Embryology 2

Question 1

Week 4 embryo

Answer 1

In week 4, the embryo is found within the amniotic cavity, which is enclosed within the amnion. This is found within the chorionic cavity, which is surrounded by a layer of extraembryonic mesoderm called the chorion. The yolk sac is attached to the midgut via the vitelline duct but contains only fluid. An allantois grows as a rudimentary diverticulum of the gut tube and fuses with the chorion.

Question 2

Endometrium

Answer 2

During pregnancy, the endometrium is called the decidua as it is shed at birth. It has 3 parts:

• the decidua basalis forms the maternal component of the placenta
• the decidua capsularis encloses the implanted embryo
• the decidua parietalis lines the walls of the uterus

Question 3

Chorion

Answer 3

The chorion is a layer of extraembryonic mesoderm, which encloses the chorionic cavity. It differentiates into two parts:

• the chorion frondosum surrounds the attachment of the umbilical chord, and becomes bushy; it forms the fetal component of the placenta
• the chorion laeve is opposite the embryo, and becomes smooth.

Question 4

Fusion of membranes

Answer 4

In weeks 4-8, the amniotic cavity expands and obliterates the chorionic cavity. The amnion fuses with the chorion laeve. The decidua capsularis stretches and disappears. By week 12, the amniochorionic membrane fuses with the decidua parietalis.

Question 5

Functions of extraembryonic membranes

Answer 5

• to store or remove waste products
• to transport nutrients
• to exchange gases (supply oxygen, remove CO2)
• to create an aquatic environment for the developing embryo

Question 6

Formation of the placenta

Answer 6

From week 4, finger-like projections reach out from the chorionic plate to the cytotrophoblast shell. They consist of central stem villi with free villi branching off. The lacunae in the syncytiotrophoblast grow larger, forming intervillous spaces filled with maternal blood.

Question 7

Types of chorionic villi

Answer 7

Primary chorionic villi are protrusions of the cytotrophoblast.
Secondary chorionic villi contain an extraembryonic mesoderm core.
Tertiary stem villi contain capillaries within the mesoderm core.

Question 8

Development of chorionic villi

Answer 8

From week 8, the cytotrophoblast layer is lost from many villi, leaving only the endothelium of the villous capillary and a thin layer of syncytium between the fetal and maternal blood.

Question 9

Functions of the placenta

Answer 9

• to transfer oxygen and nutrients
• to remove carbon dioxide and metabolic waste
• to produce hCG then progesterone to maintain the uterine lining
• to produce oestriol to stimulate the growth of the uterus and breasts

Question 10

Yolk sac

Answer 10

In egg-laying vertebrates, the ovum has a large yolk full of yolk platelets, to supply the developing embyro with the nutrients it needs.

In placental mammals, the ovum has no yolk, as the mother will supply the developing embryo with nutrients. A yolk sac forms, but it is filled with fluid, and is connected to the midgut via the vitelline duct. These are obliterated by birth.

Question 11

Mesothelium formation

Answer 11

The lateral plate mesoderm forms serous membranes lining the intraembryonic cavity. These are the splanchnic (visceral) and somatic (parietal) layers. Part of the intraembryonic cavity becomes the peritoneal cavity, so this membrane becomes the peritoneum.

Question 12

Mesentery formation

Answer 12

The mesoderm between the gut tube and the dorsal body wall forms an attachment called the dorsal mesentery. Blood vessels branch from the aorta and travel via the mesentery to supply the gut tube.

Question 13

Branches of the aorta

Answer 13

The main vessels supplying the gut tube are the:

• coeliac artery; supplies the foregut
• superior mesenteric artery; supplies the midgut
• inferior mesenteric artery; supplies the hindgut

Question 14

Division of the intraembryonic cavity

Answer 14

The intraembryonic cavity is divided into the thoracic and the abdominal cavity by a shelf of mesoderm. The septum transversum extends from the ventral body wall to meet the dorsal pleuroperitoneal folds, forming the diaphragm.

Congenital diaphragmatic hernia arises from a defect in the division of the intraembryonic cavity.

Question 15

The gut tube

Answer 15

The primitive gut tube is composed of 3 sections; the foregut, the midgut, and the hindgut. It is made of endoderm and is surrounded by mesoderm, except at the buccopharyngeal and cloacal membranes, which become the opening of the future mouth and anus.

Question 16

Division of the cloaca

Answer 16

At around week 5, a shelf of mesoderm called the urorectal septum grows towards the cloacal membrane, dividing the cloaca into two sections; the urogenital sinus and the anorectal canal.

Question 17

Midgut elongation and twisting

Answer 17

In week 5, the midgut elongates to form the primary intestinal loop. During weeks 6-9, midgut elongation and growth of the liver forces the loop to herniate into the umbilicus, where it rotates 90° anticlockwise. During weeks 10-11, the midgut rotates an additional 180° as it retracts back into the abdomen.

Question 18

Development of the stomach

Answer 18

In week 4, the stomach appears as a fusiform dilatation of the foregut. The dorsal wall grows faster than the ventral wall, creating a curve like a banana. The stomach rotates 90° to the right, creating a cul de sac called the lesser sac. The dorsal mesogastrium enlarges to form the greater omentum, while the ventral mesogastrium forms the lesser omentum.

Question 19

Development of the spleen

Answer 19

The spleen is not part of the digestive system as it is not derived from the endoderm tube. It differentiates from the mesoderm of the dorsal mesogastrium, and is connected to the stomach via the gastrosplenic ligament.

Question 20

Development of the liver and gallbladder

Answer 20

The liver develops inside the ventral mesogastrium, dividing it into the falciform ligament and the lesser omentum. The gallbladder forms from the cystic diverticulum of the liver bud.

Question 21

Development of the pancreas

Answer 21

The pancreas is formed by the fusion of the dorsal pancreatic bud and the ventral pancreatic bud, which migrates dorsally around the duodenum. The ventral pancreatic duct becomes the main pancreatic duct.

Question 22

Position of the abdominal organs

Answer 22

Initially, the entire gut tube is attached to the body wall via the dorsal mesentery. As various rotations occur, the duodenum, pancreas, ascending colon and descending colon are pressed against the dorsal body wall and merge with it, losing their mesenteries. They all become secondarily retroperitoneal.