Third to Eighth Weeks: The Embryonic Period
Overview of the Embryonic Period
• Timeline: Third to Eighth Weeks of development. This period is known as the
organogenesis phase.
• Key Concepts:
o Differentiation of the three germ layers—ectoderm, mesoderm, and endoderm—
into major tissues and organ systems.
o Organogenesis: By the end of this phase, all the major organ systems are
established, and the embryo starts to resemble the external body form of a human,
laying the foundational structures for fetal development.
Derivatives of the Ectodermal Germ Layer
General Role
• Ectodermal Germ Layer: Responsible for the development of structures and organs that
interact with the external environment.
• Main Derivatives:
o Central Nervous System (CNS) - Brain and spinal cord
o Peripheral Nervous System (PNS) - Sensory and motor neurons outside the CNS
o Sensory Epithelium: Eyes, ears, and the nasal epithelium
o Epidermis: Skin, hair, nails, sebaceous glands, mammary glands, and sweat glands.
o Specialized Structures: Tooth enamel and pituitary gland (anterior part from
ectoderm, posterior part from neuroectoderm)
Neural Induction
• Initiation: Begins during week three; notochord and prechordal mesoderm release inductive
signals to stimulate ectoderm differentiation into neuroectoderm, forming the neural plate.
• Molecular Signaling Pathways:
o Fibroblast Growth Factor (FGF) upregulates neural differentiation, FGF promotes
neural development by downregulating BMP (Bone Morphogenetic Protein) activity.
o Bone Morphogenetic Protein 4 (BMP4), a member of the TGF-β family, is
typically responsible for ventralization of ectoderm and mesoderm.
o Inhibition of BMP4:
§ BMP4 promotes epidermal fate unless inhibited.
§ Noggin, Chordin, and Follistatin, released by the notochord and primitive
node, antagonize BMP4, redirecting cells from epidermal to neural fate.
, § FGF also represses BMP: transcription, contributing to the formation of
neural ectoderm.
Neurulation
• Definition:
• Neurulation is the process by which the neural plate folds to become the neural
tube (forming the basis of the CNS).
• Begins in week three and continues into week four.
• Steps of Neurulation:
• Formation of the Neural Plate:
§ The notochord induces the overlying ectoderm to thicken into the neural
plate by secreting signals like SHH (Sonic Hedgehog) and Noggin.
• Neural Fold and Groove Development:
§ By the end of the third week, the lateral edges of the neural plate elevate,
forming neural folds, while the mid-region creates a neural groove.
§ Elevation continues, and the folds move towards each other.
• Neural Tube Formation:
§ Fusion of the folds begins at the cervical region (at the level of the fifth
somite) and progresses in both cranial and caudal directions.
§ Anterior (Cranial) Neuropore: Closes by day 25 (corresponding to the 18-
20 somite stage).
§ Posterior (Caudal) Neuropore: Closes by day 28 (25-somite stage).
§ After closure, the neural tube consists of:
§ Caudal Narrow Portion: Future spinal cord.
§ Broader Cephalic Portion: Future brain vesicles.
• Clinical Relevance: Failure of neuropore closure results in neural tube defects
(NTDs):
§ Anencephaly: Failure of cranial neuropore closure, resulting in brain
absence.
§ Spina Bifida: Failure of caudal neuropore closure.
Neural Crest Cells (NCCs)
• Formation and Migration:
o Arise from cells at the lateral edge of the neural plate.
o These cells undergo an epithelial-to-mesenchymal transition and migrate to
different parts of the body (becoming migratory and multipotent)
• Migration Pathways:
Overview of the Embryonic Period
• Timeline: Third to Eighth Weeks of development. This period is known as the
organogenesis phase.
• Key Concepts:
o Differentiation of the three germ layers—ectoderm, mesoderm, and endoderm—
into major tissues and organ systems.
o Organogenesis: By the end of this phase, all the major organ systems are
established, and the embryo starts to resemble the external body form of a human,
laying the foundational structures for fetal development.
Derivatives of the Ectodermal Germ Layer
General Role
• Ectodermal Germ Layer: Responsible for the development of structures and organs that
interact with the external environment.
• Main Derivatives:
o Central Nervous System (CNS) - Brain and spinal cord
o Peripheral Nervous System (PNS) - Sensory and motor neurons outside the CNS
o Sensory Epithelium: Eyes, ears, and the nasal epithelium
o Epidermis: Skin, hair, nails, sebaceous glands, mammary glands, and sweat glands.
o Specialized Structures: Tooth enamel and pituitary gland (anterior part from
ectoderm, posterior part from neuroectoderm)
Neural Induction
• Initiation: Begins during week three; notochord and prechordal mesoderm release inductive
signals to stimulate ectoderm differentiation into neuroectoderm, forming the neural plate.
• Molecular Signaling Pathways:
o Fibroblast Growth Factor (FGF) upregulates neural differentiation, FGF promotes
neural development by downregulating BMP (Bone Morphogenetic Protein) activity.
o Bone Morphogenetic Protein 4 (BMP4), a member of the TGF-β family, is
typically responsible for ventralization of ectoderm and mesoderm.
o Inhibition of BMP4:
§ BMP4 promotes epidermal fate unless inhibited.
§ Noggin, Chordin, and Follistatin, released by the notochord and primitive
node, antagonize BMP4, redirecting cells from epidermal to neural fate.
, § FGF also represses BMP: transcription, contributing to the formation of
neural ectoderm.
Neurulation
• Definition:
• Neurulation is the process by which the neural plate folds to become the neural
tube (forming the basis of the CNS).
• Begins in week three and continues into week four.
• Steps of Neurulation:
• Formation of the Neural Plate:
§ The notochord induces the overlying ectoderm to thicken into the neural
plate by secreting signals like SHH (Sonic Hedgehog) and Noggin.
• Neural Fold and Groove Development:
§ By the end of the third week, the lateral edges of the neural plate elevate,
forming neural folds, while the mid-region creates a neural groove.
§ Elevation continues, and the folds move towards each other.
• Neural Tube Formation:
§ Fusion of the folds begins at the cervical region (at the level of the fifth
somite) and progresses in both cranial and caudal directions.
§ Anterior (Cranial) Neuropore: Closes by day 25 (corresponding to the 18-
20 somite stage).
§ Posterior (Caudal) Neuropore: Closes by day 28 (25-somite stage).
§ After closure, the neural tube consists of:
§ Caudal Narrow Portion: Future spinal cord.
§ Broader Cephalic Portion: Future brain vesicles.
• Clinical Relevance: Failure of neuropore closure results in neural tube defects
(NTDs):
§ Anencephaly: Failure of cranial neuropore closure, resulting in brain
absence.
§ Spina Bifida: Failure of caudal neuropore closure.
Neural Crest Cells (NCCs)
• Formation and Migration:
o Arise from cells at the lateral edge of the neural plate.
o These cells undergo an epithelial-to-mesenchymal transition and migrate to
different parts of the body (becoming migratory and multipotent)
• Migration Pathways:
