Pharyngeal pouches, membranes and clefts
A sound anatomical background can only be complete with the knowledge of our somatic origins. Looking into our development in other fields such as genetics, human evolution and diet have given us vital clues that have brought science and medical knowledge into the twenty first century. Taking a look at our own recent history and though embryological development, we can better understand the human body as it is. Embryology is a science in itself and without this vital information, the placement of structures, their blood supply and innervation has no logic and no presidency.
This article aims to further the anatomists understanding of the complex pathways and structures of the head and neck that will help guide future dentists and specialists alike.
The difference between the pharyngeal pouches and clefts is simple. The pouches resemble bulging sacs whereas the clefts are grooves in between the sacs. Each of the four pouches develops from the endoderm whereas each cleft derives from ectoderm. The pharyngeal membranes exist between the clefts and pouches and are composed of external ectoderm, mesoderm, cells of the neural crest in the core and internal endoderm as a lining.
Below is a list of the pharyngeal pouches in an order corresponding to the pharyngeal arches (directed craniocaudualy), including details about each individual pouches location, embryonic structure and adult structure:
- Pouch 1: the first pouch is located opposite the first pharyngeal cleft and is separated by the first pharyngeal membrane. Its embryonic structure is the tubotympanic recess. The epithelium of the auditory tube and tympanic cavity comprise the adult structures.
- Pouch 2: the second pharyngeal membrane separates the opposing second pharyngeal pouch and cleft. The primitive palatine tonsils are embryonic forms that develop into the adult tonsillar fossa and epithelium of the palatine tonsils.
- Pouch 3: the third opposing pharyngeal cleft and pouch, separated by the third pharyngeal membrane provide an embryonic structure that divides into a dorsal and ventral part. The dorsal part migrates inferiorly in the direction of the thorax. These embryonic structures mature into the inferior parathyroid gland which arises from the dorsal part and the thymus that corresponds with the ventral part of the original structure.
- Pouch 4: the final pouch is bridged by the fourth pharyngeal membrane to the fourth pharyngeal cleft. As the third embryonic structure, this pouch divides into a dorsal and ventral part. The neural crest forms parafollicular in the ventral part of the pouch. This divided pouch becomes the superior parathyroid gland, emerging from the dorsal part and the ultimobranchial body from the ventral part during adulthood.
There are also four pharyngeal membranes and as mentioned above, these structures join the opposing clefts and pouches together. Below is a list describing each membranes location and adult structure where applicable:
- Membrane 1: The tympanic membrane is the adult structure that results from the first pharyngeal membrane that separates the first pharyngeal pouch and the first pharyngeal cleft. This is the only membrane out of the four that matures into a defined adult structure.
- Membrane 2: this membrane exits between the second pharyngeal cleft and pouch.
- Membrane 3: the third pharyngeal pouch is joined to its adjacent cleft via the third pharyngeal membrane.
- Membrane 4: the last of the four pharyngeal membranes follows the other three by binding the pharyngeal cleft and pouch, with no further development in adulthood in this particular instance.
The last list of this article will explain the locations of the four corresponding pharyngeal clefts and their existence in an adult human body:
- Cleft 1: this cleft separates the first and second pharyngeal arches and matures into the external acoustic meatus. It is the only one of the four cleft that becomes its own individual structure. The other three clefts share the same fate and merge into one structure in a postnatal human being.
- Cleft 2: the groove created between the second and third pharyngeal arches becomes the obliterated cervical sinus by the second pharyngeal arch, which grows over the cleft. As mentioned previously, the third and fourth clefts share the same fate.
- Cleft 3: the 3rd and 4th pharyngeal arches are separated by the third pharyngeal cleft.
- Cleft 4: the final pharyngeal cleft creates a groove between the fourth and sixth pharyngeal arches. It is well known that that fifth pharyngeal arch regresses almost immediately and therefore there is no corresponding cleft, pouch or membrane to mark it.
Some the most common pathological finds during the development of the pharyngeal pouches, clefts and membranes are a persistent or ectopic thymus and a branchial fistula.
Ectopic thymus is due to the migration of tissues and it is not uncommon for tissue remnants to remain along the pathway.
During a branchial fistula, the failure of the second pharyngeal arch to overgrow the third and fourth arches in a caudal direction allows the second, third and fourth pharyngeal clefts to have contact with the surface of the skin in a slim canal just anterior to the sternocleidomastoid muscle. From here, frequent lateral cervical cysts are drained.