Notes on Early Embryonic Development from Dr. Aaron Camp's Lecture

Speaker: Doctor Aaron Camp, Senior Lecturer at Sydney Medical School, University of Sydney
Focus on early developmental stages, particularly the process of gastrulation and the formation of the nervous system

At approximately 16 days post-fertilization, the embryo forms a structure called the blastula that consists of two layers:
- Epiblast: The layer that will differentiate into the trilaminar disc.
- Hypoblast: The supporting layer, not involved in forming the three germ layers.
The trilaminar disc develops during a process known as gastrulation, which leads to the formation of three primary germ layers:
- Ectoderm (blue):
- Forms the nervous system, skin, hair, and nails.
- Mesoderm (red):
- Develops into the circulatory system, skeletal system, and muscular system.
- Endoderm (yellow):
- Forms the internal organs, such as the digestive system, liver, and pancreas.

At around 18 days, the ectoderm undergoes a thickening along the midline to form the neural plate.
By 20 days, the invagination of the neural plate creates the neural groove.
The formation of the neural groove is driven by the activity of various transcription factors:
- Inhibition of certain transcription factors in the midline, while stimulation occurs laterally.
- Cells along the midline do not elongate, resulting in a deepening of the neural groove.

At 22 days, closure of the neural tube begins in the midline region. This closure is influenced by specific molecules:
- Particularly cadherin molecules, which change their expression to facilitate the pinching off of the neural tube from the surface ectoderm.
- Differential expression of different cadherin isoforms enables the separation.
Closure of the neural tube occurs between 25 and 28 days:
- Anterior neuropore closes first around 25-26 days.
- The posterior neuropore closes at approximately 27-28 days.
- This closure is critical for future neural development.

During the closure of the neural tube, neural crest cells are generated at the border of the neural plate and ectoderm.
Neural crest cells are influenced by a variety of transcription factors that dictate their fate based on gradients and timing factors:
- For instance, a prolonged exposure to Wnt and minimal Bone Morphogenic Protein leads to the formation of neural crest cells.
- These cells migrate throughout the body to create essential structures, including:
- Parasympathetic ganglia
- Dorsal roots
- Components of the facial skeleton (bones and cartilages)
- Melanocytes (pigment cells)
- Parts of the circulatory system

Following neural tube closure, the embryo enters the three vesicle stage:
- The primary vesicles are:
- Prosencephalon (forebrain)
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain)
- Additionally, the caudal neural tube is formed.
Transitioning to the five vesicle stage:
- The prosencephalon divides into:
- Telencephalon
- Diencephalon
- The mesencephalon remains unchanged.
- The rhombencephalon divides into:
- Metencephalon
- Myelencephalon
The outer layer of the neural tube ultimately develops into the structures of the central nervous system (CNS).
- Meanwhile, the lumen of the closed neural tube forms the ventricular system, which serves important functions regarding cerebrospinal fluid.

Final stages of neural tube development may encounter issues leading to neural tube defects.
Continuous folding of the neural tube leads to the development of a more advanced brain structure.
Emphasis that optimal development is not guaranteed and can be affected by various factors throughout gestation.