Unit 3
The central nervous system (CNS) and the epidermis are two major tissues derived from the ectoderm
and they are called neural ectoderm, one of the three primary germ layers established during early
embryonic development.
Central Nervous System (CNS)
• Origin and Formation: The CNS, consisting of the brain and spinal cord, originates from a portion
of the dorsal ectoderm that is specified to become neural ectoderm, forming a region called the
neural plate (which is a region of embryo). The process by which the neural plate forms the
neural tube, the precursor of the CNS, is called neurulation (neural tube formation). The neural
tube will form the brain anteriorly and the spinal cord.
• There are two main ways the neural tube forms:
○ Primary neurulation: The cells surrounding the neural plate direct it to proliferate,
invaginate, and pinch off from the surface ectoderm to create a hollow tube. This process
involves stages like the formation and shaping of the neural plate, bending of the neural
plate to form the neural groove, and the closure of the neural groove into the neural tube.
Hinge regions, including the medial hinge point (MHP) induced by the notochord and
dorsolateral hinge points (DLHPs) anchored to the surface ectoderm, play a crucial role in
the bending process.
○ Secondary neurulation: The neural tube forms from a solid cord of cells that sinks into the
embryo and subsequently hollows out (cavitates) to become a hollow tube. This is the
exclusive mode in fishes and contributes to the posterior neural tube in birds, amphibians,
and mammals.
unit 3 Page 1
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unit 3 Page 2
The central nervous system (CNS) and the epidermis are two major tissues derived from the ectoderm
and they are called neural ectoderm, one of the three primary germ layers established during early
embryonic development.
Central Nervous System (CNS)
• Origin and Formation: The CNS, consisting of the brain and spinal cord, originates from a portion
of the dorsal ectoderm that is specified to become neural ectoderm, forming a region called the
neural plate (which is a region of embryo). The process by which the neural plate forms the
neural tube, the precursor of the CNS, is called neurulation (neural tube formation). The neural
tube will form the brain anteriorly and the spinal cord.
• There are two main ways the neural tube forms:
○ Primary neurulation: The cells surrounding the neural plate direct it to proliferate,
invaginate, and pinch off from the surface ectoderm to create a hollow tube. This process
involves stages like the formation and shaping of the neural plate, bending of the neural
plate to form the neural groove, and the closure of the neural groove into the neural tube.
Hinge regions, including the medial hinge point (MHP) induced by the notochord and
dorsolateral hinge points (DLHPs) anchored to the surface ectoderm, play a crucial role in
the bending process.
○ Secondary neurulation: The neural tube forms from a solid cord of cells that sinks into the
embryo and subsequently hollows out (cavitates) to become a hollow tube. This is the
exclusive mode in fishes and contributes to the posterior neural tube in birds, amphibians,
and mammals.
unit 3 Page 1
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unit 3 Page 2
