25.6
Neurulação
A neurulação é o processo embriológico que forma os precursores do sistema nervoso central e ocorre após a gastrulação ter estabelecido as três camada…
Neurulação é o processo onde componentes do embrião pós-gastrulação se transformam para criar o futuro sistema nervoso. Após gastrulação, o ectoderma de superfície na periferia do embrião é destinado a gerar componentes de pele, enquanto o tecido mais centralizado, a ectoderme neural, irá produzir a medula espinhal e o cérebro. Sinais proteicos fazem com que esse ectoderma médio engrosse e forme a placa neural, sob a qual está uma haste condensada de células mesodérmicas conhecidas como notocorda.
O centro da placa então afixa a esta estrutura subjacente. Inicialmente, anexado a notocorda as células se assemelham a colunas. No entanto, com o tempo elas se contorcem em cunhas que, junto com outros fatores, faz com que toda a placa neural se dobre para cima e para dentro como se fosse uma dobradiça.
Simultaneamente, as bordas das placas se tornam mais proeminentes, gerando estruturas chamadas dobras neurais. Finalmente, duas dobradiças adicionais surgem, uma abaixo de cada dobra neural, e ambas presas ao ectoderma de superfície fazendo com que as bordas das placas girem para dentro. Durante este processo de neurulação primária, as dobras neurais podem se mover em direção uma a outro e finalmente tocar, coalescer, e separar-se do ectoderma de superfície adjacente.
O resultado é um cilindro vazio derivado de ectoderma chamado tubo neural que, dependendo de sua localização embrionária, irá gerar a medula espinhal ou cérebro.
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Q1: What is neurulation and when does it occur during embryonic development?
Neurulation is the embryological process that forms the precursors of the central nervous system, occurring after gastrulation establishes the three primary cell layers. During neurulation, the central portion of the ectoderm—initially a flat sheet of cells—folds upward and inward, sealing to form a hollow neural tube. The anterior neural tube develops into the brain, while the remainder forms the spinal cord.
Q2: How does the neural plate transform into the neural tube?
Protein signals cause the midline ectoderm to thicken, forming the neural plate above a condensed mesodermal rod called the notochord. Notochord-attached cells transform from columns into wedges, causing the neural plate to bend upward and inward. Neural folds form at the plate's edges and move toward each other, eventually fusing and detaching from the surface ectoderm to create the hollow neural tube.
Q3: What are neural crest cells and where do they originate?
Neural crest cells are a population located at the junction between neural and surface ectoderm. As neural folds elevate and fuse during neurulation, neural crest cells appear at the fold tips, identifiable by markers like the Pax7 transcription factor. These cells subsequently migrate throughout the embryo via cell migration to generate diverse tissue types, including peripheral nervous system components like intestinal ganglia.
Q4: How do neural crest cells migrate away from the neural tube?
Neural crest cells downregulate specific adhesive proteins, allowing them to detach from other cells in the neural tube. This reduction in cell-cell adhesion enables neural crest cells to travel throughout the embryo, populating different regions to generate various tissue types including elements of the peripheral nervous system.
Q5: What is the difference between primary and secondary neurulation?
Primary neurulation forms most of the central nervous system in humans through folding of three distinct ectoderm cell sheets. Secondary neurulation, which forms a small posterior spinal cord region, involves loosely-packed cells that condense into a medullary cord, which then hollows out and merges with the anterior primary neural tube to form a continuous structure.
Q6: What developmental consequences can result from defects in secondary neurulation?
Although secondary neurulation plays a relatively minor role in forming the human central nervous system, defects in this process can cause developmental consequences such as certain types of spina bifida. These defects occur when the medullary cord fails to properly hollow out or merge with the primary neural tube.
Q7: How does the notochord influence neural plate folding during neurulation?
The notochord is a condensed rod of mesodermal cells positioned beneath the neural plate. Notochord-attached cells initially resemble columns but transform into wedges over time. This transformation, along with other factors, causes the entire neural plate to bend upward and inward as if on a hinge, driving the folding mechanism essential to neural tube formation.