Gastrulation et Neurolation

8/8/2019 Gastrulation et Neurolation

1/25

KAPBiology DeptKenyon College

Chapter 14. Gastrulation and

Neurulation

Gastrulation

"It is not birth, marriage, or death, butgastrulation, which is truly the most importanttime in your life."

Lewis Wolpert (1986)

During gastrulation, cell movements result in a massive reorganization of the embryo from asimple spherical ball of cells, the blastula, into a multi-layered organism. During gastrulationmany of the cells at or near the surface of the embryo move to a new, more interior location

The primary germ layers (endoderm, mesoderm, and ectoderm) are formed and organizein their proper locations during gastrulation. Endoderm, the most internal germ layer, formsthe lining of the gut and other internal organs. Ectoderm, the most exterior germ layer,forms skin, brain, the nervous system, and other external tissues. Mesoderm, the themiddle germ layer, forms muscle, the skeletal system, and the circulatory system.

This fate map diagram of a Xenopus blastula shows cells whose fate is to become ectodermin blue and green, cells whose fate is to become mesoderm in red, and cells whose fate isto become endoderm in yellow. Notice that the cells that will become endoderm are NOTinternal!


2/25

from LIFE: The Science of Biology, Purves et al, 1998

Although the details of gastrulation differ between various groups of animals, the cellularmechanisms involved in gastrulation are common to all animals. Gastrulation involveschanges in cell motility, cell shape, and cell adhesion.

Below are schematic diagrams of the major types of cell movements that occur during

gastrulation.

Invagination: a sheet of cells (called an epithelial sheet) bends inward. Ingression: individual cells leave an epithelial sheet and become freely migratingmesenchyme cells.Involution: an epithelial sheet rolls inward to form an underlying layer.


3/25

from the Amphibian Embryology Tutorial

Epiboly:

a sheet of cells spreads by thinning.Intercalation: rows of cells move between one another, creating an array of cells that islonger (in one or more dimensions) but thinner.ConvergentExtension: rows of cells intercalate, but the intercalation is highly directional.



4/25

Sea urchin gastrulation



5/25

Primary mesenchyme cells undergo ingression at the onset of gastrulation, in part due tochanges in their cell-adhesion properties.

from the Sea Urchin Embryology Tutorial

The vegetal plate undergoes primary invagination to produce the archenteron (primitivegut). Primary invagination is thought to result from changes in the shape of cells in thevegetal plate.


6/25


Secondaryinvagination involves the elongation of the archenteron across the blastocoel,where it attaches near the animal pole of the embryo.



7/25

Secondary invagination is thought to involve filapodia extended by the secondarymesenchyme cells located at the tip of the archenteron. This high magnification view showsa filopodium extended by a secondary mesenchyme cell.


Secondary invagination also involves convergent extension. These images show therearrangement of a labelled clone of cells during archenteron elongation. In the image onthe left, the clone of labelled cells has smooth boundaries; by the end of gastrulation, shownon the right, the labelled cells have intercalated with neighboring unlabeled cells to generatea jagged boundary.


8/25


Xenopus gastrulation


9/25


10/25


This movie was constructed from a series ofcross-sectional images taken by confocalmicroscopy during Xenopus gastrulation. The animal pole is up, and dorsal is to the right.Use the control panel to move through the image in order to see all of cell migrationsoccuring during this complex and dynamic process!


This video show the surface of a Xenopus embryo surface during gastrulation. Early on, thdorsal lip of the blastopore forms due to the contraction of bottle cells (see below). Theblastopore continues to develop from the early "frown" until it can be observed as acomplete circular ring of involuting cells. Convergent extension closes the blastopore at theyolk plug and elongates the embryo along the anterior--posterior axis. The posterior end ofthe embryo is pointed at you.


11/25


How does the the blastopore lip form? A small group of cells change shape, narrowing atthe exterior edge of the blastula. This change in cell shape, called apical constriction,creates a local invagination, which pushes more interior cells upwards and begins to roll asheet of cells towards the interior. The constricted cells are called bottle cells, due to theirshape (like an upside down bottle in these images).



12/25


Gastrulation in birds and mammals


13/25


During gastrulation in birds and mammals, epiblast cells converge at the midline andingress at the primitive streak. Ingression of these cells results in formation of themesoderm and replacement of some of the hypoblast cells to produce the definitiveendoderm.


14/25

from Embryo Images Online

As gastrulation proceeds, the primitive groove extends anteriorly.


Across-section through the embryo allows us to observe the three germ layers that formduring gastrulation:ectoderm, mesoderm, and endoderm.


15/25



16/25


Show below are images ofhuman embryos during gastrulation,13 - 19 days post ovulationNotice the primitive streak, which is analogous to the blastopore ofXenopus.

images from the Visible Embryo

Neurulation

Neurulation in vertebrates results in the formation of the neural tube, which gives rise toboth the spinal cord and the brain. Neural crest cells are also created duringneurulation. Neural crest cells migrate away from the neural tube and give rise to a varietyof cell types, including pigment cells and neurons.

Neurulation begins with the formation of a neural plate, a thickening of the ectodermcaused when cuboidal epithelial cells become columnar. Changes in cell shape and celladhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube.The cells at the tips of the neural folds come to lie between the neural tube and the

overlying epidermis. These cells become the neural crest cells. Both epidermis andneural plate are capable of giving rise to neural crest cells.

What regulates the proper location and formation of the neural tube? The notochord isnecessary in order to induce neural plate formation.


17/25

from Patricia Phelps

Below are scanning electron micrographs of a chick embryo during neurulation.


18/25

During neurulation, somites form in pairs flanking the neural tube. Somites are blocks ofcells that form a segmental pattern in the vertebrate embryo. Somites produce cells thatbecome vertebrae, ribs, muscles, and skin.

The region where neural tube closure begins varies between different classes ofvertebrates. In amphibians such as Xenopus, the neural tube closes almost simultaneouslyalong its entire length. In birds, the neural tube closes in the anterior to posterior direction,as Hensen's node regresses. Mammalian neurulation is similar to that of birds, however thebulky anterior neural plate seems to resist closure - the middle of the tube closes first,

followed by both ends. Watch this animation ofmammalian neurulation!


19/25

This video of a living Xenopus (frog) embryo shows both gastrulation and neurulation. Youshould recognize the beginning of the film from our discussion of gastrulation. The openneural plate on the dorsal side has formed by the time the blastopore closes. The closure ofthe neural plate into a tube is accompanied by elongation of the embryo.


Animal development: Organogenesis

Organogeneis is the period of animal development during which the embryo is becoming afully functional organism capable of independent survivial. Organogenesis is the processby which specific organs and structures are formed, and involves both cell movementsand cell differentiation. Organogenesis requires interactions between different tissues.These are often reciprocal interactions between epithelial sheets and mesenchymal


20/25

cells.

The study of organogenesis is important not only because of its relevance to understandingfundamental mechanisms of animal development, but also because it may lead to medical

applications, such as the repair and replacement of tissues affected by genetic disorders,disease or injury.

Kidney development

There are three stages of mammalian kidney development: the formation of the pronephrosmesonephros, and metanephros (nephros = kidney; pro = before, meso = middle, meta =after). The metanephros is the permanent kidney found mammals (and in birds andreptiles), and forms at the region between the mesonephros and the cloaca (below).


21/25

Balinsky's 1970 figure of mesonephric and pronephric anatomy from Peter Vize

The development of the adult kidney (metanephros) provides a good example of reciprocalepithelial-mesenchyme interactions. Mature (metanephric) kidneys form from reciprocalinductions between the metanephric mesenchyme and the (epithelial) ureteric buds.

The metanephric mesenchyme forms the nephrons, which are the functional units of thekidneys, and the (epithelial) ureteric buds form the collecting ducts and ureter.

Metanephric kidney development is a multistep process.1. Mesenchyme cells induces the ureteric bud to elongate and branch.2. The ureteric bud induces mesenchyme to aggregate (transition from mesenchyme toepithelium).


22/25

images from the Kidney Development Database

3. Each aggregate forms a nephron: first a comma shape is observed, and then the S-shaped tubule, which connects to the branched ureteric bud


23/25

images from the Kidney Development Database

What is the experimental evidence for reciprocal induction?The metanephric mesenchyme doesn't condense into epithelial cells if cultured in isolation,but does if it is cultured with ureteric bud tissue. The ureteric bud doesn't branch if culturedin isolation, but does in combination with mesenchymal cells.Similar experiments using a filter to separate the tissues showed that these inductions onlywork if cell processes can extend through the filter and directly contact the responding cells

Vertebrate limb development

Vertebrate limbs develop from limb buds. The vertebrate limb bud consists of a core ofloose mesenchymal mesoderm covered by an epithelial ectodermal layer. Cells withinthe progress zone rapidly divide, and differentiation only occurs once cells have left theprogress zone. Because of this process, differentiation proceeds distally as the limbextends (that is, the proximal end of the limb develops before the distal end). The apicalectodermal ridge at tip of limb bud induces the formation of the progress zone.


24/25

Pattern formation organizes cell types into their proper locations based on positionalinformation.

Anterior-posteriorpatterning is regulated by the zone of polarizing activity, or ZPA. Thecurrent model is that proximal-distal pattern formation is regulated by the amount of time

a cell spends in the progress zone. Dorsal-ventral patterning is controlled by the overlyingectoderm.

What makes forelimbs and hindlimbs different from one another? Pattern formation isregulated by the same signals in both limbs, although these signals are interpreteddifferently. Limb-specific transcription factors have been identified, and by expressingthese transcription factors in the OTHER (wrong) limb, scientists have been able to observetransformation of the hindlimb into the forelimb, and vice-versa.


25/25

left: Misexpression ofTbx4 in the forelimb region leads to leg-like structures in this region.

right: Misexpression ofTbx5 in the hindlimb region leads to wing-like structures in thisregion.

from the Max Planck Society

Thanks to David Marcey for construction of some of the images shown above

Top

Documents

Gastrulation et Neurolation