Development of the Brain, Spinal Cord, and Face

The development of the brain, spinal cord, and face involve several complex processes that occur simultaneously to achieve correct organ development. Beginning with neurulation, the neural tube and neural crest cells form the central and peripheral nervous systems. Beginning at the 4th week, the face begins to develop as well, and through the creation of frontonasal, medial, lateral, and mandibular prominence, recognizable facial features can be observed from the 14th week onward.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Neurulation and Neural Crest Migration

The development of the brain is a specific part of gastrulation Gastrulation Both gastrulation and neurulation are critical events that occur during the 3rd week of embryonic development. Gastrulation is the process by which the bilaminar disc differentiates into a trilaminar disc, made up of the 3 primary germ layers: the ectoderm, mesoderm, and endoderm. Gastrulation and Neurulation, called neurulation, that creates the cells of the nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System.

Neurulation

Day 16 after fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week → embryonal cells belong to 1 of 3 germ cell layers:

  • Ectoderm
    • Differentiates into the neuroectoderm, creating the neural plate
    • Cell replication in the neural plate gives rise to 2 ridges (neural crests).
    • The depression between the crests is known as the neural fold.
  • Mesoderm
    • Differentiates and transforms in a tube structure called the notochord.
    • The notochord signals the neural fold to enlarge on either side of the neural groove, creating the neural tube.
  • Endoderm
Neurulation corrected

Neurulation: the differentiation and growth of the neural plate into the neural tube during the first trimester of gestation

Image by Lecturio.

Neural crest cell migration

Neuroectoderm cells migrate in waves from the neural crests to create peripheral nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System structures:

  • The 1st wave of migration creates:
    • Sympathetic ganglia
    • Parasympathetic ganglia
    • Chromaffin cells (sympathetic ganglia migrated into the adrenal medulla)
  • The 2nd wave of migration creates:
    • Posterior root ganglia
    • Schwann cells
    • Satellite cells
  • The 3rd wave of migration creates:
    • Melanocytes
  • Neural crest cells in the head and neck also form different structures:
    • Make up secondary ganglia of cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves 5, 7, 9, and 10
    • Migrate into the pharyngeal arches of the head, neck, and face to create mesenchyme, which contributes to bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones and connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue

Mnemonics

  • To quickly recall that each Schwann cell myelinates only 1 axon of the peripheral nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System, remember: “Schwone” = 1 axon.
  • To quickly recall where oligodendrocyte and Schwann cells are located, remember: COPS: CNS = Oligodendrocyte; PNS = Schwann cells.

Neural Tube

Primary vesicles

The neural tube develops 3 bulges (primary brain vesicles) at the cranial end:

  1. Prosencephalon (forebrain) splits, giving rise to:
    • The telencephalon, which becomes the cerebral cortex Cerebral cortex The cerebral cortex is the largest and most developed part of the human brain and CNS. Occupying the upper part of the cranial cavity, the cerebral cortex has 4 lobes and is divided into 2 hemispheres that are joined centrally by the corpus callosum. Cerebral Cortex. This portion of the neural canal becomes the lateral ventricles and 3rd ventricle.
    • The diencephalon, which becomes the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus, hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus, and pineal gland
  2. Mesencephalon (midbrain): also contains the cerebral aqueduct
  3. Rhombencephalon (hindbrain) splits to become:
    • The metencephalon, which becomes the pons and cerebellum Cerebellum The cerebellum, Latin for "little brain," is located in the posterior cranial fossa, dorsal to the pons and midbrain, and its principal role is in the coordination of movements. The cerebellum consists of 3 lobes on either side of its 2 hemispheres and is connected in the middle by the vermis. Cerebellum
    • The myelencephalon, which becomes the medulla oblongata
    • The remnant of the neural canal present around the metencephalon and myelencephalon, which becomes the 4th ventricle

Cerebrospinal fluid (CSF) circulatory system

  • Ependymal cells:
    • Line the central canal and the core of the neural tube
    • In lateral, 3rd, and 4th ventricles, ependymal cells become the choroid plexus.
  • Choroid plexus: filters blood and releases CSF into the ventricular system Ventricular System The ventricular system is an extension of the subarachnoid space into the brain consisting of a series of interconnecting spaces and channels. Four chambers are filled with cerebrospinal fluid (CSF): the paired lateral ventricles, the unpaired 3rd ventricle, and the unpaired 4th ventricle. Ventricular System
Table: Stages of embryonic development
Neural tube Primary vesicle stage Secondary vesicle stage Adult structures Ventricles
Anterior neural tube Prosencephalon Telencephalon Cerebrum Lateral ventricles
Anterior neural tube Prosencephalon Diencephalon Diencephalon 3rd ventricle
Anterior neural tube Mesencephalon Mesencephalon Midbrain Cerebral aqueduct
Anterior neural tube Rhombencephalon Metencephalon Pons cerebellum Cerebellum The cerebellum, Latin for "little brain," is located in the posterior cranial fossa, dorsal to the pons and midbrain, and its principal role is in the coordination of movements. The cerebellum consists of 3 lobes on either side of its 2 hemispheres and is connected in the middle by the vermis. Cerebellum 4th ventricle
Anterior neural tube Rhombencephalon Myelencephalon Medulla 4th ventricle

Flexion of the neural tube

The neural tube develops a series of bends in the sagittal plane:

  • At 3-vesicle stage:
    • Cervical flexure: between spinal cord and rhombencephalon
    • Cephalic flexure: between prosencephalon and mesencephalon
  • As the neural tube develops further: pontine flexure (between myelencephalon and metencephalon)

Development of the Spinal Cord and Brainstem

Spine

As the cephalic portion of the neural tube becomes the brain, the rest becomes the spinal cord.

  • Proliferation of neuroepithelial cells 
    • As they push outward, intermediate and marginal zones are created.
    • Marginal zone: comes in contact with sclerotomal mesenchyme that will form the meninges Meninges The brain and the spinal cord are enveloped by 3 overlapping layers of connective tissue called the meninges. The layers are, from the most external layer to the most internal layer, the dura mater, arachnoid mater, and pia mater. Between these layers are 3 potential spaces called the epidural, subdural, and subarachnoid spaces. Meninges
    • Intermediate and marginal zones fill the space inside the neural canal.
    • The cells in the marginal and intermediate zones will differentiate into neurons. 
  • The neuroepithelial tissue of the spine has a regional specialization:
    • Alar plate (sensory)
      • Axons extend to the brain.
      • Neurons in dorsal root ganglia (originally from neural crest cells) extend to the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin and back to the Alar plate.
    • Basal plate (motor): Neurons innervate myotome.
    • Sulcus limitans: separates alar plate from the basal plate
  • Top and bottom of the neural tube are closed by the roof plate and floor plate.
  • Central canal at the core of neural tube: lined by ependymal cells
  • The spinal cord fills the bony spine as the fetus develops.
    • Week 8: Spinal cord extends along the entire length of the vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column.
    • Inferior end reaches:
      • L3 level at birth
      • L1 level in adulthood
    • Spinal nerve roots that exit below L1L3 create the cauda equina
    • Conus medullaris: tapered end of the spinal cord
    • Filum terminale: connects conus medullaris to the coccygeal vertebra

Brainstem

The brainstem resembles the spinal cord in embryological organization. The basal plate gives rise to motor nuclei, while the alar plate gives rise to sensory nuclei.

  • Caudal medulla
    • Sensory nuclei are dorsal.
    • Motor nuclei are ventral.
  • Cranial medulla
    • Roof plate is more opened up (“open book” appearance).
    • The basal plate is more medially located.
    • The alar plate is more laterally located.
    • Some sensory neurons migrate anteriorly to form the olivary nucleus later in development.
  • Pons
    • Alar plate migrates to take a spot anterior to the basal plate and gives rise to the pontine nuclei.
    • Basal plate is now more posterior, but still gives rise to all the motor nuclei.
    • Cerebellum develops directly posterior to the pons from neuroepithelial cells.

Development of the Cerebral Cortex

The cortex develops from the telencephalon.

  • The neural canal at this level develops into the left and right ventricles.
  • Interventricular foramen: connects lateral ventricles to the 3rd ventricle
  • The development of lobes of the brain occurs from the ventricles outward:
    • Neuron precursor cells near the ventricles replicate rapidly.
    • Glial cells extend radial processes to provide a pathway for the neurons.
    • Neuroepithelial cells migrate laterally, passing through subventricular zone → intermediate zone → cortical plate → marginal zone
    • Different kinds of neurons stop at different points, giving rise to specialized layers.
  • 6 months: Distinctive lobes start to appear.
  • Corpus callosum: 
    • Becomes more defined as neurons from 1 cortex migrate to the other
    • Nervous structure that allows 1 side of the brain to communicate with another side
  • Gyri and sulci are not evident until near term.
  • 9 months: Brain looks like a smaller version of the adult brain.

Development of the Face

  • End of the 4th week: 1st facial structures are visible.
    • Centrally: stomodeum (early mouth)
    • Inferiorly: mandibular prominence
    • Laterally: 2 maxillary prominences
    • Superiorly: frontonasal prominence with nasal placodes
  • 5th week: Nasal placodes deepen into nasal pits surrounded by nasal prominences.
  • 6th and 7th weeks:
    • Mandibular prominences fuse → jaw Jaw The jaw is made up of the mandible, which comprises the lower jaw, and the maxilla, which comprises the upper jaw. The mandible articulates with the temporal bone via the temporomandibular joint (TMJ). The 4 muscles of mastication produce the movements of the TMJ to ensure the efficient chewing of food. Jaw and Temporomandibular Joint formed
    • Eyes are visible on the lateral side of the face (as frontonasal prominence narrows, eyes move medially).
    • Nasolacrimal groove forms: junction between frontonasal prominence and maxillary prominence, future nasolacrimal duct
    • Medial nasal prominences:
      • Grow together and fuse at the midline → stretch inferiorly
      • Fuse with maxillary prominence → form the upper lip
    • Medial and lateral nasal prominences: fuse with the maxillary prominence → form cheek and upper lip
    • Frontonasal prominence → becomes the forehead, nose Nose The nose is the human body's primary organ of smell and functions as part of the upper respiratory system. The nose may be best known for inhaling oxygen and exhaling carbon dioxide, but it also contributes to other important functions, such as tasting. The anatomy of the nose can be divided into the external nose and the nasal cavity. Anatomy of the Nose, and philtrum
  • Maxillary prominence → cheek
  • Mandibular prominence → mandible and area anterior to the ear

Clinical Relevance

The following are pathological conditions that can arise as a result of errors in the development of the brain, spinal cord, and face:

  • Hydrocephalus: blockage of the ventricular system Ventricular System The ventricular system is an extension of the subarachnoid space into the brain consisting of a series of interconnecting spaces and channels. Four chambers are filled with cerebrospinal fluid (CSF): the paired lateral ventricles, the unpaired 3rd ventricle, and the unpaired 4th ventricle. Ventricular System that causes swelling and pressure exerted on the brain. In adults, the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull is already developed, so accumulated fluid presses on the brain. In neonates, as bones have not yet completely ossified, the head circumference increases. May be caused congenitally by cerebral aqueduct stenosis.
  • Posterior fossa malformations (Arnold- Chiari malformations Chiari Malformations Chiari malformations (CMs) are a group of central nervous system (CNS) conditions characterized by the underdevelopment of the posterior cranial fossa with subsequent protrusion of neural structures through the foramen magnum. Chiari Malformations (CM)): Chiari I malformation is a congenital disorder associated with ectopic cerebellar tonsils located inferior to the foramen magnum. Children are usually asymptomatic. Chiari II malformation is caused by herniation of the cerebellar tonsils, as well as vermis, through the foramen magnum. Chiari II leads to non-communicating hydrocephalus.
  • Frontonasal dysplasia ( cleft lip and cleft palate Cleft lip and cleft palate The embryological development of craniofacial structures is an intricate sequential process involving tissue growth and directed cell apoptosis. Disruption of any step in this process may result in the formation of a cleft lip alone or in combination with a cleft palate. As the most common craniofacial malformation of the newborn, the diagnosis of a cleft is clinical and usually apparent at birth. Cleft Lip and Cleft Palate): Sonic hedgehog overactivity causes accumulation of excessive tissue in the frontonasal prominence area, resulting in a broad nose Nose The nose is the human body's primary organ of smell and functions as part of the upper respiratory system. The nose may be best known for inhaling oxygen and exhaling carbon dioxide, but it also contributes to other important functions, such as tasting. The anatomy of the nose can be divided into the external nose and the nasal cavity. Anatomy of the Nose and widely separated eyes (hypertelorism). This disorder may also cause cleft nose Nose The nose is the human body's primary organ of smell and functions as part of the upper respiratory system. The nose may be best known for inhaling oxygen and exhaling carbon dioxide, but it also contributes to other important functions, such as tasting. The anatomy of the nose can be divided into the external nose and the nasal cavity. Anatomy of the Nose and midline cleft lip due to the failure to fuse medial nasal prominences.
  • Holoprosencephaly: a disorder caused by decreased activity of the sonic hedgehog gene, resulting in narrowing of the face. More severe cases involve failure of the right and left cerebral cortexes to fully separate, as well as cyclopia. 
  • Neural tube defects Neural tube defects Neural tube defects (NTDs) are the 2nd-most common type of congenital birth defects. Neural tube defects can range from asymptomatic (closed NTD) to very severe malformations of the spine or brain (open NTD). Neural tube defects are caused by the failure of the neural tube to close properly during the 3rd and 4th week of embryological development. Neural Tube Defects: 1 of the most common congenital CNS malformations. The defects develop between the 3rd and 4th week of gestation and are often caused by folic acid deficiency. The deficiency results in improper closure of the neural plate in the embryo, mainly at the caudal or cranial ends, giving rise to anencephaly.

References

  1. Sadler, T. W. (2014). Langman’ Medical Embryology.
  2. Lindsay M. Biga et al. Anatomy & Physiology. Retrieved 21 Oct, 2020, from https://open.oregonstate.education/aandp/ 
  3. Fishman MA. Hydrocephalus. (1978). In: Neurological Pathophysiology, Eliasson SG, Prensky AL, Hardin WB (Eds), Oxford, New York.
  4. Arnold WH, Meiselbach V. (2009). 3-D reconstruction of a human fetus with combined holoprosencephaly and cyclopia. Head Face Med. doi: 10.1186/1746-160X-5-14.
  5. Shkoukani MA, Chen M, Vong A. (2013). Cleft lip – a comprehensive review. Front Pediatrics. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3873527/

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