EMBRYOL - NEURULATION AND THE FORMATION OF AXIAL STRUCTURES

The primary inductive event is the action of the chordamesoderm, or notochord, on the overlying ectoderm, resulting in the transformation of unspecialized ectodermal cells into the primordium of the central nervous system

The initial response of the induced ectoderm is to form a plate of thickened cells

These are segmentally arranged aggregations that are formed by other ectodermal cells from the junction between the neural and general cutaneous ectodermal tissues

The blocks of mesoderm on either side of the notochord soon begin to form symmetrical pairs of brick-like masses called _______, which are both major landmarks in the early embryo and the source of a number of important segmentally arranged mesodermal derivatives later in life

The somite pairs first take shape near the ______

In successive stages additional pairs of somites are formed ____ to those already laid down

From the earliest stages of formation of the nervous system, differentiation of axial structures follows a pronounced ___________

Because of the cephalocaudal gradient, processes which have already been completed in the cranial part of the embryo may be just beginning in the caudal part

_______ is considered to be the period of development starting with the first traces of formation of the neural plate and ending with closure of the neural tube

During late gastrulation, the chordamesoderm, involuting around the dorsal lip of the blastopore in amphibians, and the notochordal process, which forms from cells passing through Hensen’s node in birds and mammals, pushes cranially, just beneath the ectoderm

While the forward movement of the notochordal tissue is taking place, the ______ induce the overlying ectodermal cells, causing them to thicken and form the neural plate

A reaction which both initiates the formation of the central nervous system and causes the central longitudinal axis of the body to be established

The inductor is the _______, and the responding tissue is the ectoderm

It is essential that the inductor and the responding tissue be at the right place at the right time

Without the presence of the underlying notochord, the cells of the dorsal ectoderm do not form neural tissue but rather continue to differentiate as general cutaneous ectoderm

These are embryos in which the normal inpocketing of the archenteron is exteriorized, commonly by the concentration or types of certain ions (e.g., Li+) in the medium surrounding the embryos

The endodermal and chordamesodermal tissues lining the _______ form an outpocketing from the posterior end of the embryo

In order for neural induction to occur, the ectoderm overlying the notochordal process must be able (competent) to respond to the inductive stimulus.

During much of the period of gastrulation in amphibian embryos both the ______ ectoderm have the competence to form neural tissues when subjected to the influence of inductors

This gene is expressed in chordamesoderm, and the noggin protein is capable of converting ectoderm to neural ectoderm

Noggin has no similarity to any other known inducing factor and has been demonstrated to fulfill the requirements of the neural inductor in ______

In order to respond to the neural inductor, the dorsal ectoderm must be competent

Neural induction is associated with an increase in activity of protein kinase C a in the dorsal ectoderm

Later stages of gastrulation in amphibians are dominated by the completion of cell movements leading to the formation of the primitive gut and, in _______, the spreading of the mesodermal mantle between the ectodermal and endodermal layers of the embryo

(Neurulation in Amphibians)

Concomitant with these changes the _____ decreases in prominence

(Neurulation in Amphibians)

Morphologically, the cells responding to neural induction change their shape from a cuboidal or low columnar configuration to a high columnar form. This causes the neural tissues to rise above the surrounding ectoderm as the flattened ______

(Neurulation in Amphibians)

In the ectodermal layer, the cells of the future nervous system have begun to follow a morphological course different from that of the rest of the ectoderm. Initially all the ectodermal cells are arranged as a single sheet of low columnar cells.

(Neurulation in Amphibians)

Initially all the ectodermal cells are arranged as a ______ of low columnar cells

(Neurulation in Amphibians)

Subsequent to neural induction, the cells in an oval area overlying the notochord and future somites undergo a pronounced elongation to form the neural plate, whereas at the same time the ______ covering the rest of the embryo become flattened.

(Neurulation in Amphibians)

Soon the edges of the neural plate become raised, forming an elevated ____ on either side of a shallow, longitudinal neural groove

(Neurulation in Amphibians)

The shape of the neural plate then undergoes a pronounced transformation, becoming attenuated ________, where the spinal cord will eventually form, and remaining broader anteriorly, where the brain will form

(Neurulation in Amphibians)

This is the result of a set of morphogenetic movements caused principally by changes in the shape of cells composing the neural plate

(Neurulation in Amphibians)

During its deformation, the surface area of the neural plate decreases, but its volume remains roughly constant as the shrinkage of the _________ of the neuroectodermal cells is matched by a corresponding increase in their height

(Neurulation in Amphibians)

The shifts in the positions of cells are much more pronounced in the region of the future ________ than in that of the brain, and the magnitude of cellular displacement is correlated with the degree of change in the shape of individual cells

The neural plate does not remain flat very long

The two lateral edges of the neural folds eventually come together in the dorsal midline to form a complete _______

This is where the closure of the neural tube first occurs, and from there proceeds both cephalad and caudad

The __________ act like an internal skeleton of the cell, supporting its greatly increased height

Meanwhile, just beneath the apical surfaces of these cells are organized bundles of thin __________, which can contract, resulting in the constriction of the apical end of the cell

Integrity of the microtubules can be disrupted by exposing them to the drug ________

The organized bundles of microfilaments can be replaced by dense granular masses through the action of ________

After exposure to these inhibitors, neuroepithelial cells do not undergo their characteristic changes in shape and the neural plate remains unfolded

Four main stages have been identified in the formation of the avian neural tube. The first stage is the ________________ through neural induction

The second stage consists of _____________ through intrinsic cell rearrangements and region-specific changes in the shape of the neuroepithelial cells

A third stage in avian and most probably mammalian neural tube formation is ___________ around a median hinge point

The fourth stage of neural tube formation consists of ____________, with their fusion into a distinctive tube

The fourth stage of the formation of the avian neural tube is followed by separation of the neural tube from the general cutaneous ectoderm

At the extreme caudal end of the avian and mammalian embryo, the future _______ in the tail is formed in a different manner from the rest of the cord.

As the lateral walls of the neural folds come together and fuse, __________ begin to emerge from the dorsal part of the neural tube

These cells change their shape and properties from those of epithelial cells to mesenchymal cells, and they penetrate the _______