Neurulation - the formation of the neural plate and its closure in the neural tube in the process of embryonic development of chordates.
Neurulation is one of the key stages of ontogenesis . The germ at the stage of neurulation is called the neurula .
The development of the neural tube in the anterior-posterior direction is controlled by special substances — morphogens (they determine which of the ends will become the brain), and the genetic information about this is embedded in the so-called homeotic or homeotic genes.
For example, with the increase of its concentration, the morphogen retinic acid can turn rhombomers (segments of the neural tube of the posterior part of the brain) of one species into another.
- Neurulation in the lancelet is the buildup of ridges from the ectoderm above the cell layer, becoming a neural plate.
- Neurulation in the multilayer epithelium - the cells of both layers descend under the ectoderm alternately, and diverge centrifugally, forming a neural tube.
- Neurulation in a single-layer epithelium:
- The schizootsel type (in bony fish) is similar to the neurulation of multilayered epithelium, except for the fact that cells of one layer descend.
- In birds and mammals, the neural plate invaginates inward, and closes into the nervous tube .
Content
Roller closure
In birds and mammals in the process of neurulation, the protruding parts of the neural plate, called the nerve cusps , intersect along the entire length of the neural tube unevenly.
Usually, the middle of the neural tube closes first, and then the closure goes to both its ends, leaving as a result two unclosed parts - the anterior and posterior neuropores .
In humans, the closure of the neural tube is more complex. The dorsal region closes first, from the chest to the lumbar, the second from the forehead to the crown, the third from the front, goes in one direction, to the neurocranium, the fourth from the back of the neck to the end of the cervical, also goes to the one direction from the tailbone.
If the second site is not covered, a fatal congenital defect, anencephaly, is found . The brain does not form in the embryo.
If the fifth area is not closed, a susceptible correction is found - congenital spina bifida, or Spina bifida. Depending on the severity, spina bifida is divided into several subtypes.
Neural tube
In the process of neurulation, a neural tube is formed .
In the cross section in it immediately after the formation can be divided into three layers, from the inside to the outside:
- Ependyme is a pseudo-multilayer layer containing rudimentary cells.
- The mantle zone - contains migrating, proliferating cells, evicted from the ependymal layer.
- The outer marginal zone is the layer where nerve fibers are formed.
Plakody
Placodes , or epidermal placodes, are derivatives of the ectoderm that form at the site of contact of the neural tube with the ectoderm.
Nerve Comb
The neural crest is a collection of cells released from the dorsal portions of the neural groove during its closure into the neural tube.