BIOL 370 – Developmental Biology
Topic #14
Paraxial and Intermediate Mesoderm
Lange
In this chapter we shall feel further development of the mesoderm
and the endoderm:
• the endoderm will form the digestive and respiratory
tubulature and organs
• the mesoderm generates all the intermediate organs between
the ectoderm and endoderm
Figure 11.1 Major lineages of the amniote mesoderm (Part 1)
• Intermediate mesoderm  kidneys, gonads
• Chordamesoderm  notochord
• Paraxial mesoderm  head, somites
• somites  cartilage,
tendons,
skeletal muscle,
dermis,
endothelial cells
• Lateral plate
mesoderm 
circulatory
system, body
cavities
Figure 11.2 Gastrulation and neurulation in the chick embryo, focusing on the mesodermal
component
It is
important at
this stage to
work on
developing
an ability to
visualize
the 3-D
perspective
of the
organism
from these
sections.
Figure 11.4 Formation of new somites
Somites are bilaterally paired blocks
of mesoderm that form along the
anterior-posterior axis of the
developing embryo. An alternative
term used in place of somites is
metamere.
Figure 11.4 Formation of new somites (Part 2)
New somites are formed in the process of somatogenesis, which is both
molecular and cellular in origin. Of key interest in the somite formation
below is the use of ephrins (also known as ephrin ligands (abbreviated
Eph)). These are a family of proteins that serve as the ligands of the ephrin
receptor..
Figure 11.5 Notch signaling and somite formation (Part 4)
the Notch ligand is produced by the Delta-like 3 gene. “E”
shows a Delta-like 3 gene knockout mouse with a clearly
aberrant skeleton. White dots show the pattern of
ossification centers in both mice.
Figure 11.7 Hypothetical pathway for regulation of the clock through which an Fgf8 gradient
regulates a Wnt oscillating clock
The pathway for control of the genes regulating somite
formation are shown below. One of the most pressing
issues we have poor understanding of currently is how the
timing of these events is regulated and coordinated.
Figure 11.10 When segmental plate mesoderm is transplanted it differentiates according to its
original position
In this chronologically backwards transplant study, older donor
mesodermal tissue is transplanted to an earlier stage embryo in a
different location. The resultant donor structure is already fixed
and it continues development into its original presumptive
structures… in this case, into vertebrae now developing in the
cervical neck region.
Transverse section through the trunk of a chick embryo on days 2–4
Somites visible as the red
structures are multipotent
at this point and their
specification is dependent
upon their location
relative to paracrine
factors received from
surrounding tissues (such
as the neural tube,
epidermis, etc.)
Figure 11.11 Transverse section through the trunk of a chick embryo on days 2–4 (Part 1)
Dermamyotome – the segment of the somite that is going to form the dermis
(dermatome) and the skeletal muscle (myotome). The combined name is used
because the initial development is slower than in other somite segments.
Figure 11.11 Transverse section through the trunk of a chick embryo on days 2–4 (Part 2)
Notice now the separate dermatome and myotome.
Figure 11.12 Primaxial and abaxial domains of vertebrate mesoderm (Part 1)
Primaxial simply refers to the portion of the mesoderm that is
more “medial” and the abaxial is more distal to the center axis.
Remember from earlier discussions the term “epiblast” which is
referring to the upper layer.
Figure 11.14 Ablating Noggin-secreting epiblast cells results in severe muscle defects
Epiblastic
mesodermal cells
that are
experimentally
ablated (destroyed,
usually
chemolytically or
electrolytically)
result in severe
muscle defects
arising in the chick.
Figure 11.15 Conversion of myoblasts into muscles in culture
Notice the different paracrine factors
that will facilitate the different steps.
FGF = fibroblast growth factor
CAM = cell adhesion molecule
Figure 11.16 Schematic diagram of endochondral ossification
Chondrocytes – cartilage producing cells
Osteocytes - cells within the calcified aspect of bone
Osteoblasts - cells that synthesize bone
Osteoclasts – cells that degrade bone.
Figure 6.9 Endochondral ossification in a long bone.
Month 3
Week 9
Birth
Childhood to
adolescence
Articular
cartilage
Secondary
ossification
center
Epiphyseal
blood vessel
Area of
deteriorating
cartilage matrix
Hyaline
cartilage
Spongy
bone
formation
Bone
collar
Primary
ossification
center
1 Bone collar
Spongy
bone
Epiphyseal
plate
cartilage
Medullary
cavity
Blood
vessel of
periosteal
bud
2 Cartilage in the
3 The periosteal
center of the
forms around
hyaline cartilage diaphysis calcifies
and then develops
model.
cavities.
bud invades the
internal cavities
and spongy bone
begins to form.
4 The diaphysis elongates
and a medullary cavity
forms as ossification
continues. Secondary
ossification centers appear
in the epiphyses in
preparation for stage 5.
5 The epiphyses
ossify. When
completed, hyaline
cartilage remains only
in the epiphyseal
plates and articular
cartilages.
Figure 6.11 Long bone growth and remodeling during youth.
Bone growth
Cartilage
grows here.
Bone remodeling
Articular cartilage
Epiphyseal plate
Cartilage
is replaced
by bone here.
Cartilage
grows here.
Cartilage
is replaced
by bone here.
Bone is
resorbed here.
Bone is added
by appositional
growth here.
Bone is
resorbed here.
Steel “Bone Cages” used to lengthen legs. These were originally
developed in the Soviet Union in the 1950s to treat dwarfism.
An example of untreated acromegaly.
Figure 11.17 Endochondral ossification
In this image we see a
potential pathway for the
transition of cartilage into
bone…. The formation of
endochonral ossification.
Figure 6.17 Fetal primary ossification centers at 12 weeks.
Parietal bone
Occipital bone
Mandible
Frontal bone
of skull
Clavicle
Scapula
Radius
Ulna
Ribs
Humerus
Vertebra
Ilium
Tibia
Femur
Figure 11.18 Skeletal mineralization in 19-day chick embryos that developed (A) in shell-less
culture and (B) inside an egg during normal incubation
The shell of
the egg is the
primary
source of
calcium for
ossification of
the bird
skeleton prior
to hatching.
Figure 11.21 Scleraxis is expressed in the progenitors of the tendons
• Scleraxis expressing progenitor cells lead to the eventual
formation of tendon tissue and other muscle attachments.
• Scleraxis is also associated with embryonic tissues that develop
into tendon and blood vessels.
Aortic dissection is a disorder
that is often due to an
abnormality in scleraxis of the
aorta. The late John Ritter is
one person to have died due to
this condition.
Figure 11.22 Induction of scleraxis in the chick sclerotome by Fgf8 from the myotome
Scleraxis expressing progenitor cells lead to the eventual formation of
tendon tissue and other muscle attachments.[
Figure 11.23 General scheme of development in the vertebrate kidney
Pronephros - the most basic of the excretory organ that develops in vertebrates
Mesonephric tubules - form to attach to the mesonephros as the pronephros degenerate
Metanephros – the final mammalian kidney
Figure 11.24 Signals from the paraxial mesoderm induce pronephros formation in the intermediate
mesoderm of the chick embryo
Figure 25.3b
Figure 25.4b
Figure 11.25 Reciprocal induction in the development of the mammalian kidney
Figure 11.26 Kidney induction observed in vitro
Figure 11.32 Development of the bladder and its connection to the kidney via the ureter
End.