Fertilisation, Implantation and Gastrulation

Question 1

For sperm to be successful when should it arrive in the female reproductive tract

Answer 1

+/- 1 to 2 days form ovulation ( day 14 of the 28 day cycle )

Question 2

What happens at ovulation

Answer 2

1- Cervical mucus increases in amount, less acidic and becomes less thick to allow easier sperm transport.
2- Fallopian tube come in close contact with ovary and fimbriva sweep it into the tube
3- peristaltic waves of Fallopian tube bring ovum into ampulla of Fallopian tube where it can be fertilized

Question 3

How long can sperm survive in the female reproductive tract

Answer 3

5 days

Question 4

What is Capacitation

Answer 4

the readying of the sperm :
1- Final step of spermatozoan maturation is in the female reproductive tract
2- Binding site on plasma membrane on spermatozoan is exposed
3- spermatozoan is now available and has to binding proteins to fertilize oocyte

Question 5

Cant spermatozoa fertilize oocytes when newly ejaculated

Answer 5

No because membrane molecules are masked by plasma proteins. Glycoprotein coat has to be remodelled before sperm is ready

Question 6

Where is capacitation done

Answer 6

In the uterus and oviduct

Question 7

Explain the stages of fertilization ( binding of sperm to zona pellucid )

Answer 7

1- sperm passes through cumulus layer of oocyte
2- zona pellucida ( glycoprotein shell ) facilitates and maintains sperm binding also induces acrosome reaction
3- capacitated sperm will penetrate cumulus and bingo adhere to ZP , this triggers acrosome reaction
4- ZP opens Ca channels in spermatozoan cell membrane resulting in exocytosis of acrosomal contents

Question 8

How can sperm pass through the cumulus layer

Answer 8

1- Hyaluronidase released from sperm acrosome
2- tubule mucosal enzymes
3- flageller action

Question 9

What is the acrosome

Answer 9

modified lysosomes around the head of the spermatozoan that release acorosome enzymes to facilitate binding and penetrating of ZP

Question 10

What is the acrosome action

Answer 10

Prerequisite action for sperm to fuse with oocyte

Question 11

What happens once the sperm binds membrane of oocyte ( to stop too much sperm form entering )

Answer 11

1- cortical reaction : exocytosis of cortical enzymes into ZP
2- destroys sperm receptors on ZP & hardens the ZP
3- blocks polyspermy

Question 12

What happens when more than one sperm enters the ovum ( dispermy , triploidy )

Answer 12

Fetus nearly always aborts

Question 13

Explain stage 3 & 4 of fertilization

Answer 13

1- Fusion of plasma membranes of oocyte and sperm
2- triggers completion of meiosis II ( making a haploid )
3- head and tail of spermatozoan enters cytoplasm of oocyte

Question 14

Explain stage 5 of fertilization

Answer 14

Formation of male and female pronuclei ( hasn’t fused yet, 2 nuclei )
1- Cytoplasm of oocyte gives rise to organelles of zygote and nutrition that will be needed
2- membranes of pronuclei break down and chromosomes are arranged for mitotic cell division ( Cleavage division )
3- egg is completely fertilized and now called a Zygote

Question 15

Explain the cleavage process

Answer 15

A series of mitotic divisions that result in cells called blastomeres. After 3 divisions blastomeres undergo compaction. Compact blastomeres will divide and form 16 cell morula ( Cells start to act like a tissue and form tight junctions)

Question 16

How is a blastocyst formed

Answer 16

Morula will develop two distinct cell types and the central cavity will be filled with fluid ( blastocoel). Trophoblast cells will be pushed to outside that gives rise to placenta. Embryoblast will be the inner cell mass that gives rise to the embryo. These cells hatch from the ZP. Now cell is called blastocyst

Question 17

What is a blastocyst

Answer 17

Morula form Blastocyst which is an embryo 5-7 days after fertilization.

Question 18

What happens to the ZP after the blastocyst hatches from it

Answer 18

It disintegrates

Question 19

List the steps of In Vitro

Answer 19

1- Hormonal stimulation of egg maturation ( overstimulating ovary to get ovulation )
2- Egg is removed by laparoscopy through vagina
3- Sperm sample is collected and most active sperm is concentrated
4- in vitro fertilization
5- early cleavage in vitro
6- extra embryos frozen
7- preimplantation of up to 3 embryos

Question 20

Explain the implantation process

Answer 20

Trophoblast outer cell mass divides and differentiates into:
1- Syncytiotrophoblast outer
2- cytotrophoblasts inner

Inner cell mass embryoblalst divides :
1- epiblast
2- hypoblast

Question 21

What stage is the uterus in during implantation

Answer 21

Uterus is at the secretory phase where the blastocyst implants in the endometrium along the anterior or posterior wall

Question 22

What happens to the endometrium if fertilization doesn’t occur

Answer 22

Endometrium is shed as menstruation but basal layer stays to regenerate other layers during the next cycle

Question 23

Once the embryoblalst divides which type of cell is in the upper layer and which is the inner layer

Answer 23

Upper : Epiblast

Lower: Hypoblast

Question 24

What is the role of the synctiotrophoblasts

Answer 24

Will secrete hormone of pregnancy , HCG ( human chorionic gonadotrophin. This will signal to corpus luteum to keep producing estrogen and progesterone which will tell endometrial lining of uterus to stay in secretory phase and not go into mensis phase

Question 25

What is tested in pregnancy tests

Answer 25

HCG ( human chorionic gonadotrophin )

Question 26

What is the role of the epiblast

Answer 26

Amniotic cavity forms within it that forms the bilaminar embryonic disc structure. That is the embryo

Question 27

What is Gastrulation

Answer 27

Bilaminar disk will turn into the try laminar disc that forms the three embryonic germ layers

Question 28

At which week does the embryo consist of two flat layers

Answer 28

Week 2 ( epiblast and hypoblast )

Question 29

When does the embryo begins gastrulation

Answer 29

Week 3

Question 30

What are the three germ layers

Answer 30

1- ectoderm
2- mesoderm
3- endoderm

Question 31

Why is gastrulation important

Answer 31

It forms the founding layers of the entire human body.

Question 32

What is the first formation in gastrulation and explain what it is

Answer 32

The Primitive Streak. Made of the groove , node and pit in the epiblast layer. Only arises in caudal region ( lower region )

Question 33

What is the cloacal and oropharyngeal membrane

Answer 33

Cloacal : lower region ( anal )

Oropharyngeal : head

Question 34

What is Hensen’s node

Answer 34

The node in the primitive streak

Question 35

Explain how primitive streak is formed

Answer 35

Epiblasts migrate under epiblast layer through primitive ridge and into the groove as they loose contact with each other. They displace hypoblast cells

Question 36

What are the three germ layer made of

Answer 36

Ectoderm : the epiblast layer
Mesoderm : new middle layer formed by epiblast migration
Endoderm : the hypoblast layer that received some epiblast cells and changed

Question 37

What are the results of Gastrulation

Answer 37

1- primary germ layers are formed

2- layers organized in proper locations and creates body axes

Question 38

Explain body axes formed by germ layers

Answer 38

Axes formed are : 
1- posterior region ( caudal ) 
2- Anterior region is cranial 
3- Dorsal ventral 
4- left right

Question 39

What are the derivatives of the Ectoderm

Answer 39

1- CNS : brain and spinal cord 
2- PNS : eye, ear, nose sensory epithelia 
3- epidermis , harry, nails 
4- pituitary gland 
5- mammary glands

Question 40

What are the derivatives of the Mesoderm

Answer 40

1- connective tissue : bone, blood, cartilage 
2- smooth muscles 
3- heart 
4- blood and lymphatic vessels 
5- kidneys 
6- ovaries and testes 
7- spleen 
8- serous membranes lining body cavities

Question 41

What are the derivatives of the Endoderm

Answer 41

1-Epithelial lining of GIT
2- Respiratory tract 
3- bladder and urethra 
4- liver 
5- pancreas 
6- Thyroid and parathyroid