FOREGUT
- PRELARYNGEAL PART OF FOREGUT
- POST LARYNGEAL PART OF FOREGUT
Discussion

FOREGUT

PRELARYNGEAL PART OF FOREGUT

cephalic or pharyngeal part of foregut
- funnel shaped expansion compressed dorsoventrally
  - dorsal wall or roof
    - notochord
    - hindbrain
    - pair of dorsal aorta
  - ventral wall or floor
    - ectodermal stomodaeum
    - forward bulging of pericardial sac
    - developing heart
    - septum transversum
  - two lateral walls
    - separated from overlying surface ectoderm by
      - splanchnic mesoderm
        
        undergoes segmentation
        
        mesodermal bars / branchial or visceral arches
  - Each arch contains
    - surface ectoderm
    - core of mesoderm
    - inner lining of pharyngeal endoderm
    - extends ventrally and medially
      - along floor or ventral wall of pharynx
        
        meets the corresponding arch of opposite side
  - Between each arch
    - closely approximated membranes
      - in fishes they rupture
    - in terrestrial
      - series of ectodermal grooves called branchial or visceral cleft / ectodermal cleft
      - same on inner side endodermal pouches / pharyngeal pouches are formed
        
        FIRST POUCH
        
        dorsal part extends in to mesenchyme outwards
        
        medial part of this recess persists as auditory tube
        
        lateral part forms primitive tympanic cavity
        
        mucous lining of tympanic cavity
        
        mastoid antrum , mastoid air cells
        
        mucous layer of tympanic membrane
        
        the thin layer of mesoderm between First pouch and first cleft
        
        this region forms tympanic membrane ( tridermal in nature )
        
        SECOND POUCH
        
        THIRD POUCH
        
        FOURTH POUCH
        
        Floor of primitive pharynx
        
        developing tongue
        
        thyroid diverticulum
        
        laryngo-tracheal groove
        
        FIFTH POUCH
  - FIRST ARCH ( श - स - ष ) ectoderm - mesoderm - endoderm
    - श -
      - forms skin over the
        
        upper jaw , lower jaw and tragus of auricle
    - स -
      - Artery - first aortic arch which disappears
      - Nerve -
        
        pre trematic accompanies caudal border or preceeding arch
        
        Chorda Tympani nerve
        
        post trematic follows cephalic border of succeeding arch
        
        Mandibular nerve
      - Muscle
        
        Tensor tympani , tensor veli palatini
        
        Muscles of Mastication , Mylohyoid
        
        Anterior belly of digastric
      - Skeletal -
        
        central axis of mesodermal core is chondrified
        
        cartilaginous bar is enveloped by perichondrium
        
        dorsal maxillary process
        
        Ventral Mandibular process ( called as meckel's cartilage )
        
        dorsal part ossified to form
        
        malleus and incus
        
        ascending part regresses
        
        fibrous part persists as
        
        anterior ligament of malleus
        
        sphenomandibular ligament
        
        fibrous part is ossified to form
        
        body of mandible
        
        from mandibular foramen to mental foramen
  - SECOND ARCH
    - S -
      - Muscle -
        
        Stapedius , stylohyoid ,
        
        Platysma , posterior belly of digastric
        
        Auricular , epicranius
      - Skeletal -
        
        Cartilaginous bar is called as reichert's cartilage
        
        dorsal part of cartilage ossified to form
        
        stapes
        
        succeeding part forms
        
        styloid process of temporal bone , stylohyoid ligament
        
        Ventral part is converted
        
        lesser cornu
        
        upper part of body of hyoid bone
  - THIRD ARCH
    - S
      - muscle
        
        stylopharyngeus
      - skeletal -
        
        cartilaginous bar
        
        dorsal part dissapears
        
        ventral part is ossified
        
        greater cornu of hyoid bone and lower part of hyoid bone
  - FOURTH ARCH
    - S
      - muscle
        
        cricothyroid
      - Skeletal
        
        cartilaginous bar
        
        dorsal part dissapears
        
        Ventral part forms lamina of thyroid cartilage
  - FIFTH ARCH
    - DISSAPEARS
  - SIXTH ARCH
  - S
    - muscle
      - all intrinsic muscles except cricothyroid
    - Skeletal
      - Cartilaginous bar
        
        Dorsal part dissappears
        
        Ventral part forms cricoid and arytenoid cartilage

POST LARYNGEAL PART OF FOREGUT

Oesophageal segment
- extending from laryngeal diverticulum to fusiform dilation stomach
- buds
  - lung buds
Gastric segment
- dorsal border grows more rapidly than ventral border
  - greater curvature and lesser curvature
  - ventral border
    - suspended from anterior body wall by
      - ventral mesogastrium
        
        by hepatic bud gets divided to form
        
        lesser omentum
        
        falciform and coronary ligaments of liver
  - dorsal border
    - attached to posterior body wall
      - dorsal mesogastrium
        
        spleen develops
- it undergoes rotation vertical axis - 90 degrees
  - left surface —> ventral
    - left vagus supplies anterosuperior surface
  - right surface —> dorsal
    - right vagus supplies postero inferior
Duodenal Segment
- Two off shoots
  - liver
  - pancreas