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VI SEMESTER B. Sc ZOOLOGY PRACTICALS STUDY of EMBRYOLOGICAL SLIDES

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VI SEMESTER B. Sc ZOOLOGY PRACTICALS STUDY OF EMBRYOLOGICAL SLIDES FROG BLASTULA 1. The egg cleaves and forms blastula at 8 cell stage. 2. The blastula contains a blastocoelic cavity surrounded by unequal blastomeres. 3. The smaller blastomeres are called micromeres, found in the upper half and contain dark pigments. 4. The larger blastomeres are called macromeres, found more in the lower half and laden with yolk. 5. The lower side or vegetal hemisphere is composed of large yolky megameres. Because of their large size, the blastocoel is excentric, lying towards the animal pole. CHICK BLASTULA 1. Chick Blastula is known as Discoblastula 2. Discoblastula consists of a disc - shaped mass of blastomeres overlying a large yolk mass. 3. This blastula is the result of meroblastic discoidal cleavage 4. There is no blastocoel, instead a slit like cavity called subgerminal cavity appears in between the blastoderm and the yolk mass. 1 FROG GASTRULA 1. First part of gastrulation is the formation of a blastopore on surface of blastula 2. The cells begin to fold inward 3. Further folding of the blastopore results in a cavity called the archenteron. The future gut 4. Blastopore becomes anus 5. The blastocoel is being displaced 6. Continued morphogenic movements result in enlarging the archenteron, reducing the blastocoel and forming a yolk plug over the blastopore 7. In mature frog gastrula there are three germ layers 8. Ectoderm which are the outer cells, Endoderm which line the archenteron, and mesoderm which is in between endoderm and ectoderm CHICK GASTRULA 1. Gastrulation begins within four or five hours after the onset of incubation 2. Gastrulation in chick embryo can be divided into two phases- a) Endoderm formation and b) Primitive streak formation and movement of chordamesodermal elements. 3. Formation of endoderm: Endoderm of hypoblast develops as a single layer of cells in side of blastocoel. After the formation of endoderm, upper layer is called epiblast. 4. The second step in gastrulation is the formation of primitive streak. At the posterior region of area pellucida in the mid dorsal line primitive streak will appear as a thickened area. It starts eight hours after incubation. The thickening is because convergence of cells of blastoderm towards the centre. Usually in the early stages the primitive streak is short and broad. It gradually extends to the middle of blastoderm. 2 18 HOUR CHICK EMBRYO 1. 18 hrs chick embryos show a longitudinal thickening in the middle called primitive streak and is the first indication of axis of the embryo. 2. A fully formed primitive streak contains a primitive grove, a pair of primitive folds and primitive pit. 3. The area pellucida and area opaca are prominent 3 4. In the front, the primitive streak ends in a small pit, the primitive pit or Hensen's node. This structure is homologue with the dorsal lip of the blastopore in amphibians 5. Just in front of this node one can distinguish the formation of the notochord (chorda). This structure induces the formation of the neural plate and later the neural groove. 6. The primitive streak lies in the middle of the area pellucida as a clear line. 7. At the tail side of the area pellucida it is characterized by a primitive groove surrounded by 2 thicker walls of the blastoderm, the primitive walls. 8. The proamnion lies in front of (= pro) this head fold as a clear zone. 24 HOUR CHICK EMBRYO 1. At 24 hrs. incubation period the chick embryo is oval in shape. 2. The primitive streak is fully formed and the process of gastrulation is completed 3. The Notochord extends from the from the hensen's node as head process into the mesoderm-free area anteriorly. 4. The head fold and fore-gut develop in the embryo. 5. The mesoderm differentiates into somites, intermediate and lateral plate mesoderm. 6. In the 24 hrs chick embryo four pairs of somites are differentiated from the mesoderm. 7. The coelom begins to develop in the lateral plate mesoderm. 4 8. The blood islands appear in the area opaca. Pericardial region and primordial of heart are established. 9. The area opaca further modifies into the area vasculosa and area vitellina. 10. The neurectoderm gives rise to the neural folds and neural groove. The fusion of neural folds begins from the mid-region. 33 HOUR CHICK EMBRYO 1. Lengthening of foregut and subcephalic pocket. 2. Formation of neural tube and sinus rhomboidalis. 3. The primary division of encephalon into prosencephalon, mesencephalon and rhombencephalon. 4. Formation of neural crest cells on either side of the neural tube. 5. Development of the infundibulum as a median ventral outgrowth from the floor of prosencephalon. 6. Formation of 13 pairs of somites. 5 7. Development of heart as tubular structure lying in the midventral region to the foregut. 8. Formation of extraembryonic blood vessels in the area vasculosa. 9. Formation of intraembryonic blood vessels. 10. Disappearance of primitive streak. 48 HOUR CHICK EMBRYO 1. Appearance of cranial flexure and torsion. 2. Formation of eleven neuromeres, 3 neuromeres in prosencephalon, 2 in mesencephalon and 6 in rhobencephalon 3. Formation of secondary constrictions in the brain. 4. Completion of vitelline (extraembryonic) circulatory system. 5. Development of intra embryonic blood vessels. 6. Formation of two pairs of aortic arches. 7. Twisting of the heart and formationof chambers in it. 8. Commencement of blood circulation. 9. Formation of optic cup and lens vesicle. 10. Formation of visceral cleft. 11. Development of auditory vesicle. 12. Differentiation of 28 somites 6 13. Development of extra embryonic membranes 14. Formation of pronephros. PLACENTA OF PIG AND MAN In Eutherian mammals the embryo develops in the uterus of mother. The developing embryo will get nourishment from mother through the placenta. Placenta is not common to all mammals. It is developed well in Eutheria. The term placenta was delved from Greek word it means flat cake. Placenta is a special connective tissue, which contains the uterus of mother and foetal membranes of foetus. Functions of placenta: 1) Placenta will form a physiological barrier between mother and foetus. It will possess foetal and maternal blood mixing. 2) Placenta allows the diffusion of monosacharides, amino acids, hormones, vitamins, oxygen, carbon dioxide, water and other waste materials; because of this it supplies food, oxygen to foetus. 3) It works as an excretory organ of foetus. It releases the nitrogenous waste materials into mother blood. 4) It works as an endocrine gland. It will secretes lactogen, progesterone,etc. 5) The placenta will manufacture fructose from glucose. Allantoic Placenta of Pig The allantoic placenta consists of allantois and chorion. Allantois is a sac like outgrowth from the hindgut embryo. It is lined internally by endoderm and externally by mesoderm. As allantois grows and spreads in the extra-embryonic cavity and its mesoderm fuses with that of chorion over a somewhat restricted region. The layer formed by the fusion of allantois and chorion is termed allanoto-chorion. It becomes richly vascular and thrown into small, finger like processes, called villi. The uterine wall forms corresponding depressions, called crypts, which are penetrated by foetal villi forming allantoic placenta. Materials absorbed from the maternal blood through allantoic placenta are carried to the foetus by allantoic blood vessels. In pigs, the placenta is classified as diffuse type based on the shape and distribution of villi. 7 Chorionic Placenta of Man It occurs in man and apes and is formed only by the chorion. However, its mesoderm and blood vessels grow up to chorion whose villi enter chorion. Allantois remains small, burrows into body stalk (umbilical cord) and each of the uterine crypts forming chorionic placenta. The blastocyst actually burrows into uterine tissue to become completely surrounded by it (interstitial placentation). 8 AMNIOCENTESIS Amniocentesis is a procedure in which amniotic fluid is removed from the uterus for testing or treatment. Amniotic fluid is the fluid that surrounds and protects a baby during pregnancy. This fluid contains fetal cells and various chemicals produced by the baby. With genetic amniocentesis, a sample of amniotic fluid is tested for certain conditions - Nearly all chromosomal disorders, including Down syndrome, trisomy 13, trisomy 18, sex chromosome abnormalities (such as Turner syndrome and Klinefelter syndrome), cystic fibrosis, sickle cell disease, and Tay-Sachs disease can be identified. Neural tube defects such as spina bifida and anencephaly can also be identified by measuring the level of a substance called alpha-fetoprotein (AFP) in the amniotic fluid. With maturity amniocentesis, a sample of amniotic fluid is tested to determine whether the baby's lungs are mature enough for birth. Occasionally, amniocentesis is used to evaluate a baby for infection or other illness. Rarely, amniocentesis is used to decrease the volume of amniotic fluid. 9 CANDLING METHOD Introduction Candling is the process of holding a strong light above or below the egg to observe the embryo. A candling lamp consists of a strong electric bulb covered by a plastic or aluminium container that has a handle and an aperture. The egg is placed against this aperture and illuminated by the light. If you do not have a candling lamp, improvise. Try using a torch. Candling is done in a darkened room or in an area shielded by curtains. Determining the viability of the embryo Under the candling lamp, the embryo appears as a dark shadow with the head as a dark spot. Healthy embryos will respond to the light by moving. Sometimes the movement is very sluggish and it can take 30 to 40 seconds for the embryo to move when held under the candling lamp. This indicates the embryo is not healthy. Look carefully at the blood vessels.
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