Chapter 4: Development Week One

Question 1

There are three trimesters of prenatal development, what characterizes each trimester?

Answer 1

1. First trimester: all of the major organ systems begin to form
2. Second Trimester: Nearly complete development of organ systems. By the end of this stage fetus has human features
3. Third trimester: rapid fetal growth. in early stage most organ systems become fully functional

Question 2

What is the first week of development characterized by?

Answer 2

• fertilization
• cleavage of the zygote
• blastocyst formation
• implantation

Question 3

What happens during fertilization?

Answer 3

Genetic material from a haploid sperm cell and a haploid secondary oocyte merges into a single diploid nucleus

Question 4

Where does fertilization typically occur?

Answer 4

The fallopian tube

Question 5

What happens after the secondary oocyte is fertilized by sperm?

Answer 5

Zygote undergoes mitotic cell divisions (called *Cleavage)

Question 6

What are the cells produced via cleavage called?

Answer 6

Blastomeres. Get progressively smaller with eat division

Question 7

What is the Morula?

Answer 7

A solid sphere of cells produced from rapid successive cleavages.

• still surrounded by the zona pellucida
• still about the same size as the original zygote but now with many cells

Question 8

What happens on Day 4-5?

Answer 8

-The morula (which is moving through the fallopian (uterine) tube toward the uterine cavity) is bathed in a glycogen-rich secretion called uterine milk that provides nutrition for the developing morula

Question 9

What happens around the 32-cell stage?

Answer 9

Uterine milk fluid enters the morula

-collects between the blastomeres and reorganizes them around a cavity called the “blastocyst cavity”

Question 10

after the blastocyst cavity is formed, what is the mass of cells called?

Answer 10

Blastocyst

-now hundreds of cells, but still the same size as the zygote

Question 11

During the formation of the blastocyst, there are two cell populations that arise, what are they?

Answer 11

1. Embryoblast (inner cell mass)

2. Trophoblast

Question 12

What is the Embryoblast?

Answer 12

Type of cell found in the blastocyst

• also called “inner cell mass”
• located internally and eventually develops into the embryo and some of the extraembryonic membranes

Question 13

What is the trophoblast?

Answer 13

Type of cell in the blastocyst

• outer superficial layer of cells that forms the sphere-like wall of the blastocyst
• ultimately becomes the Outer Chorionic Sac

Question 14

What happens on the fifth day after fertilization?

Answer 14

The blastocyst is released fromt he Zona Pellucida

-blastocyst uses an enzyme to digest and hole in the zona pellucida and then squeezes through

Question 15

Why is the shedding of the zona pellucida necessary?

Answer 15

Allows for implantation to occur

Question 16

What is implantation?

Answer 16

The attachment of a blastocyst to the endometrium at about 6 days after fertilization

Question 17

There is a small period of time before the blastocyst attaches to the uterine wall, in general, how long is this and what is happening?

Answer 17

The blastocyst is free within the uterine cavity for about 2 Days
-during this, it is nourished by secretions from the endometrium

Question 18

Where does the blastocyst usually implant?

Answer 18

-The posterior portion of the fundus
or
-In the body of the uterus
*orienting itself so that the embryoblast is against the endometrium

Question 19

Following implantation, the functional layer of the endometrium takes on a new name, what is it?

Answer 19

The decidua

Question 20

There are three layers to the decidua, what are they?

Answer 20

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

Question 21

What is the decidua basalis?

Answer 21

• portion of the endometrium BENEATH the the implanting embryo
• provides large amounts of glycogen and lipids for the developing embryo and fetus
• later becomes the maternal part of the placenta

Question 22

What is the decidua capsularis?

Answer 22

portion of the endometrium that will cover the embryo after it implants in the endometrium
-disappears by about 27 weeks

Question 23

What is the decidua parietalis?

Answer 23

remaining modified endometrium that lines the noninvolvd areas of the rest of the uterus

Question 24

Summarize the main events of the first week of development.

Answer 24

1. Fertilization occurs in the fallopian (uterine) tube 12-24 hours after ovulation
2. Cleavage: rapid cell division
3. Morula is formed 3-4 days after fertilization
4. Blastocyst is formed around 5 days after fertilization
5. Implantation occurs at about 6 days after fertilization

Question 25

On what basis are the three regions of the decidua named?

Answer 25

names based on their positions relative to the site of the implanted blastocyst