Chapter 15- Lateral mesoderm
and endoderm
Recall lineages
Fig. 14.1- mesoderm lineages
Notochord
Intermediate
Kidney,
gonads
Fig. 12.4
Fig. 14.1- mesoderm lineages
Paraxial
Lateral
Circulatory,
Head Somite
Body cavity,
extraembryonic
Cartilage,
skeletal,
dermis
Somatic mesoderm -becomes body cavity wall
Coelom
-becomes the body cavity
Splanchnic mesoderm -becomes body cavity
wall and the the heart
Lateral mesoderm
How does the heart develop??
25hr
26hr
Fig. 15.3
Endocardium
1. Splanchnic
mesoderm halves
begin to merge
2. These cells differentiate into endocardium
(heart lining and valve precursors
and myocardium (heart muscles)
28hr
27hr
Myocardium
3. Endocardium tubes fuse
4. Mycocardium fuses
5. Heart begins beating even
while fusion is occurring
72hr
Blood vessel formation
Lateral mesoderm
2 steps- vasculogenesis and angiogenesis
Note: Blood vessels form independently of the heart, then link up
Some background Info
Constraints on blood vessel construction
1. Physiological- an organism must:
•
•
•
Obtain nourishment before the intestine develops
Use oxygen before there are lungs
Excrete wastes before there are kidneys
Fig. 15.13- “extra”
archs in mammal
development
2. Evolutionary•
Six pairs of aortic arches loop
out- these enable primitive fish
gills to oxygenate blood, but
these serve no obvious purpose in
mammals and birds.
3. Physical- Blood flows easier through large vessels, yet efficient diffusion
requires small vessels and slow moving blood
Solution- Large vessels branch into very small ones with overall more
cumulative volume capacity
Lateral mesoderm
Blood vessel formation
1. Vasculogenesis Fig. 15.14
Blood vessels
and blood cells
are intimately
connected
BMP
Endothelial cells line blood vessels
Angiogenic
cell cluster
(blood
islands)
Mesenchyme
Fig. 15.16
Primitive
blood cells
Endothelial cells
1. Vasculogenesis
Transcription factors in vasculogenesis
1. FGF2 is required for
hemangioblast formation
2. VEGF is required for blood
island and blood vessel formation
VEGF is a target for tumor therapy
“Tumors gotta eat”
3. Ang1 is required proper blood
vessel formation (involved in
communication between
endothelial cell and smooth
muscle)
Lateral mesoderm
Lateral mesoderm
2. Angiogenesis
Definition- Remodeling and pruning of capillary beds, arteries and veins
Note- Capillary
networks of each
organ arise within
the organ itself,
not from larger
vessels!
VEGF
plays key
role
TGFb
stabilizes
capillary
network
PDGF
recruits
pericyte cells
to ensure
flexibility of
capillaries
Lateral mesoderm
2. Angiogenesis
Arteries vs. veins??
•Arteries have EphrinB2 in cell membranes
•Veins have EphrinB2 receptor (called EphB4) in cell membranes
Arterial
Venous
(EphrinB2) (EphB4)
Fig. 15.17
Functions of the EphrinB2/EphB4 system
1. Ensure that arteries only link up with veins, not other arteries
2. Ensure capillary fusion only occurs with like cells (e.g. only
arteries with arteries)
Lateral mesoderm
2. Angiogenesis
Many organs make their own angiogenesis factors
•Example- placenta
Developing placenta secretes proliferin to promote
angiogenesis, then later secretes proliferin-related protein to
inhibit angiogenesis
Angiogenesis plays key role in tumor development
•A tumor must induce vascularization in order to enlarge
•Hence, if use a drug that inhibits this angiogenesis,
can possibly slow cure some cancers
Lateral mesoderm
Development of Blood Cells
Fig. 15.20
Stem cells – embryonic cells capable of producing many cell
types, including other stem cells
Largest population of stem cells is in the bone marrow
“Committed”
Stem Cell
(CFU-M,L)
B-cell lineage
T-cell lineage
“Differentiating”
“Differentiated”
Lateral mesoderm
Development of Blood Cells
The stem cell (CFU-M,L) also gives rise to another cell lineage
“Committed”
Stem Cell
(CFU-M,L)
B-cell lineage
“Differentiating”
“Differentiated”
Red blood cells
Platelets
Basophils
Myeloid T-cell lineage
precursor cell
Fig. 15.21
Eosinophils
Neutrophils
Macrophage
Paracrine factors that direct
blood cell formation are
termed “cytokines”
Note that this is the point of no
return- cells are committed to a
becoming only one cell type
Lateral mesoderm
Development of Blood Cells
Blood development (hematopoiesis) occurs in two phases
1. Embryonic
Angiogenic cell
cluster
• Occurs in blood islands in mesoderm
(blood islands)
near the yolk (recall fig. 15.16)
• Supplies developing embryo with oxygen
• BMP2 and 4 inhibit blood and blood
vessel formation
Fig. 15.16
• Transitory- disappear later in development
2. Definitive
•Formed in nodes of mesoderm
surrounding aorta (in a region called
the aorta-gonad-mesonephros
(AGM) region)
•Lasts the lifetime of the individual
Example- In mouse, stem
cells originate in yolk sac,
then later in AGM region
Fig. 15.24
Endoderm
Recall Fig. 12.4
Embryonic endoderm gives rise two tubes
Endoderm
Buds into
Digestive tube
Primitive gut
Endoderm
(Esophagus,stomach,small intestine,colon)
Respiratory tube
Liver, gallbladder,
pancreas
Lungs
1. Anterior endoderm – tissues are derived from pharyngeal arches
Auditory cavities
Tonsil walls
Thymus (T-cell development)
Fig. 13.1
Pharyngeal
arches
Parathyroid
Lungs (sprout form base of forth arch)
2. Posterior endoderm
The hepatic diverticulum buds
out form the foregut, then
branches to form liver, pancreas
and gall bladder
The pancreas is actually
formed by the fusion of two
distinct buds (one ventral and
one dorsal)
Liver bud Stomach
Gall Bladder
Pancreas
(ventral)
Pancreas
(dorsal)
Fig. 15.29
What directs formation of liver from the endoderm??
The notochord (and
mesenchyme)
produces factors that
prevent liver induction
The cardiac mesoderm
secretes FGF that blocks the
factors that inhibit
liver induction
Thus, FGF signals the
proximal region of the
endoderm to become
liver
Fig. 15.30
The respiratory tube
• Lungs are one of the last organs to differentiate
• Alveolar cells of the lung produce surfactant at 34 weeks gestation
• Thus, a premature infant cannot breathe properly
foregut
Pharynx
trachea
Lung buds
esophagus
Week 4 (humans) Fig. 15.31
Four problems of a land-dwelling egg
Problem
Solution
Day 2 chick
embryo
1. Desiccation
Amnion secretes amnionic fluid into embryo
2. Gas Exchange
Chorion exchanges gases
3. Nutrition
Yolk duct supplies
nutrients from
blood vessels in
yolk
4. Waste disposal- Allantois holds waste
(vestigal in humans)
Day 9 chick embryo
Fig. 15.33