Embryology Flashcards
What is Gametogenesis?
The process by which male and female germ cells become spermatozoon and oocyte respectively
Where and when does fertilisation occur?
Ampulla of the fallopian tube, usually 12 hours post ovulation
when are the meiotic and first mitotic divisions complete?
Meiotic is complete after fertilisation
the first mitotic division occurs 30 hours post-fertilisation
What stage is described at day 2 post-fertilisation?
2 cell stage - known as a zygote
What happens by day 3 post-fertilisation?
Rapid cleavage forms a MORULA which is a 16+ cell stage
-each individual cell is known as a BLASTOMERE
-migration has moved the cell to the uterine cornua
-at this stage, each blastomere is PLURIPOTENT
What has formed by day 4 post-fertilisation?
a BLASTOCYST
-composed of an Outer cell mass which forms the Trophoblast
-an inner cell mass which forms the Embryoblast
-an empty cavity known as the Blastocele
By day 6 what has happened?
The blastocyst makes initial contact with the uterine wall
-the embryoblast becomes the BILAMINAR DISC composed of Epiblast and Hypoblast
-the trophoblast becomes the Syncytiotrophoblast and the Cytotrophoblast
What occurs on day 8?
Decidualisation reaction starts with implantation
the Epiblast forms an Amniotic Cavity
What happens by day 9?
Syncytiotrophoblasts start to merge and becomes Lacunar stage
The hypoblast forms a Primitive Yolk Sac which is lined by Exocoelomic Membrane (also known as Heuser’s membrane)
What happens by day 12?
The syncytiotrophoblast has formed Lacunae which interact with endometrial sinusoids to establish blood flow
Yolk sac cells form an Extra-Embryonic Mesoderm layer (between the exocoelomic layer and cytotrophoblast)
Within the extra-embryonic mesoderm, cavities called Extra-Embryonic-Coelom or Chorionic Cavities form
When is implantation complete? and what else starts to occur at this time?
By Day 13
Cytotrophoblast starts to form Primary Chorionic vili
B-HCG is secreted by syncytiotrophoblast
What happens at day 16?
1) The allantoic diverticulum forms which forms the posterior wall of the yolk sac
2) the process of neurulation begins (formation of the brain and spinal cord)
What does the epiblast and the hypoblast become?
Epiblast becomes Ectoderm
Hypoblast becomes Endoderm
There is a potential space between them where the (Intra-embryonic) mesoderm will form
What is a Prochordal and Cloacal plate?
Two depressions of the Bilaminar disc where the ectoderm and endoderm are in contact.
They provide orientation with the Prochordal being the cranial end and the cloacal being caudal end.
They will go on to form the Mouth and Anus respectively.
What happens by day 15 of development?
At the cranial end a PRIMITIVE NODE forms and at the caudal end a PRIMITIVE STREAK forms
-Ectodermal cells migrate laterally and inferiorly from the streak to form the INTRA-EMBRYONIC MESODERM layer
-Ectodermal cells migrate from the primitive node to the prochordal plate to form the NOTOCHORD
What does the Prochordal plate become?
The Buccopharyngeal membrane (which becomes an opening to the oral cavity)
What does the Cloacal Plate become
The cloacal membrane which becomes the anal opening
What does the Notochord become?
It will fold and become the future Neural Tube (which makes brain and spinal cord)
-Nuclei Pulposis (the centre of an adult intervertebral disc, is the remnant of the notochord)
What does the Ectoderm form?
epidermis and nervous system
What does the mesoderm form?
skeletal, connective and muscle tissues
what does the endoderm form?
lining of the GI tract and respiratory systems
Which layer produces the neural plate and how is this initiated?
The notochord produces growth factors which induces Ectodermal thickening which produces the neural plate
When is the neural plate formed?
By day 18
What are the openings of the neural tube called and when do they close?
Anterior and Posterior Neuropore
-close at day 24 and 26 respectively
Where do neural crest cells come from?
They form in the neural fold and detach prior to neural plate folding prior to neural tube formation
-they form discrete aggregations and go on to form important structures
What 3 layers does the mesoderm differentiate to?
Medial = Paraxial mesoderm
-Intermediate mesoderm
Lateral = lateral plate mesoderm
-the lateral plate mesoderm is made up of 2 layers:
a) Somatic Layer (in contact with the ectoderm)
b) splanchnic layer (in contact with the endoderm)
How does the paraxial mesoderm initially differentiate?
In to blocks in a cranio-caudal direction - forms SOMITES
-these are paired on either side of the neural tube
-first pair forms on day 20, with three per day thereafter until 42-44 somite pairs exist
What do somites develop in to?
1) Dermatome (dorsal)
2) Myotome
3) Sclerotome (ventral)
What does the Dermatome generate?
Spinal meninges, skin and subcutaneous tissue
What does the Myotome generate?
Epiaxial and Hypoaxial muscles
What does the Sclerotome generate?
Vertebral column, ribs and Intevertebral discs
What does the intermediate mesoderm generate?
The kidneys and ureters
The testes and epididymis/vas deferens
The ovaries and fallopian tubes/uterus
What does the lateral plate mesoderm generate?
a) Somatic layer = parietal structures e.g parietal pleura/peritoneum
-plus sternum, limb bones and cartilage
b) splanchnic layer = visceral structures e.g visceral pleura and peritoneum
-plus adrenal cortex, lymph nodes, smooth muscle of GI tract, Cardiovascular system (endothelium), myeloid/lymphoid/erythropoid stem cells
What three key structures form from the ectoderm?
Neural Tube
Neural Crest Cells
Placodes
What key structures form from the neural tube?
Brain and spinal cord, including neural cells, glial cells, posterior pituitary, pineal gland, retinas
What are placodes and how many are there?
Areas of discrete thickening around the primitive node
-there are 3(4)
1) Olfactory placode
2) Lens placode
3) Otic Placode
What is surface ectoderm?
The rest of the ectoderm that is not neural tube/neural crest cells or placodes
-forms epithelial tissue including epidermis, nails, hair, sweat glands, linings of openings (mouth, nose, ear, inferior anus below pectinate line), Rathkeys pouch (an out pouching of nasopharynx that forms the anterior pituitary)
What main structure forms from the endoderm and what is it divided in to?
The Gut tube:
1) foregut
2) mid gut
3) hind gut
What is classed as the foregut?
Pharynx, oesophagus, stomach, first 2 parts of duodenum
What is classed as midgut?
last 2 parts of duodenum, jejunum, ileum, caecum, ascending colon, proximal 2/3 of transverse colon
what is classed as hindgut?
distal 1/3 of transverse colon, descending colon, sigmoid, rectum, anus (proximal to pectinate line)
what organs form from the foregut?
Respiratory diverticulum forms trachea, bronchi, lung parenchyma
Hepatic diverticulum forms liver and biliary tree and head of pancreas
Pancreatic diverticulum forms body and tail of pancreas
In what two planes does embryological folding occur?
Longitudinal (cephalocaudal) and Transverse/Lateral
When does embryo folding occur?
Day 21-24
When is nutrition provided by the maternal blood supply rather than the zone pellucida?
By day 12
what is the name of the process by which the blastocyst invades the endometrium?
Decidualisation
Which structures form loose attachments between the endometrium and the blastocyst?
Microvilli on the blastocyst and Pinopods on the decidua functionalis
Which structures form strong attachments between endometrium and the blastocyst?
Integrins on the trophoblast cells attach to selectins on the decidua
How does invasion happen?
The trophoblast releases metalloproteases which breaks down the Zona pellucida and starts to erode surface epithelium of the endometrium
-further release of metalloproteases by the trophoblast digests the extra-cellular matrix of the uterine decidua
What stage of development has the placenta reached by Day 9?
The syncytiotrophoblast cells divide rapidly and cell membranes break down to form a syncytium
Syncytiotrophoblast starts releasing B-HCG to support the corpus luteum to continue to make progesterone
Syncytiotrophoblast releases proteases that invade uterine blood vessels causing leaking in to stromal spaces = Lacunae formation
What happens at Day 12 of placental development?
The Lacunae mature and become Intervillous Spaces
The Hypoblast starts making Extra-Embryonic Mesoderm which sits between the Cytotrophoblast and the Amniotic Cavity
What happens in week 3 of placental development?
The somato-pleuric extra-embryonic mesoderm invades the primary villi so the villous now has a core
-this turns it into Secondary villi
By the end of week 3 this mesoderm starts breaking down in the villi and in the conecting stalk which becomes BLOOD VESSELS
What has formed in the placenta by the end of week 3?
The villi now contain blood vessels so are known as Tertiary Villi
-They contain cotyledon arteries and enable diffuse exchange between the fetal and maternal circulations across the layers of the vili
What layers make up a villous?
Fetal vessel endothelium
Connective tissue of the villous
Cytotrophoblast
Syncytiotrophoblast
What happens at 4-5 months to allow more efficient exchange across the villi?
The cytotrophoblast layer regresses so the villous is thinner and thus exchange is more efficient
What is the name for the layer of chorion over the an-embryonic pole?
Chorionic Laeve/Leaf - does NOT have villi
Which layers form a placenta?
Chorionic Frondosum (the chorion, villi, mesoderm of baby) and the Decidua of mum
What is the name for the decidua over the baby and the decidua on the free wall of the uterus?
Over the baby = decidua capsularis
Free wall = decidua parietalis
By month 4 they have fused and the uterine cavity obliterated due to the expanding fetus
What structure does the umbilical cord arise from and what makes up an umbilical cord?
The connecting stalk
-composed of 2 x umbilical arteries, 1 x umbilical vein, wharton’s jelly
What stage of division does DCDA twins happen?
By day 3 division has occured- the morula stage
When do MCDA twins happen?
Zygote division occurs between day 4 and 8
When do MCMA twins happen?
Zygote division day 9-12
When do conjoined twins happen?
Zygote division after day 12
What is a complete mole?
An empty egg is fertilised by 1 or 2 sperm (1 sperm and it divides, or 2 sperm)
-46XY (if two sperm) or XX (one sperm) but all genetic material is paternal
-only paternal cells so only produces trophoblast, no embryo
-produces diffusely swollen villi and trophoblastic hyperplasia
What is a partial mole?
Haploid normal egg is fertilised by 2 sperm - 69 XXY, 69 XXX or 69XYY
-causes focal swelling of villi and trophoblastic tissue
-there IS some embryo tissue because maternal genes are present
What is the risk of moles?
80% are benign
-15% become invasive (persistent trophoblastic disease)
-2-3% develop into choriocarcinoma
What are the two components of the skull embryologically?
the Neurocranium and the Viscerocranium
What are the two components of the Neurocranium and where do they originate?
Bones of cranial base = come from Occipital Sclerotomes and are produced via Endochondral Ossification
Skull cap = from Neural crest cells in the mesenchyme and are produced via Intramembranous Ossification
What bones in the body are made from Intramembranous ossification?
Skull cap, Viscerocranium and Clavicles
Where does the Viscerocranium come from?
Originates from the Pharyngeal arches 1+2 - via intramembranous ossification
how many fontanelles are they and what are they made of?
Unossified mesenchyme - there are 6 in total.
Anterior
Posterior
2 x anterior lateral
2 x posterior lateral
How is the vertebral column formed?
The paraxial mesoderm surrounding the notochord becomes Somites - which then becomes Dermatome/Myotome/Sclerotome
-The sclerotome differentiates around the Notochord to form the Vertebral arches
-where they fuse forms the Spinous, Costal and transverse processesW
Where do the ribs originate from?
The Costal processes of the vertebral arches
-mesenchyme grows laterally around to the ventral midline where the ribs fuse with the sternum
Where does the sternum originate?
Differentiation of ventral lateral plate mesoderm forms Cartilaginous bars
these bars via Endochonral Ossification forms the Sternum (manubrium, body and xiphoid process)
Which genes promote the formation of limb buds?
HOX genes - causes release of growth factors specifically FGF-10
What are the limb buds made up of?
Buds of ectoderm, which are then filled with buds of the Somato-pleuric layer of the lateral plate mesoderm
What is the AER and PZ?
The AER is the Apical Ectodermal Ridge - this is the distal part of the limb bud that produces LOTS of growth factor
-The PZ is the Progress Zone and this is the distal end of the Lateral Plate Mesoderm that is rapidly proliferating and thus grows the limb in a proximal-distal direction
What causes formation of digits and orientation of the extremities?
Apoptosis at intervals on the AER causes formation of digits
-The orientation of the extremity is determined by the Zone of Polarising Activity
What two areas does each myotome divide in to?
1) Ventral Hypomere
2) Dorsal Epimere
What muscles originate from the ventral hypomere?
ANTERIOR AND LATERAL MUSCLES
e.g rectus abdominis, internal/external oblique, transverse abdominis
What muscles originate from the dorsal epimere?
POSTERIOR WALL MUSCLES
e.g erector spinae
What layer does the heart tube mostly develop from?
The splanchnic layer of the lateral plate mesoderm
What do cardiac progenitor cells in the mesoderm layer become?
They transform in to Myoblasts and Blood islands (angioblasts and haemoblasts) that form a horseshoe shaped endothelial lined tube = CARDIOGENIC FIELD
What structure connects the heart tube and the pericardial cavity?
The Dorsal Mesocardium (a stalk that suspends the heart tube in the pericardial cavity)
What structures/layers arise from the Mesothelial cells of the sinus venosus?
The pericardium and coronary arteries
What are the (5)4 chambers of the primordial heart?
(1. Truncus Arteriosus)
2. Bulbus cordis
3) Primitive ventricle
4) Primitive Atria
5) Sinus Venosus
What veins does the Sinus Venosus receive?
Lateral to medial = Common cardinal vein, Umbilical Vein, Viteline vein
When is the first heart beat detectable by USS?
6 weeks
When does folding of the heart happen?
By day 28
When does partitioning of the primordial heart take place?
Starts week 4 and complete by week 8
when is the Sinoatrial node formed?
By week 5
What is the most important layer for development of the GI tract?
Endoderm
What layers cover the Gut tube and what do they become?
The Splanchnopleuric mesoderm becomes the visceral peritoneum and the Somatopleuric mesoderm becomes the parietal peritoneum
What makes something intra-peritoneal?
It is covered with peritoneum and has a Mesentary
What is primary retroperitoneal? - examples
The organ never had a mesentary
-abdominal aorta
-Inferior Vena Cava
-Adrenal glands
-Kidneys
-Ureter
-Bladder
-Oesophagus
-Lower rectum
What is secondary retroperitoneal? - examples
It did have a mesentary but during rotation it is lost
-2nd,3rd and 4th parts of duodenum
-ascending colon
-descending colon
-head and body of pancreas
Which two structures connect the foregut tube visceral peritoneum to the parietal peritoneum? and what forms in them?
The Ventral And Dorsal Mesogastrium (Mesentaries)
-In the ventral forms the Liver
-In the dorsal forms the spleen
In which direction and by how many degrees does the foregut rotate?
90 degrees clockwise - so liver moves to right and spleen to the left
What do the ventral and dorsal mesogastrium become?
Ventral mesogastrium becomes the Lesser Omentum
Dorsal Mesogastrium becomes the Greater Omentum
What is the blood supply to the foregut?
The Coeliac Trunk
What is the blood supply to the Midgut?
Superior Mesenteric Artery
What is the blood supply to the hindgut?
Inferior Mesenteric Artery
What organs make up the foregut? and what are their mesentaries?
Oesophagus - none
Stomach - greater and lesser omentum
Spleen - gastro-splenic ligament
Liver - lesser omentum and falciform ligament
Gallbladder - lesser omentum and falciform ligament
Pancreas - spleno-renal ligament
Proximal 1/2 of duodenum -hepato-duodenal ligament
What organs make up the midgut? and what are their mesentaries?
Distal 1/2 duodenum - none
Jejunum - Small bowel mesentary
Ileum - small bowel mesentary
Caecum - varies! can be intra or retroperitoneal
Appendix - mesoappendix
Ascending colon - none
Proximal 2/3 transverse colon - transverse mesocolon
What organs make up the hindgut? and what are their mesentaries?
Distal 1/3 transverse colon - transverse mesocolon
Descending colon - none
Sigmoid Colon - sigmoid mesocolon
Upper rectum (pre-pectinate line) - none
What type of muscle lines the the upper 2/3 of the oesophagus and what nerve innervates it?
Striated Muscle
Vagus nerve
What type of muscle lines the lower 1/3 of the oesophagus and what innvervates it?
Smooth Muscle
Splanchnic Plexus
What landmark signifies the joining of the foregut and midgut?
It is just distal to the origin of the Bile Duct
What structure becomes the diaphragm?
The Septum Transversum
When does physiological herniation occur?
Week 6 of embryological life
How does the midgut rotate during physiological herniation?
Upon herniation it rotates 90 degrees anti-clockwise
Upon return to the abdominal cavity it rotates further 180 degrees anti-clockwise
What is an Omphalocele?
Indirect Herniation of the abdominal contents via the umbilicus
contents covered with a layer of amnion
is associated with other structural/chromosomal abnormalities
What is Gastroschisis?
Direct herniation of the abdominal cavity via the abdominal wall
-usually paraumbilical to the RIGHT
abdominal contents lie in the amniotic cavity and are not covered by amnion
not associated with chromosomal abnormalities
genetic/environmental associations: young mothers, smoking, IUGR
-shows raised AFP
What is Meckel’s Diverticulum?
A remnant of the Vitello-Intestinal duct
-found 60cm from Ileocaecal valve
-5cm long
-affects 2% of the population
*Can be mistaken for appendicitis if it gets inflammed
-contains ectopic gastric and pancreatic mucosa
What structure forms the Urachus? - and what does the Urachus then become?
The Allantois - becomes the Urachus
-Which goes on to become the Median Umbilical ligament
What structure separates the Hindgut and the Allantois in the Cloaca? - and what structures are formed as a result?
The Urorectal Septum
-The urogenital sinus
-The anorectal canal
Describe the upper 2/3 of anal canal:
from endoderm
columnar epithelium
supplied by superior rectal artery from inferior mesenteric artery
supplied by autonomic nervous system
Describe the lower 1/3 of anal canal:
From ectoderm
Stratified squamous epithelium
Supplied by Inferior Rectal Arteries as branches of the Pudendal artery
Supplied by the Inferior Rectal Nerve
What structures form the larynx?
The Endoderm of the Pharyngeal Pouches
Mesoderm of 4th and 6th Pharyngeal arches
What forms the trachea?
Endoderm of the foregut
Mesoderm from Splanchnopleuric mesoderm
What forms the lungs?
Endoderm from the foregut
mesoderm from splanchnopleuric mesoderm
When does a lung bud form and where does this occur?
Week 4 of embryological life
-the lung bud is a diverticulum off the foregut
How do the trachea and oesophagus separate?
Tracheo-oesophageal Folds form, which then fuse and form a Tracheo-Oesophageal septum
-causing the trachea to lie ventrally and oesophagus dorsally
What is the name for the dividing process of the bronchial tree?
Branching Morphogenesis
From the Right and Left Primary Bronchi - how many secondary lung buds form?
Right has 3 secondary lung buds
Left has 2
How many tertiary bronchi? - what is the name of this stage?
Right has 20
Left has 18
The Pseudoglandular Stage
What happens between week 16 and 21? and what is this stage called for respiratory development?
Terminal bronchioles become Respiratory bronchioles through formation of alveolar ducts lined with Cuboidal Epithelium
-This is called Canalicular Stage
What happens from 26 weeks of respiratory development and what is this stage called?
Cuboidal epithelium thins to become squamous allowing for easier gaseous exchange
This is called the Saccular Stage
Which cells produce Surfactant and when does this begin?
Type 2 alveolar cells produce Surfactant from approx 24 weeks
Where does the Urinary System develop from?
Intermediate Mesoderm
What structure forms as a result of the condensing of intermediate mesoderm to form the urinary system?
Urogenital Ridge > Nephrogenic Cord
What structure develops first as part of urinary system development? -and what is this made up of?
The Pronephros
-Nephrotome
-Ducts
When does the pronephros form and when does it degenerate?
Forms start of week 4 and degenerates by end of week 4
What structure develops once the Pronephros has degenerated? - and what does the structure join to?
The Mesonephros
-joins to the Cloaca
What does the Mesonephros do?
It forms the Primitive Urinary System between weeks 5-10
Its mesonephric tubules join with glmoerular capillaries from the aorta to form Bowman’s Capsules (primitive filtration system)
-the urine produced drains down the mesonephric duct to the cloaca and out of the allantois
When does the Mesonephros start developing in to the next structure?
At week 10 the Mesonephros starts condensing to produce the Metanephric Mesoderm (Metanephric Blastema)
