✅ EMBRIOLOGÍA del SISTEMA NERVIOSO CENTRAL (Parte 2°) 🧠💥

Embryology of the Central Nervous System: Part Two

Overview of Spinal Cord Development

• The video introduces the second part of embryology focused on the central nervous system, specifically discussing spinal cord development, cell differentiation, and positional changes during embryonic development.

• The neural tube's wall consists of neuroepithelial cells that form a thick epithelium connected by junction complexes. Rapid division occurs during the neural groove phase, leading to an increase in neuroepithelial cells.

• After closure of the neural tube, neuroepithelial cells differentiate into primitive nerve cells called neuroblasts, which form the mantle layer surrounding the neuroepithelial layer and later develop into gray matter in the spinal cord.

• The outermost layer of the spinal cord is known as the marginal layer, containing nerve fibers originating from neuroblasts in the mantle layer. This area becomes myelinated and is referred to as white matter.

Structural Changes During Development

• Continuous incorporation of neuroblasts leads to ventral and dorsal enlargements on both sides of the neural tube; ventral enlargements become basal plates (motor areas), while dorsal enlargements become alar plates (sensory areas).

• A structure called the limiting sulcus demarcates between dorsal and ventral portions. The roof plate and floor plate do not contain neuroblasts but serve as pathways for crossing nerve fibers.

• An intermediate zone forms between motor and sensory areas containing neurons related to sympathetic functions within specific thoracic (T1-T12) and upper lumbar levels (L2-L3).

Histological Differentiation

• Neuroblasts arise solely from dividing neuroepithelial cells. Initially possessing a central process termed a transient axon, they migrate to form bipolar neurons before becoming multipolar neurons through further differentiation.

• As differentiation progresses, two cytoplasmic extensions emerge at opposite ends of a neuron body; one develops into an axon while others evolve into primitive dendrites.

Glial Cell Development

• Most supporting glial cells originate from glialoblasts derived from neuroepithelial cells. Following production cessation of neuroblasts, glialoblasts migrate to marginal and mantle layers where they differentiate into various types including protoplasmic astrocytes.

• Oligodendrocytes are another type of support cell formed primarily in marginal layers that create myelin sheaths around ascending and descending fibers.

Neural Crest Cells Contribution

• During mid-development stages, a third type of support cell known as microglia emerges from vascular mesenchyme when blood vessels penetrate nervous tissue.

• Neuroepithelial cells eventually stop producing both neuroblasts and glialoblasts; they then differentiate into ependymal cells lining central canals within spinal structures.

Neural Crest Cell Functions

• Neural crest cells appear along each edge during neural plate elevation; these ectodermal-originated cells extend laterally forming sensory ganglia or dorsal root ganglia among other cell types throughout development.

• Neuroblast extensions grow centrally entering dorsal roots while others project peripherally contributing to motor root formation alongside sensory ganglion creation by neural crest-derived progenitors.

Neuronal Fibers and Spinal Cord Development

Origin and Structure of Neuronal Fibers

• Las fibras en citivas se originan en neuronas fuera de la médula espinal, específicamente en los ganglios de la raíz dorsal, que derivan de células de la cresta neural.

• Las prolongaciones distales de estos ganglios se unen a las raíces ventrales para formar un nervio raquídeo, donde la fibra de la raíz dorsal tiene una función sensitiva y la raíz ventral tiene una función motora.

Division of Spinal Nerves

• Los nervios raquídeos se dividen casi inmediatamente en ramas primarias dorsal y ventral, que contienen tanto fibras sensitivas como motoras.

• Las ramas dorsales inervan musculatura axial dorsal, articulaciones vertebrales y piel de la espalda; mientras que las ramas ventrales inervan extremidades y pared del cuerpo ventral.

Positional Changes During Development

• En el tercer mes del desarrollo, la médula espinal se extiende a lo largo del embrión; sin embargo, con el tiempo, la columna vertebral crece más rápido que el tubo neural.

• Esto provoca que las raíces dorsal y ventral se desplacen oblicuamente desde su segmento original hasta el nivel correspondiente en la columna vertebral.

Adult Anatomy of the Spinal Cord

• En adultos, la médula espinal termina entre los niveles L2 o L3; el saco dural se extiende hasta S2.

• Una extensión llamada filum terminale pasa caudalmente desde el extremo terminal de la médula espinal hasta el coxis, marcando así su ruta involutiva.

Conclusion of Discussion

• La cola de caballo está formada por las raíces dorsal y ventral por debajo del extremo terminal de la médula espinal.