BIOL 370 – Developmental Biology
Topic #11
Birds and Mammals: Early Development and
Axis Formation
Lange
The amniotic egg…. eggs of those vertebrates that have an
amnion ( a water sac).
This group includes 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.
Allantois - helps the
embryo exchange
gases and handle
liquid waste.
Figure 8.1 Discoidal meroblastic cleavage in a chick egg
area pellucida - the pellucid (translucently clear) central area that immediately
surrounds a vertebrate embryo (as of a bird) formed by discoidal cleavage.
area opaca - the opaque region of yolk
In the following slide, you will see midsagital sections of chick blastoderm
tail region and then dorsal and ventral views of the whole organism.
Look for the Koller’s Sickle (in avian gastrulation, Koller's sickle is a local
thickening of cells that acts as a margin separating sheets of cells)
Look for the Posterior Marginal Zone (PMZ) which can induce a primitive
streak
Look for Hensen's Node which acts as an organizer for gastrulation in
vertebrates. This node starts at the region where the head will form and
regresses thereafter
The term Hensen’s Node is used mainly in birds. In mammals, it is usually
called the primitive knot (or primitive node) is the organizer for gastrulation
in vertebrates.
Figure 8.2 Formation of the chick blastoderm
Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo
A representation of the typical
development of the chick across
the first 24 – 26 hours of normal
development.
12 – 14 hours
15 – 17 hours
18 – 20 hours
Additionally, there is a display of
the fate map seen in the embryo
at the 18 – 20 hour stage which is
when the PRIMITIVE STREAK is
seen.
20 – 22 hours
23 – 25 hours
Four somite
stage
Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 1)
Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 2)
Notice here how the
Primitive Groove and the
Hensen’s Node are both
visible at this stage.
The primitive groove
appears on the surface of
the primitive streak, and
the anterior end of this
groove communicates (via
an aperture called the
blastophore) with the yolksac.
Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 3)
Notice how the
Hensen’s node
regresses as the
head process forms.
Figure 8.3 Cell movements of the primitive streak and fate map of the chick embryo (Part 4)
Figure 8.4 Migration of endodermal and mesodermal cells through the primitive streak (Part 2)
Significant migration occurs through the primitive streak for a variety
of cells. In this slide, focus on the hypoblast cells. These cells are
regulatory and when absent would result in multiple primitive streaks
forming.
The primitive streak is a structure that forms in the blastula during the
early stages of avian, reptilian and mammalian embryonic
development. The presence of the primitive streak will establish
bilateral symmetry.
The absence of
hypoblast results in
multiple primitive streaks
in chicken embryos.
Figure 8.6 Hox gene activation begins when the mesodermal precursor cells are still in the epiblast
Hox genes
(homeobox genes)
are a group of 1 or a
few related genes that
control the
development of the
body plan of the
embryo along the
anterior-posterior
(head-tail) axis.
Figure 8.7 Chick gastrulation 24–28 hours after fertilization
Figure 8.7 Chick gastrulation 24–28 hours after fertilization (Part 1)
Hensen’s Node – specific name give to the primitive knot in birds. The primitive
knot (or primitive node) is the organizer for gastrulation in vertebrates.
Somites - In vertebrates, somites give rise to skeletal muscle, cartilage, tendons,
endothelial cells, and dermis.
The term “metamere” is sometimes used alternatively for the term "somite“.
Figure 8.7 Chick gastrulation 24–28 hours after fertilization (Part 2)
Figure 8.8 Specification of the chick anterior-posterior axis by gravity
Notice how this affects the blastodisc position,
and how this then is portrayed in the egg that you
break open in lab.
Figure 8.10 Induction of a new embryo by transplantation of Hensen’s node
The Hensen’s node induces a new
embryo with transplantation.
Figure 8.15 Development of a human embryo from fertilization to implantation
Figure 8.16 Comparison of early cleavage in (A) echinoderms and amphibians and (B) mammals
Figure 8.22 Tissue formation in the early mammalian embryo
Figure 8.22 Tissue formation in the early mammalian embryo (Part 2)
The placenta is an organ that
connects the developing fetus to
the uterine wall to allow nutrient
uptake, waste elimination, and
gas exchange via the mother's
blood supply.
Trophoblasts are cells
forming the outer layer of
a blastocyst, which
provide nutrients to the
embryo and develop into
a large part of the
placenta.
Decidua is the term for
the uterine lining
(endometrium) during a
pregnancy, which forms
the maternal part of the
placenta. It is formed
under the influence of
progesterone and forms
highly characteristic cells.
Figure 8.22 Tissue formation in the early mammalian embryo (Part 3)
Figure 8.22 Tissue formation in the early mammalian embryo (Part 4)
• The cytotrophoblast (also called the Layer of Langhans) is the inner
layer of the trophoblast.
• It is interior to the syncytiotrophoblast and external to the wall of the
blastocyst in a developing embryo.
• The cytotrophoblast is a trophoblastic stem cell because the layer
surrounding the blastocyst remains while daughter cells differentiate
and proliferate to function in multiple roles.
• Syncytiotrophoblast is
the epithelial covering of
the embryonic
placental villi, which
invades the wall of the
uterus to establish
nutrient circulation
between the embryo
and the mother.
Figure 8.22 Tissue formation in the early mammalian embryo (Part 5)
Figure 8.24 (A) Human embryo and placenta after 50 days of gestation. (B) Relationship of the
chorionic villi to the maternal blood supply in the primate uterus
Figure 8.25 The timing of human monozygotic twinning with relation to extraembryonic membranes
Figure 8.36 Early development of four vertebrate classes
End.