SEM_09_10_Pharynx, pharyngeal arches and respiratory system_PARTE1.docx

Pharynx, pharyngeal arches and respiratory system Learning objectives Pharynx and pharyngeal arches: Consider the evolutionary changes from aquatic respiratory organs (gills) to air-breathing organs (lungs). Describe the main components of the pharynx and their connections with branchial structures...

Pharynx, pharyngeal arches and respiratory system Learning objectives Pharynx and pharyngeal arches: Consider the evolutionary changes from aquatic respiratory organs (gills) to air-breathing organs (lungs). Describe the main components of the pharynx and their connections with branchial structures. Learn about the contributions of the branchial structures to the development of some major structures of the head and neck. Respiratory system: Describe the development of the larynx, trachea and lungs. Consider briefly some congenital abnormalities in the development of the respiratory system. IRG Pharynx and pharyngeal arches The pharynx (commonly known as the throat) develops from the cranial portion of the foregut within the head, where the blind end of the foregut meets the ectoderm of an external depression called stomodeum, the future oral cavity. The region where the ectoderm and endoderm come into direct contact with each other constitutes a thin membrane called oropharyngeal membrane (also buccopharyngeal membrane). At an early stage of development, this membrane ruptures, opening the communication between the pharynx and the oral cavity. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=cSPiuW5ZPgK4tI - The pharynx. The uppermost part of the foregut is called the pharynx which blind end meets an external depression called stomodeum. In early embryos, a series of parallel structures protrudes on both sides of the ventral aspect of the pharyngeal region in a similar fashion to the branchial arches that develop in fish. - Pharyngeal or branchial arches A specific feature of the primitive pharynx begins to develop at the fourth week of gestation when cells from the neural crests migrate around the pharynx forming bilateral discrete aggregations of mesenchymal cells, located between the surface ectoderm and pharyngeal endoderm. These aggregations give rise to the formation of the six pairs of pharyngeal arches also referred to as branchial arches from the Greek term “branchia”, meaning gills, that protrude like elevation on the lateral aspect of the developing head. On the exterior, adjacent pharyngeal arches are set apart by invaginations of the surface ectoderm known as pharyngeal clefts. - Scanning electron micrograph of a human embryo at 5th week of development (Carnegie stage 14). Note the pharyngeal arches and pharyngeal clefts. On the interior, the endoderm of the lateral wall of the pharynx evaginates between the arches forming the pharyngeal pouches. The pharyngeal pouches and clefts establish contact with each other, forming branchial membranes. In fish, the six pairs of branchial arches persist as the adult gills and the branchial membranes degenerate giving rise to the gill slits, communications between the pharynx and the outside. Although the branchial arches keep developing in early embryos of air- breathing animals, the pharyngeal membranes do not break and communication between the pharynx and the exterior never occurs at these sites, at least under normal conditions. In birds and mammals, from the six primordial branchial arches, only the first four pairs can be temporally detected on the ventral surface of the head; the fifth pharyngeal arch undergoes atrophy, while the sixth arch fuses with the fourth arch, forming a fourth-sixth arch complex. At a certain stage of development of terrestrial embryos, the pharyngeal arches, clefts and pouches disappear but these branchial cell components are used in the formation of a great variety of important organs of the head and neck. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=J0cCBhYUr3Dj3C - Pharyngeal arches. In fish, the pharyngeal arches are referred to as the branchial arches that give rise to the gills. These pharyngeal arches still develop in the early embryos of earthbound vertebrates but as temporary structures which regression gives rise to numerous organs of the head and neck. Branchial clefts derivatives The first branchial cleft that lies between the first two branchial arches contributes to the formation of the external ear. The auricle (also called the pinna) is derived from three swellings or hillocks on the first arch and three swellings on the opposing surface of the second. These auricular swellings gradually merge to form the characteristic shape of the auricle. The first branchial cleft that lies in-between develops into the external auditory meatus (ear canal). The presence of a severe auricular malformation can be a useful diagnostic tool because it could anticipate associated problems in the formation of other internal derivatives of the first and second branchial arches. - Scanning electron micrograph of a human embryo at the 7th week of development (Carnegie stage 18). Note the formation of the external ear from the first pharyngeal cleft. The rest of the branchial clefts (second to fourth) are temporarily enclosed inside of the cervical sinus which eventually disappears without leaving any structure in the adult. The cervical sinus is formed when the second branchial arch extends caudally over the rest enclosing the second, third and fourth branchial clefts within an ectoderm-lined cavity. Growth and lengthening of the neck normally make the cervical sinus to disappear. If the cervical sinus persists can result in the development of palpable congenital cervical cysts, draining sinuses, or infested fistulae. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=g1OxpsgKNJZeON - Branchial clefts derivatives Only the first pharyngeal cleft remains as the external auditory canal. The rest of the pharyngeal clefts disappear inside of a transitory cervical sinus. The incomplete obliteration of the cervical sinus may result in the formation of congenital branchial cleft cysts on the lateral parts of the neck. Branchial mesenchyme derivatives Branchial mesenchyme filling each pharyngeal arch gives rise to a range of branchial derivatives that include some of the major musculoskeletal components of the head and neck. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=ZrlsPpwcvw9B8S - Branchial mesenchyme derivatives. Each pharyngeal arch has initially similar components: a cranial nerve, an artery (one of the aortic arches), and specific skeletal and muscular components. The first pharyngeal arch, also known as the mandibular arch, consists of a ventral portion, the mandibular process, and a dorsal portion, the maxillary process. In a highly simplified sketch, the lower jaw and most of the upper jaw are formed by the growth of the first pharyngeal arch. These two components enclose an invagination of the surface ectoderm to form the precursor of the mouth: the stomodeum. The ventral mandibular process forms the mandible and soft tissues of the lower jaw. Right and left sides grow cranially and fuse in the midline to form the mandibular symphysis. A plate of cartilage referred to as Meckel´s cartilage develops in the centre of each mandibular process. The proximal end of each cartilage also contributes two bones to the middle ear: the malleus (hammer) and incus (anvil). The dorsal maxillary process forms most of the upper jaw, caudal to incisor teeth. Muscular derivatives of the first pharyngeal arch include the muscles of mastication as well as some muscles of the middle ear and hyoid apparatus. The first pharyngeal arch and its derivatives are innervated by branches of the trigeminal nerve (fifth cranial nerve). - The first arch mesenchyme develops into: Bones: upper and lower jaws (mandible and maxilla) and two ossicles of the middle ear (malleus and incus). Muscles: muscles of mastication and middle ear (tensor tympani mm.) The second pharyngeal arch, also known as the hyoid arch, is smaller than the first and contains the Reichert´s cartilage. The dorsal end of Reichert's cartilage ossifies during development to form the stapes (stirrup bone) of the middle ear, while the ventral portion gives rise to most parts of the hyoid bones. Muscular derivatives of the second arch include the muscles of the auricular and facial expression as well as some muscles related to the middle ear and hyoid bones. The nerve supply to the second arch is contributed by the facial nerve (seventh cranial nerve). - The second arch mesenchyme develops into: Bones: hyoid bones and one ossicle of the middle ear (stapes). Muscles: muscles of facial expression and middle ear (stapedius mm.). The third pharyngeal arch. The cartilage of this arch completes the formation of the hyoid bones. Muscular derivatives of this branchial arch contribute to the formation of the pharyngeal muscles which are supplied by the glossopharyngeal nerve (ninth cranial nerve). The fourth and sixth pharyngeal arches merge forming the fourth-sixth complex, which gives rise to the laryngeal cartilages. Muscular derivates of this branchial arch differentiate into most the laryngeal muscles which are supplied by the vagus nerve (tenth cranial nerve). - The third and fourth arch mesenchyme develops into part of the hyoid bones, larynx cartilages and associated muscles. Pharyngeal pouches derivatives In terrestrial animals, the pharyngeal pouches disappear toward the end of the embryonic period but the endodermal tissue that lines this pharyngeal evagination is used in the formation of a number of derivatives. First pharyngeal pouch derivatives. The first pharyngeal pouch endodermal epithelium meets the ectodermal epithelium of the first pharyngeal cleft giving rise to the tympanic membrane or eardrum. Behind the eardrum, the first pharyngeal pouch enlarges to form the cavity of the middle ear or tympanic cavity. The rest of the first pharyngeal pouch form a permanent communication between the pharynx and middle ear known as the auditory tube (pharyngotympanic Eustachian tube). In horses, the auditory tube evaginates to form air-filled diverticula known as the guttural pouches (diverticulum tubae auditivae). The epithelial lining of all these derivatives comes from the endoderm of the first pharyngeal pouch which develops into a mucus-secreting membrane contiguous to the respiratory mucosa found in the nose and pharynx. Thus, effects of infection or obstruction in one area are likely to be reflected in the other areas. Second pharyngeal pouch derivatives. Part of the cavity of this bursa remains as the tonsil sinus (tonsillar fossa). The endoderm of the second bursa proliferates and fragments forming the tonsillar crypts. Finally, the mesenchyme differentiates into lymphoid tissue that is organized in the lymph nodes of the palatine tonsil. - First pharyngeal pouche derivatives. The first pharyngeal pouch forms the auditory tube, eardrum and tympanic cavity. - Second pharyngeal pouche derivatives. The second pharyngeal pouch differentiates into the pharyngeal tonsil. Third and fourth pouch derivatives. The endodermal epithelium of the third and fourth pharyngel pouches contribute to forming some important endocrine gland located in the ventral aspect of the neck like the thymus and the parathyroid glands. The epithelium of the thymus develops from two outgrowths, one on either side, of the third pharyngeal pouch. It sometimes also involves the fourth pharyngeal pouch. These extend outward and backward into the surrounding branchial mesenchyme in front of the ventral aorta. By further proliferation of the cells lining epithelium, buds of cells are formed, which become surrounded and isolated by the mesoderm to form lobules. During this stage, hematopoietic bone-marrow precursors, the thymocytes, migrate into the thymus. Normal development is dependent on the interaction between the epithelium and the hematopoietic thymocytes. Iodine is also necessary for thymus development and activity. The parathyroid gland alos develops from the edoderm of the third and fourth pharyngela pouches: The third pharyngeal pouch gives rise to the external parathyroid gland and the thymus, which develop along the ventral part of the neck. The fourth pharyngeal pouch mainly develops into the internal parathyroid gland which also migrates and becomes located laterally to the thyroid cartilage. A closely related pharyngeal derivate is also the thyroid gland which originates as a proliferation of endodermal epithelial cells on the median surface of the developing pharyngeal floor. - Third and fourth pharyngeal pouches derivatives. The endoderm of these pouches migrates to form the glandular tissue of endocrines glands in the neck such as the parathyroid glands, thymus and thyroid gland. Respiratory system The respiratory system consists of the nasal cavity, pharynx, larynx, trachea and lungs. The epithelial lining of the nasal cavity is derived from the ectoderm; this part will be considered with the development of the head. The lining of the rest of the respiratory system comes from the endoderm; besides the endodermal epithelium, the wall of the respiratory system includes a variety of vascular, supportive and muscular components, which derive from the surrounding splanchnic mesoderm (visceral mesoderm). The adult pharynx is a common passageway leading from the oral and nasal cavities in the head to the oesophagus (digestive system) and larynx (respiratory system). Therefore, The pharynx serves both respiratory and digestive functions. A series of elevations on the pharyngeal floor gives rise to the tongue while a caudal epiglottal swelling contributes to the separation of the digestive and respiratory passages. - Breathing pathway In breathing, air may flow through either the nose or the mouth before reaching the pharynx and then the larynx. The soft palate has an important role in determining the route of respiration. A long palate may narrow the opening of the larynx. The excessive length of the soft palate acts as a noisy flutter valve during relaxed breathing. Snoring occurs when these structures strike each other and vibrate during breathing. Horses (also rabbits and rodents) have a very long soft palate and they cannot breathe through their mouths under normal conditions (they are called "obligate nasal breathers"). The rearmost portion of the soft palate fits snugly around the base of the epiglottis, and the soft palate remains fixed in this position at all times except when swallowing. - Brachiocephalic obstructive airway syndrome. Certain breeds of dog with short noses or elongated soft palate are prone to suffering from brachycephalic syndrome. This syndrome includes one or a combination of 1) elongated soft palate (tissue at the back of the throat which can protrude into the airways); 2) stenotic nares (narrow nostrils); 3) everted laryngeal saccules (tissue that protrudes into the airway of the voice box); 4) hypoplastic trachea (narrow windpipe). In cases where the soft palate extends into the airways, one procedure that is now commonly performed to help relieve symptoms of the brachycephalic syndrome and prevent the long term consequences of airway collapse is to shorten (resect) the elongated soft palate. - Swallowing pathway During swallowing, the pharynx is used only for the food passage. Chewed food is gradually transported through the fauces and is collected in the back of the mouth (oropharynx) where the bolus is formed prior to swallowing. When the swallow is initiated, the soft palate elevates to separate the nasal cavity from the pharynx while gradual progressive contraction of he pharyngeal wall squeeze the food bolus into the oesophagus while the tongue moves backwardthe and the laryngeal opening is closed by the epiglottis.