Appendix B: A Taxonomic Key to Histological Organs

The following is a taxonomic key designed to facilitate the correct identification of tissue sections of organs on microscope slides. In this key, the organs are grouped into ordered categories that show degrees of similarity. The highest (first order) category is most broad and inclusive; the subcategories within each category are more restrictive; sub-subcategories are even more restrictive; and so on until the organ is correctly identified.

To use this key, systematically choose between the two first order (number 1) choices, then within the appropriate first order, select the correct second order choice (a number 2, under the number 1 selected). Proceed to narrow your choices within each successive category until you have identified the organ as specifically as possible. For example, first, looking at the lowest power image, ask "Does the organ have a lumen, or not?" (See 1�s in the key below.) Then, if the organ in question is basically solid, ask, "Does it have a cortex and medulla, or not?" (See 2�s under the "solid organ" category below.) Next, assuming the solid-appearing organ does have a cortex and medulla, ask, "Is it a lymphoid organ, or not? (See 3�s under the selected first and second categories). And so on, up through the sixth order category if necessary, until you correctly identify the organ by its unique features.

This Taxonomic Key is not meant to be memorized, nor does it represent the only logical path of thinking to correctly identify organs. It is a model of a systematic thinking process that is used in the practice of medicine and research. This intellectual process, based on learned expectations, involves art as well as science. The art lies in knowing which questions to ask in your systematic process of elimination.

1. organ basically tubular (has lumen)    
   2. simple squamous epithelium lining the lumen    
      3. luminal surface irregular with large muscular bulges and indentations heart
         4. thick walled ventricle
         4. thin walled atrium
      3. luminal surface smooth, or regular blood vessel
         4. many circularly arranged laminae elastica (lamellar units) elastic artery
         4. media mostly muscle, prominent internal elastic lamina muscular artery
         4. media < three muscle layers; internal elastic lamina arteriole
         4. tunica adventia main layer; irregular lumen vein
         4. abundant longitudinal smooth muscle in adventitia vena cava
   2. simple cuboidal epithelium lining the lumen; lumen anastomosing channels rete testis
   2. simple columnar epithelium lining the lumen    
      3. epithelium: mixture of absorptive and goblet cells    
         4. villi present small intestine
            5. submucosal glands present duodenum
            5. Peyer’s patches present ileum
            5. submucosal glands absent, Peyer’s patches absent jejunum
         4. villi absent large intestine
            5. small lumen and abundant lymphatic tissue appendix
            5. large lumen and moderate or no lymphatic tissue    
               6. taeniae coli present colon
               6. taeniae coli absent rectum
               6. internal (and external) anal sphincter present anal canal
      3. epithelium: mixture of secretory cells and ciliated cells oviduct
         4. free projections of fimbriae infundibulum
         4. height of plicated mucosa >>muscularis ampulla
         4. height of plicated mucosa similar to, or <, muscularis isthmus
      3. epithelium: a single type of cell    
         4. epithelial cells all or virtually all mucus secreting    
            5. branched plicae (glands), little smooth muscle in wall cervix
            5. epithelium forms simple tubular pits; well developed muscularis externa stomach
               6. glands with parietal and chief cells fundic stomach
               6. glands with principally mucous cells pyloric/cardiac stomach
         4. epithelial cells all tall absorptive cells gall bladder
         4. epithelial cells associated with very cellular lamina propria; thick muscularis uterus
            5. glands straight proliferative stage
            5. glands sacculated secretory stage
            5. mucosae hemorrhagic and partially sloughed menstrual stage
   2. pseudostratified columnar epithelium lining the lumen    
      3. epithelium a mixture of goblet cells and ciliated cells and of even height    
         4. cartilage an open ring trachea
         4. cartilage arranged in plates; muscularis mucosae present; no respiratory alveoli extrapulmonary bronchus
         4. cartilage arranged in plates; surrounded by respiratory alveoli intrapulmonary bronchus
      3. epithelium with stereocilia; all even height ductus epididymis
      3. epithelium with tall ciliated and short nonciliated cells; thin muscularis ductus efferentes
      3. epithelium with short streocilia; irregular lumen; very thick muscularis ductus deferens
   2. stratified squamous epithelium lining the lumen    
      3. no muscularis mucosae    
         4. with seromucous glands inner lip/oral cavity
         4. seromucous glands absent; papillae vagina
      3. thick muscularis mucosae, only longitudinal fibers esophagus
         4. muscularis externa of skeletal muscle upper esophagus
         4. muscularis externa of smooth muscle lower esophagus
   2. transitional epithelium lining the lumen    
      3. lumen wide, muscular wall interlacing bundles of smooth muscle urinary bladder
      3. lumen narrow and very irregular    
         4. standard connective tissue lamina propria ureter
         4. lamina propria is, or resembles, corpus spongiosum urethra
   2. epithelium with many cells in meiosis seminiferous tubules
   2. epithelium with seven alternating nuclear and plexiform layers eye
1. organ basically solid    
   2. cortex and medulla present    
      3. lymphatic organ    
         4. nodules present lymph node
         4. nodules absent; Hassall’s corpuscles thymus
      3. not a lymphatic organ    
         4. ova and follicles in cortex ovary
         4. glomeruli in cortex kidney
         4. prominent radial cellular cords in cortex adrenal gland
   2. cortex and medulla absent    
      3. lymphatic organ    
         4. nodules; red and white pulp spleen
         4. epithelial surface present tonsil
            5. stratified squamous nonkeratinized palatine tonsil
            5. pseudostratified with goblet cells and ciliated cells pharyngeal tonsil
            5. parakeratinized surface of the tongue lingual tonsil
      3. tubulo-alveolar acini present; intralobular ducts present    
         4. striated ducts present    
            5. all serous acini parotid gland
            5. mixed serous and mucous acini; serous demilunes submaxillary gland
         4. striated ducts absent    
            5. islets of Langerhans and centroacinar cells pancreas
            5. concretions may be present; lobules poorly defined; enlarged acini prostate gland
            5. stratified cuboidal ducts; much interlobular connective tissue mammary gland
               6. few or no secretory alveoli present nonlactating
               6. secretory alveoli present and active lactating
      3. gland with ducts absent    
         4. follicles full of colloid thyroid gland
         4. two cell types: chief and less numerous oxyphil cells parathyroid gland
         4. acidophils, basophils, and Herring bodies pituitary gland
      3. unique structure    
         4. parenchymal cells arranged in plates radiating from central canal liver
         4. numerous air spaces, lined with squamous cells present lung
         4. chorionic villi present placenta
         4. corpora cavernosa and corpus spongiosum present penis
         4. parakeratinized filiform papilla on surface tongue
         4. keratinized stratified squamous; sweat glands in dermis skin
            5. lactiferous ducts and sinuses in dermis nipple
            5. continuous with nonkeratinized s.s. epithelium with seromucous glands lip
            5. continuous with nonkeratinized s.s. epithelium, then simple columnar anal canal
         4. dentin tooth
         4. retina, cornea, iris, and lens present eye
         4. ganglion cells and Schwann cells present    
            5. unipolar ganglion cells; usually heavily myelinated axons present dorsal root ganglion
            5. multipolar ganglion cells; usually in or near organ innervated parasympathetic ganglion

Italicized organs have recognizable subdivisions

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