Overview
General information
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3 major germ layers form the initial developing embryo:
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Ectoderm
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Mesoderm
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Endoderm
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Mesoderm differentiates into:
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Paraxial mesoderm
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Intermediate mesoderm
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Lateral plate mesoderm
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Ectoderm gives rise to 3 layers:
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Neuroectoderm
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Neural crest
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Epidermis
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The head and neck are formed by:
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Paraxial mesoderm
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Lateral plate mesoderm
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Neural crest–derived mesoderm
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Ectodermal placodes
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Most of the head and neck is formed from the pharyngeal region of the embryo
Pharyngeal arches
General information
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Start forming in the 4th week of development
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Develop as blocks separated by pharyngeal clefts (formed by ectoderm)
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Initially, 6 pharyngeal arches develop, but the 5th regresses
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Arising from the endoderm are compartments called pharyngeal pouches that extend toward the pharyngeal clefts; where pharyngeal pouch endoderm meets pharyngeal cleft ectoderm is known as the pharyngeal membrane
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Help form 4 of the 5 swellings (embryonic primordia) of the face:
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2 mandibular processes (pharyngeal arch)
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2 maxillary processes (pharyngeal arch)
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1 frontonasal prominence
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Composed of
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External surface—ectoderm
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Internal surface—endoderm
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Central part—lateral plate mesoderm, paraxial mesoderm, and neural crest–derived mesoderm
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Skeletal components and associated connective tissue develop from neural crest–derived mesoderm cells
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Muscular structures develop collectively from mesoderm
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Each arch is innervated by a cranial nerve that migrates with the muscles
Derivatives of the pharyngeal arches
| Arch | Muscle(s) from Mesoderm | Cartilage Structure(s) from Neural Crest–Derived Mesoderm | Cartilage Structure(s) from Mesoderm | Connective Tissue Structure(s) from Neural Crest–Derived Mesoderm | Nerve |
|---|---|---|---|---|---|
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Trigeminal | |
| 2 (also called the hyoid arch) |
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Facial | |
| 3 | Stylopharyngeus |
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Connective tissue of the thymus and inferior parathyroid | Glossopharyngeal | |
| 4 |
|
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Connective tissue of the superior parathyroid and the thyroid | Vagus | |
| 6 |
|
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Vagus |
Pharyngeal pouches, membranes, and clefts
General information
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Pharyngeal pouches—4 develop from endoderm
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Pharyngeal clefts—each is a groove formed from ectoderm
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Pharyngeal membranes—each is composed of tissue located between a pharyngeal pouch and a pharyngeal cleft; composed of external ectoderm, mesoderm, and neural crest–derived mesoderm in the core and an internal endoderm lining
| Pouch | Location | Embryonic Structure | Adult Structure |
|---|---|---|---|
| 1 | Opposite the 1st pharyngeal cleft, separated by the 1st pharyngeal membrane | Tubotympanic recess |
|
| 2 | Opposite the 2nd pharyngeal cleft, separated by the 2nd pharyngeal membrane | Primordial palatine tonsils |
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| 3 | Opposite the 3rd pharyngeal cleft, separated by the 3rd pharyngeal membrane |
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| 4 | Opposite the 4th pharyngeal cleft, separated by the 4th pharyngeal membrane |
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Pharyngeal pouches
Pharyngeal membranes
| Membrane | Location | Adult Structure |
|---|---|---|
| 1 | Between the 1st pharyngeal cleft and the 1st pharyngeal pouch | Tympanic membrane |
| 2 | Between the 2nd pharyngeal cleft and the 2nd pharyngeal pouch | |
| 3 | Between the 3rd pharyngeal cleft and the 3rd pharyngeal pouch | |
| 4 | Between the 4th pharyngeal cleft and the 4th pharyngeal pouch |
Pharyngeal clefts
| Cleft | Location | Adult Structure |
|---|---|---|
| 1 | A groove between the 1st and 2nd pharyngeal arches | External acoustic meatus |
| 2 | A groove between the 2nd and 3rd pharyngeal arches | Obliterated cervical sinus by the 2nd pharyngeal arch, which grows over the clefts |
| 3 | A groove between the 3rd and 4th pharyngeal arches | |
| 4 | A groove between the 4th and 6th pharyngeal arches |
Cranium
General information
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Cranium (skull) is formed from
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Lateral plate mesoderm (neck region)
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Paraxial mesoderm
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Neural crest
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Cranium development is divided into 2 parts:
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Viscerocranium—forms the bones of the face (from the pharyngeal arches)
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Forms completely from neural crest
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Neurocranium—forms the bones of the cranial base and cranial vault, and the function is to protect and surround the brain and organs of special sense (olfaction, vision, auditory, and equilibrium). It can be divided into
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Membranous neurocranium (forms from neural crest and paraxial mesoderm)
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Cartilaginous neurocranium (forms from neural crest and paraxial mesoderm)
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Bony skull is formed by either of 2 mechanisms:
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Intramembranous ossification
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Endochondral ossification
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