. Functions of the Golgi Complex

1.18. Ultrastructure and Function of Lysosomes

1.19. Ultrastructure and Function of Peroxisomes

1.20. Ultrastructure and Function of Inclusions: Glycogen

1.21. Ultrastructure and Function of Inclusions: Lipid Droplets

1.22. Ultrastructure and Function of Cytoplasmic Vesicles: Endocytosis, Transcytosis, and Exocytosis

1.23. Ultrastructure and Function of Microtubules

1.24. Ultrastructure and Function of Cytoplasmic Filaments

1.25. Ultrastructure and Function of the Centrosome and Centrioles

1.26. The Cell Cycle, Mitosis, and Other Cellular Processes

1.27. Specializations of the Cell Surface: Cilia and Basal Bodies

1.1 OVERVIEW

The human body is organized into four basic tissues (epithelial, muscle, nervous, and connective) that consist of cells and associated extracellular matrix. The cell is the fundamental structural and functional unit of all living organisms. The body contains about 60 × 1012 cells—some 200 different types whose size and shape vary widely—but all have a common structural plan. The eukaryotic cell is a mass of protoplasm surrounded by an external plasma (limiting) membrane. The two components of the protoplasm are the nucleus, which holds the genome consisting of chromosomes, and the cytoplasm, a complex aqueous gel made of water (about 70%), proteins, lipids, carbohydrates, and organic and inorganic molecules. Organelles (specialized structures with functional capability) and inclusions (relatively inert, transitory structures) are in the cytoplasm. Except for mature erythrocytes, without a nucleus, most cells have one nucleus that conforms to the cell’s shape. A few cells, such as osteoclasts and skeletal muscle cells, may be multinucleated. A nuclear envelope invests the nucleus, whose substance, called chromatin, contains one or more nucleoli. Internal cell structure is modified to reflect function: Muscle cells, for example, are modified for contraction; nerve cells (or neurons), for conduction; connective tissue cells such as fibroblasts, for support; and glandular epithelial cells, for secretion.

HISTORICAL POINT

German scientists—biologist Theodor Schwann (1810–1882) and botanist Matthias Schleiden (1804–1881)—proposed the cell theory, which states that all living organisms are composed of similar units of organization called cells. For his observations on normal animal cells, Schwann is recognized as the father of modern histology. Later, renowned German pathologist Rudolph Virchow (1821–1902) proposed that disease originates in cells, not in tissues or organs. Because he was the first to use microscopes and histologic specimens as a basis for the study of pathology, he is credited as the founder of modern cytopathology. With advances in medical science more than a century later, knowing the light and electron microscopic appearance of cells has become fundamental to diagnosis, treatment, and clinical management of many common and rare diseases.

1.2 MICROSCOPES AND TECHNIQUES

Histology is the study of body tissues and cells, their constituents. Cells cannot be seen with the naked eye, so the primary tool used to study them is the microscope. It produces enlarged images of cells and enhances contrast for resolving details. Of several kinds of microscopes, two major ones are light and electron microscopes. They have different lenses and sources of illumination and provide complementary information at different levels of resolution and magnification. The ability to discriminate two points that are close together is the resolving power of a microscope. It is related to the light wavelength. A conventional light microscope uses bright-field illumination, with a resolving power of about 0.2 μm. Study specimens absorb visible light; glass lenses focus and magnify specimens. Most cells absorb very little light, so staining is needed to increase light absorption. Cells and tissues first undergo sequential processing steps. Fixation in aldehydes and dehydration in alcohols are followed by embedding in paraffin or plastic. Specimen sections (or slices) are made with a microtome, followed by staining with color dyes. The illumination source of the transmission electron microscope (TEM) is a beam of electrons, which has a smaller wavelength. The resolving power of the TEM, 0.2–0.5 nm, is about 103 greater than that of the light microscope. For the TEM, ultrathin sections are cut after specimens have been fixed and embedded in plastic. Sections are then stained with heavy metals to enhance contrast, and black-and-white, not color, images result. A scanning electron microscope (SEM) is used for thick specimens or whole cells that have been fixed, dried, and coated with a thin metal film. It provides three-dimensional surface views. A high-resolution SEM (HRSEM) allows internal morphology of cells and organelles to be discerned with great depth of focus.

1.3 DIFFERENT APPEARANCES OF CELLS ACCORDING TO TECHNIQUE

Histologic techniques provide different but complementary views of cells and thus a useful morphologic base, which can aid understanding of cell function in health and disease. Paraffin sections are routinely stained with hematoxylin and eosin (H&E) and examined with a light microscope. Cell nuclei (which are rich in nucleic acids such as DNA and RNA) have an affinity for hematoxylin (a basic dye), stain blue, and are termed basophilic. In contrast, the cytoplasm of cells and extracellular matrix typically have an affinity for eosin (an anionic dye), stain pink, and are eosinophilic (or acidophilic). With superior resolving power, a TEM provides better elucidation of cell details, such as membranes and organelles, than a light microscope. Different parts of cells have distinct affinities for metal stains used on thin sections, so resulting two-dimensional images show variations in electron density, recorded in black and white. HRSEM images of freeze-fractured cells show three-dimensional spatial relationships of organelles and inclusions.

1.4 ULTRASTRUCTURE AND FUNCTION OF CELL MEMBRANES

Membranes—semipermeable barriers that selectively regulate movement of ions, water, and macromolecules—are ubiquitous in cells. They vary in composition depending on cell type and location, but all consist of about 35% lipids, 60% proteins, and 5% carbohydrates. The cell (or plasma) membrane forms an external boundary. Intracellular membranes surround nuclei and membrane-bound organelles. Membranes are beyond the limit of resolution of a light microscope and are thus difficult to visualize without special techniques. By high-magnification electron microscopy,...