Mesoderm (3 germ layers) Flashcards
Gastrulation
Begins with the formation of the P.S.
Ends with the formation of the 3 embryonic germ layers (ectoderm, endoderm and mesoderm)
Delamination
Detachment of blastomeres from inner cell mass into blastocoel forming hypoblasts
Remaining cells proliferate and form epiblast (source of germ layers)
Hypoblast
Detached cells that line the inner side of blastocoel
Presumptive endoderm cells
Line the yolk sac
Epiblast
Remaining cells of the inner cell mass
Gives rise to the ectoderm and mesoderm
Formation of the mesoderm
Proliferation of epiblast cells
Cells move through the P.S. occupying the space ebtween epiblast and hypoblast
Formed first caudally then expands laterally and cranially
What does the mesoderm expand to form?
Paraxial
Intermediate
Lateral (splanchnic and somatic)
Extraembryonic
How is the cr. mesoderm formed?
Accumulation of multipotential cells at the cr. end of the P.S.
Forms Hansen’s/ Primitive Node, head mesoderm and plays a role in forming the notochord
Extraembryonic mesoderm
Expands between the trophoblast and extraembryonic endoderm
Continuous with embryonic germ layers
Where is the rate of mesoderm expansion the slowest?
In horses
Early embryo
Mesoderm is a loose aggregation of cells (mesenchyme)
Later embryo
Differentiation begins
Mesodermal “fate map” is possible
Lateral mesoderm
Lateral plate splits to form somatic and splanchnic mesoderm
Cavity between the two is coelom
Somatic mesoderm
Somatopleure (mesoderm + ectoderm)
Parietal
Forms pelvic and shoulder girdles, long bones of the limbs, sternum
Splanchnic mesoderm
Splanchnopleure (mesoderm + endoderm)
Visceral
Must be present for BVs to form
GI tract of thorax and abdomen
Intermediate mesoderm
Runs lengthwise along the entire “trunk” of the embryo
Later in development cell clusters (nephrotomes) and nephrogenic cord form
What does the intermediate mesoderm give rise to?
Most but not all of the urinary system and parts of the reproductive system
Paraxial mesoderm
Aggregation of mesoderm to form 2 parallel columns
Beginning cranially, transverse fissures appear and form somites
What does the somite number indicate?
the age of embryo and is constant for a species
Myotome
Forms skeletal muscles of body
Dorsal (epaxial) part of myotome
Muscles of the back
Muscles of vertebral column dorsal to transverse process
Ventral (hypaxial) part of myotome
Ventro-lateral muscles (intercostal and abdominal)
Limb muscles
Formation of the multinucleate cell
Mesenchyme –> myogenic cells –> myoblast –> cells fusion to form multinucleate cell
Sclerotome
Verterbrae and ribs
Dermatome
Dermis (not epidermis)
Mesoderm of the head
No lateral mesoderm or coelom
Mesoderm migrates to the “brachial arches”
Neural crest cells contribute (ectomesenchyme)
Occipital somites
Somites that extend only into the cd. regions of the head
Somitomeres
Rostral to occipital somites,
Formed by paraxial mesoderm
Extend only as far as the rostral extent of the notochord (cd. forebrain/ infundibulum)
Skeletal system
Somatic mesoderm
Sclerotome
Occipital somites
Neural crest cells
What do neural crest cells form?
Ectomesenchyme
Face and skull bones
How are bones formed?
Via endochondral and intramembranous ossification
Skeletal dysplasia
Cranio-facial defects- abnormal neural crest migration
T/F: After the formation of the mesoderm, P.S. regresses and has no derivative structures in the adult
TRUE
How is the notochord formed?
Mesodermal cells from Hensen’s node migrate toward the pre-cordal plate forming the notochordal plate
Plate folds to form notochord (solid mass of cells)
Formed cr. to cd. as the primitive streak regresses
Function of the notochord
Inductor for ectoderm to form neuroectoderm (neural plate, CNS, somite)
Organizing center for vertebral bodies
Formation of the neural tube
Thickening of the ectoderm overlying notochord neural plate
Folding of neural plate
Formation of neural tube
