DAA Embryo Exam 2 + MMQs

Somites arise from which mesoderm?

Paraxial mesoderm

Four mesoderm regions shown beside the neural tube

Paraxial, intermediate, lateral plate, and chordamesoderm (notochord)

Lateral plate mesoderm splits into which two layers?

Somatic mesoderm and splanchnic mesoderm

Splanchnic mesoderm gives rise to

Most of the smooth or cardiac muscle

Somatic mesoderm gives rise to

Smooth muscle found in blood vessels

Age of the embryo showing the early somites in cross section

22 days

Somitocoel cells

Cells of the developing somite that surround the somite cavity

Two parts a somite differentiates into (early)

Dermomyotome and sclerotome

Three structures a somite ultimately becomes

Myotome, dermatome, and sclerotome

Dermomyotome splits into

Dermatome and myotome

Dermatome becomes

Dermis of the skin (future dermis)

Myotome becomes

Skeletal muscle

Sclerotome becomes

Vertebrae and ribs (axial skeleton); migrates around the neural tube and notochord

Ectoderm over the somites becomes

Epidermis

Notochord becomes (in the adult)

Nucleus pulposus of the intervertebral disc

Mesenchymal contribution to the intervertebral disc comes from

Sclerotome (mesenchyme between developing vertebral bodies)

Sclerotome contribution to the neural arch

Neural arch of the vertebra

Sclerotome contribution to the centrum

Vertebral body (centrum)

Sclerotome contribution to the costal process

Costal process (rib region)

Embryo age: somites visible in the mature gastrula

19 days

Embryo age: dermomyotome and sclerotome visible, neural tube formed

22 days

Embryo age: sclerotome contributions to neural arch, vertebral body, and costal process visible

27 days

Embryo age: myotome and dermatome visible, notochord becomes future nucleus pulposus

30 days

Embryo age shown in the myotome slide with dermatome, myotome, and sclerotome

26 days

Which somite region lies closest to the neural tube and notochord?

Sclerotome

Which somite region lies closest to the ectoderm?

Dermatome (with myotome beneath it)

Number of occipital myotomes shown

4

Number of cervical myotomes shown

8

Number of thoracic myotomes shown

12

Number of lumbar myotomes shown

5

Number of sacral myotomes shown

5

Caudal myotomes that regress

Coccygeal myotomes

Age of the embryo showing segmental distribution of myotomes

6 weeks

Dorsal (epaxial) column of the myotome

Epimeres

Ventral (hypaxial) column of the myotome

Hypomeres

Source of all limb muscle connective tissue

Local mesenchyme in the limb bud

Preotic myotomes form

Extrinsic muscles of the eye

Number of preotic myotomes shown

3

Occipital (postotic) myotomes form

Muscles of the tongue

Pharyngeal arch mesoderm forms

Muscles of mastication and facial expression

Age of the embryo showing myotomes and developing muscular system (preotic/occipital slide)

About 41 days

Age of the embryo showing the myotome regions give rise to skeletal muscles

6 weeks

Age of the embryo showing developing trunk and limb musculature

8 weeks

Epimere gives rise to

Deep (intrinsic) epaxial muscles of the back

Epaxial muscles are innervated by

Dorsal ramus of a spinal nerve

Hypomere gives rise to

Body wall and limb muscles (hypaxial muscles)

Hypaxial muscles are innervated by

Ventral ramus of a spinal nerve

Epaxial vs hypaxial: which is posterior?

Epaxial is dorsal; hypaxial is ventral

Motor neuroblasts form

Primitive axons that enter skeletal muscle of the body wall

Sensory processes travel in

All nerves to muscle, in addition to cutaneous nerves

Sympathetic fibers in each spinal nerve branch supply

Arterial smooth muscle

Dorsal ramus carries sensory input from the back via

Posterior cutaneous nerve

Ventral ramus of thoracic nerves gives rise to

Lateral and anterior cutaneous nerves plus nerves to intercostal and abdominal wall muscles

Intercostal muscles derive from

Hypaxial (hypomere) trunk myotome

Rectus abdominis derives from

Hypaxial myotome (hypomere)

Layered abdominal muscles (external oblique and others) derive from

Hypomere

Psoas muscle derives from

Hypaxial myotome (hypomere)

Short and long muscles of the back derive from

Epimere (epaxial)

Erector spinae region of the back is covered by

Thoracolumbar fascia

Extensor muscles of the limb are

Hypaxial muscles supplied by posterior division nerves

Flexor muscles of the limb are

Hypaxial muscles supplied by anterior division nerves

Extensor compartment muscles are innervated by

Posterior division nerves

Flexor compartment muscles are innervated by

Anterior division nerves

Slide colour key: muscles innervated by ventral or dorsal spinal nerve rami or cranial nerves

Pink

Skeletal muscle cell development sequence

Somitic epithelium, myogenic progenitor cell, myoblast, myotube, myofiber

Three stages of skeletal muscle development

Determination, differentiation, and maturation

Myogenic progenitor cells become

Myoblasts

Myoblasts fuse to form

Myotubes (multinucleated)

Myotubes mature into

Myofibers

Determination stage genes

Myf-5, Pax-3, and MyoD

Upstream activators of Pax-3 and Myf-5 come from

Dorsal ectoderm, neural tube, and notochord

Myf-5 and Pax-3 activate

MyoD

MyoD activates

Myoblast genes and myogenin

Myogenin activates

Myotube genes and MRF-4

MRF-4 activates

Myofiber genes (maturation)

Proliferation factors that act at determination

FGF and TGF-beta

Differentiation factors act at which gene?

MyoD, driving it to myogenin

Dermatome definition

Area of skin supplied by sensory fibers of a single spinal nerve segment

Relationship of myotomes to dermatomes

Each trunk myotome also marks the territory of its dermatome, where motor and sensory fibers of the segmental spinal nerve extend

Upper limb dermatomes at 4 weeks

C3 to T2

Lower limb dermatomes at 4 weeks

L2 to S3

Preaxial border of the upper limb

Thumb (lateral/cranial) side

Postaxial border of the upper limb

Little finger (medial/caudal) side

Preaxial border of the lower limb

Big toe side

Postaxial border of the lower limb

Little toe side

Dermatome pattern shifts during limb development because

Limbs grow out and rotate, so segmental bands wrap around the limb

Limbs rotate to adult orientation at

7 weeks

Dermatome region order along the spinal cord

Cervical, thoracic, lumbar, sacral

Diaphragm nerve supply

Phrenic nerve

Phrenic nerve root levels

C3 to C5

Diaphragm muscle derives from

Myotomes of cervical somites 3 to 5

Septum transversum forms

Future anterior (central) portion of the diaphragm

Embryonic origins of the diaphragm

Septum transversum, pleuroperitoneal membranes, esophageal mesentery, and cervical myotome muscle

Why does the phrenic nerve travel so far down?

The diaphragm forms at cervical levels and descends with the heart and liver, pulling its nerve down with it

Tongue muscles are innervated by

Hypoglossal nerve (CN XII)

Infrahyoid (strap) muscles are innervated by

Ansa cervicalis

Muscle masses shown at 5 to 6 weeks

Lingual, infrahyoid, and diaphragmatic muscle masses

Embryo vs fetus on the prenatal time scale

Embryo period runs to about 2 months; fetal period runs from about 2 months to birth

1 month: sclerotome cells

Migrate around the neural tube

5 weeks: mesenchymal precartilage condensations form

Axial skeleton