Embryology Weeks 1 and 2

Question 1

_______ is a specialized process of cell division that occurs in the germ line.

Answer 1

Meiosis

Question 2

In mitosis (normal cell division), a diploid, ____ cell replicates its DNA (becoming diploid, ___) and undergoes a single division to yield two diploid, 2N daughter cells.

Answer 2

2N

4N

Question 3

In meiosis, a diploid germ cell replicates its DNA (becoming _______) and undergoes two successive, qualitatively different nuclear and cell divisions to yield _________ offspring.

Answer 3

diploid, 4N

four haploid, 1N

Question 4

Day 0: Fertilization

Once a haploid sperm fuses with the haploid egg, the egg is then called a ______.

Answer 4

zygote (diploid)

Question 5

Day 0: Fertilization
Zygote: The zygote contains a diploid complement of chromosomes and a 2N quantity of DNA. After penetration of the oocyte by the sperm, the nuclei of the oocyte and sperm swell within the zygote and are called the female and male ______, respectively. Their nuclear membranes quickly disappear as both maternal and paternal chromosomes are replicated in preparation for the first cleavage.

Answer 5

pronuclei

Question 6

Day 1 (24 Hours): Cleavage
The zygote initiates a rapid series of \_\_\_\_\_\_\_ cell division called cleavage. The cells increase in number but the zygote does not increase in \_\_\_\_. These daughter cells are called \_\_\_\_\_\_\_\_\_.

Answer 6

mitotic

size

blastomeres

Question 7

Day 3 (72 Hours): 8 Cells
Once the cells get to the 8 cell phase, they begin to undergo \_\_\_\_\_\_\_. This means they become adhered to one another by tight junctions.

Answer 7

compaction

Question 8

Day 4 (96 Hours) Morula:
By three days, the embryo consists of\_\_\_\_\_\_, and by 4 days, it consists of \_\_\_\_\_\_\_\_. The embryo at this stage is called a morula (from the Latin word morum, mulberry).

Answer 8

6 to 12 cells

16 to 32 cells

Question 9

Day 4 (96 Hours) Morula:
Compaction: With compaction, some \_\_\_\_\_\_ segregate to the center of the morula and others to the outside.

Answer 9

blastomeres

Question 10

Day 4 (96 Hours) Morula:
The centrally placed blastomeres are now called the inner cell mass or \_\_\_\_\_\_\_.

Answer 10

embryoblast

Question 11

Day 4 (96 Hours) Morula:
Whereas the blastomeres at the periphery constitute the \_\_\_\_\_\_\_. The inner cell mass gives rise to the embryo proper. The trophoblast is the primary source of the fetal component of the placenta.

Answer 11

trophoblast

Question 12

Day 4 (96 Hours) Morula:
Formation of the Blastocyst
By four days of development, the morula, consisting of about 30 cells, begins to absorb fluid. As the hydrostatic pressure of the fluid increases, a large cavity called the \_\_\_\_\_\_\_\_ forms within the morula.

Answer 12

blastocyst cavity (blastocoel)

Question 13

Day 5 (96 Hours)
Blastocyst
The embryoblast cells (inner cell mass) then form a compact mass at one side of this cavity, and the trophoblast organizes into a thin, single-layered epithelium. The embryo is now called a \_\_\_\_\_\_\_.

Answer 13

blastocyst

Question 14

Day 5 (96 Hours)
Blastocyst
The side of the blastocyst containing the inner cell mass is called the _______ of the blastocyst, and the opposite side is called the _________.

Answer 14

embryonic pole

abembryonic pole

Question 15

Implantation

The morula reaches the uterus between ______ days of development.

Answer 15

three and four days

Question 16

Implantation

Very soon after arriving in the uterus, the blastocyst becomes tightly adherent to the _________.

Answer 16

uterine lining

Question 17

Implantation

Adjacent cells of the endometrial stroma (of uterus) respond to its presence and to the progesterone secreted by the _____ by differentiating into metabolically active secretory cells called_____. This response is called the ______.

Answer 17

corpus luteum

decidual cells

decidual reaction

Question 18

Implantation

The corpus luteum will help the blastocyst ______. It produces hormones like progesterone. The pouch will wither away when there is no fertilization.

Answer 18

implant

Question 19

Formation of the Bilaminar Blastoderm (Bilaminar Embryonic Disc)

Even before implantation occurs, cells of the __________ begin to differentiate into two epithelial layers.

Answer 19

embryoblast

Question 20

Formation of the Bilaminar Blastoderm (Bilaminar Embryonic Disc)
By day eight, the embryoblast consists of a distinct external (or upper) layer of columnar cells, called the _____, and an internal (or lower) layer of cuboidal cells, called the ______, or primitive endoderm.

The epiblast is the outer layer and the hypoblast is the inner layer.

Answer 20

epiblast

hypoblast

Question 21

What is another term for hypoblast?

Answer 21

primitive endoderm

Question 22

Bilaminar Embryoblast:

The resulting two-layered embryoblast is called the _________, or ________.

Answer 22

bilaminar embryonic disc, or bilaminar blastoderm

Question 23

Bilaminar Embryoblast:

With formation of the bilaminar embryonic disc, the _____ axis of the embryo is defined (i.e. epiblast is ____, hypoblast is _______).

Blue color is epiblast and yellow color is hypoblast.

Answer 23

primitive dorsal-ventral

i.e. epiblast is dorsal, hypoblast is ventral

Question 24

Amniotic Cavity:
The first new cavity to form during the second week (the amniotic cavity) appears on day____ as fluid begins to collect between cells of the epiblast and the overlying trophoblast.

Answer 24

day 8

Question 25

Amniotic Cavity:
By day ___, the embryo is completely implanted in the uterine endometrium. The amniotic cavity is expanding, and cells from the hypoblast have begun to migrate to form ________.

Answer 25

day 9

Heuser’s membrane

Question 26

Amniotic Cavity:
The layer of epiblast cells expands toward the _______ and differentiates into a thin membrane separating the new cavity from the _______. This membrane is the lining of the amnion.

Answer 26

embryonic pole

cytotrophoblast

Question 27

What is the membrane that lines the amnion?

Answer 27

cytotrophoblast

Question 28

Primary Yolk Sac:

The migrating ________ (Heuser’s membrane) merge to form a second cavity called the primary yolk sac.

Answer 28

hypoblast cells

Question 29

Primary Yolk Sac:
Extraembryonic mesoderm is formed in the middle of the second week. On days 10-11, the space between Heuser’s membrane and the cytotrophoblast becomes filled with loosely associated _______ It forms an infrastructure that allows the mom to support the embryo.

Answer 29

extraembryonic mesodermal cells.

Question 30

Formation of the Chorion

On days 11 and 12, the ________ expands between the amnion and the cytotrophoblast.

Answer 30

extraembryonic mesoderm

Question 31

Formation of the Chorion

By days 12 and 13, the extraembryonic mesoderm splits into two layers: one coating the outside of ________, and the other lining the inside of the _________. The space between the two layers is the chorionic.

Answer 31

Heuser’s membrane

cytotrophoblast