Topic 4 Embryology pdf

He would like us to understand the steps and titles of each developmental stage, but we do not NEED to know the specific dates at which they occur

Importance

• embryology explains adult anatomy

• Explains the diseases resulting from developmental abnormalities

Differentiation & induction:

• Differentiation

  • The process by which developing cells specialize into adult cell types

• Induction

  • A process in which one cell causes another to differentiate

• Concentration gradients of various chemical messengers are often used to induce differentiation

Primary Germ Tissues:

• In week 3, embryos consist of 3 layers

  • Ectoderm

    • The outermost layer

    • Develops into epithelial and neural tissues

  • Mesoderm

    • The middle layer

    • Developers into muscle tissues dn most internal organs

  • Endoderm

    • The inner layer

    • Developers into the respiratory and GI systems along with the inner longings of a few other organs

Interrelated processes help form the head/neck:

• Neurulation

  • Forms the nervous system

• The pharyngeal apparatus

  • Forms the pharynx and surrounding structures in the neck and lower face

• Craniofacial development

  • Forms the cranium and remainder of the face

Neurulation:

• the process of neural tube formation

• Divided into:

  • Primary

    • Forms the brain, brainstem and cervical, thoracic, and lumbar spinal cord

  • Secondary

    • Forms the sacral and coccygeal spinal cord

Structures involved in Primary Neurulation:

• Neural plate

  • Thick region of ectoderm that will become the neural tub e

• Neural groove

  • A central depression formed by folding of the neural plate

• Neural folds

  • The peripheral portions of the folding neural plate

• Neuropores

  • Openings at the radial dn caudal ends of the developing neural tube

Steps in Primary Neurulation:

• Middle of week 3

1. Mesoderm induces neural plate formation

2. Center of plate invaginates

   • forms neural groove and neural folds

3. Neural folds meet in the midline and fuse into the tube

Results from primary Neurulation:

• neural tube

  • A tube formed by folding of the ectoderm

  • Will become the CNS

• Neural crest

  • Tissue pinched off form the developing neural tube

  • Cells will migrate throughout the body, becoming the PNS, pharyngeal arches and numerous other structures

How does primary Neurulation spread

• cervical region fuses in week 4

• Cranial and caudal neuropores initially open

• Fusion proceeds bilaterally

  • Tube completely closes by end of week 4

• 20 day-old

  • Cervical region fused, neuropores open

Results of Secondary Neurulation:

• Caudal eminence

  • A tail-like structure within the early embryo

• Medullary cord

  • A tube within the caudal eminence that will form the sacral and coccygeal spinal cord

How does Secondary Neurulation happen:

• Week 5-8

• Forms sacral and coccygeal regions of spinal cord

• Medullary cord develops in caudal eminence hollows out, merges with remainder of neural tube

Layers of the developing spinal cord:

• 3 layers in the neural tube, from deep to superficial:

  • Ventricular layer

    • A pseudostratified epithelium where cell reproduction occurs, producing ependymal cells, neuroepithelial cells and predecessors to other glia

  • Mantle layer

    • Neuroblasts that develop from the neuroepithelial cells

    • They will become the gray matter

  • Marginal layer

    • Formed by axons of neuroblasts

    • Will become white matter

How does induction occur in the spinal cord:

• signaling molecules are produced in opposite ends of the developing neural tube

• One is produced by ectoderm posteriorly

• The other y a structure called the notochord anteriorly

Plates of the spinal cord:

• signaling molecules lead to development of:

  • Alar plate

    • Developing gray matter in the posterior region of the neural tube

    • Will develop into posterior horn of spinal cord

  • Basal plate

    • Developing gray matter in the anterior region of the neural tube

    • Will develop into anterior horn spinal cord

  • Sulcus limitans

    • A preparation between the alar and basal plats

What are the Vesicles:

• Vesicles

  • Bulge at cranial end of neural tube

  • Will become brain and brainstem

  • Divided into primary and secondary vesicles

• Primary vesicles

  • 3 vesicles appearing during week 4

• Secondary vesicles

  • 5 vesicles that develop out of primary vesicles during week 5

• Flexures

  • Bends in-between the vesicles

Primary vesicles and what they become:

• prosencephalon

  • Becomes the telencephalon and diencephalon

• Mesencephalon

  • Middle primary vesicle

  • Becomes midbrain

• Rhombencephalon

  • Inferior primary vesicle

  • Becomes the metencephalon and myelencephalon

• Cephalic flexure

  • Between prosencephalon and mesencephalon

  • Responsible for bend in adult CNS

• Cervical flexure

  • Between rhombencephalon and spinal cord

  • Straightens in adults

Secondary Vesicles and what they become:

• week 5

• Telencephalon

  • The future cerebral hemisphere

  • Develops form prosencephalon

• Diencephalon

  • Develops from prosencephalon

• Mesencephalon Does NOT divide into secondary vesicle

• Metencephalon

  • The future pons and cerebellum

  • Develops form rhombencephalon

• Myelencephalon

  • The future medulla

  • Develops form rhombencephalon

• Pontine Flexure

  • Between metencephalon and myelencephalon

How do the plates shift in the brainstem:

• development of pontine flexure creates fourth ventricle

• Alar plates pushed from posterior positions to lateral position, creating structure of lower brainstem

• Sulcus limitans on the adult brainstem

How does the Cerebellum form:

1. Alar plate of rhombencephalon forms rhombic lips

2. Rhombic lips grow bilaterally

3. Central growth creates cerebellar nuclei

4. Peripheral growth fuses posteriorly to create cerebellum

5. Development of lobes and fists from 5th month onwards

How does the Cerebrum form:

• developing telencephalon has midline region

  • Lamina terminalis

• Commissural fibers develop here

• Lateral regions

  • Optic vesicles, telencephalon vesicles

• Cerebral hemispheres grow out form telencephalic vesicles in C-shape

• Growth pushes future cerebral cortex outwards, covering diencephalon, midbrain and hindbrain

• Basal nuclei pushed deeper by this process

• Future insula fuses to diencephalon

• Diencephalon components develop form small swellings that are invaded by neuroblasts

• the frontal and temporal poles stay in relatively the same spot

  • The material between them grown In a C-shape

Gyri and Sulci development

• Cerebrum initially smooth

• Gyri and sulci grow form 4th month until after birth

• Lobes are identifiable at 6 months

Myelination occurs:

• most myelination occurs after birth

• Motor fibers myelinated first

How do ventricles develop:

• cavity of the neural tube developed into the ventricles

Neural crest cells:

• cells which break off from neural tube during primary Neurulation and develop into a variety of tissues

  • PNS, skeletal components of pharyngeal arches, ganglia, glia, adrenal medulla, smooth muscle, connective tissue, great vessels

• Placodes

  • Thickened ectoderm that will become sensory organs and ganglia

• How does the PNS develop?

  • PNS forms for combination of neural crest cells and Placodes

  • Derivatives:

    • Anterior lobe of pituitary gland

    • Lens of eye

    • Head glia

    • Inner ear

    • Cranial nerve I and olfactory epithelium

    • Trigeminal ganglion

    • sensory ganglia of cranial nerve VII, IX, X

  • Somites

    • segmental structures

    • give rise to skin, muscle and nerves

    • each somite has 1 nerve that innervates its related skin and muscle

    • Peripheral nerve develops in somite starting in week 4

    • Motor fibers grow from basal plate

    • sensory fibers grow out of neural crest cells which form dorsal root ganglia

      • grow into alar plate and nerve

How do Cranial Nerves develop?

• cranial nerves develop over weeks 5-6

• CN III, IV, XI, XII develop like motor part of spinal nerve

• CN V, VII, IX, X develop from pharyngeal arches

• CN I from olfactory placode

• CN II form optic vesicle

• CN VIII develops as 2 sensory bundles

How does the ANS develop?

• Neural crest cells form autonomic ganglia, including paravertebral, prevertebral, and parasympathetic ganglia

• Neural crest cells also contribute to plexuses near internal organs

What clinical conditions result from development issues?

• results of defects in nervous system development depend on the timing

• Neural crest cell defects can have widespread effects due to the variety of structures they contribute to

  • i.e. PNS, great vessels, skin pigmentation, cranial bones, etc.