Epithelial tissue forms boundaries throughout the body. It covers the body surface (the epidermis of the skin), lines body cavities and hollow organs (the digestive, respiratory, and urinary tracts), lines the blood vessels and heart (endothelium), and forms the secretory parts of glands. Basically, anywhere there's a surface to cover or line, or a gland to build, you'll find epithelium.

Epithelium has five primary functions: protection (against abrasion, pathogens, chemicals, and water loss), absorption of nutrients (intestinal epithelium), secretion of substances (glandular epithelium), filtration (kidney epithelium), and sensation (taste buds or the lining of the nose). Which of these jobs a patch of epithelium actually does depends on its type and where it sits in the body.

Epithelial cells are polarized, meaning the two ends of the cell have different structures and jobs. One surface is exposed (apical) and the other is attached to connective tissue (basal). The apical surface is the free surface, facing either the lumen (open cavity) of an organ or the outside environment, and it may carry microvilli or cilia. The basal surface attaches down to the basement membrane. An easy way to keep them straight: apical faces up and out, basal faces down toward the basement membrane.

Regeneration is how the epithelium replaces cells that get worn away, damaged, or lost, by constantly making new ones through cell division (mitosis). These surfaces take constant physical and chemical wear, so old surface cells get sloughed off and freshly divided cells move up to replace them, keeping the barrier intact.

Function: rapid diffusion and filtration, because these cells form a single layer that's extremely thin and flat. Location: the air sacs of the lungs (alveoli), the walls of capillaries and the lining of all blood vessels and the heart (endothelium), the kidney's filtration membrane, and the lining of the ventral body cavities (mesothelium).

Function: protection from abrasion and physical wear. Because the cells are stacked in so many layers, the top ones can get scraped off without damaging the tissue underneath. Location: the epidermis of the skin (keratinized and dry) and the moist linings of the mouth, esophagus, and vagina (non-keratinized).

Function: secretion and absorption. Location: the tubules of the kidney, the ducts and secretory units of small glands, the surface of the ovary, and the follicles of the thyroid gland.

Function: protection and limited secretion. This is a fairly rare tissue that mainly lines the larger ducts of sweat glands, mammary glands, and salivary glands, where its layered structure protects the duct.

Function: absorption and secretion. These are tall cells that often have microvilli (finger-like projections) to boost surface area for absorption, plus goblet cells that secrete protective mucus. Location: the lining of most of the digestive tract (stomach through rectum), the gallbladder, and excretory ducts. A ciliated version lines the uterine (fallopian) tubes and bronchi, where the cilia sweep substances along the surface.

Function: protection and secretion. It is a rare tissue found in small amounts in the male urethra, the large ducts of some glands, and parts of the conjunctiva of the eye.

Function: stretching. Its dome-shaped surface cells can flatten and slide past each other, letting the tissue expand and snap back as an organ fills and empties without tearing. Location: the urinary system. It lines the bladder, ureters, and part of the urethra, letting the bladder expand as it fills with urine.

A cross section (transverse section) is a slice made perpendicular to the long axis of a structure. Picture taking a tube and slicing it crosswise: you get rings. A longitudinal section is a slice made along the long axis. Picture slicing that same tube lengthwise, down the middle. The same structure can look completely different depending on which plane it was cut in.

Simple epithelium is a single layer of cells, built for absorption, secretion, and diffusion where you want a thin barrier. Stratified epithelium is two or more stacked layers, built for protection where the surface takes abrasion. Both get named further by the shape of the cells in the apical (top) layer.

Epithelial cells divide quickly and have stem cells (mostly in the basal layer) that are always producing replacements. Even though epithelium is avascular, it sits right on top of vascular connective tissue and gets its nutrients and oxygen from it by diffusion, so it's supplied well enough to rebuild fast. That matters because these surfaces are constantly getting worn down or injured.

Connective tissue has three defining features. First, it's made of relatively few cells sitting in a lot of extracellular matrix (ground substance plus fibers), so the matrix, not the cells, makes up most of the tissue. Second, it's usually highly vascularized (it has its own blood supply), with cartilage being the main exception. Third, all connective tissue comes from the same embryonic tissue, called mesenchyme.

Epithelial cells are packed tightly together with barely any space between them and very little matrix, and epithelium has no blood supply of its own. Connective tissue cells are the opposite: sparse and spread far apart, separated by large amounts of extracellular matrix, and the tissue usually has plenty of blood vessels.

Connective tissue binds and supports other tissues and organs. It holds the body together and gives it shape (bone and cartilage), protects organs (fat cushions them, bone encloses them), handles insulation and energy storage (adipose tissue), and transports materials around the body (blood).

The actively secreting cells end in the suffix "-blast," which tells you they're building up the matrix. Once they go quiet and just stick around to maintain it, they get renamed with the suffix "-cyte." The exact blast/cyte pair depends on the tissue:

• Connective tissue proper: fibroblast becomes fibrocyte
• Cartilage: chondroblast becomes chondrocyte
• Bone: osteoblast becomes osteocyte

Collagen fibers (thick and strong, they resist pulling and stretching), elastic fibers (very thin and stretchy, they snap back to their original shape after being stretched), and reticular fibers (short, fine fibers that form a soft internal framework for soft organs).

Areolar, adipose, and reticular.

Dense regular, dense irregular, and elastic.

Blood fits the basic definition of connective tissue: it's cells (red cells, white cells, and platelets) separated by an extracellular matrix. The only twist is that blood's matrix is liquid plasma instead of something gel-like or solid, but it's still cells suspended in a matrix. Blood also develops from mesenchyme, just like the other connective tissues.

Skeletal muscle is made of long, cylindrical, striated (striped) fibers, each with multiple nuclei pushed out to the edge of the cell. It's voluntary (under conscious control) and it produces movement of the body. It attaches to the bones of the skeleton all over the body, plus places like the tongue, diaphragm, and the muscles of the face.

Cardiac muscle is made of striated, branching cells, usually with a single central nucleus each, joined to neighboring cells end to end by special junctions called intercalated discs. It's involuntary (not under conscious control), and its job is to pump blood by contracting the heart. It's found only in the heart, in a layer called the myocardium.

Smooth muscle is made of spindle-shaped cells (tapered at both ends). Each has a single nucleus in the center, and there are no striations. It's involuntary, and it pushes substances along inside hollow organs and adjusts the diameter of vessels. You'll find it in the walls of hollow organs and tubes like the stomach, intestines, blood vessels, bladder, uterus, and airways.

Smooth muscle regenerates the best. Skeletal muscle has only a limited ability to regenerate, and it patches up damage using special satellite cells. Cardiac muscle basically can't regenerate at all: when heart muscle is damaged, the body fills it in with scar (connective) tissue instead of new muscle cells.

Nervous tissue is made of neurons (cells that generate electrical impulses) and neuroglia (glial cells that support, protect, nourish, and insulate the neurons). It's found in the brain, spinal cord, and the nerves running through the body. Its function is communication and control: it picks up stimuli, processes information, and sends out signals that trigger responses in the body's tissues and organs.

Nervous tissue barely regenerates. Mature neurons usually can't divide, so neurons that are lost are rarely replaced, which is why damage to the central nervous system (brain and spinal cord) is usually permanent. The axons of peripheral nerves can regrow to a limited degree if the cell body survives and Schwann cells are there to guide the regrowth. The neuroglia, unlike neurons, can divide to replace themselves.