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.
How many pairs of spinal nerves are there?
31 segments in the body (originating from 31 pairs of somites) and therefore 31 pairs of spinal nerves.
Clinically what does a dermatome refer to?
A strip of skin supplied by a single spinal nerve.
Clinically what does a myotome refer to?
A muscle/ group of muscles supplied by a single spinal nerve
What does the paraxial mesoderm develop into?
Axial skeleton - vertebral column and ribs
Dermis
Muscles of anterior/lateral body wall
Some limb muscles
What two types of mesoderm make up the lateral plate?
Somatic
Splanchnic
What does the somatic mesoderm develop into?
Somatic means body- it develop into body structures:
the connective tissue of limbs, contributes to the A/L body wall.
What does splanchnic mesoderm develop into?
Smooth musculature, connective tissue and vasculature of the gut.
What does intermediate mesoderm develop into?
Urogenital system e.g. kidneys, ureter, gonads.
In the 4th week what types of folding occur?
- Cephalocaudal folding (head and tail fold in)
2. Lateral folding (sides fold in)
These two types of folding leave one opening. Why?
It’s the opening to the umbilicus.
What affect does craniocaudal folding have?
- The amniotic sac now surrounds the entire embryo.
- The cardiogenic area is now located in its normal location (and primordium of diaphragm).
- The connecting stalk is pulled ventrally -> true umbilical chord.
What affect does the lateral folding have?
The space between ‘the somatic and splanchnic leaves of mesoderm creates a:
- cavity inside the embryo: intraembryonic coelom
- Ventral body wall
- Part of yolk sac pinched off -> secondary yolk sac formed (creates the primordium of the gut)
What does folding acheive?
Draws together the margins of the disk:
- Creating a ventral body wall
- Pulling amniotic membrane around the disk and suspending the embryo within the amniotic sac
- Pulling connective stalk ventrally -> true umbilical chord
What has happened by the end of this fourth week?
- Neuralation :the NS has started to form.
- Segmentation: segments have appeared, assigning specific tasks to specific cells.
Organisation of the mesoderm: the embryo has folded, putting everything in its right place.
