embryo

Question 1

what enzyme activates the oocyte and what changes does this bring about within the oocyte?

Answer 1

sperm releases phospholipase c zeta (PLCz) which activates the egg to release calcium from internal stores to facilitate fertilisation

Question 2

what is the male and female pro-nucleus and when does it form?

Answer 2

the two sets of haploid chromosomes form the female and male pro-nucleus (23 chromosomes each), this occurs 4-7 hours after fertilisation. the pro-nuclei are equal in size

Question 3

what is the process of syngamy?

Answer 3

the male and female pro-nucleus migrates to the centre, and the haploid chromosomes pair up and replicate DNA for the first mitotic division the pro-nuclear membranes also break down and spindle fibres form

Question 4

what is cleavage and when does it start?

Answer 4

several mitotic cell divisions, that start on day 2. the ooplasm divides in half. during cleavage, the cell mass does not change but the number of nuclei increases

Question 5

whose genetics control the initial cell divisions and what controls it after?

Answer 5

the maternal stores of RNA lay down developmental control prior to the 4-8 cell stage. afterwards, the embryonic genome becomes activated and embryonic transcription can occur

Question 6

what is significant about day 3 of development?

Answer 6

8 cells have formed, which is the number that is required to form an entire foetus. these cells are totipotent

Question 7

what is the compaction stage of embryological development and what occurs during it?

Answer 7

day 4: the cells flatten, tight junctions occur, the outer cells become polarised and roughly 16 cells are present

Question 8

what is the presence of 16 cells in embryology called?

Answer 8

a morula

Question 9

what happens on day 5 of embryological development, and roughly how many cells are there?

Answer 9

tight junctions form between the outer cells which forms the trophectoderm. sodium is pumped in, which leads to water following by osmosis leading to a fluid cavity. there are now greater than 80 cells. the cells are now pluripotent. this cavity will continue to expand

Question 10

what happens on day 6 of embryological development?

Answer 10

due to the expansion (and enzymatic factors), the blastocyst hatches from the zona pellucida

Question 11

what is the main energy substrate in the preimplantation stages of the embryo and the blastocyst stage?

Answer 11

preimplantation stage: pyruvate
blastocyst stage: glucose

Question 12

compare the genetic control in the early cleavage with the blastocyst?

Answer 12

cleavage: maternal
blastocyst: embryonic

Question 13

compare the metabolic activity in the early cleavage with the blastocyst?

Answer 13

cleavage: low
blastocyst: high

Question 14

compare the biosynthetic activity in the early cleavage with the blastocyst?

Answer 14

cleavage: low
blastocyst: high

Question 15

compare the nutrient requirements in the early cleavage with the blastocyst?

Answer 15

cleavage: simple (low glucose, non essential amino acids and pyruvate)
blastocyst: complex (high glucose, essential and non-essential amino acids as well as vitamins)

Question 16

what nutrients does the developing embryo receive and where does it receive these from?

Answer 16

1. cumulus cells (a group of closely associated granulosa cells that surround the oocyte)
2. fallopian tube (calcium, sodium, glucose, chloride and proteins)
3. uterine secretion (iron, fat soluble vitamins and glucose)

Question 17

what growth factors and cytokines are important in embryonic growth and differentiation?

Answer 17

1. insulin-like growth factor - IGF–I and IGF–II increase cell numbers in blastocyst
2. leukaemia inhibitory factor (LIF) enhances embryo-endo interaction

Question 18

what happens when the embryo has implanted on day 10?

Answer 18

gastrulation, where three germ layers form ectoderm, mesoderm and endoderm

Question 19

how does the uterus adapt for implantation?

Answer 19

the endometrial cells change to help with the absorption of uterine fluid, which allows the blastocyst to come nearer to the endometrium to become immobilised. blood supply also improves and the maternal uterine tissue (decidua) also improves

Question 20

how long is the implantation window?

Answer 20

4 days (6-10 days post ovulation)

Question 21

how is the implantation process regulated?

Answer 21

coordination and communication between the endometrium and the developing embryo, regulated by hormones, cytokines and growth factors, proteases, genetics and cell adhesion molecules

Question 22

what are the three phases of embryo implantation?

Answer 22

1. apposition
2. attachment
3. invasion

Question 23

what is the apposition stage of embryo implantation?

Answer 23

the blastocyst adheres to the uterine lining at the area above the ICM

Question 24

what is the attachment stage of embryo implantation?

Answer 24

the adhesion to the endometrium becomes stronger, and the endometrium gets penetrated by the microvilli that protrudes from the trophoblast. the blastocyst and endometrium communicate via receptor-ligand interactions. the adhesion always occurs below the inner cell mass

Question 25

what is the invasion stage of embryo implantation?

Answer 25

the trophoblastic protrusions continue to invade the endometrium, and differentiate to form the syncytiotrophoblast

Question 26

what does the syncytiotrophoblast do?

Answer 26

erodes endometrial blood vessels till it comes into contact with maternal blood to form the chorionic villi. this is the initiation of the formation of the placenta

Question 27

what is the lacunae and its significance?

Answer 27

formed by the syncytiotrophoblast; maternal blood filled spaces where nutrients are exchanged and waste products are removed

Question 28

what is the decidual reaction stage of embryological development?

Answer 28

stromal cells adjacent to the blastocyst differentiate into metabolically active, secretory cells or Decidual Cells (under influence of progesterone)

Question 29

what do the secretions of decidual cells do?

Answer 29

contain growth factors/proteins to support growth of implanting blastocyst in the initial stages before the placenta is fully developed, which improves the vascularisation of the uterine wall

Question 30

is the decidual reaction required for implantation?

Answer 30

no, ectopic implantation can occur anywhere in the abdominal cavity

Question 31

how is the foetus protected from the mothers immune system?

Answer 31

at the same time as the decidual reaction, leukocytes in the endometrial stroma secrete interleukin-2 which prevents maternal recognition of the embryo as a foreign body during the early stages of implantation

Question 32

what is are the two roles of human chorionic gonadotropin?

Answer 32

1. ensures the corpus luteum continues to produce progesterone throughout the first trimester of pregnancy (prevents menstruation and maintains the endometrium lining).
2. immunosuppressive – has highly negative charge, may repel the immune cells of the mother & protect the foetus

Question 33

what happens to hCG levels during pregnancy?

Answer 33

levels double every 1-3 days for the first 10-12 days. a short doubling time signifies a healthy pregnancy

Question 34

what does a slow rate of hCG increase indicate?

Answer 34

early abortion, ectopic pregnancy, delayed implantation

Question 35

what are the components of in vitro culture during days 1-3?

Answer 35

water, salts, ions, lactate, proteins, no/low glucose, non essential amino acids

Question 36

what are the components of in vitro culture during days 3+?

Answer 36

water, salts, ions, pyruvate, lactate, proteins glucose, essential and non-essential amino acids, vitamins

Question 37

what are some maternal factors that influence embryo growth in vitro?

Answer 37

follicle environment

Question 38

what are some embryonic factors that influence embryo growth in vitro?

Answer 38

cleavage rate, size of blastomeres, variation in embryo metabolism

Question 39

what factors affect embryo growth in vitro?

Answer 39

exposure to light, oxygen, pH, osmolarity and culture medium

Question 40

on what day is the embryo transferred to the mother and what happens to the remaining embryos?

Answer 40

day 5. the rest are preserved by being frozen if they are of good quality and at the correct stage of development

Question 41

what is the embryo transferred to the mother with?

Answer 41

with embryo glue which increases cell-cell adhesions increasing the likelihood that the embryo undergoes apposition and attachment

Question 42

why may IVF implantation fail?

Answer 42

insufficient trophoblast invasion of maternal blood vessels, an issue with the sperm, issues with the embryo (40% IVF fertilised eggs are abnormal)

Question 43

what is recurrent implantation failure classed as?

Answer 43

failure to achieve a clinical pregnancy after transfer of at least 4 good quality embryos in at least 3 cycles, with the female being under the age of 40