Anatomy Lecture 3_ Embryology II.pdf

Week 3 Development​ 1. Describe the process of gastrulation, focusing on the location of each germ layer, the structures they form, and the of structures​ Overview of Gastrulation - Formation of the trilaminar structure that forms the embryo - Cells of epiblast migrate...

Week 3 Development​ 1. Describe the process of gastrulation, focusing on the location of each germ layer, the structures they form, and the of structures​ Overview of Gastrulation - Formation of the trilaminar structure that forms the embryo - Cells of epiblast migrate to form primary germ layers - Ectoderm - Mesoderm - Endoderm Signifies beginning of embryonic period - Orientation - independent of mothers orientation - Dorsal= back of embryo - Amnion side - Ventral- front of embryo - Umbilical vesicle side - Cranial= embryo's head end - Caudal= embryo's feet end Germ Layers and Their Locations 1. Ectoderm (Outer Layer) ○ Location: External epithelial lining ○ Structures Formed: Central Nervous System: Brain, spinal cord. Peripheral Nervous System. Epidermis: Skin, hair, nails. Sensory Organs: Eyes, ears, nose. 2. Mesoderm (Middle Layer) ○ Location: Muscle and Connective tissue ○ Structures Formed: Lateral plate- Limbs 1. Skeletal Bones and muscle 2. Body and Organ walls 3. Cardiovascular system.  Intermediate plate 1. urinary system 2. reproductive system Paraxial - Para (either side of body) animal (body axis) 1. Axial skeletal muscle and bone 2. dermis 3. Endoderm (Inner Layer) ○ Location: internal epithelial living ○ Structures Formed: Respiratory epithelium Digestive epithelium Accessory digestive organs Process of Gastrulation 1. Formation of the Primitive Streak: ○ Appears In: epiblast/embryonic ectoderm ○ Forms: cranial to caudal, from center of disc ○ Defines the body axes and is the site where cells start to migrate inward 2. Cell Migration and Formation of Germ Layers: ○ Ingression of Cells: Cells from the epiblast layer migrate forming a raised streak on the dorsal aspect of bilaminar disc. ○ Primitive Groove: canal within the primitive streak ○ Primitive node: formed by proliferation of cells at cranial end that also contains primitive pit ○ Primitive pit: within primitive node where other layers will fill in First Wave: Cells that move inward and displace the hypoblast form the endoderm. 1. Continuous with extraembryonic endoderm of umbilical vescile Second Wave: The next group of cells migrates through the primitive groove to form the mesoderm. 1. Continuous with extraembryonic mesoderm 2. Contains the mesenchyme Remaining Epiblast Cells: These cells do not migrate and remain on the surface (Dorsal Layer of epiblasts), forming the ectoderm. 1. Continuous with amnion Summary of Key Points Ectoderm forms the outer layer and gives rise to the nervous system and skin. Mesoderm forms the middle layer and gives rise to muscles, bones, the circulatory system, and more. Endoderm forms the inner layer and gives rise to the lining of the digestive and respiratory tracts. 2. Describe notochord formation, focusing on the migration of tissue, the order of events, and its role in development​ Notochord Formation: a. The notochord, a rod-like structure that provides signals to the surrounding tissue, forms from the intraembryonic mesoderm. It plays a crucial role in the development of the neural tube and the establishment of the body axis. i. Steps: 1. Mesenchymal cells migrate through primitive pit, extend cranially between ectoderm and endoderm, fusing shortly with endoderm 2. Reforms as solid notochord surrounded by embryonic mesoderm with the Lumen forming the notochordal canal in the middle 3. Vertebral column forms around notochord, Bound at cranial end by prechordal plate and caudal by cloacal membrane 4. Formation of notochord induces development of neural tube 3. Describe neurulation, focusing on the order of events and tissue derivative Neurulation (Specialization of Ectoderm): a. Following gastrulation, the ectoderm thickens and folds to form the neural tube, as well as other CNS and PNS structures  i. Neural Tube: will form central nervous system structures: brain, spinal cord (+retina) ii. Neural Crest Cells: will form peripheral, and some central, nervous system structures: spinal ganglia, meninges (+structures of head and neck) b. Steps: 1. Notochord induces overlying ectoderm to thicken and form neural plate 2. Lateral edges of neural plate rise to form neural folds, with a central depression called the neural groove 3. Neural crest cells arise from border of neural fold 4. Fusion of folds at midline and create neural tube a. Fusion begins at middle, extends both cranially and caudally b. Will give rise to brain vesicles and spinal cord 5. Neural tube detaches from ectoderm, embedded in mesoderm 6. Neural crest cells migrate dorsolaterally to form neural crest between neural tube and overlying ectoderm 7. Neural crest cells detach from neural tube 8. Openings at cranial and caudal ends (neuropores) will close in week 4 4. Explain the development of somites, focusing on their origin and the structures they form​ a. Somites i. Cubodial clumps of mesoderm ii. Development: from intraembryoic mesoderm based on location and developmental trajectory iii. Location: Paraxial mesoderm iv. Types 1. Sclerotome: bone and cartilage 2. Myotome: skeletal muscle 3. Dermatome: dermis  5. Recall where the intraembryonic coelomic spaces form and what they develop into​ a. Intraembryonic Coelom i. Formation of body cavities (paracardial, peritoneal, pleural) 1. Pericardial Cavity: Surrounds the heart. 2. Pleural Cavities: Surround the lungs. 3. Peritoneal Cavity: Surrounds the abdominal organs. ii. Development: 1. Coalescence of Spaces: The coelomic spaces form as small cavities within the lateral plate mesoderm and eventually coalesce to create a single, continuous cavity that becomes the intraembryonic coelom 1. Formation of Intraembryonic Coelomic Spaces Location: The intraembryonic coelomic spaces begin to form within the lateral plate mesoderm during the third week of embryonic development. Lateral Plate Mesoderm: This mesoderm splits into two layers: ○ Somatic (Parietal) Mesoderm: Adjacent to ectoderm. On side of amnion (dorsal side) Will form body wall ○ Splanchnic (Visceral) Mesoderm: Adjacent to endoderm. On side of umbilical vesicle (ventral side) Will form gut wall 6. List the locations and order the main events in vasculogenesis and angiogenesis and relate it to the delivery of nutrients and oxygen to the developing embryo Vasculogenesis (Formation of vessels) - Prior to third week, delivery of oxygen and nutrients occurred via diffusion across the chorion and umbilical vesicle Location: Primarily occurs in the extraembryonic mesoderm ○ 3 Places:  Umbilical vesicle Connecting stalk Chorion Allantois - extraembryonic ○ Outpouching of embryonic endoderm of umbilical vesicle that supports early blood formation and will become umbilical arteries and veins Main Events: 1. Blood Cell Formation: develop from hematopoietic stem cells (HSC) and within extraembryonic blood vessels 2. Mesodermal Cell Differentiation: Mesodermal cells differentiate into angioblasts (precursor endothelial cells). a. Surrounding mesoderm differentiate into muscular and CT layers of blood vessels 3. Blood Island Formation: Angioblasts cluster to form blood islands in the yolk sac. 4. Cavitation and Vessel Formation: blood islands undergo cavitation, leading to the formation of small lumens, which merge to form the primitive vascular network. 5. Endothelial Cell Maturation: cells adjacent to cavities further differentiate to form endothelium 6. Vessel networks: form via fusion of blood island cavities Angiogenesis (Branching of vessels) Location: Occurs in the developing embryo and in various organs and tissues, extending the primitive vascular network. Main Events: 7. Sprouting Angiogenesis: vessels sprout by Endothelial Budding into adjacent nonvascularized areas, eventually fusing with other vessels  8. Trace placental development, focusing on relationship between chorionic villi and maternal blood 1. Cells in cytotrophoblast form primary chorionic villi (in 2nd week), branch to form secondary and tertiary chorionic villi 2. Capillaries in villi fuse to form arteriocapillary network, connect to embryo via the connecting stalk a. Blood begins to flow slowly through network by the end of the third week 3. Cytotrophoblastic shell (extension of cytotrophoblast) attaches chorion to endometrium where maternal blood flows a. Gases and nutrients are exchanged between embryo’s chorionic villi and maternal sinusoids (vessels) 9. Explain cephalocaudal folding, specifically what drives the morphogenic changes and how it relates to gut formation Early 4th week Cephalocaudal folding- cranial and caudal regions fold in on themselves Cephalocaudal Folding Definition: Cephalocaudal folding- cranial and caudal regions fold in on themselves - embryo folds along its longitudinal axis, leading to the formation of the head (cephalic) and tail (caudal) regions. Driving Forces of Cephalocaudal Folding 1. Differential Growth Rates: ○ Cranial end: folds due to faster growth of neural folds. ○ Caudal end: folds due to faster growth of the caudal end of neural tube ○ This growth exceeds that of other embryonic tissues, creating a bend that pushes the head and tail regions closer together Forms & Repositions 1. Forms ○ Hindgut (tail end) i. Endoderm is incorporated into caudal embryo (forms final ⅓ of gut tube)  ○ Future urinary bladder i. incorporation of allantois (diverticulum of umbilical vesicle, yellow in figure) into caudal end of embryo 2. Repositions ○ Primordial heart and oropharyngeal membrane (future mouth) i. to ventral surface ○ Cloacal membrane (future anus) i. to tail region ○ Connecting stalk (umbilical cord) i. to ventral side of embryo 3. Formation of the Heart ○ Great vessels (aorta, vena cava, etc.) and heart form from mesenchymal cells in heart primordium (cardiogenic area) ○ Primordial heart connects to vessels in embryo, connecting stalk, chorion, and umbilical vesicles to form primordial cardiovascular system ○ Heart begins to beat day 21 or 22 ○ First organ system to reach primitive functional state. Why? i. Disc is folded, need exchange of nutrients and gases 10. Explain lateral folding including what drives the morphogenic changes and the role it plays in gut formation Early 4th week Lateral Folding Definition: Lateral (transverse) folding- left and right lateral edges of the embryonic disc move toward the midline and fuse, transforming the initially flat embryo into a more cylindrical, tube-like structure. Driving Forces of Lateral Folding 1. Differential Growth of Embryonic Structures: ○ Somites: The rapid growth of somites (blocks of mesoderm) on either side of the neural tube creates mechanical forces that pull the lateral edges of the embryo downward and inward. 2. Convergence of the Lateral Plate Mesoderm:  The lateral plate mesoderm, which is initially spread out, begins to converge toward the midline. This movement contributes to the formation of the body cavities (coelom) and plays a significant role in the folding process. Role in Gut Formation 1. Foregut (head end) and Midgut (middle) ○ ⅔’s of future digestive system, from incorporated endoderm 2. Formation of Body Cavities: ○ The lateral plate mesoderm splits into two layers: the somatic mesoderm (associated with the body wall) and the splanchnic mesoderm (associated with the gut tube). ○ The space between these layers forms the intraembryonic coelom, which will eventually give rise to the thoracic and abdominal cavities that house the heart, lungs, and gut. 11. Compare the roles of endoderm and ectoderm during folding Comparative Summary Endoderm: ○ Primarily forms internal structures, especially the gastrointestinal tract and associated organs. ○ Internalized during folding to create the primitive gut tube. ○ endoderm interacts closely with the mesoderm during folding, especially the splanchnic mesoderm, which surrounds the gut tube. ○ Plays a key role in the development of the respiratory and digestive systems. Ectoderm: ○ Remains on the exterior, forming the skin, nervous system, and sensory organs. ○ Involved in the formation of the neural tube and neural crest, crucial for nervous system development. ○ Provides protection and contributes to the development of structures involved in sensation and interaction with the environment. Teratology- Study of causes, mechanisms, and patterns of abnormal development - Certain stages of embryological development are more vulnerable to disruption than others - Teratogen exposure during the organogenic period (growth of organs between weeks 4-8) may cause major birth defects - Most critical periods is when cell differentiation and morphogenesis are at their peak 12. Recall the major germ cell derivatives for endoderm, mesoderm, and ectoderm 1. Endoderm The endoderm primarily gives rise to the internal linings of various systems and some associated organs. Major Derivatives: Epithelial lining of the gastrointestinal tract: Except for the mouth and anus (which are ectodermal in origin). Epithelial lining of the respiratory tract: Including the trachea, bronchi, and lungs. Liver and pancreas: These organs are formed from outgrowths of the primitive gut tube. Thyroid, parathyroid glands, and thymus: Derived from pharyngeal pouches. Epithelial lining of the urinary bladder and urethra. 2. Mesoderm The mesoderm gives rise to a wide range of structures, including the musculoskeletal system, circulatory system, and many internal organs. Major Derivatives: Somites: ○ Dermatome: Gives rise to the dermis of the skin. ○ Myotome: Gives rise to skeletal muscles. ○ Sclerotome: Forms the vertebrae and ribs.  Intermediate Mesoderm: ○ Kidneys and the gonads (ovaries and testes). Lateral Plate Mesoderm: ○ Somatic Layer: Forms the parietal serosa, bones, and connective tissue of the limbs. ○ Splanchnic Layer: Forms the visceral serosa, heart, blood vessels, and smooth muscle and connective tissue of the gut. Cardiovascular system: Including the heart, blood vessels, and blood cells. Lymphatic system. Adrenal cortex. Mesodermal derivatives in the reproductive system: Including the gonads (testes and ovaries) and the ductal systems (e.g., fallopian tubes, uterus, vas deferens). 3. Ectoderm The ectoderm primarily forms the nervous system, skin, and related structures. Major Derivatives: Central Nervous System (CNS): Brain and spinal cord. Peripheral Nervous System (PNS): Including cranial and spinal nerves, ganglia, and parts of the autonomic nervous system. Epidermis: The outer layer of the skin, along with hair, nails, and sebaceous (oil) and sweat glands. Sensory Organs: ○ Eyes: Retina and lens. ○ Ears: Inner ear structures. ○ Nose: Olfactory epithelium. Enamel of teeth. Mammary glands. Pituitary gland (anterior and posterior lobes). Neural crest cells: ○ Contribute to the formation of melanocytes, parts of the craniofacial skeleton, certain heart structures (e.g., the aorticopulmonary septum), and peripheral nerves.