Germ Layers in Developmental Biology
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Questions and Answers

Which bones are derived from the maxillary process of pharyngeal arch 1?

  • Mandible, malleus, incus
  • Ethmoid, sphenoid
  • Maxillary, zygomatic, squamous part of temporal bone (correct)
  • Stapes, styloid process of temporal bone

    • What type of ossification results in the formation of flat bones surrounding the brain?

  • Endochondral ossification
  • Mesenchymal ossification
  • Fibrous ossification
  • Intramembranous ossification (correct)

    • Which of the following bones is derived from the paraxial mesoderm?

  • Zygomatic bone
  • Frontal bone
  • Ethmoid bone
  • Parietal bone (correct)

    • What is the term for the gaps of connective tissue between the flat bones of the neurocranium during development?

    <p>Sutures</p> Signup and view all the answers

    Which structure is formed by multiple sutures meeting together?

    <p>Fontanelle</p> Signup and view all the answers

    What structures do the lens placodes develop into?

    <p>Eyes and associated tissues</p> Signup and view all the answers

    Which of the following is true about the nasal placodes?

    <p>They invaginate to form nasal pits.</p> Signup and view all the answers

    What is the developmental fate of the maxillary prominences?

    <p>They form the lateral upper lip and cheeks.</p> Signup and view all the answers

    Which of the following structures is associated with the first pharyngeal arch?

    <p>Malleus and incus</p> Signup and view all the answers

    What do otic placodes eventually develop into?

    <p>Internal ear structures</p> Signup and view all the answers

    Where are the mandibular prominences located in relation to the stomodeum?

    <p>Below the stomodeum</p> Signup and view all the answers

    Which of the following adult structures is associated with the pharyngeal pouches?

    <p>Thyroid gland</p> Signup and view all the answers

    What is formed from the invagination of nasal placodes?

    <p>Nasal pits</p> Signup and view all the answers

    Which germ layer is responsible for forming the facial skeleton?

    <p>Neural crest</p> Signup and view all the answers

    What is one of the adult derivatives of the fontanelles?

    <p>Parietal bone</p> Signup and view all the answers

    Which of the following structures is derived from the endoderm?

    <p>Thyroid gland</p> Signup and view all the answers

    What type of mesoderm gives rise to parts of the braincase?

    <p>Paraxial mesoderm</p> Signup and view all the answers

    Which cranial nerves are associated with ectodermal placodes?

    <p>V, VII, IX, X</p> Signup and view all the answers

    Which of the following structures does NOT derive from lateral plate mesoderm?

    <p>Skeletal musculature</p> Signup and view all the answers

    What are the two developmental processes that form the bones of the neurocranium?

    <p>Endochondral and intramembranous ossification</p> Signup and view all the answers

    Which structures are 'derived from paraxial mesoderm'?

    <p>Parts of braincase and skeletal musculature</p> Signup and view all the answers

    What is the primary role of fontanelles in infants?

    <p>To allow the skull to change shape during birth</p> Signup and view all the answers

    At what age does the anterior fontanelle typically close?

    <p>13–24 months</p> Signup and view all the answers

    Which pharyngeal arch is considered transient and disappears?

    <p>5th pharyngeal arch</p> Signup and view all the answers

    What components are found in each pharyngeal arch?

    <p>Cartilage, muscle, and nerve with blood supply</p> Signup and view all the answers

    The mastoid fontanelle typically closes at what age?

    <p>6–18 months</p> Signup and view all the answers

    From where is the hyoid derived?

    <p>Pharyngeal arch 2 and 3</p> Signup and view all the answers

    What are pharyngeal pouches?

    <p>Lateral outgrowths of the pharynx with no communication to arches</p> Signup and view all the answers

    Which structure is derived from the lateral plate mesoderm?

    <p>Laryngeal cartilages</p> Signup and view all the answers

    Which associated artery is linked to the sixth pharyngeal arch?

    <p>Pulmonary arteries</p> Signup and view all the answers

    What structure is derived from the first pharyngeal pouch?

    <p>Eustachian tube</p> Signup and view all the answers

    Which muscle is derived from the fourth pharyngeal arch?

    <p>Cricothyroid</p> Signup and view all the answers

    What is the dorsal derivative of the third pharyngeal pouch?

    <p>Inferior parathyroid gland</p> Signup and view all the answers

    What type of lining do the pharyngeal pouches possess?

    <p>Endodermal</p> Signup and view all the answers

    Which structures develop from the fourth pharyngeal pouch?

    <p>Ultimobranchial body and superior parathyroid gland</p> Signup and view all the answers

    Which of the following muscles is associated with the vagus nerve?

    <p>Muscular uvula</p> Signup and view all the answers

    At what week does facial development begin centered on the stomodeum?

    <p>Week 4</p> Signup and view all the answers

    What is the developmental fate of the nasal placodes?

    <p>They invaginate to form nasal pits.</p> Signup and view all the answers

    Which structure gives rise to the ears and associated tissues?

    <p>Otic placodes</p> Signup and view all the answers

    Which of the following is a derivative of the frontonasal prominence?

    <p>Forehead and bridge of nose</p> Signup and view all the answers

    What are the maxillary prominences primarily associated with?

    <p>Formation of the lateral upper lip and cheeks</p> Signup and view all the answers

    Which structures are formed as a result of the invagination of nasal placodes?

    <p>Nasal pits and nasal prominences</p> Signup and view all the answers

    How do lens placodes develop during embryonic growth?

    <p>They migrate medially after originating laterally to the frontonasal prominence.</p> Signup and view all the answers

    Which germ layer is responsible for forming the thymus gland?

    <p>Endoderm</p> Signup and view all the answers

    What are the bones of the viscerocranium primarily derived from?

    <p>Neural crest</p> Signup and view all the answers

    The development of cranial nerve sensory ganglia is associated with which structure?

    <p>Ectodermal placodes</p> Signup and view all the answers

    Which cranial nerve is associated with the first pharyngeal arch?

    <p>Trigeminal nerve (V)</p> Signup and view all the answers

    Which structures are formed from the endodermal lining of the pharyngeal pouches?

    <p>Parathyroid glands</p> Signup and view all the answers

    What is NOT a derivative of paraxial mesoderm?

    <p>Facial skeleton</p> Signup and view all the answers

    What is the main purpose of fontanelles in infants?

    <p>To facilitate brain growth</p> Signup and view all the answers

    Which of the following structures is associated with cranial nerves V, VII, IX, and X?

    <p>Ectodermal placodes</p> Signup and view all the answers

    What is the location of the anterior fontanelle?

    <p>Between the frontals and parietals</p> Signup and view all the answers

    What is the closure date of the posterior fontanelle?

    <p>1–2 months</p> Signup and view all the answers

    Which component is NOT associated with each pharyngeal arch?

    <p>Skeletal muscle derivation</p> Signup and view all the answers

    What are the embryonic facial prominences derived from?

    <p>Pharyngeal arches</p> Signup and view all the answers

    What is the primary function of the gaps (fontanelles) in the skull during infancy?

    <p>To allow for brain growth and skull shape adjustment</p> Signup and view all the answers

    The mastoid fontanelle is associated with which bones?

    <p>Parietal, Temporal, and Occipital</p> Signup and view all the answers

    What type of bones surround all but the base of the brain?

    <p>Flat bones</p> Signup and view all the answers

    Which of the following bones is specifically derived from the neural crest?

    <p>Sphenoid</p> Signup and view all the answers

    Which process describes the development of bones at the base of the skull?

    <p>Endochondral ossification</p> Signup and view all the answers

    What are the gaps called where multiple sutures meet during cranial development?

    <p>Fontanelles</p> Signup and view all the answers

    From which embryonic structure is the maxillary process derived?

    <p>Neural crest</p> Signup and view all the answers

    What type of ossification leads to the formation of flat bones in the skull?

    <p>Intramembranous ossification</p> Signup and view all the answers

    Which structure forms the majority of the occipital bone?

    <p>Par axial mesoderm</p> Signup and view all the answers

    Which set of structures is associated with the first pharyngeal arch?

    <p>Maxillary and mandibular processes</p> Signup and view all the answers

    What is maintained between the flat bones of the neurocranium during development?

    <p>Sutures</p> Signup and view all the answers

    Which of the following represents the outer part of the cranial vault?

    <p>Flat bone</p> Signup and view all the answers

    Study Notes

    Germ Layers

    • Mesoderm
      • Paraxial mesoderm
        • Contributes to the braincase (neurocranium)
        • Forms skeletal musculature
        • Forms dermis and connective tissues of the dorsal head
        • Forms part of the meninges
      • Lateral plate mesoderm
        • Forms laryngeal cartilages and adjacent connective tissues
    • Ectoderm
      • Ectodermal placodes
        • Form select (V, VII, IX, X) cranial nerve sensory ganglia
      • Neural crest
        • Forms facial skeleton (viscerocranium) and parts of the neurocranium and adjacent connective tissues
    • Endoderm
      • Forms the internal lining as part of the pharyngeal pouches
      • Forms endocrine structures of the neck:
        • Thymus
        • Thyroid
        • Parathyroid glands
      • Forms the epithelium of the tympanic cavity and auditory tube
    • Mesenchyme
      • Embryonic loose connective tissue derived from any germ layer

    Skull & Neck Bone Derivatives

    • Viscerocranium
      • Bones of the face
      • Derived from neural crest
      • Derived from pharyngeal arches 1 & 2
      • Pharyngeal arch 1 derivatives
        • Maxillary process
          • Maxillary, zygomatic, squamous part of temporal bone
        • Mandibular process
          • Mandible, malleus, incus
      • Pharyngeal arch 2 derivatives
        • Stapes, styloid process of temporal bone
    • Neurocranium
      • Bones surrounding the brain
      • Divided into two regions based on developmental process:
        • Flat bones - develop directly from mesenchyme (intramembranous ossification)
          • Surround all but the base of the brain (e.g., parietal)
          • Derived from neural crest:
            • Frontal
            • Squamous portion of temporal
          • Derived from paraxial mesoderm:
            • Parietal
            • Most of the occipital
        • Cartilaginous bones - mesenchyme develops into an intermediate cartilaginous stage before ossifying (endochondral ossification)
          • Bones of the base of the skull
          • Derived from neural crest:
            • Ethmoid
            • Sphenoid
          • Derived from paraxial mesoderm:
            • Portions of temporal (mastoid/petrous), base of occipital
      • Sutures & fontanelles
        • Sutures - thin gaps of connective tissue maintained between the flat bones of the neurocranium during development
          • Many sutures occur along the boundaries between bones derived from neural crest vs paraxial mesoderm
        • Fontanelles - larger gaps where multiple sutures meet
        • Functions of sutures and fontanelles:
          • Allow the skull to change shape as it passes through the birth canal
          • Allow for brain growth in early infancy
    • Neck structures
      • Neural crest derived:
        • Hyoid (Pharyngeal arch 2: lesser horn and upper body; Pharyngeal arch 3: greater horn and lower body)
      • Lateral plate mesoderm derived:
        • Laryngeal cartilages (Pharyngeal arch 4: epiglottis, thyroid; Pharyngeal arch 6: arytenoid, cricoid, other laryngeal cartilages)

    Pharyngeal Arches, Clefts, & Pouches

    • Appear in week 4-5 of development
    • Pharyngeal arches
      • Ridges of mesenchymal tissue separated by external pharyngeal clefts
      • Five pharyngeal arches (the 5th pharyngeal arch is transient and disappears, thus the last pharyngeal arch is called the 6th pharyngeal arch)
      • Composition of each arch:
        • Outer surface: ectoderm
        • Core: mesenchymal tissue (Lateral plate mesoderm, Paraxial mesoderm, Neural crest)
      • Each arch has its own:
        • Associated cartilage
        • Muscular component
        • Cranial nerve association
        • Arterial supply
      • Four embryonic facial prominences derived from the pharyngeal arches:
        • Two mandibular prominences (1st pharyngeal arch)
        • Two maxillary prominences (1st pharyngeal arch)
    • Pharyngeal pouches
      • Lateral outgrowths of the pharynx
      • Adjacent to the pharyngeal clefts but without open communication with pharyngeal arches and clefts
    • Pharyngeal clefts
      • Transient external pockets coated in ectoderm that separate each of the pharyngeal arches

    Pharyngeal Arches (Specifics)

    • 1st pharyngeal arch:
      • Muscular derivatives: Muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini
      • Associated cranial nerve: Trigeminal (V)
      • Associated arteries: Maxillary, mandibular arteries
    • 2nd pharyngeal arch:
      • Muscular derivatives: Muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric
      • Associated cranial nerve: Facial (VII)
      • Associated arteries: Stapedial artery (regresses in humans)
    • 3rd pharyngeal arch:
      • Muscular derivatives: Stylopharyngeus
      • Associated cranial nerve: Glossopharyngeal (IX)
      • Associated arteries: Common carotid artery, internal carotid artery
    • 4th pharyngeal arch:
      • Muscular derivatives: Cricothyroid, levator veli palatini, pharyngeal constrictors, palatoglossus, muscular uvula, salpingopharyngeus
      • Associated cranial nerve: Vagus (X) (Superior laryngeal branch of vagus)
      • Associated arteries: Arch of the aorta, proximal right subclavian
    • 6th pharyngeal arch (last):
      • Muscular derivatives: Intrinsic muscles of the larynx (except cricothyroid)
      • Associated cranial nerve: Vagus (X) (Recurrent laryngeal branch of vagus)
      • Associated arteries: Pulmonary arteries and ductus arteriosus

    Pharyngeal Pouches

    • Four pairs of pouches (a fifth is rudimentary in humans)
    • Epithelial endodermal lining
    • 1st Pouch
      • Forms a long process called the tubotympanic recess
      • Contact between ectoderm of the 1st pharyngeal cleft and endoderm of the pouch will eventually form the tympanic membrane
      • Derivatives:
        • Tympanic cavity (middle ear) from the distal end of the tubotympanic process
        • Stem forms the eustachian tube
        • Lining will help form the tympanic cavity
    • 2nd Pouch
      • Forms buds that penetrate adjacent mesenchyme
      • Develop into the palatine tonsil
      • The remains of the pouch become the tonsillar fossa
    • 3rd Pouch
      • Forms a dorsal and ventral wing
      • Ventral wing derivative:
        • Thymus (loses connection with the pharyngeal wall, migrates medially and inferiorly to the anterior thorax)
      • Dorsal derivative:
        • Inferior parathyroid gland
    • 4th Pouch
      • Forms a dorsal and ventral wing
      • Ventral wing derivative:
        • Forms the ultimobranchial body, which joins the thyroid gland and eventually develops into the C cells of the thyroid gland (C cells release calcitonin, a hormone that regulates calcium in the body)
      • Dorsal derivative
        • Superior parathyroid gland

    Facial Development

    • Neural crest derived
    • Begins around week 4, centered on the stomodeum (primordial oral cavity)
    • Placode - thickening of ectoderm that forms special structures
    • Main structures:
      • Stomodeum
      • Frontonasal prominence
        • Located above the stomodeum
        • Becomes forehead and bridge of the nose
      • Lens placodes
        • Lateral to frontonasal prominence, migrate medially during development
        • Develop into eyes and associated tissues
      • Otic placodes
        • Form internal ear structures
      • Nasal placodes
        • Anterolateral to the frontonasal prominence
        • Invaginate to make nasal pits, creating two nasal prominences: lateral and medial nasal prominences
      • Maxillary prominences (1st pharyngeal arch)
        • Lateral to the stomodeum
        • Becomes the lateral upper lip and cheeks
      • Mandibular prominences (1st pharyngeal arch)
        • Below the stomodeum
        • Becomes the lower lip

    Fontanelles

    • Fontanelle | Location | Closure Date | Adult Structure
    • ---------------|------------------------------------|-------------------|------------------------
    • Anterior | Frontals & Parietals | 13–24 months | Bregma
    • Posterior | Parietals & Occipital | 1–2 months | Lambda
    • Mastoid | Parietal, Temporal, & Occipital | 6–18 months | Asterion
    • Sphenoid | Sphenoid, Parietal, Temporal, Frontal | 6 months | Pterion

    Germ Layers and their contributions to Facial and Neck Development

    • Mesoderm:
      • Paraxial Mesoderm: Gives rise to parts of the braincase (neurocranium), skeletal musculature, dermis and connective tissues of the dorsal head, and part of the meninges.
      • Lateral Plate Mesoderm: Forms laryngeal cartilages and adjacent connective tissues.
    • Ectoderm:
      • Ectodermal Placodes: Develop into sensory ganglia for certain cranial nerves (V, VII, IX, X).
      • Neural Crest: Develops into facial skeleton (viscerocranium), parts of the neurocranium, and adjacent connective tissues.
    • Endoderm:
      • Forms internal lining of pharyngeal pouches.
      • Develops into endocrine structures of the neck: thymus, thyroid, and parathyroid glands.
      • Forms epithelium of tympanic cavity and auditory tube.
    • Mesenchyme: Embryonic loose connective tissue derived from any germ layer.

    Skull and Neck Bone Derivatives

    • Viscerocranium: Bones of the face; derived from neural crest and pharyngeal arches 1 and 2.
      • Pharyngeal Arch 1:
        • Maxillary Process: Develops into the maxilla, zygomatic bone, and squamous part of the temporal bone.
        • Mandibular Process: Develops into the mandible, malleus, and incus.
      • Pharyngeal Arch 2:
        • Develops into the stapes and styloid process of the temporal bone.
    • Neurocranium: Bones surrounding the brain; divided into two regions based on developmental process.
      • Flat Bones: Develop directly from mesenchyme (intramembranous ossification). Surround all but the base of the brain (e.g., parietal bone).
        • Neural Crest: Develops into the frontal bone and squamous portion of the temporal bone.
        • Paraxial Mesoderm: Develops into the parietal bone and most of the occipital bone.
      • Cartilaginous Bones: Mesenchyme develops into an intermediate cartilaginous stage before ossifying (endochondral ossification). Forms bones of the base of the skull.
        • Neural Crest: Develops into the ethmoid and sphenoid bones.
        • Paraxial Mesoderm: Develops into portions of the temporal bone (mastoid/petrous) and the base of the occipital bone.
    • Cranial Sutures: Thin gaps of connective tissue maintained between the flat bones of the neurocranium during development.
    • Fontanelles: Larger gaps where multiple sutures meet; allow for skull reshaping during birth and brain growth in early infancy.

    Pharyngeal Arches, Clefts, and Pouches

    • Pharyngeal Arches: Ridges of mesenchymal tissue separated by external pharyngeal clefts; 5 arches form.
      • Structure:
        • Outer Surface: ectoderm.
        • Core: mesenchymal tissue (lateral plate mesoderm, paraxial mesoderm, neural crest).
      • Each arch has its own associated cartilage, muscular component, cranial nerve association, and arterial supply.
      • Four embryonic facial prominences derived from these structures:
        • Two mandibular prominences (1st pharyngeal arch)
        • Two maxillary prominences (1st pharyngeal arch)
    • Pharyngeal Pouches: Lateral outpocketings(evaginations) of the pharynx; adjacent, but not communicating, with pharyngeal arches and clefts.
    • Pharyngeal Clefts: Transient external pockets coated in ectoderm that separate each pharyngeal arch.
      • The first cleft deepens, contacting the endoderm layer of the first pharyngeal pouch forming a thin membrane that becomes the tympanic membrane.
      • Remaining clefts are overgrown by the expansion of the arches.
    • Pharyngeal Arch 1 (Mandibular):
      • Derivatives: maxilla, temporal (portion), zygomatic, malleus, incus.
      • Muscles: muscles of mastication (temporalis, masseter, pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini.
      • Nerve: trigeminal nerve (CN V).
      • Artery: maxillary artery.
    • Pharyngeal Arch 2 (Hyoid):
      • Derivatives: stapes, styloid process of the temporal bone, part of the hyoid bone.
      • Muscles: stapedius, posterior belly of digastric, stylohyoid; muscles of facial expression.
      • Nerve: facial nerve (CN VII).
      • Artery: stapedial and hyoid arteries (hyoid artery gives rise to the tympanic branch of the internal carotid artery; stapedial artery atrophies, derivatives include middle meningeal, infraorbital, inferior alveolar, and other external carotid branches).
    • Pharyngeal Arch 3:
      • Derivatives: portions of the hyoid bone.
      • Muscles: stylopharyngeus.
      • Nerve: glossopharyngeal nerve (CN IX).
      • Artery: common carotid and proximal internal carotid arteries.
    • Pharyngeal Arches 4 & 6:
      • Derivatives: thyroid, epiglottis, cricoid, arytenoid, and other laryngeal cartilages.

    Neck Structures

    • Neural Crest:
      • Hyoid Bone: Lesser horn and upper body derived from pharyngeal arch 2, greater horn and lower body derived from pharyngeal arch 3.
    • Lateral Plate Mesoderm:
      • Laryngeal Cartilages: Epiglottis and thyroid cartilage from pharyngeal arch 4, arytenoid, cricoid, and other laryngeal cartilages from pharyngeal arch 6.

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