Human Embryology - BIU Medical Science Program (Week 7) PDF

Human Embryology Lesson No. 05 Week 07 of Embryo Development BIU Medical Science Program Professor Dr. Ejaz Ahmed Khan BIU Medical Science Program  Learning Outcomes After completion of this session s...

Human Embryology Lesson No. 05 Week 07 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: – Explain the Refinement and Maturation of Organ Systems during Week 7 to more advanced physiological functions. – Describe the Formation of the Cartilaginous Skeleton with the initiation of ossification in the upper limb bones, representing the early stages of skeletal maturation. – Elaborate Growth Milestones in Embryonic Length that is an embryo's Crown-Rump Length (CRL) during Week 7, progressing to approximately 15–20 mm. – Analyze the Differentiation of Limbs and Facial Structures.  Key Events of Skeletal Development: Week 7 Overview: o Formation of the cartilaginous skeleton lays the groundwork for future bone development. o Initiation of ossification in specific regions. 1. Ossification Centers Appear: o Early primary ossification centers form in the humerus, ulna, and radius. o These centers mark the beginning of ossification, transitioning from cartilage to bone. o The formation of ossification centers provides the structural framework of the body.  Key Events of Skeletal Development: Week 7 1. Formation of Cartilaginous Skeleton: o Mesenchymal cells condense to form a cartilage model in the developing limbs. o These cartilage models act as precursors to the bones, which will undergo endochondral ossification in later stages. o Chondrocytes (cartilage cells) differentiate from mesenchyme, forming the skeletal structure that will be replaced by bone as ossification progresses Skeletal Development: Week 7  Ossification Centers in Upper Limb Bones 1. Early Primary Ossification Centers: o The process begins in the humerus, ulna, and radius. These bones, which were initially cartilaginous, begin forming primary ossification centers at Week 7. o The humerus, being the first of the upper limb bones to show ossification, marks the transition point in skeletal development.  Ossification Centers in Upper Limb Bones 1. From Cartilage to Bone: o The mesenchymal cells in these bones differentiate into chondrocytes (cartilage-forming cells). As these cells multiply and mature, they form the cartilage scaffold that eventually ossifies into bone. o The formation of primary ossification centers is the first step in the gradual replacement of cartilage with bone, a process known as endochondral ossification. Formation of Cartilaginous Skeleton in Week 7 1. Mesenchymal Condensation: o Mesenchymal cells aggregate and condense into distinct cartilage models. o These models, formed in Week 7, will later undergo endochondral ossification to become the bones of the limbs. o The mesenchymal condensation is the first key step in forming the cartilaginous skeleton.  Formation of Cartilaginous Skeleton in Week 7 2. Differentiation into Chondrocytes: o Mesenchymal cells further differentiate into chondrocytes, which begin to secrete the extracellular matrix of cartilage. o The cartilage model of bones such as the humerus, radius, and ulna begin to take shape. This cartilage will eventually ossify to form long bones.  Formation of Cartilaginous Skeleton in Week 7 3. Precursor to Endochondral Ossification: o The cartilage formed during this stage serves as the template for bone development. o Endochondral ossification will later replace this cartilage with bone tissue in the coming weeks, facilitating growth and providing structure to the limbs. Ossification Centers in Upper Limb Bones  Limb Differentiation: Week 6 o Digital Rays Appear:  Thickened ridges in the hand plates mark the future fingers.  Controlled by apoptosis in the interdigital regions to sculpt the digits. o Lower Limbs: o Lag slightly behind upper limbs but show initial signs of elongation and differentiation  Limb Development: Week 7 Key Events: o Separation of Fingers:  Upper limbs develop digital rays more distinctly.  Programmed cell death (apoptosis) sculpts the separation between fingers. o Toe Rays in Lower Limbs:  Lower limbs lag behind upper limbs but show clear toe ray formation.  Future toes become visible as ridges.  Separation of Fingers in Upper Limbs 1. Digital Rays Development: o During Week 7, upper limbs show more distinct digital rays, which are thickened ridges of tissue in the hand plates and forms fingers 2. Programmed Cell Death (Apoptosis): o As the digital ray's form, programmed cell death (apoptosis) occurs in the interdigital regions (the spaces between the rays). o Apoptosis is a crucial process in the sculpting of the fingers by removing the tissue between the developing digits. This results in the separation of the fingers and the formation of the distinct shape of the hand. 3. Finger Formation: – By the end of Week 7, the limb is recognized as a hand with digits.  Toe Rays in Lower Limbs 1. Formation of Toe Rays: o The lower limbs lag behind the upper limbs and formation of toe rays during Week 7. o Toe rays appear as thickened ridges of tissue on the foot plate. 2. Toe Development: o The toe rays mark the beginning of toe formation, and as Week 7 progresses, future toes become more visible as ridges in the developing foot. o These toe rays will eventually differentiate into the individual toes, with apoptosis playing a role in separating the toes from each other. 3. Lag in Lower Limb Development: o The lower limbs are still developing the initial stages of toe formation. Face and Head Development: Week 7  Overview: o Facial features and sensory structures become more defined.  Key Events: o Prominent Eyes:  Eyes continue to enlarge and become more prominent.  Pigmentation begins to appear in the retina. o Formation of External Ear:  Auricular hillocks form around the first and second pharyngeal arches.  Begin to merge and shape the external ear (pinna). Prominent Eyes in Week 7 1. Enlargement of Eyes: o The eyes continue to enlarge and become more prominent in the developing embryo. At this stage, the eyes are now noticeably larger and more forward-facing on the face. o This enlargement is a critical aspect of craniofacial development. 2. Pigmentation of the Retina: o Pigmentation begins to appear in the retina, the light-sensitive layer at the back of the eye. This is the first sign of the functional retina and is essential for the future ability of the eye to detect light.  Prominent Eyes in Week 7 3. Development of the Eyelids: o The eyelids also start to form but are not yet fully distinct. They will develop further in the coming weeks. 4. Shape of the Eye: – The eye maintains a somewhat spherical shape, but the cornea and lens begin their early stages of differentiation. Formation of the External Ear in Week 7 1. Auricular Hillocks: o The auricular hillocks (small tissue bumps) begin to form around the first and second pharyngeal arches. These hillocks are the precursor to the external ear (pinna). o The hillocks are visible by the 7th week as small, raised areas around the future position of the ear. 2. Merging of Hillocks: o Over the course of the next few weeks, the auricular hillocks will merge and begin to take the shape of the external ear. At Week 7, the ear is still in its rudimentary form but is starting to take shape as it moves toward resembling the final pinna.  Formation of the External Ear in Week 7 3. Development of the Ear Canal: o The external auditory canal (ear canal) is not yet formed at Week 7, but the early developmental process is set into motion by the formation of the auricular hillocks and the positioning of the pharyngeal arches. 4. Functional Aspects of the Ear: o While the external ear is still rudimentary, the development of structures like the ossicles (tiny bones in the middle ear) begins in the subsequent weeks. Formation of the External Ear in Week 7 Rotation of the Midgut Loop 1. Initial 90-Degree Counterclockwise Rotation: o The midgut loop begins to rotate 90 degrees counterclockwise around the axis of the superior mesenteric artery (SMA) during week 6. o This rotation is crucial for the proper positioning of the small and large intestines within the abdominal cavity. 2. Impact on Organ Positioning: o The cranial limb moves to the right side of the body, while the caudal limb moves to the left side.  Brain Development in Week 7 Key Processes: Cerebral Hemisphere Growth: Rapid expansion of the forebrain. Neurogenesis: Increase in neural precursor cells contributing to brain complexity. Ventricular System Formation: Development of brain cavities for cerebrospinal fluid (CSF) circulation. Early Functional Insights: Neural pathways begin rudimentary connections. Formation of cranial nerves critical for early sensory functions.. Key Milestones in Brain Development Major Divisions Established: Prosencephalon (forebrain). Mesencephalon (midbrain). 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: Septation Progression: Formation of the atrial and ventricular septa. Heart Chambers Defined: Development of distinct left and right atria and ventricles. Outflow Tract Division: Further separation of the aorta and pulmonary artery. Circulatory Function: Increased complexity of blood flow patterns. Enhanced pumping efficiency supporting embryonic growth.  Key Features of Heart Development Valvular Development: Formation of atrioventricular (AV) and semilunar valves. Conduction System Establishment: Initiation of rudimentary pacemaker activity for rhythmic contractions. Vascular Network Growth: Expansion of the vascular tree connecting placenta and developing organs.