Body Plan 03: Body Folding and Placenta Formation

Question 1

What weeks constitute the true embryonic period?

Answer 1

Weeks 3-8

Question 2

When is the fetal period?

Answer 2

Weeks 8 - birth

Question 3

When does the heart start beating?

Answer 3

Start of week 4 (~day 22)

Question 4

In what two planes does the embryo fold in week 4?

Answer 4

Sagittal (cranial-caudal) and transverse (lateral)

Question 5

What generally happens during head folding?

Answer 5

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

Question 6

What generally happens during tail folding?

Answer 6

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

Question 7

Foregut

Answer 7

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

Question 8

Hindgut

Answer 8

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

Question 9

Allantois

Answer 9

A diverticulum of the hindgut that penetrates the connecting stalk and becomes the umbellical cord

Question 10

Connecting Stalk

Answer 10

The body of tissue connecting the caudal end of the embryo to the developing placenta

Question 11

What cavity grows larger during head and tail folding? What cavity gtows smaller?

Answer 11

Amniotic cavity grows larger; yolk sack grows smaller

Question 12

After folding, what structures are only located rostally to the umbellical cord?

Answer 12

Pancreas anlage, liver anlage

Question 13

After folding, what structures are only located caudally to the umbellical cord?

Answer 13

Mesenterium dorsal, allantois

Question 14

What structures does the intraembryonic celom give rise to?

Answer 14

Pericardial sac, pleural sacs, peritoneal sac, and tunica vaginalis testis

Question 15

Mesothelium

Answer 15

The epithelium that lines the sacs formed from the intraembryonic celom; they’re simple squamous

Question 16

What structures are derived from the somatopleur? For each, which germ layer do they rise from?

Answer 16

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)

Question 17

What structures are derived from the splanchnopleur? For each, which germ layer do they rise from?

Answer 17

Visceral layer of the celomic lining (lateral mesoderm), smooth muscle (lateral mesoderm), endothelium of the gut (endoderm)

Question 18

Placenta

Answer 18

Hybrid tissue of maternal endometrium and fetal trophoblast that provides for the fetus

Question 19

Chorion

Answer 19

Fetal part of the placenta consisting of extraembryonic mesoderm, the cytotrophoblast, and the syncytiotrophoblast

Question 20

Chorionic plate

Answer 20

The development of the chorion as part of the placenta to include villi

Question 21

Decidua basalis

Answer 21

The basal plate of the placenta, derived from maternal endometrium

Question 22

Placental septa

Answer 22

Extensions of the decidua basalis among the chorionic villi

Question 23

Intervillous space

Answer 23

The space where maternal blood vessels dump blood after penetrating the decidua basalis

Question 24

Placental primary villus

Answer 24

A villus containing only trophoblast cells

Question 25

Placental secondary villus

Answer 25

A villus with a mesenchymal core along with trophoblast cells

Question 26

Placental tertiary villus

Answer 26

A secondary villus that has invaded a maternal blood capillary and incorporated it in the mesenchymal core. There are three types: stem, anchoring, branch

Question 27

Stem villus

Answer 27

A type of tertiary villus that is large and directly connected to the chorionic plate

Question 28

Branch villus

Answer 28

A type of tertiary villus that are distal and is the site of substance exchange between maternal blood capilaries and umbellical blood capilaries in the intervillous space

Question 29

Anchoring villus

Answer 29

A type of tertiary villus that are distal and are connected to the decidua basalis

Question 30

How can we distinguish fetal red blood cells from maternal red blood cells?

Answer 30

Fetal red blood cells are nucleated while maternal red blood cells are not

Question 31

Umbellical cord

Answer 31

The joining of the yolk sack and body stalk during fetal folding and expansion of the amnion. The outer surface is covered with amnion

Question 32

Wharton’s Jelly

Answer 32

A mucopolysaccharide ground substance derived from extraembryonic mesoderm that is the main composition of the umbellical cord

Question 33

How many arteries and veins are in the umbellical cord?

Answer 33

One vein, two arteries