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BIO 185: Topics in Biology – Fall 2003
Developmental Biology – Outline 4
IV. Development of the Mesoderm and the Endoderm: In this section we will look
at the development of the other two primary germ layers. The ectoderm starts on the top
surface, so next we’ll look at the middle layer, or mesoderm, and then the bottom layer, or
endoderm. It is often easier to look at the tissues that these latter two structures form
simultaneously. This is true because, while they form quite different cell types, many adult
tissues contain derivatives of both. Many of the same concepts that we saw in our
discussions of the ectoderm will also be obvious here, such as induction events between
germinal components, transfer of organizing power and progressive specification. A key
difference between these layers and the ectoderm is that they start as a loose mesenchymal
migration of cells, thus their “plates” are quite different. For ease of visualization we’ll
again use the nice discoidal avian embryos as the primary model in our discussions – just
keep in mind that the same things occur in our curved embryos, it’s just way harder to
draw on the board!
A. The Paraxial Mesoderm: The primary distinctions in mesodermal tissues are their
position relative to the midline. “Paraxial” means “along the axis” or near the midline and, thus,
along the developing neural tube. This distinguishes it from regions of the mesoderm farther out
– the intermediate mesoderm and the lateral plate mesoderm. It is thought that a medial-lateral
gradient of BMP4, stronger at the lateral edge, is the source of the specification of these
mesodermal tissues.
1. The Three Subdivisions of the Paraxial Mesoderm.
a. The Prechordal Plate Mesoderm.
1. Forms the connective tissue and musculature of the head.
b. The Chordamesoderm.
1. Forms the notochord.
c. The somitic dorsal mesoderm, or paraxial mesoderm proper.
1. Forms the somites, which give rise to:
a. Vertebrae and ribs.
b. Dermis of the skin of the back.
c. Skeletal muscle of the back and the body walls.
d. Skeletal muscle of the limbs.
2. The Formation of the Somites (p. 467). The main mesodermal components that form
along the developing neural tube are the somites. We used them in the last section to
count out our position along the neural tube but they are much more than place markers.
They are the source of the non-cranial axial skeleton and of the musculature of the thorax,
abdomen, tongue and limbs. They also provide the cartilage of the spinal cord and ribs,
and the dermal layer of the skin of the back.
a. The timing and periodicity of somite formation.
1. As the primitive streak moves forward.
a. Mesenchymal cells migrate to form the middle layer.
b. Simultaneous with start of neural tube formation.
c. Presomite mesoderm due to noggin antagonism of BMP4
2. As the primitive streak moves backward.
a. Neural folds near the center of the embryo
b. Henson’s node secretes FGF8 - goes away as node does.
c. FGF8 blocks Lunatic Fringe protein expression
d. Lunatic Fringe (TF) causes expression of Notch.
e. Notch causes expression of Hairy1 in 90 second waves.
f. Each wave leads to a new somite forming.
b. Somite separation.
1. Ephs and ephrins are expressed downstream of Hairy1 (TF).
2. Appear to be related to separation but still inexact timing.
c. Somite epithelialization.
1. Mesenchymal to epithelial transformation.
a. N-cadherin expression pulls cells together
b. Integrin-fibronectin binding activates rac1 GTPase
1. Causes realingment of actin cytoskeleton
d. Specification of the somite along the Anterior-Posterior axis.
1. Somites that form cervical and lumbar vertebrae don’t form ribs.
2. Somites that form thoracic vertebrae also form ribs.
3. Primarily Hox gene dependent like the neural tube.
e. Determination of the derivatives of the somite.
1. The sclerotome forms in the ventral-medial portion of the somite.
a. Forms the cartilage of the vertebrae and ribs.
b. Undergoes a reversal to mesenchymal cells.
c. Due to sonic hedgehog from notochord and floor plate.
2. The myotome forms in the two lateral regions of the somite.
a. Form the muscle in that region of the embryo.
b. Divides into two layers: the dermamyotome.
1. Myoblasts close to neural tube form deep back muscle.
2. Myoblasts away from tube give, limb, tongue, abs.
c. Due to sonic hedgehog, Wnt’s, BMPs causing MyoD family.
3. The dermatome forms in the center of the myotome.
a. Can’t tell them apart until dermis cells migrate away.
b. Undergoes a reversal to mesenchymal cells.
c. Due to neurotrophin-3 and Wnt1 from notochord and floor plate.
f. The fate of the notochord.
1. Wherever the sclerotome forms a vertebral body the notochord apoptoses.
2. In between, these cells contribute to the intervertebral discs.
a. These are the discs that slip in back injuries.
A. The Paraxial Mesoderm (cont.)
3. Myogenesis: The Development of Muscle. (p. 473)
a. Specification and Differentiation by the bHLH Proteins.
1. Remember that there are two layers of the dermamyotome.
a. Epaxial muscles are derived near the neural tube: back muscle
b. Hypaxial muscles are farthest away: body wall, tongue, limbs
2. Both must express basic helix-loop-helix factors like MyoD.
a. Epaxial muscle requires Myf5 to activate MyoD
b. Hypaxial muscle requires Pax3
3. MyoD activates its own transcription – positive feedback loop.
a. MyoD results in determination of muscle phenotype.
b. Muscle Cell Fusion.
1. MyoD expressing cells are called myoblasts and are single nucleated.
2. Our muscles are made up of multinucleated myotubes.
a. These result from the fusion of a few to many myoblasts.
b. They align next to each other and dissolve their membranes!
3. They must leave the cell cycle to do this.
a. With a particular growth factor milieu in place, they’ll divide.
b. When they migrate into final destination they lose it and fuse.
1. Appears to depend on the local matrix
a. Fibronectin-integrin signaling.
4. Fusion is cadherin dependent and not species dependent!
a. As long as they have the right cadherin rat and mouse will fuse
b. Fusion results in myogenin expression and full differentiation.
4. Osteogenesis: The Development of Bone. (p. 474)
a. Three cell lineages give rise to bones.
1. Cranial neural crest gives face bones.
2. Lateral plate mesoderm gives bones of the limbs.
3. Somites of the paraxial mesoderm give the axial skeleton.
b. Two methods of bone formation.
1. Intramembraneous ossification.
a. Mesenchymal progenitors are converted directly into bone.
2. Endochondral ossification.
a. Mesenchymal cells go through a cartilage phase first.
c. Intramembraneous ossification of flat bones.
1. Migrating neural crest or paraxial mesoderm cells condense into nodes
a. Again integrin and cadherin related.
2. Condensation leads to osteoblast formation - bone cell commitment.
a. Osteoblasts secrete collagen-based matrix specialized for Ca++
b. Attachment to their own product gives final differentiation.
1. Osteocytes
3. Calcification proceeds outward from osteocytes.
a. Makes more mesenchymal cells condense – can happen fast!
4. Differentiation control.
a. Specification is progressive: axis position and migration path.
b. Commitment is due to bone morphogentic proteins.
c. Final differentiation: BMPs give transcription factor CBFA1
d. Endochondral ossification of long bones.
1. Migrating paraxial or lateral plate mesoderm cells commit to cartilage.
a. Local paracrine factors cause expression of Pax1 and Scleraxis
2. Mesenchymal cells condense into nodes of pre-chondrocytes.
a. Again integrin and cadherin related.
b. SOX9 transcription factor essential.
3. Rapid proliferation produces cartilage model of the bone to come.
a. Produces the exact shape of the future bone.
4. Chondrocytes hypertrophy and change their metabolism.
a. Causes a secretion of a “calcifiable” matrix.
b. Also cause a little thing called apoptosis!
5. Cells hanging out around the cartilage model become osteoblasts.
a. Invade model and calcify the matrix.
b. Accompanied by chondroclasts – eat the dying.
e. Bone remodeling.
1. Osteoclasts differentiate from macrophage progenitors.
2. Reside in bone and balance activity of osteocytes.
B. The Intermediate Mesoderm: The portion of the mesodermal layer just lateral to the
paraxial mesoderm is the intermediate mesoderm. The main claim to fame of this tissue is the
formation of the urogenital system - the kidneys, gonads and their respective duct systems. We’ll
save the gonads for a later discussion of sex-determination (might as well follow the book for
once). The formation of these organs is a highly informative introduction into the formation of
organs in general. As you’ll see it is not surprising that such a relatively large area of embryonic
tissue is devoted to such a seemingly small pursuit.
1. The Specification of the Intermediate Mesoderm. (p. 478)
a. Develops next to paraxial mesoderm which instructs it to form kidneys.
1. Cut the tissue connection – no kidneys will form.
2. Co-culture of the two tissues – kidneys will form
3. Don’t know what the signaling molecule is, yet!
b. Produces the expression of Pax2 and Pax8, both of which are necessary.
1. If either is missing the cells undergo apoptosis.
2. Progression of Kidney Types. (p. 479)
a. The kidney is very complex.
1. The nephron has over 10,000 cells of at least 12 different types.
a. Each is exactly located to the appropriate spot or the organ fails.
b. Three stages of development, the latter persists as the functioning organ.
1. The pronephros forms very early in development
a. Arises ventral to the most anterior somites
b. Mesenchyme of the intermediate mesoderm condense into tubules
c. In embryos of fish and amphibians it actually works for excretion
2. The mesonephros forms in the slightly more advanced embryo
a. The pronephric duct system degenerates by apoptosis
b. New intermediate mesoderm mesenchyme recruited for new ducts
c. Filters blood and forms urine in some but not all mammals.
d. Most of it also degenerates by apoptosis.
e. Two major parts of the mesonephros are retained into adulthood.
1. It appears to be the source of all hematopoeitic stem cells.
2. Parts remain as the vas deferens and associated ductwork.
3. The metanephros
a. The very posterior-most region of mesonephros becomes kidney.
b. Epithelial ducts and metanephric mesenchyme do the organ dance.
3. Reciprocal Interactions of Developing Kidney Tissues. (p. 481)
a. Step 1. Specification of the metanephric mesenchyme.
1. Local intermediate meseoderm must express Hox-11 and WT-1 TF’s
2. Interestingly, LeDouarain and others observe some neural crest cells here?!
b. Step 2. Mesenchyme induces part of mesonephric duct to bud out.
1. Hox-11 expression causes GDNF expression and release to give budding.
2. Mesenchymal cells also put a particular proteoglycan into their matrix.
a. Enhances continued growth in uretric buds
c. Step 3. Survival and epithelialization of mesenchyme caused by ureteric buds.
1. FGF2 and BMP7 are released from bud epithelium and stop apoptosis.
2. Mesenchymal to epithelial transformation requires FGF2, LIF and Wnt6.
A. Also released from buds.
d. Step 4. Nephron differentiation from epithelialized mesenchyme cells.
1. MET causes release of Wnt4 which acts as autocrine signal.
2. Autocrine Wnt4 causes expression of LIM-1 transcription factor.
a. Results in phenotypic shift (obviously there is more to come!)
e. Step 5. Branching of ureteric bud.
1. Epithelialized mesenchyme continues to release GDNF to cause budding.
2. TGF-b1 and collagen XVIII appear to stabilize branches once formed.
3. BMP4 restricts pattern to an appropriate spacing.
f. Step 6. Formation of the final pattern of glomeruli and collecting ducts.
1. Expression of transcription factor Fox-2b appears to limit the process.
2. Retinoic acid and FGF7 are required to the patterning as well?!?
C. The Lateral Plate Mesoderm: Now here is a tissue that I’ve come to know and love. Ah,
the lateral plate mesoderm – the source of the cardiovascular system! The heart and the blood
vessels are the first organ system to become fully functional. The normal development of which
is essential to all development beyond gastrulation. Once the cell layers get to a certain
thickness, no life is possible without the internal delivery of nutrient from the yolk or mother.
1. The Structural Divide in the Lateral Plate.
a. The Somatopleure.
b. The Splanchnopleure.
2. The Development of the Heart. (p. 492)
a. A Little Anatomy Review.
1. The heart is a muscular hollow ball about the size of your fist.
2. Four chambers contract synchronously – two atria, two ventricles.
a. The right and left atrium contract together to fill ventricles.
b. The right and left ventricles contract together to send blood out.
1. The right side fills the lungs, the left the rest of the body.
3. The vena cava flows into the right atrium
4. The pulmonary artery out of the right ventricle, the aorta out of the left.
5. Valves form between atria and ventricles, ventricles and arteries.
b. Specification and Migration of Heart Progenitors.
1. The heart forms as two tubes in the splanchnic mesoderm.
a. Some of the first cells into the streak, right behind the node.
b. Cardiogenic mesoderm is mesenchymal cells bilateral to notochord.
1. Progenitors for muscle, valves, endothelium, Purkinje fibers
2. Specification is from endodermal induction via FGFs and BMPs
a. Neural tube releases Wnts that inhibit heart, promote blood cells.
b. Endoderm also inhibits Wnts with Cerberus, Dickopf and Crescent
c. Specification depends on an FGF + Wnt inhibitor summation.
3. Migration of specified cells to the anterior is along the endodermal lining.
a. Fibronectin dependent.
b. Epithelial condensation then follows
4. The inward folding of the foregut endoderm then brings the two tubes close.
a. Interestingly, zebrafish have an active migration of cardiac cells.
c. Determination and Fusion of Cardiac Primordia.
1. Determination of myocardium is SRF and Nkx2.5 dependent
a. Followed by Mef2 and GATA-4
2. Endocardium separate out from epithelium and then reepithelialize
a. They migrate into the center of each tube to line the muscle
3. Tubes come together and fuse at ~29 hours in the chick, 3 weeks in humans
a. Both muscle and endocardium
b. Spontaneous contractions begin before the tubes are fully fused!
1. The result of cardiac specific genes expression.
4. Unfused portions at each end become presumptive inflow and outflow tracts
d. Looping of the Heart Tube and Chamber Development.
1. A very widely studied phenomenon – check it out if you’re curious (p. 495)
a. Hand-1 and Hand-2 transcription factors are key regulators
b. Looping is how the ventricles and atria move into superior-anterior.
2. Myocardium induces endocardium to EMT and make endocardial cushions.
a. Cushions separate the tube in half.
b. Two septa then grow toward cushion to partition the atria.
3. The Formation of Blood Vessels. (p. 500)
a. Constraints on Formation.
1. Physiological
a. The vasculature develops first to draw nutrient from yolk or mom.
b. Must also prepare for future independent function.
2. Evolutionary
a. First develops aortic arches to match the branchial arches.
b. Must then remodel to accommodate lungs and a single aorta.
3. Physical
a. Must form large tubes at the start to accommodate all of the blood.
1. High speed movement
b. Must have slow movement to accommodate diffusion at tissues
1. Must have more area in capillaries than arteries.
b. Vasculogenesis.
c. Angiogenesis.
d. Secondary Vasculogenesis.
e. Invasion and Replacement.