Fertilisation to Trilaminar

Question 1

where does fertilisation take place

Answer 1

typically occurs in the ampulla of the uterine tube

Question 2

describe the steps involved in fertilisation

Answer 2

1. sperm binds in a human specific interaction with the zona pellucida glycoprotein (ZP3)
2. acrosomal enzymes released from sperm head; the sperm then digests its way into egg
3. egg and sperm plasma membrane fuse and sperm contents enter the egg
4. sperm entry triggers - completion of meiosis 2, release of cortical granules by oocyte

Question 3

describe the process of cleavage and compaction

Answer 3

the zygote undergoes a series of mitotic divisions which subdivide the fertilised egg into many smaller daughter cells called blastomeres
up to the 8 cell stage, blastomere are totipotent (capable of giving rise to any cell type)

Question 4

name the different parts of the blastocyst and what they give rise to

Answer 4

trophoblast/outer cell blast - forms outer layer of blastocyst, providing nutrients to embryo and forming outer layer of blastocyst
blastocyst cavity
embryo blast/inner cell mass -(amnion and yolk sac) inner layer of epiblast and internal layer of hypoblast

Question 5

state where in the uterus the embryo implants

Answer 5

on the posterior or anterior of uterine wall (usually by day 6)

Question 6

describe ectopic implantation and explain dangers associated with it

Answer 6

implantation in a location outside the uterus - ovary, uterine tube or peritoneum - meaning the embryo does not have enough room to develop a rich blood supply causing it to rupture and to haemorrhage

Question 7

describe gastrulation

Answer 7

movement of epiblast cells through the primitive streak between epiblast and the definitive endoderm to form the 3 primary germ layers
the body axes are established by presence of primitive streak

Question 8

state the major tissues derived from ectoderm

Answer 8

epidermis of skin (hair, nails, associated glands), nervous system (brain, spinal cord, peripheral nerves)

Question 9

describe the morula

Answer 9

after the 16 cell stage, the blastomere is termed the morula - cells are confined within zona pellucida and so maximise the space available by coming into closer contact with each other and forming cell junctions, outer cells form extensive gap junctions and fluid enters through the ZP

Question 10

describe human chorionic gonadotropin (hCG)

Answer 10

produced by syncytiotrophoblast during bilaminar embryo

basis for pregnancy tests, stimulates the corpus luteum and is produced as early as the second week

Question 11

name the layers of the Trilaminar Embryo

Answer 11

ectoderm, mesoderm, endoderm
hypoblast cells are replaced by endoderm
epiblast cells are replaced by ectoderm

Question 12

state the major tissues derived from mesoderm

Answer 12

divides into 3 regions either side of the notochord - paraxial, intermediate, lateral plate (somatic layer and visceral layer)
paraxial - axial skeleton, voluntary (skeletal) muscle, parts of dermis (back)
intermediate - urogenital systems (kidney and ureter, gonads and associated structures)
lateral plate (somatic part) - most of dermis, lining of body wall, parts of limbs
(visceral part) - cardiovascular system, mesothelial covering of organs, smooth muscle

Question 13

state the major tissues derived from endoderm

Answer 13

lining of gut tube, respiratory tract, bladder and urethra

Question 14

describe the changes in the trophoblast and embryo blast during implantation - bilaminar embryo (7.5 days)

Answer 14

trophoblast divides into: cytotrophoblast and syncytiotrophoblast
embryo blast organises into 2 cell layers : epiblast (dorsal surface) and hypoblast (ventral surface)

Question 15

describe the changes in the trophoblast and embryo blast during implantation - bilaminar embryo (9 days)

Answer 15

trophoblast: cytotrophoblast develop into syncytiotrophoblasts and vacuoles appear to unite and from lacunae
embryo blast - epiblast forms into amniotic cavity, hypoblast forms into primitive yolk sac

Question 16

describe the changes in the trophoblast and embryo blast during implantation - bilaminar embryo (9 days)

Answer 16

trophoblast: cytotrophoblast develop into syncytiotrophoblasts and vacuoles appear to unite and from lacunae
embryo blast - epiblast forms into amniotic cavity, hypoblast forms into primitive yolk sac

Question 17

describe the changes in the trophoblast and embryo blast during implantation - bilaminar embryo (12 days)

Answer 17

establishment of the uteroplacental circulation, maternal blood flows into lacunae to diffuse to the embroyblast 
extraembyronic mesoderm(loose cells) develops and eventually degenerates forming the chorionic cavity

Question 18

describe the changes in the trophoblast and embryo blast during implantation - bilaminar embryo (13 days)

Answer 18

further development of the chorionic cavity and presence of connecting stalk (eventually umbilical cord), second wave of hypoblast cells migrate to form the definite (secondary) yolk sac

Question 19

describe the changes in the trophoblast and embryo blast during implantation - implanted embryo

Answer 19

uteroplacental circulation forms as syncytiotrophoblast extends and contacts maternal blood supply
the amniotic cavity is present above the bilaminar embryonic disc and the definitive yolk sac below
the chorionic cavity has developed and encircles the entire embryo