The document discusses the development and axis formation in birds and mammals, highlighting processes such as cleavage types, gastrulation, and embryonic development stages. It explains the mechanisms underlying the formation of the dorsal-ventral, anterior-posterior, and left-right axes during early development, emphasizing critical signaling centers and gene expressions involved. Additionally, it contrasts the specific developmental pathways in avian and mammalian species, including the role of the trophoblast and hox genes.

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.

5. 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.

6. 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

7. 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.

8. Gastrulation in the Avian (chick) Embryo
• The embryo of the chick blastula is two layered
1. 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.

9. • 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.

11. 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
1. The dorsal side(side facing the negative and basic albumin)
2. The ventral side (side facing the positive and the acidic subgerminal space fluid)

12. • 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.

13. 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.

14. 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.

15. 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.

16. 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

17. 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

18. 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.

19. 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.

20. 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.

21. 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.

22. • 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.

23. 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.

25. Thankyou