B. Epithelial Tissue

Epithelial Tissue

Epithelial tissues act as the body’s initial defense against chemical, biological, and physical damage. The cells in an epithelium serve as protectors, regulating permeability by selectively allowing certain materials to pass through its surface. All substances entering the body must cross an epithelium.

Simple squamous epithelium

Simple squamous epithelial cells are characterized by their thin and flat morphology, making them the thinnest among all epithelial cell types. This structure enables them to possess a large surface area exposed to the lumen on one side (known as the apical surface) and to the basement membrane (basal lamina) on the other side (the basolateral surface, as depicted in Figure 1). These scale-like cells typically feature larger, elliptically-shaped nuclei. Being a simple epithelium, the simple squamous epithelium consists of a single cell layer, ensuring direct contact of every cell with the basement membrane. Like other epithelial types, these cells are interconnected via tight junctions, forming a selective barrier essential for their function. Simple squamous epithelial cells play a role in facilitating filtration and diffusion processes. Their uncomplicated and thin structure enables the easy movement of small molecules across membranes and through the cell. For instance, molecules like oxygen and carbon dioxide can freely diffuse across simple squamous epithelia following concentration gradients. In contrast, ions rely on transmembrane protein channels for diffusion through the cells. Consequently, the composition of proteins within a specific simple squamous epithelial tissue partially dictates its function. As a result, simple squamous epithelia contribute to determining what substances can pass from the lumen into the bloodstream (where the capillary bed lies beneath the basement membrane) and vice versa.

Stratified squamous epithelium

Stratified squamous epithelia consist of multiple layers of cells positioned atop a basement membrane, with the outermost layer(s) composed of squamous cells. These tissues are typically located in areas subject to mechanical or chemical abrasion and stress, serving to shield underlying structures from damage. They are ubiquitous throughout various organ systems where the body interfaces with the external environment, including the skin, respiratory, digestive, excretory, and reproductive systems. Additionally, they provide protection against desiccation and water loss. In the digestive system, stratified squamous epithelia line the upper surface of the tongue, the hard palate, the esophagus, and the anus. They are also prevalent in the female reproductive system, specifically in the vagina, cervix, and labia majora. Furthermore, they constitute a significant component of the integumentary system, forming the outermost layer of the skin. Moreover, these tissues are present in the cornea of the eye, ensuring the integrity of delicate eye tissues. They are also found in the respiratory system, particularly in the oropharynx, which comes into contact with food behind the mouth. Finally, stratified squamous epithelia line the urethra, the pathway through which urine exits the body.

Simple cuboidal epithelium
Simple cuboidal epithelium comprises a single layer of epithelial cells with a square-like appearance in cross-section. These cells feature large, rounded nuclei positioned centrally and are directly attached to the basement membrane. Simple cuboidal epithelium serves distinct functions across various organs. In the brain ventricles and the spinal cord’s central canal, it forms the ependyma, responsible for producing cerebrospinal fluid. Within the thyroid, these cells line the thyroid follicles as follicular cells. They actively uptake iodine and synthesize precursors of thyroid hormones, subsequently releasing them into the bloodstream. On the ovarian surface, a monolayer of cuboidal epithelial cells comprises the ovarian surface epithelium, contributing to repair after ovulation and potentially aiding in ovum formation. In the kidney, this tissue lines the proximal and distal convoluted tubules of nephrons and collecting ducts. Here, the cells are specialized for selective secretion and reabsorption during urine formation. This selective reabsorption is also vital in the ducts of the male reproductive tract. Structures like the tubuli recti and rete testis, linking seminiferous tubules to the epididymis, are lined with cuboidal cells initiating fluid reabsorption and sperm concentration.

Ciliated columnar epithelium

Ciliated epithelial cells, characterized by their simple columnar shape, feature cilia that extend into the internal cavities of the structures they line. These columnar cells typically possess between 200 to 300 hair-like protrusions known as cilia, with mitochondria concentrated towards the apical region and cell nuclei mostly situated at the elongated base. Cilia, singularly termed cilium, are small hair-like projections on the surface of eukaryotic cells responsible for cell locomotion, fluid movement, and mechanoreception. Additionally, a class of microorganisms, ciliates, derives its name from these structures, utilizing them for both locomotion and feeding. Cells are interconnected through desmosomes and tight junctions, forming a semipermeable membrane that exhibits higher selectivity compared to other cell types. The nucleus of ciliated epithelial cells tends to be located closer to the basement membrane, to which the cells adhere. Tight junctions and adhesive proteins, known as desmosomes, facilitate the connection between adjacent ciliated epithelial cells. Goblet cells, often found amidst ciliated epithelial cells, derive their name from their cup-shaped appearance in cross-section. In the pulmonary system, ciliated epithelium is interspersed with goblet cells, working collaboratively to secrete mucus and form a protective mucosal layer above the epithelial surface. The coordinated action of epithelial cilia and goblet cells aids in propelling mucus away from the lungs, preventing particulate matter from causing infections.

References:

4.2 Epithelial Tissue

https://www.bio.davidson.edu/bernd/Lab/EpithelialInfoWeb/Simple%20Squamous%20Epithelium.html

Stratified Squamous Epithelium

https://biologydictionary.net/simple-cuboidal-epithelium/