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The neural tube forms from the shaping and folding of the neural plate. In primary neurulation, the surface ectoderm folds into a tube that separates from the surface. In secondary neurulation, the ectoderm forms a cord and then forms a cavity within it.
Primary neurulation is regulated by both intrinsic and extrinsic forces. Intrinsic wedging occurs within cells of the hinge regions to bend the neural plate. Extrinsic forces include the migration of the surface ectoderm towards the center of the embryo.
Neural tube closure is also a mixture of extrinsic and intrinsic forces. In humans, if the neural tube fails to close various diseases can result.
The neural crest cells arise at the lateral borders of the neural tube and surface ectoderm. They become located between the neural tube and surface ectoderm, and they migrate away from this region to become peripheral neural, glial, and pigment cells.
There is a gradient of maturity in many embryos, especially those of amniotes. The anterior develops earlier than the posterior.
The dorsal-ventral patterning of the neural tube is accomplished by proteins of the TGF-β family secreted from the surface ectoderm and roof of the neural tube, and from Sonic hedgehog protein secreted by the notochord and floor plate cells. Both types of protein appear to work through gradients.
The brain forms three primary vesicles: prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). The prosencephalon and rhombencephalon will become subdivided.
The brain expands through fluid secretion putting positive pressure on the vesicles.
The neurons of the brain are organized into cortices (layers) and nuclei (clusters).
New neurons are formed by mitosis in the neural tube. The neural precursors can migrate away from the neural tube and form a new layer. Neurons forming later have to migrate through the existing layers. This forms the cortical layers. The germinal zone at the lumen of the neural tube is called the ventricular zone. The new layer is called the mantle zone (gray matter).
In the cerebellum, a second germinal zone—the external granule layer—is formed. Other neurons migrate out of the ventricular zone on the processes of glial cells.
The cerebral cortex in humans has six layers, and the mantle zone is called the neocortex. Cell fates are often fixed as they undergo their last division. Neurons derived from the same stem cell may end up in different functional regions of the brain.
Neural stem cells have been observed in the adult human brain. We now believe humans can continue making neurons throughout life, although at nowhere near the fetal rate.
Dendrites receive signals from other neurons, while axons transmit them. The place where the signaling takes place (through the release of neurotransmitters) is called a synapse.
Axons grow from the nerve cell body, or soma. They are led by the growth cone.
The chordate and arthropod systems, though structurally very different, appear to be specified through the same set of genetic instructions.
The retina forms from the optic vesicle that extends from the brain. Pax6 plays a major role in eye formation, and the downregulation of Pax6 by Sonic hedgehog in the center of the brain splits the eye-forming region of the brain in half. If Sonic hedeghog is not expressed there, a single medial eye results.
The photoreceptor cells gather the light and transmit the impulse through interneurons to the retinal ganglion cells. The axons of the retinal ganglion cells form the optic nerve.
The lens and cornea form from the surface ectoderm. Both must become transparent.
The basal layer of the surface ectoderm becomes the stratum germinativum, or germinal layer of the skin. These cells divide to produce a stem cell and a cell committed to become an epidermal cell (keratinocyte). Stem cells appear to be able to make hair.
Paracrine factors such as TGF-α and FGF7 are important in normal skin development.
Cutaneous appendages—hair, feathers, and scales—are formed by epithelial-mesenchymal interactions between the epidermis and the dermal mesoderm.