BIU Human Embryology Lesson 04 Development Week 06 PDF

Human Embryology Lesson No. 04 Week 06 of Embryo Development BIU Medical Science Program Professor Dr. Ejaz Ahmed Khan BIU Medical Science Program  Learning Outcomes After completion of this session stud...

Human Embryology Lesson No. 04 Week 06 of Embryo Development BIU Medical Science Program Professor Dr. Ejaz Ahmed Khan BIU Medical Science Program  Learning Outcomes After completion of this session students will be able to: – Describe key events of embryonic development during the fifth week, including the growth of the brain, heart, and limb buds. – Explain the role of somites in musculoskeletal system development. – Identify the changes in cardiovascular development, including early circulation pathways and vessel formation. – Describe the formation of major organs and systems, such as the respiratory and digestive systems, during this stage.  Introduction to Week 05 Key Highlights: o Rapid Brain and Head development. o Organ primordia become more defined. Approximate Size: 7-9 mm  Key Development in Week 05 1. Brain Development: o Division of the Primary Brain Vesicles: forebrain, midbrain, hindbrain. o Cranial nerves begin to form. 2. Eye and Ear Development: o Optic vesicles appear (future eyes). o Otic pits form (future inner ears). 3. Limb Development: o Upper limb buds become paddle-shaped and Lower limb buds appear. 4. Heart Development: Septation begins in the heart chambers. Key Development in Week 05 Summary of Week 5 Brain and Cranial Nerve Development Brain Vesicles: o Forebrain: Telencephalon (cerebrum), Diencephalon (thalamus and hypothalamus). o Midbrain: Relay for sensory and motor functions. o Hindbrain: Metencephalon (pons, cerebellum), Myelencephalon (medulla).  Forebrain (Prosencephalon) Differentiation 1. Division into Two Regions: o Telencephalon:  Forms the future cerebral hemispheres.  Involved in advanced brain functions like cognition, memory, and voluntary movements. o Diencephalon:  Develops into the thalamus and hypothalamus.  Thalamus acts as a relay center for sensory information.  Hypothalamus regulates autonomic functions, including temperature and hormonal control.  Midbrain (Mesencephalon): Week 5 1. Overview: o The midbrain remains undivided in week 5. o Plays a critical role as a relay center:  Processes visual and auditory information.  Coordinates reflexive responses to stimuli. 2. Significance: o Acts as a connection between the forebrain and hindbrain. o Will later form structures like the tectum and tegmentum.  Hindbrain (Rhombencephalon) Differentiation 1. Division into Two Regions: o Metencephalon:  Forms the pons (connects cerebellum to other brain parts).  Develops into the cerebellum (coordinates motor functions and balance). o Myelencephalon:  Becomes the medulla oblongata (controls involuntary activities like breathing and heartbeat). 2. Significance: o Essential for vital body functions and motor coordination. Brain Vesicles:  Cranial Nerves Cranial Nerves: o CN I (olfactory) and CN II (optic) from the forebrain. o CN III–XII from the midbrain and hindbrain. Significance: o Lays the foundation for higher brain functions, sensory pathways, and motor coordination.  Heart Development: Week 5 Overview: o Heart undergoes complex morphogenesis to form four chambers. Key Events: o Septation of Atria:  Growth of the septum primum from the roof of the atrial chamber.  Formation of the foramen primum and later the foramen secundum. o Septation of Ventricles:  Interventricular septum grows upward from the base of the heart.  Separates the primitive left and right ventricles.  Heart Development: Week 5 o Endocardial Cushion Formation:  Cushions develop in the atrioventricular canal, contributing to valve formation. o Conotruncal Septation:  Truncus arteriosus is divided into the aorta and pulmonary trunk. Heart Development: Week 5  Overview of Heart Development 1. Significance of Week 5: o Transition from a simple tubular structure to a complex organ with chambers and valves. o Establishment of the groundwork for systemic and pulmonary circulation. 2. Key Processes: o Septation of atria and ventricles. o Formation of endocardial cushions and valves. o Division of the truncus arteriosus. Overview of Heart Development  Formation of the Atrial Septum 1. Growth of Septum Primum: o Septum primum grows downward from the roof of the common atrium. o Creates a temporary opening called the foramen primum, allowing blood flow between atria. 2. Formation of Foramen Secundum: o As the foramen primum closes, perforations in the septum primum coalesce to form the foramen secundum. o Ensures continued blood flow in fetal circulation. Formation of the Atrial Septum  Formation of the Ventricular Septum 1. Interventricular Septum Growth: o Develops as a muscular ridge from the base of the primitive ventricles. o Grows upwards toward the endocardial cushions to divide the left and right ventricles. 2. Incomplete Septation: o A temporary interventricular foramen persists until the upper membranous part forms. Formation of the Ventricular Septum  Role of Endocardial Cushions 1. Formation in the Atrioventricular Canal: o Endocardial cushions form from mesenchymal cells in the atrioventricular region. o Act as precursors for atrioventricular valves (tricuspid and mitral). 2. Function: o Contribute to the separation of atria and ventricles. o Essential for aligning the atrial and ventricular septa. Formation in the Atrioventricular Canal  Division of the Outflow Tract 1. Truncus Arteriosus Division: o Neural crest cells migrate to form the conotruncal ridges. o These ridges spiral and fuse to separate the truncus arteriosus into the aorta and pulmonary trunk. 2. Significance: o Establishes distinct systemic (aortic) and pulmonary (pulmonary artery) circulation pathways. Conotruncal ridges.  Eye and Ear Development: Week 5  Eye Development: 1. Optic Vesicles:  Appear as lateral bulges from the diencephalon.  Form the optic cup, which gives rise to the retina and optic nerve. 2. Lens Placode:  Thickening of surface ectoderm, precursor to the lens.  Ear Development: 1. Otic Pits:  Invaginations of surface ectoderm lateral to the hindbrain.  Form the otic vesicles, precursors to the inner ear structures (cochlea, semicircular canals).  Eye Development: Week 5 Key Features: o Eyes begin as optic vesicles, emerging from the diencephalon. o Interactions between the optic vesicles and surface ectoderm lead to lens formation. Key Structures Formed in Week 5: o Optic Vesicles o Lens Placode  Formation of Optic Vesicles 1. What are Optic Vesicles? o Lateral bulges emerging from the diencephalon. o Begin the development of the retina and optic nerve. 2. Steps in Development: o Evagination from the neural tube. o Interaction with the overlying ectoderm to initiate lens formation.  Optic Cup Development 1. Transformation of Optic Vesicles: o Optic vesicles invaginate to form the optic cup. 2. Key Structures from Optic Cup: o Inner layer: Becomes the neural retina (responsible for photoreception). o Outer layer: Develops into the retinal pigment epithelium.  Lens Placode Development 1. What is the Lens Placode? o Thickened ectoderm adjacent to the optic vesicle. o Precursor to the lens of the eye. 2. Steps in Formation: o Induced by signals from the underlying optic vesicle. o Begins invagination to form the lens vesicle. Eye Development: Week 5  Ear Development: Week 5 Key Features: o Ears begin as otic pits, which later form the inner ear structures. o Development influenced by the underlying hindbrain.  Otic Pit Formation 1. What are Otic Pits? o Invaginations of surface ectoderm located lateral to the hindbrain. o Precursor to the inner ear. 2. Steps in Development: o Surface ectoderm thickens and invaginates to form otic pits. o Pits deepen and eventually pinch off to create otic vesicles.  Development of Otic Vesicles 1. What are Otic Vesicles? o Hollow structures formed from the invaginated otic pits. o Precursors to the cochlea, semicircular canals, and other inner ear components. 2. Significance: o Establish the groundwork for hearing (cochlea) and balance (semicircular canals). Otic Placode, Otic Pit and Otic Vesicle Ear Development: Week 5  Limb Development in Week 5 A focus on early development of upper and lower limb buds. The role of the Apical Ectodermal Ridge (AER) in limb differentiation and outgrowth.  Limb Development: Week 5 Upper Limb Buds: o Appear as paddle-shaped structures, Derived from the mesodermal core (skeletal precursors) covered by ectoderm. Lower Limb Buds: o Begin to appear slightly later than the upper limb buds. o Start at the caudal end of the embryo. Apical Ectodermal Ridge (AER): o Specialized ectoderm at the distal tip of limb buds. o Essential for promoting outgrowth and differentiation of the underlying mesoderm. Upper and Lower Limb week 5  Development of Upper Limb Buds 1. Formation: o Appear as paddle-shaped protrusions on the lateral aspect of the embryo at the somite level C4-T1. o Core Mesoderm: Forms skeletal structures, muscles, and connective tissue. o Ectodermal Covering: Provides protective structure and aids in signaling for outgrowth. 2. Timing: o Begin forming slightly earlier than the lower limb buds.  Development of Lower Limb Buds 1. Formation: o Appear as small projections at the caudal end of the embryo, near the lumbar and sacral regions (L2-S2). o Develop later than upper limb buds but follow a similar growth mechanism. 2. Key Components: o Mesoderm Core: Precursor for the skeletal framework and musculature. o Ectoderm Covering: Supports outgrowth through molecular signaling.  Role of the Apical Ectodermal Ridge (AER) 1. What is the AER? o A thickened ectodermal ridge at the distal tip of each limb bud. o Serves as the primary signaling center for limb development. 2. Functions: o Promotes proliferation of mesodermal cells in underlying progress zone. o Ensures outgrowth and patterning of limbs along the proximal-distal axis. 3. Key Molecular Signals: o FGF (Fibroblast Growth Factors): Secreted by AER to maintain mesoderm proliferation and differentiation. Apical Ectodermal Ridge and Zone of Polarizing Activity