Nervous System Development PDF

**Development of the Nervous System:** During week 3 of development, the **trilaminar embryonic disc** consists of 3 germ layers. - Ectoderm, Mesoderm, Endoderm **Ectoderm** gives rise to the **central** and **peripheral nervous system** as well as the **epidermis** of the skin and part of our...

**Development of the Nervous System:** During week 3 of development, the **trilaminar embryonic disc** consists of 3 germ layers. - Ectoderm, Mesoderm, Endoderm **Ectoderm** gives rise to the **central** and **peripheral nervous system** as well as the **epidermis** of the skin and part of our **visual system**. **Notochord i**s a flexible, rod-like structure found in the embryos of all chordates. It provides structural support, defines the body axis, and guides development of the nervous system and vertebrae. In most vertebrates, it is replaced by the spine during development, provides longitudinal support to the embryo and stimulates the conversion of overlying surfaces. **Neural tube** forms, eventually becoming the brain and spinal cord. - The **notochord** induces the **ectoderm** to form the **neural plate**. - The edges of the neural plate rise into **neural folds**, creating a **neural groove** in the centre. - The folds **fuse**, forming the **neural tube**, with the **neural canal** inside. - The neural tube separates from the **surface ectoderm**.  **Neurulation:** - Fusion proceeds **bidirectionally.** - Cranial and caudal ends left open, forming **neuropores**. - **Neurulation** starts day 17 and is complete end of week 4 due to the **closure** of the **posterior neuropore**. This is as posterior neuropore closes two days after anterior neuropore. - **Cephalic** part of the neural tube becomes our **brain**. - **Caudal** part of neural tube becomes our **spinal cord**. - The **Neural Canal** becomes the **ventricular system** and **central canal**.  **Neural Tube Defect:** - **Exencephaly** (Failure of closure of the anterior neuropore - brain develops outside\ the skull and the cranial vault is absent or poorly formed \-\-- a precursor to\ **anencephaly** (absence of brain). - **Microcephaly** -- a small brain in a small cranium. **Failure of neural tube closure:** - Leads to **spinal Bifida** (small gap in the vertebrae) due to incomplete closing of vertebrae. Spinal cord, nerve roots and meninges may protrude outside the vertebral canal. **Neural Crest Cells:** - When the neural plate bends and fuses to form neural tube, the neural crest cells **detach** from ectoderm. The neural crest is now located **between** the **surface ectoderm** and **neural tube**. They **migrate** to numerous locations and give rise to many different structures. **Derivatives of the Ectoderm:** **Neuroectoderm: Gives rise to:\ - Neural tube --** CNS (brain & spinal cord) and MORE\ **- Neural crest --** bones of the face and skull, ganglion of cranial nerves, dorsal root ganglion,\ autonomic ganglion, meninges (pia & arachnoid), Schwann cells (produce myelin sheath in\ PNS), satellite cells (glial cells found in the ganglia of PNS) and MORE\ **Surface ectoderm: Gives rise to:**\ - **epidermis** (of skin), hair, nail, enamel\ - anterior lobe of the **pituitary gland** and MORE**\ ** **Early development of the brain:** **12 pairs of cranial nerves in humans and most arise from the brainstem.** **Optic Nerve:** During development the extension of the forebrain is called an **optic vesicle**. Which is divided into **two parts**, the distal part is known as the **distal cup** and the **optic stalk** is the proximal part. **Optic cup** becomes **retina**, **optic stalk** becomes **optic nerve**. **Development of the spinal cord:** - During the neural groove stage and immediately after closure of the neural tube -- **Neuroepithelial** cells divide rapidly into neuroepithelial layers or neuroepithelium. - Once the tube closes -- neuroepithelial cells begin to give rise the primitive nerve cells or **neuroblasts.** - Neuroblasts form the **mantle zone**. The mantle zone later forms the **grey matter**. - Outermost layer -- **marginal layer** contains nerve fibres emerging from neuroblasts in the mantle layer. - Marginal layer -- **white matter**. **As a result of continuous addition of neuroblasts to the mantle layer:**\ **Ventral thickening** is known as the **basal plates** -- contain **ventral motor horn cells** (form the motor areas of the spinal cord).\ **Dorsal thickening** is known as the **alar plates** -- form the sensory areas of the spinal cord.\ **Sulcus limitans** (a longitudinal groove) -- marks the boundary between the basal and alar plates.\ **Ventral and dorsal** midline portions of the neural tube are known as the **floor plates** and **roof plates**, respectively.\ Floor plates and roof plates: **no neuroblasts** - pathways for nerve fibres crossing from one side to the other.\ A group of neurones found between the ventral motor horn and dorsal sensory horn -- forms the **intermediate horn (lateral horn)** -- contains neurones of sympathetic portion of ANS. Present only at T1-T12 and upper lumbar (L2 or L3). **Histological Differentiation:** **Histological differentiation: Nerve Cells.** The multipolar **neuroblasts** gives rise to **adult nerve cells** (neurons). Axons of neurones in the basal plate break through the **marginal layer** and become visible on the ventral aspect of the spinal cord -- known as the **ventral roots**. They conduct impulses from the spinal cord to the muscles.\ Axons of neurones in alar plate **penetrate** the marginal layer -- **ascend** to either higher or lower levels to form **association neurones** (interneurons).  **Histological differentiation: Glial Cells.** Majority of **primitive supporting cells (glial blasts**) are formed by neuroepithelial cells after the production of neuroblasts cease.\ Glial blasts **migrate** from the neuroepithelial layers to **mantle** and **marginal layers.**\ In the **mantle layer** -- glial blasts **differentiate** into **protoplasmic astrocytes** and **fibrillar** **astrocytes** -- found between blood vessels and neurones where they provide support and serve metabolic functions.\ **Oligodendroglia cells**-- found primarily in the marginal layer. Forms **myelin sheath**.\ **Microglial cells** are a type of **phagocytic cell** in the **central nervous system (CNS)** that appear during the **second half of development**. When **neuroepithelial cells** **cease** to produce neuroblasts and glial blasts -- they **differentiate** into **ependymal cells** lining the central canal of spinal cord (and ventricles). **Myelination:**  **Brainstem** -- direct continuation of the spinal cord and may have similar organisation.\ The higher centres reflect almost none of this basic pattern -- show accentuation of the alar plate and regression of the basal plate.