Why is gastrulation so important Generation of the

Why is gastrulation so important? §Generation of the three germ layers: §Ectoderm, mesoderm, and endoderm §Generation of the basic body plan. §Specification of the axes: §Anterior and posterior §Dorsal and ventral §Left and right

Primitive streak (ingression)gastrulation is initiated ---EPIBLAST cells move towards the midline and then anteriorly. This creates the primitive streak. Anterior Posterior

3) Primitive groove - formed by wake of the primitive streak as it moves anterior 4) Hensen’s node- thickening at most anterior point of the groove -point at which cells from groove move inside blastocoel

epiblast Epithelial cell E-cadherin „slug” FGF 8 Ecadherin mesoderm endoderm hyaluronic acid and fibronectin for cell migration

The brachyury mutation was first described in mice -affects tail length and sacral vertebrae in heterozygous animals and is lethal in homozygous animals around embryonic day 10 due to defects in mesoderm formation, notochord differentiation and the absence of structures posterior to the forelimb bud). Brachyury m. RNA (T-box) expression containing transcription factor

Gastrulation Anomalies Caudal Dysgenesis (Sirenomelia) -Caudal defect: Insufficient mesoderm formation -Fused lower limbs, renal agenesis -Genetic and teratogenic mutation of Brachyury (T) gene (the brachyury mutation was first described in mice affects tail length and sacral vertebrae in heterozygous animals and is lethal in homozygous animals around embryonic day 10 due to defects in mesoderm formation)

How is handed asymmetry initiated in the embryo? Gastrulation : Defined MAIN axes in human anatomy §Specification of the axes: §Anterior and posterior §Dorsal and ventral §Left and right Anterior Posterior Dorsal Ventral Left (lateral) Right (lateral) Defined Axis Synonyms Axis runs. . . Anteroposterior Rostrocaudal, Craniocaudal, Cephalocaudal . . . from head end to opposite end of body or tail. Dorsoventral — Left-right Dextro-sinister, Sinistro-dexter §Generation of the three germ layers §Ectoderm, mesoderm, and endoderm . . . from spinal column (back) to belly (front). . from left to right sides of body.

Formation of the primitive streak also defines for the first time all major body axes. 1: cranial-caudal (or head-tail) axis, 2: dorsal-ventral (or back-belly) axis, 3: medial-lateral axis 4: left-right axis.

Anterior-posterior patterning in the embryo Lim-1 mutant embryos (headless mice)

0** head induction requires the inhibition of both BMP and Wnt signals (Glinka et al. 1997).

Molecular pathway for left-right asymmetry in the embryo Node Signals: LEFT Shh (Sonic Hedgehog) –– induces Nodal (symmetry breaking molecule) (Nodal is a signaling protein which is responsible for gastrulation, left-right patterning and induction of the primitive node. ) RIGHT : Activin – (inhibits SHH)

Situs formation The cells on the surface of the primitive node have cilia that beat

How does fluid flow influence molecular asymmetry? anterior R L Morphogen sweeping posterior -beating of cilia arround the primitive node -directional current

Situs formation : nodal flow model of left-right development Mobile cilia

situs inversus viscerum totalis (-rare human disorder, (1: 10. 000) -the handedness of all of the viscera is reversed. No functional impairments.

Medical considerations of LR asymmetry : -situs inversus (1: 10. 000, a complete mirrorimage of asymmetrically positioned organs) Kartagener’s syndrome (1: 30. 000): 1. Chronic bronchiectasis 2. Chronic sinusitis 3. Situs inversus -heterotaxia (situs ambigus) (where each organ makes an independent decision as to its situs), -severe complications right or left isomerism completely symmetrical organs; polysplenia. Kartagener’s syndrome (immotile ciliary syndrome) -hereditary disease -dextrocardia, Kartagener syndrome patients have mutations in DYNEIN genes: --immotile cilia

As gastrulation converts the bilaminar embryonic disc into a trilaminar embryonic disc, it brings subpopulations of cells into proximity so that they can undergo inductive interactions to pattern layers and specify new cell types. The first cells to move through the primitive streak and contribute to the intraembryonic mesoderm migrate bilaterally and cranially to form the cardiogenic mesoderm. Somewhat later in development, a longitudinal thick-walled tube of mesoderm extends cranially in the midline from the primitive node; this structure, the notochordal process, is the rudiment of the notochord. Migrating bilaterally from the primitive streak and then cranially, just lateral to the notochordal process, are cells that contribute to the paraxial mesoderm. In the future head region, paraxial mesoderm forms the head mesoderm. In the future trunk region, paraxial mesoderm forms the somites, a series of segmental blocklike mesodermal condensations. Two other areas of intraembryonic mesoderm form from the primitive streak during gastrulation: the intermediate mesoderm and lateral plate mesoderm. The intermediate mesoderm contributes to the urogenital system, and the lateral plate mesoderm contributes to the body wall and the wall of the gut (gastrointestinal system).