PRACTICUM IN HISTOLOGY, CYTOLOGY AND EMBRYOLOGY - 2016

Chapter 6. DIFFERENTIAL DIAGNOSTICS OF MICROSCOPIC SPECIMENS

In accordance with the educational and qualification characteristics (EQC) and educational and professional programs (EPP) for training specialists, approved by the order of the Ministry of Education and Science of Ukraine dated 16.04.03 No. 239, the main programmatic goal of studying Histology, Cytology, and Embryology is The ability to "describe, explain, recognize, and evaluate The Structure of various Organs, tissues, and systems of The Human Body, their origin and Embryogenesis."

After completing the course in histology, cytology, and embryology, students should develop a comprehensive understanding of the fine and ultramicroscopic structure, as well as the regularities of development and renewal of human Cells, tissues, and organs.

Knowledge of the histological structure of organs is critically essential for understanding the mechanisms of physiological processes and evaluating structural changes in cells, tissues, and organs during various pathological processes and diseases.

First of all, one must learn to determine whether a given section belongs to a specific tissue or organ. To achieve this, when evaluating a specimen, a student should pose a series of questions, answer them, correlate the findings, and focus their reasoning on the correct identification of the microspecimen.

Upon picking up a microspecimen, we examine it with the naked eye and tentatively determine the staining method used for the section based on its color. In most cases, a pinkish-purple color indicates hematoxylin and eosin (H&E) staining. This method is used to study epithelial, connective, and Muscle Tissues, as well as tubular and parenchymatous organs. A brown or black color indicates silver nitrate impregnation. This method is primarily used to study the peripheral and central nervous systems. A gray or violet-blue color may indicate iron hematoxylin staining.

We determine whether a section belongs to a specific tissue based on the Characteristic Features of its structure. Epithelial Tissues are characterized by a dense arrangement of cells in the form of one or several layers, cords, plates, or trabeculae. Connective Tissues feature cells and postcellular elements (fibroblasts, macrophagocytes, adipocytes, reticular cells, pigmentocytes, leukocytes) along with a strongly developed intercellular substance. In addition, Cartilage Tissues are characterized by the arrangement of a significant portion of chondrocytes within isogenous groups. And

based on the Structural Features of the intercellular substance, particularly fibers, we classify the section into a specific type of cartilage. In Bone Tissues, osteocytes reside within lacunae. Tubular bone is recognized by the presence of osteons. Muscle tissues are easily distinguished by the structural features of myocytes or muscle fibers. The Diagnostics of Nervous Tissue specimens do not present major difficulties due to the well-known structural features of Neurons.

Quite often, difficulties arise when diagnosing specimens that represent tubular structures in cross or oblique sections. In such cases, we examine the section under low magnification and attempt to determine the STRUCTURE OF THE wall layers. We begin by establishing what lines the organ from the inside. The presence of an endothelial lining indicates that the section is a Blood vessel. Next, by the presence or absence of internal and external elastic membranes, the predominance of muscular or elastic elements in the media, and The ratio of the thickness of the middle and outer coats, we determine whether the section belongs to an artery or a vein of a specific type.

Let us consider another possible scenario: a tube lined internally by an epithelium represented by one or several layers of cells. In this case, the section could be from an organ of the digestive, respiratory, or Urogenital System. We recall that simple (unilayered) cuboidal and columnar epithelia are characteristic of The Stomach, various segments of the small and Large Intestine, the Uterine tube, the Uterus, and certain segments of the spermatic ducts. In the latter case, we observe tubules on the cross-section of the organ (Epididymis) rather than as a single isolated section of a large-diameter tube. The Small Intestine is easily differentiated from the stomach by the presence of villi and goblet cells. In the large intestine, goblet cells are present while villi are absent. A significant number of lymphoid follicles are found in the lamina propria of the mucosa of the vermiform Appendix. The mucosa of the uterine tube is characterized by high folds that form a distinctive labyrinthine pattern. The endometrium of the uterus features uterine glands (crypts).

Pseudostratified ciliated epithelium is characteristic of the Trachea and Bronchi. An additional diagnostic feature for these organs is the presence of cartilage rings, half-rings, or plates in their walls.

Stratified non-keratinized epithelium is observed in cross-sections of the Esophagus. The organ appears as a tube with a stellate lumen, a well-developed underlying Connective Tissue with mucous glands (submucosa), as well as muscular and adventitial coats.

The Ureter has a structure similar to that of the esophagus. However, it should be borne in mind that, firstly, the epithelium here is transitional (urothelium); secondly, proper mucosal glands are absent; and thirdly, the muscular coat has a varying number of layers in different segments. Furthermore, in the upper part of the esophagus, it contains striated muscle fibers rather than smooth myocytes.

Additional difficulties arise when a tubular organ has a large diameter (the section is taken from an adult human organ rather than a small animal). In such cases, the specimen typically appears as a flat sheet composed of multiple layers of different tissues. If we observe mucosal epithelium, the loose connective tissue of the lamina propria, the submucosa, and smooth myocytes of the muscular coat among them, this organ may belong to the digestive, respiratory, or excretory system. Next, based on an Analysis of the epithelial lining's structural features (number of layers; shape, size, and structural/staining CHARACTERISTICS OF THE nuclei and Cytoplasm), The Nature of the relief, the presence of the muscularis mucosae, and the number and orientation of the muscular coat layers, we attempt to identify the specific organ.

If the section exhibits Stratified squamous keratinized epithelium, with the underlying connective tissue forming papillae and no muscular layers present, this indicates that the section is taken from a specific area of the Skin. Determining the exact localization is based on analyzing the fine details of the epidermal structure, skin derivatives (Hair, sebaceous and Sweat Glands), and the shape of the dermal papillae.

The presence of stratified squamous non-keratinized epithelium indicates that the section is taken from the Cornea of the eye, the Vagina, or a specific region of the Oral Cavity. Further differential diagnostics of these organs can be carried out quite easily based on their characteristic structural features.

In cases where a layered structure is absent and uniform structures are identified throughout different areas of the specimen, this likely indicates that the preparation is a section of one of the parenchymatous organs (glands, Organs of the Hematopoietic and Immune systems, Lungs, Kidneys, Gonads).

The presence of a system of excretory ducts is of key importance for identifying glandular specimens. Excretory ducts are characteristic of exocrine glands—salivary, Pancreas, mammary, and prostate. If the parenchyma of the secretory end pieces of a salivary gland consists exclusively of serocytes, it is the parotid gland. In the Submandibular salivary gland, mucocytes together with serocytes form mixed secretory end pieces (diagnosed by their characteristic demilunes of Gianuzzi). In the sublingual gland, alongside mixed end pieces,

purely mucous ones are also observed. Additional diagnostic features include: for the pancreas, scattered groups of small, pale cells known as pancreatic islets (islets of Langerhans), as well as the presence of centroacinar epitheliocytes; for the mammary gland during Lactation, wide lumina of secretory units; for the prostate, broad connective tissue septa between individual prostatic glands; and for the Liver, characteristic lobules with a central vein in the middle and radially arranged hepatic plates.

The absence of excretory ducts points toward Endocrine glands. Irregularly shaped cords of cells (epithelial trabeculae) are characteristic of the parathyroid gland and the hypophysis (Pituitary Gland). In sections of the latter, such trabeculae are present only in the anterior part—the adenohypophysis—and are absent in the neurohypophysis. The Adrenal gland is easily recognized by the clearly demarcated zones of its cortex: the zona glomerulosa, zona fasciculata, and zona reticularis.

The Thyroid Gland contains rounded follicles lined by a single layer of thyrocytes, enclosing pink colloid within their lumina.

Organs of the hematopoietic and immune systems are diagnosed by the presence of characteristic lymphoid follicles populated by numerous lymphocytes or accumulations of separately located Cell groups of the myeloid or lymphoid series. Lymph Nodes are characterized by a peripheral subcapsular arrangement of lymphoid follicles with light germinal centers (cortex), while the interior of the section features cords of lymphocytes (medulla). An unordered arrangement of lymphoid follicles within the parenchyma and the presence of a blood vessel (central arteriole) within them point to the Spleen. In the palatine tonsil, lymphoid follicles are located along the edges of crypts lined with stratified squamous non-keratinized epithelium of the oral cavity. The Thymus has a lobular structure. In each lobule, we distinguish a cortex containing thymocytes and a lighter medulla containing Hassall's corpuscles. Red Bone Marrow lacks lobules and lymphoid follicles. It is characterized by clusters of cells with nuclei of various shapes and large, isolated cells with polymorphic, bizarre-shaped nuclei—megakaryocytes.

The main structural Features of the lungs are thin-walled alveoli and the presence of large, medium, and small bronchi. Hyaline cartilage is identifiable in the walls of the bronchi.

In the Kidney, we distinguish a cortex and a medulla. Renal corpuscles are identified in the cortex, clearly outlined against the Background of cross, oblique, and longitudinal sections of nephron tubules.

The organs of The Nervous system, Sense Organs, and gonads feature a specific structure, and their Differential Diagnosis does not present any particular difficulties.