Nervous system development and critical period 3.1

Nervous system development and critical period 3.1

The development of the nervous system is influenced by three main factors

genetic instructions, intracellular signals, and interaction with the environment

genetic instructions

• The development of the nervous system is largely guided by the DNA in our genes.

• These genes contain the instructions for building and organizing the brain and spinal cord, such as when and where specific neurons should form.

• For example, certain genes trigger the formation of the neural plate, while others help guide neurons to their final locations.

intracellular signals

• Inside cells, signals are transmitted through molecules (like proteins) that help cells communicate with each other.

• These signals control processes like cell division, differentiation (turning into specific types of neurons), and migration (moving to the correct place in the body).

• An example is the role of growth factors that encourage neuron growth or the activation of genes that decide what type of neuron a cell will become.

interaction with the environment

• The environment, including chemical signals, physical surroundings, and sensory experiences, also influences nervous system development.

• For example, sensory inputs like light, sound, or touch affect how neural circuits form in the brain.

• In addition, environmental factors like nutrition, stress, or toxins can impact brain development, especially in early life.

Morula

A solid ball of cells formed by cleavage divisions of the zygote, typically consisting of 16 to 32 cells.

Blastula

An early embryonic stage characterized by a hollow sphere of cells called the blastocyst in mammals, where the inner cell mass and trophoblast are formed.

Gastrula

The stage following the blastula where cells undergo gastrulation, forming three primary germ layers (ectoderm, mesoderm, and endoderm) through invagination or other movements, setting the foundation for organ development.

ectoderm

• The outermost layer.

• It gives rise to the nervous system (including the brain and spinal cord), skin (epidermis), and structures like hair and nails.

• It also forms sensory organs like the eyes and ears.

mesoderm

• The middle layer.

• It forms structures like muscles, bones, circulatory system (heart and blood vessels), and kidneys.

• It also gives rise to the reproductive system and connective tissues.

endoderm

• The innermost layer.

• It forms the lining of the digestive tract, lungs, and other internal organs like the liver and pancreas.

• It also contributes to structures related to the respiratory and digestive systems.

Primary Neurulation

is the process by which the neural tube forms from the ectoderm and gives rise to the brain and spinal cord.

Gastrulation occurs

Day 18

• is the stage where the three primary germ layers (ectoderm, mesoderm, and endoderm) form.

• During this stage formation of notochord occurs

Notochord

• Develops from the mesoderm

• Serves as a crucial structure that defines the axis of symmetry for the body

• Plays a key role in guiding the development of the neural tube.

• Produces signals that induce the overlying ectoderm to form the neural plate.

Neurulation

• Formation of the Neural Plate which is formed by the ectoderm

• The outermost germ layer/ectoderm thickens to form the neural plate in response to signals from the notochord.

• The lateral margins of the neural plate is a flat structure that will eventually fold to form a longitudinal invagination (day 20 of gestation) called neural tube and give rise to the brain and the spinal cord.

• This process begins at the cervical region (middle of the embryo) and progresses both rostrally (toward the head) and caudally (toward the tail).

• The neural groove forms in the center of the neural plate, and the edges begin to rise and fuse together, eventually closing off the neural tube.

Completion Up to Lumbar Levels

• By Day 22-24, the neural tube has closed up to around the lumbar region (lower back).

• The anterior (head) end of the neural tube will eventually develop into the brain, and the posterior (tail) end will form the spinal cord

Role of the Notochord

• is essential for inducing the formation of the neural tube.

• It acts as a signaling center that guides the differentiation of the overlying ectoderm into the neural plate.

• The notochord also helps define the body's axial structure, playing a central role in the overall body symmetry.

Appearance of neuroblasts with the closure of the neural tube:

Neural tube → CNS

Neural Crests → PNS

Primary NEURULATION

closure of the neuropores

Tube closure

Anterior and posterior (day 24 - 26 of gestation)

Importance of folic acid

• It helps prevent neural tube defects in the developing baby, such as spina bifida.

• It also supports overall healthy cell function and DNA synthesis

Abnormal closure of the neuropores can lead to various neurodevelopmental abnormalities:

Failures in the closure of the anterior neuropore

Failures in the closure of the posterior neuropore

Failures in the closure of the posterior neuropore

• Spina Bifida (Occult or open)

Failures in the closure of the anterior neuropore

• Anencephaly

• Encephalocele

SECONDARY NERULATION

Formation of the neural tube at sacral and coccygeal levels

1. Formation of the Conus Medullaris

2. Spinal Cord Ends Between L2 and L3

3. Cauda Equina

Formation of the Conus Medullaris

• Lower end of the spinal cord.

It forms as the spinal cord elongates during development, reaching its final shape by the end of fetal development

Spinal Cord Ends Between L2 and L3

As a person grows, the vertebral column (spine) lengthens faster than the spinal cord, so the cord's end moves up to around L2 in adults, while the vertebral column continues to grow down to L3.

Cauda equina

• The cauda equina consists of nerve fibers below the medulla

• extending from the lower end of the spinal cord

• supply the lower limbs & pelvic organs

4TH WEEK OF GESTATION

FORMATION OF PRIMARY AND SECONDARY CEPHALIC VESICLES

• Prosencephalon or forebrain

• Mesencephalon or midbrain

• Rhombencephalon or hindbrain.

• Prosencephalon or forebrain

Develops into Telencephalon & Diencephalon

Telencephalon

Cerebral hemispheres

Cerebral cortex

Commissure

Diencephalon

Eyes

Pituitary

Thalamus

Hypothalamus

Pineal gland

Visual

Olfactory Placodes

 Mesencephalon or midbrain

• Cerebral peduncles

• Colliculi

• Substantia

Rhombencephalon or hindbrain

• Myelencephalon (medulla oblongata)

• Metencephalon (pons and cerebellum)

• Auditory Placodes

Developmental anomalies in during development of Prosencephalon or forebrain

Holoprosencephaly

Facial Malformations

Holoprosencephaly

• Alobar

• Semilobar

Facial Malformations

• Hypotelorism (decreased distance between two organs or bodily parts)

• Proboscis (rudimentary nasal structure)

VENTRICULAR SYSTEM

• Interconnected cavities called ventricles, which are filled with cerebrospinal fluid.

• Forms at the same time as neurulation

• 3rd ventricule

• 4th ventricule

• Two Lateral ventricules

• Central canal

How are the ventricles connected?

Lateral ventricles are connected to the third ventricle through interventricular foramen/foramen of Monro

Third ventricle is connected to the fourth ventricle through cerebral aqueduct/aqueduct of Sylvius

Hydrocephalus

• Condition where there is an excess buildup of cerebrospinal fluid (CSF) in the brain's ventricles, which can lead to increased pressure inside the skull

• occurs usually when the production, circulation, or absorption of CSF is disrupted.

PERIPHERAL NERVOUS SYSTEM DEVELOPMENT

develop from the neural crest

How does the brain and spinal cord communicate with the rest

of the body?

Through cranial and spinal nerves

31 pairs of spinal nerves

12 pairs of cranial nerves

ganglion and spinal nerve are associated with

segment of the developing embryo

Molecular signals

• Retinoic acid and FGF (Fibroblast Growth Factor) are keys in Molecular signals

• Enable the coordinated and continuous development of the neural cells of the spinal cord and those of the peripheral nervous system.

Retinoic Acid

Retinoic acid helps in the formation of the neural crest cells, which migrate to form the peripheral ganglia and contribute to the development of the PNS.

Fibroblast Growth Factor (FGF)

• During neurulation, FGF helps in neural induction (the process by which the ectoderm becomes neural tissue), neural differentiation, and the formation of the spinal cord.

• FGF plays a role in the development of the neural crest, which gives rise to structures in the PNS, including ganglia and spinal nerves.

• It influences the development of various structures along the anterior-posterior axis of the embryo.

Helps start and shape the neural tube, spinal cord, and PNS