EARLY DEVELOPMENT IN CHICK(Upto three germinal layers)

Dr. Sakshi VermaZoology Department

1. Gametes

2. Fertilization and cleavage

3. Blastulation

4. Fate Map of Chick embryo

5. Gastrulation and formation of the three germinal

layers

6. Post gastrular development

7. Extra embryonic membranes

CONTENTS

GAMETES

Gametes

Male gamete/Spermatozoon:

Size: Very Long (about 50 µm)

Spermatozoon

Head: Pointedacrosome Short cylindrical nucleus

Long, thick, cylindricalmiddle piece

Long, slender, tapering tail

Fig. 1. Spermatozoon

Female gamete/Egg: Consists of ovum(released from the ovary) and the tertiarymembranes laid around the ovum as it passes through the oviduct.

Fig. 2. Female gamete/Egg of fowl

FERTLIZATION AND CLEAVAGE

Embryonic development starts during egg formation:Fertilization

Fertilization: Fusion of sperm and ovum occurs in infundibulum before the albumen is laid round it.

In fowl polyspermry occurs but only one sperm pronucleus can fuse with the female pronucleus.

Fig 3. Passage of ovum through female reproductive tract of fowl and egg formation.

Ovulation: Release of unfertized oocyte in the infundibulum.

Cleavage:

Partial or meroblastic ,confined to the blastodisc and not extending into the yolk.

Cleavage starts soon after fertilization and by the time the egg is laid, the embryo has reached the blastula stage.

First cleavage: Vertical and radial.

Second cleavage: Similar to the first and right angle to it.

Further cleavage furrows are also vertical but occur in an irregular.

A central area of cells surrounded by a peripheral zone of unsegmented cytoplasm is formed

Fig 4. Cleavage patterns in zygote and formation of area pellucida and area opaca.

BLASTULATION

Subgerminal cavity, which iscreated when the blastodermcells absorb water from thealbumen ("egg white") andsecrete the fluid betweenthemselves and the yolk

BLASTULATION: Discoblastula

Fig 5. V.S. Blastoderm (early stages of blastulation.)

Epiblast:Death of all but superficial layer of cells in the centre of the blastoderm convert blastoderm into epiblast.Area pellucida: Clear centre of the blastoderm which has shed its deep cells. This part of the blastodermforms most of the actual embryo.Area opaca: Peripheral ring of blastoderm cells that have not shed deep cells.Marginal Zone: Thin layer of cells in between Area pellucida and area opaca.

Fig 6. Laying down of endoderm in chick embryo

• Most of the cells of the area pellucida remain at the surface, forming an "upper

layer,” called the epiblast.

• Other area pellucida cells delaminate and migrate individually into the

subgerminal cavity to form hypoblast islands (sometimes called the primary

hypoblast), disconnected clusters of 5-20 cells each.

The sheet of migrating cells (derived from deep yolky cells) pushes the hypoblast

islands anteriorly, thereby forming the secondary hypoblast, or endoblast

Fig 7. Fate map of an avian embryoprepared by radioactive thymidinemarking: hm- presumptive headmesoderm, n- notochord.

Fate Map of discoblastula

The following structures in epiblast are shown in the fate maps prepared by the use oftritiated thymidine :

• In the centre of area pellucida lies a small

area destined to produce the notochord.

• Posterior to notochord, in the median plane

is found an elongated oval area of

presumptive endoderm which will form the

gut.

• Towards the posterior edge of area pellucida lies the extra-embryonic endodermwhich will form the lining of yolk sac.

• To the right and left of the presumptive notochord and endoderm, and posterior toextra-embryonic endoderm lie the various subdivisions of presumptive mesoderm,i.e., prechordal plate or head mesoderm, mesodermal somites, lateral platemesoderm and extra-embryonic mesoderm.

• The anterior half of epiblast is the presumptive ectoderm containing centralpresumptive neural plate area, anterior to it is the presumptive embryonicepidermis and outer to it in the form of complete ring is the extra-embryonicectoderm.

GASTRULATION

GASTRULATION

Fig 8. Migration of endodermal and mesodermal cellsthrough the primitive streak

• In the chick embryo, the process of gastrulation has already started when the eggis laid and completes well into the second day of incubation.

• Primitive streak: The cells from the lateral region of the posterior epiblast of areapellucida migrate towards the centre. As a result thickening of the cell sheet atthe central posterior end of area pellucida occurs. This thickening is termed as thePrimitive streak.

• The primitive streak elongates and narrows and cover 60-75 % of the length ofthe area pellucida. thus marks the anterior posterior axis of the embryo.

• As the cells converge to form the primitive streak, a depression forms with in thestreak termed as the primitive groove, through which the migrating cells passinto the blastocoel.

• A regional thickening of cells called the Primitive Knot or Hensen’s node occur atthe end of the streak.In the centre of the node, there is a funnel shapeddepression through which cells pass into the blastocoel.

Fig 9. Surface view of chickblastoderm showing (A-C)formation and elongation ofprimitive streak: (A) 7-4 hrs., (B) 7-8 hrs., (C) 15-16 hrs. afterfertilization and (D-F)neurulation:(D) 19-22 hrs., (E) 23-24 hrs. and(F) the four-somite stage.

• The first cells to migrate through the primitive streak are those destined tobecome the foregut, i.e. presumptive endodermal cells. These cells, migrateanteriorly into the blastocoel and eventually displace the hypoblast cells in theanterior region of the embryo.

• The next cells entering the blastocoel through the Hensen’s node are thepresumptive mesodermal cells that remain between the endoderm and theepiblast to form head mesoderm and the chorda mesoderm (notochord).

• By 22 hours of incubation, most of the presumtive endodermal cells are in theinterior of the embryo, although presumptive mesodermal cells continue toingress inward for a longer time.

• Regression of the primitive streak: The second phase of gatrulation begins theprimitive streak statrs to regree moving Hensen’s node from near the centre tothe most posterior part of the area pellucida, while the mesodermal cells are stillmigrating interiorly.

• The posterior portion of the notochord is laid while the Hensen’s node movepostreiorly.

• Finally, the node regresses to its most posterior position, eventually forming theanal region in true deuterostome fashion.

• At this time, the epiblast contain only the presumptive ectodermal cells.

• The two step gastrulation process results in a distinct antero-posterior gradientof developmental maturity i.e., the anterior end has already started formingorgans, while the posterior portions of the embryo undergo gastrulation.

• Epiboly of ectodermal precursors: The presumptive ectodermal cells showepiboly to enclose yolk while the presumptive endodermal and mesodermal cellsmove inward.

• By the time the avian gatrulation is complete, the ectoderm has surrounded theyolk,the endoderm has replaced the hypoblast, and the mesoderm haspositioned itself between two regions (ectoderm and endoderm).

• The primitive pit represents the dorsal opening of the blastopore of amphibian

gatrula.

• The primitive node corresponds to the dorsal lip of the blastopore.

• The primitive groove and folds are comparable to the opposed lateral lips of

the blastopore.

• The posterior end of primitive streak are compared with the ventral region of

the blastopore (future anal opening).

• The first cells to migrate through the primitive streak are those destined to

become foregut, the situation similar to amphibians.

SIGNIFICANCE OF THE PRIMITIVE STREAK

Fig 10. Dorsal view of entire chick embryo having 4 pairs of mesodermal somites.

Fig 11. Whole mount of chick embryo of about 33 hrs. of incubation.

Embryology of chicken 33 hours of incubation:

1 = Proamnion, 2 = Prosencephalon, 3 = Mesencephalon, 4 = Rhombencephalon, 5 = Somite,6 = Eye vesicle, 7 = Foregut, 8 = Chorda (translucent), 9 = Heart, 10 = Lateral mesoderm, 11 = Spine,12 = Sinus rhomboidalis, 13 = Primitive streak,

14 = Blood islands

Embryology in chicken 48 hrs of incubation: 1 Amnion, 2 Metencephalon, 3 Mesencephalon, 4 Optic cup + lens, 5 Prosencephalon, 6 Otic vesicle, 7 Branchial arches, 8 Atrium , 9 Ventricle, 10 Lateral fold, 11 Lateral mesoderm, 12 Vitelline arteria / vein, 13 Somite, 14 Spine, 15 Tail fold

Fig 12. Whole mount of chick embryo of about 48 hrs. of incubation.

Fig 13. Whole mount of chick embryo of about 72 hrs. of incubation.

AB

Fig 13C. Whole mount of chick embryo of about 48 hrs. of incubation.

Fig 13D. Whole mount of chick embryo of about 48 hrs. of incubation.

Fig 14A. Whole mount of chick embryo of about 96 hrs. of incubation.

Fig 14B. Whole mount of chick embryo of about 96 hrs. of incubation.

Fig 14C. Whole mount of chick embryo of about 96 hrs. of incubation.

Fig 14D. Whole mount of chick embryo of about 96 hrs. of incubation.

Fig 14E. Whole mount of chick embryo of about 96 hrs. of incubation.

Chick embryo, 9 days.

Chick embryo, 13 days.

Chick embryo, 16 days.

Chick embryo, 19 days.

Newly hatched chick on day 21.

Fig 15. Specification of the chick anterior-posterior axis by gravity. (A) Rotation in the shell gland results in (B)the lighter components of the yolk pushing up one side of the blastoderm. (C) This more elevated regionbecomes the posterior of the embryo.

Fig. 16. Model by which FGFs regulate mesoderm formation and neurulation. (A) Stage XI, where thehypoblast (green) secretes Fgf8, which induces pre-neural genes ERNI and 5ox2 (blue) in the epiblast. Thecells in this domain, however, remain uncommitted. Nodal, expressed in the posterior epiblast, cannotfunction; it is inhibited by the Cerberus protein secreted by the hypoblast. (B) At around stage 1, thehypoblast is displaced from the posterior edge by the endoblast (secondary hypoblast; gold), allowingNodal to function. Nodal plus Fgf8 induces Brachyury and Tbx6 expression to specify the mesoderm andinitiate the ingression of mesoderm cells through the primitive streak (red). (C) At stage 4, continued Fgf8expression activates Churchill in the epiblast. (D) By end of stage 4, Churchill protein induces SIP1, whichblocks Brachyury and 76x6, preventing further ingression of epiblast cells through the streak. Theremaining epiblast cells can now become sensitized to neural inducers from Hensen's node .

Left-right asymmetry in the chick embryo.

• During the formation of Hensen's node in chicksinvolves Fgf8- and Shh-expressing cells rearrangingthemselves to converge on the right hand side of thenode

Therefore, the differences in gene expression can betraced back to differences in cell migration to the rightand left sides of the embryo.

Fig. 17. Model for generating left-right asymmetry in the chick embryo.

xtra-embryonic membranes also termed as foetal membranes are the extra embryonicstructures formed from blastoderm other than embryo and do not take part in theformation of embryo proper, but are external to the developing embryo.

Extra embryonic membranes

Yolk sac(encloses the

yolk)

Amnion(encloses the

amniotic fluid)

Chorion

(functions in gas exchange)

Allantois

(functions in disposal of waste product and also

contributes to gaseous exchange )

Extra Embryonic Membranes

The extra embryonic membranes are composite structures involving two germ layers.

Amnion and Chorionare composed of:

Extra-embryonic ectoderm

Somatic layer of mesoderm

Collectively known as

SomatopleureAllantois and yolk sac

are composed of:

Extra-embryonic endoderm

Splanchnic layer of mesoderm

Collectively known as

Splanchnopleure

Fig. 18. Cross section of an early chick embryo, illustrating the relationship of the germ layers.

• During neurulation, the lateral plate mesoderm splits into an outer somatic layer lyingbeneath the ectoderm and an inner layer lying outer to the endoderm layer.

• There lies the coelomic space between the above two layers.

• The ectoderm and somatic layer of mesoderm are collectively called somatopleure,while the splanchnic layer of mesoderm and endoderm form the splanchnopleure.

• During later developmental stages, the somatopleure and splanchnopleure graduallyspread outward beyond the developing embryo.

• As the body of embryo grows, it becomes separated from the foetal membranes by theappearance of body folds-cephalic, caudal and lateral folds.

• These folds limit the body of embryo and separate the embryonic region from theextra-embryonic region.

Development of Extra-Embryonic Membranes:

• The yolk sac in the beginning is joinedby a wide yolk stalk to the midgut ofthe embryo. But later on, as the bodyfolds move toward one anotherbeneath the embryo, a floor of the gutis formed except a small region of themidgut through which it communicateswith the yolk sac.

• The yolk sac is formed of the extraembryonic splanchnopleur . It is continuous with thesplanchnopleur at the proximal end and joins the extraembryonic somatopleure at theperiphery.

• Thus, the yolk sac cavity remains in continuity with the gut cavity through yolk duct of yolksac stalk. The endodermal surface of the yolk sac is thrown into folds called the yolk sacsepta that penetrate the yolk mass. The rich blood circulation develops within thesplanchnic mesoderm layer of the yolk sac. These are the paired vitelline arteries andveins, now called the omphalomesenteric blood vessels.

Fig 19. Appearance of embryonic membranes in chick embryo

Development of Yolk sac

• The endodermal cells of the yolk sac secrete digestive enzymes whichdigest the yolk. The digested yolk is collected by left and right vitellineveins, both of which open into an unpaired ductus venosus which openinto sinus venosus of the heart.

• Thus, the yolk sac serves as a digestive and absorptive surface by whichyolk is made available to the embryo. Shortly before hatching theshrivelled up remains of the yolk sac is retracted into the abdominalcavity of the embryo, and the walls of the abdominal cavity close behindit.

Fig 20. Extra embryonic membranes in chick embryo in later stage showing development of amnion and chorion.

The origin of amnion and chorion is considered together since they develop simultaneously from theextra-embryonic somatopleure. About 30 hours of incubation, the extra-embryonic somatopleure of theblastoderm rises up in front of the embryo as a fold, the fold grows and forms an arch over the embryo.

A double somatopleuric hood, called thecaphalic amniotic fold is formed which extendsextends backwardly and its caudally extendingside limbs called lateral amniotic folds arch overthe embryo from either lateral side.

A similar fold or elevation over the tail appears,called the caudal amniotic fold. All theseamniotic folds converge and fuse over theembryo, enclosing it within two sheets ofsomatopleure from all sides except the regionof yolk stalk.

The region of union of the amniotic folds iscalled the seroamniotic connection whichopens before the hatching of embryo.

Development of Amnion and Chorion

The fusion of amniotic folds produces two sac-like cavities, enclosed by two sheets of somatopleure.

The inner somatopleuric sheet becomes the amnion, enclosing the embryo within amniotic cavity filledwith amniotic fluid. While the outer sheet of somatopleure is the chorion and the cavity lying betweenthe amnion and chorion is the chorionic cavity or extra-embryonic coelom. It is lined by mesoderm ofsomatopleure and splanchnopleure.

• Amniotic fluid serves as a wtaer cusion to protect the embryo from mechanical injury.

• Neutralizes the effects of chnages in external temperature.

• Prevents dessication of the embryo.

• Prevents the adhesion of the embryo to the amniotic wall.

Functions of Amnion

• Provide space for the growth of allantois.

• Plays an important role in gaseous exchange through the porous shell.

Functions of chorion

About the third day of incubation the floor of the endodermal hindgut begins to bulge as a bladder,called the allantois. The allantoic evagination is formed from the splanchnopleure, that is, inner layer ofendoderm and an outer layer of splanchnic mesoderm. It grows rapidly and spreads into the extra-embryonic coelom, the space between the yolk sac, the amnion and the chorion.

Fig 21. Extra embryonic membranes in chick embryo almost fully developed.

The distal part of the allantois expands and remains connected with the hindgut of the embryo bymeans of a narrow allantoic stalk. When the body folds contract, separating the embryo from the extra-embryonic parts, the allantoic stalk is enclosed together with the stalk of yolk sac, forming an umbilicalcord.

As the allantois vesicle enlarges and spreadsoutward, its distal part becomes flattened andexpands between the amnion and yolk sac onone side and the chorion on the other side. Dueto this the splanchnic mesoderm of the allantoisfuses with the inner somatic mesoderm of thechorion to form an allanto-chorion.

Development of Allantois

Excretory function

• The allantois serves to store the insoluble nitrogenous waste matter, uric acid, outside the embryo proper.

Gaseous exchange

• The vascular allanto-chorion is in contact with shell membranes and it brings about the respiration of the embryo by an exchange of oxygen and carbon dioxide through the porous shell.

Functions of Chorion

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