Veterinary Histology

The roles of veterinarians in society range from clinician to researcher, food inspector to enlisted military officer. Critical to the educational training of all of these veterinarians is the fundamental understanding of basic anatomy, both gross and microscopic. The knowledge of normal anatomy provides the basis of both physiology as well as pathology. Relating gross (e.g. physical exam findings) or microscopic changes (e.g. surgical biopsy) to specific disease processes first requires firm knowledge of “normal”; hence, disease is detected as an alteration of normal!

Teaching histology in the veterinary curriculum is the challenging process of conveying to the future veterinarian the clinical relevance of microscopic anatomy, which may not be inherently appreciable to the first-year veterinary student. The authors are fortunate to instruct professional students both in the preclinical curriculum and the senior clinical rotations where the relevance is readily apparent. When a senior student looks at a cytology of an osteosarcoma, they intuitively know that the neoplastic cells observed are osteoblasts with a small proportion of non-neoplastic osteoclasts. The same goes for ultrasonography of equine ovaries where they can extrapolate the “black hole” in an ovary as a large pre-ovulatory follicle containing an oocyte and lined by granulosa cells. From the instructor’s perspective, it is easy to go overboard in terms of the amount of information conveyed to the student in this type of course, particularly if the instructor is passionate about a particular organ system! The modern veterinary medical curriculum is constantly changing and expanding. It is the instructor’s responsibility to focus on not only conveying the necessary information, but packaging this information in the context of clinical medicine. For this reason, this text focuses on conveying the basic material required for understanding the microscopic anatomy to better understand the disease processes that they will learn later in the curriculum. Included is a basic overview of veterinary histology of commonly reviewed organ systems, with the goal of illustrating important concepts of cells, tissues, and organs in a manner that we hope is not only accessible to first-year veterinary students, but serves as a reference for clinical medicine and pathology.

The textbook is meant to be utilized as both a companion to the course “Comparative Structure and Function of Tissues”, VM6530 as well as a standalone reference for basic veterinary microscopic anatomy.

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.

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 peripheral 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). Peripheral 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.