Stem Cell Module 3

What cells compose the ectoderm?

Epiblast cells that do not ingress through the primitive streak

What family of GFs is inhibited to induce ectoderm?

TGF-beta superfamily

What three categories does ectoderm become?

Surface ectoderm, neural crest, and neural tube

What does surface ectoderm become?

The epidermis, hair, nails, mouth, lens, cornea, tooth enamel, anterior pituitary, cheek epithelium

What does the neural crest become?

PNS (Schwann cells, neuroglial cells, sympathetic/ parasympathetic NS), Adrenal medulla, melanocytes, facial cartilage, dentine of teeth

What does the neural tube become?

Brain, neural pituitary, spinal cord, motor neurons, retina

What happens as the primitive streak regresses?

It leaves behind the underlying rod of notochordal tissue

What is the area proximal to the notochord called?

The neural plate

What does the neural plate become?

Fated to become neural lineage (neuroectoderm)

What do neuroepithelium cells express in humans?

Pax6, Sox2, and CDH2

What happens during primary neurulation?

The cells in the neural plate proliferate, invaginate, and pinch off from the surface to form a hollow tube

What three groups of cells does primary neurulation divide cells into?

Interior (brain and spinal cord), exterior (epidermis), and neural crest

What direction does primary neurulation occur in?

In temporal fashion down the rostro caudal axis (tail to mouth)

What are the 4 phases of primary neurulation?

Formation (shaping and folding), elevation, convergence, and closure

What is the purpose of cadherins in primary neurulation?

Cadherins participate in homotropic interactions and aid in sorting heterogenous cell populations into homogenous groups

Helps the neural tube to come together strongly

What occurs during rostrocaudal patterning? When does this happen?

Development of vesicle of the brain and spinal cord; occurs during and after neurulation

What causes rostrocaudal patterning?

The graded presence of BMP, FGF, Wnt, and retinoic acid signaling

What does regression of Hensen’s node create?

A stem zone containing biopotent neuromesodermal progenitors

What is rostrocaudal positioning indicated by the expression of?

Hox A, B, C, D

What is dorsoventral patterning first induced by and where is it secreted from?

Sonic hedgehog (Shh) which is secreted by the notochord on the ventral side

What does ectoderm secrete on neural tube that causes patterning?

BMPs

What does the gradient of BMPs and Shh do?

The gradients dictates the morphogen exposure level, which patterns neuronal progenitor subtypes.

Interkinetic nuclear migration; how does this effect mitosis?

The nucleus of a cell periodically moves between the apical and basal surfaces of developing neuroepithelia. Mitosis only takes place at the ventricular surface.

Why is “self organization” a misnomer?

Researchers need to provide the right signals at the right time

Order the following processes: Astrogenesis, gliogenesis, and neurogenesis

Neurogenesis, astrogenesis, gliogenesis

What does the lens placode induce?

The optic vesicle to differentiate into pigmented versus neural retina and signals overlying ectoderm inducing corneal differentiation

What do BMPs and Wnts induce in neural crest cells?

Pax7, Snail2, Sox9, FoxD3

When are neural crest cells generated?

During closure of the neural tube all along the rostro caudal axis.

What does the cranial neural crest become?

Cranial facial mesenchyme (cartilage, bone, neurons, glia, and connective tissue)

What does the cardiac neural crest become?

Neurons, cartilage, muscular-connective tissue wall of large arteries, septum that separates pulmonary/ aorta tract

What does the trunk neural crest become?

Dorsal root ganglia containing sensory neurons, sympathetic ganglia, aorta nerve clusters, epidermal melanocytes

What does the vagal and sacral neural crest become?

Parasympathetic ganglia of gut

What is the purpose of growth cones in axons?

The growth cones can sense and respond to similar signals as migrating cells

What directs growth of lateral motor column of the dorsal limb?

Repelled by ephrin-A5

What directs growth of lateral motor column of the ventral limb?

Repelled by Semaphorin-3F

What directs growth of medial motor column?

Attracted to dermamyotome

What does the optic disk secrete during retinotectal patterning? What type of patterning does it induce?

Netrin-1 attracts axon growth in that direction

What repels axon growth on the inner surface of the retina?

Chondrotin sulfate

What does the gradient of ephrin ligands cause? What induces this gradient?

Dictates where various neurons from different regions of the retina will terminate. Induced by a gradient in retinoic acid signaling

What transcription factors induce the neural crest?

Pax7, Snail2, Sox9, FoxD3

How is the mesoderm formed?

Brachyury (T) and Tbx6

Chordamesoderm

Notochord; only present during early development

Paraxial mesoderm

Somites that produce connective tissues of the back

Intermediate mesoderm

Urogenital system (kidney, gonads …)

Lateral plate mesoderm

Organs of the circulatory system, body cavity linings, and pelvic and limb skeleton. Limb musculature is of somitic origin

Head mesoderm

Unsegmented anterial paraxial mesorder

What causes the four mesodermal subdivisions to form?

Specified by increasing amounts of BMPs signaling along mediolateral axis (exact mechanism is unknown)

What mesoderm subdivision is BMP4 expressed at higher levels?

Lateral mesoderm

Where is Foxf1 expressed during mediolateral patterning?

Lateral mesoderm

Where is Foxc1 and Foxc2 expressed during mediolateral patterning?

Paraxial mesoderm

What does the deletion of Foxc1 and Foxc2 do?

Changes specification from paraxial mesoderm to intermediate mesoderm.

What causes unsegmented (presomitic) mesoderm proximal to neural tube to undergo somite formation?

Repression of BMP signaling, which is induced by the regression of the primitive streak

What first induces BMP signaling?

Primitive streak formation

What does the regression of Hensen’s node secrete?

Noggin

What is Noggin an antagonist for?

BMPs

What are the 5 components of somitogenesis?

1. Periodicity

2. Fissure Formation

3. Epithelialization

4. Specification

5. Differentiation

How is periodicity important to somitogenesis?

Somite generation is on a consistent cycle once the primitive streak begins to regress.

What is the ‘clock’ involved in somitogenesis regulated by?

Negative feed back loops in Notch

High levels of Fgf8 and Wnt repress expression of Delta, the ligand for Notch, this inhibits the cycle.

Retinoic acid with inhibit the Fgf8 and Wnts.

What causes a fissure in somitogenesis?

Mesp2 induced EphA4 in posterior border cells, this induces ephrin B2 in the cells across the border, and the EphB4/ephrinB2 repulsion creates a fissure.

How does epithelialization occur during somitogenesis?

Ectodermal signals act on GTPases in border cells to initiate a mesenchymal to epithelial transition, which is stabilized by N-cadherin expression. This causes epithelial cells to be on the exterior and mesenchymal cells to be in the interior.

What is specification of somites occur?

Specification along the rostral-caudal axis is governed by Hox Transcription factors. The Hox gene expression is present prior to somite formation, and once defined cannot be re-specified. Somites that form neck vertebrae cannot form ribs.

How does differentiation of somites occur?

Paracrine factors from the neural tube, notochord, epidermis, and intermediate mesoderm will influence adjacent somite regions.

Sclerotome

Ventral—medial somite cells closest to the neural tube and notochord under go epithelial to mesenchymal transition (EMT); become cartilage cells of vertebrae and major part of each rib

Dermamyotome

Remaining epithelial portion (other than sclerotome) that produces myoblasts in lateral lip portions.

Dermatome

Part of dermamyotome that cells form back dermis

Primaxial dermamytome

Myoblasts form intercostal muscles between ribs and deep muscles of the back.

Abaxial dermamyotome

Myoblasts form muscles of the body wall, limbs, and tongue

What happens to progenitors from the central dermatome that delaminate from the epithelial plate?

They join the primary myotome cells

What cells progeny account for the majority of myoblast cells that differentiate to form muscle?

Progenitors from the central dermatome that delaminate

Satellite cells

Myoblast cells that remain undifferentiated; stem cells found in adult muscles (very rare and hard to find)

Where do all skeletal musculature come from (except head)?

Dermamyotome

What does Wnt signalling in both the primaxial and abaxial dermamyotome induce transcription of?

MyoD and Myf5

What does the expression of Pax3 and Pax7 by satellite cells in the central part of the dermamyotome do?

Represses the expression of MyoD, which prevents muscle differentiation

Lack of what factors indicates that myoblasts are post-mitotic?

FGF and Shh-signaling

How do myoblasts align to form multinucleated myotubes?

Myoblasts must be post-mitotic and then they secrete fibronectin into the ECM and engage with one another through integrins. The alignment is mediated by cadherins.

What do myoblasts competent to fuse express?

Myogenin

Why do nascent myotubes secrete IL-4?

To recruit other myoblasts for further fusion and growth

Where do satellite cells sit in adults?

On the basal lamina surrounding the myotube.

What does the lateral plate mesoderm become?

Limb skeleton

Endochondral Ossification

Undergone by mesenchymal cells, where they first generate a cartilage intermediate that is then replaced by bone

Steps of osteogenesis

1. Mesenchymal cells commit to cartilage lineage

2. Cells condense into compact nodules and differentiate into chondrocytes

3. Proliferation and secretion of cartilage-specific ECM

4. Cells become post-mitotic and hypertrophic

5. Hypertrophic chondrocytes die and are replaced by osteoblasts brought along with invading vasculature

What causes Mesenchymal cells commit to cartilage lineage?

In response to Shh they express Pax1

What factor causes mesenchyme to condense during osteogenesis?

BMPs induce N-cadherin and Sox9 expression which in turn induces collagen type 2 and aggrecan (both found in ECM of collagen)

What allows chondrocytes to become post-mitotic and hypertrophic during osteogenesis?

Runx2

What transcription factor induces osteoblast differentiation?

Osterix

What part of the sclerotome becomes the syndetome?

The most dorsal part

Syndetome

Generates tendon forming cells that express scleraxis gene

What causes the syndetome cells to form?

Fgf8 secretion by myotome cells

What blocks scleraxis in sclerotomal cells?

Expression of Sox5 and 6 during to Shh signaling

Where does lateral plate mesoderm form?

Peripheral sides of the intermediate mesoderm as a single plate.

What does the lateral plate mesoderm split into ?

Splits horizontally into somatic mesoderm and splanchnic mesoderm.

Somatic mesoderm location

Top part of lateral plate mesoderm that underlies the ectoderm

Splanchnic mesoderm location

Lower layer of lateral plate mesoderm; overlies the endoderm

Where is the coelom? What does it become?

Spans from neck to torso and becomes the body cavity

What does the heart arise from?

Splanchnic mesoderm

What does the first heart field form?

The initial scaffolding and LV

What does the second heart field form?

Adds to both ends of the heart tube and produces the atria, aorta, and pulmonary arteries on one end and RV, PV, and vena cava on the other end

What inhibits heart formation during early gastrulating mesenchyme?

Neural tube secretes Wnts

What does BMPs and FGF8 secretion from anterior endoderm induce?

Cardiogenic mesoderm

What is required for activation of heart specific genes?

GATA4, Nkx2.5, Mesp1