Histología del Tejido Epitelial y Glandular

Stratified Squamous and Columnar Epithelia

Stratified squamous non-cornified epithelium is characterized by having multiple layers where the outermost surface cell defines the name of the tissue. This specific type is found in nursing individuals and is located in the oral cavity, the esophagus, the vaginal mucosa, the female urinary meatus, and the masculine urinary meatus. Additionally, it constitutes the corneal epithelium. This tissue contains no keratin; however, it has the capacity to transition into a cornified state if it is subjected to constant friction or tension.

Stratified squamous cornified epithelium is found in tissues that must endure significant friction and mechanical stress. This includes the epidermis, the oral cavity and esophagus of animals that consume hard food, the mucosa of the reticulum, rumen, and omasum, and the plantar pads of carnivores. This tissue is designed to support a great amount of weight, and its most superficial layer consists of dead cells which provide a protective barrier.

Stratified columnar epithelium typically lacks villi or cilia. It is found in large-caliber ducts of the salivary glands and in parts of the male urethra. In all stratified epithelia, the basal stratum is the only layer that touches the basement membrane, while the superficial stratum consists of cells that touch the free surface.

Pseudostratified and Transitional Epithelia

Pseudostratified columnar epithelium is technically a single layer of cells, although it appears stratified. All cells touch the basement membrane, but not all of them reach the apical surface. This tissue is typically composed of three cell types: basal cells, superficial cells, and intercalated cells. It is commonly found in the deep respiratory system, including the mucosa of the trachea, bronchi, and bronchioles, where it is often ciliated and contains goblet cells (c
lulas\;caliciformes).

Variations of pseudostratified epithelium exist for specialized functions. In the epididymis, it is found as pseudostratified columnar epithelium equipped with long microvilli. In the mucosa of the cloacal bursa in birds, it appears as pseudostratified columnar without cilia.

Transitional epithelium, also known as urothelium or epithelium of variable morphology, is characterized by its ability to transition between relaxed and distended states. It contains zones that appear stratified and others that appear pseudostratified. This tissue is primary to the urinary system, found in the bladder mucosa, the female urethra, the renal pelvis, and the mucosa of the ureters.

Functional Projections and Surface Specializations

Microvilli are surface specializations with the primary function of nutrient absorption. Their internal cytoskeleton is composed of actin filaments that maintain their rigidity and prevent movement. In contrast, cilia and flagella are designed for movement, specifically the movement of mucus across a surface. These structures utilize motor proteins, such as dynein, to facilitate their motion.

Intercellular Junctional Complexes

Membrane epithelia function as narrow barriers to regulate the passage of substances and protect underlying tissues. To support high forces of tension, these tissues rely on intercellular junction complexes which must remain intact to preserve tissue integrity. There are three primary types of these complexes.

Tight or occluding junctions ($zonula\;occludens$) are arranged like a belt in the lateral domain of the cell. They contain proteins called occludins which are membrane proteins that do not cross from side to side but maintain a seal that prevents the passage of bacteria.

Adherent junctions involve the intercellular space and utilize proteins called Cadherins, which are calcium-dependent. External to the membrane, actin filament supports provide structural stability to the Cadherins. Desmosomes ($Macula\;densa$) function like two suction cups or manchas. They consist of a support disk composed of the cell membrane and proteins called desmocollin and desmoglein, which act as biological glue for support.

Hemidesmosomes are approximately half the structure of a desmosome and serve to anchor the cell to the basal stratum. They involve proteins such as fibronectin (from Latin $fibra$ meaning fiber and $nectare$ meaning to glue) in the basal domain, working alongside integrins. Every membrane epithelium requires a basement membrane to remain attached and viable.

Communicating or gap junctions allow for the exchange of ions between cells. While rare in most epithelia, they are found in skeletal striated muscle and are critical in the heart, where they form structures called connexons to maintain coordinated rhythmic function.

The Basement Membrane and Cellular Regeneration

The basement membrane acts as the transition zone between the epithelium and the lamina propria, serving as both an anchor and a conduit for cell nutrition via fibronectin. It includes a $lamina\;densa$ containing Collagen Type IV, which forms essential links with both the basement membrane and the connective tissue of the lamina propria.

In terms of regeneration, if an epithelial cell dies, the neighboring cells widen to fill the empty space. Subsequently, regenerative cells undergo mitosis and proliferate; these new cells then allow the widened cells to return to their original size and position. In pseudostratified epithelia, basal cells are responsible for regenerating columnar and goblet cells. In stratified epithelia, the basal stratum is the source for regenerating cells both laterally and toward the upper layers.

Glandular Epithelium Classification and Origins

Glandular epithelium is a specialized tissue designed to produce and secrete useful products for the body (secrecin), as opposed to waste removal (excrecin). Glandular tissue originates from membrane epithelium. There are two main types based on how they deliver products: exocrine glands deliver products via ducts to specific sites and maintain contact with the original epithelium, while endocrine glands lack ducts and deliver secretions directly into blood capillaries, often invading deep into tissues and losing connection with the origin epithelium. Amphicrine glands possess both endocrine and exocrine functions.

Further classification depends on the number of cells (unicellular vs. pluricellular) and the location relative to the membrane (intraepithelial vs. extraepithelial).

Modes of Secretion and Chemical Composition

The mode of secretion is classified by how much cytoplasm is lost during the process. Merocrine secretion involves little to no loss of cytoplasm; examples include salivary glands, the endocrine pancreas, the thyroid, parathyroid, and sweat glands that are functional from birth. Apocrine secretion involves a moderate loss of cytoplasm that is eventually recovered, as seen in mammary glands and sweat glands that become functional during puberty (such as in the pubic area). Holocrine secretion involves the total loss of the cytoplasm as the cell dies during the process, which is characteristic of sebaceous glands.

Chemically, products can be classified as steroidal (derived from cholesterol, such as cortisol) or protein-based (such as adrenaline, noradrenaline, or other peptide hormones). Exocrine products are categorized as serous (rich in enzymes), mucous (rich in glycoproteins), composite (containing glycoproteins, fatty acids, and proteins, like sweat), or lipid-based (like sebaceous secretions or earwax).

Histological Morphology of Glands

Exocrine glands are classified by the shape of their adenomere (the portion that produces and releases secretion). Tubular adenomeres are shaped like test tubes, alveolar adenomeres are shaped like round boiling flasks, and acinar adenomeres are shaped like Erlenmeyer flasks.

Glands with a single principal duct are called simple glands. Sebaceous glands are examples of simple ramified alveolar glands. Intestinal glands are simple tubular glands. Sweat glands are categorized as simple coiled tubular glands ($tubular\;enrrollada\;simple$). Compound glands have many ducts, such as the tubulo-compound structures found in salivary and mammary glands.

Endocrine glands, which lack ducts and adenomeres, are classified histologically as follicular or cord-like ($cordonadas$). The thyroid gland is purely follicular and contains thyroid colloid to store raw materials. The pancreatic islets and the parathyroid are cord-like. The adrenal cortex and the interstitial endocrinocytes of the testis are also cord-like. The adenohypophysis is primarily cord-like, with the exception of the intermediate portion.

Modified and Sensory Epithelia

Modified epithelia are of epithelial origin but perform functions characteristic of other cell types. Myoepithelial cells are found in the alveolar and tubular adenomeres of salivary and mammary glands. Reticular epithelial cells, found in the thymus, provide support, protection, and nutrition to thymocytes, performing functions similar to connective tissue.

Sensory epithelia, or neuroepithelia, are found in sense organs and are connected to the nervous system:

• Tongue: Features filiform and fungiform papillae. It is a stratified squamous cornified epithelium containing taste buds ($yemas\;gustativas$) for flavor perception.
• Ear: The inner ear contains a simple columnar epithelium with intercalated bipolar neurons that possess microvilli for sound perception. Exposure to loud sounds can damage these microvilli, causing them to shrink.
• Eye: The retina consists of a simple cuboidal pigmented epithelium with photosensitive neurons.
• Smell: This involves a pseudostratified ciliated columnar epithelium with goblet cells and intercalated bipolar neurons for perceiving airborne particles.
• Touch: Located in the fingertips, palms, and feet, consisting of stratified squamous cornified epithelium with associated nerve endings.
• Genitals: Consists of stratified squamous non-cornified epithelium with associated nerve endings.

Introduction to Connective Tissue

Connective tissue is specialized for providing support, protection, and defense for the body.