Embryology

Question 1

Describe fertilisation.

Answer 1

Occurs in ampulla, the widest part of the uterine tube. Sperm head penetrates ovum wall and releases chromosomal content into egg. Haploid gametes fuse to form a diploid zygote, the sex of which is determined at fertilisation.

Question 2

Describe the process of implantation.

Answer 2

1. Morula (16 cells) at isthmus of uterine tube.
2. Internal cells become embryoblast, which will later form the embryo, and external cells become trophoblasts, which will later form the placenta.
3. Morula becomes a blastocyst when it becomes leaky and cells move to one side with fluid filled cavity.
4. Zona pellucida shed and blastocyst attaches to uterine wall and is implanted.

Question 3

Define gastrulation.

Answer 3

Formation of trilaminar germ discs:

Epiblast and hypoblast become ectoderm, mesoderm and endoderm.

Question 4

Describe the process of gastrulation.

Answer 4

1. Primitive streak formed from caudal proliferating cells migrating centrally. Amniotic cavity above and yolk below.
2. Some cells stay in epiblast and become ectoderm, some cells go to the middle and become mesoderm, and some epiblast cells displace hypoblast and become endoderm.

Question 5

What do the germ layers become?

Answer 5

Ectoderm - epidermis of skin, central nervous system and peripheral nervous system

Mesoderm - blood, bone, muscle, dermis, and connective tissue of respiratory and GI tracts

Endoderm - GI organs, GI epithelium and respiratory tract epithelium

Question 6

What does notochord induce?

Answer 6

Proliferation of paraxial mesoderm and development of neural folds in ectoderm.

Question 7

What are the 2 ways that blood vessels can form?

Answer 7

Vasculogenesis - cells come together to for blood islands and new vessels form from these.

Angiogenesis - new vessels branch out from existing ones.

Question 8

How is the aorta and the endocardial tubes formed?

Answer 8

At cranial end of germ disc, blood islands fuse to form a horseshoe shaped endothelial tube surrounded by myocardial muscle.

• Inner parts of horseshoe forms aorta on each side of the embryo
• Outer parts form the endocardial/heart tube via primitive cell death in a long and complex process.

Question 9

When the heart is developing, what are the 5 swellings that form to initiate this?

Answer 9

Sinus venosus - where blood enters
Primitive atrium 
Ventricle
Bulbus cordis 
Truncus arteriosus - will form arteries

Question 10

How does the heart develop from the 5 swellings?

Answer 10

1. Heart tube contracts, starting in sinus venosus, which will become SAN, and blood flows through.
2. Primitive heart tube folds and sinus venosus forms smooth parts of atria and coronary sinus.
3. Primitive atrium forms rougher more muscular parts of atrium and forms auricle pectinate muscles.
4. Primitive ventricle forms the left ventricle.
5. Bulbus cordis forms right ventricle and part where the aorta and pulmonary trunk begin.
6. Truncus arteriosus forms distal aorta and pulmonary trunk.

Question 11

Describe the process of septal development.

Answer 11

1. Endocardial cushion forms tissue in the middle.
2. Endocardial cushion forms septum intermedium.
3. Septum primum grows down to meet septum intermedium.
4. As ostium primum closes, ostium secundum appears.
5. Septum secundum grows down to the right of septum primum, leaving gap for foramen ovale.

Question 12

What can lead to ventricular septal defects?

Answer 12

Interventricular septum doe not meet septum intermedium and the gap is filled by membrane. This stage is complicated and many things can go wrong.

Question 13

What is tetralogy of fallot and its effects?

Answer 13

Septal spirals do not divide outflow completely. Involves 4 defects:

• A ventricular septal defect - where blood is passed from right to left ventricle.
• Pulmonary stenosis - narrowing
• Narrow pulmonary trunk - more force needed, right ventricle hypertrophy
• Wider aorta

Question 14

In fetal circulation, what is the function of the ductus venosus?

Answer 14

Blood from umbilical vein drains into caudal vena cava via ductus venosus.

Question 15

In fetal circulation, what is the function of the ductus arteriosus?

Answer 15

Blood crosses to the aorta from the pulmonary trunk via the ductus arteriosus.

Question 16

Describe fetal circulation.

Answer 16

1. Oxygenated blood from the placenta gets to the fetus via the umbilical vein.
2. Blood bypasses liver tissue.
3. Blood drains into caudal vena cava via the ductus venosus, and into the right atrium.
4. There is no pulmonary circulation yet so goes to the left atrium via the foramen ovale, then passing to the left ventricle and aorta.
5. Deoxygenated blood returns to the right atrium, to the right ventricle and pulmonary trunk.
6. Crosses to aorta via ductus arteriosus.
7. Blood goes around the body and then to the umbilical arteries.

Question 17

When an animal is born, what changes occur in the fetal circulation?

Answer 17

Animal takes a breath and the lungs inflate. Umbilical vessels and ductus venosus shrivel up. The 2 septums fuse together and hole closes, with remnant fossa ovalis. The ductus arteriosus becomes the ligamentum arteriosum.

Question 18

Describe how the lungs develop?

Answer 18

1. Bud from foregut called the tracheobronchial diverticulum forms 2 separate lungs.
2. Common cranial portion forms trachea.
3. Septum called tracheoesophageal septum rises between trachea and oesophagus.
4. Trachea and oesophagus share an opening near laryngopharynx.

Question 19

Describe how the bronchial tree develops.

Answer 19

Buds keep dividing, forming primary bronchi first, secondary bronchi, tertiary bronchi, and then segmental bronchi.
As terminal parts of the lung develop, respiratory bronchioles and alveoli ducts form.

Question 20

Describe the site of gas exchange at this stage of development.

Answer 20

Blood vessels and capillaries develop but do not yet make contact with alveoli. Lung epithelium is cuboidal, so gas exchange cannot occur over this - cells need to flatten to squamous and capillary network must make contact for respiration to occur across a decreased diffusion pathway.

Question 21

How does mesoderm develop and what does each form?

Answer 21

Into 3 parts:

Paraxial mesoderm - next to axis, forms musculoskeletal system.

Intermediate mesoderm - urogenital system.

Lateral plate mesoderm - forms visceral lateral plate mesoderm, which forms around gut, lungs and trachea.

Question 22

How do pleura develop?

Answer 22

Pleura derive from the structures they touch.

• Visceral pleura derives from visceral plate mesoderm.
• Parietal pleura derived from parietal plate mesoderm.

Question 23

How does the diaphragm develop?

Answer 23

The pleuroperitoneal folds develop and invaginate in from the lateral wall and start to move together. They fuse with mesentery surrounding the aorta, oesophagus, caudal vena cava and septum transversum. Septum transversum forms central tendon of diaphragm. Part of lateral wall develops into muscular periphery of diaphragm.

Question 24

How does a blastocyst form?

Answer 24

Fluid begins to penetrate the zona pellucida between the cells and eventually forms a single cavity, a blastocele and the embryo is a blastocyst.

Question 25

Describe the formation of the embryo and placenta.

Answer 25

1. Cells if the inner cell begin to mass at one pole, the embryoblast. 2. Cells of the outer cell begin to flatten and form an epithelial wall called the trophoblast, which goes on to form the placenta. 3. Epiblast and hypoblast are the 2 layers of the embryoblast. They form a flat disc within the embryo. They go on to form the embryo. 4. Cavity begins to form in the epiblast, which becomes the amniotic cavity. 5. Hypoblast produces additional cells which stat to migrate to form a new cavity, the yolk sac.

Question 26

Describe the process of gastrulation, the formation of the 3 embryonic layers.

Answer 26

1. Formation of a primitive streak, which occurs on the surface of the epiblast , running cranial to caudal. 2. Migrating layers produce ventral layer to change it from 2D to 3D. 3. Cells of the epiblast start to migrate towards the primitive streak, detach and slip beneath them and spread invagination, creating a layer of mesoderm. 4. Cells replacing epiblast layer become the ectoderm. 5. Cells replacing hypoblast layer becomes the endoderm.

Question 27

What is the notochord?

Answer 27

The dip shown in the ectoderm is where the notochord is going to form, which becomes the spinal cord and brain. It gets intercalated into the mesoderm to form a structure called a neuroectoderm.

Question 28

How does the mesoderm further develop?

Answer 28

- Cells of the mesoderm layer form a loosely woven tissue on either side of the midline. - Cells close to the midline proliferate and from a thickened plate called paraxial mesoderm. - Paraxial mesoderm divides into somites, which give rise to the segmental arrangement of the body.

Question 29

What 3 layers of mesoderm form?

Answer 29

Mesoderm remains quite thin and is called the lateral plate mesoderm (has somatic and splanchnic mesoderm layers). An intermediate mesoderm connects the paraxial mesoderm and lateral plate mesoderm. This forms the urogenital and reproductive tracts. The lateral plate mesoderm where it covers the amnion is split into somatic/parietal and splanchnic/visceral layers.

Question 30

How does lateral folding form part of the GI tract?

Answer 30

Splanchnic mesoderm becomes pinched off by this lateral folding and its gets incorporated into the body. It becomes the muscle layer of the gut tube. Endoderm becomes the lining of the gut tube, creating the lumen of the gut tube.

Question 31

How does longitudinal folding form part of the GI tract?

Answer 31

Yolk sac and endoderm layer is pinched in. Front end of the endoderm becomes the buccopharyngeal membrane, which develops the oral cavity. Tail end has the cloacal membrane that will develop into the anus. So gut tube is forming between these 2 points. Foregut and hindgut are complete tubes but the midgut doesn’t have a ventral attachment, as it still goes down into the yolk sac.

Question 32

Describe the development of the stomach.

Answer 32

1. Fusiform enlargement in the gut tube, which will become the stomach. 2. Stomach then enlarges and orientates in different ways/non-uniformly. 3. Gut tube expands in cranial abdomen to form stomach but expands more in dorsal direction producing the greater curvature. 4. Stomach is taken to a more ventral position in this displacement in reorientation, which occurs in 2 axis: rotates around the long axis so bulge becomes more lateral and rotation around the other axis so it twists within the body wall.

Question 33

Describe the development of the mesenteries.

Answer 33

1. Rotation of the gut 90˚ anticlockwise around cranio-caudal axis. Dorsal meogastrium folds back on itself, forming a doble fold called greater omentum. This hangs off the greater curvature of the stomach. 2. Rotation of gut 90˚ anticlockwise around dorso-ventral axis. Oesophagus is pushed to the left and the duodenum to the right.

Question 34

Describe the development of the liver and pancreas.

Answer 34

1. Liver appears as an endodermal diverticulum, starting to jut out at the junction between foregut and midgut. 2. The liver is an outpouching of the gut tube. 3. This outpouching also pouches to form the pancreas. 4. Has a cranial branch that forms gland tissue. This where the hepatic ducts begin forming, which goes on to become the liver. 5. Caudal branch goes on to form the gallbladder and cystic duct. 6. Initially the liver and pancreas are separate but they go on to fuse. But this does leave two separate ducts/papilla that can discharge into the duodenum. 7. Greater pancreatic duct drains both the bile duct and the pancreas because they are both coming from the initial common diverticulum. 8. The minor duodenal papilla is only discharging content from the pancreas, as it initially started on a separate bud.

Question 35

What are the species differences in the pancreatic bile ducts?

Answer 35

* Dog, horse and human – separate openings of common bile duct and accessory pancreatic ducts. * Cats, sheep and goat – pancreatic and bile ducts share common opening into the duodenum. * Cattle and pig – pancreatic ducts fuse to give one pancreatic duct, which remains separate from the bile duct.

Question 36

How does the midgut develop?

Answer 36

1. Due to rapid growth from the yolk sac, the tube actually starts to hang in this loop, forming an elongated mesentery. 2. This allows the midgut to slip out of the abdominal cavity and into what is becoming the umbilical cord in physiological herniation. 3. Allows growth of midgut outside of the embryo until the embryo is large enough to accommodate it. 4. Cranial limb of the loop will become the small intestine. This will grow much more rapidly and begins to rotate. Fixed point through the mesentery is the cranial mesenteric artery. So rotates around the axis created by the artery. It pushes across the abdomen and there is a rotation of 270˚. 5. Has a diverticulum that will become the caecum. 6. Caudal limb is going to become part of the small intestine and colon.

Question 37

Describe the development of the hindgut.

Answer 37

There is a connection at the cloacal plate. - Ectoderm is going to be the mucous membrane of the anal opening and will communicate directly with the endoderm layer of the GI tract. - In horses and ruminants, there is some secondary increase in the length of the descending colon. - In most other species, the only significant thing is the distal terminus, making sure this has broken down and that the new-born is able to pass faeces.

Question 38

Which aspects of the embryo does the urinary tract derive?

Answer 38

Most of the urinary tract, particularly the kidneys, are derived from the intermediate mesoderm at the urogenital ridge. Epithelial lining of the bladder and urethra derive from endoderm, from the cloaca.

Question 39

What are the 3 stages of kidney development?

Answer 39

1. Pronephros – very short 2. Mesonephros – in fish and amphibians/lower life forms, this stage forms the adult kidney 3. Metanephros – in reptiles, birds and mammals/higher life forms, this is where adult kidneys are developed

Question 40

Describe the pronephros stage of developing the urinary tract.

Answer 40

Occurs in cervical neck region so is more or less non-functional at this stage. Though some limited excretion can occur. Ducts pass from them and drain into the cloaca regions.

Question 41

Describe the mesonephros stage of developing the urinary tract.

Answer 41

- Develops along the urogenital ridge and lumbar region of the embryo - This is a functional kidney but will eventually be replaced by the 3rd stage kidney - Develops a very simple nephron structure that are supplied by arteries from the aorta - This can then become functional - Mesonephric duct drains any excretions into the cloaca - Paramesonephric duct develops at the same time and develops parallel to the mesonephric duct

Question 42

Describe the metanephros stage of developing the urinary tract.

Answer 42

- Develops from the urogenital ridge in the lumbar region - Forms the majority of the main part of the kidney - Ureteric bud off from the mesonephric duct goes on to form the ureter. It pushes its way into the metanephros and becomes larger, forming the collecting ducts, calices and pelvic ducts (if present in that species). - The developing metanephric tissue forms around the collecting ducts (mesonephros in origin) - Developing nephron will eventually join together with the collecting tubules - Blood vessels the nephron at the Bowman’s capsule to form the glomerulus - The kidney starts to become functional and the mesonephros/2nd stage will start to disappear, as it is not needed anymore

Question 43

Which aspect of the embryo is the bladder derived from?

Answer 43

Unlike the kidneys, the bladder develops from the gut tube, which is endoderm in origin. - So, like the gut, the muscular wall is derived from lateral plate mesoderm, which has somatic and splanchnic/visceral parts - Muscle surrounding bladder derived from splanchnic mesoderm - Lining of the bladder is from endoderm

Question 44

What is the urorectal septum in the development of the bladder?

Answer 44

The hindgut and rectum area divide from the forming bladder. There is a septum called the urorectal septum, which divides the rectum from the urinary tract. - The septum pushes its way down between these 2 structures - Ends up with developing bladder and a separate bladder - Thing can go wrong with incorrect separation or holes developing

Question 45

Describe the development of the bladder.

Answer 45

- Urethra and developing bladder will form from the allantoic bud - Most caudal part is where the urethra is going to develop and in the female, it will develop all of the urethra as it is short in the female. In males, it only forms the proximal part, as the urethra section running through the penis is formed from a different part. - Central portion expands to form the bladder itself - Most cranial part lead to the allantoic cavity into the foetus. Has urachus that closes later on development

Question 46

What is the cloaca?

Answer 46

Cloaca is common to the urinary, gastrointestinal and reproductive tracts, as they all open up to this area. In reptile and bird species, this area remains but in mammals it separates.

Question 47

Which aspect of the embryo does the bladder trigone derive from?

Answer 47

Trigone area has a different origin, which is important, because any tumours that from here will be different. It is mesoderm in origin due to convergence of mesonephric duct and bladder wall.

Question 48

What are the embryological features of the urinary tract?

Answer 48

* Median vesical ligaments – cranial tip of the developing bladder. In this is the urachus. * Lateral vesical ligaments, the umbilical arteries will eventually die back and from these. Sometimes the urachus fails to close and this is called a patent urachus. Urine drips out of the umbilical area. Sometimes the urachus can partially close and form a urachal diverticulum, which can be a possible site for infection.

Question 49

What are some disorders that can arise from development of the kidneys?

Answer 49

- Multiple renal arteries - Ectopic kidneys are ureters - Renal agenesis - 1 or both of the kidneys do not develop - Horse-shoe kidney - kidneys fuse, normally at caudal pole - Cystic kidneys - can result in urine collecting in kidney - Hypoplasia - fewer nephrons/lobules/calices develop - Duplication - 2 ureters drain the kidney rather than 1