DERIVATIVES OF GERM LAYERS
Dr Rania Gabr
OBJECTIVES
By the end of this lecture ,the student should be able
to:
• Explain the results of folding
• List the derivatives of ectoderm
• List the derivatives of endoderm
• List the derivatives of mesoderm
RESULTS OF FOLDING
1- Embryo
change into
cylinderical
embryo.
2-Transposition
between septum
transversum
and cardiogenic
plate( S.T lies
cranial then
ventral and
lastly caudal).
3- Yolk sac is reduced in size &divided into:
a- intraembryonic ( gut).
b- extraembryonic ( atrophies).
c- yolk stalk (degenerates).
4- Allantois& connecting stalk become dorsal then caudal
then ventral.
AFTER TAIL FOLD


The connecting stalk
(primordium of
umbilical cord) is
attached to the ventral
surface of the embryo.
Allantois (a
diverticulum of yolk
sac) is partially
incorporated into the
embryo as a part of
hindgut.
5- formation of
umbilical
cord.
6- The oral
membrane
was
cranially
ventral.
7- The cloacal
membrane
and allantois
was caudal
ventral.
RESULTS OF FOLDING
•The amniotic cavity  enlarged.
•The Yolk sac smaller & divided into
(intraembryonic Y.S, Yolk stalk& extra
embryonic Y.S).
•Allantois& connecting stalk  shifted
caudally.
•S.T  Shifted anterior to Cardiogenic
plate.
•The amniotic cavity  more enlarged.
•Allantois& connecting stalk  shifted
ventrally and form the umbilical cord
which contains the extra embryonic Y.S
and stalk.
•S.T  Shifted caudal to Cardiogenic
plate.
* Placenta will face the umblical cord.
DERIVATIVES OF THE ECTODERM

Ectoderm is divided into:
Surface ectoderm
Neuroectoderm
SURFACE ECTODERM DERIVATIVES
 Epidermis
 Hair
 Nails
 Sweat
of the skin
& Sebaceous glands
 Mammary glands
 Enamel of the teeth
 Lens of eye
 Epithelium of sensory organs in the inner
ear & nose
 Anterior lobe of the pituitary gland
NEUROECTODERM
Neural
Tube
Neural Crest Cells
NEURAL TUBE DERIVATIVES
Central nervous system (Brain and spinal cord)
 Peripheral nervous system
 Retina
 Sensory epithelia of nose & ear
 Pineal gland
 Posterior lobe of the pituitary gland

NEURAL CREST CELLS DERIVATIVES
 Sensory
ganglia of the spinal nerves( dorsal root
ganglia)
 Sensory ganglia of the cranial nerves
 Autonomic ganglia
 Meninges (Pia mater & Arachnoid mater) of the
brain & spinal cord
 Schwann cells: Neurolemmal sheath of peripheral
nerves
 Satellite cells
 Melanoblasts of the skin
 Suprarenal medulla (chromaffin cells)
 Several skeletal & muscular components in the head
(derived from pharyngeal arches)
DERIVATIVES OF ENDODERM
Endoderm gives rise to the epithelial lining of:

Trachea

Bronchi

Lungs
Respiratory
DERIVATIVES OF ENDODERM
Endoderm gives rise to the epithelial lining of:
 Gastrointestinal
tract
 Liver
 Pancreas
 Urinary
 Urachus
bladder
GIT
DERIVATIVES OF ENDODERM
Endoderm gives rise to the epithelial lining of:

Pharynx

Thyroid

Tympanic cavity

Pharyngeotympanic tube

Tonsils

Parathyroid glands
Pharyngeal arches
DERIVATIVES OF MESODERM
Connective tissue
 Cartilage
 Bone
 Striated & smooth muscles
 Heart
 Blood & lymphatic vessels
 Kidneys, ovaries, testes& genital ducts
 Serous membrane lining the body cavities
 Spleen & cortex of the supra renal gland

DEVELOPMENT OF SOMITES
 As
the notochord and neural tube forms
 Embryonic
proliferate:
mesoderm on each side of them
 Form
thick longitudinal columns of paraxial
mesoderm
 Each
column is continuous with intermediate
mesoderm
DEVELOPMENT OF SOMITES



Intermediate mesoderm gradually thins into a
layer of lateral mesoderm
Lateral mesoderm is continuous with the
extraembryonic mesoderm
Extraembryonic mesoderm covers the yolk sac
and amnion
SOMITES


Paraxial mesoderm differentiates and begins to
divide into cuboidal bodies called somites by the end
of the 3rd week
These blocks of mesoderm are located on each side of
the developing neural tube
SOMITES
 About
42-44 pairs of somites are present by the
end of 5th week
 They
are triangular in transverse section
 Form
distinct surface elevations on the embryo
 They
are used as one of the criteria to know
the age of the embryo at this stage
SOMITES
 First
appear in the future occipital region
 Soon
develop craniocaudally
 Gives
rise to the axial skeleton and associated
musculature
 Also
 The
forms adjacent dermis of the skin
first pair of somites appear at the
end of 3rd week : day 20
SOMITES


First appear at a short distance caudal to the cranial
end of the notochord
Subsequent pairs form in a craniocaudal sequence
INTRAEMBRYONIC COELOM



Also known as primordium of embryonic body cavity
Appears as isolated coelomic spaces in the lateral
mesoderm and cardiogenic mesoderm
These spaces soon coalesce to form a single horseshoe
shaped cavity called intraembryonic coelom
PARIETAL & VISCERAL LAYERS


Somatic or parietal layer continuous with the
extraembryonic mesoderm covering the amnion
Splanchnic or visceral layer continuous with the
extraembryonic mesoderm covering the yolk sac
PARIETAL & VISCERAL LAYERS


Somatic mesoderm with overlying embryonic ectoderm
form the embryonic body wall or somatopleure
Splanchnic mesoderm with underlying embryonic
endoderm form the embryonic gut or splanchnopleure
FATE OF INTRAEMBRYONIC COELOM
During the 2nd month, the intraembryonic coelom is
divided into 3 body cavities:

Pericardial cavity

Pleural cavity

Peritoneal cavity