Notogenesis, neurulation, somitogenesis

Course 508

Gastrulation Notogenesis, neurulation, somitogenesis

The embryonic axial structures development :

1. primitive streak

2. notochord

3. neural tube

4. somites

3 week development of primitive streak as crucial structure in

transformation of bilaminar into trilaminar embryo

1st week

from zygote to beginning of implantation

Implantation beginning – 7 ED

2nd week implantation continues

Embryo is at bilaminar blastoderm stage. The period of blastogenesis terminates by bilaminar blastoderm formation

and the period of embryogenesis begins.

Implantation is completed on ED14

3rd week development of bilaminar into trilaminar blastoderm/embryo consisting of 3 germ layers: ectoderm, mesoderm, endoderm

Epiblast and primitive streak formation

Mechanism of embryonic mesoderm and

endoderm development

Primitive streak

morphogenetic centre in mesoderm, endoderm and ectoderm development

• It regulates the epiblast cells migration and differentiation, they invaginate in its axis

• mechanism? • production of particular growth factor

s/morphogenes • FGF8- fibroblast growth factor • mechanism of its activity? • E-cadherin expression decreases in epiblast cells

All 3 germ layers develop from EPIBLAST

• ectoderm and endoderm are arranged as epithelium

• CAM + (E-cadherin)

• mesoderm is not arranged as epithelium, histologically it is connective tissue – primary mesenchyme

• CAM – (E-cadherin)

Development of notochord / chorda dorsalis

epiblast precursors, invaginating in Hensen´s primitive node, migrate toward the prechordal plate,

- creating the cord-like cellular axial structure, s.c. notochord

Body axis cranio-caudal, dorso-ventral , right-left

established during 3rd week

Cranio-caudal axis

1)Cell signaling from the area of rostral/cranial end of embryo-anterior visceral mesoderm (AVM)-expression of genes OTX2, LIM1, HESX1 (transcription factors) and factor Cerebrus - determine the head end (before the primitive streak formation)

2)primitive streak – initiated and maintained by factor Nodal (TGF-beta family)

3)Cell signaling from primitive node and notochord, gene Brachyury – dorsal mesoderm formation in the middle and caudal part of embryo (gene defect – longitudinal axis shortening – caudal dysgenesis /regression)

Dorso-ventral axis

1) mesoderm ventralisation • BMP-4 – bone morphogenetic protein • FGF-8 – fibroblast growth factor – determine,

what will differentiate in ventral mesoderm – kidneys, gonads, blood vessels

2) mesoderm dorsalisation – Noggin, Chordin, Follistatin block the BMP , that results in notochord and somites formation, neuroectoderm differentiation from ectoderm, differentiation of notochord and paraxial mesoderm

Signaling molecules and transription factors in primitive streak

First, cranial part/ head process of notochord is established. Caudal part is

established by caudal morphogenetic system (caudal end of notochord, intensively proliferating, and ectoderm close to primitive node), followed by

gradual primitive streak shortening .

Neural plate induction mechanism: interaction of notochord with adjacent ectoderm

Noggin, Chordin, Follistatin, neural inductors, for ectoderm to

neuroectoderm in the region of forebrain and midbrain Hindbrain and spinal cord are induced by WNT and FGF expressed in

the region of the caudal morphogenetic system.

Neural plate induction and neural tube development (interaction notochord/adjacent ectoderm)

Neural tube development (by activity of caudal morphogenetic system) results in the formation of

hindbrain and spinal cord

• caudal morphogenetic centre – intensive proliferating caudal end of notochord •caudal end of notochord gets beneath the primitive streak/primitive groove, that becomes determined to neuroepithelium at that region •primitive streak shortening •neural plate lengthening •neurulation (plate to tube development)

Neuroporus anterior ED 25 Neuroporus posterior ED 28

Histogenesis of neural tube wall

Neuroepithelium differentiation

Neural crest formation and its derivates

Neural tube development and intraembryonic mesoderm differentiation

Somites, 42 – 44 pairs of paraxial mesoderm, 1st pair in occipital

region, 20 ED, Segmentation: Notch, WNT, segmentational genes, retinoic acid, FGF-8, craniocaudal gradient