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DEVELOPMENT OF THE VERTEBRATE BODY PLAN
Thomas A. Marino, Ph.D. Temple University School of Medicine
DEVELOPMENT OF THE VERTEBRATE BODY PLAN
Early Development 1. Development of Ectoderm A. Neural Tube B. Surface Ectoderm 2. Development of Endoderm A. G.I. Tract B. Respiratory Tree C. Pharynx 3. Development of Mesoderm A. Paraxial B. Intermediate
Summary of Week 2
Blastocyst
Trophoblast
Inner Cell Mass
Cytotrophoblast SyncyQotrophoblast
Epiblast
Hypoblast Extraembryonic endoderm
Yolk Sac Endoderm
Amnioblasts
Embryonic Epiblast
Embryonic Ectoderm
PrimiQve Streak
Embryonic Endoderm
Notochordal Process
Notochord
MesodermExtraembryonic mesoderm
Embryonic Mesoderm
Trilaminar Embryo
Ectoderm
Endoderm
mesoderm
Notochord
So by day 18 of gestation, gastrulation is nearing completion and the trilaminar embryo will begin: 1.Neurulation 2.Lateral body folding 3.Head and tail folding !The trilaminar embryo has three layers: 1.Ectoderm 2.Mesoderm 3.Endoderm
Trilaminar Embryo
paraxial mesoderm
lateral mesoderm
intermediate mesoderm
paraxial mesodermAs the third week of gestation is coming to an end, the mesoderm layer begins to subdivide into three masses: 1.Paraxial mesoderm
1. This will become the somites. 2.Intermediate mesoderm.
1. This will give rise to much of the urogenital system.
2.Lateral plate mesoderm. 1. This will become
1. Lateral plate splanchnic mesoderm
2. Lateral plate somatic mesoderm
Ectoderm
Notochord
Neural Plate
Neurulation begins with the notochord cells migrating around the primitive node.
Ectoderm
Notochord
Neural Plate
As the notochord cells migrate in a cephalic direction, the overlying ectoderm cells will begin to differentiate into neural ectoderm or the neural plate.
Ectoderm
Notochord
Neural Plate
Prechordal Plate
Oral Plate
The notochord and the neural plate cells will grow cephalically. 1.This growth will stop in the region of the prechordal plate. 2.Cephalic to the prechordal plate is the oral plate.
1. The oral plate is ectoderm and endoderm without intervening mesoderm.
2. The oral plate demarcates the separation between the prospective oral cavity and the developing gut tube.
Neurula'on – how does neural ectoderm develop?
!•BMP gradients lead to formaQon of intermediate and lateral mesoderm.
!•Chordin, Noggin and FollistaQn form in cranial paraxial mesoderm. They bind to and inacQvate BMP.
!•They are present in notochord and paraxial mesoderm and induce ectoderm to become neural plate.
!•WNT3a and FGF do this in the caudal paraxial mesoderm.
!•Without BMP inacQvaQon the epidermal phenotype is
Chordin Noggin
Follistatin !!!FGF
WNT3a
Neural crest forma'on
Neural crest cells form in response to
!!
Intermediate levels of BMP
!!
This leads to PAX3 expression
+
FOXD3 expression is necessary for neural crest formaQon
And
SLUG expression allows for to neural crest migraQon
Chordin Noggin
Follistatin !!!FGF
WNT3a
BMP
Bone morphogene'c protein signalling and vertebrate nervous system developmentAimin Liu & Lee A. NiswanderNature Reviews Neuroscience 6, 945-‐954 (December 2005)
The CNS arises from a specialized epithelium, the neural plate (1). This process relies on the inhibiQon of bone morphogeneQc protein (BMP) signaling. Folding of the neural plate to produce the neural groove is triggered by the formaQon of a disQnct hinge point in the ventral region (the floor plate; 2). At the end of neurulaQon, the lateral edges of the neural plate fuse (3) and segregate from the non-‐neural epithelium to form a neural tube (4). The roof plate and floor plate form at the dorsal and ventral midline of the neural tube, respecQvely. The roof plate becomes a new organizing centre that produces BMPs, which provide dorsal paaerning informaQon. Neural crest cells derive from the dorsal neural tube and migrate out to form the PNS, as well as melanocytes and carQlage in the head. Neural crest cells have been shown to form at an intermediate level of BMP signaling.
Ectoderm
Neural Plate
Neural Groove
The next stage of neurulaQon in the formaQon of the neural tube. The lateral edges of the neural plate will form neural folds and a midline neural groove appears.
Fig. 1 (A) Successive images showing the progression of neural tube closure in a stylized vertebrate embryo (rostral = up).
J B Wallingford et al. Science 2013;339:1222002
Published by AAAS
Fig. 3 Multiple cell behaviors contribute to neural tube morphogenesis. Multiple cell behaviors contribute to neural tube morphogenesis. In this schematic, pink and green cells illustrate convergent extension. By exchanging neighbors specifically in the mediolateral (horizontal) axis, the sheet of cells is elongated in the anteroposterior (vertical) axis. Blue cells illustrate apical constriction. These cells do not move but rather change their shape, leading to a bend in the tissue sheet. [Schematic adapted from (7)]
J B Wallingford et al. Science 2013;339:1222002
Published by AAAS
Neural Groove Neural Fold Somites
Ectoderm
Day 20
As the third week comes to an end the neural folds become prominent and they approach one another in the cervical region.
Ectoderm
Neural Tube
Pericardial Bulge
Somites
The neural tube is first formed in the cervical region. It then conQnues to form in a cephalic and caudal direcQon.
Ectoderm
Cranial Neuropore
Caudal Neuropore
Somites
Pericardial Bulge
The last two areas to fuse are the cranial neuropore cephalically and the caudal neuropore caudally.
UNSW EmbryologyThis link takes you to the University of New South Wales embryology website where you can see an image of an embryo at 22 days of gestaQon. NeurulaQon is nearly complete.
Lateral Body Folding
• As neurulaQon occurs the embryo begins lateral body folding.
Lateral Body Folding
• Lateral body folding involves: – The ectoderm at either end meeQng in the ventral midline.
– The endoderm forming the gut tube.
Lateral Body Folding• The lateral plate mesoderm will split into somaQc and splanchnic mesoderm.
• In between the two the body cavity will form. • This space on both sides will also fuse in the midline.
Body cavity
Somatic mesoderm Splanchnic mesoderm
Lateral Body Folding• As the space between the lateral plate somaQc and splanchnic mesoderm enlarges the lateral body folding conQnues.
Body cavity
23
Lateral Body Folding
• The amnioQc cavity will also fold around the embryo.
Gut Tube
Body cavity
Amniotic Cavity
Lateral Body Folding• The two ends of the ectoderm, lateral plate somaQc mesoderm, lateral plate splanchnic mesoderm and the endoderm conQnue to migrate ventrally.
Lateral Body Folding
• The amnioQc cavity and the body cavity also migrate ventrally.
Lateral Body Folding
Gut Tube
Somatic mesoderm Splanchnic mesoderm
Body cavity
• Now the amniotic cavity surround the embryo which is surrounded by ectoderm.
• The fused body cavities are surrounded by lateral plate somatic mesoderm deep to the ectoderm
• The lateral plate splanchnic mesoderm surround the gut tube which is a fused tube of endoderm
Lateral Body Folding
Neural Tube
Paraxial mesoderm Intemediate mesodermSurface ectoderm
AmnioQc Cavity
Body Cavity
Chorionic Cavity
Gut Tube
The embryo is surrounded by the chorionic cavity which surrounds the amniotic cavity.
Lateral Body Folding
This is a very nice animaQon of lateral body folding that will give you a beaer three dimensional sense of the process. However this require the flash plugin.
UNSW Embryology
• This is an image from the UNSW embryology website. It shows a 23 – 26 day old embryo (week 6 LMP). The neural tube is fusing with embryo is undergoing lateral body folding.
• Another image can be see by clicking here.
Ectoderm
Endoderm
mesoderm
Surface Ectoderm
epidermis, hair, nails, cutaneous and mammary glands, anterior pituitary gland, enamel of teeth, inner ear, and lens
Neural Crest:
cranial and sensory ganglia and nerves, medulla of adrenal gland, pigment cells, branchial arch cartilages, head mesenchyme
Neural Tube
central nervous system, retina, pineal body, posterior pituitary
Ectoderm then develops into three main categories: surface ectoderm, neural crest derivatives and neural tube derivatives.
Endoderm
• ConQnuing the discussion of lateral body folding and adding head and tail folding The development of the endoderm can be considered as it develops into: – Pharynx – GI tract – Respiratory system – Caudal urogenital system structures
Endoderm
Ager: – Lateral Body Folding – Head and Tail Folding
The endoderm consists of: – Foregut – Midgut – Hindgut
EndodermIf a sagittal section of the embryo is made and looked at from the side one can see the ectoderm (blue), mesoderm (red) and endoderm (yellow).
Endoderm
Yolk Sac
Amniotic Cavity
Chorionic Cavity
• Dorsal to the ectoderm is the amniotic cavity and ventral to the endoderm is the yolk sac.
• The chorionic cavity surrounds these structures. • The connecting stalk connects the embryo to the placenta. • The cephalic end of the embryo is defined by the oral plate.
Connecting stalk
Oral plate
Endoderm• The cephalic end of the embryo grows the fastest and as it does the brain moves
into a cephalic position and move the oral plate ventrally and caudally. • This leaves two cul-de-sacs at either end of the gut tube called the foregut and
hindgut.
brain
Oral plate
foreguthindgut
Endoderm
Yolk Sac
Oral Plate
Cloacal Plate
foregut
midgut
hindgut
AllantoisHeart
The gut tube begins at the oral plate, starts as the foregut, continues as the midgut which is still continuous with the yolk sac and then becomes the hindgut ending as the cloacal plate.
ENDODERM
Chorionic
Cavity
Amnio'c
cavity
Yolk Sac
Uterine Cavity
AllantoisA diverticulum from the connecting stalk called the allantois connects to the hindgut and this fusion region is now called the cloaca.
Cloaca
Head
Tail
Back
Connecting S
talk
Foregut
Midgut
Hindgut
Allantois
Vitelline Stalk
The gut tube is divided into three regions: 1.Foregut (yellow) 2.Midgut
1. Cephalic end (green) 2. Caudal end (blue)
3.Hindgut (purple) 1. Cloaca
Cloaca
EndodermEctoderm
Endoderm
mesoderm
Pharynx: epithelial parts of: pharynx thyroid tympanic cavity tonsils, parathyroids
Respiratory: epithelial parts of: trachea bronchi lungs
G.I.: epithelium of G.I. tract liver, pancreas !Caudal UG system: urinary bladder urethra
The endoderm gives rise to four major components: 1.Pharynx 2.Respiratory epithelium 3.GI tract and associated glands. 4.Caudal urogenital system
GastrulaQon
Notochord
Endoderm
Ectoderm
Yolk Sac
Amnio'c CavityParaxial mesoderm !Intermediate mesoderm !Lateral plate mesoderm
The mesoderm initially is subdivided into three masses:
1. paraxial mesoderm 2. intermediate mesoderm 3. lateral plate mesoderm
Mesoderm
• The first part of the mesoderm to examine is the lateral plate mesoderm and specifically the Splanchnic mesoderm
42
!!Lateral plate mesoderm: SomaQc !!Splanchnic
Mesoderm
Blood Islands
Oral plate
Looking down on the embryo, the lateral plate splanchnic mesoderm begins to see a proliferation of cells that form blood islands. !These blood island are forming in the lateral mesoderm and continue cephalically located cephalic to the oral plate.
Blood Islands !!!Endocardial Heart Tube
The blood islands consist of angiogenic cell clusters that will become cells that will form the:
1. endothelium of blood vessels. 2. heart tube. 3. blood cells.
MesodermThree sites of early blood island formaQon: – cardiogenic area
– yolk sac – chorion and connecQng stalk
Connecting stalk
Cardiogenic area
yolk sac
Chorion
Mesoderm Endocardial heart tube
Take a cross section in the plane of the dotted line, and looking at the section from the caudal region, shows the location of the endocardial heart tubes in the splanchnic mesoderm.
MesodermThe following images demonstrate that during lateral body folding the endocardial heart tubes will come together to form the heart tube.
Endocardial heart tube
Endocardial heart tube
The endocardial heart tubes will come toward the midline. !They will be continuous with newly forming dorsal aortae.
Dorsal Aortae
By day 21the ends of the heart tube have fused and form the heart tube. It is located ventral to the gut tube. The first region to fuse is the prospective ventricular region. !At this point the heart starts to beat.
Mesoderm
Body Cavity
AmnioQc Cavity
Heart
Foregut Dorsal Aorta
The fused heart tube consists of the endocardium (red) the, myocardium from primary and secondary heart fields (green stipple) and intervening cardiac mesenchyme (jelly) in green.
!• UNSW Embryology - This is a scanning electron
microscope image of the fused heart tube. !
• UNSW Embryology 1 - This is a series of images showing the fusion of the heart tube.
Mesoderm
NotochordEndoderm
Ectoderm
Yolk Sac
Amnio'c CavityParaxial mesoderm !Intermediate mesoderm !Lateral plate mesoderm
The medial mass of mesoderm is called the paraxial mesoderm and later the somite.
Paraxial mesoderm ->Somite
Dermatome Myotome
Sclerotome
The somite will become subdivided into three components: 1. Dermatome - the connective tissue of the skin. 2. Myotome - skeletal muscle cells. 3. Sclerotome - give rise to cartilage and bone of ribs and vertebrae
SHHScleretome (PAX1)
WNT
PAX3
Dermomyotome
Initially the dermatome and myotome are fused with the dermatome in between the two masses of myotome. Signals from the notochord and neural tube influence the developmental fate of the the somite.
MYF5
Back (epaxial) muscles
MYOD
Dermis
Body wall and Extremity Muscles
The medial mass of myotome cells will become the muscles of the back. The lateral mass of myotome cells will become the muscle of the body wall and extremities.
Mesoderm
• Intermediate mesoderm will develop into components of the urinary and reproducQve systems.
56
Yolk Sac
Amnio'c Cavity
Intermediate mesoderm
Development of the Kidneys
• By day 23 intermediate mesoderm (orange and navy) is identified lateral to the paraxial mesoderm (red).
• Intermediate mesoderm is organized into: • pronephros • mesonephrose • metanephros
Mesoderm• The intermediate mesoderm gives rise to: – Kidney tubules (blue) – Ducts of the urinary and reproducQve systems
– Gonads – The gonads form from epithelium covering the intermediate mesoderm (green) and migraQng primordial germ cells. 58
Kidney tubules
DuctsBody epithelium
Primordial germ cells
Mesoderm
Intermediate Mesoderm
Urogenital system including :
1. Kidneys 2. Gonads, 3. Ducts, 4. Accessory glands
Paraxial Mesoderm
1. Skeletal muscles, 2. Skeleton (except skull) 3. Dermis of skin 4. Connective tissue
Lateral Mesoderm
1. Connective tissue of viscera. 2. Serous membranes of:
A. pleura, B. pericardium C. peritoneum
3. Blood and lymph cells 4. Cardiovascular system 5. Lymphatic system
• Early Embryology: Where are we now and where are we going. !
• View the CNN Report
60
Beginning of last menstrual period
Ovarian follicle matures
Day 0
Day 0 Proliferative phase of menstrual cycle
Ovulation Day 14
Day 1 Secretory phase of menstrual cycle.
Fertilization Day 15
Day 6 - 7 Implantation Blastocyst Day 20 - 21
Day 14 Primary villi in the placenta
Bilaminar disk Day 28
Day 15 First menstrual period missed
Gastrulation Begins Day 29
Timing of pregnancyEmbryology/ Gestational Age Clinical Age
Pregnancy loss
• Approximately 30% of the fertilized eggs are carried successfully.
• Of the 70% that are unsuccessful almost 1/3 are lost prior to implantation.
• About 40% of postimplantation pregnancies abort spontaneously,
• Clinically only 10 - 15% are observed.
Pregnancy loss• Studies on aborted material demonstrates 50 -
60% have chromosomal anomalies. • Very early losses closer to 70% • Higher spontaneous loss in older women. • Other reasons for loss:
– Genital tract abnormalities. – Infections – Endocrine and metabolic anomalies – Hematologic and immune disorders
In one month In six months In one year
Early 20's 25% 75% 94%
Late20's/early30's 15% 38-47% 70-85%
Late30's 10% 22-24% 65-70%
Chances of Conception*
* from iVillageHealth.com.
Average Time to Conception*
No. of months
Early 20's 4-5
Late 20's 5-7
Early 30's 7-10
Late 30's 10-12
* from iVillageHealth.com.
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