Epithelium is a fundamental tissue type that forms continuous sheets of cells, lining the exterior surfaces of the body and internal cavities. This tissue acts as a selective barrier, regulating the exchange of substances between the body and its environment. Studying this tissue in the laboratory mouse (Mus musculus) offers a window into the complex biological processes that govern tissue function and disease. Due to the high degree of genetic similarity and physiological conservation, the mouse is an invaluable model organism for research directly relevant to human health.
Defining Mouse Epithelium and Its Basic Structure
Mouse epithelium, regardless of location, is defined by a highly organized, polarized cellular arrangement. Epithelial cells exhibit distinct polarity, meaning their plasma membrane is divided into three domains: the apical, basal, and lateral surfaces. The apical surface faces the lumen or external environment, while the basal surface rests upon the basal lamina, a specialized layer of extracellular matrix.
The basal lamina, often referred to as the basement membrane, is a thin, dense layer composed of proteins like laminin and collagen, which anchors the epithelial sheet to the underlying connective tissue. This anchoring is facilitated by specialized structures like hemidesmosomes, which connect the cell’s internal cytoskeleton to the basal lamina. This provides structural support and serves as a signaling platform, influencing cell behavior, migration, and differentiation.
Lateral surfaces of adjacent epithelial cells are tightly linked by complex protein structures known as junctional complexes. These complexes are located just below the apical surface and are responsible for both cell-to-cell adhesion and the regulation of permeability. Tight junctions, or zonula occludens, create a selective seal, forcing transport through the cell.
Adherens junctions and desmosomes provide mechanical strength to the epithelial sheet, linking the cytoskeletons of neighboring cells to maintain tissue integrity. Adherens junctions, involving E-cadherin proteins, form a continuous belt around the cell, tethering the actin filaments of adjacent cells. Desmosomes are spot-like attachments that connect the intermediate filaments, providing resistance to shearing forces. This organization ensures the epithelium functions as a cohesive, regulated barrier necessary for its diverse physiological roles.
Diverse Roles of Epithelium Across Mouse Organs
The generalized epithelial structure is highly adapted to perform specialized functions across different mouse organ systems, often reflected in the shape of the cells themselves. For example, the epithelium of the skin (epidermis) is a stratified squamous epithelium that serves primarily as a physical barrier. The outermost layer (stratum corneum) is composed of dead, keratin-filled cells embedded in a lipid matrix, which prevents desiccation and protects against environmental pathogens.
Barrier function in the epidermis is also maintained by tight junctions in the granular layer; genetic studies in mice show that a loss of proteins like claudin-1 leads to skin barrier failure and neonatal lethality. This protective role contrasts sharply with the epithelium of the small intestine, which is specialized for absorption. The intestinal lining is a simple columnar epithelium featuring numerous microvilli on the apical surface, dramatically increasing the surface area for nutrient uptake.
In glandular tissues, the epithelium is adapted for secretion, such as in the mouse mammary gland. Here, the epithelium forms a branching network of ducts and grape-shaped secretory units called alveoli. The inner layer consists of luminal cells that synthesize and secrete milk components, while the outer layer is composed of myoepithelial cells. These myoepithelial cells contract in response to hormonal signals like oxytocin, squeezing the alveoli to eject milk through the ducts.
The respiratory system presents an adaptation for gas exchange and transport, particularly in the alveoli of the lung. This surface is lined by two main types of epithelial cells: Alveolar Type 1 (AT1) and Alveolar Type 2 (AT2) cells. AT1 cells are extremely thin, flattened squamous cells that cover over 95% of the alveolar surface, minimizing the distance for gas diffusion. Cuboidal AT2 cells are responsible for secreting surfactant, a mixture of lipids and proteins that lowers surface tension to prevent the thin alveoli from collapsing.
The Importance of Mouse Epithelium in Research
The mouse epithelium is extensively studied because it provides a reliable and manipulable system to understand human biology and disease pathology. The fundamental structure and function of epithelial tissues are highly conserved between mice and humans, making the mouse an excellent translational model. This genetic and physiological homology allows researchers to extrapolate findings from mouse studies to human conditions with a high degree of relevance.
Mouse models are indispensable for studying diseases that originate in epithelial tissues, most notably carcinomas, which are cancers arising from epithelial cells. Scientists use genetically engineered mice to introduce human cancer-related mutations into specific epithelial cell types, such as those in the colon or lung, and observe the entire process of tumor development in vivo. This allows for the testing of new therapeutic drugs and provides insights into disease progression within a whole organism.
The ease of genetic manipulation in mice is a primary reason for their utility in epithelial research. Using techniques like CRISPR-Cas9, scientists precisely edit the mouse genome to delete, modify, or insert genes within epithelial cells. This capability is applied to study disorders like inflammatory bowel disease (IBD), where researchers examine the effect of altering tight junction proteins on intestinal barrier integrity and inflammation. The resulting mouse models allow for the observation of cell-to-cell and cell-to-environment interactions that cannot be replicated in a petri dish.
Mouse models also provide a unique platform to study the epithelial response to external factors, such as environmental toxins or infectious agents. Researchers introduce these agents and then track the epithelial tissue’s injury, repair, and regeneration processes, which is particularly relevant for lung and skin disorders. The ability to conduct these controlled experiments across multiple organ systems solidifies the mouse epithelium’s standing as a standard for biomedical investigation.