Embryo

• Ectoderm


  Gives rise to the central nervous system, peripheral nervous system, sensory epithelia of the eye, ear and nose, epidermis and its appendices comprising hair and nails
• Mesoderm


  Gives rise to connecting tissue, cartilage, bone, striated and smooth muscles, blood and lymphatic vessels, heart, kidneys, ovaries, testes, genital ducts, serous membranes that are lining the body cavities as pericardium, pleura and peritoneum, spleen and the cortex of the suprarenal glands

  
• Endoderm


  Gives rise to the epithelial lining of the gastrointestinal and respiratory tracts, urinary bladder, tympanic cavity, tympanic antrum and auditory tube, parenchyma of the tonsils, thyroid gland and parathyroid glands, thymus, liver and pancreas
• Neural Crest cells


  Derived from neuroectoderm, give rise to the cells of the spinal and cranial nerves V, VII, IX and X, autonomic ganglia, ensheathing cells of the peripheral nervous system, pigment cells of the dermis, muscle, connecting tissue and bone of pharyngeal arch origin and meninges (coverings) of the brain and spinal cord
• Pharyngeal Apparatus


  Consists of: Pharyngeal arches, Pharyngeal pouches, Pharyngeal grooves/clefts, Pharyngeal membrane
• Pharyngeal Arches


  • Develop early in the fourth week as neural crest cells migrate into the head and neck region
  • The first pair of pharyngeal arches (primordium of jaws) appears as a surface elevations lateral to the developing pharynx
  • Soon other arches appear as obliquely disposed, rounded ridges on each side of the future head and neck regions
  • By the end of the fourth week, four pairs of pharyngeal arches are visible externally
  • The fifth and sixth arches are rudimentary and are not visible on the surface of the embryo
  • The pharyngeal arches are separated from each other by fissures called pharyngeal grooves
  • They are numbered in craniocaudal sequence

  
• First Pharyngeal Arch (Mandibular Arch)


  • Develops maxillary and mandibular prominences
  • Plays a major role in facial development

  
• Second Pharyngeal Arch (Hyoid Arch)

  • Contributes to the formation of hyoid bone

  
• Pharyngeal Arch Components


  • Each arch consists of a core of mesenchyme covered externally by ectoderm and internally by endoderm
  • In the third week the original mesenchyme is derived from mesoderm
  • During the fourth week most of the mesenchyme is derived from neural crest cells that migrate into the pharyngeal arches

  
• Fate of Pharyngeal Arches

  • Contribute exclusively to the formation of the face, nasal cavities, mouth, larynx, pharynx and neck
  • During the fifth week, the second pharyngeal arch enlarges and overgrows the third and fourth arches, forming the ectodermal depression called cervical sinus
  • By the end of seventh week the second to fourth pharyngeal grooves and the cervical sinus have disappeared, giving the neck a smooth contour
• Components of a Typical Pharyngeal Arch

  • An aortic arch, an artery that arises from the truncus arteriosus of the primordial heart
  • A cartilaginous rod that forms the skeleton of the arch
  • A muscular component that differentiates into muscles in the head and neck
  • A nerve that supplies the mucosa and muscles derived from the arch
• Derivatives of Aortic Arches

  • 1st pair - maxillary artery + external carotid artery
  • 2nd pair - arteria stapedia
  • 3rd pair - proximally - common carotid artery, distally - internal carotid artery
  • 4th pair - left - part of aortic arch, right - right subclavian artery
  • 6th pair - left proximally: left pulmonary artery, left distally: ductus arteriosus (Botali), right proximally: right pulmonary artery
• Derivatives of Pharyngeal Arch Cartilages

  • First arch cartilage (Meckel cartilage) - dorsal end ossifies to form malleus and incus, middle part forms anterior ligament of malleus and sphenomandibular ligament, ventral part forms primordium of mandible
  • Second arch cartilage (Reichert cartilage) - dorsal end ossifies to form stapes and styloid process of temporal bone, ventral end ossifies to form lesser cornu and superior part of hyoid bone, perichondrium forms stylohyoid ligament
  • Third arch cartilage - ossifies to form greater cornu and inferior part of hyoid bone
  • Fourth and sixth arch cartilages - fuse to form laryngeal cartilages except epiglottis
  • Fifth pharyngeal arch is rudimentary and has no derivatives

  
• Derivatives of Pharyngeal Arch Muscles

  • First arch - muscles of mastication (temporalis, masseter, pterygoideus medialis et lateralis, mylohyoideus, venter anterior digastrici, tensor tympani, tensor veli palatini)
  • Second arch - stapedius, stylohyoid, posterior belly of digastric, auricular and muscles of facial expression
  • Third arch - stylopharyngeus
  • Fourth arch - cricothyroid, levator veli palatini and constrictors of pharynx
  • Sixth arch - intrinsic muscles of larynx
• Derivatives of Pharyngeal Arch Nerves

  • Caudal two branches of Trigeminal nerve (maxillary and mandibular) supply derivatives of the first pharyngeal arch
  • Facial, glossopharyngeal and vagus nerves supply the second, third and caudal (fourth to sixth) arches respectively
  • Fourth arch is supplied by superior laryngeal branch of vagus nerve
  • Sixth arch is supplied by its recurrent laryngeal branch

  
• Pharyngeal Pouches


  • The endoderm of the pharynx lines the internal aspects of pharyngeal arches and passes into balloonlike diverticula called pharyngeal pouches
  • The pairs of pouches develop in a craniocaudal sequence between the arches
  • The endoderm of the pouches contacts the ectoderm of the pharyngeal grooves and together they form the double layered pharyngeal membranes that separate the pharyngeal pouches from the pharyngeal grooves
• Derivatives of First Pharyngeal Pouch

  • The first pharyngeal pouch expands into an elongate tubotympanic recess
  • The expanded distal part of this recess contacts the first pharyngeal groove, where it contributes to the formation of the tympanic membrane (eardrum)
  • The cavity of the tubotympanic recess gives rise to the tympanic cavity and mastoid antrum
• Derivatives of Second Pharyngeal Pouch

  • The second pharyngeal pouch is largely obliterated as the palatine tonsils develop
  • Part of the cavity of this pouch remains as the tonsillar sinus or fossa
  • The endoderm of the pouch proliferates and grows into the underlying mesenchyme
  • The central parts of these buds form crypts
  • The pouch endoderm forms the surface epithelium and the lining of the tonsillar crypts
  • At about 20 weeks the mesenchyme around the crypts differentiates into lymphoid tissue
• Pharyngeal membranes


  • Layered
  • Separate the pharyngeal pouches from the pharyngeal grooves
• Derivatives of First Pharyngeal Pouch

  1. The first pharyngeal pouch expands into an elongate tubotympanic recess

  2. The expanded distal part of this recess contacts the first pharyngeal groove, where it contributes to the formation of the tympanic membrane (eardrum)

  3. The cavity of the tubotympanic recess gives rise to the tympanic cavity and mastoid antrum
• Derivatives of Second Pharyngeal Pouch

  1. The second pharyngeal pouch is largely obliterated as the palatine tonsils develop

  2. Part of the cavity of this pouch remains as the tonsillar sinus or fossa

  3. The endoderm of the pouch proliferates and grows into the underlying mesenchyme

  4. The central parts of these buds form crypts

  5. The pouch endoderm forms the surface epithelium and the lining of the tonsillar crypts

  6. At about 20 weeks the mesenchyme around the crypts differentiates into lymphoid tissue

  7. These tissues soon organize into the lymphatic nodules of the palatine tonsil
• Derivatives of Third Pharyngeal Pouch

  1. The third pharyngeal pouch expands and develops a solid, dorsal bulbar part and a hollow elongate ventral part

  2. Its connection with the pharynx is reduced to a narrow duct that soon degenerates

  3. By the sixth week the epithelium of each dorsal bulbar part begins to differentiate into inferior parathyroid gland

  4. The epithelium of the elongate ventral parts of third pharyngeal pouch proliferates and their cavities obliterate

  5. These bilateral primordia of thymus come together in the median plane to form thymus

  6. It descends into the superior mediastenum

  7. The bilobed form of thymus remains throughout life

  8. Discretely encapsulated and each lobe has its own blood supply, lymphatic drainage and nerve supply

  9. The primordia of thymus and parathyroid glands lose their connections with the pharynx and migrate into the neck

  10. Later the parathyroid glands separate from the thymus and lie on the dorsal surface of the thyroid gland
• Derivatives of Fourth Pharyngeal Pouch

  1. The fourth pharyngeal pouch also expands into dorsal bulbar and elongate ventral parts

  2. Its connection with the pharynx is reduced to a narrow duct that soon degenerates

  3. By the sixth week, each dorsal part develops into a superior parathyroid gland

  4. It lies on the dorsal surface of the thyroid gland

  5. The parathyroid glands derived from the third pouches descend with the thymus and are carried to a more inferior position than the parathyroid derived from the fourth pouches

  6. This explains why the parathyroid glands derived from the third pair of pouches are located inferior to those from the fourth pouches
• Histogenesis of Parathyroid Gland

  1. The epithelium of the dorsal parts of the third and fourth pouches proliferates during the fifth week

  2. Forms small nodules on the dorsal aspect of each pouch

  3. Vascular mesenchyme soon grows into these nodules, forming capillary network

  4. The chief or principal cells differentiate during the embryonic period and regulate fetal calcium metabolism

  5. The oxiphil cells differentiate 5 to 7 years after birth

  6. The elongated ventral part of each fourth pouch develops into ultimopharyngeal body

  7. Its cells disseminate within the thyroid gland, giving rise to parafollicular cells

  8. They are also called "C" cells because they produce calcitonin that regulate normal calcium level in body fluids

  9. "C" cells differentiate from neural crest cells
• The Fifth Pharyngeal Pouch


  When this develops, this rudimentary pouch becomes part of the fourth pharyngeal pouch and helps to form the ultimopharyngeal body
• Pharyngeal Grooves


  1. During the fourth and fifth weeks, head and neck region of the human embryo exhibit four pharyngeal grooves or clefts on each side

  2. These grooves separate the pharyngeal arches externally

  3. Only first pair persists as the external acoustic meatus

  4. The other grooves normally obliterated with the cervical sinus as the neck develops

  
• Pharyngeal Membranes
• Development of tongue


  1. 4th week: primordia lingualia appear on inner side of pharyngeal pouches

  2. 1st arch: tuberculum impar (fades out) + 2 lateral lingual prominences
• Anomalies of tongue
• Development of the thyroid gland

  1. growth of epithelium between tuberculum impar and copula → at the level of foramen caecum

  2. growths anterior to the pharynx, with a descending path

  3. is connected to the tongue thru thyroglossus duct

  4. progressive descent anterior to the hyoid bone and cartilages of larynx

  5. within 7th week gets to its final place anterior of the trachea

  6. gets functional at the end of 3rd month
• Anomalies of thyroid gland
• Development of face


  1. facial primordia appear at end of 4th week (neural crest ectomesenchyme of 1st pharyngeal arch) around stomodeum

  2. frontonasal prominence cranially

  3. maxillary prominences laterally

  4. mandibular prominences caudally

  5. on each side develop bilateral oval thickenings of the surface ectoderm

  6. maxillary prominences enlarge (cheeks and upper jaw) and growth medially

  7. pressing medial nasal prominences to the midline → then they merge

  8. upper lip is formed by the maxillary prominences and medial nasal prominences

  9. lower lip and jaw are formed by mandibular prominences that merge in the midline

  10. nose arises from 5 sources: frontonasal prominence, 2 medial nasal prominences, 2 lateral nasal prominences

  
• Development of oral and nasal cavity

  1. stomodeum: a pit lined with ectoderm

  2. boundaries: lower processes of 1st pharyngeal arch – mandibula, on sides upper processes of 1st pharyngeal arch – maxilla, frontonasal prominence with nasal placodes from above
• Development of palate


  1. primary palate: from intermaxillary segment, by merging of both medial nasal prominences, lip component

  2. secondary palate: from lateral palatine processes of maxillary prominences, grow medially and fuse in the midline
• Separation of oral and nasal cavity
• Cleft malformations of face and palate
• Development of salivary glands
• Development of teeth


  1. 6th week: proliferation of oral epithelium (ectoderm) into surrounding ectomesenchyme

  2. dental lamina (parallell to labiogingival crest)

  3. ectoderm → enamel organ: outer enamel epithelium, stratum intermedium, stellate reticulum, inner enamel epithelium (ameloblasts)

  4. ectomesenchyme → dental papilla (odontoblasts)

  5. production of dentine: odontoblasts, procollagen → predentin → dentin

  6. production of enamel: basal surface of ameloblasts becomes secretory, enamel matrix (organic – mineralisation)

  7. development of roots: dental epithelial layers penetrate into the underlying mesenchyme