TR056/PG1007 Lecture 5 Deriva6ve of the Mesodermal Germ Layer Dr. Neil Docherty My Teaching Objec/ves • To list and describe the changes in the mesoderm following
Gastrulation
• To detail the origin and fate of the somite
• To link changes in the extra-embryonic and lateral plate
mesoderm to closure of the body wall and formation of the
intra-embryonic cavity and gut-tube
Development of The Mesoderm
ECTODERM
WEEK 3
MESODERM
ENDODERM
MESODERM
Gives rise to;
• Muscle
• Cartilage
• Bone
• Dermis
• Vascular system
• Urogenital system
• (we will look closely at nephrogenesis)
• Adrenal Cortex
The Mesoderm Day 17-21
17d
20d
19d
21d
PARAXIAL-INTERMEDIATE AND LATERAL PLATE FORMED
Paraxial Mesoderm
At the start of week 3, the paraxial mesoderm segments
cephalocaudally to form somitomeres
SOMITOMERE=Concentric whirl of mesoderm
cells arranged in a tight ball
One on each side of the midline (pair formation)
=SOMITE
3 pairs form/day
(craniocaudally)
Week 5
42 pairs
Counting the somites allows for
accurate estimation of age
Cross Section Showing Somites
Molecular Gradient Regulation of
Somitogenesis
INCREASES CRANIALLY
Retinoic acid
FGF8
INCREASES CAUDALLY
Diminishing gradient of Notch
Notch accumulation
In pre-somitic mesoderm
Somite Differentiation
Somitomeres
(mesoderm is fibroblastic)
Epithelialisation
and lumen formation
mid-line
cranial
dorsal
mid-line
caudal
ventral
Dorsomedial and
ventrolateral wall?
Ventral Medial wall?
EMT
Muscle precursors
EMT
Sclerotome
(vertebrae and ribs)
Body wall muscle
Dermatomyotome
(dermis and muscle of
Body wall, back and limb
Somite Differentiation
Molecular Regulation of Somite Differentiation
Signals arise from the
1)  The notochord
2)  Neural Tube
3)  Lateral Plate Mesoderm
Closure of The Body Wall
Intraembryonic and Amniotic Cavities
21 days
1 month
Today’s Learning Objec/ves Your learning from today should focus on being able to;
1)  Describe in detail the process of somitogenesis and list
The tissue derived from the process
2) Track the development of the amniotic cavity and the
emergence of the intra-embryonic cavity during closure of the body
wall