Development of head and neck

Development of Head and Neck

Overview

  • Instructor: Hui Zhao, Ph.D.
  • Affiliation: School of Biomedical Sciences, CU Medicine, Faculty of Medicine, The Chinese University of Hong Kong.

Learning Objectives

  • Embryonic Precursors: Understand the embryonic precursors leading to adult head and neck structures.
  • Pharyngeal Arches: Describe the role of pharyngeal arches in forming distinct structures of the head and neck.
  • Comparative Development: Compare and contrast the development of pharyngeal pouches, clefts, arches, mesoderm, nerves, and connective tissues.
  • Congenital Defects: Utilize knowledge to identify causes of congenital defects seen clinically.

Development Stages

Week 1: Fertilization to Implantation

  • Fertilization: Occurs upon the union of sperm and egg, typically in the fallopian tube.
  • Terminology:
    • Zygote: The initial diploid cell formed after fertilization.
    • Morula: A ball of cells resulting from cleavage episodes of the zygote.
    • Blastula: The morula develops into a structure with a cavity called the blastocoel.
  • Implantation: Occurs when the blastula adheres to the uterine wall by Day 6.

Week 2: Formation of Bilaminar Disc

  • Inner Cell Mass: Differentiates into a bilaminar disc comprised of:
    • Epiblast: The upper layer.
    • Hypoblast: The lower layer.
  • Cavities Formed:
    • Amniotic Cavity: Space appears above the bilaminar disc.
    • Yolk Sac Cavity: Positioned under the disc.
  • Primitive Node and Primitive Streak: Present on the epiblast and indicate the future body's midline.

Week 3: Gastrulation

  • Process: Transition from a single sheet of cells to three definitive germ layers:
    • Ectoderm: Forms epidermis, nervous system, and parts of the facial skeleton.
    • Mesoderm: Gives rise to muscles, skeleton, and connective tissue.
    • Endoderm: Forms lining of hollow organ systems.

Week 4: Neurulation

  • Neurulation: The transformation of the neural plate into the neural tube, includes:
    • Neural Plate: Initially flat structure.
    • Neural Fold: The folds of the neural plate.
    • Neural Groove: The invaginated part.
    • Neural Tube: Fully formed structure that develops into the central nervous system.
    • Neural Crest: A group of cells that will migrate to form various structures, including some cranial arches.

Germ Layer Development

  • Ectodermal Derivatives:
    • Surface Ectoderm: Forms skin and epithelia.
    • Placodes: Structure forming lens and inner ear.
    • Neural Crest: Contributes to various tissues in the peripheral nervous system.
    • Neural Tube: Forms brain and spinal cord.
  • Mesodermal Derivatives: All muscular and skeletal elements.
  • Endodermal Derivatives: Forms organs such as liver, pancreas, and thyroid.

Pharyngeal Arches

General Overview

  • Origin: Pharyngeal arches begin to develop in the 4th week with migration of neural crest cells into the head and neck region.
  • Structure Composition: Each arch contains:
    • Core: Mesenchyme covered by ectoderm externally and endoderm internally.
    • Sources of Mesenchyme:
    1. Paraxial mesoderm
    2. Lateral plate mesoderm
    3. Neural crest cells

Pharyngeal Clefts and Pouches

  • Clefts: External aspects lined by ectoderm. Labeled as Cleft 1, Cleft 2, Cleft 3, Cleft 4, and note that Cleft 6 is not visible.
  • Pouches: Internally located and lined by endoderm. Each arch aligns with a respective endodermal pouch.

Specific Pharyngeal Arches and Their Components

Arch 1 (Mandibular Arch)
  • Innervation: Trigeminal nerve (CN 5).
  • Skeletal Derivatives: Maxilla, zygomatic bone, squamous part of the temporal bone, and mandible.
  • Muscle Derivatives: Muscles of mastication (temporalis, masseter, medial/lateral pterygoids), mylohyoid, and others.
  • Vascular Component: First aortic arch develops into maxillary artery.
Arch 2 (Hyoid Arch)
  • Innervation: Facial nerve (CN 7).
  • Skeletal Derivatives: Parts of the hyoid bone, styloid process, and stapes.
  • Muscle Derivatives: Muscles of facial expression and others.
  • Vascular Component: Second aortic arch develops into stapedial artery.
Arch 3
  • Innervation: Glossopharyngeal nerve (CN 9).
  • Skeletal Derivatives: Body and greater horn of the hyoid bone.
  • Muscle Derivatives: Stylopharyngeus muscle.
  • Vascular Component: Third aortic arch forms the common carotid and part of the internal carotid arteries.
Arch 4
  • Innervation: Vagus nerve (CN 10).
  • Skeletal Derivatives: Thyroid cartilage.
  • Muscle Derivatives: Cricothyroid muscle and pharyngeal constrictors.
  • Vascular Component: Fourth aortic arch forms the subclavian artery on the right and the arch of aorta on the left.
Arch 6
  • Innervation: Vagus nerve (CN 10).
  • Skeletal Derivatives: Key laryngeal cartilages (cricoid, arytenoid, corniculate, cuneiform).
  • Muscle Derivatives: All laryngeal muscles except cricothyroid.
  • Vascular Component: Sixth aortic arch forms the pulmonary arteries.

Congenital Defects Linked to Pharyngeal Development

  • Definition: Abnormalities can arise from failure of proper arch fusion and development.
  • Cleft Lip Types: Results from fusion failure among prominences:
    • Median: fails between medial nasal prominences.
    • Unilateral/Bilateral: fails between maxillary and medial nasal prominences.
    • Oblique: fails among maxillary, medial nasal, and lateral nasal prominences.
  • Cleft Palate: Failure of palatine shelves fusion leading to a communication between oral and nasal cavities.

References

  • Sadler, T.W. – Medical Embryology.
  • Moore, K.L., Persaud, T.V.N., Torchia, M.G. – The Developing Human: Clinically Oriented Embryology.
  • Schoenwolf, G.C., Larsen, W.J. – Larsen’s Human Embryology.