Organogenesis 2 - The Pharyngeal Apparatus PDF
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Humanitas University
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Summary
This document explores the development of the pharyngeal apparatus in humans, covering its structures, origins, and contributions to various anatomical features. It explains the role of neural crest cells, paraxial mesoderm, and endoderm in the process and highlights the importance of the pharyngeal apparatus in the formation of facial structures and neck regions.
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When things go wrong in the development of the pharyngeal apparatus usually not just one problem arises —> es: book “Wonder” In fishes water is engulfed and then oxygen is extracted from it at the level of the gills, which are full of blood vessels —> this is why we developed a pharyngeal apparatus...
When things go wrong in the development of the pharyngeal apparatus usually not just one problem arises —> es: book “Wonder” In fishes water is engulfed and then oxygen is extracted from it at the level of the gills, which are full of blood vessels —> this is why we developed a pharyngeal apparatus. Even though we don’t need to extract oxygen from water we still kept our pharyngeal apparatus (which gives rise to many structures of the face and neck). During the 4th week branchial or pharyngeal arches develop around the primordial pharynx from paraxial mesoderm and due to the migration of neural crest cells The pharyngeal arches support the wall of the primordial pharynx. The pharyngeal arches are numbered 1, 2, 3, 4, 6 (5 in total, there is no number 5 though because it doesn’t develop in humans). On the outside the arches are separated by grooves or clefts, which correspond to internal pouches The formation of the pharyngeal arches is concomitant with the L pain migration of neural crest cells, so if neural crest cells don’t migrate in the right way many things can go wrong ARCHES —> they originate from the paraxial mesoderm (territory of the head, where it didn’t undergo somitogenesis) and from the migration of neural crest cells (they migrate in the inner part of the pharyngeal arches and they constitute their inner mesenchyme). POUCHES —> they originate from outpouchings of the pharynx endoderm GROOVES —> they originate from invaginations of ectoderm PHARYNGEAL ARCH ARTERIES Pharyngeal arch arteries connect the aortic sac to the dorsal aortae. Aortic arch arteries will then, after a massive rearrangement, evolve into part of the blood vessels that will supply the head, upper limbs and lungs PHARYNGEAL ARCHES: Mesenchymal core (coming from paraxial mesoderm and neural crest cells) —> the mesenchymal core will give rise to territories of dermis (head and neck, the rest of dermis originates from the dermatome arising from somites), smooth muscle (those part of the organs in the territory of the neck), skeletal elements of the head and striated muscle (like muscles for facial expression or masticatory muscles) External ectoderm Internal endoderm Each arch contains its own: Artery Cartilaginous road Nerve 1st arch (mandibular arch) —> contributes to the formation of the face —> it gives rise to two prominences around the stomodeum: maxillary prominence (future maxilla) and mandibular prominence (future mandibula) 2nd arch (hyoid arch) —> overgrows the 3rd and 4th arches and forms the cervical sinus (cavity). The cervical sinus will then disappear during the 7th week. Thanks to this overgrowth we have a smooth neck when adults (bulges are covered) DEVELOPMENT OF THE FACE The face is usually made of five prominences —> central frontonasal prominence, 2 maxillary prominences, 2 mandibular prominences. These prominences appear during the 4th week and surround the stomodeum (still closed by the oropharyngeal membrane). Frontonasal = rostral to stomodeum, maxillary = lateral, mandibular = caudal Frontonasal prominence —> on the side of it there is the formation of two placodes called nasal placodes. On the 5th week the nasal pit will be formed and on the 5th and a half week the nasal placodes will develop into a medial nasal process (in the middle) and a lateral nasal process (on the sides). e sogou.se During week 7th the nasomedial processes start merging giving rise to the philtrum of the lip Ears will develop from the ear tubercle in the upper neck and then they will move to reach their final position. Eyes arise laterally from thickenings of endoderm (optic placodes) The medial nasal processes (from nasal placodes) and the maxillary processes (from 1st arch) are important for the formation of the upper lip and palate (especially hard palate) —> this is very important because it separates the nasal cavity from the oral cavity. Babies born with problems in the formation of the palate need to have it reconstructed (otherwise food would go in airways) FORMATION OF THE PALATE (6th to 12th week) The palate is divided into —> 1. Primary palate: triangular area of the hard palate anterior to the incisive foramen and includes a portion of the alveolar ridge 2. Secondary palate: it is the remaining hard palate and all the soft palate The two medial nasal processes come together on the midline and fuse to form the primary palate (while on the outside they form the upper lip). Then the two palatine processes (or shelves, coming from the maxillary prominences) come together on the midline and fuse to form the secondary palate The primary and secondary palates then fuse together to form the definitive palate. The posterior parts of the palatine processes do not ossify —> soft palate Initially oral cavity is in communication with the nasal cavity, but after the formation of the palate they are separated Themoretheopenings theworsetheconsequences CLEFT PALATE AND CLEFT LIPS Defects in the formation of the hard palate and upper lip. The cleft palate can be anterior, posterior or both anterior and posterior, while the cleft lip can be unilateral or bilateral. There might be combinations of these such as a unilateral cleft lip with anterior palate, a bilateral cleft lip with full palate, a unilateral cleft lip with full palate or a bilateral cleft lip with anterior palate). Only one out of 5 cleft palates is inherited Many cleft palate result from non genetic factors (environmental factors) such as: Rubella (part for TORCH) and other infections (congenital) Medications Alcohol and drug abuse Cigarettes smoking Lack of vitamins during pregnancy This is a high risk period because we are around the 4th week and a woman might not be aware of her pregnancy yet (and thus consume alcohol or medications) BONES AND CARTILAGES DERIVED FROM THE ARCHES The formation of these bones will either occur through endochondral ossification (ossification of cartilage) or through direct ossification (membranous ossification) of the arch mesenchyme 1st arch —> its cartilage is called Meckel’s cartilage and its ossification will give rise to 2 of the 3 bones of the middle ear (malleus and incus). The direct ossification of the first arch instead gives rise to the maxilla, zygomatic and squamous portion of the temporal bone and the mandible. The mesenchyme of the first arch originates mostly from neural crest cells, so if they don’t migrate correctly then some of these bones might not develop correctly. 2nd arch —> here the cartilage is called Reichert cartilage and it gives rise to the stapes (3rd bone of the middle ear), styloid process of the temporal bone, stylohyloid ligament (connects the styloid bone to the hyoid bone), lesser horn and upper rim of the body of the hyoid bone (anterior portion of the neck, not linked to any other bones, it is just a doesn't articulate site of attachment for muscles and ligaments). 3rd arch —> gives rise to the rest of the hyoid bone (lower rim and greater horns) HYOID BONE ni Isaddthis.at a'interarietothe Attachment for suprahyoideal and infrahyoideal muscles Attachment for pharyngeal muscles (like the middle constrictor) Attachment for muscles of the floor of the oral cavity 4th and 6th arches —> no bones originate from here, just cartilage —> cartilage of the larynx (thyroid, cricoid, arytenoids, corniculates and cuneiform cartilages) MUSCLES DERIVED FROM THE PHARYNGEAL ARCHES (and their innervation) 1st arch —> masticatory muscles (temporalis, masseter, internal and external pterygoid), anterior belly of the digastricus, mylohyoid, tensor veli palatini, tensor tympani (helps to tense the tympanic membrane). NERVE —> mandibular branch of the 5th cranial nerve (TRIGEMINUS) 2nd arch —> mimic muscles (muscles of facial expression), stapedium, stylohyoideum and posterior belly of digastricus. NERVE —> 7th cranial nerve (FACIAL NERVE) 3rd arch —> stylopharyngeus muscle. NERVE: 9th cranial nerve (GLOSSOPHARYNGEAL) 4th arch —> circo-thyroideus, levator veli palatini, constrictor muscles of the pharynx. NERVE —> a branch of the 10th cranial nerve or vagus nerve (SUPERIOR LARYNGEAL NERVE) 6th arch —> intrinsic muscles of the larynx (except the circo-thyroideal), striated muscle of the oesophagus. NERVE —> another branch of the 10th cranial nerve or vagus nerve (RECURRENT LARYNGEAL NERVE) The organs of the upper part of the respiratory system, even though they are viscera, present striated muscles (partially under our voluntary control, we have an initial voluntary control but we can’t control each muscle one by one). All the muscles originating from pharyngeal arches are STRIATED Pharyngeal arches also contribute to the formation of the dermis and mucosa of the head and neck, which are innervate by the correspondent nerves of the arches. partofthebones frommesenchyme originate origin mesodermal of mar ifeng.mgearyme 8Th PHARYNGEAL POUCHES Pouches are endodermal pockets of the primordial pharynx. We have 4 well defined pouches that correspond externally to the grooves. Pouches and grooves are separated from each other by pharyngeal membranes 1st pouch —> it will form the tubotympanic recess, which in the adult will form the Eustachian tube (or pharyngotympanic tube) and the middle ear. A portion of the middle ear is in communication with the pharynx, which is why when we are in planes we swallow to clear our blocked ears (the Eustachian tube is distended to let air pass and make the pressure in the middle ear equal to that of the outside of the ears. 2nd pouch —> palatine tonsil (part of the Waldeyer’s ring). Lymphoid tissue doesn’t originate from the pouch (which only gives a framework) but instead it originates in the bone marrow and then reaches the 2nd pouch to organise themselves into palatine tonsils 3rd pouch —> it gives rise to the thymus and inferior parathyroid glands. The pouch starts growing inferiorly and gives rise to the precursors of the inferior parathyroid glands and thymus (behind the sternum). 4th pouch —> it is divided in a dorsal and ventral divisions. The dorsal division will form the superior parathyroid glands while the ventral division (ultimopharyngeal body) will give rise to parafollicular cells (C cells, originate from neural crest cells) The thymus will not only originate by the 3rd pouch (endoderm) but also by mesenchyme of neural crest origin (which form the cords of the thymus that divide it into lobules). After this the thymus will be colonised by prothymocytes (again, lymphoid tissue does NOT originate from endoderm). Some defects of the development of the pharyngeal apparatus can lead to problems in the immune system GROOVES (or clefts) and MEMBRANES The 2nd arch grows over the 3rd, 4th and 6th arches so the membranes get substituted 1st groove/cleft —> gives rise to the external acoustic meatus 1st membrane —> gives rise to the tympanic membrane DEVELOPMENT OF THE THYROID GLAND importantforthe development someorgans likethecornea ofBy 20 weeks the level of thyroxine and fetal thyroid stimulating hormone begin to increase, reaching adult level by 35 weeks. The thyroid gland is the first endocrine gland to develop. In the region where the tongue is forming there is a thickening of the pharynx floor (24th day) which starts to grow outside forming the thyroid diverticulum. Initially the thyroid remains connected to the floor of the pharynx through the thyroglossal duct (ex diverticulum, which then atrophies) Foremen coecum —> remnants of the thyroglossal duct (blind ended tube) vascularised very ebreeding THYROGLOSSAL DUCT REMNANTS They can give rise to cysts (which then might get filled with fluid and compress adjoining structures) or ectopic thyroid tissue (like pyramidal lobe, around 70% of people have it) FIRST ARCH SYNDROME (or TREACHER COLLINS SYNDROME) Common components of the syndrome: hypoplasia of the mandible and facial bones, malformation of external and middle ears (conductive deafness due to non-development of the bones), high or cleft palate, faulty dentition, coloboma-type defect of the lower eyelid (cut in the lower eyelid), airway blockage and frequent respiratory infections. This syndrome affects both sexes and is frequently familial (40% of cases, autosomal dominant pattern with high penetrance and extreme variability in expression). 60% of newly diagnosed cases are due to de novo mutations A defect in the development of the first arch can lead to abnormalities in the eyes, ears, mandible and palate DIGEORGE SYNDROME (or VELOCARDIOFACIAL SYNDROME) Problems in the development of the 3rd and 4th pouch — Iii > defective interaction between endoderm and neural crest cells. It can be caused by a microdeletion in the chromosome 22. It can lead to hypoplastic thymus and parathyroids. It can lead to defects in the outflow tract of the heart (interacts with the developing heart), facial abnormalities (cleft palate), decreased immunity (thymus), complications related to low levels of calcium in blood (C cells), delayed development with behavioural and emotional problems, delayed growth, reduced muscle tone… ABNORMAL POSITION OF GLANDS During the process of migration the glands might end up in places where they shouldn’t be. For example there might be cases in which a parathyroid gland is located just below the hyoid bone (undescended parathyroid gland) because it gets stuck during migration. ECTOPIC THYROID TISSUE —> During the process of migration some islands of thyroid tissue might deposit in places where they should be found. Because of the origin of the thyroid gland (base of the tongue) the heterotopic thyroid tissue can be found between the base of the tongue and the actual thyroid location (on the midline, migration path followed). There might be thyroid tissue in the central part of the neck for example. The most common type is the lingual thyroid (again, base of the tongue, 90% of cases). Thyroid tissue can also be found in thyroglossal duct cysts (25-30% of cases), in neck midline organs (larynx and trachea) and in the superior mediastinum (it keeps descending after the place where it should stop). There can also be non-midline sites (but in most cases it is found in the midline), for example in salivary glands or in the retropharyngeal space or in cervical lymph nodes. Distant sites are very rare, but sometimes thyroid tissue can also be found in the ovaries (called struma ovarii) —> in this case however we are talking about a teratoma. pregamancytoo Most of the time ectopic thyroid tissue is asymptomatic and found incidentally. The symptoms are usually related to size and location of the heterotopic tissue. For example in puberty the area with heterotopic tissue might become larger because the thyroid becomes very active (due to increased levels of thyrotropin produced by the pituitary gland), if the tissue is at the base of the tongue there might be problems in swallowing, while if it is at the pharynx there might be a blockage of airways. Hyperthyroidism deriving from ectopic thyroid tissue is rare because the heterotopia is usually not active. However, there are around 50% of cases in which the ectopic tissue is the ONLY functional thyroid tissue (the normal thyroid gland isn’t functional), so when ectopic tissue is found its functionality has to be assessed before proceeding with a surgical removal. If there is an autoimmune disease (or Neoplasms or inflammatory diseases) of the thyroid gland both the heterotopic and normal tissue might be affected (or just one of the two). It can happen that the process of obliteration of the cervical sinus (space made by the overgrowth of the 2nd arch) doesn’t take place correctly —> LATERAL CYSTS, SINUSES and FISTULAE might develop (abnormal persistence of pharyngeal grooves or pouches). Cysts —> fill-fluid cavity delimited by epithelium derived from the persistence of a pouch, groove or cervical sinus. These cysts are usually not visible until puberty (because the metabolism changes and the hormonal secretion too so the hormonal cells lining the spouse might become larger and be seen or felt). Sinuses —> problems in the closure of cervical sinuses, they’re closed on one side and open on the other (on the primitive pharynx or on the skin) Fistulae —> communication between two places that shouldn’t communicate with one another (tube open at both ends). In this case, on one side they open at the pharynx and on the other they open on the skin There might be situations in which fistulae, cysts and sinuses are present at the same time. Usually the sinuses, cysts and fistulae are located on the lateral surface of the neck in front of the anterior margin of the sternocleidomastoid muscle or in the preauricular region Swollen areas of the neck might be taken seriously but they’re not always related to catastrophic conditions (tumours). They might also have an embryological origin and not be so serious A cyst derived from a non obliteration of the thyroglossal duct is on the midline while if it is related to pouches or grooves or cervical sinus it will be lateral