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WVSOM - Embryology-3
Trilaminar Stage
| Question | Answer |
|---|---|
| Hensen's node | Serves as organizer, secreting growth factors that induce tissue type differentiation and embryogenic patterning |
| Primitive pit | Pore in center of Hensen's node |
| Primitive streak (day 15 - week 4) | Faint white trace at caudal end of embryonic disc, formed by movement of cells at onset of mesoderm formation and providing 1st evidence of embryonic axis |
| Primitive ridges | Ridge that bounds the primitive groove in early stages of embryonic development |
| Primitive groove | Lengthwise median furrow in primitive streak of the embryo |
| Situs inversus | Developmental defect when viscera in abnormal left-right orientations; probably results from defective GF genes secreted from embryonic organizer (i.e. Hensen's node) |
| Partial situs inversus | Some organs are in opposite orientations; can be fatal |
| Total situs inversus | All organs are transposed; little to no clinical consequences |
| Primitive streak expansion, regression | Begins at caudal end of germ disc; elongates towards cranial end (until day 18); extends across ~2/3 length of germ disc; streak regresses, completely disappearing by 4th week |
| Grastrulation | First movement of embryonic cell populations, serving to compartmentalize embryo into 3 germ layers - ectoderm, endoderm, mesoderm |
| How is gastrulation accomplished? | With the primitive streak |
| What cells invaginate to form the primitive groove along the caudal medial line? | Epiblastic cells |
| Endoderm (intraembryonic endoderm) | Cells that have wedged into hypoblast (from the invagination and separation of cells form the epiblast); eventually, endoderm completely replaces hypoblast by pushing it out of germ disc and into yolk sac |
| Mesoderm | Invaginating cells (from the epiblast) that do not wedge into the hypoblast |
| Ectoderm | Epiblastic cells that do not pass through the primitive streak |
| From what are all three germ layers derived? | Epiblast |
| What gives the germ disc a pear shape perimeter? | The cells that migrate into the space between the epiblast and hypoblast spread in lateral and cranial directions |
| What plate do the cells that move in the cranial direction form? | Cardiogenic (heart forming) plate |
| What else can the cranial migrating cells generate? | The notochord |
| Which cells reach the cranial extremities first? | The first cells to pass through the streak; caudual mesoderm and endoderm = younger, differentiation slightly delayed; embryos tend to develop from cranial end towards caudal |
| Sacrococcygeal teratomas | Develop when primitive streak fails to regress; most common newborn tumors; derived from pluripotent cells; consist of misture of different cell types; tumors can usually be surgically removed |
| Caudal dysgenesis | Underdevelopment of caudal features due to insufficient mesoderm in caudal-most region of embryo |
| Symptoms/abnormalities of caudal dysgenesis | Hypoplasia, fusion of lower limbs, vertebral abnormalities, renal agenesis, imperforate anus, anomalies of genital organs |
| Sirenomelia | Extreme condition of caudal dysgenesis; legs fused to produce mermaid-like fetus; common teratogenic factor = maternal diabetes |
| Notochord | Hollow, cartilaginous rod that extends through midline; phylum named after it (Chordata) |
| Notochord's role in vertebrates | Organizer |
| What is the notochord derived from? | Hensen's node |
| What differentiation does the notochord control? | Neural tube, somites |
| What happens to the notochord after it has served its purpose? | Majority of it degenerates by apoptosis; some notochordal derived cells persist in adult as nuclei pulposus of intervertebral discs |
| Formation of notochord | Epitblast cells that invaginated thru primitive pit remain contiguous, extend in cranial direction to form hollow tube (notochordal process), lumen of tube = notochordal canal |
| What region prevents the continuation of the notochordal process? | Prechordal plate (progenitor of mouth); regions contains hypoblast & epiblast celss that block further progression of process; at this point, primitive streak regression commences; occurs around day 18 |
| What does the notochord fuse with? | Underlining endoderm |
| What results from the fusion of the notochord with the underlining endoderm? | Opening to yolk sac; transitory channel (via primitive pit) connects amniotic cavity & yolk sac |
| What is the connection between the amniotic cavity & yolk sac called? | Neurenteric canal |
| The tubular process that has been flattened is called ... | Notochordal plate |
| The notochordal plate folds (reforming a tube), starting at the cranial end & progressing towards the caudal end | Notochord |
| What happens when the notochord detaches from the yolk sac endoderm? | The neurenteric canal closes |
Created by:
JaneO
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