principles of embryology Flashcards
What can birth defects be caused by?
- genetic factors
- environmental factors
- multifactorial inheritance
What are the categories of birth defects?
- malformations
- disruption
- deformation
What is a malformation? Give examples
- intrinsically abnormal developmental process
- e.g chromosomal abnormalities (trisomy 18)
What is disruption? Give example
- extrinsic interference of the normal development process
- e.g teratogenic drug
What is deformation? Give examples
- abnormal form due to mechanical factors
- e.g clubfoot (equinovarus foot)
When is the risk of birth defects highest?
around week 5
What are the developmental periods within the pre-natal period?
- embryonic period (0 - 8 weeks)
- foetal period (9 - 40 weeks)
What are the developmental periods within the post-natal period?
- neonatal (0 - 4 weeks)
- infancy (4 - 52 weeks)
- childhood ( 2 yrs - puberty)
- adolescence (11 yrs - 19)
- adulthood (19+ yrs)
What are the 5 early events in embyrology?
- gametogenesis
- fertilisation
- cleavage
- gastrulation
- formation of the body plan
What is a gamete?
cell with 23 chromosomes (haploid)
What is gametogenesis?
the formation of gametes
What is spermatogenesis?
the formation of male gametes
When does spermatogenesis start and how does it vary through time?
- starts at puberty
continues through life - decreases with age
How long does each cycle take in spermatogenesis?
around 64 days
What is the typical production rate of sperm in spermatogenesis?
6.5 million sperm per gram of testicular tissue a day
What is oogenesis?
the formation of female gametes
When does oogenesis start and end?
- starts in utero
- ends at menopause
What happens by week 28-30 in utero (oogenesis)?
primary oocytes begin meiosis but arrest in development until puberty
When is the reproductive lifespan determined in females?
in fetal life
What is fertilisation? Where does it occur?
- fusion of sperm and oocyte
- occurs in the ampulla of the uterine tube
What does fertilisation form?
a zygote
Describe a zygote
- diploid
- single cell embryo
- sex determination
What does fertilisation stimulate oocytes to do?
complete meiosis 2
What is cleavage?
repeated mitotic divisions of the zygote into blastomeres and then a blastocyte
How is the morula formed?
- rapid mitotic cell division with no increase in size
- 16+ cell forms morula
What is the zona pellucida?
gelatinous matrix around zygote, keeps cell nice and tight (keeps us safe)
What is the trophoblast?
after many divisions, the outer layer in touch with the zona pellucida gets called the trophoblast
What does the trophoblast do?
- forms embryonic portion of the placenta
Describe the trophoblast
- lateral cell surface express tight junctions, gap junctions and desmosomes
- basal Na/K ATPase pump
What happens in ‘hatching’?
shedding of the zona pellucida allow future interaction with the endometrial lining of the uterus
What are some assisted reproductive technologies?
- in-vitro fertilisation (IVF)
- intracytoplasmic sperm injections (ICSI)
- gamete or zygote intrafallopian transfer (GIFT/ZIFT)
What happens in in IVF?
- trigger super ovulate (FSH, clomiphene citrate)
- oocyte harvested and partnered with sperm in vitro
- zygote cultured then implanted in uterine cavity
List the components of a zygote at the beginning?
- polar bodies
- pronuclei
- zona pellucida
- oolemma
List the components of a blastocyte?
- zona pellucida
- inner cell mass (embryoblast)
- trophoblasts
- blastocyst cavity
What does the inner cell mass (embryblast) differentiate into during implantation?
- epiblasts and hypoblasts
- they form the bilaminar germ disc
What do the trophoblasts differentiate into during implantation?
- syncytiotrophoblast
- cytotrophoblast
What does the blastocyte do before implantation? When does it finally implant?
- floats for around 72 hours before implantation
- implants 6-8 days post ovulation
What are the two steps that happen in the implanted embryo?
- establish primordial uteroplacental circulation
- formation of the chorionic cavity
How does the implanted embryo establish uteroplacental circulation?
- maternal capillaries become sinusoids
- further invasion of syncytiotrophoblast ruptures capillaries and cells
- lacunae flood with maternal blood and nutrients
In the implanted embryo, how is the chorionic cavity formed?
- extra-embryonic mesoderm fills space between primitive yolk sac/amniotic cavity and cytotrophoblast cells
- large cavities form and become connected
What is an extra-uterine (ectopic) pregnancy?
abnormal implantation of embryo
What percentage of pregnancies are ectopic
2%
What percentage of pregnancy related maternal mortality is due to ectopic pregnancy?
9%
What is gastrulation and when does it occur?
- movement of epiblast cells through the primitive streak to form 3 primary germ layers
Which cells do the epiblasts replace to then form the endoderm?
hypoblasts
What is the ectoderm made from?
what’s left of the epiblast layer
What does the ectoderm go on to become?
- epidermis of skin (hair, nails, associated glands)
- nervous system
What does the intraembryonic mesoderm become?
it divides into 3 layers
Which 3 layers does the intraembryonic mesoderm become?
- paraxial
- intermediate
- lateral plate
What 2 layers does the lateral plate mesoderm divide into?
- somatic layer (dorsal)
- visceral layer (ventral)
What does the paraxial mesoderm become?
- axial skeleton (vertebrae)
- voluntary (skeletal) muscle
- part of dermis (back)
What does paraxial mesoderm undergo further differentiation to become?
somites
What do somites differentiate into?
- dermomyotomes
- sclerotomes
What do dermomyotomes do?
they form connective tissue and skeletal muscle
What do sclerotomes do?
they form bone and cartilage (vertebral arch)
What does the intermediate mesoderm become?
urogenital systems
- kidneys, ureter, trigone of bladder
- gonads, uterus + uterine tubes, superior portion of vagina, ductus deferens
What does the somatic part of the lateral plate mesoderm become?
- most of dermis (not in central back)
- lining of body walls
- bones of limbs
What does the visceral part of the lateral plate mesoderm become?
- cardiovascular system
- mesothelial covering of organs
- smooth muscle
What does the endoderm become?
- lining of gut
- lining of respiratory tract
- lining of bladder and urethra
