Week 6

E Lecture: Pharyngeal Arch Formation

Learning Objectives

• To appreciate that:

  • (a) The head of the mammalian embryo is characterized by the presence of pharyngeal arches.

  • (b) The three germ layers are represented in these arches but are organized in a different way from other body regions.

  • (c) Each arch derivative shares the same innervation and blood supply.

Development of the Neural Structures

• Neurogenesis Stages:

  • Neural Plate: Flat layer of ectoderm that will differentiate into the neural tube.

  • Neural Groove: A depression in the neural plate that will fold to form the neural tube.

  • Neural Folds: Elevated edges of the neural groove that will come together to form the neural tube.

  • Primitive Node and Streak: Structures forming during gastrulation that are critical for the organization of body axes.

Figures and Illustrations

• Figure 16.1: Dorsal view of a developing embryo at the stage when neurulation commences, illustrating key structures such as the neural plate, groove, and folds.

• Figure 16.2: Sections through the embryo at sequential stages of primary neurulation:

  • A: Formation of the neural groove and the location of neural crest cells.

  • B: Formation of neural folds.

  • C: Formation of the neural tube.

Molecular Factors in Neural Development

• Signaling Molecules:

  • Bmps (Bone Morphogenetic Proteins):

  • Bmp-4, Bmp-7 involved in neuroectoderm differentiation.

  • Shh (Sonic Hedgehog):

  • Secreted by the notochord, plays a critical role in the patterning of the neural tube.

  • Role of Notochord: Induces the formation of the neural tube and adjacent structures; it releases Shh that affects the adjacent neural crest cells, influencing their differentiation.

Morphological Changes During Neurulation

• Closure of Neural Groove:

  • Formation of the neural tube progresses through the closure of the neural groove at the rostral and caudal neuropores.

  • Role of Amnion and Somites:

  • Development of surrounding structures during neural tube formation, contributing to embryonic organization.

Ectodermal Derivatives

• Ectoderm Formation:

  • Surface Ectoderm: Gives rise to the epidermis and its associated glands.

  • Neuroectoderm:

  • Composed of neural crest cells (NCC) and the neural tube.

  • Neural Tube: Develops into the Central Nervous System (CNS), including the brain and spinal cord.

  • Neural Crest Cells (NCC):

    • Migrate to various regions, including the pharyngeal arches to form mesenchyme.

Structural Anatomy of Pharyngeal Arches

• Definition and Composition:

  • The pharyngeal arches are six paired aggregations of mesenchyme composed of a mix of mesoderm and neural crest cells.

  • Each arch is encased laterally by a pharyngeal cleft (from surface ectoderm) and medially by a pharyngeal pouch (from endoderm).

  • Development order is cranio-caudal, with Arch 1 appearing first.

  • Differential Growth: Leads to obliteration of Arch 5 and merging of Arches 4 and 6.

Arch Derivatives

• Composition and Functions of Arches:

  • Each arch contains a specific set of muscle, bone, and cartilage derivatives, unique pouch and cleft derivatives, and associated cranial nerves.

Detailed Breakdown of Pharyngeal Arch Derivatives:

• First Arch (Mandibular):

  • Muscles: Muscles of mastication, mylohyoid, rostral belly of digastricus.

  • Bone/Cartilage: Mandible, maxilla, auricle of ear, malleus, incus.

  • Pouch Derivatives: Auditory tube, guttural pouch.

  • Cleft Derivatives: External auditory meatus.

  • Innervation: Trigeminal nerve (V).

• Second Arch (Hyoid):

  • Muscles: Muscles of facial expression, caudal belly of digastricus.

  • Bone/Cartilage: Auricle of ear, stapes, most of hyoid.

  • Pouch Derivatives: Palatine tonsils.

  • Cleft Derivatives: None.

  • Innervation: Facial nerve (VII).

• Third Arch:

  • Muscle: Stylopharyngeus muscle.

  • Bone/Cartilage: Part of hyoid bone.

  • Pouch Derivatives: Parathyroids III, stroma of thymus.

  • Cleft Derivatives: None.

  • Innervation: Glossopharyngeal nerve (IX).

• Fourth and Sixth Arches:

  • Muscle: Constrictors of pharynx, intrinsic muscles of larynx.

  • Bone/Cartilage: Most cartilages of larynx.

  • Pouch Derivatives: Parathyroids IV, stroma of thymus.

  • Cleft Derivatives: None.

  • Innervation: Cranial and recurrent laryngeal branches of vagus (X).

Autonomic Nerve Supply to the Head

• Parasympathetic Innervation:

  • Supplied by cranial nerves III, VII, and IX.

• Sympathetic Innervation:

  • Originates from the thoracic spinal cord, travels up sympathetic trunk, and goes through the middle ear cavity to its targets.

Detailed Distribution of Vagus Nerve (X)

• This nerve has multiple branches:

  • Proximal and distal ganglion distribution affecting the auricular nerve, pharyngeal nerve, and more.

Glossopharyngeal Nerve (IX)

• Functions Include:

  • Sensory and motor functions related to swallowing and taste, and it has distinct branches servicing various head regions.

Conclusion

• The interaction between neural crest cells and mesoderm is crucial for the proper formation of the pharyngeal arches, with a complex interplay of structures, nerves, and functions that are vital for embryonic development.