The early development of chick: cleavage and gastrulation
Zoology Lesson: The early development of chick: cleavage and gastrulation Lesson Developer: Dr Meena Yadav College/Department: Maitreyi College, University of Delhi
Institute of Life Long Learning, University of Delhi
0
The early development of chick: cleavage and gastrulation
TABLE OF CONTENTS
Introduction Cleavage in chick Gastrulation The hypoblast The primitive streak Formation of endoderm and mesoderm Regression of primitive streak Cell movement in the ectoderm Avian Embryo: Axis Formation pH and dorsal-ventral axis Gravity and anterior-posterior axis Left-right axis Gene expression on the right side of primitive streak Gene expression on the left side of primitive streak Summary Exercise/practice Glossary References Suggested readings Useful web links
Institute of Life Long Learning, University of Delhi
1
The early development of chick: cleavage and gastrulation
INTRODUCTION There have been many animal models for studying embryology. Aristotle was the first scientist to follow the early development of the chick till 3-weeks. Since then, chick has become a favorite organism for embryological studies. Advantages of chick as the model organism for such studies are: 1. It is accessible all around the year and is easy to breed. 2. Its developmental stage can be accurately predicted at any given temperature. This makes it possible to collect large number of embryos at same stage of development 3. The embryos can be surgically manipulated 4. Since the stages of its development are similar to mammals, it provides a suitable medium to study various stages of development and also provides an insight into the development of mammalian embryos.
CLEAVAGE IN CHICK In birds, fertilization occurs in the oviduct of the female just before the albumin and shell are laid upon it. The chick egg is telolecithal i.e. the cytoplasm has been pushed to one corner of the egg and the rest of the egg is filled with yolk. A small disk of the cytoplasm is present on top of a mound of yolk (Figure 1).
Figure 1: The chick egg Source: Author Institute of Life Long Learning, University of Delhi
2
The early development of chick: cleavage and gastrulation The cleavage in eggs of the birds is discoidal meroblastic. The cleavage divisions occur only in the cytoplasm present at the animal pole in the form of a blastodisc, its diameter being only 2-3 mm. The first cleavage occurs at the center of the blastodisc and other cleavage divisions follow soon, to create a single layer of cells called blastoderm (Figure 2). The cleavage divisions do not extend into the yolk, so the peripheral cells remain in contact with yolk and the rest of the cells are also connected with each other.
Figure 2: Cleavage in chick egg Source: Author
Later the vertical and equatorial divisions continue in the blastoderm and produce a sheet of tissue consisting of six to seven layers. These cells are now connected with each other by tight junctions. As the divisions continue, the blastodermal cells absorb fluid from the albumin and secrete it such that the fluid accumulates between the cells and between the cells and yolk. The space between the yolk and blastoderm is called subgerminal cavity. The cells present in the deep center of the blastoderm die and are shed, thus creating a one-cell thick area pellucida. This part forms the actual embryo while the peripheral ring of the blastoderm does not shed the cells and form the area opaca (Figure 3). Marginal zone or marginal belt is the name given to a thin layer of cells between the area pellucida and area opaca.
GASTRULATION Institute of Life Long Learning, University of Delhi
3
The early development of chick: cleavage and gastrulation
Gastrulation is an early event during embryogenesis in vertebrates where the group of cells, formed after cleavage divisions arrange themselves into three layers i.e. ectoderm, mesoderm and endoderm. These three layers are precursors of future organs in the vertebrate body. However, the events of gastrulation differ slightly in all vertebrates. Here, we will discuss events that occur during avian gastrulation.
The hypoblast In hen, at the time of laying eggs, the blastoderm contains as many as 20,000 cells. Most of the cells of the area pellucida remain at the surface of the animal pole and form the epiblast, while the rest of the cells of the area pellucida delaminate themselves and migrate into the subgerminal cavity, where they form polyinvagination islands or primary hypoblast. These cells exist in clusters of 5-20 cells each. The posterior margin of the blastoderm contain a local thickening of cells called as Koller’s sickle. As soon as the delamination of the hypoblast cells starts, a sheet of the cells from the Koller’s sickle region starts migrating to the anterior side and join the polyinvagination islands, thus forming secondary hypoblast (Figure 3).
Institute of Life Long Learning, University of Delhi
4
The early development of chick: cleavage and gastrulation
Figure 3: Migration of cells to form two layers i.e. epiblast and hypoblast with blastocoel separating them Source: Author
VALUE ADDITION: Things to know Heading: Cell movements of primitive streak of the chick embryo Body Text:
Institute of Life Long Learning, University of Delhi
5
The early development of chick: cleavage and gastrulation
Cell movements of the primitive streak of the chick embryo. (A-C) Dorsal view of the formation and elongation of the primitive streak. The blastoderm is seen at (A) 3–4 hours, (B) 7–8 hours, and (C) 15–16 hours after fertilization. The early movements of the migrating epiblast cells are shown by arrows. (D-F) Formation of notochord and mesodermal somites as the primitive streak regresses, shown at (F) 19–22 hours, (E) Institute of Life Long Learning, University of Delhi
6
The early development of chick: cleavage and gastrulation 23–24 hours, and (F) the four-somite stage. Fate maps of the chick epiblast are shown for two stages, the definitive primitive streak stage (C) and neurulation (F). The endoderm has already ingressed beneath the epiblast, and convergent extension is seen in the midline. Source: http://www.devbio.biology.gatech.edu/?page_id=52 CC
The blastoderm now has two layers i.e. epiblast and hypoblast, separated by a cavity called blastocoel. The entire avian embryo is formed from the cells derived from the epiblast. The hypoblast cells form the portions of the extraembryonic membranes like yolk sac and stalk, which links the yolk mass to endodermal digestive tube. The epiblast cells, thus, form the embryo and some extraembryonic membranes.
VALUE ADDITION: Things to know Heading: Signal for initiation of primitive streak formation Body Text:
Institute of Life Long Learning, University of Delhi
7
The early development of chick: cleavage and gastrulation
Source: http://www.mun.ca/biology/desmid/brian/BIOL3530/DEVO_04/ch04f11.jpg
The primitive streak The presence of a primitive streak is characteristic feature of the avian, reptilian and mammalian gastrulation. The primitive streak appears as a thickening of the epiblast at the posterior side near the Kolliker’s sickle. This thickening is the result of the ingression of endodermal precursor cells from epiblast into the blastocoel and also the migration of cells from the lateral regions of the epiblast towards the center. As the migration of these cells continues towards the primitive streak, the streak elongates Institute of Life Long Learning, University of Delhi
8
The early development of chick: cleavage and gastrulation towards the anterior side or future head. Simultaneously, the secondary hypoblast cells continue to migrate towards the anterior side. The primitive streak extends to 60-75% of the length of area pellucida. The primitive streak is important in establishing the axes of the embryo. It lies length wise in an anterior-posterior axis; the cells migrate from its dorsal side to the ventral side and it also separates left side from the right side. The cells that are closer to the primitive streak form the medial (central) structures while cells away from it form the distal (lateral) structures.
Figure 4: Migration of cells towards the primitive streak and formation of primitive groove Source: Author The cells keep on coming towards the primitive streak. As they pile up, some of them start converging at the center of the streak from both sides. This causes formation of a groove at the center of the streak called as primitive groove (Figure 4). This groove then serves as the opening through which the migrating cells pass into the blastocoel. Thus, the primitive groove is similar in function to the amphibian blastopore.
Institute of Life Long Learning, University of Delhi
9
The early development of chick: cleavage and gastrulation
VALUE ADDITION: Things to know Heading: Migration of cells during chick gastrulation Body Text:
Source: http://s723.photobucket.com/user/alphamalik/media/47_13ChickGastrulation.jpg.html The anterior end of the primitive streak contains a thickening of the cells known as primitive knot or Hensen’s node. This node contains a funnel shaped depression at the center, called primitive pit, which acts as a passage for the cells to pass to the blastocoel (Figure 5). The Hensen’s node is analogous to dorsal lip of the blastopore in amphibians. The above figure shows that once the primitive streak is formed, its cells start migrating through the primitive groove and primitive pit into the blastocoel. These cells migrate anteriorly and form the foregut, head mesoderm and notochord. Another population of cells passes through the lateral portions of the primitive streak and gives rise to most of the endodermal and mesodermal derivatives.
Institute of Life Long Learning, University of Delhi
10
The early development of chick: cleavage and gastrulation
Figure 5: Side view of the primitive streak stage during chick embryogenesis Source: Author The cells that enter the inside of the avian embryo ingress as individuals after they have undergone epithelial-mesenchymal transformation. The cells at the Hensen’s node and at the entire length of the primitive streak are broken from the basal lamina (delamination) and are released into the embryo. This is thought to be assisted by a 190-kDa protein called scatter factor. This protein is secreted by the cells themselves as they enter the primitive streak. The epithelial cells are converted into the mesenchymal cells in several ways by the scatter factor. It is thought to downregulate the expression of E-cadherin and thus, prevents its function as well.
Institute of Life Long Learning, University of Delhi
11
The early development of chick: cleavage and gastrulation
VALUE ADDITION: Things to know Heading: Formation of primitive streak and endodermal layer in chick Body Text:
Source: https://www.ucl.ac.uk/cdb/research/stern/stern_lab/Fig3_copy.jpg Institute of Life Long Learning, University of Delhi
12
The early development of chick: cleavage and gastrulation
VALUE ADDITION: Things to know Heading: Function of E-cadherin Body Text: E-cadherin is a cell-adhesion molecule and helps in attachment of adjacent cells with the help of catenins and actins.
Source: http://www.clinsci.org/cs/118/0451/cs1180451a02.gif Formation of endoderm and mesoderm The cells start migrating towards the primitive streak and eventually enter the blastocoel. (1)
The first cells to start migration form the pharyngeal endoderm of the foregut. Once they enter the blastocoel, they migrate anteriorly and displace the hypoblast cells, such that the hypoblast cells become confined to a region in the anterior portion of the area pellucida. This specific region is called germinal crescent, and contains the precursors of the germ cells. These precursor cells then migrate to the gonads through the blood vessels.
(2)
The next group of cells entering the blastocoel through the Hensen’s node, also move anteriorly, but not as far as those of the pharyngeal endoderm. They establish themselves between the endoderm and the epiblast to form prechordal plate and head mesenchyme. All these cells move anteriorly and Institute of Life Long Learning, University of Delhi
13
The early development of chick: cleavage and gastrulation push up the anterior midline region of the epiblast to form the head process. This places the head of the embryo anterior (rostral) to the Hensen’s node. (3)
The next cells in line become the chordamesoderm (notochord) cells. These cells extend anteriorly to the presumptive midbrain and meet the prechordal plate. The chordamesoderm extends from some anterior portion of Hensen’s node till some posterior portion of it. The chordamesoderm at the level of the Hensen’s node and caudal to it form the hindbrain and trunk respectively.
(4)
Simultaneously, the cells continue to migrate from the lateral sides into the primitive streak. As they enter the blastocoel through the primitive groove, they separate into two layers: (i) The deep layer joins the hypoblast in the middle and displaces the hypoblast cells to the sides. These cells give rise to the endodermal organs of the embryo and most of the extraembryonic membranes. The hypoblast cells give rise to rest of the extraembryonic membranes. (ii) The second layer spreads itself between the endoderm and the epiblast and forms a loose layer of the cells. These middle layer cells form the mesoderm and thus the mesodermal derivatives and some extraembryonic membranes.
After 22 hours of incubation, most of the presumptive endodermal cells lie at the interior of the embryo while the presumptive mesodermal cells continue their migration inwards.
Regression of the primitive streak While the mesodermal cells continue their migration, the primitive streak starts to retreat such that the Hensen’s node moves from the center of area pellucida to a posterior position. Thus, it leaves the dorsal axis of the embryo and the notochord. The notochord is laid down starting from the future midbrain towards the posterior side as the Hensen’s node retreats. The formation of the anterior and rest of the notochord is contributed by different set of cells: the anterior part of the notochord is formed by the cells that entered through the Hensen’s node. The posterior notochord (after somite 17 in chick) is formed from the mesodermal tissue, which was formed from the cells ingressed through the lateral sides of the primitive streak. The posterior portion of the notochord extends till the tail of the embryo. The regression of the primitive streak continues till the Hensen’s node reaches the posterior most portion and forms the anal region.
Institute of Life Long Learning, University of Delhi
14
The early development of chick: cleavage and gastrulation
VALUE ADDITION: Things to know Heading: Prospective cardiac cell migration to form heart tube in chick Body Text: “In avian embryos, the prospective cardiogenic cells are located in the epiblast and primitive streak (Figure). Within the epiblast, these cells are bilaterally distributed on both sides of the primitive streak, caudal to the node. A few hours later (PS5), the rostral half of the primitive streak, with the exception of the node, contains the prospective cardiogenic cells (Figure 1), and also the cells that are going to form the endoderm which underlies the pre-cardiac mesodermal cells at both sides of the embryo. Progeny of cells from this region will contribute to all layers of the heart tube, including endocardium, myocardium, and parietal pericardium. Later (HH4) in gastrulation (Figure 1), pre-cardiac cells from the epiblast invaginate through the primitive streak, and move bilaterally and cranially, to form the bilateral cardiogenic mesoderm, located in the anterior position, and constituting the primary cardiac field, also called the heart-forming region. At this moment, the pre-cardiac mesoderm is surrounded by the adjacent endoderm, which comes from the more cranial part of the primitive streak, and which plays a crucial role during cardiac specification. From stage HH7 begins the organization of the heart rudiment pre-cardiac splachnic mesoderm, which will form both of the primitive endocardial tubes, surrounded by the underlying endoderm. From stage HH9, both primitive endocardial tubes fuse in the mid-line to form the primitive heart tube, structurally organized in concentric layers of endocardium and myocardium.”
Institute of Life Long Learning, University of Delhi
15
The early development of chick: cleavage and gastrulation
Fig. Schematic diagrams showing the pre-cardiac cells, from stage HH3 (PS3) to stage HH10, in the developing chick embryo. Red indicates the mesodermal pre-cardiac cells, and green the endodermal cells that are related to cardiogenesis. The section shows the migration of epiblast cells ingressing through the primitive streak at stage HH4. Source: Carmen López-Sánchez, Virginio García-Martínez. Molecular determinants of cardiac specification
Cardiovascular
Research185-195.DOI:
http://dx.doi.org/10.1093/cvr/cvr127
http://cardiovascres.oxfordjournals.org/content/cardiovascres/91/2/185/F1.large.jpg
At this point of development, all the presumptive endodermal cells and the mesodermal cells lie inside the embryo and the epiblast represents the presumptive ectodermal cells. As the cells migrate and take their respective future positions in the embryo like head, notochord etc., the anterior-posterior axis is established in the embryo. The cells at the anterior portion of the embryo are ahead in their development and start forming the organs even as the cells at the posterior portion are still undergoing gastrulation. Thus, there is an obvious spatial and temporal difference in the stages during the chick embryogenesis. For the next few days, the anterior portion runs ahead with the development as compared to the posterior portion.
Institute of Life Long Learning, University of Delhi
16
The early development of chick: cleavage and gastrulation
VALUE ADDITION: Things to know Heading: Regression of the primitive streak in chick Body Text:
Source: http://www.mun.ca/biology/desmid/brian/BIOL3530/DEVO_04/ch04f10.jpg
Cell movement in the ectoderm (epiboly= 4 days) At the time when the endodermal and mesodermal precursor cells are migrating to their prospective positions in the embryo, the ectodermal precursor cells start proliferating. These cells start surrounding the yolk by their migration called epiboly. This process takes about 4 days to complete. The precursor ectodermal cells migrate underneath the vitelline envelope, however, only the cells of the outer edge of area opaca attach to the vitelline envelope. These cells are unique as they can extend numerous large cytoplasmic processes (500 µm) into the vitelline envelope. These processes act as the locomotor apparatus (called as filopodia) of these marginal cells and help them in pulling other ectodermal cells around the yolk. The filopodia bind to a protein present in the vitelline envelope called fibronectin. This binding helps in the migration of ectodermal cells beneath the vitelline envelope. If experimentally, the contact between the marginal cells and fibronectin is broken, the Institute of Life Long Learning, University of Delhi
17
The early development of chick: cleavage and gastrulation epidermal migration stops as filopodia retract. At the end of gastrulation in chick, the three germ layers take their respective positions, as in other vertebrates: i)
The ectoderm surrounds the yolk and forms the outermost covering of the
embryo. ii)
The endoderm replaces the hypoblast
iii)
The mesoderm establishes itself between these two layers.
Avian embryo: Axis formation The signals for the axis formation in chick embryo come from the start of the embryogenesis i.e. cleavage. The various factors affecting the formation of avian axis are:
pH and dorsal-ventral axis The first axis that is formed is the dorsal-ventral axis, which differentiates the back and belly side of the chick in the embryo. It is critical for the formation of the first new layer of cells i.e. hypoblast. This axis is established due to the cleaving blastodermal cells. These cells create a pH gradient between the albumin above the blastodisc (pH= 9.5, basic) and subgerminal space below it (pH=6.5, acidic) (Figure 6). The blastodermal cells transport water and Na+ ions from the albumin into the subgerminal space, thus creating a membrane potential difference of 25 mV across the epiblast cell layer. The membrane potential is positive at the ventral side of the epiblast. This exchange of ions separates the two sides of the epiblast: i)
One side having negative membrane potential and facing basic albumin. This
side becomes the future dorsal side ii)
The other side having positive membrane potential and facing the acidic
subgerminal space fliud. It becomes the future ventral side of the embryo. If the pH of the two sides is reversed or the membrane potential is reversed, the axis formation is also reversed experimentally.
Institute of Life Long Learning, University of Delhi
18
The early development of chick: cleavage and gastrulation
Figure 6: pH and dorsal-ventral axis formation Source: Author Gravity and anterior-posterior axis The gravity plays an important role in converting radially symmetrical blastoderm into bilaterally symmetrical embryo. When the ovum is passing through the reproductive tract of the hen, it is rotated for 20 hours in the shell gland. The rate of the spinning is 10-12 revolutions per hour. This spinning causes the rearrangement of the yolk components such that its lighter components lie beneath one side of the blastoderm. Due to this, that portion of the egg tips up and is the place from where the formation of the primitive streak initiates. This part then becomes the posterior portion of the embryo. The molecular signals that make this part of the egg as posterior portion and initiate gastrulation are not fully understood. Any part of the marginal zone can initiate formation of the primitive streak. It has been proved by experiments (Spratt and Haas, 1960) in which the blastoderm is separated into many parts, each having its own marginal zone. Each part was able to form its own primitive streak. Once the posterior marginal zone (PMZ) is formed in the marginal zone, it prevents the other regions of the marginal zone to initiate formation of any other primitive streak and also allows the gastrulation to commence. Thus, it appears that PMZ may contain cells analogous to those of Nieuwkoop center of the amphibians, which give signals for the initiation of gastrulation. If a part of the posterior marginal zone tissue (not including Koller’s sickle) is grafted at the anterior region, it induces the formation of the primitive streak and Hensen’s node. At this region, the TGF-β signaling and β-catenin localization in the nucleus, coincide. The posterior region of the embryo is the only source of Vg1. Institute of Life Long Learning, University of Delhi
19
The early development of chick: cleavage and gastrulation
Important points to remember: S.No. 1
Important points of chick gastrulation The Hensen’s node is formed by the cells from the epiblast and the middle layer cells of the anterior Koller’s sickle.
2
Posterior portion of the Koller’s sickle form the posterior portion of the primitive streak
3
Dorsal mesoderm: forms the central nervous system in the ectoderm overlying it.
4
The cells of the Hensen’s node and its derivatives: they secrete Chordin, Noggin and Nodal proteins. They inhibit the action of the BMPs and dorsalize the ectoderm and mesoderm.
The avian Hensen’s node is analogous to the amphibian dorsal blastopore lip because: i)
It is the site where the gastrulation begins
ii)
It is the region whose cells become chordamesoderm
iii)
It is the region whose cells can establish new embryonic axis when transplanted
into new locations of the gastrula. iv)
If the Koller’s sickle is transplanted into a new embryo, it causes formation of
new axes v)
The middle layer cells of the Koller’s sickle express goosecoid as do the cells of
Spemann organizer in amphibians. The neural induction in chick is different from that of fishes or amphibians. Unlike in amphibians, in chick embryos, ectopic expression of chordin in non-neural epiblast cells do not cause neural induction and BMPs do not inhibit neural induction. The important factor that causes positive regulation of neural induction in chicks are Fibroblast Growth Factors (FGFs). They are produced in Hensen’s node and primitive streak and generate neuronal phenotypes in the epiblast. The signals controlling the anterior neuron production are still unknown but studies by Diaz and Schoenwolf (1990) and Darnell et al (1999) suggest that anterior visceral endoderm may provide such signals.
Left-right axis Institute of Life Long Learning, University of Delhi
20
The early development of chick: cleavage and gastrulation The vertebrate body is distinctly divided into left and right axis, and so are its internal organs located on either of the sides. For example, heart and spleen generally lie on the left side of the body while the liver occupies the right side. There are two major proteins that regulate the left and right axis formation: i)
Nodal (a paracrine factor)
ii)
Pitx2 (a transcription factor)
The mechanisms by which these two genes i.e. nodal and pitx2 regulate the left-right axis formation varies in different groups of vertebrates. Some of the common genes expressed during the lefty-right axis formation are (Figure 7): Gene expression on the right side of the primitive streak: a)
Sonic hedge-hog: the expression of this gene occurs on both the sides of the
primitive streak. However, once the primitive streak reaches its maximum length, transcription of its mRNA stops on the right side of the embryo. This is due to presence of a protein activin and its receptor, which actually downregulate the expression of shh gene. But at the same time activin signaling activates the expression of fgf-8. FGF-8 prevents the expression of the caronte gene. In the absence of caronte gene, the BMPs are able to block the expression of two genes i.e. nodal and lefty-2. b)
Snail gene (cSnR): The suppression of the nodal and lefty-2 genes activates
the snail gene, expressed exclusively on the right side of the embryonic organs. Gene expression on left side of the primitive streak: On the left side of the body, the lefty-1 gene is expressed and it blocks the expression of fgf-8, while shh activates the caronte gene. The protein Caronte, is a paracrine factor that inhibits BMPs so that nodal and lefty-2 genes remain activated. Also, due to downregulation of BMPs, the lefty-1 is also expressed properly on the ventral midline structures. Nodal and Lefty-2 activate pitx2 but suppress snail. The protein, Lefty-1 in the ventral midline prevents the Caronte signal from passing to the right side of the embryo. Pitx2 plays an important role in creating asymmetry in the embryo as has been seen in Xenopus. If we experimentally induce the expression of nodal or pitx2 on the right side of the embryo, the symmetry of the embryo is reversed or randomized on the left or right side. If any of the steps of the left-right axis formation are blocked or interfered, the symmetry is randomized. Institute of Life Long Learning, University of Delhi
21
The early development of chick: cleavage and gastrulation
Figure 7: Different genes involved in the left-right axis formation in chick Source: Author VALUE ADDITION: Things to know Heading: Cross-section of the chick embryo at primitive streak stage Body Text:
Institute of Life Long Learning, University of Delhi
22
The early development of chick: cleavage and gastrulation
Source: http://www.mun.ca/biology/desmid/brian/BIOL3530/DB_03/fig3_17.jpg
Some results of the experiments during chick gastrulation:
If follistatin, a blocker of activin is added, the asymmetry conferred by shh disappears and the position of the heart tube is randomized.
If beads soaked in activin are placed on the left side of the Hensen’s node, the Institute of Life Long Learning, University of Delhi
23
The early development of chick: cleavage and gastrulation expression of the sonic hedgehog gene is suppressed. This blocks the expression of the nodal. Under such circumstances, the heart may be present on either left or the right side.
If certain cell are grafted on the right side of the Hensen’s node, which secrete sonic hedgehog, the above condition results.
In all these cases, nodal is induced symmetrically in the lateral plate of mesoderm and the heart has equal chances of being present on either of the sides. Once the three germ layers have occupied their respective positions in the embryo, the organogenesis can begin.
Although, several genes or factors that play role during
gastrulation and in the axes formation have been identified and their role ascertained, there still lie several lacunae in the in-depth understanding of the exact mechanisms of events occurring during avian gastrulation.
Institute of Life Long Learning, University of Delhi
24
The early development of chick: cleavage and gastrulation
SUMMARY 1. The chick egg is filled with huge amount of yolk and little cytoplasm at the point of sperm entry. 2. The cleavage divisions start at the blastodisc and after subsequent divisions form a single layer sheet of cells called blastoderm 3. The cleavage divisions form two distinct regions in the embryo i.e. area opaca and area pellucida. The region between them is known as marginal zone. 4. The formation of the primitive streak and gastrulation starts at the posterior marginal zone. 5. The primitive streak is formed from the deep cells of the posterior marginal zone and anterior epiblast cells. 6. The primitive streak moves anteriorly and simultaneously Hensen’s node is also formed 7. The cells that migrate through the Hensen’s node into the blastocoel form the chordamesoderm (notochord). These cells extend upto the presumptive midbrain where they meet the prechordal plate. 8. The prechordal induces the formation of the forebrain; chordamesoderm forms the midbrain, hindbrain and spinal cord. 9. Cells that enter the blastocoel from the lateral side of the primitive groove form the endoderm and in the process they displace the hypoblast. 10. The mesodermal cells follow the endodermal cells into the blastocoel. 11. The ectoderm, present on the surface, surround the entire yolk by epiboly 12. The dorsal-ventral axis in birds is dependent on the pH difference on two sides of the embryo while gravity determines the anterior-posterior axis. 13. The left-right axis is dependent on the expression of the nodal gene on the left-side of the embryo. Nodal gene activates the pitx2 gene which leads to formation of the structures on the left side of the body.
Institute of Life Long Learning, University of Delhi
25
The early development of chick: cleavage and gastrulation
EXERCISE/PRACTICE Q.1. Give contribution of the following scientists: i)
Spratt and Haas
ii) Diaz and Schoenwolf Q.2. Define i) Hansen’s node iii) Primitive streak
ii) Primitive groove iv) Koller’s sickle
Q.3. Fill in the blanks: i)
The formation of ______________________________ in the marginal zone prevents the initiation of formation of primitive streak.
ii) The gene whose expression is exclusively upregulated on the right side of the embryo is _______________________________. iii) FGF-8 prevents the expression of ___________________ gene. iv)
_____________________________ blocks the action of activin.
v)
The central nervous system in the chick is formed by the ________________________________________________.
vi) _______________________ in chick is known as the equivalent of amphibian dorsal blastopore lip. vii) The ovum spins a t a rate of ________________________ while moving through the hen’s reproductive tract. viii)
The membrane potential across the epiblast cell layers is ______________________________________________.
ix) The cells of the area opaca produce cytoplasmic processes known as ______________________ into the vitelline envelope. x)
The hypoblast cells contribute to the formation of ________________________________________________.
Q.4. Draw a comparison between avian Hensen’s node and amphibian dorsal lip of the blastopore. Q.5. List the genes involved in the development of structures on the left and right side of the chick embryo.
Institute of Life Long Learning, University of Delhi
26
The early development of chick: cleavage and gastrulation
Q.6. Label the parts in the following figures: (a)
(b)
Institute of Life Long Learning, University of Delhi
27
The early development of chick: cleavage and gastrulation
Institute of Life Long Learning, University of Delhi
28
The early development of chick: cleavage and gastrulation
Q,6. Write short notes on the following: i)
PMZ
ii) Polyinvagination islands iii) Epiboly of the ectoderm iv) Formation of the Hensen’s node v) Role of pH in dorsal-ventral axis formation vi) Regression of primitive streak Q.7. Discuss the influence of gravity on the anterior-posterior axis formation. Q.8. True/False i.
Cleavage in avian eggs is holoblastic
ii. Nodal and Pitx2 are involved in left-right axis formation in chick iii. Blastoderm is a 5-7 layer thick layer of cells iv. The space between yolk and blastoderm is called subgerminal cavity v. E-cadherin helps in cell signaling in chick embryo vi. Marginal zone lies between area opaca and area pellucida vii. Scatter factor helps in delamination of cells from primitive streak viii.Koller’s sickle lies on the anterior side of the chick embryo ix. Blastocoel is lined by epiblast and hypoblast x. The secondary hypoblast includes cells from epiblast and Koller’s sickle
Institute of Life Long Learning, University of Delhi
29
The early development of chick: cleavage and gastrulation
GLOSSARY Area opaca: The cells at the peripheral ring of the blastoderm are not shed and form area opaca. Area pellucida: The deep cells of the blastoderm die and are shed, giving rise to single layer of cells called area pellucida. These cells form the actual embryo Blastocoel: The cavity between epiblast layer and hypoblast layer Blastoderm: A single layer of cells formed after cleavage divisions in chick at the animal pole. Blastodisc: The name given to small amount of cytoplasm in the avian eggs. Epiblast: The cells of the area pellucida are known as epiblast cells Filopodia: The cytoplasmic processes of the precursor ectodermal cells at the outer edge of area opaca, which maintains contact with vitelline envelope. They act as the locomotor apparatus and help pulling other ectodermal cells around the yolk during epiboly. Germinal crescent: The specific region at the anterior portion where the hypoblast cells get accumulated and form the precursors of the germ cels Hensen’s node/primitive knot: Thickening of the cells at the anterior portion of the primitive streak Koller’s sickle: Local thickening of the cells at the posterior margin of the blastoderm Marginal zone/marginal belt: The thin layer of cells between area pellucida and area opaca. Polyinvagination islands/ Primary hypoblast: The cells delaminating from the epiblast and exist in the groups of 5-20 each Posterior marginal zone (PMZ): The cells that initiate gastrulation and start the formation of the primitive streak, thereby inhibiting other cells of the marginal zone to initiate other primitive streaks. Primitive groove: The groove formed at the centre of the primitive streak due to ingression of cells Primitive pit: A funnel shaped depression at the centre of the Hensen’s node Primitive streak: Thickening of the cells at the middle line of the blastoderm in anterior-posterior direction, starting from the posterior side near Koller’s sickle Scatter factor: A protein (190-kDa) secreted by the cells themselves as they enter the primitive streak and is required for their delamination from it Subgerminal cavity: The space between yolk and blastoderm
Institute of Life Long Learning, University of Delhi
30
The early development of chick: cleavage and gastrulation Telolecithal eggs: The eggs which contain large amount of yolk and the small amount of cytoplasm lies at one end.
REFERENCES 1.
Developmental Biology: S.F. Gilbert 7th Ed.
SUGGESTED READINGS 1.
An introduction to embryology: B.I. Balinsky and B.C. Fabian
USEFUL WEB LINKS 1. http://www.ncbi.nlm.nih.gov/books/ 2. https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Ea rly_Embryology_of_the_Chick_5 3. http://www.devbio.biology.gatech.edu/?page_id=52
Institute of Life Long Learning, University of Delhi
31
