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Nerve & Muscle Tissue
What are the core concepts of histology?
Histology Topics for Lincoln Scholars Program

Every beginning biology student learns three fundamental principles which together comprise Cell Theory: 

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.)

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

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CAVEAT:  Several of the generalizations above have peculiar exceptions. 
A few of the more important exceptions are listed below; others are mainly curiosities. 

  1. Germ cells (spermatogonia and oocytes) do not really fit into any of the "basic tissue type" categories.
  2. 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.
  3. Epithelial parenchymal cells of several endocrine glands do not form proper surfaces that would lie between separate fluid compartments.
  4. 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.
  5. Epithelial podocytes of the kidney do not attach tightly to one another, thereby permitting free passage of fluid across the epithelial boundary.
  6. 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.
  7. The connective tissue matrix of blood is plasma, which lacks those matrix materials (fibers, glycosaminoglycans) which normally hold connective tissue matrix in place.
  8. In the connective tissue matrix of bone, impermeable mineral replaces tissue water, thereby forcing several other special adaptations of the tissue.

Comments and questions:

SIUC / School of Medicine / Anatomy / David King
Last updated:  18 June 2023 / dgk