Extraembryonic membranes

What are extraembryonic membranes?

Membranes that support embryo growth and survival but do not become part of the adult organism.

Which animals have the four extraembryonic membranes?

Amniotes: birds, reptiles, and mammals.

Why are fish and amphibians excluded from amnioties?

Their life cycles are tied to water.

What are the four extraembryonic membranes?

Amnion, chorion, yolk sac, and allantois.

What is a somatopleure?

Ectoderm + somatic mesoderm.

What is a splanchnopleure?

Endoderm + splanchnic mesoderm.

Which membranes form from somatopleure?

Amnion + chorion.

Which membranes form from splanchnopleure?

Yolk sac + allantois.

What is the main function of the amnion?

Surrounds the embryo, keeps it moist, and prevents desiccation.

What is the main function of the chorion?

Gas exchange in egg-laying amniotes; forms the embryonic part of the mammalian placenta.

What is the main function of the yolk sac?

Digests yolk and provides nutrients to the embryo.

What is the main function of the allantois?

Waste storage in birds and reptiles; contributes to gas exchange with the chorion.

Which extraembryonic membrane appears first?

The yolk sac.

What is the main role of the yolk sac in birds and reptiles?

Nutrition through digestion and absorption of yolk.

How does the chick yolk sac develop in chicks?

The primary hypoblast expands around the yolk.

What pockets develop as the chick embryo grows?

Sub-cephalic and subcaudal pockets.

What produces the sub-cephalic and subcaudal pockets?

Formation of body folds.

What do cephalic and caudal folds accomplish?

They pinch the embryo away from the yolk, separating foregut and hindgut.

Which part of the gut remains connected to the yolk?

The midgut, through the yolk duct.

Which folds form around the chick embryo?

Lateral folds, a caudal fold, and cephalic folds.

What do yolk sac blood vessels do?

Secrete hydrolytic enzymes to break down yolk.

Why are hydrolytic enzymes released by yolk sac vessels?

To break down yolk for absorption and distribution.

What happens to the chick yolk as development proceeds?

It diminishes as it is consumed and disappears after hatching.

How does the mammalian primary yolk sac form?

Hypoblast endoderm lines the interior of the trophoblast.

What happens to the mammalian yolk sac when there is little or no yolk?

It diminishes and is incorporated into the umbilicus.

What do mammalian yolk sac mesoderm cells produce?

Blood islands that provide early blood vessels.

How do amnion and chorion folds begin in birds and reptiles?

The cephalic region folds backward toward the yolk, forming a head fold in the somatopleure.

What forms the tail fold in birds and reptiles?

A caudal bend.

How do lateral folds contribute to amnion and chorion formation?

They grow around the embryo and meet at the midline.

What forms when the cephalic & caudal folds fuse?

Scar-like thickenings called the (chorio)amniotic raphe.

Which membrane is outermost in birds and reptiles?

The chorion.

What does the bird and reptile chorion interact with?

The shell, for gas exchange.

Which membrane is innermost in birds and reptiles?

The amnion.

What does the amnion directly contact?

The embryo, creating an aqueous environment.

How does the amnion protect the embryo?

It maintains a moist environment and prevents desiccation.

How does the mammalian chorion form?

The trophoblast, including cytotrophoblast, associates with mesoderm after implantation.

How does the mammalian amniotic cavity form?

Cavitation in the bilaminar disc, separating epiblast and hypoblast.

How does the mammalian amniotic cavity expand?

Mesoderm cells spread laterally and fuse, allowing the cavity to expand around the embryo.

What happens to the mammalian amnion as the embryo grows?

It expands until it eventually fuses with the chorion.

How does the bird and reptile allantois originate?

Evagination from the ventral wall of the hindgut.

Which tissue combination forms the allantois?

Splanchnopleure.

Where does the allantois extend?

Into the extraembryonic coelom.

What happens as the allantois expands?

It grows around the embryo.

How does the chorioallantoic membrane form?

Splanchnic mesoderm + somatic mesoderm.

What is the function of the chorioallantoic membrane?

Highly vascularized tissue responsible for gas exchange through the shell.

What is another function of the bird and reptile allantois?

It serves as a waste reservoir.

How does the mammalian allantois originate?

As an outpocketing from the hindgut with a reduced lumen.

What do mammalian allantoic mesodermal cells contribute to?

The umbilicus.

What does the mammalian chorioallantoic membrane form?

A disc-like placenta for gas and nutrient exchange.

What is implantation?

The process by which the mammalian embryo attaches to and becomes embedded in the endometrium.

What supports the embryo nutritionally in eutherian mammals?

The placenta.

What eventually happens to the embryo during implantation?

It becomes completely encased by the endometrium.

What allows the syncytiotrophoblast to burrow into the endometrium?

Release of hydrolytic enzymes.

When do implantation lacunae form?

Around day 12.

What fills the implantation lacunae?

Fluids that create blood pools.

What develops from the syncytiotrophoblast around day 13?

Finger-like projections that extend into the endometrium.

What forms primary chorionic villi?

Cytotrophoblast cells extend into the finger-like projections.

How do primary villi become anchored?

Cytotrophoblast cells penetrate the syncytiotrophoblast and anchor villi to the endometrium.

What forms secondary chorionic villi?

Mesoderm pushes into the cytotrophoblast projections.

What forms tertiary chorionic villi?

A vascular core develops in the villus from mesoderm.

When do tertiary villi form?

Around days 21–24.

Where are chorionic villi most concentrated?

In the decidua basalis.

How does fetal tissue relate to maternal blood at the villi?

Fetal tissue is bathed in maternal blood, but the blood does not directly mix with fetal blood.

What is the decidua?

The endometrium during pregnancy.

What is the decidua parietalis?

The unperturbed uterine lining opposite the implantation site.

What is the decidua basalis?

The uterine lining directly connected to the embryo at the implantation site.

What is the decidua capsularis?

The uterine lining that covers the embryo.

What is the placenta?

A highly developed mammalian organ for fetal nutrition and respiration.

What tissues form the mammalian placenta?

Embryonic chorion and maternal uterine tissue from the decidua basalis.

What is the connecting stalk?

A mesodermal connection between the embryo and cytotrophoblast.

What happens to the extraembryonic coelom by about day 21?

It expands, with the embryo connected to the cytotrophoblast by the connecting stalk.

How does the umbilical cord form?

Amnion growth compresses the allantois, yolk sac, and connecting stalk together.

What happens to the extraembryonic coelom as the amnion expands?

It diminishes as the amnion grows.

What happens to the amnion and chorion as development proceeds?

The expanding amnion eventually fuses with the chorion.

What happens to villi beneath the decidua capsularis?

They lose blood supply and diminish.

What is the chorion laeve?

The smooth chorion with few or no villi.

What is the chorion frondosum?

The highly vascularized chorion with many villi near the implantation site.

Which decidua covers the chorion laeve?

Decidua capsularis.

Which decidua covers the chorion frondosum?

Decidua basalis.

Which two structures form the placenta?

Chorion frondosum + decidua basalis.

What does human chorionic gonadotropin (HCG) do?

Maintains the corpus luteum so it continues progesterone secretion.

What does human chorionic somatomammotropin (HCS) do?

Promotes maternal growth and breast development for milk production.

What do estrogen and progesterone do during pregnancy?

Maintain the uterine lining and prevent further ovulation.

What does human placental growth hormone do?

Maintains elevated maternal blood glucose levels.

How do nutrients reach the embryo through the placenta?

They pass from maternal blood through the placenta and umbilical cord.

What does relaxin do?

Dilates the birth canal and loosens pelvic ligaments; may promote oxytocin production.

What does oxytocin do?

Causes uterine contractions and milk release.

What is parturition?

The process of giving birth.

What is the afterbirth?

The placenta, umbilical cord, and membranes expelled after the fetus is born.

What determines the type of placenta?

Placenta type is species-specific.