Embryology - mesoderm, segmentation and folding (week 4) Flashcards
Which component of the embryo directs the conversion of overlying ectoderm to neuroectoderm?
notochord
Notochord signals cause overlying ectoderm to thicken into what slipper-shaped component of the embryo?
Neural plate
How does the neural tube form?
The neuroectoderm in the edges of the neural plate multiplies so much that the edges of the neural plate elevate out of the plane of the disk and curl towards each other, creating the neural tube. The ectoderm then closes over the top of this tube.
What are the four new differentiated mesoderm types that form?
- Somatic mesoderm - to do with body structures
- Splanchnic mesoderm - to do with the organs
- Paraxial mesoderm - para to the neural tube
- Intermediate mesoderm
What are somites?
Somites are division of the body of an animal or embryo. They are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo in segmented animals.
Why are somites useful in dating the age of embryos?
They have a regular, predictable appearence e.g. first pair appear at day 20 in the occipital region.
Somites appear in a craniocaudal sequence. How many appear per day?
3 pairs/day
How many somites are present by the end of week 5? How many are present in the adult?
43-44 pairs
some disappear, leaving 31 pairs
What do somites look like when they first appear?
A regular block of mesoderm cells arranged around a small cavity.
What happens during “organised degeneration” of somites?
The ventral wall breaks down, leading to the formation of the schlerotome (hard section of tissue).
What happens to the dorsal portion of the degenerated somite?
It forms a combined dermomyotome (consisting of myotome and dermotome).
What happens to the dermamyotome?
The myotome proliferates and migrates and the dermatome disperses - but both still stay affiliated to their original somite.
Name the three somite derivatives
Dermatome
Myotome
Sclerotome
These are the beginnings of the muscular-skeletal system.
What does dermatome differentiate into?
Dermis (skin)
What does myotome differentiate into?
Muscle
What does scleratome differentiate into?
Bones (sclero- means hard)
What is the position of the dermomyotome relative to the sclerotome?
Dermomyotome is dorsal relative to the sclerotome.
In what relation to each other are, the derivatives of the dermomyotome, the back muscles to the ventral body wall muscles & limb muscles?
The back muscle are medial to the ventral body wall muscles & limb muscles.
In what relation to each other are the vertebral body and the vertebral arch (both derivatives of the sclerotome)?
The vertebral body is medial to the the vertebral arch.
In what relation to each other are the proximal and distal ribs?
The proximal ribs are medial to the distal ribs.
In what relation to each other are the vertebral body and vertebral arch to the proximal and distal ribs?
The ribs are ventral to the vertebrae.
What are the implications of segmentation?
- It gives rise to repeating structures (vertebrae, ribs, intercostal muscles and spinal chord segments (functionally not anatomically segmented)).
- Guides innervation.
What is an epimere?
The dorsal portion of a somite, from which are formed muscles innervated by the dorsal branch of a spinal nerve for that segment. These muscles are the skeletal muscle of the back.
What is an hypomere?
The ventral portion of a somite, from which are formed muscle innervated by the ventral branch of a spinal nerve for that segment. These muscle include the lateral and anterior muscle of the abdomen and thorax.
