Chick Embryo Labs, con't.
C. Serial Cross-Sections of the 48-hour chick embryo.
N.B.: Photos 3.77-3.101 in Schoenwolf are relevant to the following descriptions.
At 48 hours the head has become bent. The sections are still cut perpendicular to the length of the body , and the mesencephalon is therefore the first part of the body to appear in the series. Farther back the forebrain and hindbrain will appear in the same section. In making your initial survey of the series, focus your attention of the embryo itself; the membranes will be considered later.
Proceeding back from the first section, identify the parts of the brain. Observe the otic vesicles. Are they still open to the exterior? Find a section through the middle of the developing eyes. The optic vesicle has invaginated, forming an optic cup. Moving the slide back and forth, study the structure of the lens vesicle, which is still connected to the outer ectoderm. The part of the brain between the optic cups is the diencephalon. Find the optic stalks. Under the head is a narrow open space which is part of the future mouth cavity, formed by the folding down of the head against the ventral surface of the neck. Ventrally the mouth cavity is bounded by the first pair of branchial or pharyngeal or visceral arches, the mandibular arches, which will become the lower jaw. Tracing back and forth you will find a place where the two mandibular arches are joined together only by a narrow strip of tissue, which is the oral plate. What is responsible for the apparent change in position of the plate since 33 hours?
The cavity on the other side of the oral plate is the pharynx. Posterior to the first arch you will see the first visceral pouch, which is an extension of the pharyngeal endoderm toward the outside (see Fig. 12-3 from Patton and Carlson). The first visceral groove is the corresponding depression in the outer ectoderm; the thin strip of tissue where the endoderm of the pouch meets the ectoderm of the groove is the closing membrane.
Proceeding posteriorly you will come to the pair of branchial arches (= hyoid arches), behind which are the second visceral pouches and grooves with closing membranes. The third and more posterior visceral arches are equivalent to the gill arches of fish and amphibians. Of course these arches never produce gills in reptiles, birds and mammals, but they invariably appear in embryos. You will later see how this potential gill apparatus does produce a number of essential structures in higher vertebrates.
The second aortic arch is the blood vessel in the hyoid arch. Find the second aortic arches opening into the dorsal aortae. Running posteriorly you will find that both the first and second arches proceed on the ventral side from a common vessel, the ventral aorta, which is very short in the chick embryo and which can be traced back to the heart. Note that the heart is still suspended below the gut by the dorsal mesocardium. The heart is partly surrounded by the body folds, which later meet on the left side to form the complete pericardial cavity. At the tip of the head, note the thickened nasal placodes.
The ventral aorta passes directly into the bulbus arteriosus. On which side of the embryonic body does the bulbus lie? Tracing back, follow the bulbus into the ventricle, which is the large chamber lying across the entire width of the body. On the side opposite the bulbus the ventricle is continuous with the atrium. Note that the endocardium is much further away from the myocardium in the ventricle than in the atrium; this difference presages the future differences in thickness of the muscular wall of the two chambers. Behind the atrium is a median chamber, which is attached below the esophagus by means of a mesocardium. This chamber is the sinus venosus. What are the two large vessels which enter the sinus from each side of the body wall? Follow the sinus back until it breaks down into the two omphalomesenteric veins.
Now return to the aortic sac and trace the two sets of aortic arches until they enter the dorsal aortae. Follow the aortae back from the point of entrance of the first aortic arches and find where the two vessels come together to form the single median aorta. Farther posterior, however, the vessel is still double. Note the paired intersegmental arteries arising from the dorsal surface of the aorta. Find the place where the two omphalomesenteric arteries spring from the aortae and spread out over the yolk sac. How far back do the aortae extend beyond this point?
Find a section where the gut is long and narrow. The ventral extension is the liver diverticulum. The section of the heart into which the liver seems to be growing is the sinus venosus, which can be identified by the fact that the large common cardinal veins enter its dorsolateral corners. Following the sinus back you will see that it is formed by the union of the two omphalomesenteric veins. At this level, find the anterior intestinal portal, the first point where the gut is still open ventrally.
In the open gut region note the large dorsal aorta under the notochord. The small blood vessels to each side are the posterior cardinal veins, the first intraembryonic veins to appear. Immediately under the posterior cardinal vein is the pronephric duct; this is now known as the mesonephric (Wolffian) duct, and the kidney tubules which may be found along the duct are the mesonephric tubules. (The mesonephros forms from the part of the nephrotome immediately behind the pronephros.) The beginnings of the mesonephros will be seen as small masses of nephrogenous tissue, derived from the nephrotome, clustering around the duct. Over a considerable length of the body, mesonephric tubules have already differentiated from part of the nephrogenous tissue. (The duct is lateral to the tubules.) Find a mesonephric tubule opening into the duct. Examine the first two or three tubules and see whether you can find one with a nephrostome, or opening into the coelom. Nephrostomes are characteristic of pronephric tubules, which do not have internal glomeruli, but the first few mesonephric tubules generally possess a coelomic aperture. Follow the mesonephros back and observe that it is a very long structure. Toward its posterior end, tubules are not yet formed. How far back does the duct reach?
Transfer your attention to the posterior end of your series, determine whether the primitive streak is still present. Has the hindgut begun to close on the ventral side of your specimen?
The formation of the amnion and chorion will now be described. In the tail region note that the somatopleure spreads out flat at each side of the body. Running forward, find the place where the extraembryonic somatopleure is thrown into folds (amniotic folds) at each side of the body . Still farther anteriorly these folds make contact with one another over the dorsal surface of the body to form the hood-like amnion. The more anterior parts are already enclosed in this membrane. The amnion eventually forms a completely closed sac enveloping the entire embryo; it becomes filled with fluid and thus provide an excellent medium for supporting the soft, delicate embryonic tissue.
Where the amniotic folds meet, the corresponding layers fuse and so cut off a layer of somatopleure external to, and separate from, the amnion; this layer is the chorion , which will be found at the level of the liver diverticulum and probably further posteriorly. The chorion continues to spread peripherally and eventually envelopes the amnion and yolk sac. How may the chorion and yolk sac be distinguished in sections?
Although the major part of the amnion is formed of folds which proceed backward from the anterior, a portion of it is contributed by folds which arise around the tail, at the beginning of the third day, and grow forward. The final closure of the amnion is then effected by the meeting of the anterior and posterior folds.
Draw sections through:
D. Whole Mount 56-hour chick embryo.
[Refer to Figures Figure 12-1 , Figure 12-2, Color Figure Figure 12-5 from Foundations of Embryology, 3rd ed. by B. M. Patton and B. Carlson (1974), Mc-Graw Hill, Inc. ] In the period between this embryo and the preceding one, most of the trunk has been laid down. The large head has turned on itself around the midbrain, the bend thus form being the cephalic flexure. As a result of this flexing process some of the surface ectoderm is folded in between the ventral surfaces of the head and neck. This infolded pocket becomes the mouth cavity, which is therefore lined with ectoderm. At the same time the body has twisted so that it lies partly on one side. This twisting starts at the head and proceeds gradually backward. How far back is the body turned in your specimen? Which side was turned toward the yolk?
The brain now has a number of subdivisions, as in the frog embryo. The forebrain is represented by the small telencephalon, in front of the eyes, and the larger diencephalon, to which the eyes are connected. The undivided mesencephalon is the portion on which the cephalic flexure centers. The hindbrain or rhombencephalon consists of a series of bulges called rhombomeres. The caudal rhombomeres are covered by a transparent roof. Recent cell lineage studies have shown specific relationships between specific rhombomeres and the forming cranial nerves.
The optic vesicles have by infolding of their outer surfaces reached the optic cup stage. Notice the lens. Find the otocyst (auditory vesicle) adjacent to the rhombencephalon; you may be able to see a short extension by which it is still connected to the exterior. The entire head is now covered by a transparent membrane, the amnion. See if you can find the posterior margin of this structure.
The growth of the heart has caused the organ to become doubled on itself. The blood still enters through the omphalomesenteric veins into the sinus venosus (only a small part of which can be seen), passes forward to the atrium, and then goes to the ventricle, which is the expanded part farthest from the body. Leaving the heart, blood flows forward in the bulbus arteriosus, which soon breaks up into the aortic arches. (Note: You cannot study a large, complex structure like the 56 hour heart by focusing on only one level. Using your fine focus adjustment, shift your focus gradually from the lower to the upper surface of the heart. Try to interpret what you see at the different levels in terms of a three-dimensional structure. You may find it helpful to shape the heart in modeling clay available in the lab.)
Each aortic arch runs through a visceral arch on the side of the pharynx. These visceral arches are masses of mesoderm bounded inside by endoderm and outside by ectoderm. Between the arches the ectoderm comes close to the endoderm in the thin, transparent visceral grooves. The first groove is posterior to the first arch the second groove is posterior to the second arch, and so on. How many grooves do you see? How many arches? (In embryos which develop gills, the arches give rise to the gills and the grooves to the gill slits. In land-living forms, however this potential gill apparatus has a different fate.)
Note the great lengthening of the trunk since 33 hours. How many somites are there now? Is there still a segmental plate? Around the twentieth somite you will see two large blood vessels running out of the body. These are the omphalomesenteric arteries. What do you suppose is their function?
Draw this mount in pencil. Label the structures identified in the text.
This page was posted 16 March 2003