“THIS LITTLE BUN IS ALMOST DONE” - The Process of fetal brain development – By Dr. Vandana Patel (PT)

 

“Birth does not mark a particular milestone in the development of the brain. The brain continues to develop throughout the lifespan.”

 

 

Following fertilization of the ovum, rapid cell division takes place. As we have already studied during the journey of zygote formation; that the epiblast cellular layer results in formation of amnionic cavity. From the floor of this amnionic cavity embryonic disc starts emerging which comprises three layers of cells: the ectoderm, mesoderm and endoderm.

 The inner layer (endoderm) will transform into the internal organs (e.g., digestive and respiratory systems) of the body while the middle layer (mesoderm) will form the musculature and skeletal systems. The outer layer (ectoderm) evolves into a variety of structures of the Central Nervous System (CNS).

 The development of CNS begins by thickening of ectoderm layer. This thickening is attributed to increase in the height of ectodermal cells (shape changes from cuboidal to tall columnar) as well as migratory movements of the cells. This thickened layer of ectoderm is known as the neural plate.

 Furthermore, two ridges on each side of this neural plate continue to grow, which gives rise to two longitudinal neural folds, forming a neural groove in between.

 

                                               

  

  

                                          

 

During this process of neural plate shaping the broad and short neural plate becomes narrowed transversely and elongated rostro-caudally. The folds increase in height, curve toward each other and get fused to form rudiment of the neural tube. This process of formation of initial structure of CNS is called as neurulation. Neurulation commences toward the end of the third week of gestation. The neural tube converts into a cylindrical shape.

 After the fusion of neural folds, the formed neural tube separates from ectoderm and becomes buried in the mesenchyme below the surface. During this process, aggregations of cells from the neural folds come to lie on the supero-lateral margins of the tube. They are called as neural crest cells which forms neural crest.

The neural tube grows to form the central nervous system consisting of the brain and the spinal cord, while the neural crest forms the peripheral nervous system consisting of autonomic, cranial and spinal ganglia and nerves. The lumen of neural tube later becomes the central canal of the spinal cord and the ventricles of brain.

The head portion of the neural tube becomes the brain while the middle portion becomes the brain stem. The head or cephalic portion further differentiates into the forebrain, midbrain and hindbrain. For instance, by about the fifth week, the neural tube differentiates into the three primary structural units of the brain: the proencephalon (forebrain), the mesencephalon (midbrain) and the rhombencephalon (hindbrain).

 By the seventh week two additional structures are formed. The two additional structures are created when the prosencephalon and the rhombencephalon divide in two. The prosencephalon divides into the cerebral hemispheres (the telencephalon) and the thalamus, hypothalamus, epithalamus and pineal bodies (the diencephalon), while the rhombencephalon divides into pons, cerebellum (the metencephalon) and the medulla (the myelencephalon).

 

Let us conceptualize fetal CNS development to the construction of a house. The same way that a blueprint guides house construction, an individual’s genome serves as a blueprint for the brain. Some of the DNA in the genome creates proteins that build structures, while others are ‘timing genes’ that manage the sequencing of the building process. Neurons and glial cells function as the foundational materials of bricks, wood and cement. Axons, dendrites and synaptic connections among neurons serve as the wiring for electricity and the telephone. 

                                                                                                                                      

 

 

 

 

REFERENCES:

Martin RP, Dombrowski SC. Prenatal exposures: Psychological and educational consequences for children. Springer Science & Business Media; 2008 Feb 1.

Polin RA, Fox WW, Abman SH. Fetal and Neonatal Physiology E-Book. Elsevier Health Sciences; 2011 Aug 13.

Hepper P. prenatal development. In Slater A, Lewis M, editors, Introduction to Infant Development. 2nd edition. Oxford University Press. 2007. p. 41-62