Body Plan 03: Body Folding and Placenta Formation Flashcards
What weeks constitute the true embryonic period?
Weeks 3-8
When is the fetal period?
Weeks 8 - birth
When does the heart start beating?
Start of week 4 (~day 22)
In what two planes does the embryo fold in week 4?
Sagittal (cranial-caudal) and transverse (lateral)
What generally happens during head folding?
The head rotates 180, bringing the precardiac cavity and the cardiac tube caudal to the oropharyngeal membrane, and defining the foregut between the notochord and the cardiac structures which is closed by the oropharyngeal membrane and opened to the yolk sack
What generally happens during tail folding?
The neural tube grows quicker than the notochord, causing the caudal end to fold down. It defines the hindgut that is sealed off by the cloacal membrane and opens into the yolk sac. The allantois of the hindgut penetrates the connecting stalk to become the umbellical cord
Foregut
The foremost section of the gut that is defined with head folding. It is closed off by the oropharyngeal membrane and opens into the yolk sac
Hindgut
The endmost section of the gut that is defined with tail folding. It is closed off by the cloacal membrane and opens into the yolk sac
Allantois
A diverticulum of the hindgut that penetrates the connecting stalk and becomes the umbellical cord
Connecting Stalk
The body of tissue connecting the caudal end of the embryo to the developing placenta
What cavity grows larger during head and tail folding? What cavity gtows smaller?
Amniotic cavity grows larger; yolk sack grows smaller
After folding, what structures are only located rostally to the umbellical cord?
Pancreas anlage, liver anlage
After folding, what structures are only located caudally to the umbellical cord?
Mesenterium dorsal, allantois
What structures does the intraembryonic celom give rise to?
Pericardial sac, pleural sacs, peritoneal sac, and tunica vaginalis testis
Mesothelium
The epithelium that lines the sacs formed from the intraembryonic celom; they’re simple squamous
What structures are derived from the somatopleur? For each, which germ layer do they rise from?
Epidermis (ectoderm), epaxial dermis (paraxial mesoderm via dermomyotome of somite), hypaxial dermis (parietal layer of lateral plate mesoderm), Skeletal muscles (paraxial mesoderm via dermomyotome of somite), parietal layer of the celomic lining (lateral mesoderm)
What structures are derived from the splanchnopleur? For each, which germ layer do they rise from?
Visceral layer of the celomic lining (lateral mesoderm), smooth muscle (lateral mesoderm), endothelium of the gut (endoderm)
Placenta
Hybrid tissue of maternal endometrium and fetal trophoblast that provides for the fetus
Chorion
Fetal part of the placenta consisting of extraembryonic mesoderm, the cytotrophoblast, and the syncytiotrophoblast
Chorionic plate
The development of the chorion as part of the placenta to include villi
Decidua basalis
The basal plate of the placenta, derived from maternal endometrium
Placental septa
Extensions of the decidua basalis among the chorionic villi
Intervillous space
The space where maternal blood vessels dump blood after penetrating the decidua basalis
Placental primary villus
A villus containing only trophoblast cells
