Life Before Birth

Question 1

what is embryology?

Answer 1

the study of development of an embryo from the stage of fertilisation until the fetal stage

Question 2

preimplantation stage

Answer 2

week 1

Question 3

embryonic stage (organogenesis)

Answer 3

weeks 2-8 (where development of placenta happens)

Question 4

fetal stage (growth and development)

Answer 4

weeks 9-38

Question 5

cleavage

Answer 5

mitotic divisions of fertilised egg, overall size remains the same. It is allowed passage through the narrowest part of the uterine tube (ISTHMUS) and is surrounded by a tough glycoprotein coat (ZONA PELLUCIDA)

Question 6

morula

Answer 6

around 4 days after fertilisation, cells maximise contact with each other, forming a cluster of cells held together by tight junctions which helps with communication and signalling between the cells. it enters the uterus

Question 7

blastocyst formation

Answer 7

the first sign of cellular differentiation, inner cell mass goes on to form the embryo and extraembryonic tissues
outer cells trophoblasts contributes to the placenta
as the embryo enters the uterine cavity, fluid enters via the zone pellucid into the spaces of the inner cell mass. a fluid filled blastocyst cavity forms

Question 8

what typically happens on day 7?

Answer 8

the blastocyte hatches out of the zone pollucida

Question 9

why does the blastocyte need to hatch?

Answer 9

it begins to run out of nutrients and needs to implant

Question 10

how does the blastocyte hatch?

Answer 10

Icm cells undergo proliferation and the fluid builds up in the cavity, eventually resulting in the blastocyte hatching from the zone pellucid to facilitate implantation

Question 11

when does implantation occur?

Answer 11

in week 2 of development

Question 12

what is implantation?

Answer 12

the interaction between the embryo and the endometrial layer of the uterus

Question 13

what do inner cell mass cells form

Answer 13

the embryo

Question 14

what do trophoblast cells form

Answer 14

the placenta

Question 15

what is a blastocyst?

Answer 15

consists of two cell populations, inner cell mass and trophoblasts

Question 16

when does decidualisation occur?

Answer 16

when the blastocyst makes contact with the endometrium of the uterus

Question 17

what happens during decidualisation?

Answer 17

it is a process that results in several changes in the endometrium to prepare for pregnancy. it happens in the stromal cells of the uterus. these cells will form the maternal complex of the placenta. the process triggers the production of several molecules and promotes the trophoblast cells to become invasive.

Question 18

what are cytotrophoblasts?

Answer 18

a single layer of cells closest to the inside of the embryo

Question 19

what is the syncytiotrophoblast?

Answer 19

more invasive and extensive than the cytotrophoblasts. it is the outer layer of cells.

Question 20

what is the bilaminar disc?

Answer 20

the 2 layers formed from the inner cell mass, the epiblasts and hypoblasts

Question 21

how is the diffusion of oxygen, waste and nutrients via the blood supply established?

Answer 21

towards the end of week 2, the implanting syncytiotrophoblast cells communicate with the maternal side of the placenta and establish a connection to able this diffusion

Question 22

what is hCG (human chorionic gonadotrophin)

Answer 22

a hormone produced by the end of week 3 by the syncytiotrophoblasts which is then secreted into the urine

Question 23

what happens during week 2 of development?

Answer 23

implantation occurs, creating a connection between the embryo and the maternal endometrium.
the inner cell mass forms two laters making up the bilaminar disc, the epiblast and the hypoblast.
the trophoblast layer differentiates to form two placental layers, the cytoptrophoblast and the invasive syncytiotrophoblast

Question 24

what is an ectopic pregnancy?

Answer 24

where implantation occurs in the wrong place

Question 25

what can cause an eptopic pregnancy ?

Answer 25

clot transit through the uterine tube, premature hatching of the blastocyte or common implication of IVF

Question 26

what are extra-embryonic membranes?

Answer 26

membranes that are derived from the blastocyte but do not contribute to the embryo

Question 27

amnion

Answer 27

continuous with the epiblast of the bilaminar disc. lines a structure called the amniotic cavity which is filled with fluid and acts to protect the developing embryo this is present up until birth

Question 28

chorion

Answer 28

double layered membrane formed by the trophoblast and the extra embryonic membranes it lines a structure called the chorionic cavity which is seen in early pregnancy but disappears due to the expansion of the amniotic cavity. it forms the fetal component of the placenta

Question 29

yolk sac

Answer 29

continuous with the hypoblast of the bilaminar disc. it is important in nutrient transfer in weeks 2-3 but disappears completely by week 20 important in blood cell formation and formation of the gut

Question 30

allantois

Answer 30

forms as an outgrowth of the yolk sac it contributes to the umbilical arteries and connects to the fetal bladder in non human mammals it acts as a reservoir for fetal urine

Question 31

what is gastrulation

Answer 31

the process of cell division and migration resulting in the formation of three germ layers

Question 32

what is the primitive streak?

Answer 32

a thickened area of cells which appears at the tail end of the cells. a small bulge at the end called the primitive node

Question 33

how are the 3 layers formed?

Answer 33

the epiblast evaginates (cells move towards the primitive streak and move down under the epiblast and spread out again) and creates 3 new layers of cells. this is controlled by fibroblast growth factor 8 which is made by primitive streak

Question 34

what are the 3 layers (outside to inside)

Answer 34

ectoderm, mesoderm, endoderm

Question 35

what produces the ectoderm?

Answer 35

the top layer of epiblasts

Question 36

what are some examples of ectoderm derivatives?

Answer 36

- epidermis of skin - nervous system - tooth enamel - epithelial lining of mouth and anus - cornea and lease of eye

Question 37

what are some examples of mesoderm?

Answer 37

- skeletal system - muscular system - dermis of skin circulatory and lymphatic system

Question 38

what are some examples of endoderm?

Answer 38

- epithelial lining of digestive track - liver epithelial lining of resp system - thymus - Thyroid glands and parathyroid glands

Question 39

totipotent cells?

Answer 39

total power, eg zygote has the ability to make all types of cells

Question 40

pluripotent cells?

Answer 40

can make very many types of cells within the body. eg inner cell mass of blastocyte as these cells can give rise to three germ layers. another example is embryonic stem cells

Question 41

multipotent cells?

Answer 41

can make multiple cell types, eg haematopoietic stem cells can differentiate into many types of stem cells

Question 42

what cells form the primitive streak?

Answer 42

epiblast cells that migrate to the midline then form the primitive streak

Question 43

what do the hypoblast cells surround?

Answer 43

the yolk sack

Question 44

what do the epiblast cells form?

Answer 44

the cavity known as the amniotic cavity

Question 45

what is the primitive streak?

Answer 45

elongates from caudal to cranial

Question 46

what happens to epiblast cells that migrate beyond the primitive streak and into the hypoblast?

Answer 46

become the endoderm

Question 47

what cells make up the ectoderm?

Answer 47

the remaining epiblast cells that do not enter the hypoblast

Question 48

what cells make up the mesoderm?

Answer 48

the majority of epiblast cells that occupy the space between the epiblast and hypoblast layers differentiate to become the mesoderm

Question 49

what is the first mesodermal tissue to form?

Answer 49

notochord

Question 50

where are somites derived from?

Answer 50

paraxial mesoderm

Question 51

what are the 3 segments of the mesoderm after it changes shape (medial to lateral)?

Answer 51

paraxial mesoderm intermediate mesoderm lateral plate mesoderm

Question 52

what are the different parts of the lateral plate mesoderm?

Answer 52

the one closest to ectoderm? - somatic layer of the lateral plate mesoderm the one closest to the endoderm - sphlanic layer of the lateral plate mesoderm

Question 53

what do the somites turn into?

Answer 53

dermatome myotome -top sclerotome - bottom

Question 54

what does the paraxial mesoderm eventually turn into (dorsal to ventral)?

Answer 54

dermatome myotome sclerotome