Intraembryonic Mesoderm

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)