General patterns of evolution and ontogenesis
The evolutionary development of the nervous system is closely linked to the fundamental laws of individual development. According to the concept of phylembryogenesis proposed by A.N. Severtsov, evolutionary modifications of ontogenetic processes aim to increase the adaptation of organisms at the adult stage.
During phylogenesis, certain structures may undergo reduction. A crucial pattern of this process is that reduction never affects the earliest stages of organ formation. A classic example is the development of the notochord in higher vertebrates: it is laid down at early stages but subsequently disappears.
If the normal mechanism of reduction fails and does not occur during embryogenesis, the adult animal develops an organ that should have disappeared. This phenomenon is known as atavism.
Primordium and formation of the neural tube
The formation of the nervous system begins at the early gastrula stage. The localization of the primordium is strictly defined—it occurs within the ectoderm on the dorsal side of the developing embryo.
The first major stage of embryonic development demonstrates common features of the entire phylum Chordata and consists of the formation of the neural tube. This process proceeds under strict genetic control. The main trigger is the Zfp521 gene, which activates a whole group of specific genes responsible for transforming ordinary ectoderm cells into specialized neuroectoderm.
The process of neural tube formation can be divided into three consecutive stages:
- Neural plate. On the dorsal side of the embryo, a strip of neuroectoderm differentiates and separates within the ectoderm.
- Neural groove (sulcus). The cells of the plate change shape, causing the neural plate to fold and bend inward into the embryo.
- Neural tube. The edges of the formed neural groove gradually approach each other and finally fuse. A closed tube with an internal cavity called the neurocoel is formed.
If the normal course of the third stage is disrupted (if the neural groove fails to fuse), a severe developmental pathology develops — rachischisis (failure of neural tube closure).
Differentiation and structure of the brain
The second stage of nervous system development reflects features exclusive to the subphylum Vertebrata. At this stage, complex differentiation of the initially homogeneous neural tube occurs into two main divisions:
- Brain
- Spinal cord
The structure of the brain follows a single fundamental plan in all vertebrate animals. It always consists of exactly five regions. These regions are arranged strictly in the same sequence, although the degree of their anatomical and functional development varies significantly across different animal classes.
Sequence of brain regions (from rostral to caudal):
- Forebrain
- Diencephalon
- Midbrain
- Hindbrain (includes structures such as the pons and cerebellum)
- Medulla oblongata
Briefly: answers to topical questions
What structures are part of the hindbrain?
The hindbrain includes the pons and the cerebellum.
- Hindbrain (Metencephalon) — forms at the five-vesicle stage.
- Pons — a structural component of the hindbrain.
- Cerebellum — a structural component of the hindbrain.
Both of these structures develop as a result of the differentiation of the hindbrain vesicle (Rhombencephalon), which divides into the metencephalon and myelencephalon.
What are the primary brain vesicles called at the three-vesicle brain stage?
At the three-vesicle brain stage, the primary brain vesicles are called the forebrain, midbrain, and hindbrain.
- Forebrain vesicle (Prosencephalon) — the first brain vesicle.
- Midbrain vesicle (Mesencephalon) — the second brain vesicle.
- Hindbrain vesicle (Rhombencephalon) — the third brain vesicle.
These three regions are formed as a result of the division of the primary swelling at the rostral end of the neural tube.
From which primary brain vesicle does the diencephalon develop?
The diencephalon develops from the forebrain vesicle.
- Forebrain vesicle (Prosencephalon) — differentiates at the five-vesicle stage.
- Diencephalon (Diencephalon) — forms from the caudal part of the forebrain vesicle.
The cranial part of the same vesicle subsequently gives rise to the telencephalon (Telencephalon).
The brain regions are easy to remember by their first letters (from rostral to caudal): Telencephalon, Diencephalon, Mesencephalon, Metencephalon, Myelencephalon.
Class and exam questions
From which germ layer does the nervous system develop?
The nervous system always develops from the ectoderm (the outer germ layer) on the dorsal side of the embryo at the early gastrula stage.
What is phylembryogenesis according to Severtsov?
These are evolutionary modifications of individual development (ontogenesis) processes that lead to an increase in the overall adaptation of sexually mature animal forms.
What is the role of the Zfp521 gene in nervous system development?
The Zfp521 gene is a key regulator in the early stages. It activates a group of genes that trigger the transformation of ordinary ectoderm into neuroectoderm.
What is rachischisis?
Rachischisis is an embryonic developmental pathology characterized by a failure of the neural groove margins to fuse, leading to unclosed neural tube.
What else to study on this topic
- Mechanisms of notochord reduction in higher vertebrates
- Genetic regulation of ectoderm transformation into neuroectoderm
- Evolutionary significance of neurocoel formation
- Differences in the degree of development of the five brain regions across various vertebrate classes
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Phylogenesis of the chordate nervous system" and 3,793 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the section "Phylogeny of the vertebrate nervous system"
The entire section "Phylogeny of the vertebrate nervous system"Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.