Histology of Epithelial Tissue

Cells are the fundamental units of living organisms and are organized into tissues, which are collections of similar cells performing a specific function. Tissues further assemble into organs, which are functional units composed of different tissues working together.

Epithelial tissues, also known as epithelium, are sheet-like layers of closely packed cells that form the coverings and linings of body surfaces and cavities. They are essential for protection, secretion, absorption, and filtration.

Epithelial tissue, being composed mostly of cells with minimal intercellular space, exhibits high cellularity. This allows for efficient communication and coordinated action among the cells.

Epithelial cells have distinct apical and basal surfaces. The apical surface faces the free space, while the basal surface attaches to the underlying connective tissue. This polarity reflects specialized functions of each surface.

The basal lamina is a thin, acellular layer underlying the basal surface of epithelial tissues. It acts as a structural foundation and filter, regulating the movement of substances between epithelial cells and underlying tissues.

Epithelial tissues lack blood vessels and receive their nourishment from diffusion of nutrients from capillaries in the underlying connective tissue. This explains their avascular nature.

Epithelial tissue performs various essential functions, including absorption (taking in nutrients), transport (moving substances), secretion (releasing substances), barrier protection, sensory reception, filtration, and reproduction.

Epithelial tissue can be broadly classified based on the shape of its cells and whether they are arranged in a single layer (simple) or multiple layers (stratified). This classification provides a general understanding of tissue function.

Endothelium refers specifically to the lining of blood vessels and lymphatic vessels, playing a crucial role in blood flow and circulation.

Mesothelium refers to the lining of body cavities, such as the peritoneal, pleural, and pericardial cavities. It helps reduce friction and support organ movement.

A single layer of flat cells tightly joined together. Their nuclei are disk-shaped.

Lines the heart, blood vessels, and lymphatic vessels.

Lines the peritoneal, pleural, and pericardial cavities.

A single layer of cube-shaped cells with a central nucleus.

Found in the walls of exocrine glands, the parenchyma of most glands, and lining most kidney tubules.

A single layer of rectangular cells, taller than wide, with an oval nucleus near the base.

Lines the gastrointestinal tract, gallbladder, uterus, oviducts, and large ducts of glands.

Multiple layers of cells with varying shapes, but the top layer is flat.

A single layer of rectangular cells that appears to be multiple layers due to nuclei at different levels. All cells rest on a common basal lamina, and shorter cells can differentiate into taller ones.

This type of epithelial tissue has multiple layers of cells with different shapes. Its defining feature is the presence of dome-shaped, binucleated cells.

Microvilli are extensions of the free (apical) surface of cells, especially common in absorptive cells like those in the kidney and small intestine. These tiny protrusions increase the surface area by about 30 times.

Stereocilia are NOT cilia, but extremely long microvilli. Found primarily in the inner ear, they are non-mobile and play a role in sensory reception.

The free surface modifications are specialized structures found on the plasma membrane of epithelial cells. These modifications enhance the functions of the cells, facilitating processes like absorption, secretion, and protection.

Cells that produce and secrete substances, like enzymes, mucins, and steroids.

These cells have a lot of rough endoplasmic reticulum (rER) at their base, indicating active protein synthesis.

These cells produce mucins, which are glycoproteins and proteoglycans that act as lubricants or barriers.

These cells have many smooth endoplasmic reticulum (sER), which is involved in lipid and steroid synthesis.

These cells have folded plasma membranes and many mitochondria, indicating active pumping of ions across the membrane.

Aggregates of secretory cells that release their products through ducts.

Exocrine glands that release their products through ducts to the surface of the skin or into body cavities.

Exocrine glands that release their products through ducts associated with hair follicles.

These are specialized extensions of a cell's microtubule network (cytoskeleton). Their internal structure resembles a centriole, with a '9 + 2' complex of microtubule doublets. They are found in the respiratory tract and fallopian tubes, where they move in coordinated waves to ensure fluid moves in one direction only. This movement is possible due to the sliding microtubule hypothesis, where dynein, a protein, uses ATP to ratchet up/down adjacent microtubules.

This is a long isolated cilia found only on sperm cells. It functions to move fluid over the surface of the cell and in cell motility.

These are rigid apical areas found only in cells lining the urinary tract. When the bladder is empty, they fold down into the cell to increase the luminal surface area. When the bladder is full, they unfold and increase surface area.

This is a fuzzy layer on the surface of cells. It is made up of proteins, glycoproteins, and sugar residues. It helps with cell recognition, adherence, immune system activity, and enzyme activity.

This is a process where stratified squamous epithelial cells fill with protein (keratin) as they migrate towards the surface. This results in a tough and waterproof layer.

These are made of extracellular membrane bound granules that form sheets between cells in the upper epithelium.

Keratinization occurs as cells move apically and begin to express specialized proteins that interact with keratin filaments in the cytoplasm. This process forms keratin.

The top layers of cells in keratinization are eventually shed.

Watery solution secreted by sweat glands to cool the skin.

Sweat glands that release their product via vesicle pinching off the cell surface.

Sweat glands that release their product along with part of the cell.

A single-celled exocrine gland that secretes mucin.

A glycoprotein that dissolves in water to form mucus.

A common inflammatory skin condition with various causes.

A condition characterized by thickened keratin and nuclear remnants in the skin.

A condition characterized by abnormal thickening of the stratum spinosum.