1Developmental Biology - Biology 4361Lecture 14 - Endodermal and Mesodermal OrgansDecember 1, 2005Endodermal derivatives - endoderm forms the inner epithelium of the gut and derivatives - all internal organs are constructed with both endoderm and mesoderm components- e.g. connective tissues, blood vessels, muscles of organs are mesodermal - gastrulation places endoderm in an inner aspect with relation to ectoderm and mesoderm- amphibians - archenteron forms the inner-most sphere- other vertebrates - endoderm forms disc adjacent to yolk or adjoining cavity (yolk sac) - post-gastrulation movements (mammal):- flanks bend ventrally (lateral folding); form gut- craniocaudal flexion - rapid extension of neural plate bends embryo anteriorly- vitelline duct formed as folding and flexion reduce area between gut and yolk sac- ends of digestive tube closed (temporarily)- cranial end - buccophayngeal membrane- caudal end - cloacal membrane- gut appendages (act as positional landmarks)- tracheal rudiment - ventral outpocketing near cranial end- will form trachea and eventually lungs- pharynx - between buccopharyngeal membrane and tracheal rudiment- foregut - trachea to rudiments of liver and pancreas- midgut, hindgut - extend from liver/pancreas rudiment to cloacal membranePharyngeal arches - series of pharyngeal pouches develop from endoderm in neck area; endoderm displacesmesenchyme; forms pouch under ectoderm- endoderm induces pharyngeal cleft (thin-walled depression) in overlying ectoderm- mesenchymal tissue between clefts forms pharyngeal arches - pharyngeal arches are the bases for gill slits and gills - also act as precursors for jaws in primitive fish - all vertebrate embryos form several pairs of pharyngeal arches; each contain - precartilage cells (neural crest origins)- premuscle mesenchyme- blood vessel- cranial nerve - first pharyngeal arch embryonic cartilage: palotoquadrate cartilage and mandibular2cartilage- in fish - proximal portions persist as quadrate and articular bones- form primitive jaw joint - second pharyngeal arch embryonic cartilage: hyomandibular cartilage and hyal cartilage- in fish - hyomandibular cartilage develops into bone that connects jaw to braincase - in mammals: primary jaws replaced by secondary generation of jaw bones:- lower jaw - dentary bone- upper jaw - maxillary and squamosal (portion of temporal (skull) bone)- mammals - vestiges of palatoquadrate and mandibular cartilage persist as incus andmalleus (middle ear bones; see below)- also, hyomandibular cartilage (bone) retained as stapes (third middle ear bone)- hyal cartilage retained as the hyoid bone - supports base of tongue- third, etc. pharyngeal arch cartilage contributes to hyoid and larynx - mammals: pharyngeal pouches and clefts form, but do not perforate into slits; other functionshave evolved- 1st pharyngeal pouch - forms middle ear (containing middle ear bones - ossicles)- connection between pouch and pharynx forms pharyngotympanic (Eustachian)tube - equalizes pressure between pharynx and middle ear- 1st pharyngeal cleft forms outer ear- tympanic membrane (eardrum) formed where 1st pouch meets 1st cleft- 2nd pouch endoderm forms palatine tonsil (tonsil); gland structure - lymphoid tissue- 3rd pouch forms thymus gland and lower (inferior) portion of the parathyroid gland - note - both glands lose connections with the pharynx; move caudally andmedially to adult positions- thymus - below thyroid- parathyroids (2 upper, 2 lower) - on and within thyroid- 4th pouch - upper (superior) portion of parathyroid gland- also forms ultimobranchial body (produces calcitonin; incorporated intothyroid (higher vertebrates) or as distinct postbranchial body in birds andlower vertebrates)Note - 2nd to 4th pharyngeal clefts disappear in mammals during developmentPhylotypic Stage - vertebrate organ rudiments present in similar spatial arrangement (basic bodyplan) after organogenesis- e.g. after organogenesis, vertebrate embryos have similar body plans: - notochord- brain rudiments- sense organs- gut3- heart- NOTE - similar phenomenon in insects, other phyla or subphylums- after phylotypic stage, organ development may diverge using same rudiments : e.g.Middle ear formation- fish - hyomandibular bone (from dorsal part of 2nd pharyngeal arch) anchors jaw tobraincase; also acted to:- transmit sound (vibrations) to inner ear and brain areas involved with gravityperception and proprioception (body position, movement)- thus functions secondarily in hearing- in terrestrial vertebrates - jaws attach directly to braincase; hyomandibular bone nolonger needed as jaw brace- became more slender (in accord with hearing function)- hyomandibular cartilage becomes the columella bone in amphibians, reptiles,birds- single middle ear ossicle- vertebrate (except mammals) jaws: primary jaw joint formed by quadrate andarticulare bones- these bones are formed from 1st pharyngeal arch: palatoquadrate cartilageand mandibular cartilage, respectively - mammalian jaw: secondary jaw added to primary jaw joint;- formed between dentary bone (part of the lower jaw) and squamosal(part of temporal (skull) bone in the upper jaw)- evolution of jaws toward exclusive use of secondary jaw joint- primary jaw components; (palatoquadrate and mandibularcartilage) evolved into ossicles; incus and malleus,respectivelyEndodermal lining of intestine and its appendages [remember, endoderm makes up just a portion of most internal organs; the bulk of the organ isusually of mesodermal origin] - tracheal rudiment - branches off foregut- forms two lateral buds; buds divide into branches- three branches on right, two on left (to accommodate heart)- branches develop into bronchi- bronchi further divide into bronchiole- epithelial cells at end of bronchiole develop into sacs called alveoli- epithelial cells transform from cuboidal to squamous (increased surface area)- become intimately associated with capillaries; site of gas exchangeNOTE - only inner epithelium of trachea, bronchi, etc. and alveoli are endoderm; all other4structures (cartilage, blood vessels, connective tissues, etc.) are mesodermal - esophagus, stomach, anterior duodenum - develop from portion of foregut betweentracheal rudiment and liver/pancreas rudiment- midgut (at posterior duodenum; liver/pancreas) forms small intestine- epithelium specialized to take up nutrients- midgut also makes up small portion of colon- hindgut - colon, rectum, anal canal-