Talk:Lecture - Mesoderm Development
Mesoderm Development
Introduction
File:Mesoderm_001_icon.jpg</wikiflv> | This animation shows the migration of mesoderm throughout the embryonic disc during gastrulation. |
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).
Mesenchyme
- Embryonic connective tissue, describes the cell morphology (Histology is not epithelial organization)
- epithelial to mesenchymal transitions
- mesenchymal to epithelial transitions
Intermediate Mesoderm
- lies between paraxial and lateral mesoderm
- generates urogenital system
- Wolffian duct, kidney
- MH - covered in Kidney Development Lecture/Laboratory
Lateral Plate Development
- lying at the surrounding edge of he embryonic disc
- a cavity begins in this week to form within the mesoderm itself
Intraembryonic Coelom
- small spaces (vacuoles) begin appearing within the lateral plate mesoderm
- small spaces enlarge forming a single cavity within the lateral plate mesoderm
- divides lateral plate mesoderm into 2 parts at about day 18-19
- this cavity is called the Intraembryonic Coelom
- coelom is a general term for a "cavity" and can lie within the embryo (intraembryonic) and outside the embryo (extraembryonic)
- later anatomical spaces within the embryo and fetus can also be described as coeloms
- when the embryonic disc folds the intraembryonic coelom will form all 3 major body cavities:
- Pericardial
- Pleural
- Peritoneal
Somatic mesoderm
The intraembryonic coelom divides the lateral plate into 2 portions
- closest to ectoderm
- body wall osteogenic, chrondrogenic and fibrogenic
- except ribs and scapula
Splanchnic mesoderm
- closest to endoderm
- heart, smooth muscle of GIT and blood vessels
Heart
- prechordal splanchnic mesderm forms cardiac mesoderm (week 4)
- splanchnic mesoderm lying above the notochord
- initially as a simple tube
- first organ to form in the embryo
Somite Formation
Early somite induction signals in the mouse |
- ball forms through epithelialization and interactions (cell-cell, cell-extracellular matrix, ECM) fibronectin, laminin
- has 2 populations of cells - peripheral columnar and central mesenchymal
- early somite has cavity- somitocoel, cavity is lost during growth
- 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?
- Somite Differentiation
- Compartmentalization accompanied by altered patterns of expression of Pax genes within the somite
Somite initially forms 2 main components
- ventromedial- sclerotome forms vertebral body and intervertebral disc
- dorsolateral - 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 ‚"shifts"
Dermomyotome
- later divides into dorsal dermatome and ventral myotome
- (MH - This topic of muscle and skeleton development will be covered in 2 later lectures Musculoskeletal Development and Limb Development)
- lateral myotome edge migrates at level of limbs
- upper limb first then lower
- mixes with somatic mesoderm
- dermotome continues to contribute cells to myotome
Myotome
- Myotome component of Somite
- epaxial myotome (dorsomedial quarter) forms the dorsal epimere (erector spinae)
- hypaxial myotome (dorsolateral quarter) forms the ventral hypomere, 3 primary muscle layers which are different at neck, thorax and abdomen
Muscle
- Myoblast determining transcription factor MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed
- basic Helix Loop Helix
- from 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
Chicken Model - Somite formation
- Chicken Stages -regular appearance of somites (somitomere to somite) allowed early experimenters to accurately stage the embryo Chicken Development
- Advantages - accessible, easy to manipulate, limb grafts/removal, chimeras, developmental processes
- 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 Hamburger Hamilton Stages
- Other vertebrate animal models - Fish (Zebrafish), Frog (Xenopus)
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
- mainly growth of surrounding tissues not movement of sclerotome
- notochord forms nucleus pulposus of intervertebral disc (IVD)
Mesoderm Overview
Week 3
Trilaminar embryo Compare this week 3 trilaminar embryo with the week 4 embryo. (Note - 2 these images are not to scale) |
Week 4
Scanning electron micrograph of a cross-section of a human embryo at week 4 (stage 11). Note the mesoderm structures now present and their relative position and size within the embryo. Compare the mesoderm structures to those formed by ectoderm (neural tube and epidermis) and endoderm (epithelia of developing gastrointestinal tract). |
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