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I. Gut Formation and Derivatives

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1 I. Gut Formation and Derivatives
Embryonic endoderm: gut and its derivatives Mesoderm: connective tissues, blood vessels, muscles of gut and associated organs

2 A. Yolk sac 1. analogous to the archenteron seen in amphibians 2. endoderm forms disk adjacent to yolk or a yolk sac

3 3. flanks bend together ventrally
- closes off endoderm - creates gut - called lateral folding

4 4. lateral folding enhanced by craniocaudal flexion
- driven by rapid extension of neural plate

5 5. vitelline duct - originally wide opening between gut and yolk sac reduced to narrow passage

6 6. ends of gut tube are temporarily closed
- buccopharyngeal membrane - cloacal membrane

7 B. Trachea 1. ventral out-pocketing near cranial end of digestive tube 2. pharynx - area between buccopharyngeal membrane and tracheal rudiment

8 3. tracheal rudiment: forms 2 lateral buds
- bronchi - continue to enlarge to form lungs - epithelial lining of respiratory tree (endoderm) - other tissues (cartilage, CT, blood vessels: mesoderm)

9 C. Pharyngeal pouches 1. bulge out laterally from pharyngeal endoderm 2. displace surrounding mesenchyme 3. pharyngeal cleft - formed as pouch approaches overlying ectoderm

10 4. pharyngeal arches - columns of tissue that remain between pouches and clefts 5. primitive vertebrates: pouches and clefts fuse to form slits - gills and jaws in primitive fish 6. mammals: no slits - pouches and clefts assume many functions

11 7. first pouch: gives rise to middle ear bones
- maintain connection to pharynx (auditory tube)


13 8. tympanic membrane - where the first pouch meets the first cleft 9. second pouch: palatine tonsil 10. fish: first pouch becomes primary jaw joint

14 10. fish: first pouch becomes primary jaw joint
- mammal middle ear bones still resemble primitive jaw - retraceable in all mammals - recapitulation

15 Recapitulation: 1. primitive vertebrates: adult structures similar to embryonic stuctures 2. advanced vertebrates: adult stage is farther removed from embryonic stages

16 All vertebrate embryos look similar after organogenesis

17 Vertebrate embryos: - similar basic body plan - notochord and brain rudiments, sense organs, heart, gut - all have similar spatial arrangement

18 Called phylotypic stage:
- stage shared by all species in a phylum or subphylum - also seen in insects and other arthropods - why reinvent the wheel?

19 D. Gut 1. foregut - esophagus and stomach 2. midgut - small intestine 3. hindgut - colon, rectum, and anal canal - endoderm invaginates to form bladder and urethra (just the epithelial inner lining)

20 Endoderm and ectoderm give rise to mainly epithelial structures
- skin, lining of gut structures Mesoderm: surrounds epithelial layers of internal structures - walls of blood vessels and gut structures Mesoderm forms some epithelia and large amounts of mesenchyme - includes axial, paraxial, intermediate mesoderm, and the lateral plates

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