15. Embryonic Development (TT) Flashcards
What part of which germ layer do the urinary and genital systems originate from?
Intermediate mesoderm
Compare excretion in humans and aquatic vertebrates, and how this relates to the structure of the urinary system.
Aquatic vertebrates:
- Ammonia is used mostly, since it is simple to produce
- However, it is toxic, so lots of water is required to excrete it, and therefore it is only really used by aquatic animals
Humans:
- Urea is used
- The kidney is more important in terrestrial vertebrates because it allows excretion while conserving animals
Summarise the two important functions of the kidney.
- Removal of waste
- While conserving water
What are the three stages of kidney development in mammals?
[EXTRA]
There is a progression of 3 stages:
- Pronephros
- Mesonephros
- Metanephros
These form in the craniocaudal sequence.
What happens to each of pronephros, mesonephros and metanephros in humans?
- Pronephros and mesonephros degenerate during embryogenesis
- Metanephros forms the definitive kidney (IMPORTANT)
The definitive kidney forms from…
The metanephros of intermediate mesoderm.
What is the cloaca?
- It is the point at the caudal end of the embryo through which the urinary, intestinal and genital systems exit.
- Humans have a cloaca during development, but it later partitioned into a separate rectum and bladder.
What are the mesonephric ducts in kidney development?
- Two epithelial tubes that run craniocaudally along the entire embryo
- There are formed by mesenchymal to epithelial transitions in the intermediate mesoderm
- They drain the mesonephros in the embryo into the cloaca, and they give rise to the ureteric buds in the formation of the metanephros
Describe the formation of the pronephros, mesonephros and mesonephric ducts. [EXTRA]
Pronephros and mesonephric ducts:
- Near the start of the 4th week, mesenchymal to epithelial transitions in the intermediate mesoderm lead to the formation of two epithelial tubes, known as mesonephric ducts. They begin in cervical region, but inductive mechanisms cause them to extend in the caudal direction.
- Simultaneous to this formation, the pronephros forms also form in the cervical region of the intermediate mesoderm, medial to the mesonephric ducts.
- The pronephros takes the form of a series of hollow epithelial buds, but it is not functional and within two days of its formation, it begins to degenerate.
Mesonephros
- As the mesonephric ducts develop caudally, they induce the intermediate mesoderm caudal to the pronephros to form into mesonephric buds, which begins around day 25.
- Within each bud, mesonephric tubules form, forming a total of around 40 tubule pairs, although earlier tubules regress as more caudal ones develop, and by the end of the 5th week there are only 20 pairs (in the lumbar region).
- Each of these tubules is similar in structure and function to the nephrons in the definitive adult kidney. The most cranial mesonephric tubules fuse on their lateral side to the mesonephric ducts, which have extended towards the cloaca and fused with its ventral side (around day 26), draining the tubules.
- The tubules are functional and produce urine between approximately the 6th and 10th weeks, although in females they regress. In males, the mesonephric duct forms the genital duct in the adult.
When does the metanephros start to develop?
Around the end of week 4 (day 28).
What are the two portions of the definitive kidney? What part of the intermediate mesoderm is each derived from?
- Excretory portion (nephron) -> Metanephric mesenchyme
- Collecting portion -> Ureteric bud
Describe the formation of the metanephros.
- The metanephros begins to develop around the very end of week 4 with the formation of the ureteric buds at the caudal section of each mesonephric duct. This occurs due to induction from the metanephric mesenchyme, which is intermediate mesoderm in the sacral region.
- The ureteric buds each enter the metanephric mesenchyme occurs by day 32.
- The epithelial-mesenchymal interaction between the ureteric bud (epithelial) and metanephric mesenchyme induces the ureteric bud to begin branching (branching morphogenesis). Bifurcation continues until around the 32nd week.
- There is also a reciprocal induction, which is the way in which the ureteric buds induce the mesenchyme to form glomerular units.
- The branching forms the ureter, pelvis, major and minor calyces, and collecting tubules (see other flashcard for this)
What two tissues does the metanephros form from?
Ureteric bud and surrounding mesenchyme
Describe how these structures are formed by the branching of the ureteric bud:
- Pelvis
- Major and minor calyces
- Ureter
- Collecting ducts
- Ureter -> This is the part of the ureteric bud before any branching occurs
- Pelvis -> Formed by the first bifurcation
- Major calyces -> After 4 generations of branching by simple bifurcation, the branches formed coalesce to form the major calyces
- Minor calyces -> Further branches coalesce to form the minor calyces
- Collecting ducts -> Form at the tips of the minor calyces by further branching
Is the process of branching of the ureteric bud continuous?
No, there are periods of branching and periods of coalescing (which are required to form the calyces).
Describe how nephrons develop in the metanephros.
- First appear around 10 weeks, in the distal region of the metanephros
- The ampullae at the tip of each collecting duct interact with adjacent mesenchyme and induce nephron formation.
- Nephric vesicles form ‘comma’ and then ‘S’ shaped tubules. These ultimately form a Bowman’s capsule, proximal convoluted tubules and loops of Henle.
- Ampullae extend deeper into cortex and continue to branch, inducing the formation of more nephrons .
What is Wt1 and what is its importance?
- Wilm’s tumour suppresor gene
- Expressed in the early metanephric mesenchyme for induction of other tissues
- It is required for the initial formation of the ureteric bud from the mesonephric duct
What is Wilm’s tumour?
- A form of kidney cancer that is common in children
- It is frequently caused by alterations to the Wt1 gene
- Results in a palpable mass, pain in the abdomen, poor appetite and fever.
- It is highly responsive to treatment.
Aside from Wt1, what are two factors that are important in inductive processes in kidney development? What does each do?
- Ret (expressed in the ureteric bud) -> Important in branching morphogenesis + Transduces extracellular signals to inside the cell by phosphorylating tyrosine in downstream targets
- Gdnf (expressed in the metanephric mesenchyme) -> Important in branching morphogenesis + It is the ligand for Ret receptor
They have strikingly complementary expression in the two tissues.
When in development do the kidneys ascend and how does this happen?
- Between the 6th and 9th weeks, the metanephric kidney ascends to the lumbar region from its original caudal position
- Caused primarily by differential growth -> The kidneys stay in place while the embryo lengthens
- It remains retroperitoneal (behind the peritoneum)
- It changes its blood supply to progressively higher segmental arteries (branches from the aorta) as it ascends
What happens when a kidney fails to ascend in development?
[EXTRA]
It can result in a pelvic kidney.
What is horseshoe kidney?
[EXTRA]
When the kidneys fuse before they ascend, so they are trapped under the inferior mesenteric artery.
What does the bladder form from?
- Lower end of allantois
- Caudal ends of mesonephri ducts
- Urogenital sinus
What is the allantois?
It is an outgrowth from the hindgut.
How does the allantois relate to the cloaca?
The lower allantois is continuous with the cloaca because the allantois is an outgrowth of the hindgut.
What are the parts of the cloaca and how are they formed?
- Urogenital sinus -> Goes on to form part of the bladder (UG sinus is continuous with the allantois)
- Anorectal canal -> Connected to the GI tract
The urorectal septum separates these two.
Describe the formation of the bladder.
- The urogenital sinus (more cranial part of the cloaca) grows larger, forming part of the bladder
- The urorectal septum grows to separate the urogenital sinus from the anorectal canal (meaning that in the future there is a separate exits for the urogenital and digestive tracts)
- The lower part of the allantois also goes on to form part of the bladder because it is continuous with the urogenital sinus
- The ends of the mesonephric ducts merge into the developing bladder on the dorsal side, bringing the opening of the ureter into the bladder wall
Describe the formation of the trigone on the bladder.
- Between weeks 4 to 6, the distal parts of the mesonephric duct and forming ureter are absorbed into the endoderm-lined bladder, forming the mesodermal trigone.
- However, UB derived cells subsequently undergo apoptosis and are replaced by bladder derived cells -> This means that the lining of the bladder is ultimately mostly endodermal, despite having had mesodermal contribution at some point
Describe the formation of the urethra.
- While the superior portion of the urogenital sinus forms the bladder, around day 53 the inferior portion forms the pelvic urethra
- In males, the pelvic urethra forms the membranous urethra in females, and the membranous and prostatic urethra in males
- The distal expansion of the urogenital sinus forms the vestibule of the vagina in females, and the penile urethra in males
When is the development of the metanephric kidney finished?
It is functional by 12 weeks.
What do the kidneys produce in the embryo?
The amniotic fluid -> Supports the foetus from mechanical shock
What is the foetal excretory organ?
Placenta -> Nitrogenous waste in the fetal blood is transferred across the placenta to the maternal blood
Describe a morphological change in the kidneys that occurs after birth.
- Before birth, the kidneys produce a large amount of dilute urine
- After birth, the loops of Henle lengthen so that the urine becomes more concentrated
For development of the genital system, see flashcards on section 13.
Do it!
What germ layer is the gut tube formed from predominantly?
Endoderm
The gut tube is formed from the endoderm. How is this germ layer formed?
During gastrulation, cells of the epiblast colonise the hypoblast to form the endodermal layer.
Describe the formation of the primitive gut tube and how this then differentiates.
During the 4th week, embryonic folding occurs:
- The amnion folds down around the embryo, squeezing the neck of the yolk sac.
- The anterior edges of the amnion fuse, forming the body cavity and the gut tube is the endoderm-lined space inside.
Draw how the yolk sac changes shape during development.
- Initially, the yolk sac has a goldfish bowl shape
- When the foregut and hindgut form due to embryonic folding, the tube narrows down leaving a vitelline duct
What does the vitelline duct connect in development?
Yolk sac and the mid-part of the gut tube.
The fusion of the amnion’s anterior sides during embryonic folding surrounds the embryo in what? Show this on a diagram.
It surrounds the embryo in amniotic fluid.
Label this.
Embryonic folding results in the formation of what cavity in the embryo?
Intraembryonic cavity a.k.a. coelom or coelomic cavity
What germ layer is the dorsal mesentry made of?
Mesoderm
Of what germ layer origin is the lining of the gut tube?
Endodermal
After embryonic folding, what are the two types of mesoderm in the coelomic cavity?
- Visceral (splanchnic) mesoderm lines the outside of the gut tube
- Parietal mesoderm lines the body wall
After embryonic folding, what is the inner and outer lining of the gut tube?
- Inner lining -> Endoderm
- Outer lining -> Visceral mesoderm
What structure partions the coelomic cavity and what does it go on to form? [IMPORTANT]
- Septum transversum
- It forms the diaphragm
Which side of the body wall is the gut tube attached to and what by?
- Dorsal body wall, by the dorsal mesentry
- It is also attached to the ventral body wall at the level of the stomach and liver by the ventral mesentery, within which the liver develops
Across which cavity does the septum transversum form?
Coelomic cavity
Draw the formation of the septum transversum.
Note that the gaps at the posterior side are filled by pleuroperitoneal membranes that complete the septum, which forms the future diaphragm.
What are the three main sections of the gut tube? What does each comprise?
- Foregut
- Stomach
- Liver
- Pancreas
- Part of the duodenum
- Midgut
- Small bowel
- Up to halfway along the descending colon
- Hindgut
- Last part of the descending colon
- Rectum
Draw a diagram to show the different structures in each segment of the gut in the developing embryo.
Describe the important structures of the gut tube that are found at the each end of the gut tube.
At the rostral end:
- Gut tube opens at the mouth (stomodeum)
- Buccopharyngeal membrane -> This is a segment where the endoderm is in direct contact with the ectoderm
At the caudal end:
- Gut tube opens at the anus (cloaca)
- Cloacal (anal) membrane -> This is a segment where the endoderm is in direct contact with the ectoderm.
What does the cloacal (anal) membrane end up as in after development?
Hilton’s white line in the anal canal.
What are the two important sites of direct contact between the endoderm and ectoderm in the gut tube that you need to know about?
- Buccopharyngeal membrane (at cranial end)
- Anal (cloacal) membrane (at caudal end)
How does patterning of the gut tube occur?
- Differential expression of hox genes in the early developing gut mesoderm -> Causes differentiation of the mesoderm.
- Differences in gut epithelial structure are thought to arise by interactions between the mesoderm and endoderm -> i.e. the mesenchymal structures induce the formation of corresponding epithelial structures
How many hox genes are involved in patterning of the gut tube?
In mammals there are four sets (hox a-d) and they are expressed in a rostral-to-caudal pattern along the developing gut tube.
What is some experimental evidence for the importance of hox genes in patterning the gut tube?
In situ hydridisation can be used to show the different levels of hox gene expression at different points along the gut tube.
In what part of the gut tube are hox genes expressed? What is the result of this?
- In the mesoderm that surrounds the endodermal lining of the gut tube
- This means that the mesoderm induces the endoderm to form different structures
In gut tube development, is it the surrounding mesoderm that patterns the endodermal lining of the gut tube, or is it vice versa? What is the evidence for this?
- This means that the mesoderm that surrounds the endodermal lining of the gut tube induces the endoderm to form different structures
- The evidence from this comes from placing a section of mesoderm from a different part of the body near a section of endoderm from a different part of the gut tube -> The mesoderm induces formation of the endoderm from the corresponding organ
Describe the development of the very cranial end of the gut tube (i.e. the pharynx).
- The mouth forms at the rostral end of the gut tube at the stomodeum
- The pharyngeal specialisations (e.g. tongue) develop from the pharyngeal arches and pouches
Remember to revise the structures that are derived from the pharyngeal pouches that relate to the gut tube differentiation.
- The pharyngeal pouches are endodermal structures, so they are involved in the formation of many of the upper gut tube structures.
- See the flashcards on this.
What are the buccopharyngeal and cloacal membranes?
- Points of direct contact between the endoderm and the ectoderm
- They go on to rupture and form the two ends of the alimentary tract.
- At the rostral end, the gut tube opens at the stomodeum (the site of the future mouth), while at the caudal end it opens at the cloaca (the future anus).
Describe the formation of the lungs. [IMPORTANT]
- Just caudal to the pharyngeal pouches, there is a single midline bud from the gut tube that goes on to form the lungs.
- Just like in patterning of other gut tube structures, the surrounding mesoderm is involved in patterning the endodermal lining of the gut tube:
- Wnt signals from the surrounding mesoderm cause local up-regulation of Nkx2.1 in the endoderm
- This Nkx2.1 is the first signal of bud formation
- Bud growth is stimulated by FGF10 (and BMP4 helps)
- Shh confines action of the FGF10 to the tip of the bud, so that there is outgrowth rather than growth in all directions
- When the bud is of a certain size, ECM deposition (of fibronectin and HSPG) along the midline causes branching morphogenesis
What is the name of the process that occurs during lung bud outgrowth? What signalling molecules allow this process to occur?
- Branching morphogenesis
- FGF10, BMP4, Shh
What sort of developmental abnormalities of the lungs can occur?
Since the lungs bud from the gut tube in development, the two may remain connected in an abnormal way. For example, the oesophagus may remain connected to the trachea at birth.
What defines what is foregut, midgut and hindgut in development?
Each section has its own blood supply.
Describe the blood supply to the foregut, midgut and hindgut in development.
- Foregut -> Celiac artery
- Midgut -> Superior mesenteric artery
- Hindgut -> Inferior mesenteric artery
On the diagram, the brown is the dorsal mesentery, while the yellow is the gut tube!
Describe the venous drainage of the different sections of the gut.
It is all via the hepatic portal vein.
At what level is there also a ventral mesentry that connects to the gut tube? What are the different parts?
- It is at the level of the stomach and liver
- The two parts are:
- The part between the stomach and liver -> Lesser omentum
- The part between the liver and the anterior abdominal wall -> Falciform ligament
Label this.
It is worth noting that in the embryo the liver is closely apposed to…
The diaphragm.
Which part of the gut tube does the stomach form from and what is its blood supply?
Foregut -> Supplied by the celiac artery.
Describe the rotation of the stomach.
Forms from the foregut. It involves two rotations:
- Clockwise rotation (90*) around the craniocaudal axis, with the left side of the stomach becoming anterior.
- Anticlockwise rotation around the dorsoventral axis due to the lengthening of the left side of the stomach.
What structures form as a result of the rotation and expansion of the stomach?
- Lesser sac
- Greater omentum -> Fuses to the mesentery that surrounds the transverse colon.
What is the effect of the first stomach rotation on nerve position?
Moves the left and right vagus nerves to anterior and posterior position respectively.
Describe and draw the formation of the lesser sac, greater sac and greater omentum. [EXTRA]
- The rotation of the stomach 90* clockwise causes the dorsal mesentry to start folding
- Eventually this forms a pocket (called the lesser sac) with just a small hole (epiploic foramen) keeping it continuous with the remainder of the abdomen
- The dorsal mesentery is also affected by this rotation and bulges out into the abdomen.
- The two sides of the dorsal mesentery later fuse to form a sheet of mesentery attached to the greater curvature of the stomach and to the transverse colon, called the greater omentum.
Just watch this video:
https://www.youtube.com/watch?v=nTWjL69IwT0
Which mesenteries are the greater and lesser omenta derived from?
- Greater omentum -> From the dorsal mesentery
- Lesser omentum -> From the ventral mesentery
What is the greater omentum and what heppens to it?
- It is sheet that forms when the dorsal mesentry is stretched by the stomach rotatation, begins to droop down and becomes double folded.
- It then fuses with the mesentry surrounding the transverse colon.
When does differentiation of the gastric mucosa occur and how?
- Around week 8 -> Hydrochloric acid production begins prior to birth.
- Differentiation requires epithelial-mesenchymal interactions
What signalling factors are important in the differentiation of the gastric mucosa?
- Hox B6 is expressed in the underlying stomach mesoderm
- Sox9 is expressed in the differentiating epithelial layer
What is pyloric stenosis? What are the symptoms? [IMPORTANT]
- Hypertrophy of the muscle that surrounds the pyloric sphincter
- It can result in projectile vomiting
Which organs form from gut diverticula that undergo branching morphogenesis?
- Liver
- Pancreas
- Lungs
What drives the formation of the liver?
FGF signals from the adjacent cardiac region.
Describe the formation of the liver.
- The liver forms due to FGF signalling from the cardiac region inducing a hepatic field in the endoderm at around the level of the foregut-midgut junction
- In the 4th week, a hepatic diverticulum develops, which gives rise to:
- Hepatic bud
- Forms the liver by branching morphogenesis
- More cranial
- Cystic bud
- Forms the gall bladder
- More caudal
- Hepatic bud
- Bile production starts around week 12 and the liver also has a major role in haematopoiesis in the developing embryo.
When does the hepatic diverticulum arise?
In week 4.
What does the expansion of the liver lead to?
- As it grows into the septum transversum, the mesoderm of the septum transversum becomes the ventral mesentery.
- The ventral mesentery will form the lesser omentum and the falciform ligament.