Somites Flashcards
What are somitomeres?
A concentric whirl of mesoderm cells arranged in a tight ball. There is one on each side of the midline (pair formation) - somite.
Describe the distribution of the somites in the regions of the body.
4 somites in the occipital region.
8 in cervical
12 in thoracic
5 in lumbar
5 in sacral
3 in coccygeal
What parts do somites differentiate into? What do they eventually form?
The scelerotome and dermomyotome.
Sclerotome forms the vertebrae and rib cartilage while the dermomyotome includes dermis and muscle zones.
How is the differentiation of somites into sclerotome induced?
The ventralmedial part of the somite epithelial cells will be induced by shh and noggin, from the floor of the neural tube and the notochord.
This causes the ventral part of the somite to express pax1 and form the sclerotome.
How is the differentiation of somites into dermomyotome induced?
The dorsolateral part of the somite will be induced by wnt members from the ectoderm and low shh from the notochord.
This activates pax3 to form the dermomyotome.
How does the dermatome and myotome form?
Mesenchymal ends of the dermomyotome migrate underneath the epithelial layer, forming 2 layers. These are the dermatome and myotome.
What regions will the sclerotome divide into?
Ventral, dorsal and lateral.
Describe the formation of the vertebrae.
The sclerotome will split into rostral and caudal segments due to an outgrowth of a nerve. They then fuse to form the vertebrae.
Chondrification and ossification centres are observed at around 6 weeks, whilst bones begin developing at around 7 weeks.
What are epimeres and hypomeres?
Epimere - epaxial muscles in the back.
Hypomere - hypaxial muscles of the lateral and ventral wall in the thorax and abdomen.
What signalling molecules direct formation of epaxial muscles?
Wnts direct the dorsomedial portion of the somite to form epaxial (back) muscles.
What signalling molecules direct formation of hypaxials?
Wnt and Bmp4 signals the dorsolateral portion to form hypaxial (limb and body wall) muscles.
What does the myotome differentiate into?
From the myotome comes the differentiation of the dorsal part called epimer and a ventral part called hypomer.
What is the mesenchyme?
The mesenchyme is a type of CT in the embryo. It’s a jelly-like substance made by mesenchyme cells shaped like a star.
They will differentiate into many types of primitive cell lines, such as fibroblasts, chondroblasts and osteoblasts.
What are the types of bone cells? Where do they arise from?
Osteoblasts - arise from mesenchymal stem cells.
Osteocytes - arise from mesenchymal stem cells.
Osteoclasts - arise from the hematopoietic system.
What are the ossification processes?
Ossification
Endochondral ossification
Intramembranous ossification
