Development of Chick

Embryology of Gallus domesticus has been extensively studied because the eggs are larger, easily available at all times of year and incubated easily. It provides basis for understanding the early differentiation of the organ system and the fundamental process of body formation, which is common to all vertebrates.

Avian (Bird) Egg

Gametes

The gametes consist of sperm and egg. The sperm of fowl is very long. The egg as laid by hen, consists of ovum released from the ovary and the albumens, two shell membranes (outer and inner shell membrane) and shell. The ovum consists entirely of yolk (eggs with large amount of yolk is called microlecithal egg). The cytoplasm lies as a small disc, the blastodisc, at the animal pole. 

Fertilization

Fertilization is internal. It takes place in the front part of the oviduct, before the secretion of albumen around the ovum.

Incubation

After fertilization, the egg is laid within 24 hours, but the true egg is the mature ovum. Subsequent development of the zygote occurs during the female's incubation period, which must be maintained consistently for 21 days. When artificially incubating the egg, the temperature is carefully regulated between 36°C to 38°C.

Cleavage

Immediately after fertilization, the egg undergoes a series of mitotic divisions called cleavage. The cleavage furrows are confined to the blastodisc, not extending at all into the yolk. This type of cleavage is called discoidal cleavage.

Cleavage starts soon after fertilization and continues as the egg passes down the oviduct. This often takes about 12 to 16 hours, so that by the time the egg is laid, the embryo has reached the blastula stage.

The first cleavage furrow is vertical. The second cleavage furrow is similar to first, but is at right angle to it. The third cleavage runs horizontally parallel to the surface and thus, cuts underneath the cytoplasm and separates it from the yolk. Further cleavage furrows are also vertical, occur in an irregular manner, and there is increase in the number of cells. The furrows do not affect the entire blastodisc.

Chick cleavage stages

With the result a central area of cells surrounded by a ring of unsegmented cytoplasm the marginal zone (periblast) is produced. Later cleavages occur in the marginal zone as well as in the central zone.

Morula

Cleavage results in the formation of rounded closely packed mass of cells. These cells are called blastomeres..

Blastocoel

Blastula

The morula stage is short lived. Horizontal cleavage change blastodisc into a regular one or more layers of cells, called blastoderm. In the centre of the blastoderm, the blastomeres. are smaller and completely defined while those at the periphery, are flattened and larger. A fluid filled space called blastocoel (sub-germinal cavity) appears beneath the central cells of the blastoderm, separating them from underlying yolk. Further horizontal cleavages make the blastoderm several layers thick.

The blastoderm also grows peripherally, gradually spreading over the yolk. The marginal cells bf the blastoderm remain in contact with the yolk called zone of junction to engulf and digest the yolk. The blastoderm now shows two distinct regions, a large central transparent area pellucida and a narrow , peripheral opaque area opaca. The area pellucida appears transparent as it over lies the blastocoel (sub-germinal cavity) and the area opaca looks dark because it over lies the yolk . The two areas can be observed if the egg is seen from the above by transmitted light. The embryo at this stage is called blastula.

Chick, Presumptive Area

Presumptive Areas

The presumptive areas of blastula lie within the area pellucida. Beginning from the future posterior end, these area are; a small disc of presumptive endoderm, a broad band of presumptive lateral plate mesoderm, two narrow lateral bands of presumptive somatic mesoderm, a narrow band of presumptive notochord, a large area of presumptive neural plate and very large presumptive ectoderm. They latter surround the other areas on all sides, except the posterior. Outside the embryonic ectoderm is extra-embryonic ectoderm.

Gastrulation

Gastrulation in the chick involves four important processes,

(1) Formation of endoderm,

(2) Formation of mesoderm,

(3) Formation of notochord,

(4) Formation of neural tube.

These processes, though overlap to a certain extent, occur in the order named. They involve two events (i) cell movement and (ii) cell division.

The blastoderm splits into two layers the epiblast and hypoblast. The epiblast is an upper layer of cells and is mainly presumptive ectoderm and mesoderm. Hypoblast is the lower layer of cells and is mainly presumptive endoderm. At this stage, the central cells of blastoderm can be separated from the yolk.

Formation of Endoderm

The presumptive endoderm cells migrate into the blastocoel (sub-germinal cavity). Here they spread forwards and laterally, so that the blastocoel (sub-germinal cavity) soon acquires a complete floor of embryonic endoderm. Later, the embryonic endoderm grows peripherally and meets the yolky extra embryonic endoderm.

Primitive Streak

As the presumptive endoderm moves in from the surface, the presumptive lateral plate mesoderm lying just ahead of it quickly moves backward and towards the median line to take place. This results in heaping up of the presumptive lateral plate mesodermal cells in the middle line, forming a slight ridge the primitive streak in the posterior region of area pellucida. At anterior end of the primitive streak, appears a small depression, the primitive pit, and in front of it develops an elevation the Hensen's node or primitive knot. A narrow depression, the primitive groove appears along the middle of the primitive streak.

T.S of embryo

The primitive streak is thus, the result of convergence of mesodermal and notochordial cells towards the middle line and the area pellucida becomes pear shaped.

Formation of Mesoderm 

The presumptive lateral plate mesoderm   streams into the primitive groove and sinks down into the blastocoel (sub-germinal cavity). Here, it fans out laterally forming a sheet (third layer) on either side between the endoderm and the upper layer of cells. The presumptive somatic   mesoderm follows presumptive lateral plate mesoderm.

Gastrulation In The Chick

However, it remains as two bands close to the middle line, seprated by the notochord.

Formation of Notochord

The presumptive notochord cells roll over the edge of the Hensen's node and through the primitive pit, sink into the blastocoel (sub-germinal cavity). Here they extend forward as a strip in the middle line beneath the surface cells.

Formation of Neural Tuba

With the sinking in, of the presumptive mesoderm and notochord, and retreat of the Hensen's node, the two wings of the presumptive neural plate move towards one another and meet in the medial line to form the neural plate in front of the Hensen's node. The lateral margins of the neural plate rise up as neural folds which unite to form the neural tube. The neural tube encloses a cavity, the neurocoel, and opens out by neuropore. In 24 hours chick embryo, the folding of the neural plate is clearly visible. The embryo is termed as neurula. With the formation of neural tube, there is formation of central nervous system. This entire process is neurulation.

T-S Of Embryo Is Showing Notochord

Differentiation of Mesoderm

The mesoderm occurs first as a pair of solid sheets. Each sheet consists of a thicker medial somatic mesoderm and a thinner outer lateral plate mesoderm. The somatic mesoderm shortly after the formation of the head fold, segments transversely to form paired blocks, the somites lying on the sides of the notochord. Somites are seen in 25-26 hours embryo. The lateral plate mesoderm splits up into two layers upper somatic mesoderm and lower splanchnic mesoderm, with a space between them. The cavity formed between somatic and splanchnic mesoderm is coelom. The splanchnic mesoderm of the area opaca and of the outer part of the area pellucida develops blood capillaries. All the development up to this takes place on the first day of incubation.

Folding Off The Chick Embryo

Later Development of Chick

Further development of chick involves three main processes.

(a) Folding off of the embryo.

(b) Formation of organs.

(c) Formation of embryonic membrane.

 

(a) “Folding off' of the embryo: It commences at the anterior end of the area pellucida, by the formation of head fold having an endodermal pocket the so called “foregut”. Lateral fold next appear, under the sides of the embryo by a tail fold, which includes the hind gut.

(b) Organogenesis: It is beyond the scope of this book to discuss organogenesis in chick.

(c) Embryonic membrane: The blastoderm not only gives rise to the paired blocks, the somites lying on the sides of the embryo, but certain structures that lie outside the embryo. These are called embryonic membranes. These include yolk sac, amnion, chorion and allantois.

Hatching

On the 20th day of incubation, the chick ruptures the inner shell membrane adjacent to the air space by means of the beak. On the same day or next, the chick repeatedly strikes the shell by a sharp horny process. The shell eventually breaks and the young-feathered chick emerges.

Cleavage In Chick Is Called Discoidal Type, Why?

Cleavage in a chick embryo is referred to as discoidal cleavage because it occurs only in the yolk-containing region of the egg. Unlike other animal embryos, the yolk of a bird egg is massive and highly concentrated with nutrients that the developing embryo needs for growth and development. Therefore, in the early stages of development, the chick embryo forms a disc-like structure on top of the yolk, known as the blastoderm.

During discoidal cleavage, the cells divide rapidly at the periphery of the blastoderm, forming a disc-shaped cluster of cells that sits on top of the yolk. The cells in the center of the disc, which are in direct contact with the yolk, do not divide because the yolk is a nutrient-rich, but inert, substance that does not support cell division. Thus, the embryo's development is limited to the peripheral cells of the blastoderm until the formation of the primitive streak when gastrulation occurs, and the cells begin to migrate into the yolk to form the germ layers.

This type of cleavage is called "discoidal" because it only affects the disc-like structure of the blastoderm that sits on top of the yolk, and not the entire egg.