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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|
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 is internal. It takes place in the front part of the oviduct, before the secretion of albumen around the ovum.
The egg is laid 24 hours after fertilization, but the real egg is . the mature ovum. Further development of the zygote takes place, when the egg is incubated by the female. Incubation must continue steadily for 21 days. In incubating ' egg artificially, the incubations are regulated at temperature between 36°C to 38°C.
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.
Cleavage results in the formation of rounded closely packed mass of cells. These cells are called blastomeres.
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|
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 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.
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.
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.
Labels: Development of Chick
There is general pattern of development in all vertebrates. The process can be divided into four stages: (1) Cleavage (2) Gastrulation (3) Organogenesis (4) Growth.
After a sperm penetrates an egg, the egg quickly surrounds itself with an enveloping coat, the fertilization membrane, and a series of cell division called cleavage begins.
A cycle of repeated mitotic division continues until a spherical mass of cells known as morula is formed. The cells do not grow between the cell divisions. As cleavage or division continues, cells begin to move apart, so that spaces appear among the cells in the centre of the mass. Cells keep pulling away from the central area, forming a fluid filled cavity known a blastocoel. This hollow-sphere embryo which develops at the end of cleavage is called a blastula. The cells of the morula and blastula are called blastomeres.
Cell division continues but the whole embryo remains in same size at this early stage, the cells are becoming smaller as cleavage progresses.
Then the embryo enters a phase called gastrulation. During this phase there is rearrangement of cells. The essential feature is the formation of three layers of cells, with each layer capable of developing into special tissue.
Once gastrulation has occurred, the whole body has an outer coating the ectoderm (ecto; outside), and an inner lining the endoderm (endo; inside). Between the ectoderm and endoderm, new cells, proliferate, filling the part between the two original layers and eventually building the bulk of the embryo. This is the mesoderm. Ectoderm, endoderm and mesoderm are called germinal layers.
After formation of the three germinal layers, different cells of these layers start differentiating to form different organ rudiments. This phase constitute the beginning of organogenesis. The three germinal layers give rise to the structures as shown in the table below.
It is a long phase. All the basic organ rudiments increase in size and also undergo many physiological changes. As a result fully functional organism is ready to start life on its own, then the embryo either hatches or is born.
Example: Hatching in birds and reptiles, birth in eutherian mammals (a mammal whose young develop within the womb surrounded by a placenta. Subclass: Eutheria).
It takes place before gastrulation. The two main types of cleavage is spiral cleavage and radial cleavage
Spiral cleavage is holoblastic when the egg contains little or no yolk (isolecithal egg), all the eggs divide at each cleavage.
|Types of Cleavages|
It is characterized by the axes of the cleavage spindles being oblique to the polar axis of the egg or blastula, rather than at right angles or parallel to it. Thus, the blastomeres become spirally arranged.' This type of cleavage is of protostomes i.e. seen in platyhelminthes (flatworms), nemerteans (Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms"), annelids, mollusks and arthropods. The blastomere even if separated as early as the first or second cleavage division, will only develop into that part of the animal that they would have formed if left in the entire egg. Thus, their fate is determined in some way very early in development so the spiral cleavage is called determinate cleavage.
It is also holoblastic. The plane of cleavage is always either at right angle or parallel to the polar axis of the blastula. This type of cleavage is indeterminate, the fate of the various parts of the developing embryo being essentially plastic (moldable) until after gastrulation has occurred. This form of cleavage is seen in deuterostomes (Deuterostomes (taxonomic term: Deuterostomia ; from the Greek: "second mouth") are a superphylum of animals) consisting of echinoderms, hemichordates and chordates.
In the case of determinate blastulae, the process is the moving into place the preordained parts while in the undetermined embryos it is the actual laying of the germ layers and in the initiating of determination for later development.
In the protostomes the mouth arises either from the blastopore itself or from a new opening very close to the original blastopore. In the deuterostomes on the other hand the mouth is completely new structure and the blastopore forms the anus.
|Various types of gastrulation|
In the spirally cleaved blastula, on completion of gastrulation the product of division of cells are the sites of invagination (Invagination is the enfolding of one part within another part of a structure, a folding that creates a pocket), grow into blastocoel to lay down the third germ layer, the mesoderm The various types of gastrulation is shown in figure . After gastrulation mesoderm formation rapidly takes place.
Development may be direct or through metamorphosis. Metamorphosis may be complete or incomplete in arthropods or through many larval stages in echinoderms.
The fusion of egg and sperm constitutes the process of fertilization and triggers an orderly increase in complexity called development. During development, the single celled zygote divides by mitosis to produce millions of cells, each in a prescribed place and with a defined shape and function. Initially the developing organism or embryo grows inside a protective space which may be a hard-shelled egg, a jellylike globe or a womb. But development continues long after hatching or birth, as the new individual matures to adulthood and reproduces. As we explore the marvels of animal development, three themes will recur (to come up at intervals).
(1) Development is an ordered sequence of irreversible steps.
(2) Each development step depends on previous events.
(3) Each cell expresses different genes at appropriate time.