Human Embryology Week 7 Quiz

Human Embryology - Week 7

• Learning Outcomes: Students will explain organ refinement and maturation, describe cartilage skeleton formation, elaborate growth milestones (CRL), and analyze limb and facial structure differentiation. CRL at week 7 is approximately 15-20mm.

Skeletal Development: Week 7

• Overview: Cartilage skeleton formation is the groundwork for future bone development. Ossification begins in specific regions.
• Ossification Centers Appear: Early primary centers develop in the humerus, ulna, and radius. This marks the transition from cartilage to bone, providing skeletal structure.
• Formation of Cartilaginous Skeleton: Mesenchymal cells condense to create cartilage models in developing limbs. These cartilage models are precursors to bones that will undergo endochondral ossification. Chondrocytes (cartilage cells) form the skeletal structure, which will later be replaced by bone.

Ossification Centers in Upper Limb Bones

• Early Primary Ossification Centers: The process starts in the humerus, ulna, and radius (initially cartilaginous), creating primary ossification centers by week 7. The humerus is the first to show ossification, marking a key point in skeletal development.
• From Cartilage to Bone: Mesenchymal cells differentiate into chondrocytes (cartilage-forming cells). These cells multiply and mature, forming the cartilage scaffold that later ossifies into bone. The formation of primary ossification centers kickstarts the gradual replacement of cartilage with bone (endochondral ossification).

Formation of Cartilaginous Skeleton in Week 7

• Mesenchymal Condensation: Mesenchymal cells aggregate and condense into distinct cartilage models. These models form in week 7 and will later undergo endochondral ossification to become limb bones. This process is the initial step in forming the cartilage skeleton.
• Differentiation into Chondrocytes: Mesenchymal cells develop into chondrocytes, which produce the cartilage extracellular matrix. Cartilage models (e.g., humerus, radius, ulna) take shape. This cartilage will eventually ossify into long bones.
• Precursor to Endochondral Ossification: The cartilage created during this stage acts as the template for bone development. Endochondral ossification will replace the cartilage with bone in the coming weeks, allowing limb development.

Limb Development: Weeks 6 & 7

• Digital Rays Appearance: Thickened ridges in the hand plates mark the future fingers, and are sculpted by apoptosis in the interdigital regions.
• Lower Limbs: Slightly lag behind the upper limbs, but show initial signs of lengthening and differentiation. Toe rays appear as thickened ridges on the foot plate.
• Separation of Fingers: Upper limbs exhibit digital rays more prominently, with programmed cell death (apoptosis) separating the finger tissue. This process is crucial in shaping the distinct hand structure.
• Toe Rays in Lower Limbs: Lower limbs lag. Future toes appear as ridges, and apoptosis assists in separation between toes, setting them apart to develop.

Face and Head Development: Week 7

• Overview: Facial features and sensory structures become more distinct.
• Prominent Eyes: Eyes enlarge, with pigmentation beginning to appear in the retina.
• Formation of External Ear: Auricular hillocks form around the first and second pharyngeal arches, and combine to shape the external ear.

Prominent Eyes in Week 7

• Enlargement of Eyes: Increasing size of developing eyes, notably more pronounced and forward-placed. This is essential to craniofacial development.
• Pigmentation of the Retina: Appearance of pigmentation in the retina (light-sensitive layer). This signifies the functional retina and is enabling light detection.
• Development of the Eyelids: Eyelids begin to form, but are not yet fully developed. Growth will continue through succeeding weeks.
• Shape of the Eye: Maintaining a roughly spherical shape, with the cornea and lens at early differentiation stages.

Formation of the External Ear in Week 7

• Auricular Hillocks: Small, tissue bumps (auricular hillocks) form around the first and second pharyngeal arches. These precursors to the external ear are visible by week 7.
• Merging of Hillocks: The auricular hillocks combine to gradually shape the external ear (pinna) over several weeks. Rudimentary form is established, but development continues for a more refined external appearance.
• Development of the Ear Canal: Although the ear canal hasn't fully formed by week 7, the process to establish it begins. The positioning of the pharyngeal arches and the presence of the auricular hillocks support development of the ear canal.
• Functional Aspects of the Ear: Basic development, including the middle ear ossicles, begins in week 7.

Rotation of the Midgut Loop

• Initial 90-Degree Counterclockwise Rotation: The midgut loop rotates 90 degrees counter-clockwise around the superior mesenteric artery (SMA) by week 6. Placement of the small and large intestines within the abdominal cavity is determined by this movement.
• Impact on Organ Positioning: The cranial limb moves to the right side, and the caudal limb to the left, establishing the overall position of digestive organs.

Brain Development in Week 7

• Key Processes: Rapid expansion of the forebrain (cerebral hemisphere growth); increase in neural precursor (neurogenesis) contributing to brain complexity; development of brain cavities for cerebrospinal fluid (CSF) circulation (ventricular system formation).
• Early Functional Insights: Rudimentary neural pathways establish connections; formation of cranial nerves crucial for early sensory functions.

Key Milestones in Brain Development

• Major Divisions Established: Prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).
• Neural Tube Refinement: Advanced folding and segmentation for future brain regions.
• Impact on Facial Structure: Brain growth drives cranial and facial differentiation.

Heart Development in Week 7

• Cardiovascular Highlights: Atrial and ventricular septa begin to form (septation progression). Distinctive left and right atria and ventricles develop (Heart chamber definition).
• Outflow Tract Division: Aorta and pulmonary artery further separate (Outflow tract division).
• Circulatory Function: Increased complexity of blood flow; enhanced pumping efficiency.

Key Features of Heart Development

• Valvular Development: Formation of atrioventricular and semilunar valves.
• Conduction System Establishment: Rudimentary pacemaker activity initiates rhythmic contractions.
• Vascular Network Growth: Expansion of the vascular tree connecting the placenta and developing organs.