130 BICD Embryos, Genes, and Development
Control of sensory and motor axon projection by netrin in the worm embryo
Dark blue: UNC-6 expression
UNC-6: Netrin UNC-5: Component of repulsive receptor UNC-40: Component of attractive receptor
Control of commissural axon projection by Slit and Robo in the fly embryo
Comm: Attractive cue Slit: Repulsive cue Robo: Slit receptor
Commissural axons never leave the midline in Slit mutant embryos
all CNS neurons
all CNS neurons
Migrating epiblast cells that disperse into the blastocoel give rise to the mesoderm of the chick embryo
Distinct subdivisions of the mesoderm arise at different positions relative to the neural tube
Subdivisions of the Embryonic Mesoderm
chordamesoderm: notochord paraxial (somitic or dorsal) mesoderm: connective tissues of the back (bone, muscle, cartilage, dermis) intermediate mesoderm: urinary system and genital ducts lateral plate mesoderm: heart, blood vessels, blood cells; lining of body cavities; non-muscle mesodermal components of limbs head mesenchyme: connective tissues and musculature of the face
Fate of the notochord
Provides axial integrity of early embryo Induces formation and patterning of neural tube; helps specialize sclerotome Most degenerates; remainder forms intervertebral discs
Development of paraxial mesoderm
segmental plate (birds); unsegmented mesoderm (mammals) somitomeres: circular whorls of cells somites: separate blocks of cells
neural crest cells
somite
Somites form as repeating blocks of paraxial mesoderm
somitomere
Cyclical expression of the chick Hairy1 gene is associated with formation of somite boundaries
expression of Hairy1
Hairy1 expression is controlled by cyclical activation of the Notch signaling pathway
EphrinB2 and EphA4 receptor are expressed in complementary patterns in unsegmented mesoderm as somites bud off
EphA4 receptor mRNA
The A/P identity of segmental plate mesoderm is determined before somitogenesis
Somite commences differentiation with the commitment of cells to the sclerotome or the dermamyotome
Subdivisions of somites
sclerotome: chondrocytes (cartilage cells); vertebrae and ribs dermatome: mesodermal portion (dermis) of dorsal skin myotome: musculature of back, abdominal walls, and limbs
The two-layered dermamyotome subdivides into the epaxial and hypaxial myotome and the dermatome
Subdivisions of myotome
epaxial (closest to neural tube): epaxial muscles (deep muscles of the back) hypaxial (furthest from neural tube): hypaxial (body wall) muscles, muscles of limbs and tongue
Patterning of the somite by cell signaling
Subdivisions of lateral plate mesoderm in the frog embryo
Derivatives of lateral plate mesoderm
somatic (parietal) mesoderm: associated with ectoderm splanchnic (visceral) mesoderm: associated with endoderm coelom (body cavity): pericardial (heart), pleural (thorax), and peritoneal (abdomen) cavities
Comparison of mesoderm development in chick and frog embryos
Formation of the chick heart from splanchnic lateral plate mesoderm
Cardiogenic mesoderm is induced by BMP and FGF signals from adjacent anterior endoderm
Fusion of the endocardial primordia creates the inner lining of the heart (endocardium) Fusion of the mesocardium forms a continuous myocardium that will give rise to heart muscle
Cardia bifida caused by surgical prevention of mesodermal fusion at the ventral midline
Somatic lateral plate mesoderm provides the precursors to the skeletal elements of the limb
Functions of the endoderm
Participates in the formation of mesodermal organs (notochord, heart, blood vessels) Constructs the linings of the digestive and respiratory tubes
Major derivatives of the endoderm
Digestive tube: esophagus, stomach, small intestine, colon; tonsils, thyroid, thymus, parathyroid glands (pharynx); liver, pancreas, gallbladder (caudal to stomach) Respiratory tube: trachea, bronchi, alveoli of lungs