What are the core concepts of histology?Histology Topics for Lincoln Scholars ProgramEvery beginning biology student learns three fundamental principles which together comprise Cell Theory:
- All living things are made of cells.
- All cells are fundamentally alike.
- All cells come from pre-existing cells.
Tissue biology (i.e., histology) contributes one additional organizing principle:
The cells of our bodies are organized into four different types of tissue:
Each of these four basic tissue types has its own distinctive character, a set of properties which unites all examples of that tissue type and distinguishes it from examples of the other three tissue types.
Practically every cell in the body belongs to one of the four basic tissue types. (Note caveat #1 below.)
- Epithelial tissue is the tissue of surfaces. Epithelial cells are attached to one another like bricks in a wall or tiles on a floor. But there is no mortar; the cell membrane of each epithelial cell is attached directly to the membrane of each adjacent epithelial cell. A basal lamina separates epithelial tissue from underlying connective tissue.
Epithelial tissue forms a continuous surface which separates inside from outside. Each epithelial cell is polarized, with one end facing outward and the other end facing inward. Typically, there would be connective tissue "beneath" the epithelium and the rest of the world "above" the epithelium. Because this epithelial surface follows many routes deep into the body, the directions "beneath" and "above" (or "inside" and "outside") must be understood relative to the epithelium itself. Epithelium lines various tubular organs (e.g., gastrointestinal, respiratory, urinary, and reproductive tracts), with branches into various glandular organs.
Glands might appear to be solid organs rather than epithelial surfaces. But the apparent solidity of a gland masks the fact that the bulk of most glands consists of epithelial cells, with each such cell forming part of an elaborately-shaped surface, a boundary separating what is properly inside the body (i.e., "beneath" the epithelium, where capillaries travel through connective tissue) from what is outside the body ("above" the epithelium, where secreted fluids flow away from the epithelial surface into the lumen of epithelially-lined ducts).
(Epithelial tissue in a few special locations departs from the characteristics listed above; note caveats 2, 3, 4, 5, 6 below.)
- Connective tissue is the tissue of extracellular materials, called matrix. Cells of connective tissue live within the midst of the matrix.
Connective tissue matrix includes proteinaceous fibers (tough collagen and stretchy elastin) as well watery ground substance, which is the principal arena for inflammation.
Connective tissue cells include fibroblasts and their close kin, which manufacture matrix materials. Connective tissue also includes several cell types with immune function, such as lymphocytes, which can wander more or less freely within the connective tissue matrix.
Cells and matrix elements can vary in proportion, resulting in wide variation in connective tissue texture. Extreme variants include blood and bone (note caveats 7, 8 below.), but all varieties of connective tissue share an underlying commonality of cell types and matrix components which is quite different from any of the other three tissue types.
- Muscle tissue is the tissue of bulk movement. Muscle tissue consists of elongated muscle cells, capable of powerful contraction.
- Nervous tissue is the tissue of rapid, long distance communication. Nervous tissue is composed of nerve cells, many of which have incredibly long cytoplasmic extensions whose length can be measured in inches or even in feet. Nerve cells are intimately associated with one another at specialized sites called synapses. Apart from such contact at synapses, most nerve cells are intimately surrounded by the support cells of nervous tissue.
Most organs include tissues of all four types. And wherever each tissue occurs, it shares that tissue's defining characteristics (with occasional special exceptions; see caveats below).
These, then, are the core concepts of histology: Four basic tissue types, together with the characteristic properties of each type, muscle, nervous, epithelial, and connective. To "unpack" these concepts in much greater detail, see basic tissue types or follow any of the links in the text above.
Understanding how certain organs work -- notably the kidney and the lung and the liver -- requires some detailed knowledge of microanatomy that is unique to each organ; for example, podocytes and juxtaglomerular apparatus in the kidney, or interalveolar septa in the lung and bile canaliculi in the liver. But only a tissue-level perspective can organize that knowledge and make it far easier to comprehend, by revealing how all three of these organs share with pancreas the same basic glandular tissue architecture, of epithelial parenchyma supported by connective tissue stroma.
For a brief essay on how the four basic tissues are arranged in relation to one another to form organs, see "What are the most common patterns of tissue organization?"
For a much longer and more formal introduction to the basic concepts of histology, see "Basic tissue types."
Histology Topics for Lincoln Scholars Program
CAVEAT: Several of the generalizations above have peculiar exceptions.
A few of the more important exceptions are listed below; others are mainly curiosities.
- Germ cells (spermatogonia and oocytes) do not really fit into any of the "basic tissue type" categories.
- Although usually classified as epithelial tissue, endothelial and mesothelial tissues separate different fluid compartments within the body rather than separating underlying tissues from the outside world.
- Epithelial parenchymal cells of several endocrine glands do not form proper surfaces that would lie between separate fluid compartments.
- The configuration of epithelial hepatocytes in liver is unique, closely related to the liver's triple function as blood filter, endocrine gland, and exocrine gland. In liver, there is no basal lamina separating parenchyma from stroma.
- Epithelial podocytes of the kidney do not attach tightly to one another, thereby permitting free passage of fluid across the epithelial boundary.
- The epithelial nature of the lens of the eye and the pigmented epithelium of the retina is far from obvious in the mature organ, but these structures are distinctly epithelial in their embryonic development.
- The connective tissue matrix of blood is plasma, which lacks those matrix materials (fibers, glycosaminoglycans) which normally hold connective tissue matrix in place.
- In the connective tissue matrix of bone, impermeable mineral replaces tissue water, thereby forcing several other special adaptations of the tissue.
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Last updated: 18 June 2023 / dgk