Development (Chapter 4 in Principles of Human Anatomy)

Question 1

What happens in the First Week of Prenatal Development?

Answer 1

1. A zygote is formed as the result of fertilization of the egg by sperm
2. Cells are produced by rapid mitotic cell divisions called cleavage
   - the progressively smaller cells produced via cleavage are called blastomeres
3. A series of divisions occur once ever 20 hours with blastomeres decreasing in size with each division
4. THREE days after fertilization, a ball of cells, called the morula, is formed

Question 2

How is the blastocyst formed?

Answer 2

1. Morula enters the uterine tube and after 4-5 days is bathed by uterine milk secreted by the endometrium
2. At the 32-cell stage, this fluid enters the morula to form a fluid-filled blastocyst cavity (blastocoel)
3. The developing mass is now called a blastocyst
4. about 5 days after fertilization, the blastocyst hatches from the zona pellucida

Question 3

The blastocyst has two types of cells, what are they?

Answer 3

1. embryoblast or inner cell mass: which becomes the embryo

2. Trophoblast: will form the extra-embryonic structures - ultimately forms the outer chorionic sac

Question 4

What is secreted by the trophoblast and what does it cause?

Answer 4

Trophoblast secretes Human Chorionic Gonadotropin (hCG)

• sustains the corpus luteum so progesterone will continue to be produced
• progesterone is necessary to sustain the uterine wall

Question 5

When does the blastocyst implant into the endometrium?

Answer 5

aprox Day 6

Question 6

Describe the process of implantation

Answer 6

• Process by which the blastocyst attaches to and embeds within the endometrium
• occurs about 6 days after fertilization
• as the blastocyst implants (usually on the posterior wall of the fundus (or body of the uterus)) it is oriented so that the inner cell mass faces the endometrium

Question 7

After implantation, what is the endometrium called?

Answer 7

The decidua

Question 8

What are three regions of the Decidua (named after positions relative to the site of the implanted blastocyst)?

Answer 8

1. Decidua basalis
2. Decidua capsularis
3. Decidua parietalis

Question 9

What is the decidua basalis?

Answer 9

portion of endometrium located between the embryo and the stratum basalis - becomes the Maternal part of the placenta

Question 10

What is the decidua capsularis?

Answer 10

portion of the endometrium located between the embryo and the uterine cavity

Question 11

What is the decidua parietalis?

Answer 11

remaining modified endometrium that lines the non-involved areas of the rest of the uterus

Question 12

What happens in the 2nd Week of Development?

Answer 12

Development of the:

1. Trophoblast
2. Bilaminar embryonic disc
3. Amnion
4. Yolk sac
5. Sinusoids (lacuna network)
6. Extraembryonic mesoderm and chorion

Question 13

When does Gastrulation begin?

Answer 13

When implantation is completed (3rd week)

Question 14

Describe the development of the trophoblast

Answer 14

• The trophoblast develops into 2 layers
  1. Outer syncytiotrophoblast
  2. Inner cytotrophoblast

Question 15

What is the syncytiotrophoblast and what is its role in development?

Answer 15

Layer of the trophoblast

-Secretes enzymes that enable blastocysts to penetrate into the endometrium

Question 16

Describe the development of the bilaminar embryonic disc

Answer 16

the embryoblast (inner cell mass) develops into a hypoblast and epiblast
-the hypoblast and epiblast together make up the bilaminar embryonic disc