SIU SOM Histology INTRO
Nervous Tissue
Overview of the Four Basic Tissue Types
Epithelial Tissue covers body      surfaces (epi, on + thelium, surface).  Epithelial tissue      consists of cells attached to one another to form an uninterrupted layer of      cells that separates the underlying tissues from the outside world.  The      body's epithelium not only covers its obvious surfaces (such as the epidermis      of the skin and the linings of respiratory, urinary, and digestive tracts)      but also extends into all of the complex invaginations which form lungs, kidneys,      sweat glands, digestive glands, liver, etc.  Epithelial      tissue provides the essential functions of protection; containment of body      fluids; and transport in and out across body surfaces (absorption and secretion).  Embryonically, most epithelial tissues are derived      either from ectoderm (e.g., epidermis) or endoderm (e.g., epithelium of trachea      and lung).  [ More ] [ Examples ]
Connective Tissue supports other      tissues.  Connective tissue consists of several cell types and extracellular      products which, together, provide essential functions of mechanical reinforcement,      immune surveillance, transport/diffusion of nutrients and wastes, and energy      storage (fat).  Embryonically, connective tissues      derive from mesoderm or mesenchyme.  [ More ] [ Examples ]
Nervous Tissue is      responsible for rapid long-distance signalling, coordination, and "thinking".  Nervous tissue consists of highly specialized nerve cells and support      cells which are derived from embryonic neuroectoderm and neural crest.  [ More ]
Muscle Tissue is specialized      for gross movement by means of cellular contraction.  Embryonically,      muscle derives from mesoderm or mesenchyme. [ More ]
 A note on pathology nomenclature:  The names      of neoplasms reflect the fundamental nature of their source tissues.  Thus      a carcinoma is a cancer of epithelial origin, while a sarcoma is a cancer      of mesenchymal (connective tissue or muscle) origin. 
Parenchyma / Stroma:  The parenchyma of an organ consists of that tissue which conducts the specific      function of the organ and which usually comprises the bulk of the organ.  Stroma is everything else -- connective tissue , blood vessels,      nerves, ducts.  The parenchyma / stroma distinction provides a      convenient way to circumvent the listing of tissue types when discussing an      organ.  
Examples:
The parenchyma of the kidney is epithelial        tissue (renal tubules and corpuscles).  The blood vessels,        nerves, and supporting connective tissue of the kidney comprise the stroma.
The parenchyma of the spleen is connective tissue (mostly lymphocytes and other        blood cells).  The supporting fibrous connective tissue of the spleen        comprises the stroma.
The parenchyma of the heart is muscle        tissue (cardiac muscle cells).  The nerves, intrinsic blood        vessels, and connective tissue of the heart comprise the stroma.
The parenchyma of the brain is nervous tissue (nerve cells and glia).  The blood vessels within the brain and the connective tissue associated        with these blood vessels are stroma.
The parenchyma of the malignant neoplasm is cancer        cells.  Other tissues, including blood vessels, which grow to support        the tumor are stroma.
Parenchyma is interesting.  Because organ-specific function usually      centers on parenchymal cells, histological (and physiological) accounts often      emphasize parenchyma. Unfortunately, stroma is commonly ignored as      just boring background tissue.  
Pay attention to the stroma.  No organ can function without      the mechanical and nutritional support provided by the stroma.  If an      organ is inflamed, the signs of inflammation appear      first in the stroma.  (For an example of inflammation from liver ,      see WebPath .)
Historical note:  Ignoring        inconspicuous tissue features can have consequences.  Stromal capillaries        are seldom evident in tissue specimens.  Nothing calls them to one's        attention, so they are often ignored and forgotten.  Unfortunately,        just such inattention may have delayed for decades the realization that        interfering with tumor vasculature might powerfully inhibit tumor growth.
Philosophical note:  The concept of "four basic        tissue types" provides a simple (and powerful) framework for organizing        and learning a great wealth of detail.  This concept is more than just        a convenient intellectual construct.  There is        a real boundary layer, the basal lamina (visible microscopically        with appropriate stain), which separates tissues of different types.
Nevertheless,  nature does remind us every so often        that her organization is not bound by our simplifying concepts.  Although        most tissues do correspond closely to one of the four basic tissue types,        the usual caveat ("all rules have exceptions") does apply.
Exceptions include germ cells, which don't fit the scheme          at all, and several specialized varieties of connective tissue which masquerade          as epithelium, such as synovial membranes of joint capsules and stria          vascularis of the inner ear.
Endothelium and mesothelium are special cases, usually          classified as epithelial even though they derive from mesenchyme.
The best way to appreciate the basic tissue organization of the body is through      examples in which this organization is displayed in relatively simple form.  
Each of the following examples illustrates the distinct difference between epithelial tissue and connective tissue .  Click on a thumbnail image for an annotated enlargement.
Skin