Development
High-Yield Summary
- Neurulation sequence: gastrulation (3 germ layers) → notochord signals ectoderm above it → neural plate → neural groove/folds → fuse into the neural tube (becomes CNS); neural crest cells form alongside and migrate widely.
- Notochord SIGNALS (never becomes brain/spinal cord); neural tube BECOMES the CNS — don't mix up the two roles.
- Neural crest cells (multipotent) become: PNS components, skin pigment cells, facial structures, and part of the adrenal medulla.
- Spina bifida = incomplete neural tube closure; folic acid lowers risk. TORCH = classic infectious teratogens (Toxoplasmosis, Other, Rubella, CMV, Herpes).
- Teratogen effects depend on timing — same exposure, different developmental stage, very different consequences (e.g., thalidomide → limb defects if exposed during limb formation).
- Babinski reflex: toes fan out + big toe extends = NORMAL in infants (corticospinal tract still developing), PATHOLOGICAL in adults (upper motor neuron damage).
- Gross motor skills (large muscles — sitting, crawling, walking) vs. fine motor skills (small/precise — pincer grasp, writing). Receptive language (understanding) outpaces expressive language (producing) early on.
Key Terms
- Neurulation
- The process forming the neural tube, foundation of the brain and spinal cord.
- Notochord
- Mesoderm-derived signaling center directing nervous system development; never becomes CNS tissue itself.
- Neural crest cells
- Multipotent cells from the neural fold edges that migrate to form PNS parts, skin pigment cells, facial structures, part of the adrenal medulla.
- Teratogen
- Any substance or exposure that interferes with normal fetal development.
- Pincer grasp
- Picking up small objects between thumb and index finger; emerges ~15 months, signals refined voluntary motor control.
- Vocabulary explosion
- Period of rapid vocabulary growth in early childhood.
Neurulation Sequence
- 1Gastrulation forms three germ layers: ectoderm, mesoderm, endoderm.
- 2Notochord (from mesoderm) signals the ectoderm above it.
- 3That ectoderm thickens into the neural plate.
- 4The neural plate folds inward, forming the neural groove and neural folds.
- 5Neural folds fuse and pinch off to form the hollow neural tube (→ CNS); neural crest cells form alongside and migrate throughout the embryo.
Primitive Reflexes
| Reflex (Trigger) | Response / Typical Disappearance |
|---|---|
| Stepping (held upright, feet on surface) | Alternating stepping motions; ~2–3 months |
| Tonic neck/fencing (head turned to one side) | Same-side arm/leg extend, opposite bend; ~3–4 months |
| Moro/startle (loss of support, loud sound) | Arms extend then return; ~4–6 months |
| Rooting (touch to cheek/mouth corner) | Head turns toward stimulus; ~4–6 months |
| Grasp (pressure in palm) | Fingers wrap around stimulus; ~4–6 months |
| Sucking (palate/tongue stimulation) | Rhythmic sucking; ~6 months |
| Babinski (stroking sole of foot) | Toes fan out, big toe extends up; ~12–24 months |
Common MCAT Trap
- Notochord signals; neural tube becomes the CNS — don't swap their roles.
- Babinski flips by age: toe fanning/big toe extension is NORMAL in an infant, PATHOLOGICAL in an adult. Toes curling down is normal in an adult.
- Teratogen severity depends heavily on timing of exposure, not just the substance itself — same exposure can hit different organ systems depending on developmental stage.
Quick Recall
What does the notochord actually become in the adult body?
What are 4 things neural crest cells give rise to?
What does the TORCH acronym stand for?
Is a Babinski response (toes fan out, big toe extends) normal or abnormal in a healthy adult?
Which typically comes first in language development: understanding or producing language?