Bird early development

Avian embryology:

Chicken development can be used to model the development of birds and repltiles. What is commonly reffered as the egg of  a chicken is, in the reality, the true egg, plus a variety of membranes that protect the egg. The yellow portion of the chicken egg is the single cell produced in the chicken ovary. This egg is released into the oviduct , where fertilization may occur, following fertilization and fluids collect around the egg. A vitelline membrane covers the surface of the true egg. The “white” consists of water and a protein called albumen. This watery environment protects the egg from mechanical damage and drying. Albumen is a source of nutrients(in addition of the yolk of the egg)and is eventually consumed during development .two denser strands of albumen (called chalazas)attach to the inside of the shell and to the egg, and suspend the egg in the Center of watery albumen. The shell is made of calcium carbonate impregnated with protein. Thousands of tiny pores (40 to 50micro meter in diameter)in  the shell permit gas exchange  between the embryo and the outside. On the inside of the shell are two shell membranes.an air pocket forms between these membranes at the rounded end of the shell. The air pocket enlarges during development, as air moves through pores in the shell to replace water loss.as hatching approaches, the chick penetrates the air pocket with its beak, the lungs inflate , and the chick begins to breath from the air sac ,while still exchanging gases across vascular extra embryonic membranes.

 

Early cleavages and Gastrulation:

Cleavage of the chicken egg is meroblastic .a small disk of approximately sixty thousand cells at the animal end of the egg develops and is the blastoderm. The blastoderm is raised off the yolk, leaving a fluid filled space analogous to the blastocoel of the amphibian blastula.

Proliferation and movement of blastoderm cell sort the cells into two layers:

• Epiblast

• Hypoblast

The epiblast is the outer layer of cells, and the hypoblast is the inner layer. The movement of blastoderm cells is the beginning of gastrulation. The female reproductive tract release the egg at about this time.

A medial linear invagination, called the primitive streak, gradually extends anteriorly. A depression, called Henson’s node. Form at the anterior margin of primitive streak and marks the beginning if an invert migration of epiblast cells. Comparable to involution of the amphibian’s gastrula .the primitive streak is, therefore, analogous to the dorsal lip of blastopores. This migration occurs during a dramatic posterior movement of Henson’s node. Migrating cells form mesoderm, what is left of the epiblast on the surface of embryo is the ectoderm, and they hypoblast and the hypoblast form the endodermal lining of the gut tract. The three germ layers are now arranged above the surface of yolk.

Following gastrulation, notochordal cells separate from the overlying neural ectoderm, and form neural tubes.in Addition mesoderm, which originally formed as solid blocks of cell originate into smites and split to form the coelom.

The embryo lifts off the yolk when the margins of the embryo grow downward and meet below the embryo. A correction between the embryo and the yolk is retained and is called the yolk stalk. Blood vessel develops in the yolk stalk and carry nutrient from the yolk to the embryo.

 

The Development of Extra embryonic Membranes:

Extra embryonic membranes of amniotes include the yolk sac, the amnion, the chorion, and the allantois. Reptiles and birds have large quantity of yolk that becomes enclosed by a yolk sac. The yolk sac develops from a proliferation of the endoderm and mesoderm around the yolk. The yolk sac is highly vascular and distributes nutrient to the developing embryo.

The neural tube stages, the ectoderm and mesoderm on both side of embryo lift off the yolk and grow dorsally over the embryo. As these membranes meet dorsally, they fuse and form an inner amnion and outer chorion. The amnion encloses the embryo in a fluid filled sac. This amniotic cavity protect against shock and dying. The chorion is nearer the shell becomes highly vascular, and aid in gas exchange.

  Development of Extra-embryonic membranes:

Development in a closed environment presents a problem of waste disposal. Accumulation of nitrogenous waste products in the embryo or unconfined in the shell, would be lethal for the embryo. The immediate breakdown product of proteins is highly toxic ammonia. The ammonia is converted to less toxic form, uric acid which is excreted and stored in the allantois, a ventral outgrowth of the gut tract. Uric acid is a semisolid, and thus, little water is wasted. The allantois gradually enlarges during development to occupy the region between the amnion and the chorion. In addition, the allantois becomes highly vascular and functions with the chorion in gas exchange.

The fate of mesoderm:

Following gastrulation in birds, reptiles and mammals all three primary germ layers have formed. Of three layers, the fate of mesoderm is the most complex.

                                     

Mesoderm forms all of the supportive tissues of vertebrates, including connective tissues (bone, cartilage and blood) and muscles. The supportive tissues are frequently associative with derivatives of other primary germ layers. For example the inner lining of the gut is endodermal, but mesodermally derived structures, such as smooth muscle, blood and blood vessels make up the bulk of that system.

Another important development at this stage is the way the cells to change to allow the production of the different types of cellsthat make up the tissues. By the time the egg is laid  the embryo consists of many cells differentiating into the various tissues, organs and body systems.