3rd Week of Development

Question 1

What is the 3rd week of development characterized by?

Answer 1

Gastrulation

Question 2

What is gastrulation?

Answer 2

The process that establishes all 3 germ layers in the embryo (ectoderm, mesoderm, endoderm)

Question 3

Gastrulation begins with the formation of what?

Answer 3

The primitive streak

Question 4

Where does the primitive streak appear?

Answer 4

Caudally in the median plane of the dorsal aspect of the embryonic disc

Question 5

The primitive streak results from what?

Answer 5

The proliferation and movement of cells of the epiblast to the median plane of the embryonic disc

Question 6

How does the primitive streak elongate?

Answer 6

by addition of cells to its caudal end

Question 7

The cranial end of the primitive streak proliferates to form what?

Answer 7

The primitive node

Question 8

What is the primitive pit?

Answer 8

A depression in the center of the primitive node, connecting to the notochord

Question 9

What is the primitive groove?

Answer 9

a shallow groove that appears in the primitive streak

Question 10

What can we identify as soon as the primitive streak appears?

Answer 10

The embryo’s cranicaudal axis,cranial and caudal ends, dorsal and ventral surfaces, and right and left sides

Question 11

How and the primitive groove and primitive pit formed?

Answer 11

From the invagination of epiblastic cells

Question 12

Describe how endoderm, mesoderm, and ectoderm form

Answer 12

Once the epiblast cells have invaginated, some displace the hypoblast, thereby creating the embryonic ENDODERM. Other epiblast cells come to lie between the epiblast cells and newly created endoderm to form the embryonic MESODERM. Cells remaining in the epiblast the form the ECTODERM.

Question 13

What gives rise to the 3 germ layers of the embryo?

Answer 13

The epiblast

Question 14

The _____ gives rise to all tissues and organs in the embryo.

Answer 14

epiblast

Question 15

What is a sacrococcygeal teratoma?

Answer 15

Type of germ cell tumor that contains tissues derived from pluripotent primitive streak cells

Question 16

The embryonic disc is bilaminar at what 3 sites by the end of the 3rd week?

Answer 16

1) Oropharyngeal Membrane
2) In the median plane cranial to the primitive node, where the notochord process is located
3) Cloacal membrane

Question 17

If the embryonic disc is bilaminar what can’t happen and why?

Answer 17

Mesenchymal cells cannot migrate between the endoderm and ectoderm because they are fused

Question 18

What will the paraxial mesoderm become?

Answer 18

The structures right along the midline (vertebral clumn and associated muscles)

Question 19

What will intermediate mesoderm become?

Answer 19

Kidneys and urogenital system

Question 20

What will the lateral plate mesoderm become?

Answer 20

the body wall

Question 21

What will the extrembryonic mesoderm become?

Answer 21

The chorion

Question 22

Differentiation occurs in a _______ direction as gastrulation continues.

Answer 22

Cephalocaudal

Question 23

What is the allantois?

Answer 23

A small diverticulum from the caudal wall of the yolk sac (umbilical vesicle) that extends into the connecting stalk

Question 24

When does the allantois appear?

Answer 24

Approximately day 16

Question 25

What is the importance of the allantois in humans?

Answer 25

It remains very small, but allantoic mesoderm expands beneath the chorion and forms umbilical arteries that will serve the placenta

Question 26

What is the urachus?

Answer 26

The proximal part of the original allantoic diverticulum that extends from the bladder to the umbilical region

Question 27

What represents the urachus in adults?

Answer 27

Median Umbilical Ligament

Question 28

How do secondary villi form?

Answer 28

Mesodermal cells penetrate the core of th eprimary villi and grow toward the decidua

Question 29

What are tertiary villi?

Answer 29

Mesodermal cells in the core of the secondary villus begin to differentiate into blood cells and smell blood vessels and the villus is now called tertiary

Question 30

Why does the heart begin to beat around the same time tertiary capillaries are formed?

Answer 30

Capillaries in the tertiary villi make contact with capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk. These vessels then establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo.