Human Embryology - Week 6 Development

Human Embryology - Week 6 Embryo Development

• This lesson details embryo development during the fifth week.
• Students will be able to describe key events, including brain, heart, and limb bud growth.
• Somites' role in musculoskeletal system development will be explained.
• Cardiovascular development, including early circulation pathways and vessel formation, will be identified.
• The formation of major organs and systems, such as respiratory and digestive, during this stage will be described.

Week 5 Embryonic Development Overview

• Rapid brain and head development occurs.
• Organ primordia become more defined.
• Approximate size: 7-9 mm
• Brain development: Division of the primary brain vesicles (forebrain, midbrain, hindbrain). Cranial nerves begin forming.
  • Eye development: Optic vesicles appear (future eyes). Otic pits form (future inner ears).
• Limb development: Upper limb buds become paddle-shaped, with lower limb buds appearing. Heart development: Septation begins in the heart chambers.

Embryonic Period Overview

• Duration: First 8 weeks after conception.
• Major embryological events: Organs formed from three primary germ tissues, emergence of basic body plan.
• Timelines: 25 days (3 weeks), 3.5 mm; 36 days (5 weeks), 10 mm; 56 days (8 weeks), 30 mm

Week 5 Brain and Cranial Nerve Development Summary

• Brain Vesicles: Forebrain (Telencephalon - cerebrum; Diencephalon -thalamus and hypothalamus), Midbrain, Hindbrain (Metencephalon- pons, cerebellum; Myelencephalon- medulla)..
• Midbrain: Relay center for sensory & motor functions.
• Hindbrain: Controls involuntary activities (breathing, heartbeat).

Forebrain (Prosencephalon) Differentiation

• Division into two regions:
  • Telencephalon: Forms the future cerebral hemispheres, involved in advanced brain functions (cognition, memory, and voluntary movements).
  • Diencephalon: Develops into the thalamus and hypothalamus, with the thalamus acting as a sensory relay center and the hypothalamus regulating autonomic functions (temperature and hormonal control).

Midbrain (Mesencephalon) Week 5

• Midbrain remains undivided in week 5. A critical relay center, processing visual and auditory information, coordinating reflexive responses to stimuli.
• Acts as a connection between the forebrain and hindbrain, later forming structures like the tectum and tegmentum.

Hindbrain (Rhombencephalon) Differentiation

• Division into two regions:
  • Metencephalon: Forms the pons (connects cerebellum to other brain parts), and the cerebellum (coordinates motor functions and balance).
  • Myelencephalon: Becomes the medulla oblongata (controls involuntary activities like breathing and heartbeat).
• Essential for vital body functions and motor coordination.

Cranial Nerves

• CN I (olfactory) and CN II (optic) from the forebrain.
• CN III-XII from the midbrain and hindbrain.
• Lay the foundation for higher brain functions, sensory pathways, and motor coordination.

Heart Development: Week 5

• Heart undergoes complex morphogenesis to form four chambers.
• Key Events:
  • Septation of Atria:
    • Septum primum grows from the atrial roof
    • Formation of foramen primum and secundum
  • Septation of Ventricles:
    • Interventricular septum grows from the heart base
    • Separates primitive left and right ventricles
• Endocardial Cushion Formation: Cushions develop in atrioventricular canal, contributing to valve formation
• Conotruncal Septation: Truncus arteriosus divides into the aorta and pulmonary trunk.

Overview of Heart Development

• Transition from a simple tubular structure to a complex organ with chambers and valves.
• Establishment of the groundwork for systemic and pulmonary circulation.
• Key processes: Septation of atria and ventricles, formation of endocardial cushions and valves, division of the truncus arteriosus, blood flow and anatomical descriptions.

Eye and Ear Development: Week 5

• Eye development
  • Optic Vesicles: Appear as lateral bulges from the diencephalon, forming the optic cup which gives rise to the retina and optic nerve.
  • Lens Placode: Thickening of surface ectoderm, precursor to the lens.
• Ear development
  • Otic Pits: Invaginations of surface ectoderm lateral to the hindbrain forming otic vesicles which are precursors to the inner ear structures (cochlea, semicircular canals).

Limb Development in Week 5

• Upper Limb Buds: Appear as paddle-shaped structures derived from mesoderm (skeletal precursors), covered by ectoderm
• Lower Limb Buds: Appear slightly later than the upper limb buds, starting at the caudal end of the embryo.
• Apical Ectodermal Ridge (AER): specialized ectoderm at the distal tip of limb buds, essential for promoting outgrowth and differentiation of underlying mesoderm.

Role of Apical Ectodermal Ridge (AER)

• AER acts as the primary signalling center for limb development, promoting proliferation of mesodermal cells and ensuring outgrowth and limb patterning along the proximal-distal axis.
• Fibroblast Growth Factors (FGF) are secreted by AER to maintain mesoderm proliferation and differentiation.

Explore the key events of embryo development during Week 6. This quiz covers brain, heart, and limb bud growth, as well as the formation of major organs and systems. Gain insights into the role of somites in the musculoskeletal system and early cardiovascular development.