Class 17 Thursday, 2 April 2020

Gastrulation & Neurulation

Exams will be graded and returned on Tuesday

We will go over questions and answers Xenopus: simpler, fertilized egg produces embryo only – no maternal interactions, no extra-embryonic tissues – useful for looking at major morphogenic processes but “unique” (timing / size / speed - non-placental)

Mouse: human (mammalian) relevance but distinctive differences

As an example, we will return to nine-banded armadillo, which always produces monozygotic quadruplets (good for embryonic noise studies) videos (time-lapse) Xenopus Richard Harland (UC Berkeley) 2: The Cellular Basis of Gastrulation watch through ~ 12:30 What happens if Dvl does not function?

Two receptor complexes are formed on opposite sides of a cell: • Frizzled forms a complex with Flamingo, disheveled (Dvl), and Diego • on the other side the complex consists of Flamingo, Van Gogh, and Prickle. • The extracellular parts of these complexes interact controlling cellular polarization Absence of Wnt: GSK3 binds Axin & APC to form the β-catenin destruction complex.

• Some Wnts stimulate Frizzled & LRP5/6, inhibit GSK3 & stabilize β-catenin • GSK3 inhibition also leads to target gene derepression by promoting TCF3 phosphorylation by homeodomain-interacting protein kinase 2 (HIPK2)- mechanism involves β-catenin • Other Wnts use distinct receptors in addition to Frizzled - control the cytoskeletal organization through core planar cell polarity (PCP) proteins, small GTPases (Rho/ Rac/ Cdc42), and c-Jun amino- terminal kinase (JNK).

Double-gradient model of embryonic axis formation in Xenopus: perpendicular activity gradients of Wnts and BMPs regulate head- to-tail and dorsal–ventral patterning.

Patterning begins at gastrula stages, but for clarity, it is depicted in an early neurula. The formation of head, trunk and tail requires increasing Wnt activity. mouse / human (placental mammal) pre-implantation mouse embryos from the 2- cell stage to over 100-cells as the expanded blastocysts hatch from their zona pellucidas

compaction: time lapse video Implantation of the blastocyst

` In toto live imaging of mouse development from gastrulation to early organogenesis

schematic gastrulation video

schematic “embryo folding" video Gastrulation - establishment of outer inner between cells

outer cell layer - ectoderm inner layer - endoderm between ectoderm / endoderm layers - mesoderm • Prior to implantation, both human and mouse embryos similarly undergo cell divisions culminating in the development of a blastocyst comprising a discernible ICM and TE. • Mouse embryonic genome activation begins at the 2-cell stage. • The mouse blastocyst forms between days 3 and 4. • Pre-implantation blastocysts comprise an outer layer of trophectoderm (TE) cells, which form the trophoblast lineage of the placenta, and an inner cell mass (ICM) that segregates into epiblast (Epi) and primitive endoderm (PE) layers. Epiblast cells give rise to future fetus, whereas the PE gives rise to extra-embryonic endoderm (ExEn) cells that will form the yolk sac. • In the mouse, the TE gives rise to a proliferative stem cell pool of extra-embryonic ectoderm (ExEc) cells that bud off differentiated polyploid trophoblast giant (TG) cells. • Prior to implantation, both human and mouse embryos similarly undergo cell divisions culminating in the development of a blastocyst comprising a discernible ICM and TE. Mouse • Embryonic genome activation in human begins at ~4- to 8-cell stage on day 3. • Compaction and blastocyst formation are delayed in human embryos (compared to mouse); human blastocysts form between days 5 and 6. • Pre-implantation blastocysts comprise an outer layer of trophectoderm (TE) cells, which form the trophoblast lineage of the placenta, and an inner cell mass (ICM) that segregates into epiblast and primitive endoderm layers. • In human TE gives rise to villous cytotrophoblast (VCT) cells, a multinucleated syncytium (Syn) and extravillous trophoblast cells anti-bmp READ Submit one question / call for clarification via beSocratic