Early Embryonic Development & Implantation

Question 1

What is the blastocyst?

Answer 1

A hollow ball of cells that surround a cavity filled fluid formed during early embryonic development

Question 2

What is the inner cell mass?

Answer 2

The group of about 30 cells in the blastocyst that develop into the embryo; aka embryoblast

Question 3

What is the inner cell mass composed of?

Answer 3

Stem cells that will differentiate into the different body cells to form the embryo.

Question 4

What stops the endometrium breaking down?

Answer 4

High levels of oestrogen and progesterone stops the endometrium breaking down, and so the menstrual cycle ceases.

Question 5

What produces oestrogen and progesterone during the early stages of pregnancy?

Answer 5

The corpus luteum until developing placenta can take over role after ~ 8-12 weeks.

Question 6

What is the embryonic period?

Answer 6

The first two months of pregnancy after 2nd month, developing individual called a foetus.

Question 7

Summarise a zygote’s cell differentiation

Answer 7

A zygote develops from an unspecialised totipotent cell to a blastocyst, and then to an embryo and its membranes.

Question 8

What makes the primary germ cells?

Answer 8

While the blastocyst is implanting in lining of uterus, during the 3rd week of development, inner cell mass undergoes changes as cells change to multipotent.

Question 9

What is the ectoderm?

Answer 9

Outermost germ layer - form outer layers of the body, such as the epidermis of skin, hair and mammary glands

Question 10

What is the mesoderm?

Answer 10

The middle germ layer that allows the formation of the stomach and intestines - incl. the skeleton, muscles, heart etc.

Question 11

What is the endoderm?

Answer 11

The innermost germ layer - forms lining of the digestive system as well as the lungs and thyroid.

Question 12

What happens after a sperm fertilises an oocyte?

Answer 12

A zygote is formed which then has the potential to develop into a complete embryo
- fertilised oocyte = totipotent stem cell

Question 13

What is the morula?

Answer 13

A solid ball of cells formed as the zygote undergoes cleavage

Question 14

What happens at Day 3 of fertilisation?

Answer 14

Cleavage stage: Zygote divides twice/thrice creating daughters called blastomeres

Zygote is dividing, doesn’t grow bigger in size/ mass yet.

Question 15

What happens at Day 4 of fertilisation?

Answer 15

1. Arrives in the uterus.
2. Morula stage: When this number reaches a cluster of ~ 16
3. Cells inside zygote continue to divide, and number of cells doubles each time.
4. Morula = totipotent - can different into all cell types both embryonic/extra embryonic
5. Floats in uterus for a couple days nourished by nutrients stored in egg cytoplasm + uterine milk = endometrium secretions.

Question 16

What happens at Day 5/6 (Blastocyst Stage) of fertilisation?

Answer 16

1. 100 cells = cells start to arrange themselves around a fluid-filled cavity, forming early blastocyst/blastula.
2. Consists of pluripotent inner cell mass = embryoblast forms all tissues of the human body
3. Outer shell: Trophoblast - nourishes embryo, later becomes chorion/placenta - support/development of foetus
4. Secretes enzyme that dissolves membrane surround embryo so it can hatch and be implanted

Question 17

What happens at Day 8/9 of fertilisation?

Answer 17

1. Implantation: Blastocyst becomes attached to the endometrium
2. At site of contact, superficial cells of trophoblast fuse together to form the syncytiotrophoblast - grows into endometrium/deriving nutrition from it.
3. Endometrium grows over and envelops blastocyst

Question 18

What is implantation?

Answer 18

The process whereby an embryo sinks into the soft endometrium to become firmly attached to the uterus wall.

Question 19

What does implantation enable?

Answer 19

Enables blastocyst to gain nourishment for growth and development by absorbing nutrients from glands/blood vessels of uterine lining

Question 20

What happens at Day 10 of fertilisation?

Answer 20

1. Embryoblast starts to form two-layered embryonic disc in middle-endoderm/ectoderm called blastocyst
2. With blastocyst and amniotic cavity on each side
3. End of 2nd week, 2 layers grow around cavity on respective side and form yolk sac and amnion.

Question 21

What happens at Day 16 of fertilisation?

Answer 21

1. Gastrulation: 2 layered disc transforms into 3 germ layers called gastrula
2. Lose pluripotency and become multipotent
3. 1st set of cells displaces hypoblast = endoderm
4. 2nd set of cells fills in space between endoderm/epiblast = mesoderm
5. Remaining epiblast cells form the ectoderm

Question 22

What are the embryonic membranes?

Answer 22

Form in the embryonic period outside the embryo to protect and nourish it - amnion, chorion, yolk sac and allantois.

Question 23

What is the amnion?

Answer 23

The membrane that forms a cavity around the embryo; it secretes amniotic fluid into the cavity to protect the embryo

Question 24

What is the purpose of amniotic fluid?

Answer 24

1. Protects embryo against physical injury by acting as a shock absorber.
2. Helps maintain constant temperature
3. Allows developing embryo, and later foetus, to move freely.

Question 25

What is the Chorion?

Answer 25

An embryonic membrane that eventually forms part of the placenta - formed from outer cells of blastocyst together with a layer of mesodermal cells

Question 26

What is the purpose of the chorion?

Answer 26

Surrounds the embryo and the other three embryonic membranes.

Question 27

What happens to the chorion as the amnion enlarges?

Answer 27

It fuses with inner layer of chorion. Eventually, the chorion becomes main part of the foetal portion of placenta.

Question 28

What is the purpose of the yolk sac and allantois?

Answer 28

They form the outer structure of the umbilical cord.

Question 29

What is the placenta?

Answer 29

Organ that supplies nutrients to/ removes wastes from, the foetus; produces hormones, including oestrogen and progesterone

Question 30

What is a placentas additional purpose?

Answer 30

Also serves as an endocrine organ, producing several hormones necessary to maintain pregnancy

Question 31

When does the foetal part of the placenta develop?

Answer 31

As the blastocyst is implanted in the endometrium

Question 32

What are chorionic villi?

Answer 32

Finger-like projections that develop from the chorion and contain numerous blood vessels; grow to form part of the placenta.

Question 33

What happens as the villi grow into the endometrium?

Answer 33

1. They penetrate the endometrium 2. Become surrounded in the mother's blood - the foetal and maternal blood don't mix because separated by laters of cells.

Question 34

How does the exchange of materials happen in the placenta?

Answer 34

Diffusion and active transport.

Question 35

How does diffusion take place in the placenta?

Answer 35

Oxygen/nutrients from the mother's blood diffuse into the foetal blood, and wastes leave the foetus by diffusing into the maternal blood. - large number of villi provide large SA substances can pass. - fully developed placenta SA = 16m2

Question 36

What is the umbilical vein?

Answer 36

1. Carries blood from the placenta, through the umbilical cord, back to the foetus. 2. Mother's blood enters the placenta through the uterine arteries, flows through the blood spaces where substance exchange occurs/leaves again through uterine veins

Question 37

What is the umbilical cord?

Answer 37

The cord that attaches the foetus to the placenta. Made up of two umbilical arteries and a single umbilical vein.

Question 38

What is the umbilical arteries?

Answer 38

Two arteries within the umbilical cord that carry blood to the capillaries of the chorionic villi.