You will study in this lab the early embryological development of
the starfish and the chicken. These 2 organisms represent extremes with
regard to how cleavage and gastrulation take place.
Objectives: Upon the completion of this lab, the student should:
1. Recognize the different stages of Starfish development.
2. Be able to describe the stages of Starfish development.
3. Recognize the different stages of Chicken development.
4. Be able to describe the different stages of Chicken development.
5. Know the different highlighted structures and their function on the
chick development slides.
Starfish: The starfish is a marine organism and the eggs and sperm are shed into the seawater for external fertilization. The early cleavage stages occur in the sea water and eventually a free-swimming larval stage hatches from the egg. The echinoderms have small eggs with a small amount of yolk so there is complete cleavage of the fertilized egg (zygote) and the 2 blastomeres formed are equal in size. Gastrulation occurs by invagination of a portion of the outer layer of the blastula to form the primitive gut (archenteron) and the coelom is formed next from 2 pouches from the primitive gut. Invagination means the infolding of a portion of the outer layer of the blastula in the formation of a gastrula.First examine this slide containing the early stages of starfish development under scanning power. Note that all of the different stages of development are present on this slide. Next examine the different stages in more detail under low power. Find the stages listed below. Make drawings of the stages and label all stages.
- Unfertilized egg - note that the nucleus and nucleolus are visible in the unfertilized egg
- Fertilized egg (zygote) - the nucleus is no longer visible; the fertilization membrane is present
- 2-cell stage, 4-cell stage
- Morulas - 8-cell stage, 16-cell stage
- Blastulas - 32-cell stage, blastocoel
- Late Blastula - shows thickened layer of cells at one end, the so-called vegetal pole
- Early Gastrula - blastocoel, blastopore, primitive gut (archenteron)
- Late Gastrula - endoderm, ectoderm, coelomic pouches (mesoderm)
- Bipinnaria - the bipinnaria larva (bilateral symmetry) develops into a brachiolaria larva; this larva undergoes metamorphosis to become the adult starfish, which has radial symmetry.
(Click on the eggs at the Chickenscope web site (link above) to view photomicrographs of chick development)
The term "egg" in the case of the chicken is applied to the entire entity laid by the hen, while the egg cell, which is released from the ovary, is called the ovum. The ovum consists of the yellow yolk plus a small yolk-free area called the blastoderm (blastodisc, germinal disc). The nucleus of the ovum is in the blastodisc. The blastodisc appears as a small whitish area on the upper surface of the yolk.Albumen is added to the ovum as it moves down the hen's oviduct. A viscous albumen is first added and is twisted into a pair of strands as the ovum passes down the oviduct. Each strand of viscous albumen is called a chalaza (plural is chalazae). A watery albumen is added to the ovum further down the oviduct. Eventually 2 shell membranes and a calcareous shell are added to form the complete "egg". Note: The yolk serves as a food source, while the albumen serves as a source of protein and water.
Because of the large amount of yolk present in the hen's egg, cleavage, morphogenesis and differentiation are confined to the blastoderm. Initially the blastoderm becomes several cell layers thick and a cavity, called the subgerminal cavity, is formed under this layer. This stage of the embryo is comparable to the starfish morula. As cleavage continues and more cells are formed there is eventually a splitting of the blastoderm to form 2 layers, a dorsal epiblast (ectoderm) and ventral hypoblast (endoderm). This embryo stage corresponds to the starfish blastula and the cavity separating these 2 layers is called the blastocoel. Development to this stage takes place while the egg is still in the oviduct of the hen. Further development will takes place only if the egg is brooded by the hen or placed in an incubator at 37 degrees C.
Gastrulation occurs by a process of involution. Involution is the curling inward and ingrowth of a group of cells. Cells of the blastoderm surface migrate posteriorly and medially and involute (turn in) along a line called the primitive streak. These involuted cells will form the mesoderm germ layer. As gastrulation progresses, the anterior end of the streak moves posteriorly, so the anterior region of the embryo is formed first. The primitive streak is functionally the same as the blastopore of the starfish gastrula. The 3 germ layers (ectoderm, endoderm and mesoderm) are present following gastrulation by involution. The coelom results from a splitting of the lateral mesoderm . The involuted cells form the notochord anterior to the primitive streak and the lateral mesoderm (somites) laterally.
CAUTION: Use only scanning and low power of the microscope with this whole mount slide; NEVER use high power.NOTE: The primitive streak is the area where the cell involute to form the mesoderm germ layer. The primitive streak is functionally comparable to the blastopore of the starfish embryo or the frog embryo. The notochord is derived from the mesoderm germ layer. The somites are derived from the mesoderm germ layer.
- Examine the slide of the whole mount under scanning and low power and referring to the image of the 30-hr chick embryo whole mount identify the: primitive streak, notochord, neural tube, somites, vitelline vein, heart, brain (forebrain, midbrain, and hindbrain), posterior border of the foregut, and optical vesicles.
Serial Section A
Serial Section B
Serial Section C
Serial Section D
Serial Section E
Serial Section F
Serial Section GThis slide contains cross-sections of an entire embryo laid out in rows (some slides are in groups of two). Examine the slide under scanning power and note the rows of cross-sections. The sections are laid out on the slide so that you can go down one row and then pick up the next appropriate section on the adjacent row, and so on. The advantage of a serial arrangement of sections is that it is possible to follow the path of certain vessels, such as the dorsal aortae or to visualize the formation of particular structures, such as the dorsal hollow nerve cord and foregut.
In our study of the 33-hr chick embryo we will note in particular the development of the circulatory system and the nervous system, both of which appear early in development. We will also take note of the early development of the digestive tract.
- Circulatory system (mesoderm origin): Examine the slide under low power and locate the heart in a section such as F. Now examine sections posterior to the heart and find where the 2 vitelline veins coming from the surface of the yolk empty into the heart. Next examine sections more posteriorly and follow the path of the vitelline veins out onto the surface of the yolk. Continue to examine the slide under low power and find the 2 dorsal aortae (the 2 vessels above the foregut) in a cross-section such as "E". Now examine sections more anteriorly. Next, you should find where the 2 dorsal aortae turn ventrally and become the 2 ventral aortae (somewhere between sections F and G) coming from the heart. Next go back to section E and examine section posterior to E and follow the dorsal aortae and find where these 2 vessels eventually become the vitelline arteries extending out onto the surface of the yolk.
- Nervous system (ectoderm origin): By following the serial arrangement of cross-sections on this slide, it is possible to visualize the process of neurulation (formation of the nervous system) in the chick embryo. Examine the sections under low power and find a cross-section such as B where the nervous system is simply a groove called the neural plate - this is the beginning of the neurulation process. Now examine sections more anteriorly and find sections where the sides of the neural plate have started to fold inward to form the neural folds (cross-section C). Next examine sections anterior to the neural folds to visualize how the neural folds eventually close the form the dorsal hollow nerve cord. Continue to examine sections more anteriorly and note that the most anterior region of the nerve cord differentiates to form the initial 3 regions of the brain (hindbrain, midbrain, forebrain). We will cover more on brain development in 48-hr and 72-hr whole mounts of chick embryos.
- Digestive Tract (endoderm origin): Foregut formation - The head of the embryo lifts off the surface of the yolk early in development. The lifting of the head forms the so-called head fold which is evident in the 30-hr chick embryo. Examine a cross-section such as G and observe the head is lifted off the surface of the yolk. The cavity (see section f) formed from the head fold becomes the foregut. Look at the free-hand drawing below to visualize how the foregut is formed. Later in development a posterior fold will form which will lift the posterior part of the embryo off the surface of the yolk and result in the formation of the hindgut.
CAUTION: USE SCANNING POWER ONLY to avoid breaking the cover slip. Examine the anterior portion of the embryo and note that the body of the embryo has undergone a rotation so that the left side lies against the surface of the yolk. The brain has differentiated into 5 regions labeled I-V on the image of the 48-hr chick embryo whole mount . Examine the embryo and identify these 5 regions and note that the eye (optic cup and lens) is in region II and the auditory vesicle in region V. The neural tube is closed back to the level of the remaining portion of the primitive streak.
- Examine the embryo in the region of the head and heart and note that the heart has increased in size and has undergone a twist. Identify the ventral aortae leaving the heart. The ventral aortae give rise to 4 aortic arches on each side of the embryo. Identify the 4 aortic arches and the 3 pharyngeal slits on one side of the embryo.
- Examine the portion of the embryo posterior to the heart region and note the somites and the vitelline arteries extending out onto the yolk surface.
CAUTION: USE SCANNING POWER ONLY to avoid breaking the cover slip. Examine the anterior portion of the embryo and note the rotation of body at the anterior end. Identify the 5 regions of the brain and the eyes in region II and the auditory vesicles in region V.
- Examine the embryo in the region of the head and heart and identify the ventral aortae leaving the heart. The ventral aortae give rise to 4 aortic arches on each side of the embryo. Identify the 4 aortic arches and the 3 pharyngeal slits on one side of the embryo.
- Examine the portion of the embryo posterior to the heart region and note the somites, the vitelline veins and the vitelline arteries. The vitelline veins and vitellin arteries extend out onto the yolk surface.
- Examine a plastic mount under scanning power and identify the eyes, auditory vesicles, aortic arches, pharyngeal slits, anterior limb buds, posterior limb buds, and the allantois