Lecture notes for Monday, February 19 Ectoderm   early neurulation closure of neural tube Ectoderm subdivides by folding, to form 3 subdivisions: Neural Tube Ectoderm: Which itself subdivides to form the following. Brain Spinal cord Motor nerves (one segmental motor nerve per somite). Preganglionic Autonomic Nerves Neural Retina Pigmented Retina   Neural Crest Ectoderm, which differentiates into many diverse cell types Sensory nerves, dorsal root ganglia (one per somite) Postganglionic autonomic nerves Melanocytes, and other mesenchymal pigment cells. Schwann cells (but not oligodendrocytes). Facial skeleton (cell types that would be mesodermal in any other part of the body!)   Somatic Ectoderm, most of which becomes epidermis Some parts in the head become placodes. A pair of olfactory placodes become nerves of nose. A pair of lens placodes become the lenses of the eyes. A pair of otic placodes become the inner ear (semi-circular canals, cochlea, etc.). In fish and amphibians, the lateral line system develops from placodes. The inner ear uses neuromast cells to detect sound, gravity & water flow; the lateral line system also uses neuromast cells to detect flow.     olfactory placode in an amphibian; the dark spots are pigment cells   cross-section of chick embryo showing lens and otic placode   developing eye in a chick embryo         Neuromast cells, right above here Semi-circular canals in a living Xenopus tadpole   Otoliths in the same living Xenopus tadpole These granules of calcium carbonate are embedded in a gel, and detect which way is down, by pressure on neuromast cells.   Chick eye-cup with lens   Elongation of lens fibers   Lens of a mammal eye: Each cell is extremely long, extending from the posterior side of the lens, almost all the way to the front Pigment cells in amphibian skin   Embryonic chick eyeball, with retina surrounding it. The groove is the location of the optic nerve.   Axons extending in tissue culture Axons in tissue culture on a rubber substratum