What are the core concepts of histology?Histology Topics for Lincoln Scholars Program
Every beginning biology student learns three fundamental concepts of cell biology: All living things are made of cells. All cells are fundamentally alike. All cells come from pre-existing cells. Together these three assertions comprise Cell Theory, one of the cornerstones of biology.
Cells are organized into tissues. But far fewer students understand the fundamental organizing principles of tissue biology, the discipline known as "histology."
Put most simply, our bodies are composed of four different kinds of stuff, traditionally regarded as the four basic tissue types.
To elaborate a bit further, each of the four basic tissue types has its own distinctive character, a set of properties which unites all examples of that tissue type while distinguishing them from examples of the other three tissue types. Practically every cell in the body belongs to one of these four tissue types (note caveat below). Most organs include tissues of all four types.
The four basic tissue types are muscle tissue, nervous tissue, epithelial tissue, and connective tissue.
- 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 which 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.
- 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 the several epithelial cells which surround it.Epithelial cells attached to one another form a continuous surface which separates what lies on either side of that epithelial surface. Typically, there would be connective tissue on the inside and the rest of the world on the outside. This epithelial surface follows many routes deep into the body, lining 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 parenchymal cell forming part of an elaborately-shaped surface within the gland, a boundary separating what is properly inside the body (i.e., "beneath" the epithelium, where capillaries travel through connective tissue stroma) 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).
- Connective tissue is the tissue of extracellular materials. Those extracellular materials are called the matrix of the connective tissue. The cells of connective tissue live within the midst of the matrix. Significantly, connective tissue forms an arena in which the drama of inflammation plays out.Connective tissue matrix typically includes proteinaceous fibers (tough collagen and stretchy elastin) embedded in "ground substance," which is mostly water. (The water is held more-or-less loosely in place by glycosaminoglycans.)
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.
These several cell types and matrix materials can vary hugely in proportion, yielding varieties of connective tissue which differ tremendously in texture. At the extreme, these varieties include blood and bone (note caveat below).All varieties of connective tissue share an underlying commonality of cell types and matrix components which is quite different from that of epithelial, nervous, or muscle tissue.
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
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CAVEAT: Several of the generalizations above have peculiar exceptions. A few of these exceptions are important; others are mainly curiosities. Some of the more important exceptions are listed below.
Below are a few significant exceptions to the core-concept generalizations outlined above.
- 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.
- In the liver, there is no basal lamina separating parenchyma from stroma.
- The configuration of epithelial parenchyma in liver is unique, closely related to the liver's dual function as both an endocrine and an exocrine gland.
- 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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of Medicine / Anatomy / David
Last updated: 4 September 2022 / dgk