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Embryology II
Gastrulation, neurulation, folding
Question | Answer |
---|---|
What is gastrulaiton? | Process by which three germ layers are established |
What are the 3 germ layers? | Ectoderm, mesoderm, and endoderm |
What is the bilaminar layer? | Hypoblast and epiblast |
What does the bilaminar embryonic disc turn into? | Trilaminar embryonic disc |
What is the embryo referred to as during gastrulation? | Gastrula |
What is occuring at the beginning of the 3rd week? | Formation of primitive streak |
What is the primitive streak | Thickened linear band in the median plane of the dorsal aspect of the embryonic disc |
How is the primitive streak formed? | Results from proliferation and movement of cells of epiblast to the median plane of the embryonic disc |
What happens to the cranial end of the primitive streak? | Proliferates to form a primitive node |
What happens to the primitive streak? | End proliferates to form a primitive node, narrow groove (primitive groove) develops in primitive streak that is continuous with a small depression in the primitive node - primitive pit |
How is the mesoderm formed? | Cells leave deep surface of streak and start to move between epiblast and hypoblast - forming mesenchyme which forms mesoblast which forms the embryonic mesoderm |
What is mesenchyme? | Embryonic CT which forms supporting tissues of the embryo such as CT |
What is mesoblast? | Undifferentiated mesoderm |
What does the mesoblast have the potential to form? | Muscles, vessels, and CT |
What is the mesoderm derived from? | Epiblast |
How is the endoderm formed? | Cells from epiblast, as well as from primitive node and other parts of primitive streak, displace hypoblast, forming embryonic endoderm in roof of umbilical vesicle |
How is the ectoderm formed? | Cells remaining in epiblast form embryonic ectoderm |
What are all 3 germ layers derived from? | Epiblast |
What can mesenchymal cells derived from primitive streak differentiate to? | They are pluripotential cells that can differentiate into diverse types of cells - fibroblast, chondroblast, osteoblast |
What eventually happens to the primitive streak? | Diminishes in relative size and becomes an insignificant structure in the sacrococcygeal region |
When does the primitive streak disappear? | By end of 4th week |
How is the notochordal process formed? | Some mesenchymal cells dive into the primitive pit and migrate cranially to form cord |
How is the notochordal canal formed? | Notochordal process acquires a lumen and becomes the notochordal canal |
What is the role of the notochord? | Gives rigidity, provides signals for development of axial muscoloskeletal structures and CNS, contributes to intervertebral discs |
How does the notochordal process grow? | Cranially between ectoderm and endoderm until it reaches the prechordal plate |
What is the prechordal plate | Small circular area of columnar endodermal cells where ectoderm and endoderm are fused |
what does the prechordal plate give rise to? | Endoderm of oropharyngeal membrane located at future site of oral cavity |
How does the notochordal process elongate? | Invagination of cells from the primitive pit |
How does the primitive pit help form the notochordal canal? | Extends into notochordal process forming notochordal canal |
What shape is the notochordal process after forming the canal? | Cellular tube that extends cranially from the primitive node to the prechordal plate |
What happens to the floor of the notochordal process? | Fuses with underlying embryonic endoderm |
What happens after the layers of the notochordal process have fused | Degenerate, resulting in formation of openings in the floor of the notochordal process - |
What do the openings in the floor of the notochordal process allow? | Allows communication with umbilical vesicle |
Do the openings in the floor of the notochordal process remain? | No, become confluent and floor of notochordal canal disappears |
What happens after the floor of the notochordal canal disappears? | Remains of the notochordal process form a flattened grooved notochordal plate |
How is the notochord formed? | Beginning at the cranial end of embryo, notochordal cells proliferate and notochordal plate infolds to form notochord |
What happens to the notochord after it is formed? | Becomes detached from endoderm of the umbilical vesicle, which again becomes a continuous layer |
What is the function of the notochord in the beginning of embryo? | Primary inductor (signaling center) |
What happens to the notochord later in embryo? | Developing notochord induces overlying embryonic ectoderm to thicken and form neural plate |
What is the primordium of the CNS? | Neural plate |
What is the functio of the prechordal plate? | Will become oropharyngeal membrane, prevents further notochord growth, signaling center for controlling development cranial structures |
How is the cardiogenic mesoderm formed? | Mesenchymal cells from primitive streak that migrate around prechordal plate to form |
What is the premordium of the heart? | Cardiogenic mesoderm |
How is the cloacal membrane formed? | Forms caudally to primitive streak |
What is the future site of the anus? | Cloacal membrane |
What is the alantois | Small, vascularized diverticulum from the caudal wall of umbilical vesicle, extending into connecting stalk |
What is the function of alantois? | Early blood formation and bladder development - blood vessels become umbilical arteries. Small portion persists as urachus that extends from bladder to umbilical region - becomes umbilical ligaments in adults |
What does the ectoderm of the neural plate give rise to? | CNS and retina |
What does the neural plate correspond to at first? | Corresponds in length to underlying notochord (rostral to primitive node) |
What happens to the neural plate as the notochord elongates? | Neural plate broadens and extends cranially as far as the oropharyngeal membrane |
What happens on day 18 to the neural plate? | Invaginates along its central axis to form a longitudinal median neural fold and then neural groove |
What is happening at the end of the 3rd week to the neural plate? | The neural folds move together and fuse, converting the neural plate into the neural tube |
What is the primordium of the CNS? | Neural tube |
What are neural crest cells? | Subset of neuroectoderm cells that lose affinity to epithelium and neighboring cells |
How do neural crest cells migrate? | Dorso-laterally on each side of the tube |
Where do neural crest cells migrate to? | Widely throughout mesenchyme |
What is the neural crest | Flattened mass of neural crest cells |
When is neurulation complete? | 4th week |
How are paraxial mesoderm formed? | Cells derived from primitive node form thick longitudinal column of cells - each column is continuous laterally with intermediate mesoderm |
What is lateral mesoderm continuous with? | Extraembryonic mesoderm covering umbilical vesicle and amnion |
What happens to the paraxial mesoderm? | Differentiates, condenses, and begins to divide into paired cuboidal bodies - somites |
Where are the somites located? | Each side of the developing neural tube |
What do somites give rise to? | Axial skeleton and associated musculature as well as to the adjacent dermis of skin |
How does the primordium of the intraembryonic coelom appear? | Isolated coelmoic spaces in the lateral mesoderm and cardiogenic mesoderm |
How is the intraembryonic coelom formed | Isolated spaces in lateral mesoderm and cardiogenic mesoderm soon coalesce to form a single horseshoe shaped cavity - |
What does the intraembryonic coelom divide? | Lateral mesoderm into two layers |
What are the two layers of the lateral mesoderm? | Somatic (parietal) and splanchnic (visceral) layers |
What is the somatic (parietal) layer? | Lateral mesoderm located beneath ectodermal epithelium and continuous with extraembryonic mesoderm covering amnion |
What is the splanchnic (visceral) layer? | Lateral mesoderm located adjacent to endoderm and continuous with extraemryonic mesoderm covering umbilical vesicle |
What is the somatopleure? | Somatic mesoderm and overlying embryonic ectoderm that forms embryonic body wall (somatopleure) |
What is the splanchnopleure? | Splanchnic mesoderm and underlying embryonic endoderm forming the embryonic gut (splanchnopleure) |
What happens during the 2nd month to the intraembryonic coelom? | Divided into 3 body cavities - pericardial, pleural, peritoneal |
How does the embryo fold during the 3rd week? | Along cranial caudal direction and laterally - resembling more like a tube than flat disk |
As the embryo folds cranially, what begins to appear | Brain vesicles and a few somites |
As the embryo folds laterally, what begins to appear? | Body wall is formed |
What forms the foregut? | Part of endoderm of umbilical vesicle incorporated into embryo |
Where is the foregut located | Between brain and heart |
What separates the foregut from the stomoderum | Oropharyngeal membrane |
What is the stomodeum? | Primordium of the mouth |
Where is the septum transversum and what does it do? | Lies caudal to heart where it develops into central tendon of the diaphragm and separates abdominal cavity from thoracic cavity |
Where is the pericordial coelom after folding? | Ventral to heart and cranial to the septum transversum |
What projects over the cloacal membrane as embryo grows? | Caudal eminence |
What makes up the hindgut? | Part of endodermal germ layer |
What is the connecting stalk attached to? | Ventral surface of embryo and the allantois is partially incoporated into embryo |
What is the primordium of the umbilical cord? | Connecting stalk |
What produces lateral folding? | Growing spinal cord and somites |
How is the midgut formed? | As abdominal walls form, part of endoderm germ layer is incorporated as midgut |
Is there still a wide connection between the midgut and umbilical vesicle after lateral folding? | No, it is reduced to an amphaloenteric duct |
What happens to the region of attachment of the amnion to the central surface of the embryo after lateral folding? | Reduced to a relatively narrow umbilical region |
What does the ectoderm give rise to? | CNS, PNS, sensory epihelia (eyes, ears, nose), epidermis and its appendages, mammary/pituitary/subcutaneous glands, enamel of teeth, melanocytes, muscle, CT, bones of pharyngeal arch origin, suprerenal medulla, meninges |
What is neural crest cells derived from? | Ectoderm |
What does mesoderm give rise to? | CT, cartilage, bone, striated and smooth muscle, heart, blood, lymphatics, KD, ovaries/testes, genital ducts, seroud membranes, spleen, cortex of suprarenal glands |
What does endoderm give rise to? | Epithelial lining of the Gi and respiratory tracts, urinary bladder, most of ureter, tympanic cavity; tympanic antrum; pharyngotympanic tube; parenchymal of tonsils; thyroid and parathyroid glands; thymus; LV; pancrease; |