The fertile hen’s egg can be used to cultivate and propagate various types of viruses. Because of the ability to alter their tropism and to adapt to a new host species, many viruses become capable of growing in chick embryo tissues wherein they frequently attain a much higher concentration than in the tissues of the natural host.
STRUCTURE OF AN EGG
The extra-embryonic membranes of the chick embryo arise from three germinal layers: the endoderm, mesoderm and ectoderm (Fig. 1). The dorsal somatopleure consists of ectoderm on one side and mesoderm on the other side while the splanchnopleure consists of mesoderm and endoderm. By a process of folding, the somatopleure gives rise to the chorion and amnion while the allantois and yolk sac membranes develop from the splanchnopleure. The amnion arises from the head and caudal regions of the embryo, the membrane being reflected back to form the chorion. the amniotic membranes grow rapidly and fuse to form the amniotic sac by the 5th day. The allantois grows out as a bud from the hind gut of the embryo and enlarges rapidly. By the 10th day the allantois becomes attached to the outer layer of the amniotic sac and the inner layer of the chorion to form the chorioallantoic sac (CAS) which separates the chorion from the amnion. The fused chorionic and allantoic membranes are referred to as the chorioallantoic membrane (CAM).
Because the CAS represents a diverticulum of the gut, it serves as the excretory receptacle for the embryo. It contains from 5 to 10 ml of fluid with dissolved solids, the solution being clear in early stages but becoming turbid after the 12th day due to the presence of urates. The CAM is the respiratory organ of the embryo and thus is richly supplied with blood vessels. The embryo is surrounded by the amniotic sac and lies bathed in about 1 ml of amniotic fluid. The amniotic fluid, which contains much of the albumin in the egg, serves as a source of protein which is ingested during swallowing movements the embryo is seen to make from the 9th day onward.
The air-sac is present in the blunt end of the egg. Underlining the shell is the fibrous egg shell membrane. In the beginning stages of development, the chick embryo can be recognized with difficulty as a small dark area attached to the yolk sac. After 4-5 days the embryo can be readily detected by candling. After the 10th day, the embryo development, rapidly increase in size and feathers appear. The respiratory tract develops between the 12th and 15th days. If the egg remains uninoculated and is maintained in a humid 38oC environment, it will hatch on the 21st day of life.
The methods described below for the inoculation of the chick embryo do not comprise a complete list but represent those that are practiced most commonly. Likewise, while there are a number of techniques for inoculation by each of the routes listed, only the one most widely used is described.
Chlamydia and rickettsia grow readily in the yolk sac (YS) membranes. Although some of the smaller viruses are inoculated by the YS route, they invade and replicate in the tissues of the embryo itself rather than in the YS tissues.
a.Candling and drilling.
Fertile eggs that have been incubated for 5 to 7 days are suitable since the YS is relatively large at this time. The eggs are candled and the boundary of the air sac penciled in. The shell over the air space, which is referred to as the shell cap, is disinfected by an application of iodine to one small area. When the iodine is dried, a hole is made through the shell over the center of the natural air space by means of a drill or egg punch.
b.Inoculation and incubation.
By means of a syringe fitted with a one and one-half to two inch 23 gauge needle, the inoculum is deposited in the YS by passing the needle through the hole in the shell cap and directing it...