Embryogenesis

Question 1

Embryology begins at:

Answer 1

fertilization

Question 2

Before fertilization, the oocyte is stalled where?

Answer 2

• the beginning of the second meiotic division.

Question 3

Sperm capacitation:

Answer 3

• biochemical changes that occur to the head of the sperm that renders it able to fertilize the oocyte.
• occurs in the female uterus and uterine tube.

Question 4

What reaction allows sperm to penetrate corona radiata and zona pellucida?

Answer 4

• acrosomal reaction
  
  • in vitro it takes about 20 sperms acrosomal reaction to penetrate the zona pellucida and allow one sperm to fertilize.

Question 5

Once a sperm reaches the zona pellucida, what occurs?

Answer 5

• The sperm then releases its acrosomal enzymes, which dissolve some of the zona to allow the sperm to reach the oocyte.

Question 6

Sperm fusion with the oocyte plasma membrane causes:

Answer 6

• an influx of calcium, which causes the cortical reaction.

Question 7

What is the cortical reaction?

Answer 7

• Enzymes from the cortical granules render the zona pellucida impermeable to other sperm, preventing polyspermia.
  
  • polyspermia = multiple fertilization
  • caused by the influx of calcium following sperm penetration of the oocyte plasma membrane.

Question 8

Following the cortical reaction, what occurs?

Answer 8

1. Sperm extrudes its contents into the oocyte.
   
   • ONLY nucleus, centrosome, & microtubules.
   • Sperm have no organelles, mitochondria, or RNA.
2. Oocyte completes the 2nd meiotic division.

Question 9

All the events taking place in the zygote for the first 2-3 weeks following fertilization are controlled by:

Answer 9

• mRNA already in the oocyte at the time of fertilization (maternal mRNA).

Question 10

Following the oocyte completing the 2nd meiotic division, what occurs?

Answer 10

• sperm and oocyte nuclei fuse to form zygote
  
  • restoration of dipoid (2n) cell

Question 11

What event is the beginning of embryonic development?

Answer 11

• zygote formation when the sperm and oocyte nuclei fuse

Question 12

What occurs immediately following zygote formation?

Answer 12

• Sex of the embryo is determined.
• 1st mitotic division begins.

Question 13

Sequence of events during the cortical reaction:

Answer 13

1. sperm fuse to oocyte plasma membrane
2. local calcium concentration increase
3. calcium tsunami
4. cortical granule fusion to plasma membrane, preventing polyspermia

Question 14

Where does fertilization normally take place?

Answer 14

• the ampula of the uterine tube

Question 15

What is the zone pellucida?

Answer 15

• a glycoprotein membrane surrounding the plasma membrane of the oocyte

Question 16

The mitotic divisions occurring the first 4 days following fertilization are:

Answer 16

• symmetric, synchronous, and contained within the zona pellucida.
  
  • day 4 = morula (32 cells)

Question 17

What occurs on post-fertilization day 5?

Answer 17

• blastocyst forms (hollow ball)
• blastocyst, releasing enzymes, hatches from the zona pellucida.

Question 18

Blastocyst structure:

Answer 18

• outer cell mass = trophoblast
• inner cell mass = embryoblast
• hollow cavity

Question 19

Trophoblast:

Answer 19

• the outer cell mass of the blastocyst that will go on to form the placenta.

Question 20

Embryoblast:

Answer 20

• the inner cell mass of the blastocyst that contains the “embryonic stem cells” that will go on to form the embryo.

Question 21

On post-fertilization day 6, what occurs?

Answer 21

• implantation of the blastocyst on the body of the uterine wall

Question 22

How does implantation occur?

Answer 22

• the cells of the trophoblast invade the lining of the body of the uterine wall

Question 23

While invading the lining of the uterine wall during implantation, what else are the trophoblast cells doing?

Answer 23

1. fuse together and differentiate into a cytotrophoblast and a syncytiotrophoblast.
   
   • allows for regional specialization of protein production.
2. syncytiotrophoblast begins producing human chorionic gonadotropin (hCG).

Question 24

Implantation any place other than the body of the uterus is an:

Answer 24

• ectopic pregnancy

Question 25

What is the most common site of ectopic pregnancy?

Answer 25

* ampulla of the uterine tube

Question 26

What is the most common site of ectopic pregnancy in the peritoneal cavity?

Answer 26

* the rectouterine pouch

Question 27

When is human chorionic gonadotropin (hCG) detectable in the blood following pregnancy?

Answer 27

* implantation * when the trophoblast implants into the body of the uterine wall, differentiates into a syncytiotrophoblast, and begins producing hCG.

Question 28

What are the two roles of the syncytiotrophoblast?

Answer 28

* produces hCG at the time of implantation. * invades the uterine lining and induces vascular changes.

Question 29

Follwing implantation, the embryoblast differentiates into:

Answer 29

* epiblast and hypoblast.

Question 30

The hypoblast of the embryoblast sits on top of:

Answer 30

* the blastocyst cavity, which becomes the yolk sac

Question 31

The epiblast of the embryoblast sits below:

Answer 31

* the aminotic cavity

Question 32

When is dorsal and ventral first defined in the embryogensis?

Answer 32

* at the time of epiblast and hypoblast development during the second week.

Question 33

During what week doe the syncytiotrophoblast begin to form lacunae (hollow spaces)?

Answer 33

* second week * lacunae fill with maternal blood establishing a primitive utero-placental circulation in week 3.

Question 34

When is the utero-placental circulation first noticeable?

Answer 34

* the third week

Question 35

Connecting stalk:

Answer 35

* suspends the embryo, with its amnion and yolk sac, in the chorionic cavity * stalk formed by extraembryonic mesoderm * formed by the end of the third week

Question 36

At the end of the third week, the hypoblast and epiblast cells adhere to each other in only 2 locations. Which two?

Answer 36

buccopharyngeal membrane and cloacal membrane

Question 37

When does zygotic mRNA transcription begin?

Answer 37

* the beginning of the fourth week

Question 38

What is the first indication in the fourth week that zygotic transcription has begun?

Answer 38

* The primitive streak (groove) appears at the caudal end of the dorsal surface of the epiblast.

Question 39

The primitive streak (groove) initiates:

Answer 39

gastrulation

Question 40

Gastrulation:

Answer 40

* turns a bilaminar embryo into a trilaminar embryo

Question 41

Morphologic assymmetry begins at the primitive groove, and is due to what protein?

Answer 41

sonic hedgehog (ssh)

Question 42

Cells in the pimitive node/pit:

Answer 42

* all have a single cilia. * cilia sweeps fluid from the right hand side of the embryo to the left hand side. * fluid contains vesicles containing sonic hedgehog (ssh).

Question 43

The cilia of the cells of the primitive pit cause sonic hedgehog (ssh) to:

Answer 43

* migrate from the right to the left side of the embryo. * ssh stimulates gene expression on the left hand side of the embryo BEFORE the right hand side.

Question 44

ssh stimulates gene expression on what side of the embryo first?

Answer 44

* left-hand side. * due to primitive pit cells' cilia. * sets up the morphological asymmetry of the body.

Question 45

"sidedness" of the embryo is determined at what point in embryogenesis?

Answer 45

* gastrulation * when the primitive pit's cells' cilia stimulates movement of ssh-containing vesicles to the left side of the embryo.

Question 46

Ciliopathy:

Answer 46

* can cause errors in the distribution of ssh during gastrulation * gene expression is in altered locations, morphological changes occur. ## Footnote **SITUS INVERSUS**

Question 47

Delamination of the primitive groove gives rise to: (epithelial to mesenchymal)

Answer 47

* a three layer embryo: * ectoderm = outer region * mesoderm = middle region * endoderm = inner region * the cells of the primitive groove migrate in between the epiblast and the hypoblast. They completely replace the hypoblast.

Question 48

The ectoderm, mesoderm, and endoderm all arise from:

Answer 48

* the epiblast * the hypoblast is completely replaced by primitive groove cells

Question 49

The epiblast cells do not invade:

Answer 49

* the buccopharyngeal or cloacal membranes * because this is the only place where epiblast and hypoblast cells tightly adhere to one another.

Question 50

How is the notochord established?

Answer 50

* epiblast cells of the primitive node migrate to the buccopharyngeal membrane on the midline

Question 51

The notochord induces development of:

Answer 51

* the nervous system * after gastrulation

Question 52

What is the only remnant of the notochord in adults?

Answer 52

nucleus pulposus

Question 53

What results from the union between the sperm and the oocyte?

Answer 53

* diploid (2n) complement of chromosomes * cortical reaction * formation of second polar body * completion of the oocyte’s 2nd meiotic division

Question 54

Similarities between the buccopharyngeal membrane and the cloacal membrane:

Answer 54

* both have no mesodermal layer * both predict site of the future opening of gut tube * both visible as a shallow depression in the ectoderm * both will rupture