Endodermal and Mesodermal Organs

Developmental Biology – Biology 4361

December 5, 2005

Part 1

Mammalian gastrulation

Figure 14.2

Gut formation – lateral folding

­ the gut closes by lateral folding

Mammals ­ endoderm is open to the adjacent yolk or to the yolk sac

Gut formation – craniocaudal flextion

Figure 14.3

Lateral folding is enhanced by craniocaudial flexion: ­ rapid extension of neural plate bends embryo

The wide opening between gut and yolk sac is reduced to narrow duct = vitelline duct

18 d 24 d

30 d

21 d

28 d 18 d

4 weeks 5 weeks

Endodermal derivatives

Figure 14.4

­ pharyngeal pouches ­ trachea rudiment ­ connects to the lungs ­ liver and pancreas rudiments ­ urinary bladder (with allantois)

Gut derivatives:

­ mesenchymal (mesoderm) cells will surround the tube to form muscles, connective tissue, cartilage

­ endodermal cells generate only the lining of the digestive tube Gut sections: pharynx – forgut – midgut – hindgut

Pharyngeal pouch formation

­ endoderm displaces mesoderm ­ endoderm induces cleft ­ mesenchyme forms arches

Figure 14.5

generalized vertebrate embryo shark adult

­ in fish pharyngeal pouches (arches) develop into spiracle, gills

­ also jaws! (primitive fish)

Pharyngeal arch formation

Adapted from Gilbert 1994, p. 284

neural crest cell migration

Figure 14.1 Human embryo 31 d

pharyngeal arch: endoderm mesoderm

Pharyngeal arches

A. Arches 1, 2; buccopharyngeal membrane remnant (arrow) B. Mesoderm core lined by ectoderm (arrows), endoderm (arrowheads)

A B

Phayrngeal arch: ­ precartilage cells (NC) ­ premuscle mesenchyme ­ blood vessel ­ cranial nerve

Figure 14.1 Human embryo 31 d

stomatodeum

cardiac bulge

Pharyngeal arches – frontal view

Phylotypic Stage ­ Vertebrates

Figure 14.9

­ developmental stage that is very similar for all species of a phylum

Phylotypic stage:

­ notochord ­ brain rudiments ­ sense organs ­ gut ­ heart

Similar body plans:

Why?

Class Agnatha – jawless fishes

Figure 14.6

Pharyngeal arch cartilage

­ develop into gills & jaws in primitive fish ­ have acquired other functions in terrestrial vertebrates

Pharyngeal arches

Developing shark skull

Mammalian jaw and ossicle development

Figure 14.7 1st pharyngeal arch cartilage forms primary jaw joint (vertebrates, exc. mammals)

Mammals ­ secondary jaw joint replaces primary jaw joint; ­ formed by dentary and squamosal bones.

Parts of primary jaw cartilage persists as middle ear bones: ­ mandibular cartilage → malleus (hammer); ­ quadrate cartilage → incus (anvil)

also ­ hyomandibular cartilage (anchors jaw to braincase) → stapes

10 week fetus adult

secondary

primary

Figure 15.23 in SF Gilbert: Developmental Biology 7 th ed, 2003; see Figure 14.8 in Kalthoff

Pharyngeal pouch derivatives ­ human

1st → auditory cavities of middle ear and Eustachian tubes

2nd → walls of the tonsils (gland = lymphoid tissue)

(NOTE ­ thyroid gland forms from an unpaired thyroid primordium between the 2nd pair of pouches)

3rd → dorsal ­ lower (inferior) parathyroid glands

→ thymus

4th → upper (superior) parathyroid glands

→ ultimobranchial (postbranchial) body

4 weeks 5 weeks

Endodermal derivatives

Figure 14.4

­ pharyngeal pouches ­ trachea rudiment – forms and connects to lungs ­ liver and pancreas rudiments ­ urinary bladder (with allantois)

Gut derivatives:

Figure 14.11

Hindgut/cloaca ­ human

­ tracheal rudiment forms branches ­ three on right ­ two on left (humans)

­ tracheal rudiment forms bronchi

­ bronchi form bronchioli

­ alveoli form at ends of bronchi

Endoderm forms the inner epithelium of trachea, bronchi & lungs

Mesodermal forms connective tissue, blood vessels, cartilage of the trachea and lungs

Lung development

Figure 14.10

Mesoderm

Mesoderm forms both epithelia and mesenchyme

(NOTE ­ endoderm and ectoderm form mostly epithelial cells)

Major subdivisions: ­ axial – prechordal plate & notochord

­ paraxial – presomitic plates, somites

­ intermediate – kidney, gonads

­ lateral plate – splanchnic, somatic, extraembryonic

SF Gilbert: Developmental Biology 7 th ed, 2003

Major lineages of the mesoderm

Mesoderm differentiation

Figure 14.12

SF Gilbert: Developmental Biology 7 th ed, 2003

Major lineages of the mesoderm ­ axial

Figure 14.13

Axial mesoderm – notochord formation

­ in vertebrates (exc. mammals) – notochord replaced by vertebral column

­ in mammals – notochord remnants remain in intervertebral discs

notochord ­ posterior head, in neck, trunk and tail ­ dorsal rod of cartilage­like connective tissue ­ persists in primitive marine Chordata

­ urochordates (tunicates) ­ cephalochordates (brachiostomes)

prechordal plate (anterior head region) ­ forms mesenchyme ­ contributes to cranial cartilage

Axial mesoderm forms along dorsal midline

tunicate larva, adult

notochord

SF Gilbert: Developmental Biology 7 th ed, 2003

Major lineages of the mesoderm ­ paraxial

Paraxial mesoderm – somite formation

Figure 14.14

­ presomitic plates form somitomeres ­ somitomeres ­ “whorls” of mesenchymal cells

­ paraxial mesoderm forms somites

­ presomitic plates form as node regresses

­ somitomeres mature into somites

Regulation of somite formation

Hairy1 expression wave

Hairy1

­ periodic expression of Eph receptor tyrosine kinases ­ periodic expression of ephrins (Eph ligands)

­ Eph targets include Notch signaling pathway

­ Notch also regulates the periodic expression of Hairy1 ~ 90 min periodicity

­ Notch directs the placement of somite borders

­ Eph/ephrins involved in cell­cell repulsion

Figure 14.6 in SF Gilbert: Developmental Biology 7 th ed, 2003

Epithelialization of somites

Epithelialization: solid mesenchymal mesoderm transforms into hollow epithelial ball

­ synthesize extracellular matrix proteins; e.g. fibronectin, N­cadherin (see above) ­ cells form tight junctions between basal lamina ­ cells polarize: sub­apical surface (inward); basal membrane (outside)

Figure 14.16

(a) somite – forms epithelial sac

(c) epaxial & hypaxial myotome form myotome ­ epaxial – dorsal trunk muscles ­ hypaxial – detach from myotome; form

limb muscles, ventral trunk muscles

(d) dermatome cells become mesenchymal; form dermis

(b) ventral & medial cells separate; migrate towards notochord, neural tube = sclerotome; cartilage precursors of vertebrae ­ remainder of somite = dermamyotome

­ dorsomedial marginal lip = epaxial myotome ­ ventrolateral marginal lip = hypaxial myotome

Somite derivations

Figure 14.17

Somite Patterning

Figure 14.18

Somites are exposed to signals from surrounding organ rudiments:

ventralizing: sonic hedgehog (Shh) ­ also medialize ­ dose­dependent:

Shh high = sclerotome Shh lower = myotome

dorsalizing: Wnt family ­ antagonizes Shh

lateralizing: bone mophogenic protein (BMP­4) ­ inhibits dorsalizing signal (noggin?)