2010 Lecture 4

From Embryology

Week 3 Development

Introduction

Inner cell mass

This lecture will continue from the second week into the third week and discuss early placentation, gastrulation and notochord formation. Note that we will be covering only the early events of placentation and a later lecture will cover this topic in more detail.

File:Week2_001_icon.jpg</wikiflv> This animation shows the process of implantation, occurring during week 2 of development in humans.


(More? Development Animation - Implantation)

Lecture Overview

  • Understand broadly the events of week 2-3 of human development
  • Understand the process early placentation, villi formation
  • Understand the process of gastrulation
  • Understand the process of axis formation

Early Placentation

Development Animation - Implantation

Early placenta anchoring villi

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

  • Syncitiotrophoblasts - 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. The development of trophoblast villi extending into the uterine decidua.

There are three stages of villi development:

  • Primary Villi - cytotrophoblast
  • Secondary Villi - cytotrophoblast + extraembryonic mesoderm
  • Tertiary Villi - cytotrophoblast + extraembryonic mesoderm+ blood vessels

There are two main types of early villi:

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

Maternal endometrium

  • Decidual reaction

Gastrulation

Development Animation - Mesoderm | Gastrulation

Embryonic Disc showing primitive streak

Gastrulation, (Greek = belly) means the formation of gut, but has been used in a more looser sense to to describe the formation of the trilaminar embryo. The epiblast layer, consisting of totipotential cells, derives all 3 embryo layers: endoderm, mesoderm and ectoderm. The primitive streak is the visible feature which represents the site of cell migration to form the additional layers.

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.

  • primitive 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
  • primitive streak - region of cell migration from the epiblast layer forming sequentially the two germ cell layers (endoderm and mesoderm)

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, 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

Development Animation - 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. This signaling role patterns many different tissues (neural plate, neural tube, somites, endodermal organs). It has its own sequence of development from a primitive axial process and is a developmental feature not present in the adult anatomy.

  • 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.

Movies

Week2 001 icon.jpg Mesoderm 001 icon.jpg Notochord 01 icon.jpg Notochord 02 icon.jpg Chorion 001 icon.jpg Amnion 001 icon.jpg Week3 folding icon.jpg
Implantation Mesoderm Notochord Notochord Chorionic Cavity Amniotic Cavity Week 3

References

Textbooks

  • The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Chapter 3
  • Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Chapter 3

Online Textbooks

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Course Content 2010

Embryology Introduction | Cell Division/Fertilization | Lab 1 | Week 1&2 Development | Week 3 Development | Lab 2 | Mesoderm Development | Ectoderm, Early Neural, Neural Crest | Lab 3 | Early Vascular Development | Placenta | Lab 4 | Endoderm, Early Gastrointestinal | Respiratory Development | Lab 5 | Head Development | Neural Crest Development | Lab 6 | Musculoskeletal Development | Limb Development | Lab 7 | Kidney | Genital | Lab 8 | Sensory | Stem Cells | Stem Cells | Endocrine | Lab 10 | Late Vascular Development | Integumentary | Lab 11 | Birth, Postnatal | Revision | Lab 12 | Lecture Audio | Course Timetable


Cite this page: Hill, M.A. (2019, May 24) Embryology 2010 Lecture 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_Lecture_4

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G