Embryology parts 1-4 Flashcards
When is conceptus referred to as foetus
8 weeks
For how long does development within fallopian tube usually occur
Preimplantation development normally occurs within the Fallopian tube (oviduct) over a period of ~6 day
What development happens in fallopian tube
Define morula
Series of cleavage divisions, which sequentially double the number of cells in the conceptus (2, 4, 8, 16 cells) to produce a ball of undifferentiated cells (the Morula)
What does the morula differentiate into
First level of differentiation, into
BLASTOCYST
What is blastocyst
From morula.
Still has zona pellucida
Outer layer of cells=trophoblast
Inner cell mass
Fluid-filled cavity
T/f the zona pellucia implants into the lining of the uterus
F:
What happens to blastocyst
The Blastocyst then hatches from the Zona Pellucida (within which it has developed up to this time, about day 6 after fertilisation), and begins to implant in the uterine lining
By what day is the blastocyst implanted
complete about 10 days post-fertilisation
What is the structure of the blastocyst after it has implanted into uterus
he inner cell mass, which was a group of undifferentiated cells has become a bilayer disk, composed of hypoblast and epiblast cells
What is the importance of the bilayer in the blastocyst
his bilayer disk gives rise to all the tissues of the human fetus, through a complex series of changes.
What is the first change that occurs to the blastocyst
Gastrulation. Day 14-18 PA
Converts bilayer–> trilaminar embryo with 3 layers of germ cells:
- Ectoderm
- Mesoderm
- Endoderm
How are each of the 3 layers formed in gastrulation
The ectoderm originates from the epiblast cells.
Epiblasts also differentiate into mesoderm cells.
These mesoderms cells move into the space between epiblast and hypoblasts.
The mesoderm cells form mesoderm layer, but also differentiate into endoderm cells.
Hypoblast layer lost by apoptosis
What does ectoderm give rise to
skin and the central nervous system
What does mesoderm give rise to
muscles, blood, skeleton, heart and kidney
What does the endoderm give rise to
gut, lungs and liver
T/F tissues usually originate from a single germ layer type
F…. Muscular and vascular tissue are generally of mesodermal origin, so tissues are normally a mixture of germ layer types (e.g. muscle in the skin and gut).
What stage is next from gastrulation
Neurulation. Occurs before gastrulation is complete.
What is neurulation. What controls it
ifferentiation of the Ectoderm (Epiblast) to generate the central nervous system (Brain and Spinal cord), under the control of the notocord in the mesoderm of the developing embryo.
Outline the formation of the neural tube
development of the neural plate on the ectoderm; this develops two folds, which increase in size until the meet over the neural groove and fuse to form the neural tube
When does fusion of the neural plate occur
during week 4 of development.
Now the neural tube is formed
What cell groups are outside the embryo proper
primordial germ cells (PGC) –> in yolk sac endoderm at caudal end
cardiac and vascular progenitors–> in primary heart field at cranial end
Where do each of the cell types that were outside the embryo proper move to
PGCs to hindgut
Heart progenitors under head of embryo
Which folding occurs in the third week of development
LATERALLY –> the sides curve round to fuse at the front (ventral midline)
ANTEROPOSTERIOR –> folds the PGCs into the hind gut, and the developing heart progenitors under the head of the embryo
What sacs are there, dorsal and ventral to embryo
Dorsal is the amnionic cavity
Ventral is the yolk sac
What happens in the transverse plane in day 19-28
Fig 5.2.8
You can see the ecto, endo and meso derm
The ectoderm is undergoing formation of neural tube
The amniotic cavity is just an extension of ectoderm
The yolk sac is continuous with the endoderm
There is a visceral part of mesoderm which is in contact with the endoderm/yolk (i,,e the pink layer around the yellow from the first image)
Then there is a somatic or parietal layer of mesoderm which comes to surround the endoderm and the visceral layer
Look back to year 1 abdo anatomy
Note that in the foregut and the hindgut, it will look like E and the gut tube closes
In the midgut, there is still connection to the yolk sac, which can be seen in D
What happens in longitudinal plane
Image 5.2.9
Left is cranial
Right is caudal
The top of the image is dorsal and the bottom is ventral
You can see how the heart progenitors have moved from the angiogenic cell cluster (outside of embryo proper) into the embryo by AP folding
You can also see how there is movement of the PGCs from outside to behind the foregut
When is all the lateral and transverse foling occurring
Week 3-4
By what time have the precursors of all internal tissues have been laid down
End of week 4
Development of what happens in month 2 of development
Urogenital, cardiac, facial and lung and limb
Where do the limbs grow out from
lateral plate mesoderm rapidly under control of special signalling regions
When does limb formation begin and end
Forelimb bud appears at d27/8
Hindlimb bud at d29
both fully formed d56
What was thalidomide and what did it effect
Morning sickness drug
Affected upper limb most
Mechanism of thalidomide action
it damages developing blood vessels, thus depriving the adjacent cells of nutrients and preventing their proper growth and development
Why did thalidomide affect upper limb
In humans, it seems that the timing of thalidomide administration (8 weeks of pregnancy onwards, as this is the starting point for severe cases of morning sickness), matches with upper limb development (Figure 5.3.1), 6 weeks post-fertilisation.
upper limb blood vessels seem to be particularly sensitive to thalidomide,
What could thalidomide be used for now
thalidomide has been found to be a useful and effective treatment in some cancers, and also for some complications of leprosy
Where do digits arise from
Digital rays on the flat hand plate
These grow out in response to sonic hedhog protein
If the sonig hedghodg protein is replicted, you could get mirror image of a hand
Stages of kidney development
Pronephros is the most immature form of kidney
Mesonephros, an intermediate phase
Metanephros is most developed and persists as the definitive adult kidney.
What do gonads arise from
intermediate mesoderm within the urogenital ridges of the embryo
What do genital ducts arise from
paired mesonephric and paramesonephric ducts
T/F embryonic gonads begin differentiating immediately after fertilisation
F … Gonads show no differentiation in development until about Week 7 post fertilisation
What is development of male reproductive system dependent on
he activity of sex-determining region Y (SRY) protein, coded for by the SRY gene on the Y chromosome.
Which ducts give rise to female and male genital ducts
The mesonephric ducts give rise to MALE genital ducts
The paramesonephric ducts give rise to FEMALE genital ducts
Primative forms of kidney do not contribute to final kidney that develops from metanephros T/F?
During human development, primitive forms of kidney develop (pronephros and metanephros), which do not contribute to the final kidney that develops from the metaneophros
Where do the kidneys move from and do during development and when
When are the gonads developing
Weeks 6-9
You have the mesonephros (which DOES NOT become the kidney, it is just a primative form) attached ti the bladder via mesonephric duct (from which male gonads develop)
Then the kidneys (arise from metanephros) are connected to the bladder by ureters. They are receiving blood from bifurcation of aorta
The kidneys then ascend up along the mesonephros and form a new arterial supply at a higher up level. The blood supply comes from furhter and furhter up the aorta as the kidneys move up (i.e. former renal arteries degraded).
They end up immediately under the suprarenal glands
As kidney is moving up, the mesonephros is shrinking, and then gonads developed from the mesopnephric duct which the mesonephros was attached to
Differentiate the kidney and the ureter during development
he ureters, which connect the kidneys to the bladder, extent in length during this process, retaining the kidney-bladder connections
in contrast the kidneys form new connections with the developing arterial system as they move, so that renal arteries break down and re-form during this process.
Outline renal abnormalities in development
One kidney may be retained in the pelvis
Retention of an extra artery (or another problem) may obstruct (partly or fully) the ureter, and cause enlargement of the renal pelvis
kidneys form separately, but may fuse to form a horseshoe kidney (Figure 5.4.3B); the extra tissue makes it impossible for it to move, so it will remain
T/f one functional kidney always sufficies both in development and adulthood
F:
All these abnormalities may compromise kidney function. In an adult, one functional kidney may suffice, but this may not always apply during development.
What is the wolffian system
The mesonephric ducts give rise to MALE genital ducts (Wolffian system)
What is the mullerian system
The paramesonephric ducts give rise to FEMALE genital ducts (Mullerian system)
T/f the gonads and reproductive tracts are different from the start of development
F:
The gonads and reproductive tracts are indifferent up until 7 weeks of development; differentiation is influenced largely by the presence or absence or SRY (on the Y chromosome)
What happens if SRY +, what if -
If SRY+, then development proceeds along the male path
If SRY-, then development proceeds along the female path
Where do the ducts involved in genital tract development come from.
Where do the gonad precursors develop
What are gonad precursors surrounded by
So the mesonephric and paramesonepric will become genital tracts, and then there are precutrsors which bvecome the gonads too!
Within the mesonephros, the mesonephric and paramesonephric ducts develop, and are readily identifiable by week 5 post fertilisation
At the same time, the gonad precursor is developing from the mesonephric mesoderm, and is covered by coelomic epithelial cells.
in 5.4.4 diagram just look at where the mesonephric and paramesonephric ducts are
The paramesonephric duct is medial to the mesoneprhic duct
t/f the gametes which will end up in the gonad, develop in the mesonephric mesoderm, along with the gonads themselves.
f In parallel with the developing reproductive tissues, the primordial germ cells (PGC) are following a separate developmental pathway.
Which layer do the gametes develop from
They develop different from the gonads
In parallel with the developing reproductive tissues, the primordial germ cells (PGC) are following a separate developmental pathway.
PGC will give rise to the gametes within the gonads
They originate in the epiblast
Where do PGCs migrate after developing in the epiblast
migrate to the caudal part of the yolk sac
Once the main caudal structures of the embryo proper have developed
the PGC migrate through the hind-gut and dorsal mesentery to the mesonephros and thence to the developing gonads (a result of antero-posterior folding, remember!)
What does the indifferent gonadal system look like and when is it formed by
By week 7
Coming off of the urogenital sinus (basically the cloaca, separates into urogenital sinus and the anus just before this. UG sinus gives rise to bladder and then urethra and other stuff in women), so the mesonephric duct comes from the UG sinus and into the mesonephric mesoderm (Wollfian)
The paramesonephric duct comes off the UG sinus (or bladder shown in other images) next to the mesonephric mesoderm (mullerian)
In the presence of SRY, when will male gonadal development start and why
Weeks 7-8 (weeks 9-10 gestational age)
Because hCG levels peak at this time, and they stimulate the testosterone needed for male development
What are the key regulators of male gonadal devlopment
- Testosterone from Leydig cells (under stimulation from hCG from maternal circulation). Support development of wollfian ducts, which give rise to male reproductive tract
- AMH (anti-Mullerian hormone) produced from sertoli cells, to regress mullerian ducts
What causes regression of mullerian ducts in males
estis Sertoli cells produce anti-Mullerian hormone (AMH), which causes the regression of the Mullerian (paramesonephric) ducts.
When does the female gonadal development begin in the absence of SRY
8-9 post fertilisation (later than men, which is 7-8 weeks)
What is an important factor in the development of the early indifferent genitalia
The key regulator seems to be dihydrotestosterone (DHT), a potent androgen that is produced from testosterone originating in the Leydig cells of the testis.
Most common maldevelopments in males
(a) the inability to produce the appropriate hormones (testosterone and anti-Mullerian hormone (AMH)
or
(b) the inability of target tissues to respond to these hormones (cognate receptors)
What causes an inability for male embryo to respond to AMH or testosterone
defects in the cognate receptors.
What is androgen insensitivity syndrome
- No virilisation of external genitalia (they look like female); lacking mesonephric (woolfian) ducts; testis structure variable; do not descend
DUE TO mutant androgen receptor
- AMH from sertoli cells normal, so mullerian (paramesonephric) ducts regress. No female structures such as uterus of oviducts are present.
What is congenital adrenal hyperplasia
IN ENDO!!!
Mutation in cytp450 21 hydroxylase.
Means limited cortisol production from adrenal fetus.
Lack of cortisol leads to ACTH output from piutuitry, and overstimulation of foetal adrenals, which make eak androgens (e.g. androstenedione)
Cause partial virilisation of genitalia
Why is only genitalia and not internal systems affected in congenital adrenal hyperplasia
The internal systems are female, as there is no SRY (no testicular development), no male ducts (no testosterone) and female ducts develop (no AMH, as no Sertoli cells).
