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Exam 4 Study Guide Outline
I.
ORGANOGENESIS: PARAXIAL AND INTERMEDIATE MESODERM
a. STUDENT LEARNING OBJECTIVES:
i. Understand the MESODERM forms all of the ORGANS between ECTODERMAL WALL and
ENDODERMAL TISSUES.
ii. Understand the PARAXIAL MESODERM form structures at the BACK of the embryo surrounding the
spinal cord, including the somites, and the derivative cartilage, bone, muscle, and dermis.
iii. Understand the INTERMEDIATE MESODERM forms the structures of the UROGENITAL TRACT,
including the kidneys, gonads, ductwork, and the adrenal cortex.
iv. Understand that the MESODERM helps both the ECTODERM and the ENDODERM form their own
tissues.
b. FIGURE 11.2: The Mesoderm forms during GASTRULATION and NEURULATION, same as Ectoderm
i. During gastrulation the mesodermal layer between the endoderm and ectoderm is formed
ii. The formation of the mesodermal and endodermal tissues occurs synchronously to the neural tube
formation
iii. The notochord extends beneath the neural tube, from the base of the head to the tail
iv. On either side of the neural tube are THICK BANDS OF PARAXIAL MESODERM called ;
1. SEGMENTAL PLATE ( in chick embryos)
2. UNSEGMENTED MESODERM ( in other vertebrate embryos)
v. The cells of the paraxial mesoderm will form SOMITES as the primitive streak regresses and neural
folds begin to gather.
c. FIGURE 11.1 (PART 1): More Lineages of the Mesoderm
i. 4 REGIONS OF THE TRUNK MESODERM:
1. Chordamesoderm:
a. Central region of the trunk mesoderm
b. Cells in this region forms the NOTOCHORD
2. Paraxial Mesoderm:
a. Flanking both sides of the notochord
b. *Cells in this region form SOMITES
i. 3 MAJOR COMPARTMENTS OF SOMITES
1. Sclerotome
a. Forms the vertebrae + rib cartilage
2. Myotome
a. Forms the musculature of the back, rib cage, and ventral
body
3. Dermamyotome
a. Contains dermal cells, which generate the dermis of the
back
b. Contains skeletal muscle progenitor cells that migrate into
the limbs (limb muscles).
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ii. SMALLER COMPARTMENTS FORMED BY 3 MAJOR SOMITE COMPARTMENTS
1. Syndetome
a. The most dorsal sclerotome cells
b. Generates the tendons
2. Arthrotome
a. The most internal sclerotome cells
b. Generates the vertebral joints/disc + proximal ribs
3. Unnamed
a. The most posterior sclerotome cells
b. Generates dorsal aorta + intervertebral arteries
3. Intermediate Mesoderm
a. Further away from the notochord
b. Forms the urogenital system, which consist of the kidneys + gonads
4. Lateral Plate Mesoderm
a. Farthest away from the notochord
b. Forms the heart, blood vessels, and blood cells, lining of body cavities, and all
mesodermal compartments (except muscles)
c. Forms embryonic membranes
d. Figure 11.8 (Part 2): Paraxial mesoderm is made up of head mesoderm and somites
i. Head mesoderm is anterior to the trunk mesoderm
ii. Head mesoderm consist of unsegmented paraxial mesoderm + prechordal mesoderm
iii. Head mesoderm region forms the muscles and connective tissues of the head and eyes
1. Forms under the direction of transcription factors
iv. Suffers different disease states
e. SOMITOGENESIS  Formation of Somites
i. General Overview:
1. First somites appear in the anterior portion of the trunk.
2. New somites “bud off” from the rostral end of the presomitic mesoderm
3. Somite formation begins as paraxial mesoderm become somitomeres
4. Somitomeres become compacted + split apart as fissures separate
them into immature somites
5. Mesenchymal cells that make up the immature somites change:
a. Outer cells join the epithelium layer
b. Inner cells remain mesenchymal
6. Number of somites is an indicator of how far development has occurred
ii. Periodicity:
1. Periodic formation of somites is inherited to the cells of the mesoderm
2. Somite formation depends clock and wave mechanism
a. Notch and Wnt signals like a clock
b. FGF signals sweep rostral-to-caudal in a wave, which sets the somite boundaries
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3. DELTA NOTCH ARE EXPRESSED AT PRESUMPTIVE BOUNDARIES:
a. When a small group of cells from a region at the posterior border at the
presumptive somite boundary is transplanted into a region of unsegmented
mesoderm, a non-boundary is created.
i. Cells instruct the cells anterior to them to epithelialize and separate
ii. Usually, non-boundary cells don’t induce border formation, BUT it can if
active NOTCH protein is electroporated into those cells.
b. NOTCH has been implicated in SOMITE FISSIONING by MUTATION
i. Segmentation defects are mutant for components of the notch pathway.
ii. Components include Notch proteins and its ligand Delta proteins
iii. Mutations affecting the notch signaling are responsible for aberrant
vertebral formation.
c. NOTCH signaling leads to the expression of Notch genes in the posterior region of
the forming somites, which is anterior to the fissure.
i. Notch gene is transcribed in a cyclic fashion and function as an autonomous
segmentation.
d. NOTCH CONTROLS THE WAVELIKE EXPRESSION OF hairy 1
i. Where Notch is expressed Hairy -1 stays on long term
ii. The posterior edge is the edge that signals separation
iii. HOW:
1. The entire process of 1 somite formation takes 90mins
2. The posterior portion of the chick embryo somite (S1) buds off from
the presomitic mesoderm.
3. Expression of the hairy 1 gene is in the caudal half of this somite,
the posterior portion of the presomitic mesoderm, and a thin band
next to caudal half of the next somite.
4. Caudal fissure separates the new somite from the presomitic
mesoderm.
5. The posterior region of hairy 1 extends anteriorly
6. The newly formed somite is now called S1, which retains the
expression of hairy 1 in its caudal half
7. Posterior domain of hairy 1 moves anteriorly and shortens
8. Former S1 is called S2, which undergoes differentiation
9. Formation of S1 is completed
iii. FISSURE FORMATION
1. This the separation from unsegmented mesoderm
2. The FGF wavefront sets up an oscillation in Wnt and Notch signaling as it passes
3. The Notch expression gives final position of Hairy-1
4. Hairy-1 causes EPHRIN expression which repels neighbors
iv. EPITHELIALIZATION OF SOMITES
1. The same posterior edge start mesenchymal to epithelial transition:
2. HOW:
a. Occurs immediately after somatic fission
b. Cells of the new somites are randomly organized as a mesenchymal mass
c. Cells are compacted into an outer epithelium and internal mesenchyme
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d. Ecotodermal signals cause peripheral somatic cells to undergo mesenchymal to
epithelial transition
3. N-cadherin, rho family , and actin change involvement:
a. Epithelialization of each somite is stabilized by synthesis of extracellular matrix
protein fibronectin and adhesion protein N-cadherin
b. N-cadherin links the adjoining cells into an epithelium, while fibronectin matrix acts
alongside the Ephrin and Eph to promote the separation of the somites from each
other.
v. SPECIFICATION OF PARAXIAL MESODERM
1. Though somites look identical, they will form different structures
2. Somites are specified due to Hox genes they express
3. Hox genes are active in the segmental plate mesoderm before it becomes organized into
somites
4. Example:
a. Ribs are generated only by the somites forming the thoracic vertebrae
b. Specification of the thoracic vertebrae occur early in development
c. Segmental plate mesoderm is determined by its position in the anterior-posterior
axis before somitogenesis.
d. If one isolates the region of chick segmental plate that will give rise to ta thoracic
somite and transplants this mesoderm into the cervical region of a younger embryo.
e. The host embryo will develop ribs in its neck.
vi. DETERMINATION AND DIFFERENTIATION IN SOMITES
1. All of the cells of the somite are competent to form all the derivative cell types:
a. Cartilage, bone, muscle, tendons, dermis, vascular cells, meninges
2. Their fate depends on their position near the neural tube, notochord, epidermis and
intermediate mesoderm
3. THE FIRST STEP:
a. Involves the induction of sclerotome
b. Paracrine factors from the surrounding tissues (neural tube, notochord, epidermis,
and intermediate mesoderm) influence the regions of the somite adjacent to them.
c. As somites mature, the regions become committed to forming certain cell types.
d. Ventral-medial cells of the somite undergo mitosis, lose their round epithelial
characteristics, and become mesenchymal cells again.
e. Portion of these somites give rise to the sclerotome
i. They become vertebral cartilage
f. The remaining portion leaves dermamyotome epithelium
4. THE SECOND STEP:
a. Involves the segregation of the dermamyotome
b. Central and bilateral myotome surrounds the dermatome
c. The 2 lateral portions of this epithelium are myotomes , which will form muscle cells
d. The central region, the dermatome, forms the back dermis, brown fat.
e. Laterteralmyotomes, muscle precursor cells migrate beneath the dermamyotome
and produce a lower layer of muscle precursor cells called myoblast
f. Myoblast in the myotome close to the neural tube form the primaxial muscles:
i. Includes intercostal muscles
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f.
g. Myoblast further from the neural tube produce the abaxial muscles:
i. Adominal muscles, tongue, limbs
h. The central portion proliferates madly and makes most cells
5. PRIMAXIAL AND ABAXIAL DOMAINS FO VERTEBRATE MESODERM:
a. The boundary between the primaxial and abaxial muscles and between the somite
derived and lateral plate derived dermis is called the lateral somatic frontier
MECHANISMS OF TISSUE FORMATION FROM SOMITES
i. WHAT IT INVOLVES:
1. Myogenesis: Muscle formation
2. Osteogenesis: Bone formation
3. Vascualar Replacement in the Dorsal Aorta
4. The Syndetome: Tendon Formation
ii. MYOGENEIS:
1. WHAT IT INVOLVES:
a. The paraxial, abaxial and central somite
b. Cells in the center that give rise to satelie cells
i. Maybe stem cells, maybe committed progenitors
ii. Remain viable for the life of the organism
iii. Exit cell cycle upon injury and differentiate to muscle
c. Classical skeletal muscle differentiation
i. Paracrine signals induce MyoD, Myf-5
ii. TFs for muscle genes and for themselves
2. FIGURE 11.15 :
a. Adult muscle cells (myotubes) are large and multinucleated
i. Keep in mind: muscle satilite cells don’t express MyoD until injury
b. HOW MUSCLE FUSION OCCURS:
i. This is the conversion of myoblast into muscles in culture
ii. The determination of myotome cells are due to paracrine factors
iii. Committed myoblasts divide in the presence of growth factors FGFs, but
show no obvious muscle-specific proteins.
iv. When growth factors are used up, the myoblasts cease dividing, align, and
fuse myotubes contract.
c. IN CULTURE:
i. Muscle fusion will occur in culture, no matter what species.
iii. OSTEOGENESIS
1. WHAT IT INVOLVES:
a. Four different sources of bone:
i. Somites form the axial skeleton
ii. Lateral plate mesoderm form the limb skeleton
iii. Cranial neural crest forms the bones of face and head
iv. Mesodermal mesenchyme in patella, periosteum
b. Two different processes:
i. Endochondrial ossification in the first 2
ii. Intramembraneous ossification in the second 2
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2. FIGURE 11.16: ENDOCHONDRAL OSSIFICATION
a. Endochondrial means “within cartilage”
b. Involves the formation of vertebrae ribs, pelvis, and limbs
c. STEPS:
i. Mesenchymal cells commit to becoming cartilage cells (chondrocytes)
ii. Committed mesenchyme condenses into compact Nodules
iii. Nodules differentiate into chondrocytes and proliferate to form the
cartilage model of bone.
iv. Chondrocytes undergo hypertrophy and apoptosis while they change and
mineralize their ECM.
v. Apoptosis of chondrocytes allows blood vessels to enter
vi. Blood vessels bring in osteoblast, which bind to the degenerating
cartilaginous matrix and deposit bone matrix.
vii. Bone formation and growth consist of ordered arrays of proliferating,
hypertrophic, and mineralizing chondrocytes.
viii. Secondary ossification centers also form as blood vessels enter near the tips
of the bone.
d. FIGURE 11.18:Skeletal mineralization in 19-day chick embryo
i. Mineralized ECM for bone differentiation is important
ii. It uses calcium carbonate of the egg’s shell as its calcium source
iii. The circulatory sys of the chick embryo translocates120 mg of calcium form
the shell to the skeleton
iv. When chick is removed for their shells and grown in plastic wrap, the
cartilaginous skeleton fails to mature to bony tissue.
e. FIGURE 11.19: Endochondrial Ossification of Vertebrae
i. Sclerotome mesenchyme are attracted by notochord and neural tube
secretions
ii. As motor axons extend toward muscles through sclerotome and split them
rostral-to caudal
iii. The caudal end of one then recombines with the rostral end of the nest to
form the bone model and then bone
3. VASCULAR REPLACEMENT IN THE DORSAL AORTA
a. Blood vessels are a sinlge layer of endothelium surrounded by multiple layers of
smooth muscle
b. The dorsal (or descending) aorta forms a primary model by vasuculaogenesis and
then both the endothelium and smooth muscle are replaced by somite.
i. Same occurs with the ascending aorta by neural crest cells
c. HOW THIS OCCURS:
i. At an early stage, the primary dorsal aorta is of the lateral plate origin. A
subpopulation of sclerotome cells becomes specified by NOTCH in the
posterior half of somites as endothelial precursors.
ii. Chrmoattractantsmafe in the primary dorsal aorta cause these cells to
migrate through the somite to the aorta.
iii. The sclerotome cells take up residence in the dorsal region of the vessel
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iv. These cells spread along the anterior-posterior and dorsal-ventral axes,
ultimately occupying the entire region of the aorta.
v. The primary aortic endothelial cells become blood cell precursors.
4. THE SYNDETOME: TENDON FORMATION
a. Tendon joins bone to muscle. The last row of sclerotome is induced by the overlying
myotome to differentiate into those connectors
b. UNDERSTAND HOW IT OCCURS: Use Figure 11.22
i. The dermatome, myotome, and sclerotome are established before the
tendon precursors are specified. Tendon precursors (syndetome) are
specified in the dorsalmost tier of sclerotome cells by FgF8 received from
the myotome.
ii. Pathway by which FgF8 from the muscles precursor cells induce the
subjacent sclerotome cells to become tendons.
iii. Sundetome cells mugrate along the developing vertebrae. They
differentiate into tendons that connect the ribs to the intercostal muscles.
iv. FORMATION OF THE KIDNEYS FROM INTERMEDIATE MESODERM
1. WHAT IT INVOLVES
a. The adult kidney is very complex
i. A single nephrom has 10,000 cells, 12 cell types
ii. Each is positioned exactly for its job relative to others
b. The embryo increasingly needs to filter blood
i. IM mesoderm 1st forms the organizer, the pronephric duct
ii. This tissure then induces three stages of kidney
iii. The first 2 are transitory, third persists
2. FIGURE 11.23: General scheme of development in the vertebrate kidney
a. Nephric duct is the primitive organizer: Wolffian Duct
b. Pronephros is functional in fish, amphibians, not in mammals, then degenerates
c. Mesonephros is functional in some mammals, including humans, degenerates in
females, forms epididymous and vas deferens
3. FIGURE 11.25: Metanephros formed by reciprocal induction with Wolffian Duct
a. HOW IT OCCURS:
i. As the embryo develops anterior-posterior
ii. There is an increase need for filtration
iii. Wolffian duct forms
iv. Buds out forms Pronephros
v. Pronephros degrades anteriorly
vi. Forms a new one posteriorly
vii. Forms mesonephros
viii. A bud from the IM mesoderm mesenchyme interplay
ix. Recruits mesenchyme, and stimulates it to bud
x. This forms renal side of the kidney
xi. Develop complex structures needs to be filled with blood vessels
xii. Metamehpros degrades.
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II.
ORGANOGENESIS : LATERAL PLATE MESODERM
a. FIGURE 12.1: Lateral Plate Mesoderm
i. Terminology:
1. SOMATOPLEURE:
a. Somatic mesoderm plus ectoderm
2. SPLANCHNOPLEURE
a. Splanchnic mesoderm plus endoderm
3. COELOM:
a. Body cavity that forms between them
b. COELOM:
i. Eventually left and right cavities fuse into one
ii. Runs from neck to anus in vertebrates
iii. Portioned off by folds of somatic mesoderm
1. Pleural cavity: Surrounds the thorax and lings
2. Pericardial Cavity: Surrounds the heart
3. Peritoneal Cavity: Surrounds the abdominal organs
c. FIGURE 12.1: Mesodermal development in frog and chick embryos
i. HOW A CHICK IS MADE INTO A FROG:
1. Neurula-stage frog embryos, have progressive development of the mesoderm and coelom.
2. When the chick embryo is separated from its yolk mass, it resembles the amphibian neurula
at a similar stage
d. HEART DEVELOPMENT
i. The heart is the first organ to function in the embryo and the circulatory system is the first
functional system.
1. HeartArteriesCapillariesVeinsHeart
e. Before the embryo can get very big it must switch from nutrient diffusion to active nutrient transport.
f. FIGURE 12.8: HEART DEVELOPMENTS
i. Understand HOW the following formations occur:
1. Tube formation
2. Looping
3. Chamber formation
g. Heart development: TUBE FORMATION
i. FIGURE 12.2:
1. The presumptive heart cells are specified but not determined in the epiblast
2. These cells migrate together near the node
3. The outflow forming cells migrate first, then the inflow forming cells migrate second.
ii. FIGURE 12.4:
1. The cardiogenic mesoderm migrates out of the mesodermal layer tpwards the endoderm to
form endocardial tubes on either side
2. At the same time the endoderm is folding inward
3. The endoderm continues folding inward until it forms its own tube, which drags the two
endocardial primordial close to each other
a. The endocardial tubes are surrounded by myocardial progenitors
b. When the endocardial tubes get close enough, they fuse together
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iii. FIGURE 12.5
1. If you mess with endoderm migration or signaling, you end up with 2 hearts
iv. HEART TUBE CELL BIOLOGY
1. Splanchnic mesoderm cells express cadherins and form anepithelial sheet for their inward
migration – MET
2. The presumptive endocardial cells undergo EMT to migrateaway from the sheet and
another MET to form tubes
3. The cells in the original mesodermal sheet form the myocardium
4. The myocardial epithelium fuses first and the two endocardialtubes exist together inside for
a while before fusing
5. Both the rostral end (outflow) and caudal end (inflow) remain asunfused double tubes
6. The heart beat starts spontaneously as myocardial cells express
7. the sodium-calcium pump - before fusion is even complete
h. HEART DEVELOPMENT: LOOPING and CHAMBER FORMATION
i. HOW IT OCCURS:
1. Left-right asymmetry is due to Nodal and Pitx2
2. Looping requires:
a. Cytoskeletal rearrangement
b. Extracellular matrix remodeling
c. Asymmetric cell division
3. Heart valves keep the blood from flowing back into the chamber it was just ejected from
4. The septa separate the two atria and the two ventricles
5. The truncusarteriosis, or outflow tract, also becomes septated allowing one great artery to
flow from right ventricle to lings and the other from left ventricle to the body.
ii. VALVES:
1. TRICUSPID VALVE: is between the right atrium and right ventricle
2. PULMONARY or PULMONIC VALVE: is between the right ventricle and pulmonary artery
3. MITRAL VALVE: is between the left atrium and left ventricle
4. AROTIC VALVE: is between the left ventricle and aorta
iii. FIGURE 12.9: Formation of chambers and valves
1. In utero, the foramen ovale allows right left shunting of blood
2. HOW IT OCCURS:
a. Endocardial cushions form and fuse
b. Septa grow toward cushion
c. Valves form from myocardium
i. EMBRYONIC CIRCULARTORY SYSTEMS
i. All of the blood must circulate outside of the embryo for oxygenation
ii. Understand How this occurs:
1. Blood pumped through the dorsal aorta passes over the aortic arches and down into the
embryo.
2. Some blood leaves the embryo through the vitelline arteries and enters he yolk sac.
3. Nutrition and oxygen are absorbed, and the blood returns though the vitellineviens to reenter the heart through the sinus venosus.
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j.
REDIRECTION OF HUMAN BLOOD FLOW AT BIRTH
i. The expansion of air into the lungs causes pressure changes that redirect the flow of blood in the
newborn infant.
ii. The ductusarteriosus squeezes shut, breaking off the connection between the aorta and the
pulmonary artery, and the foramen ovale, which is a passageway between the left and right atria,
also closes.
iii. This way pulmonary circulation is separated from systemic circulation.
k. BLOOD VESSELS DEVELOPMENT
i. The vessels form independently of the heart
ii. They form for embryonic needs as much as adult
1. Must get nutrition before there is a GI tract
2. Must circulate oxygen before there are lungs
3. Must excrete waste before there are kidneys
4. They do these through links to extraembryonic membranes
iii. The vessels are constrained by evolution
1. Mammals still extend vessels to empty yolk sac
2. Birds and mammals also build six aortic arches as if we had gills, eventually settling on a
single arch
iv. The vessels adapt to the laws of fluid dynamics
1. Large vessels move fluid with low resistance
2. Diffusion requires small volumes and slow flow
3. Highly organized size variance controls volume
4. And superbranching smaller vessels control speed
v. UNDERSTAND AORTIC ARCHES OF THE HUMAN EMBRYO: USEFIGURE 12.13 AS A REFERENCE
vi. FIGURE 12.13: VASCULOGENESIS AND ANGIOGENESIS:
1. Vasculogenesis is the de novo differentiation of mesoderm into endothelium
2. If is followed by the endothelium recruiting smooth muscle cell coat
3. How this occurs:
a. Vasculogenesis involves the formation of blood islands and the construction of
capillary networks from them.
b. Angiogenesis involves the formation of new blood vessels by remodeling and
building on older ones.
c. Angiogenesis finishes the circulatory connections begun by vasculogenesis.
d. The major paracrine factors involved in each step are shown at the top of the
diagram, and their receptors are beneath them.
vii. FIGUE 12.15: VASCULOGENESIS
1. Start in the extraembryonic mesoderm as well as in the large embryonic blood vessels
2. Understand the process: use Figure 12.15 as a reference
viii. ACQUISITION OF A SMC LAYER OCCURS IN STAGES:
1. Recruitment of SMC progenitors
2. Condensation of early SMC
3. Final differentiation of intermediate SMC
4. Late SMC is formed
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ix. FIGURE 12.14 : ANGIOGENESIS
1. Angiogenesis is the growth and remodeling of the 1st vessels in response to blood flow and
tissue-derived recruitment signals
x. PROCESS OF VASCULAR DEVELOPMENT
1. Vasculogenesis
a. De novo endothelial differentiation
b. Tubular plexus formation
2. Establishment of blood flow
3. Angiogenesis
a. Extensionof existing endothelium
b. Remodeling of vessel size
c. Acquisition of a medial SMC layer
xi. SECONDARY VASCULOGENESIS
1. Proepicardial Organ (PEO) forms from splanchnic mesoderm overlaying the liver
2. PEO contacts the ventricle and migrates as epicardium
3. Subset of epicardial cells delaminate towards myocardium
4. These undergo MET to form cornonary endothelium
5. Coronary arterires then plug intot he aorta where nerves are
xii. KNOWN EMBRYONIC LINEAGES OF VASCULAR SMOOTH MUSCLES
1. Ectomesenchymal: neural crest-derived
2. Epimesenchymal: epicardium-derived
3. Mesomesenchymal: local mesoderm-derived
xiii. FIGURE 12.21: ARTERIES ALIGNED WITH PERIPHERAL NERVES
1. It is common phenomemnon for arteries and verves to form together
2. Veins and nerves do not follow one another
xiv. LYMPHATIC DRAINAGE FORMS FROM JUGULAR VEIN
1. Sprouts as lymphatic sacs by angiogenesis
2. Continues to form secondary drainage system
3. Major conduit for immune cells
l. WHERE DO THE HEMATOPOIETIC STEM CELLS OF THE ADULT BONE MARROW COME FROM?
i. Use figure 12.23 as a reference
ii. Splanic mesoderm of aorta-gonad-mesonephros (AGM) region in embryo
iii. Hemogenic endothelium form sclerotome
iv. Hemogenic endothelium from may sites
v. Understand the process:
m. DEVELOPMENT OF THE ENDODERM:
i. THE DIGECTIVE TUBE:
1. Anterior endoderm forms anterior intestinal portal
2. Posterior endoderm forms posterior intestinal portal
3. Midgut goes through expansion and contraction to yolk
4. Each end has ectodermal cap, then forms an entrance
ii. THE DERIVATES:
1. 4 pharyngeal pouches form head and nech structures
2. Floor between 4tth pair buds out to form respiratory tube
3. Gut tume forms esophagus, stomach, SI, LI, rectum
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iii.
iv.
v.
vi.
vii.
4. Gut tube buds out to form liver, gall bladder, pancreas
FIGURE 12.27: Formation of glandular primordial from the pharyngeal pouches
1. The cranial neural crest cells migrate through this endoderm and contribute component
structures around them
FIGURE 12.33: Partitioning of the foregut into the esophagus and respiratory diverticulum
1. Localized Wnt/B-Catenin and retinoic acid causes budding
a. Wnt signaling is also required to form the lungs from the digestive tract.
b. 2 Wnts in the earliest stages of lung development, cause the accumulation of Bcatenin in the region of the gut tube that will become the lung and trachea.
c. This allows the separation from the gut tube of the trachea and the allows the
development into the lungs
FIGURE 12.35: SUFACTANT
1. Normal-time birth is signaled from the lungs
2. The immune system relays a signal from the embryonic lung
3. Surfactant proteins activate macrophages in the amnionic fluid to migrate into the unterine
muscles, where the macrophages secrete IL 1B.
4. IL1B stimulates production of an enzyme that triggers the production of the prostaglandin
hormones responsible for initiation uterine muscle contractions and birth.
FIGURE 12.28: REGIONAL SPECIFICATION OF THE GUT ENDODERM AND SPLACHNIC MESODERM
1. The anterior-posterior specification of the gastrointestinal tract
a. Occurs through reciprocal interactions
b. Regional transcription factors are seen prior to interactions with the mesoderm, but
are not stabilized.
2. RECIPROCAL INDUCTION:
a. Simultaneous Anterior-Posterior specification of both endoderm and mesoderm
b. Shh in the endoderm
c. Reception of Shh in the mesoderm
d. Hox gene expression of the mesoderm
e. Specification of mesoderm
f. BMPs, FGFs, made by mesoderm
g. Reception of paracrine signals by endoderm
h. Specification and differentiation of endoderm
i. Paracrine factor in the endoderm
j. Differentiation of mesoderm
FIGURE 12.30: Positive and negative signaling in the formation of the hepatic endoderm
1. Mesoderm also induces liver bud
2. Dorsal endoderm: hepatogenesis blocked
3. Hepatic region of gut endoderm epressing a-fetal protein and albumin
4. HOW:
a. The ectoderm and notochord block the ability of the endoderm to express liverspecific genes.
b. The cardiogenic mesoderm, through FGFs promotes liver specific gene transcription
by blocking the inhibitory factors induced by the surrounding tissues
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viii. FIGURE 12.29 PANCREATIC DEVELOPMENT IN HUMANS
1. At 30 days, the ventral pancreatic bud is close to the liver primordium
2. By 35, days it begins migrating posteriorly and comes into contact with the dorsal pancreatic
bud during 6th week of development.
3. In most individuals, the dorsal pancreatic bud loses its duct into the duodenum, however, in
about 10% of the population, the dual duct system persist.
n. EXTRAEMBRYONIC MEMBRANES
i. What’s involved:
1. Adaptation for development on dry land
a. As the body starts to develop epithelium expands to isolate embryo within them
2. Four sets of extraembryonic membranes
a. Somatopleure forms amion and chorion
b. Splanchnopleure forms yolk sac and allantois
ii. FIGURE 12.36 Extraembryonic membranes of the chick
1. Somatopleure forms amnion and chorion
2. Splancnopleure forms yolk sac and allantois
3. The amnion folds up to cover the embryo and keep it from drying out
a. The cells of the amnion secrete water
4. The chorion surrounds the entire embryo and controls gas exchange
a. In birds and reptiles it lines shell
b. In mammals it forms the placenta
5. The yolk sac expands to surround yolk (even w/o one)
6. The allantoic membrane creates a space for waste storage
a. Birds and repiltes require it
b. We don’t use it for waste but it contributes to our umbilical cord.
III.
LATE DEVELOPMENT: DEVELOPMENT of the TETRAPOD
a. PATTERN FORMATION (How limb growth is regulated)
i. Limbs show an amazing aspect of development
ii. Similarities:
1. Architectural symmetry in all 4 limbs
2. Size symmetry in the opposite limb
3. Symmetry in growth rate and development
iii. Differences:
1. Forelimb-Hindlimb: rostral-caudal asymmetry
2. Mirror imagery in opposite limbs: left-right asymmetry
3. Structural polarity in 3 axes per limb:
a. Proximal-distal, Anterior-Posterior, Dorsal-Ventral
4. Pattern Formation occurs in 4 dimensions
b. FIGURE 13.1: SKELETAL PATTERN FORMATION
i. The pattern to muscles, tendons, cartilage, vessels, and nerves
ii. The digits are numbered 2,3,4.
iii. The cartilage condensations forming the digits appear similar to those forming digits 2,3,4 of mice
and humans.
c. FIGURE 13.18 Deletion of Limb Bone Elements by the Deletion of ParalogousHox Genes
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i.
ii.
iii.
iv.
Hox genes play a role in specifying the location where limbs will form
Hox genes also specify whether a mesenchymal cell will become stylopod, zeugopod, or autopod
Hox genes also specify the ZPA and the digits
DELETION of Limb Bones:
1. Hox9 and Hox10 specify the humerus (stylopod), Hox11 specify zeugopod, and Hox 12 and
Hox13 specify the autopod
2. Knocking out the genes will inhibit specifications of the limb
d. Molecular Specification of the Pattern
i. Morphogenetic Rules Cross Species:
1. Grafts of retile or mammal can induce avian limb
2. Frogs limb buds can pattern salamander limbs
3. Regeneration of salamander limbs follows the pattern
ii. Axis Formation is Driven by Specific Molecules:
1. Proximal-Distal axis governed by FGF FAMILY
2. Anterior-Posterior axis governed by Shh
3. Dorsal-Ventral axis governed by Wnt7a (mostly)
e. FIGURE 13.2: FORMATION of the LIMB FIELD in the MESODERM
i. Mesodermal cells give rise to a vertebrate limb
ii. The Limb field is the region that represents all cells in the area capable of forming a limb
iii. The center of the disc give rise to the Limb
iv. Adjacent to the disc forms Peribrachial Flank Tissue (back of arm) + Shoulder girdle (anterior end)
v. The black ring of cells will not contribute to the limb unless the other cells are lost
1. Normally, the black ring does not form the limb
2. Only if the cells (ii-iv) were removed, the black ring will contribute to form the limb
3. If the black ring were removed, no limb will develop
f.
FIGURE 13.3 (PART 1): EMERGENCE OF THE LIMB BUD
i. Bulge is due to proliferation and migration of somatic mesoderm for bone and abaxialmyotome for
muscle.
ii. How:
1. The proliferation of mesenchymal cells from the somatic region of the lateral plate
mesoderm accumulate under the ectoderm tissues and create a limb bud.
2. These cells causes the limb bud to bulged out and develop the skeletal elements of the limb.
a. In different vertebrates, the limb differ with respect to somite level it arises from
and the position with respect to the level of Hox gene expression.
3. The lateral plate mesoderm can induce the myoblast to migrate out of the somites and
enter the limb bud to form limb musculature.
g. FIGURE 13.4 :Multilimb Pacific Tree Frog (Hylaregila)
i. It is the result of infestation of the tadpole-stage developing limb buds by trematode cyst.
ii. Parasite infestation splits the “Limb Field”
iii. Becauseall cells in the “Limb field” are competent for all limb cells, multiple limbs develop
h. LIMB FORMATION Is a RECIPROCAL INDUCTION BETWEEN MESODERM and ECTODERM
i. PROXIMAL-DISTAL PATTERNING
1. As mesodermal mesenchyme forms the limb bud, it tells overlying ectoderm to form
APICAL ECTODERMAL RIDGE (AER)
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i.
j.
2. AER then directs changing limb development in nearby mesoderm called the
PROGRESS ZONE (PZ)
ii. HOW :
1. Mesenchyme cells enter the limb fiels and secrete Fgf10.
2. Fgf10 induces the overlaying ectoderm to form the AER.
3. AER runs along the distal margin of the limb bud and becomes major signaling center for
developing limb.
a. ROLES of the AER:
i. Maintains the mesenchyme beneath it in a proliferating state, which
enables the linear growth of the limb.
ii. Maintains the expression of molecules, which generate the anteriorposterior axis
iii. Interacts with the dorsal-ventral axis and anterior-posterior axis, so that
each cell is given instructions to differentiate.
4. The Proximal-distal growth and differentiation of the limb bud are due to the series of
interactions between AER and limb bud mesenchyme beneath it.
5. The PZ is the distal mesenchyme and its proliferative activity extends the limb bud.
FIGURE 13.11: THE AER is NECESSARY for WING DEVELOPMENT
i. Removal of the AER at progressively later times
ii. Images show how important the AER is
iii. If AER is removed from an early-stage wing bud, only a humerus forms.
iv. If AER is removed slightly later, humerus, radius, and ulna form.
ANTERIOR –POSTERIOR PATTERNING
i. FIGURE 13.15:
1. The ZONE OF POLARIZING ACTIVITY (ZPA) is a piece of mesoderm at the junction of the
young limb bud and the body wall
2. When a ZPA is grafted to anterior limb bud mesoderm, duplicated digits emerge as a mirror
image of the normal digits.
ii. HOW:
1. Anterior-posterior axis is specified by a small block of mesodermal tissue near the posterior
junction of the young limb bud and the body wall.
2. When tissues from a young limb bud is transplanted to a position on the anterior side of
another limb bud, the number of digits of the resulting wing is doubled.
3. These extra set of structures are mirror images of the normal structures.
4. The polarity has been maintained, but signals are coming from both anterior and posterior
directions
5. This regions of mesoderm is called ZONE OF POLARIZTING ACTIVITY
iii. FIGURE 13.16: Build a ZPA by making cells express Shh
1. Figure 13.16A:
a. The sites of Shh expression in the posterior mesoderm of the chick limb buds
b. Transplantation experiments define these regions as ZPA.
2. Figure 13.16B:
a. The association between ZPA and Shh.
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b. Secretion of Shh protein is sufficient for polarizing activity
c. Embryonic chick fibroblast were transfected with a viral vector containing the Shh
gene.
d. The gene become expressed, translated, and secreted in these fibroblast, which
were then inserted under the anterior ectoderm of an early chick bud.
e. Mirror-image digit duplication were the results.
iv. FIGURE 13.21: Concentration and Timing of Shh is Critical
1. HOW AND WHY?
a. Shh-secreting cells from digits 4 and 5 can contribute to the specification of digigts 2
and 3 in the mouse limb.
b. In early mouse hindlimb bud, the progenitors of digit 4 (green dot), and the
progenitors of digit 5 (red dot) are both in the ZPA and express Shh (light green).
c. At later stages, the cells forming digit 5 are still expressing shh it he ZPA, BUT cells
forming digit 4 no longer do,,
d. When digits are formed, cells in digit 5 will have seen high levels of Shh protein for a
loner time that the cells in digit 4.
2. Schematic:
a. Digits 4 and 5 are specified by the amount of time they were exposed to Shh in an
autocrine fashion.
b. Digit 3 is exposed to Shh both in an autocrine and paracrine fashion.
c. Digit 2 is specified by the concentration of Shh its cells received by paracrine
diffusion
d. Digit 1 is specified independently of Shh
v. FIGURE 13.22:
1. The Shh pattern is mage more intricate by response in the WEBBING
2. Secondary BMP signals makes all digits different
3. What happens:
a. Mechanism of Shh works to establish a digit’s identity involves BMP pathway
b. The identity of each digit is specified by the webbing between the digits.
c. That region of mesenchyme undergoes apoptosis
d. The interdigital tissue specifies the identity of the digit forming anteriorly to it.
i. When webbing was removed between digits 2 and 3, digit 2 changed to
structure of digit 1.
ii. When webbing was removed between digit 3 and 4, digit 3 formed digit 2
4. How it happens:
a. Webbing could be altered by changing BMP levels
b. Adding beads containing Noggin (a BMP anatagonist) in the webbing between digits
3 and 4, digit 3 transforms to digit 2
c. Adding beads containing Noggin (a BMP anatagonist) in the webbing between digits
1 and 2, digit 2 transforms to digit 1
k. DORSAL-VENTRAL PATTERNING
i. FIGURE 13.23:
1. The knuckles vs palm axis is determined by the overlaying ectoderm
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a. Because the dorsal-ventral polarity of the limb bud is determined by the ectoderm
encasing it.
2. Reverse the patterning, reverses the covering
ii. HOW:
1. Wnt7a gene is expressed in the dorsal ectoderm
2. Wnt7a induces activation of Lim1 gene in the dorsal mesenchyme
3. Lim1 gene encodes a transcription factor for specifying dorsal fates in the limb
iii. If there is a deficiency in Wnt7a:
1. The ventral tendons and footpads are duplicated on the dorsal suface.
l. FIGURE 13.26: PATTERNS of the CELL DEATH in LEG PRIMORDIA of DUCK and CHICK EMBRYOS
i. DUCK LEG PRIMORDIUM:
1. Minimal cell death
ii. CHICK LEG PRIMORDIUM:
1. Extensive cell death
a. After a certain stage, chick cells between the digits are programed to die
iii. HOW:
1. Cell death is genetically programed
2. The difference between the digits is the presence or absence of cell death between the
digits.
3. Signal for apoptosis is provided by BMP proteins
iv. IN ADDITION:
1. The ulna and radius are separated from each other by interior necrotic zone and anterior
and posterior necrotic zones
2. They further shape the ends of the limb
m. FIGURE 13.28 INHIBITION of CELL DEATH by INHIBITING BMPs
i. Usually, the interdigital regions of the chick feet undergo BMP-mediated apoptosis
ii. The webbing can be preserved is Germlin-soaked beads or BMP inhibitors are placed in the
interdigital regions
iii. This would inhibite BMP-mediated apoptosis in the interdigital regions
iv. Generating ducklike pattern
n. FIGURE 13.30 Tiktaalik
i. A fish with wrist and fingers, lived in shallow waters ~ 375 years ago
1. Have mobile wrist and a substrate supported stance that flex the elbow and shoulder.
ii. Joints are complex:
1. Ligaments
2. Tendons
3. Fibrous capsule
4. Lubrication
5. Immune System
iii. Movement and muscular force are required to form the joints
iv. HOW JOINTS ARE FORMED;
1. BMP proteins are made in the perichondrial cells surrounding the condensing chondrocytes
and promotes further cartilage formation.
2. The regions between the bones where BMP proteins are expressed will form the joints
3. Wnt proteins and blood vessels are also critical in joint formation
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a. Mesenchymal cells will not form nodules of cartilage-forming tissues in the presence
of blood vessels
b. WntprotienssustatinB-catenin and suppresses genes that characterize precartilage
cells