Thursday, March 20, 2014

Pharyngeal Slits

Pharyngeal slit arrangements of invertebrate chordates

The pharyngeal slits are among the four defining characteristics of chordates and make their appearance in all vertebrates at some point in their life.  They were originally part of an invertebrate suspension-feeding device.  This mechanism is well illustrated in modern animals by the adult tunicates, or sea squirts, and amphioxus, an invertebrate chordate that appears much like a present day fish.  These animals are all filter feeders that rely on straining minute food particles from waters that pass through holes in the pharynx.

A shark's pharyngeal gill slits

The pharynx is a cavity that exists immediately behind the mouth.  In filter-feeding chordates and vertebrate fish the pharynx is perforated with a variable number of holes that allow a current of water to pass through the mouth and into the pharynx, then, out through its holes or pharyngeal slits.  Small, hair-like structures called cilia create a beating motion in invertebrate chordates that induces this flow of water.  The pharynx itself is lined with mucus that is used to snare the suspended food particles.  Cilia complete the feeding process by moving the food-enriched mucus to the animal’s esophagus and then digestion in its gut.  Vertebrate fish subsist on larger prey and rely on muscular action, not the rhythmic beating of cilia, to produce the current that enters by way of their mouth and exiting through their pharyngeal perforations.  In this case the water supplies the animal with oxygen and not the nutrition from food.

Embryonic land vertebrates have unperforated pouches

This perforated pharynx feeding mechanism of ancient chordates provided the framework for the evolution of subsequent features that include the pharyngeal muscular pump, internal gills and vertebrate jaws.  Along the arches that separate the individual pharyngeal slits began the development of tiny plates or folds of tissue.  Over time this tissue became increasingly vascularized, harboring beds of capillaries that were rich with blood.  The role of capturing food along this tissue became secondary to providing respiration for these increasingly large, and active, animals.  The larger body size and the higher metabolic rate required for an active predator meant the need for respiratory efficiency beyond that of small, sedentary organisms.  For the first time the term pharyngeal gill slits could be accurately applied to these specialized structures of vertebrate fish.  Muscular pharyngeal pumping forced water over these gills, enabling oxygen to be absorbed by the animal while carbon dioxide would be diffused from the gills into the passing current. 

Pharyngeal arches have new roles rather than disappear

While pharyngeal slits persist into adulthood with bony fish as gills, their embryonic form in most land animals is overgrown and no longer appear in their initial form or role.  The pockets in the embryonic pharyngeal cavity of vertebrate tetrapods never break through to become slits.  Instead they remain grooves, or pouches, that give rise to other structures, including the Eustachian tube, middle ear cavity, tonsils, parathyroid glands and other tissues associated with the lower jaw and neck. 

The fact the each cell in the body of an organism carries the animal’s complete DNA blueprint undoubtedly contributes to the species’ ability to differentiate as needs change over time.  Structural tissue has the potential to become respiratory tissue which, in turn, may evolve into a hormonal producing gland.  This statement itself is probably an error in its simplification but both fossil and genetic evidence clearly illustrates the transformational talent that life forms exhibit as they change to meet the new challenges presented over geologic time.  

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