Lecture - Week 3 Development

2010 Lecture | placentation | gastrulation | Notochord

Objectives

Germ layers

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

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

Links: Development Animation - Implantation

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)

Links: Development Animation - Mesoderm | Gastrulation

Epithelial to Mesenchymal Transition

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".

Co-ordinator Note

Dr Mark Hill

ANAT2341 Embryology S2 2011

Lecture content is made available on the Monday of the associated lecture week.
Some guest lecturers will provide additional handouts or lecture notes.
For background information see Embryonic Development and Systems Notes
2011 Timetable | 2011 Students

--Mark Hill 06:47, 20 July 2011 (EST)

Course Content 2011

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Cite this page: Hill, M.A. (2024, April 30) Embryology Lecture - Week 3 Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Week_3_Development

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