Brain embryogenesis

Initial Stage and Structure Formation

The formation of the central nervous system starts with the creation of a primary swelling at the most rostral (cranial) end of the neural tube. This complex process begins as early as the 22nd day of embryonic development. It is at this moment that the foundations of the future forebrain are laid down.

Simultaneously with the growth of the neural tube, the visual apparatus forms: specialized structures—optic cups—evaginate on the sides of the future diencephalon. By the fourth week of development, the lens epithelium begins to invaginate. It is important to note that the division of the primary optic vesicles is critical: if it is disrupted, the fetus develops a severe developmental pathology known as cyclopia. By the fifth week, the embryo progresses to the next stages of active organogenesis.

Three-Vesicle Stage

Soon, the primary expansion of the neural tube divides into three distinct regions, forming the three-vesicle brain stage. At this stage, the basic major divisions of the future brain are established:

• Forebrain (Prosencephalon);
• Midbrain (Mesencephalon);
• Hindbrain (Rhombencephalon), also referred to as the primary hindbrain.

This stage is fundamental for the proper development of the entire nervous system. If developmental timing is disrupted (heterochrony) and the rostral part of the neural tube arrests growth precisely at the three-vesicle stage, a severe malformation—atelencephaly—develops. The primary consequence of this pathology is the complete absence of the cerebral hemispheres and subcortical nuclei.

Five-Vesicle Stage

As the embryo grows (during the period when its length reaches 10.2–13.6 mm), further differentiation occurs. The three-vesicle stage transitions into the five-vesicle brain stage:

• The primary forebrain (Prosencephalon) undergoes division into the telencephalon (Telencephalon) and diencephalon (Diencephalon).
• The primary midbrain (Mesencephalon) does not divide and retains its original name, forming all midbrain structures.
• The primary hindbrain (Rhombencephalon) gives rise to two structures: the metencephalon (Metencephalon), which will later include the pons and cerebellum, and the medulla oblongata (Medulla oblongata). A distinguishing feature of the medulla oblongata is that it lacks a sharp inferior border and transitions smoothly into the spinal cord tissue.

When the fetus measures between 50 and 130 mm, a progressive and very rapid growth of the telencephalic structures (Telencephalon) is observed. The cerebral hemispheres expand so intensively that they begin to physically cover the underlying structures (diencephalon and mesencephalon). Concurrently, the neural tube forms characteristic flexures. As a result, the formed brain acquires a developed cortex, corpus callosum, and all necessary brainstem structures.

Transformation of the Neurocoele into Ventricles

The neural tube is initially a hollow structure. Its internal space, called the neurocoele, does not disappear during embryogenesis, but undergoes complex transformation. The ventricular system of the brain is formed from the cavities of the primary brain vesicles:

1. In the growing cerebral hemispheres (Telencephalon), the cavities of the neurocoele expand significantly, forming the left and right lateral ventricles. As an interesting comparative anatomy fact, in fish these spaces are not separated from each other and form a single common cavity.
2. The cavity of the diencephalon (Diencephalon) transforms into the third ventricle.
3. In the region of the midbrain (Mesencephalon), the wide cavity narrows significantly, forming a narrow cleft. This structure is called the cerebral aqueduct (or aqueduct of Sylvius) and serves as a channel connecting the ventricles to each other.
4. The cavity in the area of the medulla oblongata (Medulla oblongata) expands and forms the fourth ventricle. Inferiorly, it communicates directly with the central canal of the spinal cord, ensuring fluid circulation within the central nervous system.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed anatomy of the telencephalon and cerebral hemispheres
• Spatial formation of brain tube flexures
• Development of the cerebral cortex and corpus callosum
• Comparative anatomy of the development of the ventricular system in vertebrates

Other topics in the section "Phylogeny of the vertebrate nervous system"

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