Intraembryonic MesodermIntraembryonic Mesoderm
OriginOrigin: The epiblastic cells : The epiblastic cells from the from the primitive streak primitive streak (groove)(groove)
The newly formed cells The newly formed cells migrate migrate ventrally, laterally & ventrally, laterally & craniallycranially between the epiblast between the epiblast and hypoblastand hypoblast
At the margins of the At the margins of the embryonic disc, the embryonic disc, the intraembryonic mesoderm intraembryonic mesoderm merges with the merges with the extra-extra-embryonic mesodermembryonic mesoderm
By the end of 3rd week, mesoderm lies between embryonic ectoderm and endoderm everywhere, EXCEPT in the region of:
Buccopharyngeal membrane (fused prechordal plate + ectoderm)
Cloacal membrane, as the embryonic ectoderm & endoderm are fused at these regions
NotochordNotochordNotochord is a rod of
mesenchymal cells located in the
midline extending cranially
from the primitive node to the buccopharyngeal membrane
Formation of NotochordFormation of Notochord Origin: Primitive node/pit Like the primitive streak, the
primitive pit cells proliferate and then migrate cranially in the midline, toward the buccopharyngeal membrane, and form a rod like notochordal process
The notochordal process becomes canalized forming a hollow tube, the notochordal canal, which communicates with the amniotic cavity at the primitive pit.
Formation of NotochordFormation of Notochord cont’d cont’dThe floor of the tube
and the underlying endoderm fuse and then break down, forming a notochordal plate
The notochordal plate becomes continuous with the endodermal layer.
Formation of NotochordFormation of Notochord cont’d cont’dA temporary
communication is established between the amniotic cavity and the yolk sac, termed the neurenteric canal.
Notochordal plate Notochordal plate folds to form the folds to form the notochordnotochord, , which gets separated from the underlying which gets separated from the underlying endodermendoderm. .
Functions of NotochordFunctions of NotochordDefines primordial axis of the embryoProvides rigidity to the embryoServes as a basis for the development of the axial
skeletonIndicates the future site of the vertebral
bodies/column Regulates differentiation of surrounding
structures including the overlying ectoderm and the mesoderm
Fate of NotochordFate of NotochordDegenerates and
disappears as the bodies of the vertebrae develop
The part that lies between the vertebral bodies persists as the nucleus pulposus of each intervertebral disc
Remnants of notochordal tissue give rise to tumors called Chordomas
Differentiation of the Differentiation of the Intraembryonic MesodermIntraembryonic Mesoderm
Induced by the notochord
Differentiates into the: Paraxial mesoderm Intermediate cell
mass Lateral plate
mesoderm
Ectodermal DerivativesEctodermal Derivatives
Dr. Zeenat Zaidi
The NeurulationThe NeurulationIt is the process by which the neural tube is
formed. The stages of neurulation include the formation of:
Neural plateNeural groove Neural folds & their fusionNeural crest cellsNeural tube
Begins during early part of the 4th week (22-23 days)
Ends by the end of 4th week (27 days)Is induced by the notochord
The NeurulationThe Neurulation
Under the inducing effect of the developing notochord, the overlying ectodermal cells thickens to form the neural plate
The neural plate first appears:
Cranial to the primitive node and
Dorsal to the developing notochord & the mesoderm adjacent to it
As the notochord forms & elongates:
The embryonic disc elongates and becomes club-shaped
The neural plate broadens and extends cranially as far as the buccopharyngeal membrane, and later on grows beyond it
On 18th day: the neural plate invaginates to form neural groove & neural folds
Neural fold
Some neuroectodermal cells along the crest of the neural fold differentiate as the neural crest cells.
Neural crest cells
Neural fold
By the end of 3rd week, the neural folds move to the midline and fuse to form the neural tube
The fusion begins in the future cervical region and then extends both in cranial and caudal direction
Following fusion of the neural folds, the neural crest cells get separated and move laterally to form the sensory neurons of the spinal (dorsal root) ganglia
The neural tube separates from the surface ectoderm, lies in the midline, dorsal to the notochord
Neural tube is open at both ends, communicating freely with the amniotic cavity.
The cranial opening, the rostral neuropore closes at about 25th day & the caudal neuropore closes at about the 27th day
The cranial ⅓ of the neural tube represent the future brain
The caudal ⅔ represents the future spinal cord
Congenital Anomalies of the Nervous Congenital Anomalies of the Nervous SystemSystem
• Disturbance of neurulation may result in severe abnormalities of the brain and the spinal cord
• Most defects are the result of non-closure or defective closure of the neural tube:
• In the brain region (e.g. anencephaly)• In the spinal cord regions (e.g. spina
bifida)• High level of alpha-fetoprotein (AFP) in the
amniotic fluid is a strong sign of neural tube defects
EctodermEctoderm
Surface ectodermNeuroectoderm
Surface Ectoderm DerivativesSurface Ectoderm Derivatives
Epidermis of the skin Hair Nail Sweat & Sebaceous glands Mammary glands Enamel of the teeth Lens of eye Internal ear Anterior lobe of the pituitary gland
NeuroectodermNeuroectoderm
Neural TubeNeural Crest Cells
Neural Tube DerivativesNeural Tube Derivatives
Central nervous systemPeripheral nervous systemRetinaSensory epithelia of nose & earPineal glandPosterior lobe of the pituitary gland
Neural Crest Cells Neural Crest Cells DerivativesDerivatives
Sensory ganglia (cranial & spinal) Autonomic ganglia Meninges (Pia mater & Arachnoid mater) of
the brain & spinal cord Schwann cells Satellite cells Melanoblasts Suprarenal medulla (chromaffin cells) Several skeletal & muscular components in
the head (derived from pharyngeal arches)