Embryology

💫 Pharyngeal arches

Pharyngeal arches are transient bulges that appear on the lateral aspect of the embryonic head region between day 27 to 29. Between weeks 5 to 8, the mesenchyme differentiates into skeletal, muscular, and vascular derivatives.

Pharyngeal pouches are endodermal evaginations between the arches, giving rise to internal structures. 

• First pouch gives rise to the tympanic cavity and auditory tube
• Second pouch gives rise to the lining of the palatine tonsils and the tonsillar fossa
• Third pouch gives rise to the inferior parathyroids and thymus 
• Fourth pouch gives rise to the superior parathyroids and ultimobranchial body

Pharyngeal clefts normally regress except the first cleft which forms the external auditory meatus. 

Persistence or malformation of pharyngeal arch structures yield congenital syndromes such as Treacher‑Collins (1^st arch neural‑crest dysfunction), Di George (3^rd & 4^th pouch aplasia), and branchial cleft cysts/fistulae. During the 6th week of gestation, the second pharyngeal arch overgrows the third and fourth arches, creating a temporary ectoderm-lined depression known as the cervical sinus. This sinus is meant to fuse and be reabsorbed. A branchial cleft cyst is a congenital fluid-filled sac that forms when the cervical sinus from this second arch fails to fully obliterate.

🌙 Derivations of the pharyngeal arches

Remember that CN 5 (V3) - 7 - 9 derive from the 1st - 2nd - 3rd arches. The SLN and RLN of CN 10 is derived from the 4th and 6th arch.

So if a patient presents with unilateral facial weakness, the affected nerve would be the facial CN VII of the second pharyngeal arch.

🦴 Germ Layers & Vertebral Development

• Vertebrae develop from sclerotome, a derivative of the paraxial mesoderm (somites).
  • Failure of complete formation or segmentation of the sclerotome leads to hemivertebra, an important cause of congenital scoliosis.
• Ectoderm forms skin and nervous system structures.
• Endoderm forms gut and intestinal epithelial linings.
• Notochord induces vertebral column development but does not form vertebrae directly.
• Neural tube develops into brain and spinal cord

📚 Exam-style questions

• The muscles of mastication develop from which pharyngeal arch?
  • [[First::Muscles of mastication: temporalis, masseter, medial/lateral pterygoids, supplied by CN V₃]]
  • Second
  • Third
  • Fourth
  • Sixth

1. ➡️ Muscles of mastication, tensor tympani, anterior digastric, mylohyoid
2. ➡️ Muscles of facial expression, posterior digastric, stylohyoid
3. ➡️ Stylopharyngeus
4. ➡️ Cricothyroid, pharyngeal constrictors, levator veli palatini
5. ➡️
6. ➡️ All intrinsic laryngeal muscles except the cricothyroid

• Which cranial nerve is associated with the second pharyngeal arch?
  • Facial
  • ((Trigeminal::CN V₃, arises from the first pharyngeal arch))
  • ((Glossopharyngeal::CN IX is from the third arch, which forms the stylopharyngeus))
  • ((Vagus::Associated with the fourth and sixth arches, forming laryngeal and pharyngeal muscles))

1. ➡️ CN V₃
2. ➡️ CN VII
3. ➡️ CN IX
4. ➡️ CN X, SLN
5. ➡️
6. ➡️ CN X, RLN

• From which arch does the nerve that innervates the muscles of facial expression originate?
  • First arch
  • Second arch
  • Third arch
  • Sixth arch

• A patient presents with unilateral facial weakness. From which pharyngeal arch does the affected nerve originate?
  • ((First arch::First arch gives rise to muscles of mastication: temporalis, masseter, medial/lateral pterygoids, supplied by CN V₃))
  • [[Second arch::Remember that CN 5_C, 7, 9, 10, 10 derive from the 1^st, 2^nd 3^rd , 4^th and 6^th pharyngeal arches respectively.]]
  • ((Third arch::Third arch gives rise to stylopharyngeus muscle, innervated by CN IX))
  • ((Sixth arch::Sixth arch gives rise to all intrinsic laryngeal muscles except the cricothyroid, innervated by the recurrent laryngeal n.))

1. ⏩ Trigeminal CN V₃
2. ⏩ Facial CN VII
3. ⏩ Glossopharyngeal CN IX
4. ⏩ Vagus CN X, SLN
5. ⏩
6. ⏩ Vagus CN X, RLN

• A 21-year-old man undergoes an uncomplicated tonsillectomy for recurrent tonsillitis. Postoperatively, he complains of otalgia despite no ear pathology. The nerve responsible for this referred pain originates from which pharyngeal arch?
  • First arch
  • Third arch
  • Fourth arch
  • Fifth arch

The glossopharyngeal CN IX originates from the 3^rd pharyngeal arch. It supplies the tonsils, posterior third of the tongue, and the tympanic plexus. Referred otalgia after tonsillectomy occurs due to shared sensory innervation between the tonsillar fossa and the middle ear via CN IX, from the 3^rd pharyngeal arch.

• What is the embryological arch of origin of the ductus arteriosus?
  • ((First arch::Contributes to maxillary arteries))
  • ((Second arch::Becomes the stapedial artery, not involved in ductal formation))
  • ((Third arch::Forms common carotid and internal carotid arteries))
  • ((Fourth arch::Forms part of the aortic arch (left) and right subclavian (right)))
  • [[Sixth arch::The left sixth pharyngeal arch artery persists to form the ductus arteriosus, connecting the pulmonary artery to the aortic arch in the fetal circulation.]]

1. ➡️ Maxillary arteries
2. ➡️ Stapedial artery
3. ➡️ Common carotid and internal carotid arteries
4. ➡️ Aortic arch, right subclavian
5. ➡️
6. ➡️ Ductus arteriosus

• What is the embryological arch of origin of the left and right pulmonary artery?
  • Second arch
  • Third arch
  • Fourth arch
  • Sixth arch

• The inferior parathyroid gland is derived from which structure?
  • First pouch
  • Second pouch
  • [[Third pouch::The third pouch gives rise to the inferior parathyroids and thymus.]]
  • Fourth pouch

• A cervical sinus forms from which branchial arch?
  • First arch
  • Second arch
  • Fourth arch
  • Third pouch

• A branchial cyst is a derivative of which structure?
  • First arch
  • [[Second arch::A branchial cleft cyst is a congenital fluid-filled sac that forms when the cervical sinus fails to fully obliterate It is a remnant of this embryonic structure that persists after birth.]]
  • Fourth arch
  • Third pouch

• A pharyngeal pouch (Zenker’s diverticulum) arises through a weak area in the posterior pharyngeal wall. Between which two muscles does it occur?
  • Thyrohyoid and cricopharyngeus
  • Inferior constrictor and thyropharyngeus
  • Thyropharyngeus and cricopharyngeus
  • Stylopharyngeus and cricopharyngeus

• Where is a pharyngeal (Zenker’s) pouch located?
  • Below inferior constrictor
  • Through inferior constrictors
  • Between superior and middle constrictor
  • Between middle and inferior constrictor

The Killian dehiscence is a triangular area of weakness formed by the oblique fibres (thyropharyngeus), and the transverse fibres (cricopharyngeus) of the inferior pharyngeal constrictor. Upper GI endoscopy is contraindicated for pharyngeal pouch and an UGI fluoroscopic swallowing study is done instead.

• Hemivertebra develops from which embryological structure?
  • [[Mesoderm::Mesoderm develops into a range of tissues including muscles, bone, cartilage, blood, connective tissues, the circulatory system, the urogenital system and dermis of the skin and the lining of the body cavity.]]
  • ((Ectoderm::Ectoderm forms skin and nervous system structures.))
• ((Endoderm::Endoderm forms gut and intestinal epithelial linings.))
• ((Notochord::Notochord induces vertebral column development but does not form vertebrae directly.))
• ((Neural tube::Neural tube develops into brain and spinal cord.))

• Failure of development of the caudal portion of the metanephros results in which condition?
  • Horseshoe kidney
  • [[Renal agenesis::The metanephros gives rise to the definitive kidneys and if the caudal portion of the metanephros fails to develop properly, it can result in renal agenesis, meaning one or both kidneys fail to form.]]
  • Polycystic kidney disease
  • Ureter agenesis 
  • Double ureter 

• A child presents with epispadias and bladder exstrophy. What is the embryological origin of the bladder?
  • [[Urogenital sinus::Endodermal structure giving rise to bladder (except trigone which derives from mesonephric duct). ]]

• Hypospadias results from a developmental defect involving which of the following structures?
  • [[Urogenital fold::Hypospadias occurs due to failure of the urogenital folds to fuse properly.]]

• An atrioventricular septal defect results from a developmental defect in which embryological structure?
  • [[Endocardial cushion::Endocardial cushions are crucial for forming the lower atrial septum, upper ventricular septum, and the atrioventricular valves, and their failure to fuse properly leads to the characteristic defects seen in AVSD.]]

• During fetal development, at what gestational age do the intestines return to the abdominal cavity after physiological herniation into the umbilical cord?
  • 8 weeks
  • [[10 weeks::During normal embryonic development, the midgut herniates into the umbilical cord around week 6 due to rapid growth and limited intra-abdominal space. By week 10 to 11, the intestines retract back into the abdomen. Failure of this process results in omphalocele.]]
  • 16 weeks
  • 20 weeks

• Which of the following is the axis of gut rotation during development? 
  • Inferior mesenteric artery
  • Left gastric artery
  • [[Superior mesenteric artery::The midgut herniates into the umbilical cord around week 6, then returns to the abdomen by week 10. While outside the abdominal cavity, the midgut rotates 270° counterclockwise around the superior mesenteric artery.]]

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