Embryology of the Muscular and Skeletal Systems Flashcards
Define somites
Somites are segments of paraxial mesoderm (form alongside the notochord)
Somites form the Dermatome, Myotome, and Sclerotome
Define Dermatome, Myotome, Sclerotome
Dermatome: forms from somites (paraxial mesoderm); migrates to form the dermis layer (medial layer of skin)
Myotome: forms from somites (paraxial mesoderm); migrates everywhere to form skeletal muscles
Sclerotome: forms from somites (paraxial mesoderm); forms “hard” structures like bones (axial skeleton) and cartilage
Define Intracartilaginous/Endochondral Ossification
Which bones use this method to form?
One of two ways in which bone can form!
Sclerotome-derived bones (most bones! e.g. vertebrae, ribs, sternum) all use this method!
1. mesenchyme condenses
2. CHONDROBLASTS in the mesenchyme form a CARTILAGE MODEL of the bone
3. the cartilage model becomes ossified by osteoblasts
Define Intramembranous Ossification
Which bones use this method to form?
One of two ways in which bone can form!
Importantly, used to form SKULL
- mesenchymal sheets (membranes) condense but do not form cartilage
- osteoblasts differentiate and deposit BONE SPICULES
Define epiphysial growth plate
this is a cartilage plate that keeps forming new cartilage model cells (on the epiphyseal side) that are ossified (on the diaphysial side) and cause long bones to continue growing until about age 20
Define achondroplasia
“dwarfism”: premature or abnormal ossification of cartilages, particularly affecting long bones (short limbs, short stature)
Define scoliosis
Scoliosis is a lateral curvature of the spine caused by (for the purposes of this class) an error in vertebral body formation or fusion, for example hemivertebra
Define Epaxial and Hypaxial myotome
epiblast -> mesoderm -> paraxial (along notochord) mesoderm -> somites -> dermatome/myotome/sclerotome -> myotome -> epaxial and hypaxial divisions
Epaxial division: gives rise to EXTENSORS of the trunk region (e.g. erector spinae)
Hypaxial division: gives rise to the FLEXORS of the trunk region (including muscles of the ventral body wall)
What gives rise to skeletal muscle?
What gives rise to smooth (visceral) muscle?
Most but not all skeletal muscle arises from the myotome (from somites from paraxial mesoderm).
Smooth muscle around the gut arises from splanchnic (visceral) mesoderm, which is the inner lining of the Intraembryonic Coelom (with midgut, bit of yolk sac, inside of it)
What is prochordal mesoderm?
What does it form and how does it form this?
Prochordal mesoderm is a region of mesoderm that is at the rostral end, rostral to the oropharyngeal membrane.
- Cavitation splits it into dorsal (somatic) and ventral (splanchnic) halves.
- Splanchnic prochordal mesoderm forms angiogenic cells.
- The angiogenic cells coalesce into a solid tube.
- Canalization of the tube forms a pair of hollow endocardial tubes which will form the inner lining of the heart.
ALSO: prochordal mesoderm will become CARDIAC MUSCLE
What are the branchial arches and what will they become?
NEURAL CREST cells migrate and form “gills” (branchial, or pharyngeal, arches)
These will become MOST MUSCLES IN THE HEAD AND NECK
What is branchial arch mesenchyme derived from?
from neural crest
What is the Apical Ectodermal Ridge?
An area of ectoderm at the tip of a forming limb that is an INDUCTIVE structure! It sends out Fibroblast Growth Factors 8 & 10 that induce the different structures of the limb.
How are digits formed?
The AER splits and induces the formation of digits…
DIGITAL RAYS are sculpted by utilizing apoptosis between each “fingertip” of apical ectodermal ridge.
What are AMELIA and MEROMELIA?
These are errors in limb induction or pattern formation.
Amelia: complete
Meromelia: partial
What are POLYDACTYLY and SYNDACTYLY?
These are errors in digital ray formation and/or the programmed cell death between rays.
Polydactyly: too many digits
Syndactyly: too few digits (fused)
What parts of the musculoskeletal system are derived from somites and what parts are not?
Derived from somites:
dermis (from dermatome)
skeletal muscle (from myotome)
most of the axial skeleton (from sclerotome)
Not derived from somites:
- -cranial vault (from head mesoderm)
- -visceral smooth muscle (from splanchnic mesoderm of lateral plate)
- -smooth muscle of blood vessels and lymphatics (from somatic mesoderm of lateral plate)
- -cardiac muscle (from prochordal mesoderm)
- -head and neck muscles (from branchial arch mesenchyme from neural crest)
What are the embryonic origins of the axial and appendicular skeleton?
axial skeleton: sclerotome-derived cells form vertebrae, ribs, sternum via intracartilaginous ossification
head mesoderm forms cranial vault via intramembranous ossification
appendicular skeleton: made of ectoderm, somatic mesoderm, and paraxial mesoderm via intracartilaginous ossification
What is the fate of the notochord?
the notochord is replaced by sclerotome cells from somites (-> vertebrae) and the remnant of the notochord is the NUCLEUS PULPOSUS, in the intervertebral discs
Why are the vertebral bodies considered INTERSEGMENTAL structures?
The boundary between vertebrae shifts by half a segment.
- Sclerotome cells migrate to notochord in a segmented fashion.
- The caudal halves of each segment form cartilages.
- The rostral halves stay less condensed.
- The caudal half of one segment ossifies with a rostral portion of the next segment back.
- The intervertebral disc is formed from the rostral portion that was not ossified with the caudal segment ahead of it.
What is the contribution of the branchial arch mesenchyme to the musculoskeletal system?
Branchial arch mesenchyme (neural crest-derived) froms most muscles in the head and neck.
What types of tissues participate in limb formation?
Muscles of limbs are formed from MYOTOME cells (from the somites from paraxial mesoderm).
Appendicular skeleton is made of:
ectoderm
somatic mesoderm
paraxial mesoderm
What is the developmental origin of cardiac muscle?
prochordal mesoderm
What is the developmental origin of smooth muscle?
splanchnic mesoderm from lateral plate -> visceral smooth muscle
somatic mesoderm from lateral plate -> smooth muscle of blood vessels and lymphatics
