EMBRYOL Chapter 6 - Gastrulation and the Formation of the Germ Layers

Following the period of cleavage and the formation of a blastula, the embryo embarks on one of the most critical periods in its development—__________

Cleavage ---> Blastula ---> _______

is characterized by profound but well-ordered rearrangements of the cells in the embryo.

The major morphogenetic movements during gastrulation result in the _____________________ to a stage characterized by the presence of three germ layers, but other morphogenetic movements continue into later stages.

Major types of morphogenetic movements seen in early embryos (gastrulation part)

Inpocketing of sheet of cells (e.g. archenteron formation in amphioxus)

Outpocketing of a sheet of cells (e.g. exogastrulation)

Rolling around a corner of an expanding outer layer of cells and spreading over an internal surface (e.g. Cell movements through the amphibian blastopore)

Spreading of a cell sheet (e.g. Spreading of outer cells toward amphibian blastopore)

Sinking of individual cells from a surface into an area (e.g. Primary mesenchyme formation in sea urchin blastula)

Separation of a second sheet from an original single sheet (e.g. Formation of primary hypoblast in avian embryos)

Migration of cells as single individuals through their own motility (e.g. migration of neural crest cells

Embryo's two main strategies for dealing with gastrulation

In amphioxus, gastrulation consists of an inpocketing or ____ of a blastula

The new cavity in the double-walled cup is termed ______

the rudimentary alimentary cavity of an embryo at the gastrula stage.

Opening from the outside into the archenteron is called

In ___________ the original single layer of the blastula has been rearranged to form two layers

outer cell layer is called

inner layer of the early gastrula is composed of cells belonging to the _____

middle germ layer

The _______________________ begin in the late blastula stage with the separation of the primary mesenchyme from the wall (vegetal plate) of the blastula

prominent projections

The main feature of gastrulation in the sea urchin is the formation of the ______

1. The first stage is dominated by the inpocketing, or invagination, of cells at the vegetal pole and their elongation into the blastocoel

2. The second stage of archenteron formation is characterized by the presence of a population of secondary mesenchymal cells, which become distinguishable at the innermost tip of the archenteron

3. In the third stage, the tip of the archenteron makes contact with the blastocoel wall

After the archenteron reaches the opposite wall of the blastula, the resulting bilaminar layer soon ruptures to form the ________. At the other end, the blastopore becomes the _______.

After the emergence of the oral opening and anus, the embryo then begins to assume a triangular appearance, and as the skeleton arises from the primary mesenchyme, armlike structures form and the embryo is well on the way to becoming a __________.

This groove, called the _______, represents the site at which cells move into the interior of the embryo

The upper margin of the blastoporal groove is known as the _________ and will later become the major organizing region (Spemann organizer) of the embryo

The mass of yolk left presenting at the blastopore is known as the _____

Gastrulation movements are _______ among the major amphibian groups

Much of the process of gastrulation consists of _____________ at the blastopore.

As cells turn inward around the lips of the blastopore, _____________

Around most of the ventral margins of the blastopore and extending down onto the ventral part of the embryo the _________ is rolled into the interior of the embryo and eventually comes to line its archenteron or primitive gut

Most of the cells passing over the dorsal lip of the blastopore are often called ______ because they eventually give rise to the notochord and cephalic mesoderm

When involution has just begun, the early archenteron is lined by _______ on its dorsal surface and elsewhere by endoderm

In ________ embryos, part of the dorsal surface of the archenteron remains lined by chordamesodermal material until late in gastrulation

Convergent extension of the gastrulating embryo is due in large part to a process of _________ that begins around the midgastrula period.

In _____________, lateral cells insert processes between cells located in the medial part of the layer and ultimately become totally interposed between them

Through mediolateral intercalation, a short and broad mass of cells becomes long and thin, and the gastrulating embryo as a whole begins to ___________________

As they move into the interior of the embryo, the ___________ of the amphibian embryo migrate over the inner surface of the roof of the blastopore

The _________, which begins to be deposited in the early blastula stage, has a high concentration of fibronectin molecules.

As the _______ pass around the rim of the dorsal lip, they become the endodermal lining of the archenteron

The _______, which are so prominent in early gastrulation, decrease in height and ultimately become indistinguishable from other endodermal cells of the endodermal lining.

it appears that _____ move toward the blastopore, turn inward at its lips, and then migrate away from the blastopore as the definitive middle mesodermal germ layer

The _____________ does not remain static during gastrulation

The remainder of the ectoderm will form the epidermis of the skin and is therefore called __________

By the end of gastrulation the entire outer surface of the embryo is covered by ___________, whereas the future endoderm and mesoderm have come to lie entirely within

(Gastrulation in birds) a thin sickle-shaped mass of cells or _________ takes shape at the posterior end of the embryo

From Roller’s sickle, a second generation of hypoblastic cells (secondary hypoblast) pushes __________, compressing and folding the primary hypoblast ahead of it

__________ germ cells are found in the primary hypoblast

The secondary hypoblast forms extraembryonic endoderm, principally the __________.

Formation of the primary and secondary hypoblast can be considered a ___________ phenomenon

_______ and formation of the definitive embryonic germ layers begin with the appearance of a condensation of cells in the posterior part of the epiblast

The appearance of the primitive streak is the result of an ______________________ (Eyal-Giladi and Wolk, 1970), and its orientation is a reflection of the intrinsic polarity of the underlying hypoblastic layer.

The early primitive streak initially elongates in both a cephalic and a caudal direction

At the cephalic end of the primitive streak, closely packed cells form a local thickening known as

The main period of gastrulation begins with the establishment of the ___________ and ____________

In ____, gastrulation is accomplished by the coordinated passage of individual cells from the exterior into the interior of the embryo (ingression) rather than the immigration of integral sheets of cells

The first cells to pass through the area of the anterior part of the primitive streak are future embryonic endodermal cells.

As time goes on, a progressively greater percentage of the cells which pass through the node are destined to be incorporated into the mesoderm and a correspondingly smaller number lodge in the endoderm

The most extensive invagination of mesodermal cells occurs along the length of the ________, where the coherent layer of mesodermal cells that is formed expands parallel to the underlying layer of the embryonic endoderm

Besides the primitive streak, the other major site of mesoderm formation is through ________, where a rod of mesodermal cells directed cephalad lies in the midline of the embryo in the track of the regressing primitive streak

The mesodermal rod becomes the ________

The epiblast is composed of ____________cells

The deeper parts of the cells are bound together by spotlike junctions known as _________, which are involved in cell-to-cell communication.

When the cells of the epiblast enter the primitive groove, they undergo a ________________, becoming to some extent bottle-shaped in a manner reminiscent of the bottle cells seen in amphibian gastrulation

During this change of shape, the _________ begin to break up and lose their circumferential arrangement at the apex of each cell.

Shortly after the first prospective notochordal cells are laid down, the primitive streak and Hensen’s node undergo a regression toward the ______ end of the embryo

the mammalian embryo contains almost no yolk

As the hypoblast forms, the remainder of the inner cell mass can be called the ________

the ______ contains the cells that will ultimately migrate through the primitive streak and become the definitive endodermal and mesodermal germ layers of the embryo

Formation of the primitive streak in mammalian embryos follows a pattern quite similar to that in _____ embryos

Not long after the hypoblast is established, the remaining cells of the inner cell mass become more regularly arranged and are collectively called the _________

In mouse, the early hypoblast is called ___________

Primitive endoderm in ______ initially forms a single layer beneath the inner cell mass

(in mouse (rodents))

The single layer beneath the inner cell mass created by the primitive endoderm soon spread out beneath the ________ to form the endodermal layer of the parietal yolk sac.

(in mouse (rodents))

From an early stage, the trophectoderm can be subdivided into two sections

A subdivision of trophectoderm that overlies the inner cell mass

A subdivision of trophectoderm that surrounds the blastocyst cavity

In ______, the inner cell mass undergoes a transformation that is strikingly different from its course in other mammals

Immediately above the primitive ectoderm, the cells of the polar trophoblast form a relatively massive ____________.