Develop. of spinal cord

🖇 I. Embryonic Structure of the Neural Tube

📌 Four Primary Parts:

• Dorsal Part: Alar Plate (Alar Lamina)
• Ventral Part: Basal Plate (Basal Lamina)
• Roof Plate: Connects Alar and Basal plates dorsally
• Floor Plate: Connects Alar and Basal plates ventrally (appears blue in diagrams)

🚨 Important Note on Roof and Floor Plates:

• Contain NO cells
• Function: Allow cells to pass through only
• Do NOT form structural elements

🖇 II. Three Layers of Neural Tube Wall

(Listed from innermost to outermost)

📌 1. Ependymal Layer (Innermost/Germinal Layer)

• Most medial, central, and innermost layer
• Source layer or "factory" of cells (Exam Point)
• Lined by primitive ependymal cells
• Contains two types of primitive cells (Exam Point):
• • Neuroblasts: Destined to form neurons
  • Spongioblasts: Destined to form glial cells (CNS connective tissue)

📌 2. Mantle Layer (Intermediate Layer)

• Formed by migration of Neuroblasts and Spongioblasts from Ependymal layer
• Densely packed (congested) with cells
• Later forms the Grey Matter of the spinal cord

📌 3. Marginal Layer (Outermost Layer)

• Outermost layer of neural tube wall
• Appears pale/light-colored (lacks cell bodies)
• Composed of axons (nerve cell processes) from Mantle layer
• Eventually forms White Matter of the spinal cord

🖇 III. Functional Specification of Neural Tube Plates

🟠 Alar Plate (Dorsal) - SENSORY (Exam Point)

• Forms the sensory part of spinal cord
• Develops into Posterior Horn (Sensory Horn)
• Contains sensory cells
• Produces sensory axons

🟠 Basal Plate (Ventral) - MOTOR (Exam Point)

• Forms the motor part of spinal cord
• Develops into Anterior Horn (Ventral Horn)
• Contains motor cells (Anterior Horn Cells)
• Produces motor axons

📌 Development of Horns and Boundaries:

• Thickening: Both Basal and Alar plates thicken due to cell migration and concentration
• Sulcus Limitans: Forms on internal wall surface
• • Marks boundary between sensory (Alar) and motor (Basal) regions (Exam Point)

🟣 Formation of Lateral Horn:

• Some neuroblasts concentrate between Posterior horn (sensory) and Anterior horn (motor)
• Forms the Lateral Horn
• Contains autonomic nerve cells (sympathetic and parasympathetic)

📌 Fissures and Septa Formation:

• Anteriorly: Anterior Median Sulcus forms
• Posteriorly: Two sides fuse forming Posterior Median Septum
• These structures separate right and left spinal cord halves

🖇 IV. Histogenesis and Cell Lineage

📌 Derivatives from Ependymal Cells:

From Neuroblasts:

• Differentiate into mature Nerve Cells (Neurons)

From Spongioblasts:

• Differentiate into CNS connective tissue cells (Glial cells):
• • Oligodendrocytes
  • Fibrous Astrocytes
  • Protoplasmic Astrocytes

🚨 Microglia - Special Origin (Exam Point):

• ONLY CNS connective tissue cells with mesodermal origin
• NOT derived from primitive neuroblasts/spongioblasts
• Sometimes called undifferentiated stem cells

🖇 V. Peripheral Nerve Formation

📌 Myelination in PNS:

• Performed by Schwann cells
• Schwann cells originate from Neural Crest cells (via migration)

🟠 Spinal Nerve Components:

Ventral Root (Motor):

• Contains axons (efferent processes)
• Originates from motor cells in Anterior Horn (Mantle layer of Basal Lamina)

Dorsal Root (Sensory):

• Contains sensory fibers (afferent processes)
• Originates from cells associated with Posterior Horn (Mantle layer of Alar Lamina)
• Dorsal Root Ganglion located on dorsal root

Formation of Spinal Nerve:

• Sensory + Motor roots combine → Mixed Spinal Trunk
• Spinal trunk divides into:
• • Ventral Ramus
  • Dorsal Ramus
• Ventral Rami form nerve plexuses (cervical, brachial, lumbar, sacral)

🖇 VI. Spinal Cord Clinical Notes and Anatomy

📌 Grey vs. White Matter Arrangement:

Spinal Cord:

• Gray matter: central/internal
• White matter: surrounds gray matter (peripheral)

Brain (Cerebrum/Cerebellum):

• Gray matter (cortex): superficial
• White matter (medulla): central/internal

🚨 Note: Spinal cord arrangement is REVERSE of brain arrangement

🟣 Clinical Note: Poliomyelitis (Infantile Paralysis)

• Common before vaccination era
• Polio virus specifically attacks Anterior Horn Cells (AHCs)
• Damage causes Lower Motor Neuron (LMN) lesion
• Interrupts motor orders from pyramidal tract
• Results in muscle paralysis

📌 Sensory Pathways (Tractology):

Study of: Ascending (sensory) and descending (motor) tracts

Types of Sensation:

• Fine touch
• Crude touch
• Discrimination (one vs. two points)
• Proprioception
• Vibration
• Cold
• Heat
• Temperature
• Pain

Stereognosis: Ability to recognize objects by touch without vision

Sensory Pathway Route:

• Entry: Dorsal Root
• Through: Dorsal Root Ganglion
• Into: Posterior Horn
• Then: Ascends via tracts

🖇 VII. Post-Natal Ascent of Spinal Cord

🚨 Developmental Changes in Cord Length (Exam Point):

3rd Month Intrauterine:

• Spinal cord occupies entire length of vertebral column

At Birth:

• Cord tip at level of 3rd Lumbar vertebra (L3)

Adult Position (Reached 3 Months Post-Birth):

• Final position: 1st or 2nd Lumbar vertebra (L1 or L2) (Exam Point)

📌 Termination Points:

Vertebral Column Structure:

• Total: 33 vertebrae
• • 7 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 fused sacral
  • 4 fused coccygeal

Meninges Termination:

• Dura Mater (outer layer): terminates at S2 (Second Sacral vertebra)
• Arachnoid Mater (middle layer): terminates at S2

Filum Terminale:

• Fibrous terminal extension of cord
• Attaches to First Coccygeal vertebra

🖇 VIII. Congenital Anomalies - Spina Bifida

📌 Definition:

• Failure of vertebral arch to fuse in midline
• Arch formed when pedicles and laminae (from posterior vertebral body surface) meet and fuse

🚨 Classification of Spina Bifida (Exam Point):

🟠 1. Spina Bifida Occulta:

Cause:

• Failure of vertebral arch halves to fuse in median plane on one side

Location:

• Most common: Lumbar 5 (L5) or Sacral 1 (S1)

Clinical Features:

• Often asymptomatic (no illness)
• Found in 10% of population
• Many people unaware they have it
• May have tuft of hair over affected skin area

🟠 2. Spina Bifida with Meningocele:

Extent:

• Usually involves more than one vertebra

Presentation:

• Appears as cyst-like sac

Contents:

• Sac is empty
• Contains only meninges (no neural tissue)

🟠 3. Spina Bifida with Meningomyelocele:

Severity:

• Most extensive and severe lesion

Contents:

• Sac contains:
• • Part of the spinal cord
  • Spinal nerves
  • Meninges

Clinical Features:

• Patients suffer from associated health problems
• Most severe form with neurological complications