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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 Ectodermi. 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 tailiv. 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 Mesodermi. 4 REGIONS OF THE TRUNK MESODERM :

1. Chordamesoderm:a. Central region of the trunk mesodermb. Cells in this region forms the NOTOCHORD

2. Paraxial Mesoderm:a. Flanking both sides of the notochordb. *Cells in this region form SOMITES

i. 3 MAJOR COMPARTMENTS OF SOMITES 1. Sclerotome

a. Forms the vertebrae + rib cartilage2. Myotome

a. Forms the musculature of the back, rib cage, and ventral body

3. Dermamyotomea. Contains dermal cells, which generate the dermis of the

backb. 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 cellsb. Generates the tendons

2. Arthrotomea. The most internal sclerotome cellsb. Generates the vertebral joints/disc + proximal ribs

3. Unnamed a. The most posterior sclerotome cellsb. Generates dorsal aorta + intervertebral arteries

3. Intermediate Mesoderma. Further away from the notochordb. Forms the urogenital system, which consist of the kidneys + gonads

4. Lateral Plate Mesoderma. Farthest away from the notochordb. 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 somitesi. Head mesoderm is anterior to the trunk mesoderm

ii. Head mesoderm consist of unsegmented paraxial mesoderm + prechordal mesodermiii. Head mesoderm region forms the muscles and connective tissues of the head and eyes

1. Forms under the direction of transcription factorsiv. Suffers different disease states

e. SOMITOGENESIS Formation of Somitesi. 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 mesoderm3. Somite formation begins as paraxial mesoderm become somitomeres4. Somitomeres become compacted + split apart as fissures separate

them into immature somites5. Mesenchymal cells that make up the immature somites change:

a. Outer cells join the epithelium layerb. 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 mesoderm2. Somite formation depends clock and wave mechanism

a. Notch and Wnt signals like a clockb. 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 separateii. 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 1i. Where Notch is expressed Hairy -1 stays on long term

ii. The posterior edge is the edge that signals separationiii. HOW:

1. The entire process of 1 somite formation takes 90mins2. 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 anteriorly6. The newly formed somite is now called S1, which retains the

expression of hairy 1 in its caudal half7. Posterior domain of hairy 1 moves anteriorly and shortens8. Former S1 is called S2, which undergoes differentiation9. Formation of S1 is completed

iii. FISSURE FORMATION1. This the separation from unsegmented mesoderm2. The FGF wavefront sets up an oscillation in Wnt and Notch signaling as it passes3. The Notch expression gives final position of Hairy-14. Hairy-1 causes EPHRIN expression which repels neighbors

iv. EPITHELIALIZATION OF SOMITES1. The same posterior edge start mesenchymal to epithelial transition:2. HOW:

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a. Occurs immediately after somatic fissionb. Cells of the new somites are randomly organized as a mesenchymal massc. Cells are compacted into an outer epithelium and internal mesenchymed. Ecotodermal signals cause peripheral somatic cells to undergo mesenchymal to

epithelial transition3. 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 MESODERM1. Though somites look identical, they will form different structures2. Somites are specified due to Hox genes they express3. Hox genes are active in the segmental plate mesoderm before it becomes organized into

somites4. Example:

a. Ribs are generated only by the somites forming the thoracic vertebraeb. Specification of the thoracic vertebrae occur early in developmentc. 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 SOMITES1. All of the cells of the somite are competent to form all the derivative cell types:

a. Cartilage, bone, muscle, tendons, dermis, vascular cells, meninges2. Their fate depends on their position near the neural tube, notochord, epidermis and

intermediate mesoderm3. THE FIRST STEP:

a. Involves the induction of sclerotomeb. 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 cartilagef. The remaining portion leaves dermamyotome epithelium

4. THE SECOND STEP:a. Involves the segregation of the dermamyotomeb. Central and bilateral myotome surrounds the dermatomec. The 2 lateral portions of this epithelium are myotomes , which will form muscle cellsd. The central region, the dermatome, forms the back dermis, brown fat.

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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

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 cells5. 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

f. MECHANISMS OF TISSUE FORMATION FROM SOMITESi. WHAT IT INVOLVES:

1. Myogenesis: Muscle formation2. Osteogenesis: Bone formation3. Vascualar Replacement in the Dorsal Aorta4. The Syndetome: Tendon Formation

ii. MYOGENEIS:1. WHAT IT INVOLVES:

a. The paraxial, abaxial and central somiteb. Cells in the center that give rise to satelie cells

i. Maybe stem cells, maybe committed progenitorsii. Remain viable for the life of the organism

iii. Exit cell cycle upon injury and differentiate to musclec. Classical skeletal muscle differentiation

i. Paracrine signals induce MyoD, Myf-5ii. 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 injuryb. HOW MUSCLE FUSION OCCURS:

i. This is the conversion of myoblast into muscles in cultureii. 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. OSTEOGENESIS1. WHAT IT INVOLVES:

a. Four different sources of bone:i. Somites form the axial skeleton

ii. Lateral plate mesoderm form the limb skeletoniii. Cranial neural crest forms the bones of face and head

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iv. Mesodermal mesenchyme in patella, periosteumb. Two different processes:

i. Endochondrial ossification in the first 2ii. Intramembraneous ossification in the second 2

2. FIGURE 11.16: ENDOCHONDRAL OSSIFICATIONa. Endochondrial means “within cartilage”b. Involves the formation of vertebrae ribs, pelvis, and limbsc. 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 entervi. 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 importantii. 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 Vertebraei. Sclerotome mesenchyme are attracted by notochord and neural tube

secretionsii. As motor axons extend toward muscles through sclerotome and split them

rostral-to caudaliii. The caudal end of one then recombines with the rostral end of the nest to

form the bone model and then bone3. 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 cellsc. HOW THIS OCCURS:

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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 vesseliv. 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 FORMATIONa. Tendon joins bone to muscle. The last row of sclerotome is induced by the overlying

myotome to differentiate into those connectorsb. 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 MESODERM1. WHAT IT INVOLVES

a. The adult kidney is very complexi. A single nephrom has 10,000 cells, 12 cell types

ii. Each is positioned exactly for its job relative to othersb. The embryo increasingly needs to filter blood

i. IM mesoderm 1st forms the organizer, the pronephric ductii. This tissure then induces three stages of kidney

iii. The first 2 are transitory, third persists2. FIGURE 11.23: General scheme of development in the vertebrate kidney

a. Nephric duct is the primitive organizer: Wolffian Ductb. Pronephros is functional in fish, amphibians, not in mammals, then degeneratesc. Mesonephros is functional in some mammals, including humans, degenerates in

females, forms epididymous and vas deferens3. 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 filtrationiii. Wolffian duct formsiv. Buds out forms Pronephrosv. Pronephros degrades anteriorly

vi. Forms a new one posteriorlyvii. Forms mesonephros

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viii. A bud from the IM mesoderm mesenchyme interplayix. Recruits mesenchyme, and stimulates it to budx. This forms renal side of the kidney

xi. Develop complex structures needs to be filled with blood vessels xii. Metamehpros degrades.

II. ORGANOGENESIS : LATERAL PLATE MESODERMa. FIGURE 12.1: Lateral Plate Mesoderm

i. Terminology:1. SOMATOPLEURE:

a. Somatic mesoderm plus ectoderm2. SPLANCHNOPLEURE

a. Splanchnic mesoderm plus endoderm3. COELOM:

a. Body cavity that forms between themb. COELOM:

i. Eventually left and right cavities fuse into oneii. Runs from neck to anus in vertebrates

iii. Portioned off by folds of somatic mesoderm1. Pleural cavity: Surrounds the thorax and lings2. Pericardial Cavity: Surrounds the heart3. Peritoneal Cavity: Surrounds the abdominal organs

c. FIGURE 12.1: Mesodermal development in frog and chick embryosi. 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 staged. HEART DEVELOPMENT

i. The heart is the first organ to function in the embryo and the circulatory system is the first functional system.

1. HeartArteriesCapillariesVeinsHearte. 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 formation2. Looping3. Chamber formation

g. Heart development: TUBE FORMATIONi. FIGURE 12.2:

1. The presumptive heart cells are specified but not determined in the epiblast2. These cells migrate together near the node3. The outflow forming cells migrate first, then the inflow forming cells migrate second.

ii. FIGURE 12.4:

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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 inward3. The endoderm continues folding inward until it forms its own tube, which drags the two

endocardial primordial close to each othera. The endocardial tubes are surrounded by myocardial progenitorsb. When the endocardial tubes get close enough, they fuse together

iii. FIGURE 12.51. If you mess with endoderm migration or signaling, you end up with 2 hearts

iv. HEART TUBE CELL BIOLOGY1. Splanchnic mesoderm cells express cadherins and form anepithelial sheet for their inward

migration – MET2. The presumptive endocardial cells undergo EMT to migrateaway from the sheet and

another MET to form tubes3. The cells in the original mesodermal sheet form the myocardium4. The myocardial epithelium fuses first and the two endocardialtubes exist together inside for

a while before fusing5. Both the rostral end (outflow) and caudal end (inflow) remain asunfused double tubes6. The heart beat starts spontaneously as myocardial cells express7. the sodium-calcium pump - before fusion is even complete

h. HEART DEVELOPMENT: LOOPING and CHAMBER FORMATIONi. HOW IT OCCURS:

1. Left-right asymmetry is due to Nodal and Pitx22. Looping requires:

a. Cytoskeletal rearrangementb. Extracellular matrix remodelingc. Asymmetric cell division

3. Heart valves keep the blood from flowing back into the chamber it was just ejected from4. The septa separate the two atria and the two ventricles5. 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 ventricle2. PULMONARY or PULMONIC VALVE : is between the right ventricle and pulmonary artery3. MITRAL VALVE : is between the left atrium and left ventricle4. AROTIC VALVE : is between the left ventricle and aorta

iii. FIGURE 12.9: Formation of chambers and valves1. In utero, the foramen ovale allows right left shunting of blood2. HOW IT OCCURS:

a. Endocardial cushions form and fuseb. Septa grow toward cushionc. Valves form from myocardium

i. EMBRYONIC CIRCULARTORY SYSTEMS

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i. All of the blood must circulate outside of the embryo for oxygenationii. 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 re-

enter the heart through the sinus venosus.

j. REDIRECTION OF HUMAN BLOOD FLOW AT BIRTHi. 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 heartii. They form for embryonic needs as much as adult

1. Must get nutrition before there is a GI tract2. Must circulate oxygen before there are lungs3. Must excrete waste before there are kidneys4. They do these through links to extraembryonic membranes

iii. The vessels are constrained by evolution1. Mammals still extend vessels to empty yolk sac2. Birds and mammals also build six aortic arches as if we had gills, eventually settling on a

single archiv. The vessels adapt to the laws of fluid dynamics

1. Large vessels move fluid with low resistance2. Diffusion requires small volumes and slow flow3. Highly organized size variance controls volume4. And superbranching smaller vessels control speed

v. UNDERSTAND AORTIC ARCHES OF THE HUMAN EMBRYO: USEFIGURE 12.13 AS A REFERENCEvi. FIGURE 12.13: VASCULOGENESIS AND ANGIOGENESIS:

1. Vasculogenesis is the de novo differentiation of mesoderm into endothelium2. If is followed by the endothelium recruiting smooth muscle cell coat3. 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.

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vii. FIGUE 12.15: VASCULOGENESIS1. Start in the extraembryonic mesoderm as well as in the large embryonic blood vessels2. Understand the process: use Figure 12.15 as a reference

viii. ACQUISITION OF A SMC LAYER OCCURS IN STAGES:1. Recruitment of SMC progenitors2. Condensation of early SMC3. Final differentiation of intermediate SMC4. Late SMC is formed

ix. FIGURE 12.14 : ANGIOGENESIS1. Angiogenesis is the growth and remodeling of the 1st vessels in response to blood flow and

tissue-derived recruitment signalsx. PROCESS OF VASCULAR DEVELOPMENT

1. Vasculogenesisa. De novo endothelial differentiationb. Tubular plexus formation

2. Establishment of blood flow3. Angiogenesis

a. Extensionof existing endotheliumb. Remodeling of vessel sizec. Acquisition of a medial SMC layer

xi. SECONDARY VASCULOGENESIS1. Proepicardial Organ (PEO) forms from splanchnic mesoderm overlaying the liver2. PEO contacts the ventricle and migrates as epicardium3. Subset of epicardial cells delaminate towards myocardium4. These undergo MET to form cornonary endothelium5. Coronary arterires then plug intot he aorta where nerves are

xii. KNOWN EMBRYONIC LINEAGES OF VASCULAR SMOOTH MUSCLES1. Ectomesenchymal : neural crest-derived2. Epimesenchymal : epicardium-derived3. Mesomesenchymal: local mesoderm-derived

xiii. FIGURE 12.21: ARTERIES ALIGNED WITH PERIPHERAL NERVES1. It is common phenomemnon for arteries and verves to form together2. Veins and nerves do not follow one another

xiv. LYMPHATIC DRAINAGE FORMS FROM JUGULAR VEIN1. Sprouts as lymphatic sacs by angiogenesis2. Continues to form secondary drainage system3. 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 embryoiii. Hemogenic endothelium form sclerotomeiv. Hemogenic endothelium from may sites

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v. Understand the process:m. DEVELOPMENT OF THE ENDODERM:

i. THE DIGECTIVE TUBE:1. Anterior endoderm forms anterior intestinal portal2. Posterior endoderm forms posterior intestinal portal3. 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 structures2. Floor between 4tth pair buds out to form respiratory tube3. Gut tume forms esophagus, stomach, SI, LI, rectum4. Gut tube buds out to form liver, gall bladder, pancreas

iii. FIGURE 12.27: Formation of glandular primordial from the pharyngeal pouches1. The cranial neural crest cells migrate through this endoderm and contribute component

structures around themiv. FIGURE 12.33: Partitioning of the foregut into the esophagus and respiratory diverticulum

1. Localized Wnt/B-Catenin and retinoic acid causes buddinga. 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 B-

catenin 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 lungsv. FIGURE 12.35: SUFACTANT

1. Normal-time birth is signaled from the lungs2. The immune system relays a signal from the embryonic lung3. 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.vi. FIGURE 12.28: REGIONAL SPECIFICATION OF THE GUT ENDODERM AND SPLACHNIC MESODERM

1. The anterior-posterior specification of the gastrointestinal tracta. Occurs through reciprocal interactionsb. 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 mesodermb. Shh in the endodermc. Reception of Shh in the mesodermd. Hox gene expression of the mesoderme. Specification of mesodermf. BMPs, FGFs, made by mesodermg. Reception of paracrine signals by endodermh. Specification and differentiation of endodermi. Paracrine factor in the endoderm

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j. Differentiation of mesoderm vii. FIGURE 12.30: Positive and negative signaling in the formation of the hepatic endoderm

1. Mesoderm also induces liver bud2. Dorsal endoderm: hepatogenesis blocked3. Hepatic region of gut endoderm epressing a-fetal protein and albumin4. HOW:

a. The ectoderm and notochord block the ability of the endoderm to express liver-specific genes.

b. The cardiogenic mesoderm, through FGFs promotes liver specific gene transcription by blocking the inhibitory factors induced by the surrounding tissues

viii. FIGURE 12.29 PANCREATIC DEVELOPMENT IN HUMANS1. At 30 days, the ventral pancreatic bud is close to the liver primordium2. 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 them2. Four sets of extraembryonic membranes

a. Somatopleure forms amion and chorionb. Splanchnopleure forms yolk sac and allantois

ii. FIGURE 12.36 Extraembryonic membranes of the chick1. Somatopleure forms amnion and chorion2. Splancnopleure forms yolk sac and allantois3. The amnion folds up to cover the embryo and keep it from drying out

a. The cells of the amnion secrete water4. 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 itb. We don’t use it for waste but it contributes to our umbilical cord.

III. LATE DEVELOPMENT: DEVELOPMENT of the TETRAPODa. PATTERN FORMATION (How limb growth is regulated)

i. Limbs show an amazing aspect of developmentii. Similarities:

1. Architectural symmetry in all 4 limbs2. Size symmetry in the opposite limb

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3. Symmetry in growth rate and developmentiii. Differences:

1. Forelimb-Hindlimb: rostral-caudal asymmetry2. Mirror imagery in opposite limbs: left-right asymmetry3. Structural polarity in 3 axes per limb:

a. Proximal-distal, Anterior-Posterior, Dorsal-Ventral4. Pattern Formation occurs in 4 dimensions

b. FIGURE 13.1: SKELETAL PATTERN FORMATIONi. 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

i. Hox genes play a role in specifying the location where limbs will formii. Hox genes also specify whether a mesenchymal cell will become stylopod, zeugopod, or autopod

iii. Hox genes also specify the ZPA and the digitsiv. 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 limbd. Molecular Specification of the Pattern

i. Morphogenetic Rules Cross Species:1. Grafts of retile or mammal can induce avian limb2. Frogs limb buds can pattern salamander limbs3. Regeneration of salamander limbs follows the pattern

ii. Axis Formation is Driven by Specific Molecules:1. Proximal-Distal axis governed by FGF FAMILY2. Anterior-Posterior axis governed by Shh3. Dorsal-Ventral axis governed by Wnt7a (mostly)

e. FIGURE 13.2: FORMATION of the LIMB FIELD in the MESODERMi. 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 limbiii. 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 limb2. Only if the cells (ii-iv) were removed, the black ring will contribute to form the limb3. If the black ring were removed, no limb will develop

f. FIGURE 13.3 (PART 1): EMERGENCE OF THE LIMB BUDi. Bulge is due to proliferation and migration of somatic mesoderm for bone and abaxialmyotome for

muscle.ii. How:

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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 developh. LIMB FORMATION Is a RECIPROCAL INDUCTION BETWEEN MESODERM and ECTODERM

i. PROXIMAL-DISTAL PATTERNING1. As mesodermal mesenchyme forms the limb bud, it tells overlying ectoderm to form

APICAL ECTODERMAL RIDGE (AER)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 anterior-posterior 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.i. FIGURE 13.11: THE AER is NECESSARY for WING DEVELOPMENT

i. Removal of the AER at progressively later timesii. 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.

j. ANTERIOR –POSTERIOR PATTERNINGi. 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

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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

directions5. This regions of mesoderm is called ZONE OF POLARIZTING ACTIVITY

iii. FIGURE 13.16: Build a ZPA by making cells express Shh1. Figure 13.16A:

a. The sites of Shh expression in the posterior mesoderm of the chick limb budsb. Transplantation experiments define these regions as ZPA.

2. Figure 13.16B:a. The association between ZPA and Shh.b. Secretion of Shh protein is sufficient for polarizing activityc. 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 Critical1. 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

diffusiond. Digit 1 is specified independently of Shh

v. FIGURE 13.22: 1. The Shh pattern is mage more intricate by response in the WEBBING2. Secondary BMP signals makes all digits different

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3. What happens:a. Mechanism of Shh works to establish a digit’s identity involves BMP pathwayb. The identity of each digit is specified by the webbing between the digits.c. That region of mesenchyme undergoes apoptosisd. 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 24. How it happens:

a. Webbing could be altered by changing BMP levelsb. Adding beads containing Noggin (a BMP anatagonist) in the webbing between digits

3 and 4, digit 3 transforms to digit 2c. Adding beads containing Noggin (a BMP anatagonist) in the webbing between digits

1 and 2, digit 2 transforms to digit 1

k. DORSAL-VENTRAL PATTERNINGi. FIGURE 13.23:

1. The knuckles vs palm axis is determined by the overlaying ectoderma. 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 mesenchyme3. 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 EMBRYOSi. DUCK LEG PRIMORDIUM:

1. Minimal cell deathii. CHICK LEG PRIMORDIUM:

1. Extensive cell deatha. After a certain stage, chick cells between the digits are programed to die

iii. HOW:1. Cell death is genetically programed2. 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 zones2. They further shape the ends of the limb

m. FIGURE 13.28 INHIBITION of CELL DEATH by INHIBITING BMPsi. Usually, the interdigital regions of the chick feet undergo BMP-mediated apoptosis

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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 regionsiv. Generating ducklike pattern

n. FIGURE 13.30 Tiktaaliki. 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. Ligaments2. Tendons3. Fibrous capsule4. Lubrication5. Immune System

iii. Movement and muscular force are required to form the jointsiv. 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 joints3. Wnt proteins and blood vessels are also critical in joint formation

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