Development of chick embryo- structure of egg,cleavages,fate map and primitive streak

Shri Shankaracharya Mahavidyalaya,Junwani,Bhilai
Development of check Embryo
Dr. Sonia Bajaj
(Head of Department)

DEVELOPMENT OF CHICK EMBRYO
• Introduction:
• Chick belongs to the class-Aves.
• Sexes are separate.
• Male bird is the Cock-has a pair of testes
• Female bird is the hen-has only one ovary located on the left side
• The hen is oviparous
• Fertilization is internal
Structure of Egg- Hen's egg is large. It is 3cm. in diameter and 5cm. in length. It
contains enarmous amount of yolk. Such egg is called macrolecithal egg. The egg
is oval in shape. The ovum contains a nucleus. It is covered by yolk free
cytoplasm. It is 3mm. in diameter. It is seen on the animal pole. The entire egg is
filled with yolk. This yolk has alternative layers of yellow and white layers. They
are arranged concentrically around a flask shaped structure called latebra. Below
the blastodisc the neck of latebra expands. This is called nucleus of pander. Yellow
yolk got its colour because of carotenoids White yolk layers are thin and yellow
yolk layers are thick. Yolk is a liquid. It contains 49% water and 33%
phospholipids 18% proteins, vitamins, carbohydrates are present.

The entire ovum is covered by plasma membrane. It is called plasmalemma. It is lipoprotein layer. This is
ovum is covered by egg membranes.
Primary membranes: These membranes develop between oocyte and follicle. The primary membranes
are secreted by follicle cells. It is called vitelline, membrane is come from two origins. Inner part is
produced by ovary. Outer part is from the fallopian tube.
Secondary membranes: Oviduct secretes secondary membranes. Above vitelline membrane albumen is
present. It is white in colour and it contains water and proteins. The outer layer of albumen is thin. It is
called thin albumen. The middle layer of albumen is thick. It is called thick albumen, or dense albumen.
The inner most albumen is very thick. It develops into chalazae. The chalazae are called balancers. They
keep the ovum in the centre.
Shell membranes: Above the albumen two shell membranes are present. Towards the broad end of egg, in
between the shell membranes an air space is present. This air space is formed when egg is laid cooled
from 60°C to lesser temperature.

Shell: Above the shell membranes a shell is present, it is porous in nature. It is calcareous. This porous
shell is useful for exchange of gases. In a freshly laid hen's egg shell is soft. Very soon it becomes hard.
Laying of the egg: Between 9 A.M. and 3 P.M., the egg is expelled from the cloaca of hen. At the time
of laying formation of endoderm is completed. For further development it is to be incubated.
Incubation: When the egg is laid, the development is stopped. For further development it is to be kept
at 38°C. This is done by hen by sitting over the egg. This is called incubation. Artificially eggs are
incubated in incubators. For the hatching of egg 21 days are required.
In the upper region of oviduct fertilization will takes place. One sperm will penetrate into hens egg and
fertilizes with the egg. The fertilized egg will travel through oviduct. It takes nearly 22 hours. Hence the
early development of egg will take place in oviduct.

CLEAVAGE
Cleavage is restricted to blastodisc and the yolk remains uncleaved.
Such cleavage is called meroblastic or discoidal cleavage. The
central part of blastodisc is whitish and circular. It is surrounded by a
darker marginal zone known as the periblast, which merges with the
underlying white yolk.
I.Cleavage: After five hours of fertilization the first cleavage will
appear. It is confined to the centre of blastodisc. It is meridional. lt
cannot completely divide the blastodisc. Blastomeres are not formed.
II.Cleavage: It takes place at right angles to first cleavage. Even
because of second cleavage clear blastomeres are not formed.
III.Cleavage: It is vertical and parallel to the first division. It is in
the two sides of first division. As a result of this division eight
blastomeres are formed. But they do not show boundaries.
IV.Cleavage: It takes place in such a way that eight central
blastomeres and eight peripheral blastomeres (marginal blastomeres)
will form. Only at this stage of division definite cells are formed. The
central eight cells are completely separated from yolk. After fourth
cleavage the cleavage are irregular and a blastoderm is formed.
Discoidal meroblastic
cleavage in a chick
embryo. Four stages (A-D)
viewed from the animal
pole. E: An early-cleavage
embryo viewed from the
side

Development of chick embryo- structure of egg,cleavages,fate map and primitive streak

Central and marginal cells : After fourth cleavage, the planes of further cleavage furrows
become irregular, rapid and involve both central and peripheral cytoplasm and result in the
formation of- -central cells and marginal cells.
1. The central cells: Lie in the centre of blasto disc. These cells have complete cell
boundaries but are still continuous with the underlying yolk.
2. The marginal cells: Lie on the periphery of central cells. These have incomplete cell
boundaries being connected with the marginal periblast on the sides and with the yolk on
the outer surface.
Morula and periblast formation: After 32 cell stage, the horizontal or latitudinal cleavage
furrows are laid down first in the region of central cells and later in the marginal zone. This
establishes a superficial single layer of cells completely separated from the egg cytoplasm
beneath. This layer forms the blastoderm. Its cells rest upon a layer of cells which are
continuous on their deep faces with the underlying yolk. The lower layer of incomplete
cells is called central periblast.

BLASTULA
These cells will undergo further division quickly. Hence above the segmentation cavity
mass of cells will be present, in several layers. These cells have complete boundaries. The
cells present towards the periphery are not separated from yolk. They are called marginal
cells. This region is called zone of junction.
Discoblastula:
In chick, the fully formed or mature is a discoblastula. It consists of
1. A central multicellular multilayered blastoderm free from the underlying yolk and
central periblast. It forms area pellucida.
2. Underlying blastocoel or sub germinal cavity that separates blastoderm from the
underlying central periblast and yolk.
3. Marginal periblast tissue -It is present on the periphery of central blastoderm. It is
called germ wall. It may be differentiated into-
(i) Inner zone of distinct cells, which divide rapidly and contribute cells to the periphery
of growing cellular blastoderm.
(ii) Outer peripheral zone of syncytial germ cells, which is also called zone of junction. It
lies in intimate contact with yolk.

Area Pellucida & Area opaca
The central cell mass of the blastoderm will be in four to five layers, they are lifted
from the yolk. Hence the central part of the blastoderm is free from yolk. This
region is transparent. It is called area pellicida, which is destined to become the
embryo proper. At the zone of junction the cells are in contact with that region is
opaca. That region is called area opaca .

Fate maps of discoblastula:
The blastoderm in discoblastula of chick consists of two
parts: The area pellucida and area opaca, the area opaca
(area vitellina) does not form any part of the embryo
proper. It forms non or extra embryonic ectoderm which
during later developmental stages forms extra embryonic
membranes and blood vessels (area vasculosa) to involve
in the digestion of the yolk. Hence, the fate map is
restricted to area pellucida. Moreover fate map of both
epiblast and hypoblast are to the made out separately.
The epiblast has material for ectoderm, mesoderm and
also for endoderm, the hypolast is form exclusively of
endoderm material.
• The anterior 2/3rd of area pellucida is prospective
ectoderm consisting of prospective epidermis and
extra embryonic ectoderm.
• A crescentric area is prospective neural area.
• Next to neural area is the prospective notochord
• It is followed by prechordal mesoderm.
• On the side of prechordal region is the lateral plate
mesoderm.
•

Formation of the two-layered blastoderm of the chick embryo

Primitive streak
Introduction: Hen lays egg while the embryo is preparing for gastrulation. In
Chick, the process of gastrulation is prolonged and highly modified due to the
presence of yolk. The gastrular movements responsible for the establishment of
embryonic membranes are: convergence, streaming, involution, epiboly,
elongation.
Gastrulation is separated into three distinct processes or phases which occur in
a chronological sequence. These are:
(1) Formation of endoderm,
(2) Formation of mesoderm
(3) Formation of embryonic axial structure primitive streak.

Cell movements
of the primitive
streak of the
chick embryo.
Drawings to the
right show fate
maps of the
epiblast in ‘B’
and ‘F

1. Separation of endoderm or hypoblast: Presumptive endoderm or hypoblast is
Formed during gastrulation by the delamination or large yolky cells from the
inner surface of epiblast. These form a distinct layer in the sub germinal cavity
above yolk. A number of theories have been proposed. The important ones are :
1. By the involution of marginal cells through temporary pore, present in the zone
of junction.
2. According to Jacobson (1938) hypoblast is formed by the invagination of
epiblast at the posterior margin of blastoderm.
3. According to Peter (1938) hypoblast is formed by the infiltration of cells from
the epiblast.
The individual cells detach and get arranged within the sub germinal cavity.
The layer Thus, formed is hypoblast or presumptive endoderm.
4.Spratt (1946) suggested that delamination of cells from epiblast is responsible
for the formation of hypoblast.
• A number of processes appear to be responsible in the formation of hypoblast.

Site of origin of hypoblast cells:
There are different views as regards to the site from which hypoblast cells arise. According
to Pasteles (1945) the hypoblast cells arise from all over the area pellucida of blastoderm.
But Peter believed that cells arise from future posterior side and near the margin of
blastoderm. Hypoblast gives rise to the gut lining .
2. Formation of mesoderm: The cells of embryonic epiblast consist of prospective
notochord, mesoderm and ectoderm. The cells of notochord and mesoderm migrate inward
and form a layer in between the epiblast and hypoblast. These movements are evident in the
form of primitive streak. Thus, the three embryonic layers are established .The uppermost
or the epiblast now forms the ectoderm the media layer mesoderm and the lower one
endoderm.
3. Formation of primitive streak: The prospective endoderm having migrated to the
definite position, the central area of blastoderm is left with prospective somatic mesoderm,
prospective prechordal mesoderm, neuroectoderm and the epidermal ectoderm.
Next formative movements are concerned with the segregation and carrying in of
prospective mesodermal and chordal material. These cells begin to leave the under surface
of blastoderm and converge in the middle line between epiblast and hypoblast at the
posterior end in the area pellucida, this mass movement is initiated at about 6th 7th hour.

(a) Formation of initial primitive streak: The lateral place mesoderm
begins to convergence towards the middle line from a point about a
quarter of the way from the anterior end of the area pellucida towards the
growth centre in the posterior quadrant, to initiate in the formation of a
thickening called a primitive streak. The next material to converge in the
middle line is the somatic mesoderm followed by pre chordal und
notochordal materials which are rolled in and are incorporated in the
primitive streak in the last. These cells swing backward in circular orbits.
At the same time the posterior end elongates in a forward direction. By the
eight hour the cells of the posterior radius have begun to heap up and form
a short broad longitudinal strand which is thicker than the rest of epiblast
of area pellucida .This is the initial primitive streak.
(b) Formation of definite primitive streak : By the 19th hour of incubation
there is forward streaming or stretching in the primitive streak, owing to the
addition of more and more mesodermal cells at its forward end Increased by its
active elongation the streak steadily pushes out across the area pellucida and in
so doing, it becomes very much narrow. The posterior end of the streak also
elongates and thus a much elongated about 3/5 of the entire length of area
pellucida narrow and sharply delimisted definite primitive streak is formed.

(c) Homology of primitive streak: Formation of primitive streak in the middle line
is considered homologous to the formation of lateral lips of blastopore in amphibian
gastrula. It means primitive streak of chick is homologous with the closed
blastopore. The reason for such a homology is as follows-
(i) Both primitive streak and lateral lips of blastopore develop by the migration of
presumptive chorda mesodermal and mesodermal elements.
(ii) The head process is equivalent to the notochord and axial mesoderm of
amphibian embryo.
(iii) The neurenteric canal in amphibian embryo originates by the fusion of neural
folds over the anterior part of blastopore. In chick though the neurenteric canal is
absent, a bit occur at this point, corresponding to blastopore.
(iv) In some birds, something like neurenteric canal is formed at the anterior end of
streak. It shows similarity to frog.
(v) Cells about the dorsal lip of early blastopore possess remarkable inductive
capacity and induce formation of nerve chord, etc.

24 HRS. CHICK EMBRYO
1. At 24 hrs. incubation period the chick embryo is
oval in shape.
2. The primitive streak is fully formed and the
process of gastrulation is completed .
3. The Notochord extends from the from the hensen's
node as head process into the mesoderm-free area
anteriorly.
4. The head fold and fore-gut develop in the embryo.
5. The mesoderm differentiates into somites,
intermediate and lateral plate mesoderm.
6. In the 24 hrs. chick embryo four pairs of somites
are differentiated from the mesoderm.
7. The coelom begins to develop in the lateral plate
mesoderm.
8. The blood islands appear in the area opaca.
Pericardial region and primordial of heart are
established.
9. The area opaca further modifies into the area
vasculosa and area vitellina.
10. The neurectoderm gives rise to the neural folds
and neural groove.The fusion of neural folds begins
from the mid region.

33 HRS CHICK EMBRYO
1. Lengthening of foregut and subcephalic
pocket.
2. Formation of neural tube and sinus
rhomboidalis.
3. The primary division of encephalon into
prosencephalon, mesencephalon and
rhombencephalon
4. Formation of neural crest cells on either
side of the neural tube.
5. Development of the infundibulum as a
median ventral outgrowth from the floor of
prosencephalon.
6. Formation of 13 pairs of somite's.
7. Development of heart as tubular structure
lying in the midventral region to the foregut.
8. Formation of extra embryonic blood vessels
in the area vasculosa.
9. Formation of intraembryonic blood vessels.
10. Disappearance of primitive streak.

48 HRS. CHICK EMBRYO
1. Appearance of cranial flexure and torsion.
2. Formation of eleven neuromeres,
3neuromeres in prosencephalon, 2 in
mesencephalon and 6 in rhobencephalon.
3. Formation of secondary constrictions in
the brain.
4. Completion of vitelline (extra embryonic)
circulatory system.
5. Development of intra embryonic blood
vessels.
6. Formation of two pairs of aortic arches.
7. Twisting of the heart and formation of
chambers in it.
8. Commencement of blood circulation.
9. Formation of optic cup and lens vesicle.
10. Formation of visceral cleft.
11. Development of auditory vesicle.
12. Differentiation of 28 somite's.
13. Development of extra embryonic
membranes
14. Formation of pronephros.

72 HOURS CHICK EMBRYO:
1.The cranial flexure is maximum.
2. Appendage rudiments present.
3. Optic cups with distinct, closed
retina and lenses.
4. Auditory vesicles connected to the
ectodermal aperture through ductus
endolymphaticus.
5. Head fold grows back and lies
between 10 to 18 somites.
6. Pharynx and four pharyngeal
pouches formed.
7. Somites 35 pairs.

EVENTS IN EMBRYONIC DEVELOPMENT
Before Egg Laying:
Fertilization
Division and growth of living cells
Segregation of cells into groups of special function (tissues)
Between Laying and Incubation:
No growth; stage of inactive embryonic life
During Incubation:
First day:
16 hours - first sign of resemblance to a chick embryo
18 hours - appearance of alimentary tract
20 hours - appearance of vertebral column
21 hours - beginning of nervous system
22 hours - beginning of head
24 hours - beginning of eye
Second day:
25 hours - beginning of heart
35 hours - beginning of ear
42 hours - heart beats

Third day:
60 hours - beginning of nose
62 hours - beginning of legs
64 hours - beginning of wings
Fourth day - beginning of tongue
Fifth day - formation of reproductive organs and differentiation of sex
Sixth day - beginning of beak
Eighth day - beginning of feathers
Tenth day - beginning of hardening of beak
Thirteenth day - appearance of scales and claws
Fourteenth day - embryo gets into position suitable for breaking shell
Sixteenth day - scales, claws and beak becoming firm and horny
Seventeenth day - beak turns toward air cell
Nineteenth day - yolk sac begins to enter body cavity
Twentieth day - yolk sac completely drawn into body cavity; embryo
occupies practically all the space within the egg except the air cell
Twenty-first day - hatching of chick

References-
Modern text book – R.L.Kotpal
Jantu Vigyan- S.M. Sexsena
Jantu Vigyan- Dr.H.N. Baijal

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