Question 1. Derivatives of 1 pharyngeal arch. (or) Derivaties of first pharnyngeal arches.
Answer:
- First pharyngeal arch is called manidbular arch.
Skeletal derivative:
- Dorsal part- Gives rise to incus and malleus.
- Ventral part – Surrounded by developing mandible.
- Part of it forms anterior ligament of the malleus and the sphenomandibular ligament.
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- Mesenchyme – Forms maxilla, mandible, zygomatic, bone, palatine bone and part of the temporal bone.
- Cartilage:
- Meckel’s cartilage.
- Muscles:
- Temporalis, masseter, medial and lateral pterygoid.
- Mylohoid.
- Anterior belly of diagastric.
- Tensor palatine, tensor tympani.
- Nerve: Mandibular division of trigeminal nerve.
Question 2. Derivative of Second Pharyngeal Arch.
Answer:
Second Pharyngeal Arch Cartilage:
- Stapes
- Styloid process
- Stylohyoid ligament.
- Lesser cornua of hyoid bone.
- Superior part of body of hyoid bone.
Second Pharyngeal Arch Muscles:
- Muscles of facial expression.
- Buccinator, auricularis, frontalis, platysma, orbicularis oris, orbicularis oculi.
- Posterior belly of disgastric.
- Stylohyoid.
- Stapedius.
- Nerve: facial nerve.
- Synonym: Hyoid arch.
Question 3. Derivatives of Third Pharyngeal Arch.
Answer:
- Third Pharyngeal Arch Cartilage:
- Greater cornua of hyoid bone.
- Lower part of thebody of hyoid bone.
- Muscles stylopharnygeus.
- Third Pharyngeal Arch Nerve: Glosso-pharyngeal.
Question 4. Pharyngeal pouches. (or) Formation and fate first pharyngeal pouch.
Answer:
- In the interval between any two adjoining pharyngeal arches, the endoderm extends outwards in the form of a pouch called endodermal/pharyngeal pouches.
- The endodermal pouches take part in the formation of several important organs.
Question 5. Development of thyroid gland and its anomalies.
Answer:
Development Thyroid Gland :
- The thyroid gland develops mainly from the thyroglossal duct.
- The medial ends of the two mandibular arches are separated by a midline swelling called the tuberculum impar.
- Behind the tuberculum, the epithelium is thickened which soon gets depressed to form a diverticulum called thyroglossal duct
- The diverticulum grows down in the midline into the neck.
- Its tip soon bifurcates.
- Proliferation of the cells of the bifid end gives rise to the two lobes of the thyroid gland.
- Parafollicular cells are derived from the caudal pharyngeal complex which is derived from the fourth and fifth pharyngeal pouches.
Thyroid Gland Anomalies:
1. Anomalies of shape.
- Pyramidal lobe arises from the isthmus or from one of the thyroid lobes.
- Isthmus may be absent.
- One of the lobes may be small or absent.
2. Anomalies of Position.
- Lingual thyroid – under tongue mucosa.
- Intra-lingual thyroid – embedded in tongue musculature.
- Suprahyoid thyroid – above the hyoid bone.
- Infrahyoid thyroid – below the hyoid bone.
- Intrathoracic thyroid – lie in the thorax.
3. Ectopic Thyroid Tissue.
- Thyroid tissue has been observed in the larynx, trachea, oesophagns, pons, pleura, pericardium and ovaries.
4. Remnants of the Thryoglassal Duct.
- Thyroglossal cyst- occur along the course of the duct.
- Thyroglossal fistula – opening at foramen caecum.
- Carcinoma of the thyroglossal duct.
Question 6. Development of Mandible.
Answer:
- About 4th week of intrauterine life, pharyngeal arches are laid down on the lateral and ventral aspects of foregut.
- Mandible is developed from first pharyngeal arch is called mandibular arch.
- The mandibular processes of both sides grow towards each other and fuse in the midline.
- This results in formation of lower lip and lower jaw.
Development of Condylar Process:
- It occurs of 5thwek of intrauterine life.
- Mesenchymal condensation occurs above the ventral side of developing mandible.
- Cartilage is formed at 10th week which undergoes ossification to form condylar process.
Development of Coronoid Process:
- At 10-14 week of interauterine life, secondary accessory cartilages.
- This gets incorporated into expanding ramus and forms coronoid process.
Development of Mental Region:
- Ossification of 1 or 2 small cartilages occurs on either side of symphysis.
- It forms the mental region.
Question 7. Meckel’s Cartilage.
Answer:
- It is formed around 41st-45th day of intrauterine life.
- First structure to be formed is trigeminal nerve.
- Mesenchymal condensation occurs through.
- Activation of neurotrophic factors.
- Induction osteogenesis.
- Ossification occurs starting from bifurcation of inferior alveolar nerve.
- This ossification spreads below and around the nerve.
- Intramembranous ossification spreads dorsally and ventrally resulting in formation of body and ramus of mandible.
- This ossification stops at the site which forms lingula.
- Remaining Meckel’s cartilage forms.
- Mental ossicles
- Incus of sphenoid.
- Anterior ligament of malleus
- Sphenomandibular ligament.
Question 8. Write the persistent structures of Meckel’s cartilage.
Answer:
- Malleus and incus.
- Mental ossicles.
- Anterior ligament of malleus.
- Sphenomandibular ligament.
- Spine of sphenoid.
Question 9. Enumerate the muscles derived from second branchial arch.
Answer:
- Muscles of facial expression.
- Stylohyoid.
- Posterior belly of digastric.
- Stapedius.
Question 10. Derivatives of Third Branchial Arch
Answer:
Third Branchial Arch Skeletal component:
- Greater cornua of hyoid bone
- Lower part of the body of hyoid bone
Third Branchial Arch Muscles:
- Stylopharyngeus
Third Branchial Arch Nerve:
- Glosso-pharyngeal
Question 11. Development of Thyroid
Answer:
- The thyroid gland develops from a median endodermal thyroid diverticulum
- This grows down in front of the neck from the floor of the primitive pharynx
- The lower end of the diverticulum enlarges to form the gland
- The rest of the diverticulum remains narrow and forms thyroglossal duct Most of it soon disappears
- Remnants of it forms thyroglossal cysts or fistulas
Question 12. Development of Thymus
Answer:
- The thymus develops from the endoderm of third pharyngeal pouch
- After separation from the inferior parathyroid rudiment, each thymic rudiment has two parts
- Thinner cranial part
- It forms cervical part of thymus
- Thinner cranial part
- Broader caudal part
- Broader parts of both the sides enters the thorax and become united to each other by connective tissue
- The endodermal cells of the thymus are invaded by vascular mesoderm
- This mesoderm partially breaks up the thymic tissue into isolated masses
- This results in its lobulated surface
Question 13. Reichert’s Cartilage
Answer:
- It is cartilage of second pharyngeal arch
- It is composed of two distinct cartilagenous segments joined by a faint layer of mesenchyme
Reichert’s Cartilage Derivatives:
- Stapes of middle ear – formed from dorsal ends of cartilage
- Lesser cornua and body of hyoid bone – formed from ventral portion of cartilage
- Styloid process of temporal bone
- Stylohyoid ligament – formed from cartilage lying between hyoid bone and styloid process
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