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Overview of chick development

Chicks have been a favorite developmental system since Aristotle, primarily because they are easily obtained and much of later development is very similar to humans. (Gilbert, 6th edition, figure1.2)

Reptile and bird eggs are the culmination of an evolutionary trend towards producing fewer eggs with greater maternal investment in the form of yolk. The entire "yellow" of a chicken "egg" is actually a single cell, the oocyte. Yolk proteins are synthesized in the liver and loaded into the growing oocyte. Technically, it is telolecithal, which means that the yolk is concentrated at one end. In fact, it takes up the majority of the cell and the non-yolky cytoplasm is pushed to a small spot at the top.

The vast amount of yolk has several important consequences for development.
First, it puts physical constraints on the division and movement of cells.
Second, it provides a food source that is essentially "outside" the embryo, so avian embryos can do something that many other embryos can't; they can grow. The egg contains 7.2 g of protein, 6 g of fat and 40 ml of water.

The eggs of reptiles and birds are also laid on land. Unlike amphibians, they aren't dependent on water to lay eggs. This allows greater parental care of the eggs while the embryos are developing, but it requires a mechanism to prevent them from drying out. The embryo develops within a fluid-filled sac known as the amnion, and the membranes and shell of the egg make it resistant to desiccation.

Obviously, sperm would have a hard time penetrating a chicken egg after it was laid, so fertilization has to be internal. However, the oocyte must be fertilized 15 - 40 minutes after ovulation. Males deposit a packet of sperm in the cloaca of the female. This can be stored for several days, and sperm are released when an egg is ovulated and travel up the single oviduct. Because the structure is so large, only one functional ovary develops.

The sperm fertilizes the oocyte, which then completes meiosis II. The oocyte is surrounded by a noncellular vitelline membrane that helps keep the cell from breaking. A second layer is added in the oviduct. As the egg travels down the oviduct, several types of albumin is added. The runny "white" of the egg provides a source of water and protein and contains antibacterial proteins. Thicker albumin cords keep the yolk in the middle. This is surrounded by the shell membranes and the calcium carbonate shell.

The non-yolky cytoplasm that will give rise to the embryo is a flat disc on one end. The initial cleavage planes are meridional, starting at the top (known as the Animal pole) and dropping through the cytoplasm until it hits the lipid-rich yolk. The cleavage furrow can't progress into the yolk, so it stops, leaving the cell open to the yolk on one side. This type of incomplete cleavage is known as meriblastic. Eventually, equatorial cleavage divisions wall the cells off from the yolk and create a disc of cells, the blastoderm. (Gilbert 11.8)

The cells of the blastodisc form an epithelium, a flat sheet of cells connected to each other by tight junctions. This forms a barrier. The blastodisc cells secrete fluid between themselves and the yolk. This creates a cavity, the subgerminal space. The center of the blastodisc, over the cavity, is only one or a few cells thick. This region is known as the area pellucida because it is translucent. Around the periphery, the blastodisc is thicker and touches the yolk, which makes it darker. This region is known as the area opaca. At the time of laying, the embryo is a blastodisc containing approximately 60,000 cells. (Gilbert 11.9)

The selective transport properties of the epithelial cells of the blastodisc create a charge and pH difference between the subgerminal space and the surrounding albumin. (Inside pH 6.5, outside pH 9.5) These differences polarize the cells and establish the Dorsal-Ventral axis of the embryo.

The Anterior-Posterior axis appears to be formed by the action of gravity on the blastodisc as it travels down the oviduct. More cells pile up in the marginal zone on one side which defines the future posterior. (Gilbert 11.3)

The blastodisc splits in two (delaminates). The Posterior Marginal Zone cells (also known as Koller's sickle) migrate anteriorly above the yolk to form a second layer, the hypoblast. They are joined by cells dropping down the upper layer, now called the epiblast. The space between the epiblast and hypoblast is analogous to the blastocoel. The hypoblast does not contribute cells to the embryo proper, but does contribute to some of the extraembryonic membranes. In addition, it helps to initiate gastrulation. (Gilbert 11.9)

The chick embryo is basically a flat disc, which makes it a challenge to create a three-layered tube-within-a-tube structure of the basic vertebrate body plan.

As the Posterior Marginal Zone cells migrate anteriorly in the hypoblast, the overlying epiblast cells converge towards the mid-line above them. The thickened area created is known as the primitive streak. (
Gilbert 11.10) Initially, the primitive streak is a wedge-shaped region in the posterior of the epiblast. As more cells converge, the streak elongates and becomes two raised ridges with a groove in between.

Epiblast cells move towards the groove as an epithelium, but when they reach it, they loose cell attachments and ingress through the streak into the blastocoel. The first cells that ingress through the streak drop all of the way down and displace the hypoblast cells along the midline to form the embryonic endoderm. Later cells move into the space between the hypoblast and epiblast and become the mesoderm.(Gilbert 11.11)

When the primitive streak reaches its maximum length, the cells of the most anterior region of the streak appear morphologically distinct. This region is known as Hensen's node. The cells that ingress through it form the notochord and foregut endoderm. Hensen's node appears to be analogous to the dorsal lip of the blastopore in amphibians. (Gilbert 11.12 & 11.16)

The primitive streak regresses towards the posterior of the embryo. As it does so, it deposits a line of notochord precursors along the dorsal midline of the embryo. Two thick bands of mesodermal precursors that form the somites are left on either side of the notochord by cells that ingress on either side of Hensen's node. Cells that migrate through the streak father from the node become heart, kidney and gonads, lateral plate mesoderm and finally the blood and extraembryonic mesoderm.

In other words, the D-V polarity of the mesoderm is related from the position in the streak and the distance from the node. Prior to migration and ingression, the cells of the blastoderm appear to be totipotent. (Gilbert 11.14)

As the streak regresses, the ectoderm cells along the midline thicken to form the neural plate. The neural plate folds into a tube and the ectodermal cells slide over it. Gastrulation proceeds in a AÆP fashion; structures are forming in the anterior region while cells are still ingressing through the primitive streak in the posterior. (Gilbert 12.3)

A set of chick embryos sectioned through equivalent regions shows the sequential bending and folding of the neural tube. You can see a similar series if you section the same embryo, with a well developed closing tube in the anterior and earlier stages towards the posterior. (Gilbert 12.5)


Morphogenesis of the embryo
Head-folds

Reptiles and birds lay eggs on land, not in water like amphibians. This is possible because the embryos is surrounded by a fluid-filled sac known as the amnion that prevents desiccation. The blood supply of the embryo is connected to the chorion which lines the inside surface of the egg and serves as the site for gas exchange. Since the embryo is walled off from the outside world, it can't dispose of the wastes formed from nitrogen metabolism. To prevent the buildup of urea, the embryo converts it to uric acid and stores it in a sac called the allantois. Eventually, it also helps to carry blood to the chorion. The yolk sac grows over the yolk and transports nutrients to the embryo. Unlike sea urchin and frog embryos, the chick embryo is able to increase in mass many fold because it has an outside source of food. (Gilbert 2.22)

After gastrulation and the establishment of the basic body plan, the major organ systems are formed. Some organs, such as the brain, are formed exclusively from descendants of one tissue type. However, organogenesis typically involves interactions between multiple groups of cells, often from more than one germ layer, brought together by the movements of gastrulation. The sense organs are formed from thickened regions of the ectoderm, known as placodes, influenced by their proximity to the brain.