Pharyngeal arches and congenital malformation

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Lecture Introduction

Lecture Title: Pharyngeal arches and congenital malformationPresented by: Dr. Munira Xaymardan

Learning Objectives

• Describe the pharyngeal arches and their contributions to:

  • Head and neck structures, including the development of facial features and structures.

  • Cranial nerves that innervate various regions of the face and neck, essential for sensory and motor functions.

  • Muscles contributing to facial expressions, chewing, and other vital movements.

  • Other tissues such as bones, ligaments, and connective tissues that provide structural and functional support to the head and neck.

Frontonasal Prominence and Stomodeum

Key Structures:

• Frontonasal prominence: The precursor to the forehead, bridge of the nose, and nasal septum.

• Cardiac bulge: An early indicator of heart development located in the thoracic region, influencing later development of structures.

• Stomodeum: The primitive oral cavity that forms during the 4th week of embryonic development, marking a key developmental milestone.

Developmental Timeline:

• Week 4: Formation of stomodeum occurs, allowing for the differentiation of the oral cavity. This week is critical for the initial stages of craniofacial development.

Pharyngeal Arches Overview

• Ectoderm Contributions:

  • Dermal coverage: The outer layer of skin around the arches.

  • Sensory placodes: Special thickenings that form the sensory organs, such as the inner ear.

  • Neural crest cells (ectomesenchymal): These cells contribute to many structures, including bones and nerves.

• Mesoderm Contributions:

  • Muscle: Various muscles of the head and neck arise from the mesoderm of pharyngeal arches.

  • Bone/cartilage: Forms the skeletal elements, including the jaw and cartilage structures.

  • Vascular progenitors: Contribute to the blood vessels associated with the head and neck region.

• Endoderm Contributions:

  • Inner lining: Forms the lining of the throat and associated structures, essential for respiratory and digestive systems.

Development of Pharyngeal Arches

Pharyngeal arches develop in a staggered manner, each arch contributing distinct structures to the developing fetus. Each arch is characterized by an infiltrate of cells from both neural crest and mesoderm, leading to diverse tissue formations. In humans, there are 5 pairs of visible pharyngeal arches, the “5th arch” is not visible in adult anatomy.

List of Arches:

1. First Arch

2. Second Arch

3. Third Arch

4. Fourth Arch

5. Sixth Arch

Anatomy of Pharyngeal Structures

• Pharyngeal Arches: The main portion contains mesenchymal tissues, muscle, bone progenitors, and nerves necessary for forming various structures.

• Clefts: Grooves located on the external surfaces of the pharyngeal arches, important for various developmental processes.

• Pharyngeal Pouches: Internal structures located opposite the clefts, crucial for the formation of certain glands and other anatomical features.

First Pharyngeal Arch

Key Elements:

• Maxilla and Mandibula: These bones form the upper and lower jaw.

• Trigeminal Nerve (CN V): The principal nerve responsible for sensation in the face and motor functions such as biting and chewing.

Muscles Innervated by CN V:

• Temporalis

• Masseter

• Pterygoid muscles

• Mylohyoid

• Anterior belly of digastric

• Tensor tympani

• Tensor veli palatini

Innervation:

• Anterior 2/3 of the tongue: Responsible for sensory functions such as taste.

Second Pharyngeal Arch

Nerves:

• Facial nerve (CN VII): Responsible for the muscles of facial expression.

• Vestibulocochlear nerve (CN VIII): Involved in hearing and balance.

Muscles:

• Facial expressions (e.g., smiling, frowning)

• Posterior belly of digastric

Bone Contributions:

• Hyoid bone: Vital for anchoring the tongue.

• Part of the temporal bone: Important for hearing structures.

Pouches and Clefts Formation

• First Pouch (between the 1st and 2nd arches):

  • Structures include the internal acoustic meatus, tympanic membrane, and Eustachian tube.

• First Cleft (between the 1st and 2nd arches):

  • Contributes to the external acoustic meatus and tympanic membrane, key for hearing.

Third, Fourth, and Sixth Arches

Third Arch:

• Nerve: CN IX (Glossopharyngeal nerve)

• Structures: Common carotid artery and internal carotid arteries: Essential for blood supply to the brain and neck.

Fourth Arch:

• Structures: Form the superior parathyroid glands and epiglottic cartilages aiding in swallowing and protecting the airway.

• CN X (Vagus nerve): Plays a crucial role in autonomic control of the heart and digestive tract.

Fifth Arch:

• Incorporates into the fourth arch during development, thus playing a supportive role.

Sixth Arch:

• Structures: Responsible for the intrinsic muscles of the larynx, crucial for voice production, and cricoid cartilages.

• CN X (Recurrent laryngeal nerve): Vital for motor innervation of the laryngeal muscles.

Tongue Contributions by Arches

• First Arch:

  • Anterior 2/3 of the tongue, providing sensory innervation via CN V.

• Third and Fourth Arches:

  • Posterior 2/3 of the tongue, innervated by CN IX and CN X, responsible for taste and texture.

Pharyngeal Pouches and Clefts

• Pouches:

  • 3rd Pouch: Forms the thyroid and parathyroid glands.

  • 4th Pouch: Contributes to the parathyroid gland, essential for calcium homeostasis.

• Clefts:

  • Other clefts typically disappear during development. If unresolved, may develop into a branchial cleft cyst (usually located at the anterior border of the sternocleidomastoid muscle).

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Reproduction and communication of material message restated in accordance with Copyright Act 1968.