#08 Flashcards
During gastrulation the embryo converts from a bilaminar disk to a trilaminar embryo
comprised of three germ layers,
Endoderm: The innermost of the three primary germ layers of an embryo,
developing into the gastrointestinal tract, the lungs, and associated structures.
* Mesoderm: The middle embryonic germ layer, lying between the ectoderm and the
endoderm, from which connective tissue, muscle, bone, and the urogenital and
circulatory systems develop.
2
* Ectoderm: The outermost of the three primary germ layers of an embryo, from which
the epidermis, nervous tissue, and sense organs develop
The nervous system then develops in stages, some of which overlap in time:
- Neural induction and patterning: The neural plate forms from surface ectoderm,
then patterned into dorsoventral and anteroposterior domains - Neurulation: The neural plate folds up and the lateral edges fuse dorsally forming
the neural tube - Vesicle formation: The neural tube bends and expands to form characteristic
swellings called vesicles - Neuronal proliferation: Neuroblasts undergo mitosis in an area adjacent to
ventricles - Neuronal migration: Post-mitotic neurons travel from the subventricular zone to
their proper position within the maturing brain - Axon outgrowth and pathfinding: Neurons extend axons to innervate targets,
establishing roles in cell-cell signaling - Neuronal differentiation: Discrete populations of neurons acquire their unique
characteristics - Myelination
- Post-natal development
During the first stage, the neural plate forms from m, and is then
patterned into dorsoventral and anteroposterior domains.
m: surface ectoderm
Week 3: On the a side of the embryo, local mesodermal structures (e.g., the
notochord) secrete b which induce overlying c cells to
become d
a dorsal
b growth factors
c ectoderm
d neural plate
- Ectoderm cells secrete a which acts back on these same cells (i.e., in an autocrine fashion) to block neural development.
- The b, a dorsal mesoderm derivative, secretes noggin and chordin, two molecules which bind a and prevent its action
- Absent a signaling, ectoderm cells adopt a neural fate and become c.
a: Bone Morphogenic Protein 4 (BMP4)
b notochord
c neural plate
Tissues flanking the nervous system secrete growth factors which instill regional pattern
on the nervous system. This process begins at the neural plate stage but continues during
neurulation. The signals are usually a, defined as having dose-dependent
effect on the fate of responding cells; i.e., a higher dose induces one fate, and a lower
dose a different one.
a morphogens
Stage 2: neurulation
After neural induction, the neural plate folds up and the
lateral edges fuse dorsally forming the neural tube
In the adult, the spinal cord remains a tube while the
brain undergoes several characteristic bends and
swellings resulting from cellular proliferation
The most lateral domain of the neural plate becomes the
neural crest, a population of
pluripotent precursor cells.
Anencephaly
failure of the anterior neuropore to close, resulting in a major portion
of the brain, skull, and scalp being absent. This defect is generally incompatible with
life and infants delivered with this disorder typically do not live more than a few
hours or days after birth.
Rachischisis
failure of the posterior neuropore to close; the neural tube is not fully
“rolled up”. Rachischisis patients have motor and sensory deficits, chronic infections,
and disturbances in bladder function. The defect
often occurs with anencephaly.
Spina bifida:
Essentially a less severe form of
rachischisis, spina bifida defects involve
incomplete formation of the vertebrae and
meninges surrounding the spinal cord
Ventral root (motor) fibers of each spinal nerve innervate a group of skeletal muscles
called a
myotome.
Upper limb –
elbow flexors a
elbow extensors b
hand muscles c
a: C5-6
b: C7
c: C8/T1
Lower limb –
hip flexors a
knee extensors b
ankle dorsiflexors c
ankle plantarflexors d
a: L2
b: L3
c: L4
d: S2
Diaphragm – a
Pelvic diaphragm/perineum –
b
a C3/4/5
b S2/3/4
