Embryology | Week 3&4 development and derivatives of ectoderm | First Aid Lectures | Lec4 #Drjamil
Channel: Dr. Jamil Lectures
Date: 07/03/2023

In this lecture we will discuss about gastrulation,neurulation,Neural tube,derivatives of neural tube ,dervatives of Ectoderm and neural crest cells derivatives.

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Development up until week 3
❖ Differentiation of Bilaminar Disk/Gastrulation
● At the end of 2nd week (day 14) epiblast cells become thickened and form primitive
streak →At the end of the primitive streak,there is formation of node towards the
cranial end→ this node is called primitive node→Eventually, some cells in the center
of the primitive streak and primitive node start dying →This forms a cavity called the
primitive groove and primitive pit respectively → Some cells in the edge of the
primitive groove start secreting chemicals like Fibroblast Growth Factor-8 towards the
lateral sides
● FGF-8 binds on nearby epiblast cells via FGF-8 receptors and activates the SNAIL-1
proteins present inside the epiblast cells → activated SNAIL-1 inhibits the formation
of e-cadherins →As e-cadherins is responsible for sticking of cells together and
prevents from free movement,so inhibition of e-cadherins allows the epiblast cells to
move freely → this is called epithelial migration →freely moving epiblast cells will
move through the primitive groove towards ventral, and reach at the site where the
hypoblast is to replace it with freely moving epiblast cells → It changes the hypoblast
into endoderm (first layer to form)
● After the hypoblast is replaced by endoderm,the more FGF-8 is
released causing more epiblast cells to migrate and move
through the primitive groove→Then they move downward,lateral
and forward towards cranial side and fill up the area between the
epiblast layer and the hypoblast, forming a new layer called
mesoderm → At the end stage the epiblast cells differentiate
themselves and turn the epiblast into ectoderm →Now the
bilaminar disc is transformed to a trilaminar disc → This whole
process is called gastrulation.
● Neurulation →As more FGF-8 is released,causing the
ectodermal cells to continue migration,but now migrating cranially
through the primitive pit towards the prechordal plate (not through
primitive groove)→ This forms a chord called the notochord, that moves underneath
the ectoderm but above the endoderm → notochord is also called axial mesoderm
→The notochord secretes proteins and growth factors which stimulates the ectoderm
cells to proliferate and become thickened around the primitive streak and node →it
forms a specialized structure called the neural plate → The neural plate keeps
proliferating and then invaginates along its central axis,It forms a groove called
neural groove → The lateral longitudinal folds on each side of neural plate are called
neural fold →The neural folds come closer and fuse with each other that gives rise to
a tube,called neural tube→The lateral parts of the neural fold differentiate and gives
rise to neural crest cells that detach once the neural folds fuse with each other →
This process is called neurulation.

● Primordial germ cells are formed from epiblast or yolk sac at the end of week 2 and
start gamete formation (gametogenesis) at the end of week 5.
❖ Derivatives of each layer of Trilaminar disk
1. Ectoderm
● It forms skin and nervous system
2. Mesoderm
● It has further 3 parts.
➔ Paraxial part that forms axial skeleton
➔ Intermediate part that forms kidneys and gonads
➔ Lateral part that forms splanchnic part (for GI organs) and Somatic part.
3. Endoderm
● It forms the lining of GI tract, accessory organs and glands.
1. Ectodermal Derivatives
★ Neural Tube Derivatives
● As ectodermal cells give rise to neural tube (neurulation) → neural tube remains
open near prechordal plate (near cranial side) → this opening is called anterior
neuropore →It usually closes around day 24/25 with sufficient folate/folic acid
(Vitamin B9) supplementation.
● Neural tube also remains open near the caudal side and this opening is called
posterior neuropore→ It usually closes around day 26-28 with sufficient folate/folic
acid (Vitamin B9) supplementation.
● If closure of these neuropores fails,then it results in spina bifida (defect of spine that
is usually apparent at birth),anencephaly (birth defect in which a baby is born without
some parts of brain and skull).


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