Developmental Fate: A Comprehensive Overview of Ectoderm, Mesoderm, and Endoderm Derivatives
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Name
of different germ layers:
1. Ectoderm 2. Mesoderm and 3. Endoderm
Ectoderm
Parts of ectoderm: surface ectoderm, neuro-ectoderm,
neural crest, ectodermal placode
Derivatives
of Surface ectoderm: epidermis of skin
Derivatives
of Neuro-ectoderm: it has two parts: neural tube and neural crest
Derivatives
of neural tube: Brain
and Spinal cord
Derivatives
of neural crest:
1.
Connective
tissue and bones of face and skull
2.
C
cells of the thyroid gland
3.
Cono-truncal
septum in the heart
4.
Cranial
nerve ganglia
5.
Schwann
cells
6.
Spinal
ganglia,
7.
Sympathetic
ganglia
8.
Parasympathetic
ganglia
9.
Meninges,
10.
Melanocytes
Derivatives
of Ectodermal placode:Lens placode: give rise to lens.
Otic
placode :it form otic vesicles.
It is divided into two parts
i)
Ventral component – give rise saccule and cochlear duct
ii)
Dorsal component forms the utricle, semicircular canals and endolymphatic duct
Nasal
placode: nose and nasal cavity
Mesoderm:they are two
types
Intra-embryonic mesoderm and extra-embryonic
mesoderm
Parts
of intra-embryonic mesoderm:
Paraxial mesoderm, intermediate mesoderm and lateral
plate mesoderm, septum transversum
Formation of intra embryonic mesoderm
At first the intra embryonic mesodermal
cells are distributed as a loose sheet on each side of the notochord in between
the ectoderm and endoderm . then intra embryonic mesoderm subdivided into three
parts,
Paraxial mesoderm: on each side of the
notochord and beneath the developing neural plate.
Intermediate mesoderm: middle part of mesoderm.
Lateral plate mesoderm: it is continuous
with extra embryonic primary mesoderm.
1.
Paraxial
mesoderm:
It is a thick plate of mesoderm located on each side
of the midline. They organized into segment and form somitomeres (1-7) in head region which
contribute mesoderm to the pharyngeal arches and somites(42-44 pairs )in
occipital and caudal region
Each
somite has three parts
i)
Myotome- give rise to epimeric and hypomeric muscle tissue
ii)
Sclerotome – give rise bone and cartilage of
vertebral column
iii)
Dermatome – subcutaneous tissue of body
2.
Intermediate mesoderm: developed
genitourinary system
Urinary
system: kidney, ureter,
Genital
system: In case female para-mesonephric
duct developed from intermediate mesoderm –
Derivatives
from para-mesonephric duct: uterus, uterine tube and ovary, upper part of
vagina
In
case male mesonephric duct
developed from intermediate mesoderm
Derivatives
from mesonephric duct – testes, vas deferens)
3.
Lateral plate mesoderm: it has two parts –
i) somatopleuric layer: develops
parietal layer of serous sac like parietal layer of pleura, peritoneum,
pericardium and muscles of body wall
ii)
Splanchnopleuric layer: develops viceral
layer of serous sac like visceral layer of pleura, peritoneum, pericardium,
smooth muscle and connective tissue of GIT.
Septum
transversum
The septum
transversum is a sheet of mesoderm
that appears on day 22 rostral to the developing heart.
Derivatives of
septum transversum
The cranial part of
the septum transversum gives rise to the central tendon of the diaphragm and
The fibrous pericardium
The caudal part of the septum transversum is form stroma of
liver, the ventral mesentery of
the foregut, the lesser
omentum, the visceral peritoneum of the liver and
the falciform ligament
Derivatives
of endoderm:
Lining
epithelium of GIT, lining epithelium of respiratory tract, lining epi of
urinary bladder.
Parenchyma
of thyroid gland, parathyroid, liver and pancreas
Fig
: formation of neural tube, intra embryonic mesoderm, primitive gut tube
Foldings
of the embryo
Up
till now the trilaminar germ disc is flattened and pear shaped .
Cranio-caudal
folding appear due to rapid growth of
the central nervous system
Lateral
folding appear due to rapid growth of somites . due to this folding primitive
gut tube is formed
During
these process of embryonic folds the growth of yolk sac is retarded and the
amniotic cavity enlarges and cover whole fetus
Stages
of development.
Other Developments in
the Embryo
Several other major developments that
occur during the embryonic stage are summarized chronologically below, starting
with the fifth week after fertilization
.
Week Five
By week five
after
Fertilization,
the embryo measures about 4 mm (0.16 in.) in length and has begun to
curve into a C shape. During this week, the following developments take place:
- Grooves
called pharyngeal arches form. These will develop into the face and neck.
- The inner
ears begin to form.
- Arm buds
are visible.
- The liver, pancreas, spleen,
and gallbladder start to form.
Week Six
By week six
after fertilization,
the embryo measures about 8 mm (0.31 in.) in length. During the sixth
week, some of the developments that occur include:
- The
eyes and nose start to develop.
- Leg
buds form and the hands form as flat paddles at the ends of the arms.
- The
precursors of the kidneys begin to form.
- The stomach starts
to develop.
Week Seven
By week seven,
the embryo measures about 13 mm (0.51 in.) in length. During this
week, some of the developments that take place include:
- The lungs
begin to form.
- The arms
and legs have lengthened, and the hands and feet have started to develop
digits.
- The lymphatic
system starts to develop.
- The primary
prenatal development of the sex organs begins.
Week Eight
By
week eight — which is the final week of the embryonic stage —
the embryo measures about 20 mm (0.79 in.) in length. During this
week, some of the developments that occur include:
- Nipples
and hair follicles begin to develop.
- External
ears start to form.
- The
face takes on a human appearance.
- Fetal
heartbeat and limb movements can be detected by ultrasound.
- All
essential organs have at least started to form.
