Axis formation in birds and mammals

types of eggs , types of cleavage in birds and mammals,early development in birds and mammals, axis formation in birds and mammals

1. Axis formation in Birds and Mammals PG Biotechnology Department-Plant Biology and Biotechnology Cell and developmental biology

2. Introduction A multicellular organism develops from a single cell (the zygote) into a collection of many different cell types, organized into tissues and organs. Development involves cell division, body axis formation, tissue and organ development, and cell differentiation (gaining a final cell type identity).

3. Amniotic Eggs-Found in birds , reptiles and mammals Amniotic eggs contain membranes that are designed to allow survival on land: amnion – the membrane that allows the embryo to float in a fluid environment to avoid desiccation yolk sac – enables nutrient uptake and development of the circulatory system chorion – contains blood vessels that exchange gasses with the external environment.

4. Types of Cleavage Meroblastic Cleavage : Eggs having high yolk content (telolecithal eggs) the cytoplasm and nucleus are restricted to animal pole region and the cleavage plane continuously divides this region . Yolk region does not get divided. When viewed from the pole a disc is appeared (as the mass of cells float around the yolk)this disc is known as the blastodisc and the cleavage is known as meroblastic cleavage . Example : Found in eggs of birds Holoblastic Cleavage : The cleavage planes divides the zygote completely then it is known as holoblastic cleavage . Example : found in eggs of mammals .

5. Early development in Birds (Chick) Domestic chicken is used in embryological studies Easily accessible and easily raised Meroblastic cleavage- Blastodisc seen First cleavage furrow appears in the blastodisc Other cleavage form the single layered blastoderm Equatorial and vertical cleavages:- five- six layered blastoderm

6. Dicoidal Meroblastic Cleavage in birds Area Pellucida - Deep cells present in the centre of the blastoderm that are shed and disc leaving behind is area pellucida . Area Opaca – the peripheral ring of blastodermal cells that has not shed their deep cells constitute the area opaca. Marginal Zone or Marginal Belt : a thin layer between area pellucida and area opaca is the marginal zone. They become important in determining the cell fate of early chick development.

7. Gastrulation in the Avian (chick) Embryo The embryo of the chick blastula is two layered Upper layer- epiblast 2. Lower layer – primary hypoblast Formation of the Secondary Hypoblast Koller’s Sickle : In avian gastrulation, Koller's sickle is a local thickening of cells that acts as a margin separating sheets of cells. Primitive streak :After 3-4 hrs of incubation the process of condensation and convergence of cells at the posterior end of the epiblast gastrulation and germ layer formation begins. After 12 hrs of incubation a streak of cell layer is seen which is known as the primitive streak.

8. After 16 hrs of incubation the streak acquires a definite shape and this stage is known as primitive streak stage . A groove appears within the streak cephalocaudally known as the primitive groove . At the anterior end the streak cells condense and the region becomes very thick and this region is known as the Hensen’s node or primitive knot . It acts a organizer for gastrulation and starts regressing when formation og head starts. Migration of cells from primitive streak : after the formation of primitive streak the epiblast cells migrate through it into the blastocoel. In this way the continuous flow of migratory cells through the node down into the blastocoel and migrate anteriorly forming the endoderm, the notochord and cephalic mesoderm. After cell migration the primitive streak is almost disappeared and a portion of it is seen in tail bud and partly with the cloaca.

9. Axis formation in the Avian (chick) embryo The role of pH in forming the dorsal-ventral axis : Dorsal Ventral axis is critical to the formation of the hypoblast and to the further development. This axis is established when the cleaving cells of the blastoderm establish a barrier between the basic ( pH-9.5 ) albumin above the blastodosc and acidic( ph-6.5 ) subgerminal space below it. A potential difference of 25mV across the epiblast cell layer due to the transportation of water and sodium ions from the albumin to the subgerminal cavity. This distinguishes the two sides of epiblast The dorsal side(side facing the negative and basic albumin) The ventral side (side facing the positive and the acidic subgerminal space fluid)

10. The role of gravity in forming the anterior posterior axis : The conversion of the radially symmetrical blastoderm into a bilaterally symmetrical structure is determination by gravity.

11. Left-right axis formation The distinction between left and right sides is regulated by two proteins :the paracrine factor Nodal and the transcription factor Pitx2 . Right side: the transcription of sonic hedgehog gene ceases due to the expression activin on the right side of the embryo. This in turn activates the expression of fgf8 which in turn prevents the transcription of the caronte gene. In the absence of caronte, bone morphogenetic proteins (BMP’s) which block the expression of the nodal and lefty-2 . This activates the snail gene (cSNR) that is characteristics of the right side of the avain embryonic organs.

12. Left side: the lefty-I protein blocks the expression of fgf8 while sonic hedgehog activates caronte . Caronte is a paracrine factor that prevents BMPs from expressing the nodal and lefty-2 genes , and also inhibits BMPs from blocking the expressin of lefty-1 on the ventral midline structures. Nodal and Lefty-2 activate Pitx2 and repress snail (cSNR) Pitx2 is crucial in directing the asymmetry of embryonic structures. The left side structures starts forming.

13. Early development in Mammals Mammals undergo holoblastic cleavage which is rotational and the slowest amongst the other members of the animal kingdom. The first cleavage is a normal meridional division; however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially . This type of cleavage is known as rotational cleavage.

14. Compaction: 8­cell stage ­ tight junctions between outside cells  seal off inside of sphere. Morula  – 16­cell stage ­ small group of internal cells; inner cell mass (ICM)  . ­ICM will form the embryo proper  ­ larger group of external cells; trophoblast  ( trophectoderm )  ­ trophoblast will form extraembryonic structures  ­ secretes hormones causing uterus to retain foetus. ­ Cavitation – trophoblast secretes fluid into morula (via Na + pumps)  ­ creates blastocoel ­ hydrostatic pressure pushes ICM to one end 

15. Blastocyst Hatching & Implantation Zona pellucida  prevents adhesion to uterine wall (premature adhesion = ectopic pregnancy)   Trophoblast attaches to uterine wall ­ forms the chorion – embryonic  portion of the placenta  Trophoblast secretes proteases  ­ digests uterine ECM  -­ blastocyst implants ICM – forms the embryo proper  ­ also, the yolk sac, allantois, and amnion

16. Mammalian anterior-posterior axis formation Two Signalling Centres : one in the node “ the organizer ” and the other in the anterior visceral endoderm. The node is responsible for the formation of all the body parts. The two signalling centres are said to be responsible for the formation of the brain. The node produces the chordin and noggin ,while the anterior visceral endoderm expresses several genes that are necessary for head formation. These include the genes for transcription factors Hesx-1 , Lim-1 , and Otx-2 as well as the gene for the paracrine factor Cerberus . The anterior visceral endoderm is established before the node, and primitive streak always forms on the side of the epiblast opposite this anterior site.

17. Patterning the Anterior-Posterior Axis: The Hox Code Hypothesis It is specified by the expression of hox genes once gastrulation begins. These genes are homologous to the homeotic gene complex (Hom-C) of the fruit fly . The Hom-C are arranged in the same order (as in Drosophila) as their expression pattern along the anterior-posterior , the most 3´ gene ( labial ) being required for producing the most anterior structures, and the most 5´ gene ( AbdB ) specifying the development of the posterior abdomen.  Mouse and human genomes contain four copies of Hox complex per haploid set ,located o four different chromosomes ( Hoxa through Hoxd in the mouse , HOXA through HOXD in humans). The mammalian Hox/HOX genes are numbered from 1 to 13 , starting from that end of each complex that is expressed most anteriorly. The equivalent genes in each mouse complex such as Hoxa-1, Hoxb-1 and Hoxd-1 are called paralogous chromosomes.

18. Expression of Hox gene along the dorsal axis Hox gene expression can be seen along the dorsal axis ( in the neural tube, neural crest, paraxial mesoderm, and surface ectoderm)from the anterior boundary of the hindbrain through tail. The hox gene expression is said to be the reason for specifying the different regions. In general, the genes of paralogous group 1 are expressed from the tip of that tail to the most anterior border of the hindbrain. 2 genes are expressed throughout the spinal cord, but the anterior limit of expression stops two segments more caudally than that of paralogue 1 genes. The higher numbered Hox paralogues are expressed solely in the posterior regions of the neural tube, where they also form a “nested” set.

19. The Dorsal-Ventral Axis Very little is known about the mechanisms. In mice and human, the hypoblast forms on the side of the ICM that in contact with the trophoblast . Dorsal – ventral axis is defined by the embryonic-Abembryonic axis of the blastocyst.

20. Figure showing the relationship between the animal-vegetal axis of the egg and the embryonic-abembryonic axis of the blastocyst. The polar body marks the animal pole of the embryo . The dorsal-ventral axis of the embryo appers to form at right angles to the animal-vegetal axis.

21. The Left-Right Axis Mammalian body is not similar Heart formation begins in the midline left side of chest and loops in the right. The spleen in the left side of abdomen. Major lobe of the liver is seen in the right side of abdomen Large intestinal loops right to left as it transverses the abdominal cavity. Right lung has one more lobe than the left lung The distinction between left and right sides of the axis formation begins in the ciliary cells of the node.

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