Embryology and congenital malformations Flashcards
why study human embryology ?
history of prenatal origin understand brith defects and cogenital abnormalities
understand adult anatomy
obstetrics and paediatrics specialities
understand adult illness or diseased with developmental origin
reproductive technologies
advise patients - issues with reproduction, birth defects, parental development, in vitro fertilisation, stem cells and cloning
trimesters
1st, 2nd, 3rd
3 month periods in 9 months
Pre-embryonic period
Fertilisation of the egg of the 3rd development with implantation of the conceptus
embryonic period
period of organogenesis
from the beginning of 4th week to end of 8th week of development - where the germ layers (Ectoderm, Mesoderm and Endoderm) are formed and they give rise to specific tissues and organs
end of embryonic period
main organ system have been established
(embryonic period = can include the pre-embryonic - first 8 weeks following ovulation and fertilisation resulting in pregancy)
period of the foetus/ feral period
3rd month to birth - period of maturation of embryonic organ systems and tissues
gametogenesis
The process of the production of male and female gametes or sex cells (sperm and oocyte or egg) from the primordial germ cells (PGCs) via meiotic cell division in the gonads (testes and ovaries)
spermatogenesis stems
Primordial germ cell in embryo
spermatogonial stem cell
spermatogonium
primary spermatocyte
(meiosis 1)
secondary spermatocyte
(meiosis II)
early spermatid
(cell differentiation)
sperm
Sertoli cells
provide nutrients
oogenesis steps
primordial germ cell
oogonium
primary oocyte (present at birth - arrested in prophase of meiosis I)
(complete meiosis I and onset of meiosis II)
secondary oocyte (arrested at metaphase of meiosis II)
(ovulation / sperm entry)
(complete meiosis II)
fertilised egg
primordial germ cells
specialised stem cells - give rise to germ line and are formed a generation earlier when the parents were embryos - handed over reproductive ability by parents before you were born
principles of gametogenesis for development of human body structure
- gametogenesis reduced the chromosomal number of the gametes from a diploid (46) to haploid (23)
- gametogenesis results to an enhanced genetic variability in the gametes through a random recombination of genetic material on homologous maternal and paternal chromosomes
- spermatogenesis does not start in the male until puberty and occurs in seminiferous tubules of the testes
- oogenesis starts from feel life in the female - occurs in the ovary and completed at puberty
errors in gametogenesis
chromosomal abnormalities could result in birth defects or spontaneous abortions
errors in spermatogenesis
lead to number of spermatozoa morphological abnormalities that could affect male fertility = sperm count clinical investigation
fertilisation
male and female gametes fuse = form a zygote
where does fertilisation occur
ampullarf region of uterine tube (oviduct)
capacitation
sperm conditioning process within the female reproductive tract (uterine tube) in prep for fertilisation for fertilisation of the ovum
only capacitated sperm can pass through cells and undergo acrosome reaction
acrosome reaction is induced by what
zona proteins
after binding of the acrosomal region of the sperm with the zone pellucida of the oocyte
leads to release of enzymes needed to penetrate the zone pellucida
when do cortical and zone reactions happen
after release of acrosome enzymes (across)
what is the cortical reaction
sperm can penetrate the zone pellucida
sperms contact with plasma membrane of the oocyte leads to release of lysosomal enzymes form cortical granules in the plasma membrane = become impenetrable to other permatozoe
what is the zona reaction
changed in permeability of the zona pellucida
the enzymes alter the properties including structure and composition of the zona pellucida to prevent polyspermy
what is the purpose of cortical and zona reactions
to ensure only one sperm penetrates the egg
what could male infertility be due to
could result from the quality and quantity of spermatozoa ejaculated
when does cleavage occur
1 week after fertilisation
what is cell cleavage
rapid mitotic divisions of the large zygote to produce an increase in numbers of smaller daughter cells - blastomeres
does cleavage result in cell growth
no there is no increase in protoplasmic mass
egg remains 1 - more divisions so proportions become smaller
what increases after cleavage
the nucleocytoplasmic ratio increases after cleavage - as cytoplasm is partitioned - smaller cells
what does cleaver transform the zygote into
multicellular embryo = morula from single large cell (zygote)
A solid ball of cells
12-16 cells
in about 3 days after fertilisation
Parthenogenesis
process where an unfertilised egg develops to a new individual
Parthenogenesis
process where an unfertilised egg develops to a new individual
Morula
A solid ball (mulberry) of 12-16 cells (blastomeres) following cleavage
Compaction
a process of cells (blastomeres) reorganisation and segregation into inner cell mass (embryo blast) and outer cell mass (trophoblast) following cleavage
what does compaction lead to
The establishment of inside-outside polarity and increased maximised cell-to-cell contact
Blastocyst stage
Stage when the morula developed a fluid filled cavity (Blastocoel) with a compact inner cell mass at one side (embryonic pole) enclosed by a thin, single layered epithelium of trophoblast
when does blastocyst happen
2nd week following fertilisation
what is an embryonic germ disc
Cluster of embryonic cells (inner cell mass - embryoblast) at the embryonic pole of the blastocyst that give rise to tissues of the embryo proper and constitutes the germ disc formed within the 2nd week
what is embryonic germ disc used for
placenta and other fatal membrane formations
after segregation of blastomeres
outer cell mass forms the trophoblast
what are the bilaminar embryonic germ disc layers
embryo blast (germ disc) differentiated into 2 layers
outer layer = epiblast
inner layer = hypoblast (primitive endoderm)
role of bilaminar embryonic germ disc
· Defines the primitive dorsal-ventral axis of the embryo
· Epiblast = Dorsal
Hypoblast = Ventral
stem cell
undifferentiated cell that has the ability to form specilised cell types
embryonic or adult stem cell
pluripotent cell
has ability to form all mature cell types in the body except placental and extra embryonic cells
it can’t form a whole organism
multipotent cell
has the ability to form more than one closely related mature cell types in the body but not as varied as pluripotent cells
examples of multipotent cells
cord blood/bone marrow stem cells form erythrocyte, leucocyte, platelets
totipotent cell
Have the ability to form all differentiated cell types in the body including the placental and extra embryonic membrane cells
it could form a whole organism
e.g zygotę and first few generations of blastomeres
3 key events in human embryology
- Gametogenesis
- fertilisation
-cleavage
what is the primitive streak
a transient thickened longitudinal midline structure at the caudal end of the epiblast of the bilaminar embryonic germ disc
when does primitive streak form
day 15 of developing human embryo
what is a primitive groove
a narrow tough like depression with slightly bulging regions on either side on the primitive streak
what is the primitive node
Cephalic (cranial) end is slightly elevated expanded area
surrounds a small circular depression called primitive pit that is continuous caudally with primitive groove
what happens in gastrulation
Process of epiblastic cell movement (ingression or invagination) through primitive streak
leading to transformation of blaminar germ disc into trilaminar germ disc
what layers are in a trilaminar germ disk
ectoderm
mesoderm
endoderm
what cells form what layers in gastrulation and when
formation of the primitive streak (15th day)
- epiblastic cell ingress or invaginate to form the mesoderm and endoderm
- remaining epiblastic cells become the ectoderm
goals of gastrulation
- form the primitive streak
- establish all major body axes in 4th week of development
- cranio caudal axis
- mediolateral axis
- dorsoventral axis
- left right axis
- formation of embryonic body plan
how are malformations such as caudal dysplasia,, caudal regression syndrome, sacral anagenesis and sireomlia caused
errors in gastrulation
dextrocardia
when heart is on right side of body not left
laterality and heart defects
disruption of the laterality pathway in specifying left and right sidedness in the progenitor heart cells causes many different types of heart defects:
- ventricular septal defects
- atrial septal defects
- double outlet right ventricle
ectodermal germ layer
outside
organs and structures
enamel of teeth
oral and anal canal
sensory epithelium of ear, nose, eye
what germ layer forms the neural tube
Induced to form the Neuroectoderm = formation of the neural tube (via nerulation)
Rise of neural crest cells and pituitary gland
mesoderm is made up of what 3 parts
paraxial mesoderm
intermediate mesoderm
lateral plate mesoderm
what are somitomeres
in paraxial mesoderm
segmental blocks (called neuromeres in head in association with neural plates)
what is the mesenchyme
connective tissue in head
what are somites
mature somitomeres
neck (occipital) region downwards = the somitomeres form segmented pair of blocks of mesoderm on either side of the developing neural tube
when and where does the first pair of somites arise
ccipital region of the embryo at 20th day of development
how many somites arise per day
New somites appear craniocaudally at the rate of about 3 pairs per day
how many somites at the end tf the 5th week and where
At end of the 5th week - 42 to 44 pairs of somites formed
4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8-10 coccygeal pairs of somites
how can you tell the age of an embryo
counting somites numbers during a period of development because somites are formed with specific periodicity
(1st pair = day 20, add 3 pairs extra each day)= somite age of the embryo
intermediate mesoderm contributes to what structures
Contribute in the formation of the structures of the urogential system = primordial germ cells, gametes and gonads
lateral plate mesoderm is divides what into 2 layers
parietal (somatic)
visceral (splanchnic)
what does the parietal layer line
intraembryonic cavity walls
mesoderm from parietal layer and ectoderm together form
-The lateral body wall
-The dermis of the skin in the body wall and limbs, bones and connective tissue of the limbs and sternum
-The costal cartilage, limb muscles and most of the body wall muscles
-The mesothelial or serous membranes (parietal layer) that line -The peritoneal, pleural and pericardial cavities and secrete serous fluid
Mesoderm of the visceral layer and endoderm together form
- wall of the gut tube
- serous membrane (visceral layer) that surrounds organs
- blood cells and blood vessels
Endodermal Germ layer
inner layer
epithelial lining of respiratory tract, urinary bladder, urethra and auditory tube
pharynx
walls of gut derived from
mesoderm
how does gastrulation begin
formation of morphogenetic centre = primitive streak - day 15
what does gastrulation involve
movement and migration of epiblastic cells
ingress (invaginate) through primitive streak
form mesodermal germ layers between epiblast and hypoblast
what does formation of mesodermal germ cell layer lead to
transformation of the hypoblast into a endodermal germ cell layer and epiblast = ectodermal germ cell layer
what does hypoblast become
endodermal germ cell layer
what does epiblast become
ectodermal germ cell layer
what is the result of forming primitive streak and completion of gastrulation
establishes major body axes
- cranio caudal
- medio lateral
- dorso ventral
- right to left axes
what is morphogenesis
- shaping processes in an embryo
how is morphogenesis controlled
- controlled by cell behaviours
- result in differential tissue growth
-Cell behaviours = cell shape, size, position, number and adhesivity
what causes dsymorphogenesis
Interference with differential tissue growth in an embryo = occasioned by genetic mutation, tetrogen exposure or a combination
defamation vs malformation
deformation - secondary morphologic defects that are imposed upon an organ or body part due to mechanical forces over a period of time (indirect effect)
malformation = environmental or genetic factors
club foot caused by
Clubfeet due to compression in the amniotic cavity as a result of insufficient fluid (Oligohydramnios)
disruptions vs defects
Disruptions = morphological alterations of already formed structures due to destructive processes
Defects produced by amniotic bands
Cleft lip
something external impacting on normal development = kills and eats up tissue in amniotic band = sawing through upper lip
malformation syndrome
When dysmorphogenesis occurs as a patten of well characterised multiple primary malformations appearing together in a predictable fashion in one or more tissues due to a specific underlying single or common case = Syndrome
Down syndrome
extra chromosome
reduced muscle tone - hypotonia
upwards/ down start to eye
enlarged tongue
hyper flexibility
foetal alcohol syndrome
mother drinking alcohol
short palpebral tissue lengths
thin upper lip
brain damage
teratogen
any factor or agent that causes birth defect or congenital malformation or congenital anomaly
what factors determine the capacity of an agent (teratogen) to produce a birth defect
- genotype of conceptus and maternal genome
- development stage at time of teratogen
- dose and duration of exposure to teratogen
morphogenesis can be
normal development - normogenesis
abnormal development = dysorphogenesis
morphogenesis can be
normal development - normogenesis
abnormal development = dysorphogenesis
tetrogens can
inhibit biochemical or molecular process or pathway or alter cellular behaviours
