2009 Lecture 5

From Embryology
Revision as of 11:53, 9 August 2009 by S8600021 (talk | contribs)

Mesoderm Development

Carnegie stage 9 showing somite formation
Carnegie stage 9 scanning electron microscope image showing somite formation

]]

Introduction

We have seen the following processes during early human development so far: fertilization and blastocyst development in the first week, implantation in the second week, early placentation and bilaminar to trilaminar in the third week. In the third to fourth week we will now follow the development of the trilaminar embryo as each layer begins to differentiate into the primordia of different tissues within the embryo. From this point onward the lectures will not be in a strict timeline format as we will have to follow each layer (ectoderm, mesoderm, endoderm) forward through its early development, and then jump back to discuss the next layer.

This lecture will look at mesoderm development and formation of the body cavities.

Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.

Coelom, meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The intraembryonic coelom is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities (pericardial, pleural, peritoneal).

Objectives

  • Understanding of events during the third week of development
  • Understanding the process of notochord formation
  • Understanding the process of early somite development
  • Understanding the process of body cavity formation
  • Understanding the future fate of mesoderm components
  • Brief understanding of early heart formation

UNSW Embryology Links

Lecture Summary

The following text is extracted and modified from 2008 lecture slides and should be used as a "trigger" to remind you of key concepts.

Lecture Overview - Mesoderm

  • Axial mesoderm - notochord
  • Paraxial mesoderm- somites
  • Intermediate mesoderm
  • Lateral Plate mesoderm
    • Somatic, Splanchnic
  • Cavity is called a Coelom (Intraembryonic, ?Extraembryonic)
    • Intraembryonic Coelom forms in lateral plate mesoderm and forms 3 major body cavities
  • partitioning and segmentation
  • adult products, axes
    • MH - Limb, Heart, Bone, connective tissue and later development will be covered in future lectures
  • UNSW Embryology - 2008 Lecture Slides http://embryology.med.unsw.edu.au/Science/ANAT2341lecture06.htm | Week 3 Development http://embryology.med.unsw.edu.au/Notes/week3.htm | Week 4 Movies http://embryology.med.unsw.edu.au/Movies/week4.htm | Skeletal Muscle Development http://embryology.med.unsw.edu.au/Notes/skmus.htm
  • Mesenchyme
    • Embryonic connective tissue
    • Describes the cell morphology
    • Histology - not epithelial
  • Mesoderm
    • generated from epiblast cells migrating through the primitive streak
    • epiblast cells expressing fibroblast growth factor (FGF2)
    • forms a layer between ectoderm and endoderm with notochord down midline
    • present before neural tube formation
  • divides initially into 3 components
    • Paraxial
    • Intermediate
    • Lateral plate
  • Intermediate Mesoderm
    • lies between paraxial and lateral mesoderm
    • generates urogenital system
    • Wolffian duct, kidney
    • MH - covered in Kidney Development Lecture/Laboratory
  • Lateral Plate Mesoderm
    • divides into 2 parts at about day 18-19
  • Intraembryonic coelom
    • forms 3 body cavities- pericardial, pleural, peritoneal
  • Splanchnic
    • heart and smooth muscle of GIT and blood vessels
  • Somatic
    • body wall osteogenic, chrondrogenic and fibrogenic
    • except ribs and scapula
  • Paraxial Mesoderm
    • lies adjacent to notochord
    • Forms 2 components
      • Head - unsegmented paraxial mesoderm
      • Body - segmented paraxial mesoderm
    • Generates trunk muscles, skeleton, dermis of skin, blood vessels, connective tissue
  • Segmented Paraxial Mesoderm
    • segments called somites
    • first pair of somites (day 20)
    • segmentation imposes a pattern on
    • nerves, vasculature, vertebra....
    • somites appear in ordered sequence cranial to caudal
    • appearance so regular used to stage the embryo
    • Hamburger & Hamilton 1951- chicken
  • thought to be generated by a "clock" (1 pair every 90 minutes)
  • neural tube begins to close at 4th somite level
  • 44 pairs of somites
  • Somite Formation
    • Somite formation - Chick embryo
    • Somitomere to Somite
    • Chicken Stages -regular appearance of somites allowed early experimenters to accurately stage the embryo
    • Advantages - accessible, easy to manipulate, limb grafts/removal, chimeras, developmental processes
  • Chicken
    • taxon-Gallus gallus
    • develops and hatches in 20-21 days
    • Fertilized eggs easily maintained in humidified incubators
    • Embryo Staged growth
    • Series of Embryonic Chicken Growth
    • Hamburger & Hamilton J. Morphology, 88 49 - 92, 1951
    • [1]
  • See also UNSW Embryology - Other Embryos
  • Zebrafish, Xenopus
  • Paraxial Segmentation
  • Stage 13/14 Embryo
  • Somites
    • ball forms through epithelialization and interactions (cell-cell, cell-extracellular matrix, ECM) fibronectin, laminin
    • has 2 populations of cells
      • peripheral columnar
      • central mesenchymal
    • early somite has cavity- somitocoel
      • cavity is lost
      • will contribute to sclerotome
  • somite enclosed by ECM connected to nearby tissues
  • Somite Specification
    • Different segmental level somites have to generate different segmental body structures?
    • somite has to form different tissues?
  • Segmental Plate Mesoderm
  • Somite Differentiation
  • Compartmentalization accompanied by altered patterns of expression of Pax genes within the somite
  • forms 2 main components
    • ventral- sclerotome
      • forms vertebral body and intervertebral disc
    • dorsal- dermomyotome
      • forms dermis and skeletal muscle
  • Somite Axial Specification
    • rostro-caudal axis appears regulated by Pax/Hox expression
    • family of DNA binding transcription factors
  • Movie: Somite Development
  • Sclerotome
    • sclerotome later becomes subdivided
    • rostral and caudal halves separated laterally by von Ebner's fissure
      • half somites contribute to a single vertebral level body
      • other half intervertebral disc
    • therefore final vertebral segmentation ‚Äúshifts‚Äù
  • Muscle
    • MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed
    • MyoD myoblast determining transcription factor
    • basic Helix Loop Helix
    • form myotome
  • Myotome component of Somite
    • epaxial myotome (dorsomedial quarter)
      • dorsal epimere (erector spinae)
    • hypaxial myotome (dorsolateral quarter)
      • ventral hypomere
      • 3 primary muscle layers
    • different at neck, thorax and abdomen
  • Chick Embryo Mesoderm
  • Body Musculature - Myotome derivatives-mouse embryo
  • Lateral Plate Mesoderm
  • Limb Musculature
  • Dermomyotome- Muscle (MyoD)
  • MyoD Pax 3
  • Somite Differentiation
    • migrating neural crest cells enter cranial half, will form dorsal root ganglia (DRG)
    • sclerotome bulges ventro-medially towards notochord, then surround and engulf notochord
      • not movement of sclerotome, growth of surrounding tissues
      • notochord forming nucleus pulposus of IVD
  • Dermomyotome
    • lateral myotome edge migrates at level of limbs
    • upper limb first then lower
    • mixes with somatic mesoderm
    • dermotome continues to contribute cells to myotome
  • Muscle Development Abnormalities
    • Duchenne Muscular Dystrophy
      • Embryonic muscle development normal and changes occur postnatally
      • X-linked dystrophy, large gene encoding cytoskeletal protein - Dystrophin
      • progressive wasting of muscle, die late teens
    • Becker Muscular Dystrophy, milder form, adult onset
  • Axial Segmentation
  • Somite Specification Signals
  • Mesoderm Movies

References

Textbooks

  • The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Mesoderm Ch15,16: p405-423, 426-430 Body Cavities Ch9: p174-184
  • Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Mesoderm Ch11 p311-339 Body Cavities Ch6 p127-146

Additional Textbooks

  • Before We Are Born (5th ed.) Moore and Persaud Ch16,17: p379-397, 399-405
  • Essentials of Human Embryology Larson Ch11 p207-228
  • Human Embryology Fitzgerald and Fitzgerald Body Cavities Ch5 p29-32, Ch7 p47,48
  • Human Embryology and Developmental Biology ?Carlson Ch9,10: p173-193, 209-222 Body Cavities Ch5 p29-32, Ch7 p47,48

Online Textbooks

Search

UNSW Embryology Links

Glossary Links

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Next Lecture

Lecture 6 | Course Timetable

Dr Mark Hill 2009 UNSW CRICOS Provider Code No. 00098G