Chapter 15- Lateral mesoderm and endoderm

Recall lineages

Fig. 12.4

Fig. 14.1- mesoderm lineages

Intermediate

Kidney, gonads

Paraxial

Head Somite

Cartilage,skeletal, dermis

Lateral

Circulatory,Body cavity,

extraembryonic

Notochord

Fig. 14.1- mesoderm lineages

Somatic mesoderm

Splanchnic mesoderm

Coelom -becomes the body cavity

-becomes body cavity wall and the the heart

-becomes body cavity wall

How does the heart develop??

1. Splanchnic mesoderm halves begin to merge

2. These cells differentiate into endocardium (heart lining and valve precursorsand myocardium (heart muscles)

25hr 26hr

Endocardium

3. Endocardium tubes fuse4. Mycocardium fuses

Fig. 15.3

27hr 28hr

5. Heart begins beating even while fusion is occurring

72hr

Myocardium

Lateral mesoderm

Blood vessel formation

Note: Blood vessels form independently of the heart, then link up

Constraints on blood vessel construction1. Physiological- an organism must:

• Obtain nourishment before the intestine develops• Use oxygen before there are lungs• Excrete wastes before there are kidneys

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

Fig. 15.13- “extra” archs in mammal

development

2 steps- vasculogenesis and angiogenesis

Some background Info

Lateral mesoderm

1. Vasculogenesis

Blood vessels and blood cells are intimately

connected

Endothelial cells line blood vessels

Fig. 15.14

Angiogenic cell cluster(blood islands)

Endothelial cellsMesenchyme

Primitive blood cells

Fig. 15.16

Blood vessel formation

BMP

Lateral mesoderm

Transcription factors in vasculogenesis

1. FGF2 is required for hemangioblast formation

2. VEGF is required for blood island and blood vessel formation

3. Ang1 is required proper blood vessel formation (involved in communication between endothelial cell and smooth muscle)

1. Vasculogenesis

VEGF is a target for tumor therapy

“Tumors gotta eat”

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

TGFstabilizes capillary network

PDGF recruits pericyte cells to ensure flexibility of capillaries

Lateral mesoderm

2. AngiogenesisArteries vs. veins??

•Arteries have EphrinB2 in cell membranes

•Veins have EphrinB2 receptor (called EphB4) in cell membranes

Functions of the EphrinB2/EphB4 system1. Ensure that arteries only link up with veins, not other arteries2. Ensure capillary fusion only occurs with like cells (e.g. only

arteries with arteries)

Arterial(EphrinB2)

Venous(EphB4) Fig. 15.17

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

Stem cells – embryonic cells capable of producing many cell types, including other stem cells

Largest population of stem cells is in the bone marrow

Fig. 15.20

Stem Cell (CFU-M,L)

“Committed” “Differentiating” “Differentiated”

B-cell lineage

T-cell lineage

Lateral mesoderm

Development of Blood Cells

Fig. 15.21

Stem Cell (CFU-M,L)

“Committed” “Differentiating” “Differentiated”B-cell lineage

T-cell lineage

The stem cell (CFU-M,L) also gives rise to another cell lineage

Myeloid precursor cell

Red blood cells

Platelets

Basophils

Eosinophils

Neutrophils

Macrophage

Note that this is the point of no return- cells are committed to a becoming only one cell type

Paracrine factors that direct blood cell formation are termed “cytokines”

Lateral mesoderm

Development of Blood Cells

Blood development (hematopoiesis) occurs in two phases

1. Embryonic

• Occurs in blood islands in mesoderm near the yolk (recall fig. 15.16)

Angiogenic cell cluster

(blood islands)

Fig. 15.16

• Supplies developing embryo with oxygen• BMP2 and 4 inhibit blood and blood vessel formation• 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

Lateral mesoderm

Endoderm

EndodermEmbryonic endoderm gives rise two tubes

Digestive tube(Esophagus,stomach,small intestine,colon)

Respiratory tube

Buds into Liver, gallbladder, pancreas

Lungs

Primitive gutEndoderm

Fig. 13.1Pharyngeal

arches

Auditory cavities

Tonsil walls

Thymus (T-cell development)

ParathyroidLungs (sprout form base of forth arch)

Recall Fig. 12.4

1. Anterior endoderm – tissues are derived from pharyngeal arches

2. Posterior endoderm

The hepatic diverticulum buds out form the foregut, then branches to form liver, pancreas and gall bladder

StomachLiver bud

Gall Bladder

Pancreas(ventral)The pancreas is actually

formed by the fusion of two distinct buds (one ventral and one dorsal)

Pancreas(dorsal)

Fig. 15.29

Fig. 15.30

The cardiac mesoderm secretes FGF that blocks the factors that inhibitliver induction

The notochord (and mesenchyme) produces factors that prevent liver induction

What directs formation of liver from the endoderm??

Thus, FGF signals the proximal region of the endoderm to become liver

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

Fig. 15.31

foregut

Lung buds

Pharynx

trachea

esophagus

Week 4 (humans)

Four problems of a land-dwelling egg

1. Desiccation

SolutionProblem

Amnion secretes amnionic fluid into embryo

2. Gas Exchange

4. Waste disposal-

3. Nutrition

Chorion exchanges gases

Allantois holds waste (vestigal in humans)

Yolk duct supplies nutrients from blood vessels in yolk

Day 2 chick embryo

Day 9 chick embryo

Fig. 15.33