3rd week of development
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The main events-
1.
Appearance of primitive streak.
2.
Gastrulation.
3.
Formation of the notochord.
4.
Growth of embryonic disc.
5.
Establishment of body axes.
6.
Tertiary chorionic villi are formed by the end of the
third week.
Fig : Tri-laminar germ disc
Primitive streak: it is a groove formed in the epiblast at the
caudal end of the bi-laminar germ disc. Through primitive streak epiblast cells
migrate to form endoderm and mesoderm during gastrulation.
Gastrulation
It is process of forming the three
primary germ layers from the epiblast involving movement of the cells through
the primitive streak to form endoderm and mesoderm.
Fig: Gastrulation
Notochord
It is the primordial longitudinal
axis of the embryo extends from the cranial end of primitive streak to
prochordal plate. Around the notochord the
axial skeleton is formed.
Time
period of formation
Start from 16 days, and is completely formed by the beginning of the
fourth week.
Fate of notochord
It
is disappears but its remnants are persists as the nucleus pulposus of the
intervertebral disks and apical ligament of dens of 2nd cervical
vertebra.
Importance of notochord
The
notochord is the defining structure of the phylum chordates, and has essential
roles in vertebrate development.
Functions of notochord
It
acts as organizer and induces the development of neural tube.
It
forms the central axis of developing embryo.
It
serves as the basis for the axial skeleton (vertebral column) but the notochord
does not give rise to the vertebral column.
Formation
of notochord
1. Appearance
of primitive streak
2. The primitive streak's
cranial end proliferates to form a primitive node
3. Cells in the primitive node
multiply and migrate cranially between the ectoderm and endoderm, eventually
reaching the prochondral plate. The notochondal process is a solid cord of tissue.
4. A small depression called a
primitive pit develops in the primitive node and extends into the notochordal
process, converting it to a notochordal canal.
5. The notochordal canal's
floor fuses with the endoderm. Then it crumbles and form the neuroenteric canal
which communication with the amniotic cavity and the yolk sac cavity.
7. The remainder of the
notochordal canal flattens and forms the notochondal plate.
8. The notochordal plate folds
and forms a tube
9. Tube cell proliferation results in the formation of a cord known as the definitive notochord.
Formation of neural tube
The notochord induces the overlying
ectoderm to differentiated into neuro-ectoderm, which forms the neural tube and
the process of formation of the neural
tube is known as neurulation.
Steps of formation of neural tube
1. The ectoderm overlying the notochord
changes and forms neuro-ectoderm
2. These cells proliferate to form a
thick plate called neural plate which extends in midline from primitive knot to
prochordal plate / buccopharyngeal membrane.
3. the margin of the neural plate gets
elevated due to the growth of para-axial mesoderm which is situated two sides
of notochord.
4. these elevated structure is known as
neural folds, and some cells differentiate into neural crest cells
5. neural groove is developed in between
two neural folds
6. Gradually,
the neural folds approach each other in the midline, where they fuse and neural
crest cells start to disassociate from neuro ectoderm.
7. Fusion
of neural folds begins in the cervical region (fifth somite) and give rise
neural tube which is open at both ends known as anterior and posterior cranial
neuropores
8. Neural
tube is communication with amniotic
cavity through the anterior and posterior cranial neuropores
9. Gradually
the neural tube proceeds cranially and caudally and
10. anterior cranial neuropore close at 25th days becomes lamina terminalis and if it is
fail to close it result upper neural tube defects (NTDs) e.g. anencephaly
11. posterior
cranial neuropore close at 28th days and if it is fail to close it
result lower neural tube defects (NTDs)
e.g. spina bifida
12. the rostral part of neural tube
becomes the adult brain and the caudal part becomes adult spinal cord
13. the lumen of the neural tube gives rise to the ventricular system of brain and central canal of spinal cord.
Formation of neural crest
Neural crest, group of embryonic cells that
are pinched off during the formation of the neural tube, they are located
within the neural folds. The cells of
the neural crest migrate to numerous locations in the body ( both cranial
region and trunk region) and contribute to the formation of diverse structures, mostly associated with the nervous system.Cranial neural crests
migrate into pharyngeal arches.
Cranial
neural crest cells differentiate into the following adult cells and structures.
1. Pharyngeal arch skeletal and
connective tissue components
2.
Bones of neurocranium
3.
Pia and archnoid mater
4.
Parafollicular (C) cells of thyroid gland
5.
Conotrucal septum/ aortic-pulmonary septum
6.
Odontoblasts (dentin of teeth)
7.
Sensory ganglia of cranial nerves (CN) V, VII, IX, X
8. Parasympathetic ganglia (ciliary
,otic, submandibular and pterygopalatine
Trunk
neural crest cells
These neural crest cells extend
from somite 6 to the most caudal somites and migrate in a dorsolateral ,
ventral and ventrolateral direction throughout the embryo.
Trunk
neural crest cells differentiate into following adult cells and structures
1. Melanocytes
2. Schwann cells
3. Chromaffin cells of adrenal medulla
4. Dorsal root ganglia
5. Sympathetic chain ganglia
6. Pre-vertebral sympathetic ganglia
7. Enteric parasympathetic ganglia of
the gut (meissner and Auerbach CN X)
8. Abdominal / pelvic cavity
parasympathetic ganglia
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