Introduction
Epithelia constitute one of the four basic tissues of the animal body and perform essential functions related to protection, absorption, and the exchange of substances, being fundamental for the maintenance of tissue integrity and homeostasis (Junqueira & Carneiro; Ross & Pawlina).
From an anatomical and functional perspective, epithelia cover external surfaces and line internal cavities and ducts, acting as selective barriers and specialized surfaces. Their correct microscopic identification allows the veterinarian to interpret biopsies, cytologies, and tissue lesions with greater diagnostic accuracy (Zachary & McGavin).
Definition of epithelial tissue
Epithelial tissue is defined as a group of cells closely bound to each other, with little or no extracellular matrix, that covers body surfaces and delineates internal cavities. These cells exhibit marked structural and functional polarity, with well-defined apical, lateral, and basal domains, a feature essential for their function (Junqueira & Carneiro; Ross & Pawlina).
Epithelia may derive from the three embryonic germ layers: ectoderm, mesoderm, and endoderm. This diversity of embryonic origin explains the wide variety of epithelia present in the different organs and systems of domestic animals (Ross & Pawlina).
General characteristics of epithelia
Epithelial cells are very closely packed and joined by specialized junctional complexes, such as tight junctions, adherens junctions, and desmosomes. This organization provides mechanical resistance, cellular cohesion, and allows the formation of effective selective barriers against physical, chemical, and biological agents (Junqueira & Carneiro).
Epithelia lack their own blood vessels; therefore, their nutrition and oxygenation depend on the diffusion of substances from the underlying connective tissue. This characteristic is key for histological recognition and for understanding their vulnerability to circulatory alterations (Ross & Pawlina).
Many epithelial cells exhibit a high rate of cellular turnover, allowing rapid regeneration in response to injury or physiological wear. This property is especially evident in epithelia subjected to constant friction or environmental exposure, such as the skin and mucosae (Junqueira & Carneiro; Zachary & McGavin).
Classification of epithelia
The classification of epithelial tissue is based mainly on morphological criteria, such as the number of cell layers and the shape of the most superficial cells. These criteria reflect the structural adaptation of the epithelium to specific functions within each organ or system (Ross & Pawlina).
Covering epithelia
Simple epithelium consists of a single layer of cells, which facilitates the direct exchange of substances between the epithelial surface and the underlying tissues. This type of epithelium is located in regions where diffusion, absorption, and filtration functions predominate (Junqueira & Carneiro; Ross & Pawlina).
According to cell shape, it can be classified as simple squamous, cuboidal, or columnar epithelium. Simple squamous epithelium has flattened cells that allow rapid exchange of gases and liquids, as occurs in pulmonary alveoli and vascular endothelium. Simple cuboidal epithelium actively participates in metabolic and transport processes, such as in renal tubules. Simple columnar epithelium specializes in absorption and protection and is characteristic of the gastrointestinal tract (Junqueira & Carneiro; Ross & Pawlina).
Stratified epithelium is composed of two or more layers of cells, which confers a predominantly protective function. Only the basal cells maintain direct contact with the basement membrane, while the superficial layers act as a barrier against friction and mechanical aggression (Ross & Pawlina).
The most representative type is stratified squamous epithelium, which may be keratinized or non-keratinized. The keratinized form constitutes the epidermis and provides effective protection against desiccation and trauma, whereas the non-keratinized form lines mucosae such as the oral cavity and esophagus, where resistance without loss of moisture is required (Junqueira & Carneiro; Zachary & McGavin).
Pseudostratified epithelium appears to be composed of multiple layers due to the irregular arrangement of cell nuclei; however, all cells contact the basement membrane, which differentiates it from true stratified epithelium (Ross & Pawlina).
It is characteristic of the respiratory tract and often presents apical specializations such as cilia, which facilitate the movement of particles and secretions, playing a fundamental role in the defense mechanisms of the respiratory system (Junqueira & Carneiro; Zachary & McGavin).
Transitional epithelium is exclusive to the urinary system and is characterized by its ability to adapt to changes in volume without losing its barrier function. Its superficial cells modify their shape according to the degree of organ distension (Ross & Pawlina).
This epithelium allows structures such as the urinary bladder and ureters to repeatedly expand and contract, maintaining impermeability to urine and protecting the underlying tissues (Junqueira & Carneiro; Zachary & McGavin).
Clinical importance in veterinary medicine
Epithelial alterations such as hyperplasia, metaplasia, dysplasia, and neoplasia are common in veterinary pathology. Their histological recognition is essential for establishing accurate diagnoses, evaluating prognosis, and guiding clinical treatment (Zachary & McGavin).
Damage to the epithelium can compromise vital functions such as intestinal absorption, renal filtration, or the protection of body surfaces, directly affecting the health and welfare of animals (Junqueira & Carneiro; Ross & Pawlina).
Conclusion
Epithelial tissue represents a key structural and functional component in animal organisms. Its study in veterinary histology provides the necessary basis for understanding normal tissue organization and pathological changes, strengthening the clinical, diagnostic, and academic practice of the veterinarian (Junqueira & Carneiro; Zachary & McGavin).
References
Junqueira, L. C., & Carneiro, J. Basic Histology. McGraw-Hill.
Ross, M. H., & Pawlina, W. Histology: A Text and Atlas. Wolters Kluwer.
Zachary, J. F., & McGavin, M. D. Pathologic Basis of Veterinary Disease. Elsevier.