BIU Lesson 05 Week 08 Human Development PDF

Human Embryology Lesson No. 05 Week 08 of Embryo Development BIU Medical Science Program Professor Dr. Ejaz Ahmed Khan BIU Medical Science Program  Learning Outcomes After completion of this sess...

Human Embryology Lesson No. 05 Week 08 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 the key developmental milestones in organogenesis at Week 8, (Gastrointestinal, Heart, and Kidney systems). – Identify the stages of limb differentiation at Week 8, including the development of upper and lower limbs. – Analyze the process of heart septation and circulation during Week 8 and explain its functional implications for the developing embryo. – Assess the impact of developmental disruptions in gastrointestinal, kidney, and cardiovascular systems at Week 8, and discuss potential clinical conditions. Introduction to Week 8 of Embryonic Development Key Highlights: o Marks the end of the embryonic period. o Embryo reaches a size of approximately 25–30 mm (Crown-Rump Length). o All major organ systems are established in their rudimentary forms. o Transition from embryo to the fetal stage.  End of the Embryonic Period in Week 8 Week 8 marks the completion of the embryonic period of development. During this time, the embryo has developed the foundational structures of major organ systems such as the heart, brain, liver, and kidneys. The embryonic period is characterized by rapid differentiation of tissues and organogenesis (formation of organs). The transition to the fetal stage is important because it shifts the focus from the formation of organs to growth and functional refinement.  Embryo Size in Week 8: 25–30 mm (Crown-Rump Length) The embryo measures approximately 25-30 mm in length. Head and body proportions are more defined: o The head remains large in proportion to the body, but the neck begins to develop. o The limbs are elongated, and fingers and toes are becoming more distinct. External features like the eyes, nose, and mouth are taking shape. Growth rate slows down compared to earlier weeks, but differentiation of organ systems continues  Major Organ Systems Established in Rudimentary Forms All major organ systems have developed their basic structure: 1. Nervous System: o The brain has developed its basic regions, and spinal cord formation continues. o Motor neurons begin to grow, though the embryo is not yet capable of coordinated movements. 2. Cardiovascular System: o The heart is fully formed with four chambers. It has a consistent heartbeat. o Blood vessels are continuing to form, and circulation is active.  Major Organ Systems Established in Rudimentary Forms 3. Musculoskeletal System: o The limbs have distinct fingers and toes, and the cartilaginous skeleton continues to form. o Ossification has started in some bones, particularly the long bones of the arms and legs. 4. Digestive System: o The intestinal loop continues its development, and the stomach, liver, and pancreas are forming.  Major Organ Systems Established in Rudimentary Forms 5. Respiratory System: o The lungs begin to develop their basic structure, but the fetus is not yet capable of breathing air. 6. Reproductive System: o The genitalia are becoming more defined, though differentiation between male and female genitalia is still rudimentary.  Transition from Embryo to Fetus (Week 8) The fetal period begins at Week 9 and marks a shift from the embryo’s formation of organs to the fetus’s growth and maturation. Key transitions include: 1.Growth: The embryo will now grow in size and mass as it prepares for the final stages of gestation. 2.Functional Refinement: Organ systems, while rudimentary in the embryo, begin to take on their final form and start functioning more efficiently. 3.External Features: The external ear, eyes, and face continue to refine, and the limbs grow in length and complexity. Lower Limb Development in Week 6 4. Increased Mobility: The embryo will start to show some signs of movement, although it’s not fully developed enough for significant activity. The embryo’s transition to the fetal stage involves the cessation of rapid organogenesis and the start of growth, refinement, and functional maturation.  Progress in facial Structure and External Features Key Events: o Fusion of Eyelids:  Eyelids grow and fuse to protect the developing eyes.  Fusion is temporary and eyelids will reopen in the fetal period. o Formation of External Genitalia:  Indifferent genitalia begin to show the first signs of differentiation.  Genital tubercle elongates (early stages of external genitalia development). Formation of External Genitalia in Week 8 1. Indifferent Genitalia: o The indifferent genitalia (early stage of genital development) start to show the first signs of differentiation. o At this stage, the external genitalia have not yet taken on male or female characteristics, and both sexes appear similar. 2. Genital Tubercle Elongation: o The genital tubercle begins to elongate, which is the first step in the formation of external genitalia. o This process marks the early development of the penis in males or clitoris in females.  Significance of Week 8 Face and Neck Development 1. Facial Features Become Defined: o By the end of Week 8, the embryo’s face with visible eyes, nose, and mouth. This is a critical step in the facial maturation process,. 2. Preparation for the Fetal Stages: o The eyelid fusion and early genitalia formation signify the preparation of the embryo for the transition into the fetal period, where growth of features like the face and neck will continue. o The development of the face and neck structures is integral to the embryo’s overall appearance, making it more recognizable.  Limb Development: Week 8 Overview: o Limbs become more defined, with differentiation of fingers and toes. Key Events: o Fingers and Toes Well-Defined:  Webbing between digits disappears due to apoptosis.  Nails begin to develop as tiny ridges at the tips. o Coordinated Movements:  Early spontaneous limb movements start. Significance: o Movement is an early sign of neuromuscular coordination. Coordinated Limb Movements in Week 8 1. Early Spontaneous Limb Movements: o These movements are facilitated by the development of muscles and nerves in the limbs. o Although these movements are not yet voluntary, they are crucial for the development of neuromuscular coordination. 2. Neuromuscular Coordination: o The initial movements help to establish communication between the nervous system and the muscular system. o These movements are reflexive and occur as part of the early motor development process.  Internal Organ Development: Week 8 Key Events: o Kidneys Begin Functioning:  Rudimentary kidneys (metanephros) start producing urine.  Urine contributes to the amniotic fluid composition. o Intestinal Loop Retraction:  The midgut herniation retracts into the abdominal cavity.  Positions the intestines in their normal anatomical location.  Kidneys Begin Functioning at Week 8 1. Rudimentary Kidneys (Metanephros): o By week 8, the metanephros (the permanent kidneys) begin to function, marking the first stages of renal development. o Metanephros is the final stage in kidney development, and it begins producing urine. 2. Urine Production: o The kidneys start producing urine, which is a significant event in renal maturation. o The urine produced by the kidneys is an important component of the amniotic fluid.  Kidneys Begin Functioning at Week 8 1. Contribution to Amniotic Fluid: o The urine produced contributes to the composition of amniotic fluid, providing a stable environment for growth and development. Significance: The initiation of kidney function is vital for the preparation of the embryo’s excretory system, which will continue to develop through the fetal stage. Amniotic fluid composition plays a crucial role in supporting fetal development and protecting the embryo.  Intestinal Loop Retraction in Week 8 1. Midgut Herniation: o Prior to week 8, the midgut undergoes hernitation into the umbilical cord due to the limited space in the abdominal cavity. o This herniated loop of intestines allows for rapid gut growth during earlier stages. 2. Retraction of the Midgut: o By week 8, the midgut herniation retracts back into the abdominal cavity. o This retraction allows the intestines to move into their normal anatomical positions within the abdominal cavity  Intestinal Loop Retraction in Week 8 3. Normal Anatomical Positioning of the Intestines: o The small and large intestines are now positioned properly within the abdomen, preparing them for further differentiation and functional maturation in later fetal development. Significance: Retraction of the midgut is crucial for the proper alignment of the intestines in preparation for digestive function after birth. The retraction marks the completion of one of the most critical stages of gastrointestinal development.