Exam 4- Lateral Plate Mesoderm (CV system/Limbs)

what specifies lateral plate mesoderm

high levels of BMP signaling

2 layers of lateral plate mesoderm

somatic and splanchnic, separated by coelom

somatic mesoderm

external layer, forms body wall and bones of limbs

splanchnic mesoderm

internal layer, forms cardiovascular system and mesoderm of digestive system

heart forms from

splanchnic lateral mesoderm at anterior of embryo

heart development steps

specification of heart field

migration of heart fields into thorax

fusion of heart fields across midline to form single heart

cell differentiation and formation of simple tube heart

further development of mammalian heart

heart field

population of lateral mesodermal cells capable of forming heart tissue

heart field specification

cardiac crescent forms in anterior splanchnic mesoderm, cells competent to form all heart specific cell typeshe

heart field specification signals

specified by high BMP, Wnt inhibited by Cerberus from AVE, activates NKx2- 5 and mesp 1

heart field differentiation

Knx2-5 and mesp1 master regulators of cardiac development

Conservation of heart development

BMP to Nkx2-5 to Gata, Mcf2, and Hand

heart field migration to thorax

migrates as 2 groups, one on left and one on right, guided by fibronectin gradient in endoderm ECM

fusion of heart fields across midline

lateral folding brings heart field closer but not sufficient for fusing, additional cell migration and signaling interactions implicated in fusion

disruption of fusion across midline

cardiac bifida, 2 heart phenotype

gene involved in fusion of heart across midline

Miles apart encodes gene that regulates interactions between fibronectin and receptors, FoxP4 TF expressed in endoderm

development of early heart

develops as tube with single central lumen, cell lining lumen differentiate into endocardium, external cells differentiate into myocardium, cardiomyocytes begin coordinated contractions immediately after differentiation

further heart development in mammals

must be patterned along A-P and L-R axes for control of blood flow and appropriate attachments, single lumen subdivided into 4 chambers, atrial and ventricular septums form, heart undergoes looping

A-P patterning of heart

posterior high levels of RA specify atria, repress TBX5

anterior lower levels of RA specify ventricles, activate TBX5

L-R patterning of heart

Pitx2 asymmetrically localized on left side of body, activates Hand TFs in heart, Hand1 patterns left, Hand2 patterns right

Vitamin A defficiency

regulates RA signaling, affects A-P patterning, atria differentiation and formation affected

Septa of heart

form to separate chambers, disruptions cause congenital heart defects, can be caused by Nkx2-5 mutations/Tbx5 mutations/neural crest defects

neural crest contributions to septa of heart

form wall between pulmonary artery and aorta, small contribution to ventricular septum where it connects to arteries

truncus arteriosis

incomplete separation between aorta and pulmonary artery

limb development requires cooperative interactions between these 2 tissues

somatic lateral mesoderm, ectoderm

tetrapod limb

must grow outward, patterned along 3 axes to form distinct functional structures

anterior posterior limb patterning

thumb to pinky, big toe to pinky toe

proximal to distal limb patterning

shoulder to finger, ankle to toes

dorsal to ventral limb patterning

back of hand to palm, top of foot to sole

steps of limb development

establish 4 limb fields

distinguish forelimbs from hindlimbs

outgrowth from limb fields to produce limb buds

patterning of limb along 3 axes

development of structures of mature limb

specification of limb fields

regions of lateral mesoderm capable of initiating limb growth, determined by hox gene expression

hox gene expressed in anterior limb field

Hox4/5 and Tbx5

hox gene expression in posterior limb field

Hox9/10 and Tbx4

gene expression within limb field

hox genes activate Tbx, activate FGF, induce FGF expression in overlying ectoderm

does ectopic expression of Tbx4 convert forelimb to hindlimb

no, not sufficient

specification of hindlimbs in mammals

Tbx4 and Pitx1

holt oram syndrome

mutations in Tbx5 affect forelimb development, defects in arms and heartf

formation of limb buds

driven by AER, FGF diffuses back to mesoderm, stimulates cell proliferation and outgrowth of limb bud

apical ectodermal ridge

FGF producing ectodermal regions that form signaling centers

mechanisms of limb outgrowth

proliferation of limb field lateral plate mesoderm

attraction of migrating myoblasts into limb bud

proliferation of what part of the mesoderm drives limb outgrowth

distal

mechanism of proximal-distal limb patterning

driven by AER, regulated by RA and FGF gradients, RA and Meis1/2proximal, FGF distal

3 distinct structures along P-D axis

stylopod, zeugopod, autopod

stylopod

specified by Meis1/2

zeugopod and autopod

specified by hox genes

disruption of P-D patterning

mutations in Hox13 cause brachydactyly, short digits

A-P limb patterning

most pronounced in digits, patterning by zone of polarizing activity/Shh

ZPA

Shh, necessary and sufficient for ZPA function

activation of Shh in posterior limb bud to form ZPA

FGF8 from AER activates Shh expression in ZPA, requires activation by posteriorly expressed hox genes

AER and ZPA have a ___ signaling relationship

reciprocal

how Shh patterns A-P axis

diffuses away from ZPA as traditional morphogen

Shh autocrine signal that regulates Shh expressing cells

autocrine Shh signal

some ZPA cells stay posterior some move away

digit 1

thumb, no Shh exposure

Digit 2

low Shh concentration

Digit 3

low Shh concentration, brief Shh expression

Digit 4

more extended Shh expression

Digit 5

pinky, continuous Shh expression

Shh regulates ____ signaling

BMP

how BMP gradient patterns digits on A-P axis

BMP in interdigit mesoderm regulates digit differentiation, more BMP results in posterior fates

where is the ZPA located

mesoderm

role of apoptosis in limb development

occurs in several locations, BMP signaling activates programmed cell death in interdigit mesoderm

defects in apoptosis

inhibition of BMP blocks cell death, tissue retention

syndactyly

defects in interdigit apoptosis, linked to defects in Hh pathway

later steps on limb development

bone and muscle formation, innervation, vascularization

bones form from

lateral mesoderm, develop via endochondral ossification

skeletal muscles develop from

migrating myoblasts from somitic myotome

nerves and ganglia form from

neural crest cells

blood vessels are derived from

splanchnic lateral mesoderm, extend into limbs via angiogenesis

evolution of limb development

loss of distal ray fins, expansion of skeletal elements, patterning along P-D axis, reduction of digit number

how fins become limbs

change in expression of P-D patterning genes

reductions in digit number

decreased for better grips, linked to hox gene expression and Shh pathway

webbed feet for swimming

increased Gremlin expression inhibits BMP

bat forelimbs modified for flight

increased length of digits, maintains interdigit tissue

bird forelimbs modified for flight

decrease number of digits, altered structure of digits, growth of feathers

loss of limbs

mutation in enhancer that regulates Shh expression causes loss of Shh expression in ZPA, AER absent, outgrowth stopped