Veterinary Histology (Martin, Meek, and Willebtry)

The purpose of this textbook is to provide an introductory, yet comprehensive, source of information on epidemiology for veterinary students, researchers, and practitioners. There has not been a textbook that presents analytic epidemiology as a science, basic to veterinary medicine’s efforts in health management (herd health) as well as in clinical medicine.

The cell is the smallest and most basic unit of life and makes up all living organisms, whether unicellular or multicellular. In multicellular organisms, several cell types can interact to form specialized tissues and organs. Cells contain multiple organelles, or subcellular structures, that each have distinct structures and specific functions. Functions of these organelles are often dictated by the structure and are dependent on the location (tissue) of the cell and the physiological and disease status of the organism.

Cell Membrane

The cell membrane acts as the barrier between inside and outside of the cell. It is composed of an asymmetric phospholipid bilayer with embedded proteins and carbohydrates and is approximately 7 nm thick. The phospholipids in the membrane can move through lateral diffusion, flexion, or rotation, a process called the fluid mosaic model. The asymmetric bilayer serves an important function for the cell by regulating molecular diffusion across the membrane based on charge and size. The two general categories of proteins present in the cell membrane are referred to as integral or paramembrane proteins. Integral proteins form hydrophobic bonds with lipids and other integral proteins. These proteins penetrate partially or through lipid bilayer and may be glycoproteins  (oligosaccharides attached to N-terminal end of protein). Paramembrane proteins are less hydrophobic than integral proteins. They may associate with lipid polar headgroups or integral proteins via H-bonds or ionic interactions.

Some proteins exist as receptors located within the membrane, with functions ranging from activating downstream intracellular signaling when bound by ligand or acting as channels to allow ions to flow into or out of the cell. Other functions of the membrane are to subdivide the cytoplasm within the cell and increase surface area of the cell. Within the membrane are structures named lipid rafts. These areas are enriched in cholesterol, sphingolipid, and proteins. Lipid rafts are important to the cell for signal transduction across the membrane.

The nucleus contains and confines the genetic material of a eukaryotic cell. Prokaryotic cells do not have a defined nucleus, but rather have genetic material within the cytoplasm. The nuclear envelope is the cellular component that surrounds and defines the nucleus. This structure is composed of two membranes that are continuous with the rough endoplasmic reticulum that will be further discussed later in this chapter. Within the nuclear envelope are nuclear pores, donut-shaped symmetric ring structures that allows selective transport of materials such as RNA or proteins, lipids, and carbohydrates into or out of the nucleus. However, a nuclear localization sequence is necessary before transport can occur. The fibrous lamina is the third component of the nuclear envelope. The fibrous lamina is composed of lamin and membrane-associated proteins and is found on the inner surface of the inner nuclear envelope. This component is responsible for overall nuclear stability and organizing nuclear events such as DNA replication and mitosis.