ectoderm. mesoderm, endoderm

Ectoderm

• The notochord secretes substances including noggin and chordin which inhibit the growth factor BMP-4, causing the overlying ectoderm cells form the neural plate or neurectoderm

• In week 4, the flat neural plate rolls up into the neural tube

• The neural tube will form the brain and spinal cord

• Other ectoderm (in the presence of BMP-4) becomes epidermis

• The otic and lens placodes are thickenings of ectoderm that will form the labyrinth of the ear and the lens of the eye

• Ectoderm also gives rise to subcutaneous glands, pituitary gland and tooth enamel

Mesoderm

• Mesoderm condenses into 3 columns on each side

• Paraxial mesoderm forms paired segments: somites - appear in a craniocaudal sequence; ‘segmentation clock’ depends on cyclic expression of several genes in mesoderm

• Each somite divides into: sclerotome; myotome; dermatome; molecular signals from neural tube and notochord (sonic hedgehog and noggin) induce sclerotome to differentiate

• Intermediate mesoderm forms urogenital structures

• Lateral plate mesoderm pulls apart at the edges of the germ disc - to form a visceral/splanchnic layer lining yolk sac/organs and a parietal/somatic layer lining inside of body wall

• Growth of somites causes lateral folding of the embryo and encloses intraembryonic cavity

Endoderm

• Development of the brain causes cephalocaudal folding of the embryo – encloses part of the endoderm-lined cavity inside the embryo as the primitive gut tube

• Foregut finishes blindly at buccopharyngeal membrane

• Midgut still attached to yolk sac (outside the body of the embryo) via yolk sac duct/vitelline duct

• Hindgut finishes blindly at cloacal membrane