early fetal development

Question 1

when is fertilization age

Answer 1

measured from time of fertilization

Question 2

when is gestational age

Answer 2

calculated from time of the beginning of the last menstrual period

determined by:

• fertilization day +14 days
• early ultrasound and compare to embryo charts

Question 3

What is the Carnegie stage

Answer 3

23 stages of development of embryo- based on features not time

can compare development with other species

Question 4

what is the embryogenic stage (14-16 days post fertilization)

what is the embryonic stage (16-50 days post fertilization)

what do these two stages both form

Answer 4

when two populations of cells are determined- pluripotent cells (contribute to fetus) and extraembryonic cells (contribute to support structures eg placenta)
when embryo is established from oocyte- morula just turned to blastocyst

establishment of germ layers and different tissues

first trimester

Question 5

what is the fetal stage (50-270 days)

what stage is this part of

Answer 5

major organ systems are developed, migrate to final location
foetus viability established (survival outside the womb)

second trimester

Question 6

what are the stages of embryo development, starting from oocyte

Answer 6

ovulated oocyte (1cell ) 
zygote (1 cell)
cleavage stage embryo (2-8cell)
Morula (16 cells)
blastocyst (200-300cells)

Question 7

what is the embryo dependant on to get through the first stage of divisions

when are they synthesised and stored

what happens when there is a failure to store or interpret these properly

Answer 7

Maternal mRNA and proteins

during oocyte development

impairs embryonic development

Question 8

what happens during the maternal to zygotic transition

Answer 8

transcription of embryonic genes (zygotic genome activation)

inc. protein synthesis

organelle maturation

Question 9

what happens past the 8 cell stage

Answer 9

compacted morula becomes blastocyst

spherical cells turn wedge shaped

outer cells pressed against zona and become polarised

formation of blastocyst cavity

Question 10

how to outer cells connect to each other and how does this affect their connection to the inner cells

Answer 10

form tight gap junctions and desmosomes

form a barrier against diffusion to the inner cells

Question 11

In a blastocyst, what is:

• zona pellucida
• inner cell mass
• trophoectoderm
• blastocoel

Answer 11

hard protein shell, protects embryo, prevents polyspermy

pluripotent embryonic cells contributing to foetus tissue

extraembryonic cells that contribute to support structures

fluid filled cavity osmotically formed due to trophectoderm pumping Na+ into cavity

Question 12

why does blastocyst need to exit zona pellucida

how does this occur

Answer 12

to implant it into uterus

enzymatic digestion
cellular contraction

Question 13

what does the trophectoderm split into

how do syncitiotrophoblasts form

what do syncitiotrophoblasts do

what do cytotrophoblasts do

Answer 13

syncitiotrophoblast and cytotrophoblast

by trophoblast cells fusing together

destroys local cells in endometrium of uterus
creates interface between embryo and maternal blood supply

provide source of syncitiotrophoblast cells

Question 14

what does the inner cell layer split into

what is the function of each

Answer 14

epiblast and hypoblast

epiblast: form fetal tissue
hypoblast: form yolk sac (extraembryonic)

Question 15

when does the bi laminar embryonic disc form

how does this form

what do amnion cells do

what do syncitiotrophoblasts secrete at this stage

Answer 15

day twelve

cells are separated from epiblast and form aminotic cavity

form extra embryonic membranes

HCG

Question 16

Describe the process of gastrulation

Answer 16

a thickened structure forms along the midline of the epiblast called the primitive streak

the primitive streak expands at the cranial end forming the primitive node which has a circular depression inside called the primitive pit

the depression continues along the midline of the epiblast towards the caudal end forming the primitive groove

epiblast cells on top of the groove detach from epiblast and migrate towards groove, go inwards through the groove into the interior of the embryo- invagination

they displace hypoblast cells and form a new germ layer called the endoderm

the epiblast cells that didn’t migrate through the groove are called the ectoderm and the cells between the endoderm and ectoderm are called the mesoderm

Question 17

what organs are derived from the endoderm

what organs are derived from the ectoderm

what organs are derived from the mesoderm

Answer 17

thyroid, lung, GI Tract, Liver, Pancreas

CNS, neural crest, skin epithelia, tooth enamel

bone, cartilage
blood (endothelial cells, red and white blood cells)
muscle ( smooth, skeletal, cardiac)
gonads, kidneys, adrenal cortex

Question 18

What is the notochord

where does it form

what does it do

what is the neural plate

Answer 18

rod like tube structure formed from cartilage like cells

formed along the embryo midline, under the ectoderm

acts as the organizing centre for the neurulation and mesoderm development

Question 19

describe the process of forming the neural tube

Answer 19

notochord releases growth factors and signalling molecules which organises the embryo to form the neural system

it directs the neural plate ectoderm to invaginate and form a groove

creates two ridges called the neural folds that runs along the cranial caudal axis

neural crest cells develop in the neural folds

neural folds move towards each other over the neural groove, forming a hollow tube (the neural tube) when the folds fuse

the neural tube is overlaid with epidermis

the neural crest cells in the neural folds increase in number and migrate to other areas of embryo

Question 20

What days do the head end and the tail end of the neural tube close?

what condition arises due to failure of the closure of the tail end of the neural tube

what condition arises due to failure of closure of the head end of the neural tube

the closure of the head end precedes what developing?

Answer 20

day 23 and 27

spina bifida - when the neural tube is open at birth, usually the lower spine, varying severity (0.4-5-1000 births)

anencephaly- absence of most of the skull and brain (1/10,000 births)

formation of brain structures

Question 21

give examples of the derivatives of the neural crest cells

Answer 21

cranial- cranial nerves

cardiac- aortic arch, cardiac nerves

trunk- dorsal root ganglion, sympathetic ganglion, melanocytes. aortic nerve

clusters, adrenal medulla

vagral and sacral- PNS ganglia, Enteric ganglia

Question 22

what do defects in neural crest cell migration lead to?

Answer 22

pigmentation disorders
failure to innervate GI tract
deafness
cardiac and facial defects

Question 23

what is somitogenesis

what are somites

Answer 23

formation of somites

paired blocks of paraxial mesoderm flanking the neural tube and notochord

Question 24

what is the process of somitogenesis?

what aspects of somite formation is species specific?

Answer 24

blocks of paraxial mesoderm condense and bud off in pairs
one on each side of the neural tube
begins at the cranial end and continues towards the caudal end

appearance of somites
numbers of somite pairs
rate of budding of somite pairs

Question 25

what two types of tissue do somites form

what do these tissues go on to form

Answer 25

schlerotome, dermomyotome

schlerotome- vertebrae and rib cartilage
dermomyotome - forms dermatome and myotome
- dermatome- dermis of skin, fat and connective tissues of neck and trunk
- myotome- muscles of the embryo

Question 26

what day does formation of the gut tube occur and what two types of folding occurs

how does the folding help form the primitive gut

what does the primitive gut then go on the form

Answer 26

day 16, ventral folding (head and tail ends curl together) and lateral folding ( left and right sides of embryo roll)

this pinches off part of the yolk sac to form the primitive gut

foregut, midgut, hindgut

Question 27

what are the derivatives of the foregut

what are the derivatives of the midgut

what are the derivatives of the hindgut

Answer 27

eosophagus, stomach, upper duodenum, liver, gall bladder, pancreas

lower duodenum, remainder of small intestine, ascending colon, first two thirds of transverse colon

last third of transverse colon, descending colon, rectum and anal canal

Question 28

what day and where does the heart form from

what occurs at day 22 of heart formation

what occurs from 6 weeks gestational age

Answer 28

tube in mesoderm, day 19

beating and pumping blood commences around day 22

fetal heartbeat is detected from 6 weeks gestational age

Question 29

what week does lung development start, and where from

what happens at the end of the fourth week

Answer 29

fourth week, lung bud- an endodermal structure adjacent to the foregut

lung bud splits into two, progressively branches through development

Question 30

where do the gonads form from and what do they form

what happens to XY embryos due to what gene

what happens to XX embryos and what is it enforced by

Answer 30

mesoderm, initially bipotential (neither ovary or testes) structures known as gonadal/genital ridges

due to SRY gene on y chromosome, directs gonadal cells to become sertoli cels, triggering testis development, leydig cell formation and testosterone production

due to absence of SRY gene, gonadal cells adopt a granulosa cell fate and ovary development, requires reinforcement by FOXL2