18. Early Embryonic Development - The Fate of the Mesoderm Flashcards
What has happened by the end of week 3 in early embryonic development?
The embryo has gastrulated and the bilaminar disk is converted to a trilaminar disk. The axes have been set and the three germ layers have developed: ectoderm, mesoderm and endoderm.
What is the objective of the mesoderm?
Notochord formation for neurulation, organisation of the mesoderm into somites, intermediate and lateral plate mesoderm.
What is the objective of segmentation?
To form dermatomes and myotomes.
What is the notochord responsible for?
Releasing signals to surrounding ectoderm.
What are the two parts of neurulation?
Notochord - driven induction of ectoderm to form the nervous system.
Neural plate - folds to develop the brain and spinal cord from the neural tube.
Where is the notochord found?
Between the ectoderm and endoderm.
How does neurectoderm form?
The notochord directs conversion of overlying ectoderm to neurectoderm.
How is the neural tube formed?
Cells at the edges of the neural plate so they rise up and curl round to meet each other and form the tube.
What is the paraxial mesoderm?
The mesoderm found either side of the axis.
What is the intermediate mesoderm?
The mesoderm found inbetween the axis and the edge.
What is the somatic mesoderm?
The mesoderm that goes on to give the skeletal muscle and is to do with body and body structures.
What is the splanchnic mesoderm?
The mesoderm to do with viscera/ organs, e.g. intestines.
What is the intraembryonic coelom?
The space inside the embryo (new cavity in gastrulation).
What are somites?
The organisation of paraxial mesoderm into segments.
When does the first somite appear in embryonic development?
At day 20.
Why can somites be used to age the foetus?
Because they have very predictable presentation. The first appears at day 20 and then 3 pairs for every day after that until there are 42-44 pairs by the end of week 5. Some then disappear, leaving 31 pairs total.
What is organised degeneration in embryonic development?
The ventral wall of somites breaks down leading to the formation of the sclerotome (gives rise to hard tissue).
What are the following derivatives of somites?
a. Dermatome
b. Myotome
c. Sclerotome
a. Skin section/ dermis
b. Muscle section/ muscle
c. Hard tissue section/ bones.
Why is having 31 pairs of somite important for later development?
There are 31 pairs of spinal nerves.
What are the two meanings of dermatome and myotome (developmental and clinical)?
Dermatome = part of the somite that gives rise to the dermis (dev.) or the strip of skin supplied by a single spinal nerve (clin.) Myotome = gives rise to the muscles (dev.) or the muscle/groups of muscles supplied by a single spinal nerve (clin.).
What is the cardiogenic area in embryonic development?
The future heart.
What is the buccopharyngeal membrane in embryonic development?
The future mouth, the opening to the oral cavity.
How does embryonic folding happen?
Cephalocaudal folding driven by the size of the neurotube - the head (cranial end) folds under, then the tail folds under.
Lateral folding driven by the size of the developing somite - the sides fold under.
What happens to the cardiogenic field and buccopharyngeal membrane in embryonic folding?
They are both folded into the inside in a new cavity, with the ectoderm facing outside still.
What does folding achieve in embryonic development?
It draws together the margins of the disk to create a ventral body wall, pull amniotic membrane around the disk, pull connecting stalk ventrally.
What has happened by the end of the fourth week in embryonic development?
The nervous system has started to form, segments have appeared and the embryo has folded to put everything in the right place.
