Human Embryology Week 4 Overview

Human Embryology - Week 4 Development

• Learning Outcomes: Students will understand neural tube development, somite formation, heart development, pharyngeal arch formation, and limb bud development during week 4 of embryo development.

Neurulation: Neural Tube Formation (Week 4)

• Neural tube formation is critical for central nervous system (CNS) development.
• The neural plate folds into a neural tube, forming brain and spinal cord.
• Cranial closure begins at the head end of the tube.
• Caudal closure occurs towards the tail end of the tube.
• Neural crest cells form from neural tube folds, developing into peripheral nerves and facial bones.
• Clinical Correlation: Problems with neurulation can lead to spina bifida and anencephaly.

Somite Formation (Week 4)

• 20-25 somites form during week 4, giving rise to various tissues and structures.
• Somites are blocks of mesoderm alongside the neural tube.
• They contribute to axial skeleton (vertebrae and ribs) and skeletal musculature.
• Somites define the segmented body plan of the embryo.
• Sclerotome: Forms vertebrae, ribs, and intervertebral discs (axial skeleton).
• Dermatome: Forms the dermis of the skin (segmental body regions).
• Myotome: Forms skeletal muscles of the back, trunk, and limbs (epaxial and hypaxial muscles)

Heart Development (Week 4)

• The primitive heart tube starts beating in week 4, initiating blood circulation.
• The heart tube initially develops linearly but loops to form heart chambers.
• Blood is pumped from the primitive heart to various parts of the embryo for nutrient and oxygen delivery.

Pharyngeal Arches (Week 4)

• The first two pairs of pharyngeal arches appear in week 4.

• These arches eventually give rise to head and neck structures, including bones, muscles, and nerves.

• 1st arch: Develops into maxilla, mandible, and muscles of mastication.

• 2nd arch: Develops into stapes of ear and facial expression muscles.

• Cartilage Derivatives: Malleus, Incus, Mandible, Ligaments

• Muscle Derivatives: Mastication, Mylohyoid, Tensor, Facial Expression, Stapedius, Stylohyoid

• Nerve Supply: Mandibular (CN V3), Facial (CN VII)

• Arterial Derivatives: Maxillary, Stapedial, Hyoid

• These arches form various head and neck tissues.

Limb Bud Formation (Week 4)

• Limb buds form in week 4, marking the start of limb development.
• Upper limb buds appear slightly earlier than lower limb buds.
• Limb buds are composed primarily of mesoderm, which develops into bones and muscles, and ectoderm, which develops into skin.
• Limb buds elongate and differentiate into arms and legs, with fingers and toes developing later in embryonic development.

Summary of Key Events (Week 4)

• Neurulation forms brain and spinal cord.
• Somites form axial skeleton and musculature.
• Heart begins beating and blood circulation starts.
• Pharyngeal arches form head and neck structures.
• Limb buds initiate arm and leg development.