Gastrulation

Gastrulation - The point in embryogenesis where the basic organization of the organism is established. A.Establishment of the three basic germ layers Ectoderm, Mesoderm, Endoderm B. Groups of cells that will give rise to specific organ systems and tissues are moved into the right position both externally and internally. C. Groups of cells that will influence each others differentiation are positioned so they can have an effect on each other.

How are layers of tissue established? How do cells get to their proper positions for development? In part, this is accomplished by various types of Morphogenetic Movements. QUESTIONS?

Morphogenetic Movements Epiboly

Morphogenetic Movements Invagination, Evagination

Involution Morphogenetic Movements Epiboly over a fold Involution

Morphogenetic Movements Delamination

Morphogenetic Movements Ingression

Morphogenetic Movements Cell Migration

Morphogenetic Movements “Intercalation: rows of cells move between one another, creating an array of cells that is longer (in one or more dimensions) but thinner.” “Convergent Extension: rows of cells intercalate, but the intercalation is highly directional.” - involved in formation of the primitive streak in avian and mammalian embryos.

Differential Growth Morphogenetic Movements time

ingression - primary and secondary mesenchyme invagination - initial formation of the archenteron convergent extension - lengthening of the archenteron Starfish/Sea urchin gastrulation

Amphibian Gastrulation (Urodeles, e.g. salamanders) Urodele blastula

hr Urodele

hr Urodele

hr Urodele

28 hr

38 hr

15 hr of time lapse, 48 min/sec

Shell Blastodisk Chalazae Thin albumin Yolk Air pocket Shell membrane

The morphogenetic movement that results in elongation of the primitive streak is convergent extension. z

Chicken (Avian) Gastrulation Formation of the primitive streak Morphogenetic movement involved in the formation of the primitive streak is convergent extension.

PRIMITIVE STREAK Actually, this is a human embryo. We have the same type of gastrulation as a chicken embryo, yet we do not have a large yolky egg. Why might this be?

Establishing the Endoderm Extraembryonic endoderm Primary Hypoblast Secondary Hypoblast Embryonic endoderm Derived from cells that are originally in the epiblast that ingress through the primitive streak region and then migrate and insert into the hypoblast layer.

Chicken (Avian) Gastrulation - primary and secondary hypoblast

Yolk Chicken (Avian) Gastrulation Fate map as gastrulation is beginning Yolk

Ingression during avian gastrulation 1. Hensen’s Node 2. Primitive groove Two regions of ingression:

2nd wave - head mesoderm 3rd wave - chordamesoderm Later, * Hensen’s node will contribute to somites as well *Some cells from Hensen’s node will remain in the epiblast and contribute to neural plate Timing (waves) of ingression: Hensen’s node - initial waves 1st wave - foregut endoderm (mostly as primitive streak forms)

Primitive groove 1st wave - rest of embryonic endoderm (mostly as primitive streak forms) 2nd wave - mesoderm (embryonic & extraembryonic) Ingression during avian gastrulation

Chicken (Avian) Gastrulation

embryonic endoderm groove Embryonic ectoderm Chicken (Avian) Gastrulation

Chicken (Avian) Gastrulation - embryonic endoderm

Chicken (Avian) Gastrulation Establishment of the primary germ layers

Chicken (Avian) Gastrulation See animation on digital lab manual

Chicken (Avian) Gastrulation