Lecture 12 - Gastrulation and Morphogenesis Flashcards
Define gastrulation
The process by which the bilaminar embryo is transformed into one that has 3 germ layers. Each of these layers will give rise to different parts of the embryo
What are the three germ layers created via gastrulation derived from?
The epiblast proper
What will the ectoderm differentiate to become? (2)
Skin
Nervous system
What will the mesoderm differentiate to become? (3)
Bones
Muscles
Mesenteries
What will the endoderm differentiate to become? (2)
Lining of gut
Large glands (e.g liver and pancreas)
When does cellular movement during gastrulation occur?
~15 days after fertilisation
What occurs during cellular movement during gastrulation?
Primitive streak forms
What does the primitive streak define?
The midline and caudal end of an embryo
What is the difference between caudal and rostral?
Caudal - near tail end
Rostral - near front end
Describe the formation of the primitive streak.
Cells in epiblast start migrating towards the midline on the caudal end of the epiblast and then downwards towards the hypoblast
What happens to the primitive streak as more cells join the stream of downwards movement?
The movement of the primitive streak elongates rostrally
What happens to the primitive streak when fewer cells are actively migrating?
The primitive streak shortens
Is the primitive streak a structure?
The primitive streak is not technically a structure in itself.
Describe the formation of the three embryonic layers.
Endoderm: first cells to move through the primitive streak move downwards towards the hypoblast, and then sideways underneath the epiblast. This pushes the hypoblast towards the sides of the embryo and the inner wall of the blastocyst cavity/blastocoele. The first wave of cells replace the hypoblast in the embryonic disk and gives rise to the endoderm.
Mesoderm: second wave of migrating cells moves between endoderm and epiblast. Loosely packed mesoderm is known as mesenchyme.
Ectoderm: remaining epiblast is known as the ectoderm
During gastrulation, the embryonic disc elongates to form a(n) _______ shape.
Oblong
What is the name of the depression found at the rostral end?
Oropharyngeal membrane
What does the oropharyngeal membrane become?
The mouth opening
Which end does the oropharyngeal membrane come from?
Rostral end
What is the name of the depression found at the caudal end?
The cloacal membrane
What does the cloacal membrane become?
The opening of the anus
What end of the embryonic disc does the cloacal membrane come from?
The caudal end
How many regions within the mammalian embryo contribute to establishing the head and tail ends?
2 - known as “centres”
Does the oropharyngeal membrane and cloacal membrane contain mesoderm?
No, these structures are devoid of mesoderm.
What is another name (or two) for a centre?
Organising centre
Organiser
What is the role of an organising centre?
Organising centres secrete molecules that act at a distance and influence the differentiation of different body parts
Name two examples of organisers in the embryo
The node of the primitive streak (primitive node)
The anterior visceral endoderm (AVE)
What does an agonist do?
It initiates a response
What does an antagonist do?
It inhibits a response
What are HOX genes responsible for?
They encode transcription factors which regulate body formation
What is fate mapping/lineage?
Understanding embryonic origin of various tissues by establishing the correspondence between cells at one stage of development and their progeny at later stages.
What is the primitive node? What does it do?
An organiser that is responsible for the formation of the body, and gives rise to the notochord
What do the AVE and primitive node contribute to?
The formation of an embryonic head
What is the notochord?
A transient medial mesodermal structure that influences the development of surrounding tissue
At what time do organs begin to form?
During the 4th week of development
What occurs during morphogenesis?
The embryo undergoes dramatic changes in shape and size
What is the overall product of embryonic folding?
The embryo transforms from a trilaminar disc to a 3D structure
What is embryonic folding the result of?
Differential growth - some parts of the embryo grow at an increased rate relative to other parts
Along what axis does longitudinal bending occur?
The rostro-caudal axis
What is the process of longitudinal bending?
The flat disc of the trilaminar embryo bends along the rostro-caudal axis
The head and tail move downwards, endoderm moves towards middle
A tube formed with blind ends which will eventually become the gut
Along what axis does transverse folding occur?
The transverse axis
Describe the process of transverse folding
Transverse folding occurs simultaneously with longitudinal bending
The lateral edges of the embryo move downwards and eventually meet along the midline - the embryo is now tube shaped
Fusion begins at the cranial and caudal ends and progress like a zipper towards the mid-trunk
By the end of embryonic folding, what is the shape of the embryo?
The embryo is a closed tube except for a small opening connecting the inside of the gut to the yolk sac
What does the 3D structure of an embryo 1 month post-fertilisation resemble?
It resembles that of a typical vertebrate.
What are characteristics of a typical vertebrate that can be seen in the embryo at the miniature vertebrate stage? (3)
Surrounded by ectoderm
Endoderm surrounds a hollow tube (will become walls of gut)
Mesodermal structures developing between ectoderm and endoderm
Have the organs fully developed by the time the embryo reaches the miniature vertebrate stage?
No, organ formation has begun but still possesses a rudimentary structure, and will continue to develop.
