2010 BGD Lecture - Development of the Embryo/Fetus 1

Introduction

--Mark Hill 20:53, 3 May 2010 (EST) I am currently updating the notes that will appear here before the lecture on Thursday this week for 2010. Previous 2008 Lecture

Begin by reviewing the recent Foundations Lecture and Practical.
This lecture covers conceptus development from fertilization to implantation to trilaminar embryo formation.
The lecture will also introduce early fetal membranes and placentation.

For those that cannot wait, here are some images to get you started......

Week 1 and 2

Gastrulation

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)

gastrulation

Trilaminar embryo SEM

Trilaminar embryo

Notochord

Movie - notochord 1 | - notochord 2

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".

Early Placentation

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.

Somitogenesis

Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.

Coelom, meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The intraembryonic coelom is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities (pericardial, pleural, peritoneal).