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Nerve & Muscle Tissue
What are the most common patterns of tissue organization?
Histology Topics for Lincoln Scholars Program

CAVEAT:  As they say, all rules have exceptions.  The account on this page should be regarded as a broad-brush overview, a caricature which captures some essential architecture of most organs but simplifies or elides many details that can be easily appreciated later, with further study of particular organs.

Esophageal mucosa

Everywhere, the four basic tissue types stand in the same relation to one another.  Epithelium always rests upon, or is supported by, connective tissue.  Peripheral nerves are surrounded by, or embedded within, connective tissue.  Muscle tissue is enmeshed by connective tissue.  Blood vessels (organs in their own right, built from all four basic tissues) travel within (surrounded by) connective tissue. 

These relationships hold true throughout the body, so that connective tissue can be found practically everywhere, along with the muscles, nerves, and vessels that lie within it and the epithelium which lies upon it. 

Some incidental vocabulary:  The connective tissue that lies closest to an organ which it surrounds is commonly called adventitia.  If the organ faces a body cavity (such as the intestine within the peritoneal cavity), the enwrapping connective tissue is called serosa.  If the surrounding connective tissue forms a tough enveloping sheet, it called an organ sheath.  Much of remaining "everywhere" connective tissue is commonly called fascia, or just "ordinary connective tissue."

Organs themselves are commonly built according to one of just two basic patterns of tissue architecture, the "tubular organ plan" and the "glandular organ plan"  (These two plans might also be nicknamed "layers and lumps.")

Each organ has its own special details.  Nevertheless, learning these two basic patterns -- the "tubular organ plan" and the "glandular plan" -- will cover a lot of territory all at one go, without the necessity of learning a special plan for each organ separately.

In both patterns, epithelium is supported by, or rests upon, connective tissue.  In the tubular organ plan, epithelium forms the innermost lining of the tube.  In the glandular plan, a vast area of epithelial surface (the parenchyma) is packed into a compact volume.

Tubular organs

Layers of esophageal wall

The basic "tubular organ plan" consists of three principal layers -- in effect, tubes within tubes -- the mucosa, the submucosa, and the muscularis externa.  A fourth layer is also sometimes included; this is the serosa or the adventitia, depending on whether or not the organ is adjacent to one of the body cavities.

Layers of the tubular organ plan are clearly visible along most of the length of the gastrointestinal tract.  These layers present their simplest appearance in the esophagus.  For a more detailed description of the tubular organ plan, with additional images, see Tissue Layers of the GI Tract.

Glandular organs

Glandular architecture is nature's way to pack a vast amount of epithelial surface into a relatively small volume.  Glandular architecture gives a large number of epithelial cells access to the outside world, without actually exposing any of those cells to external hazards.  A few organs not usually thought of as glands, notably the lung and the kidney, also share this same glandular organ plan (see below). 

The epithelium of a gland follows the basic rules of epithelial tissue.  Glandular epithelium forms a surface separating the outside of the epithelium from connective tissue on the other side.  And the glandular epithelium is supported upon and sustained by the connective tissue that lies immediately beneath the epithelium. 

Frog skin

The basis for glandular architecture is exemplified by the extremely simple gland illustrated at left.  At an orifice along an epithelial surface, that surface turns inward, forming a pore lined by duct epithelium.  At its inner end, the duct is closed by a small ball of epithelial cells, called an acinus.  The acinar cells are secretory, releasing their secretion into a lumen at the center of the acinus.  The acinar lumen leads into the lumen of the duct.  At the other end of the duct, at the pore, the duct lumen opens to the outside world.  In the cartoon diagram at right, duct cells are colored blue and secretory epithelial cells are colored orange

The real trick to making a gland of substantial size, one which gives the appearance of a solid organ, is for a relatively large duct to branch again and again into many small twigs.  The end of each twig is capped by its own small ball of secretory cells.  (Think of grapes in a bunch.)  Thus the ends of many, many little twigs of duct are capped by many, many little aciniBy branching repeatedly, a duct system can accommodate an extremely large number of parenchymal cells.  The result can be rather similar in shape to an extremely large bunch of very tiny grapes (if the branching grape stems were hollow ducts).

Only secretory cells (orange) and duct cells (blue) are shown in the cartoons above.  Stroma (connective tissue with blood vessels) occupies the area of background color.  In a real gland (below), the acini are typically packed much more closely together (like the grapes in the bunch), such that supporting stroma is often inconspicuous (like the spaces between grapes in a bunch).  In most actual glands, stroma occupies only a very small proportion of the overall bulk of the gland.  Most of the gland's bulk is parenchyma (i.e., the secretory epithelial cells).

Parenchyma of parotid gland

In the image of glandular parenchyma at left, many balls of secretory cells (acini) fill most of the field of view.  Circled in blue are a few acini which happen to be fairly neatly cut in this section.  Surrounding each acinus is the stroma, inconspicuous in these images, containing capillaries which provide material support for secretion.  One acinus is enlarged at right.  In this enlargment, the sides of individual parenchymal cells are marked with white, and the central lumen of the acinus is indicated by an arrow.  The central lumen of each acinus lies "outside" the epithelium and communicates through the duct tree with the outside world.

For a more detailed description of the glandular organ plan, with additional images, see Histology of Glands

Lung and kidney also share this basic glandular plan.  In the lung, gas-exchange alveoli occupy the place of acini, with cuboidal secretory parenchymal cells transformed into extremely thin squamous epithelial cells.  In the kidney, it is cuboidal epithelium of the duct tree that is elaborated into renal parenchyma, with peculiar renal corpuscles in the position of glandular acini.  In both of these organ, glandular-style stroma provides vascular support.

For a much longer and more formal introduction to histology, see this page.

Histology Topics for Lincoln Scholars Program

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SIUC / School   of Medicine / Anatomy / David   King
  Last updated:  14 August 2022 / dgk