Embryologically, the brainstem comes from three primary brain vesicles: mesencephalon, metencephalon, and myelencephalon. The midbrain develops from the mesencephalon, the pons (and the cerebellum) come from the metencephalon, and the medulla oblongata arises from the myelencephalon. Together, these give the brainstem's main structures—the midbrain, pons, and medulla. The cerebellum, thalamus, hypothalamus, and cortex come from other embryonic divisions, so they aren’t the brainstem itself. That’s why referencing mesencephalon, metencephalon, and myelencephalon best explains the brainstem’s origin.

Peripheral nerves and many cranial structures come from neural crest cells, a population that forms at the border between the neural tube and the epidermis during early neurulation. These cells are highly multipotent and migrate to many destinations. They differentiate into the Schwann cells that wrap and insulate peripheral nerves, as well as the neurons and glia of the peripheral nervous system, including autonomic ganglia. They also contribute to craniofacial structures such as bones and cartilage of the face, along with other derivatives like certain pigments and adrenal medulla cells. In contrast, neural plate cells produce the neural tube that becomes the central nervous system; mesoderm generates muscles, bones, and other connective tissues; endoderm lines the gut and associated organs. So the population responsible for peripheral nerves and many cranial structures is neural crest cells.

The choroid plexus does this. It lines the brain’s ventricles with a specialized epithelium that forms tight junctions between cells, creating the barrier between blood and cerebrospinal fluid and regulating what substances reach the CSF. Beyond forming the barrier, the choroid plexus actively secretes CSF and releases signaling molecules and growth factors into the CSF, providing essential cues for neural progenitor proliferation, differentiation, and neuronal migration during CNS development. The other options don’t fit this role: the cerebral cortex is the tissue where neurons reside, not the barrier-forming structure; the meninges are protective coverings rather than the source of the blood-CSF barrier; cortex-specific glia aren’t the structure responsible for establishing this barrier or delivering developmental signals in this context.

Guidance of commissural axons at the midline relies on a signal that lures growing tips toward the floor plate. Netrin-1 is secreted by the midline structure and creates a gradient that axons expressing DCC receptors interpret as an attractive cue, drawing them toward and across the midline. After crossing, they encounter Slit proteins that bind Robo receptors, producing repulsion to prevent re-crossing. SHH mainly patterns ventral neural tube identity rather than acting as a midline attractant, and WNT signaling has other guidance roles but is not the classic attractant for this crossing event. So netrin-1 best fits as the midline attractant guiding commissural axons.