Lecture - Week 3 Development

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Week 3 | Placentation | Gastrulation | Notochord

Introduction

In this lecture I will talk about week 3 of embryonic development. I will cover early placentation, gastrulation and embryonic folding.


Lecture - Print PDF

Objectives

Germ layers
  • Understand the process early placentation, villi formation
  • Understand broadly the events of week 3 of human development
  • Understand the process of gastrulation
  • Understand the process of axis formation and embryo folding
  • Brief understanding of early cardiogenesis.

Lecture Resources

Movies
Mesoderm 001 icon.jpg
 ‎‎Week 3 Mesoderm
Page | Play
Notochord 01 icon.jpg
 ‎‎Week 3 Notochord
Page | Play
Notochord 02 icon.jpg
 ‎‎Week 3 Notochord
Page | Play
Week3 folding icon.jpg
 ‎‎Week 3
Page | Play
Amnion 001 icon.jpg
 ‎‎Amniotic Cavity
Page | Play
Nodal-cilia-001-icon.jpg
 ‎‎Nodal Cilia
Page | Play
Quail HH stage 2 fibronectin movement.jpg
 ‎‎Gastrulation ECM
Page | Play
Gastrulation planar cell move icon.jpg
 ‎‎Planar Movement
Page | Play
Mesoderm migration movie 1 icon.jpg
 ‎‎Mesoderm Move
Page | Play
References
UNSW Embryology logo
Hill, M.A. (2020). UNSW Embryology (20th ed.) Retrieved January 21, 2021, from https://embryology.med.unsw.edu.au
Textbook cover The Developing Human, 9th edn.
Moore, K.L., Persaud, T.V.N. & Torchia, M.G. (2011). The developing human: clinically oriented embryology (9th ed.). Philadelphia: Saunders.
The following chapter links only work with a UNSW connection.
Textbook cover Larsen's human embryology 4th edn.
Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R. & Francis-West, P.H. (2009). Larsen's human embryology (4th ed.). New York; Edinburgh: Churchill Livingstone.
The following chapter links only work with a UNSW connection.
ECHO360 Recording
Link added after Lecture.
ECHO360 icon.gif

Links only work with currently enrolled UNSW students.

Early Placentation

Early placenta anchoring villi

The trophoblast layer has now differentiated into two morphologically distinct cellular layers.

  • Syncytiotrophoblasts - form a multinucleated cytoplasmic mass by cytotrophoblast cell fusion and both invade the decidua and secrete hCG
  • Cytotrophoblasts - form a cellular layer around the blastocyst, proliferates and extends behind syncitiotrophoblasts

Early Utero-Placental exchange

  • transfer of nutrition from maternal lacunae filled with secretions from uterine glands and maternal blood from blood vessels.
  • development of trophoblast villi extending into the uterine decidua.
  • ongoing process with placental development (covered in future lecture)
primary villi

There are three stages of villi development:

  1. Primary Villi - cytotrophoblast
  2. Secondary Villi - cytotrophoblast + extraembryonic mesoderm
  3. Tertiary Villi - cytotrophoblast + extraembryonic mesoderm+ blood vessels

There are two main types of early villi:

  1. Anchoring villi - attached to decidua
  2. Floating villi - not attached to decidua, floating in maternal lacunae.


Links: Development Animation - Implantation

Gastrulation

Embryonic Disc

  • Gastrulation, (Greek = belly) means the formation of gut.
  • used in a more looser sense to to describe the formation of the trilaminar embryo.
  • Epiblast layer - consists of totipotential cells forming all 3 embryo layers (germ layers) that form the entire embryo.
    • Historically, gastrulation was one of the earliest observable morphological event occurring in the frog embryo.
    • Currently, the molecular and physical mechanisms that regulate patterning and migration during this key event are being investigated in several different animal models.
    • In humans, it is proposed that similar mechanisms regulate gastrulation to those found in other vertebrates.
Stage7 features.jpg

Embryonic Disc showing primitive streak

Mesoderm 001 icon.jpg
 ‎‎Week 3 Mesoderm
Page | Play
Trilaminar embryo.jpg
  1. Ectoderm (epithelium) - forms the central and peripheral nervous system, parts of the sensory systems, and the epithelium of the skin.
  2. Mesoderm (connective tissue) - forms the body connective tissues: blood, bone, muscle, connective tissue skin, gastrointestinal and respiratory tracts.
  3. Endoderm (epithelium) - forms gastrointestinal tract organs and the epithelium of the gastrointestinal and respiratory tracts.
This is a very simplified explanation of the 3 layer contributions, many body tissues have contributions from all 3 origins.


Primitive Streak

The primitive streak is the visible feature which represents the site of cell migration to form the additional layers.

  • Midline of embryonic disc running between the primitive node to near the edge of the disc at the caudal end (near the connecting stalk).
  • Region of cell migration from the epiblast layer forming sequentially the two germ cell layers (endoderm and mesoderm).
  • Cells that do not migrate form the ectoderm.
Stage7 primitive streak labelled.jpg

Primitive Node

  • (Hensen's node) region in the middle of the early embryonic disc epiblast from which the primitive streak extends caudally (tail).
    • nodal cilia establish the embryo left/right axis
    • axial process extends from the nodal epiblast
Nodal-cilia-001-icon.jpg
 ‎‎Nodal Cilia
Page | Play
Stage7 axial process.jpg
Links: Development Animation - Mesoderm | Gastrulation

Epithelial to Mesenchymal Transition

Chicken-gastrulation2.jpg Chicken-gastrulation3.jpg
  • Epithelial cells (organised cellular layer) which loose their organisation and migrate/proliferate as a mesenchymal cells (disorganised cellular layers) are said to have undergone an Epithelial Mesenchymal Transition (EMT).
  • Mesenchymal cells have an embryonic connective tissue-like cellular arrangement
    • cells that have undergone this process may at a later time and under specific signaling conditions undergo the opposite process, mesenchyme to epithelia.
  • In development, this process can be repeated several times during tissue differentiation.

This process occurs at the primitive streak where epiblast cells undergo an epithelial to mesenchymal transition in order to delaminate and migrate.

MH - there are a number of common cellular changes that occur during embryonic development at different times and in different tissues, which we can classify into "developmental mechanisms".

Notochord

Stage7 axial process
  • The notochord is a structure which has an early mechanical role in embryonic disc folding and a major signaling role in patterning surrounding embryonic tissue development.
  • A developmental feature not present in the adult anatomy.
  • This signaling role patterns many different tissues (neural plate, neural tube, somites, endodermal organs).
  • Has own sequence of development from a primitive axial process
    • axial process an initial epiblast hollow epithelial tube which extends in the midline from the primitive pit, cranially in the embryonic disc (toward the oral membrane).
    • neuroenteric canal is a transient communication between the amnionic cavity and the yolk sac cavity formed by the axial process.
    • notochordal plate forms from the axial process merging with the endoderm layer.
    • notochord forms from the notochordal plate which then separates back into the mesoderm layer as a solid column of cells lying in the midline of the embryonic disc and running rostro-caudally (head to tail).
  • An alternate name for the notochord is "axial mesoderm".

MH - Much of our knowledge of this structure comes from the study of animal models of development.

Notochord 01 icon.jpg
 ‎‎Week 3 Notochord
Page | Play
Notochord 02 icon.jpg
 ‎‎Week 3 Notochord
Page | Play


Links: Notochord Movie | Mesoderm

Embryo Folding

  • embryonic disc now has 3 germ layers (ectoderm, mesoderm, endoderm)
  • this flat 3 layer disc then begins to fold ventrally at the edges
  • eventually forming a "hollow tube" of these 3 germ layers
Stage7 folding.jpg

Embryonic Disc Folding - all edges of the embryonic disc will fold ventrally, forming a rostro-caudal "C" shaped tube.

Week3 folding icon.jpg
 ‎‎Week 3
Page | Play

Cardiogenesis

  • Heart - heart in week 3 begins as paired heart tubes.
  • formed from prechordal splanchnic mesoderm.

Heart Tube Fusion.jpg

Timeline

Human development timeline graph 01.jpg

Human development timeline graph icon.jpg
 ‎‎Human Timeline
Page | Play
Human development 001 icon.jpg
 ‎‎Human Development
Page | Play
Next: Lab 2 - Fertilization/Week 1 and 2

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