12. Yolk sac, Allantois and twinning Flashcards
(18 cards)
When does yolk sac development begin in humans?
Around day 9 in humans
This marks the early implantation phase.
What are the layers formed during yolk sac development?
Parietal and visceral endoderm layers
These layers are formed as endodermal cells migrate out over cytotrophoblast cells.
What is the primary function of the yolk sac in mammals?
Nutrient transfer/storage
Important from day 15 to 3 months in humans.
What significant structures develop in the yolk sac?
First blood vessels, endothelial cells, hematopoietic stem cells
Blood islands also form here.
What are the differences in yolk sac formation between humans and mice?
Humans form a small pouch; mice grow down into yolk sac and rotate
In humans, yolk sac remains separate; in mice, it surrounds the embryo.
What type of placenta do placental mammals have?
Chorio-allantoic placenta
This involves the fusion of allantois with chorion.
When does allantois development begin in humans?
Day 16-17 in humans
Allantois develops as an outpouching from the hindgut.
What does the allantois become as development proceeds?
Extension from bladder, forming urachus
The urachus connects the bladder to the umbilicus.
What are the components of the umbilical cord?
Connection to yolk sac, allantois, umbilical blood vessels
Includes two arteries, one vein, and sometimes a loop of intestine.
What is an umbilical hernia?
Loop of intestine remains in umbilical cord
Usually resolves with growth but may require surgery if large.
What is Meckel’s diverticulum?
Remnant of yolk sac attachment on intestine
Present in approximately 2% of the population, usually asymptomatic.
What are the extra-embryonic membranes present in fish?
Only yolk sac
Fish do not have other extra-embryonic membranes.
What is the main function of the chorion in reptiles and birds?
Primarily for gas exchange
In these species, yolk sac and allantois are larger and more important.
What are the types of monozygotic twin formation based on timing?
Early, middle, late division
Each type has different characteristics regarding placentas and amnions.
What occurs during early division of monozygotic twins?
Complete separation of embryos
Results in separate placentas, amnions, and chorions.
What is tetragametic chimerism?
Fusion of two separate embryos
Rare and can result in conditions like true hermaphroditism.
What is microchimerism?
Transfer of cells between twins with fused placentas
Occurs in a notable percentage of dizygotic twins.
What is freemartinism?
Infertility in female twin due to hormones from male twin
Affects 0.5% of calves born and is identifiable by farmers.
