15. Embryonic Development Flashcards
Give some reasons for studying embryology?
- Model system for cell/molecular biology
- Understanding congential defects
- Model for regenerative medicine
What are some model systems used in embryology?
- Xenopus (frog)
- Chick
- Mouse
- C elegans
- Zebrafish
Give an example of a gene that is conserved between species.
- Pax6 is conserved in humans, mice, zebrafish and drosophilia
- A mutation in the Pax6 gene results in eyeball abnormalities in all of these species
Name some advantages of the fly as a model system for embryology.
- Cheap
- Short lifespan
- Easy to manipulate
Name some advantages of the mouse as a model system for embryology.
- Foremost mammalian model
- Short gestation period
- Highly similar development to humans
Name some advantages of C elegans as a model system for embryology.
- Cheap
- Have fixed no. of cells -> Exact development + position of each cell has been mapped
Name some advantages of the chick as a model system for embryology.
- Accessible -> Grows in eggs
Name some of the important aspects of development.
- Cell differentiation
- Patterning
- Induction
- EMT and MET (epithelial to mesenchymal transitions and mesenchymal to epithelial transitions)
- Morphogenesis
- Cell death
Are signalling pathways ever reused in the development?
Yes, the same signalling pathways appear in many places in the human body.
What are the main important stages in embryonic development?
- Fertilisation (Day 0)
- Pre-embyronic stage (Day 0)
- Implantation
- Bilaminar germ disc
- Embryonic period (3rd week)
- Gastrulation + Axis formation
- Neurulation
- Embryonic folding
- Somitogenesis
- Limb bud formation
- Organogenesis
- Fetal period (8th week)
- Growth
- Maturation
- Birth
What is the product of fertilisation?
Diploid zygote
Define fertilisation.
The joining of an egg and sperm to produce a diploid zygote.
Describe the process of fertilisation.
- Occurs in ampullary region of oviduct (the far end, near the ovary) – Note: Oviduct = Fallopian tube
- In order to fertilise oocyte, spermatozoa need:
- Removal of glycoprotein coat (capacitation) -> This is calcium dependent
- Binding to zona pellucida -> Activates acrosome reaction, where acrosome (a Golgi-derived organelle in the head of the spermatozoon) releases enzymes to break through the zona pellucida
- As spermatozoon enters the zona pellucida, the membrane of the spermatozoon fuses with the membrane of the oocyte, releasing the spermatozoon nucleus into the oocyte
- At the same time, the cortical granules of the oocyte release their contents, making the zona pellucida impenetrable to other spermatozoa
- Entry of the spermatozoon nucleus stimulates the oocyte to complete the second meiotic division (meiosis has not yet been completed) -> It is now called a definitive oocyte and the two nuclei are called the female pronucleus and male pronucleus (the name for the nucleus during fertilisation)
- Since it has been fertilised, the definitive oocyte can also be called the zygote
- The two pronuclei approach each other and duplicate their DNA, ready for the first mitotic division
- Pronuclear membranes break down and the chromosomes line up for metaphase
- First cell division takes place
What percentage of sperm reach the cervix and what happens to them there?
- About 1%
- Ovulation induces them to move further into the oviduct
What are the technical names for the sperm and egg?
- Sperm = Spermatozoon (pl. spermatozoa)
- Egg = Oocyte
Where does fertilisation occur?
Ampullary region of oviduct (a.k.a. fallopian tube)
(far end, near the ovary)
Draw out the path of spermatozoa to the oocyte.
What two things must happen in order for the spermatozoon to enter the oocyte?
- Removal of glycoprotein coat (capacitation) -> This is calcium dependent
- Binding to zona pellucida -> Activates acrosome reaction, where acrosome (a Golgi-derived organelle in the head of the spermatozoon) releases enzymes to break through the zona pellucida
What prevents multiple spermatozoa fertilising the oocyte?
Cortical granules (just inside the plasma membrane) release their contents, making the zona pellucida impenetrable.
Describe the process of meiosis involved in generating the female pronucleus.
- The process of meiosis is halted just before the second meiotic division
- When the male pronucleus enters the oocyte, this triggers the second meiotic division -> This creates the female pronucleus
How is the first mitotic division different to normal mitotic divisions?
The DNA duplication occurs before the male and female pronuclei even fuse.
What are pronuclei?
The nuclei of the gametes during the process of fertilisation.
Draw a diagram of an oocyte and spermatozoon.
What is a zygote?
The union of the sperm cell and the egg cell. Also known as a fertilized ovum.
What is cleavage?
A series of mitotic divisions without cell growth.
Describe the process of cleavage.
- Cleavage is a series of mitotic divisions without cell growth
- All of the cells during cleavage are called blastomeres
- Cleavage occurs with all of the cells still within the zona pellucida, while travelling down the oviduct
- At the 8 to 16 cell stage, outer cells begin to form tight junctions with each other, which is called compaction -> This leads to the morula structure
- Morula (from the Latin for mulberry) -> 16 to 32 cells
- Inner cell mass (ICM) (a.k.a. embryoblast) -> On the inside of the morula -> Loses totipotency so can only form embryo
- Trophoblast -> On the outside of the morula -> Keeps totipotency and goes on to form placenta
- Blastocyst (not to be confused with blastula: blastocyst is what a blastula is called in mammals)
- Blastocystic cavity (a.k.a. blastocoel) -> Formed by the absorption of fluid
- Inner cell mass (ICM) (a.k.a. embryoblast) -> Is forced to one side of the cavity as a compact mass
- Trophoblast -> Is now a single-layered epithelium around the outside
- The zona pellucida degenerates and the blastocyst is able to hatch out
What are the cells in a cleaving morula or blastocyst called?
Blastomeres
What is compaction?
In the 8 to 16 cell phase of cleavage, outer cells start to form tight junctions with each other.
What are the different structures seen during cleavage? Draw diagrams.
- Morula (from the Latin for mulberry) -> 16 to 32 cells
- Inner cell mass (ICM) (a.k.a. embryoblast) -> On the inside of the morula -> Loses totipotency so can only form embryo
- Trophoblast -> On the outside of the morula -> Keeps totipotency and goes on to form placenta
- Blastocyst (not to be confused with blastula: blastocyst is what a blastula is called in mammals)
- Blastocystic cavity (a.k.a. blastocoel) -> Formed by the absorption of fluid
- Inner cell mass (ICM) (a.k.a. embryoblast) -> Is forced to one side of the cavity as a compact mass
- Trophoblast -> Is now a single-layered epithelium around the outside
Clinical relevance: Describe the different types of twins and how they arise.
- Dizygotic twins -> Two seperate blastocysts, which form embryos that may have a fused placenta but do not share blood supply
- Monozygotic twins I -> ICM splits, which form embryos that have a common placenta and sometimes share a blood supply
- Monozygotic twins II -> Splitting occurs anywhere between two-cell to morula stage, which form embryos that may share placenta but do not share blood supply
Give some examples of the clinical relevance of fertilistaion and cleavage.
- Twins
- IVF
- Cloning
- Embryonic stem cells
When does the developing embryo lose the zona pellucida?
- After it is a blastocyst
- Blastocyst ‘hatches’ from the disintegrating zona pellucida around day 5
- This allows for implantation
Describe the process of IVF.
- Hormonal stimulation of mature oocyte formation produces several mature follicles
- Collection of oocytes
- Placement in Petri dish and fertilisation in vitro
- Cleavage of zygotes in medium until 4 to 8 cell stage is reached
- Transfer to up to 5 embryos into uterine cavity using a catheter
During implantation, what does the blastocyst adhere to?
Endometrium (uterus wall)
Describe the process of implantation. Draw diagrams.
- Morula reaches uterus by day 3/4 of development
- Blastocyst hatches from zona pellucida and adheres to endometrium lining (uterus wall)
- This triggers decidual reaction: endometrial stroma (connective tissue) responds to blastocyst and progesterone from the corpus luteum and turns into decidual cells (secretory cells) -> These secrete lots of glycogen and mucus for immunological protection
- The uterine wall becomes more vascularised and oedematous (swollen)
- Trophoblast near the uterus begins to grow into the uterine wall, forming a syncytiotrophoblast that starts pulling the blastocyst into the uterine wall
What is the decidual reaction?
It is part of the implantation process:
- The stroma (connective tissue) in the endometrium responds to binding of the blastocyst as well as progesterone from the corpus luteum
- Stroma cells differentiate into decidual cells (secretory cells) -> These secrete lots of glycogen and mucus for immunological protection
Give some clinical relevance of implantation.
Ectopic pregnancy:
- When a blastocyst implants outside the uterus, for example in the pericardial cavity or on the surface of the oviduct, the epithelium coming in contact with the blastocyst will still react, becoming more vascularised
- This allows the blastocyst to survive outside the uterus and start to develop, but it will not reach term
- This may be threatening to the mother due to, for example, haemorrhage
What does the trophoblast do during implantation?
- At one pole of the trophoblast, the syncytiotrophoblast and cytotrophoblast form from trophoblast cells
- Syncytiotrophoblast cells are the cells that initially invade the endometrium, drawing the blastocyst into the uterine wall -> Ruptures capillaries so creating an interface between maternal blood and embryonic fluid for passive transfer. Also secretes a lot of hCG to maintain corpus luteum
- Cytotrophoblast cells line the inside of the syncytiotrophoblast and will eventually form the foetal portion of the placenta
What things trigger the decidual reaction?
- Progesterone from the corpus luteum
- HcG from the synctiotrophoblast assists with this by maintaining the corpus luteum
What are hydratidiform moles and what are the two main types? [EXTRA]
- Growth of an abnormal fertilized egg or an overgrowth of tissue from the placenta
- Embryos derived from two male haploid genomes (androgenotes) give rise to largely placental tissue
- Embryos derived from two female pronuclei (gynogenotes) give rise to largely embryonic tissue
What is the first structure that is formed after implantation?
Bilaminar germ disc
What does the bilaminar germ disc form from?
Inner cell mass (ICM)
What are the two layers of the bilaminar germ disc?
- Hypoblast
- Epiblast
What are the two cavities that surround the bilaminar germ disc?
- Amniotic cavity
- Yolk sac
Which layer of the bilaminar germ disc gives rise to the amnion and which gives rise to the yolk sac?
- Epiblast -> Amnion
- Hypoblast -> Yolk sac
What is the amnion and amniotic cavity?
- The amnion is a membrane that closely covers the embryo when first formed.
- It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus.
Which cell group forms the placenta?
Trophoblast
Describe the formation of the amniotic cavity and yolk sac.
Amniotic cavity:
- Develops between the epiblast and cytotrophoblast
- Cells from the epiblast form amnioblasts that line the cavity -> This is called the amnion
- The amniotic cavity will eventually engulf the entire embryo
Yolk sac:
- Cells from the hypoblast migrate out in the two waves
- Wave 1: Forms the primary yolk sac
- The exocoelomic (Heuser’s membrane) that lines the primary yolk sac is formed by the migrating hypoblast cells
- Wave 2: transforms the primary into the secondary yolk sac
After initial implantation by the synctiotrophoblast and cytotropohblast, how does the embryo implant further?
- The emrbyo keeps embedding further into the endometrium until the cytotrophoblast surrounds the embryo and a coagulation plug is formed
- Vacuoles in the synctiotrophoblast cells fuse to give lacunae
What are the extraembryonic mesoderm and how does it form?
- The extraembryonic mesoderm is the layer of mesoderm that forms just inside of the cytotrophoblast (between the cytotrophoblast and amnion + lining of primitive yolk sac)
- It is formed by the migration of cells from the edges of hypoblast and epiblast
What are the chorionic cavity and how does it form?
- Chorionic cavity = Extraembryonic coelom
- It is the cavity that forms within the extraembryonic mesoderm, between the cytotrophoblast and the embryo
Describe how the secondary yolk sac forms from the primary yolk sac. What structures form alongside this?
- The trophoblast grows much faster than the bilaminar disc
- This gives rise to the chorionic cavity and secondary yolk sac
- The connecting stalk left behind forms the umbilical cord
What happens to the lacunae in the synctiotrophoblast?
They fill with blood due to the decidual reaction prompting greater vascularity.
Summarise the main events just after implantation, up to the formation of the secondary yolk sac.
Implantation:
- Morula reaches uterus by day 3/4 of development
- Blastocyst hatches from zona pellucida and adheres to endometrium lining (uterus wall)
- At one pole of the trophoblast, the syncytiotrophoblast and cytotrophoblast form from trophoblast cells:
- Syncytiotrophoblast cells are the cells that initially invade the endometrium, drawing the blastocyst into the uterine wall -> Ruptures capillaries so creating an interface between maternal blood and embryonic fluid for passive transfer
- Cytotrophoblast cells line the inside of the syncytiotrophoblast and will eventually form the foetal portion of the placenta
- The decidual reaction is triggered by progesterone from the corpus luteum and hCG from the synctiotrophoblast assists with this by maintaining the corpus luteum
- Decidual reaction: Endometrial stroma (connective tissue) turns into decidual cells (secretory cells) -> These secrete lots of glycogen and mucus for immunological protection
- The uterine wall becomes more vascularised and oedematous (swollen)
- Bilaminar germ disc forms from the ICM -> The two layers are the hypoblast and epiblast
Amniotic cavity forms from the epiblast:
- Develops between the epiblast and cytotrophoblast
- Cells from the epiblast form amnioblasts that line the cavity -> This is called the amnion
- The amniotic cavity will eventually engulf the entire embryo
Yolk sac forms from the hypoblast:
- Cells from the hypoblast migrate out in the two waves
- Wave 1: Forms the primary yolk sac
- The exocoelomic (Heuser’s membrane) that lines the primary yolk sac is formed by the migrating hypoblast cells
- Wave 2: transforms the primary into the secondary yolk sac
Later implantation:
- The embryo keeps embedding further into the endometrium until the cytotrophoblast surrounds the embryo and a coagulation plug is formed
- Vacuoles in the synctiotrophoblast cells fuse to give lacunae -> These fill with blood due to increased vascularity
- The extraembryonic mesoderm is the layer of mesoderm that forms just inside of the cytotrophoblast (between the cytotrophoblast and amnion + lining of primitive yolk sac) -> It is formed by the migration of cells from the edges of hypoblast and epiblast
- The trophoblast grows much faster than the bilaminar disc
- This gives rise to the chorionic cavity and secondary yolk sac
- The connecting stalk left behind forms the umbilical cord
Compare the body axes in the adult and embryo.
What does the epiblast give rise to eventually?
All embryonic tissues.
What is gastrulation?
A complex series of movements during which the three germ layers establish their appropriate topological positions and the basic body plan emerges.
What are germ layers?
An operational definition describing three cell layers/types that give rise to different organ systems in the body.
What germ layer are mesenchymal cells derived from?
- Mesenchymal cells can derive from any germ layer
- They form a loosely organised tissue, which frequently differentiates subsequently into specific cell types (eg. mesenchymal cells of the sclerotome will form the vertebral column); mesenchymal cells in skull development
What are the 3 definitive germ layers?
- Ectoderm (on the side of the amniotic cavity)
- Mesoderm
- Endoderm
Describe the process of gastrulation.
- Gastrulation begins with the formation of a primitive streak, which is a shallow marked line along the epiblast.
- This begins at what is now defined as the caudal end of the epiblast and runs along the midline.
- At the cranial (or rostral) end of the streak, a circular region called the primitive node forms, which contains a depression called the primitive pit.
- This depression continues down the primitive streak, forming a long, linear primitive groove. The primitive groove along with the primitive pit mark sites to which epiblast cells migrate and travel towards the mesoderm (a process known as ingression), beginning the process of creating a trilaminar embryonic disc.
- Endoderm is formed by the invasion of the hypoblast. Cells leaving the primitive streak displace the hypoblast cells and gradually form the endoderm.
- Mesoderm forms when the epiblast cells migrate laterally or cranially after passing through the primitive groove, before they reach the endoderm. This forms a layer of cells between the endoderm and the epiblast. A
- Ectoderm is essentially what remains of the epiblast after gastrulation is complete.
What gastrulation begin with?
Begins with the formation of a primitive streak, which is a shallow marked line along the epiblast.
Describe how gastrulation defines the body axis.
- Gastrulation begins with the formation of a primitive streak, which is a shallow marked line along the epiblast.
- This occurs at what is now defined as the caudal end (relating to the “tail) of the epiblast.
- Aside from determining the cranial-caudal axis, the streak is defined as running as medially as possible, so the medial-lateral direction is perpendicular to this.
- Right and left are also defined when looking down at the ectoderm from the amniotic fluid, when the caudal end is closest to the viewer.
What does the primitive node act as?
An organiser.
What determines what an ingressing cell passing through the primitive streak becomes?
The time at which it passes past the primitive node and through the primitive streak.
What are some defects in primitive node and streak activity?
What transition do cells undergo when they ingress during gastrulation?
EMT (epithelial-to-mesenchymal transition)
Which direction does the primitive streak move in during gastrulation?
Caudally
What are the two broad categories of structure that the ectoderm goes on to form?
What is neurulation?
The formation of the neural tube.
Describe neurulation.
- The notochord (in the mesoderm) along the midline induces the ectoderm above it to differentiate into the neural plate
- The neural plate folds downwards so that the two sides of it fuse
- This produces a tube called the neural tube
- The cells just above it are called the neural crest
What transition do neural crest cells undergo?
EMT (epithelial to mesenchymal transition)
What is the neural plate?
The area of ectoderm along the midline that goes of to form the neural tube.
What induces neurulation?
Notochord (in the mesoderm just below the neural plate)
What are two examples of neural tube closure defects?
In summary, what does the nervous system form from? [IMPORTANT]
Neuroectoderm
What are the derivatives of the neural tube?
CNS
Summarise the derivatives of the neural crest. [IMPORTANT]
- PNS
- Dorsal root ganglia
- Autonomic ganglia
- Cranial nerve ganglia
- Enteric nervous system
- Schwann cells
- Skin -> Melanocyte
- Cranial bones
- Eyes
Give an example of a condition caused by failure of neural crest cell migration.
In general, summarise the different types of tissue that mesoderm goes on to form.
- Muscle
- Connective tissue
- Blood, etc.
What are the different types of mesoderm?
- Prechordal (head)
- Axial (notochord)
- Paraxial (somites)
- Intermediate
- Lateral plate
What is another name for the notochord?
Axial mesoderm
What are some roles of the notochord?
- Induces the neural plate (and therefore neural tube)
- Patterns the neural tube in the dorso-ventral axis
- Forms primitive axis around which axial skeleton is laid down
What does the notochord go on to form? [IMPORTANT]
Nucleus pulposus of intervertebral discs
How are the neural tube and associated structures patterned in the craniocaudal axis?
Check this -> Thought it was Hox genes but it might be Shh
How is the notochord patterned in the dorsoventral axis?
- Shh, initially from the notochord, but subsequently from the most ventral cells of neural tube, forms a concentration gradient across neural tube
- It induces different ventral fates in neural tube, while BMPs regulate dorsal fates
How does the sclerotome give rise to the vertebrae and ribcage?
The medioventral portion of each somite migrates and the cells surround the notochord.
Describe how the somites lead to segmentation of the peripheral nervous system.
Describe the positions of the sclerotome, dermatome and myotome within the somite.
- Sclerotome -> Medioventral
- Myotome -> Dorsomedial and dorsolateral
- Dermatome -> Rest of somite
Describe spina bifida.
Draw a diagram to show the signalling centres involved in patterning the different parts of a somite.
Draw the structure of a limb bud, including the axes.
Describe how limb outgrowth and patterning occurs in the proximodistal axis.
Patterning:
- The AER induces the proximodistal axis, where the later a cell is in contact with the AER, the more distal the limb structure that it forms.
Outgrowth:
- Specific Hox gene expression leads to the release of T-box transcription factors.
- These stimulate FGF10 (signalling protein) production in the lateral plate mesoderm.
- This in turn induces the ectoderm to thicken into an apical ectodermal ridge (AER).
- The AER stimulates the continued mesenchymal proliferation (positive feedback) by producing FGF8 and FGF4.
Describe how limb patterning occurs in the anteroposterior (craniocaudal) axis.
- The zone of polarising activity (ZPA) is a region of mesenchymal cells on the posterior side of the limbs.
- It patterns the structures along the anteroposterior axis, such as the order of the digits, by sonic hedgehog (Shh) gradients, which affect the transcription of Hox genes.
Describe how limb patterning occurs in the dorsoventral axis.
The dorsoventral signalling centre is in the dorsal ectoderm, which functions by Wnt7a signalling that induces transcription factor activity in the dorsal mesenchyme (especially activity of Lmx1).
What is the result of removal of the AER experimentally?
What are amelia, meromelia and phocomelia
and what causes them?
What is the ZPA and what does it do?
- Zone of polarizing activity
- An area of mesenchyme at the posterior (caudal) part of the limb bud.
- Contains signals that pattern the anterior/posterior axis.
Draw a diagram to show how Shh acts as the morphogen of the ZPA.
Draw cleft foot and hand and how it occurs.
Describe the process of dorsoventral patterning in limb formation in more detail.
What is responsible for left-right assymetry of the body in development?
The primitive node:
- Cells of the node have prominent cilia, which rotate, creating flow
- Fluid flows from one side to the other generating asymmetry
- Nodal flow leads to asymmetric gene expression
In cranial (longitudinal) folding, what structures are moved where?
In the cranial to caudal direction at the cranial end (before folding):
- Septum transversum -> Mesoderm that will fold into the thoracic region -> Forms diaphragm and part of stomach and duodenum
- Cardiogenic region (will form the heart) -> Moved to a position on the ventral side, inserting it into the thoracic region
- Oropharyngeal membrane (area where the ectoderm and endoderm are directly in contact without mesoderm in the middle) -> Head folding moves this to the ventral side where the future mouth will be -> Eventually forms the mouth
In caudal (longitudinal) folding, what structures are moved where?
In the caudal to cranial direction of the caudal end (before folding):
- Allantois -> Part of endoderm that is contained within the connecting stalk -> When the connecting stalk is moved by folding, it merges with the neck of the yolk sac -> Allantois becomes part of the urinary system
- Cloacal membrane -> Analogous to the oropharyngeal membrane in that it is a direct joining of the endoderm with the ectoderm, without a mesoderm layer in between -> Becomes the future rectum
Draw and describe the process of lateral folding.
- Embryonic disk grows, while the yolk sac does not grow -> This results in a mushrooming effect
- Sides of amniotic sac move dorso-ventrally and latero-medially, so that they fuse at the ventral midline
- Lateral plate mesoderm has split into the somatic (upper, continuous with the amniotic mesoderm, will form the body wall) and splanchnic (lower, continuous with the yolk sac mesoderm, will form the surrounding of the organs) mesoderm -> Upon folding, the endoderm and mesoderm layers become fused to the corresponding layers of the other side
- The space between the somatic and splanchnic mesoderm is initially open to the chorionic cavity, but upon fusion of the amniotic sac side, this cavity becomes enclosed within the embryo and is now called the intraembryonic coelom
- As folding occurs and the amniotic cavity sides fuse, the yolk sac is pinched to give the gut (which is surrounded by the intraembryonic coelom)
What important cavities are formed by lateral embryonic folding?
- Gut tube
- Intraembryonic coelom
- Amniotic cavity surrounds embryo
Describe how the shape of the yolk sac changes during embryonic folding.
Starts off looking like a goldfish bowl but then the neck narrows.
What is the gut tube suspended in? What surrounds the gut tube and lines the cavity that it is suspended in?
- Gut tube is suspended in the coelomic cavity by the dorsal mesentry
- The gut tube is surrounded by splanchnic (visceral) mesoderm
- The body wall is lined internally by somatic (parietal) mesoderm
Describe some congenital defects of the ventral body wall.
What happens to the lateral plate mesoderm during embryonic folding?
It divides into splanchnic (visceral) mesoderm and somatic (parietal) mesoderm.
Describe how the umbilical cord is formed.
The umbilical cord is the red and yellow tube you can see in the third picture.
What are the two main body cavities and how are they formed?
- Pericardial cavity (thoracic)
- Peritoneal cavity (abdominal)
They are separated (almost completely) by the septum transversum during embryonic folding.
During development, the two main body cavities are the pericardial (thoracic) and peritoneal (abdominal). Which of these is then partitioned further and how?
The pericardial cavity is partitioned into the pericardial and pleural cavities by outgrowth of the pleuropericardial folds from the body wall:
- Pleuropericardial folds grow and fuse medially subdividing the primitive pericardial cavity into a definitive pericardial cavity and two pleural cavities
- The root of the pleuropericardial folds then migrate around to the anterior body wall, forming the definitive pericardium
Describe the formation of the diaphragm.
- Septum transversum forms the amuscular central tendon
- Pericardioperitoneal canals closed by pleuroperitoneal membranes that contribute to the posterior musculature
- Dorsal oesophageal mesentery (L1-L3) give rise to the diaphragmatic crura
- Body wall musculature form the circumferential muscular rim (segmental thoracic innervation (T7-T12)
Describe the formation of the lungs. [COVERED LATER SO DW]
What structure does heart formation begin with? How does it form?
Horseshoe-shaped cardiogenic region (a.k.a. cardiac cresent):
- This forms when angiogenic clusters of blood vessels coalesce to form heart fields
- These eventually form a cresent shape
- The two sides are the left and right heart tubes
Describe how the left and right heart tubes form a single heart tube.
Lateral embryonic folding causes the two sides to fuse along the midline.
Label this early heart tube. Which way does blood flow through it?
Blood flows caudal to cranial.
What are the different layers of the early heart tube? [IMPORTANT]
- Endothelium -> This is the initial structure that forms the heart
- Cardiac jelly -> This is the ECM secreted by the myocardium that lies between the endothelium and epicardium
- Myoepicardium -> This is the mesoderm that surrounds the endothelium of the heart
Describe heart looping.
The atria move behind the linear heart tube and come to lie superior to the primitive ventricle.
Label this heart after looping.
Describe how left-right asymmetry of the heart is created. [EXTRA]
Describe the formation of the endocardial cushions. What does their formation allow?
The endocardial cushions form between the primitive atria and ventricles:
- Endocardial cushions begin as mesenchymal masses on the superior and inferior walls of the heart (noting that the heart is at an angle in the adult chest, so that the left wall is superior).
- Eventually, they form a barrier between the atria and ventricles, with only two atrioventricular canals remaining unclosed.
- The endocardial cushions enable septation to occur.
Describe the process of atrial septation.
- The septation of the atria occurs by the formation of the septum primum, which grows down from the atrial wall to fuse with the endocardial cushions.
- After this, holes form in the septum that converge to form the ostium secundum, a hole in the septum.
- To the right of the septum primum, the septum secundum forms, with a large hole called the foramen ovale.
- The ostium secundum and foramen ovale allow blood flow from the right atrium to the left atrium (bypassing the developing pulmonary system), but not vice versa.
- The foramen ovale is sealed closed later in development.
Describe the septation of the outflow tract of the heart.
- The septation of the ventricles carries on in the truncus arteriosus, with the septa turning throughout the outflow tract due to the blood flow over them.
- The spiral ridges in the outflow tract are called conotruncal ridges -> These fuse together, dividing the outflow tract
As a result, the truncus arteriosus later divides into the aorta and pulmonary trunk, which end up twisted around each other.
What are the 4 characteristics of the Tetralogy of Fallot?
- Pulmonary Stenosis
- Interventricular Septal Defect
- Over-riding Aorta
- Hypertrophy of Right Ventricle
Draw the circulatory system at 28 days.
Describe what happens to each of the aortic arches.
The aortic arches start to remodel to form the separate aortic and pulmonary trunks at end of 4th week:
- Arch I and II regress quickly
- Arch III forms the carotid arteries
- Arch IV and VI form aortic arch and pulmonary trunk
Describe how the venous system changes in the embryo.
The system is initially bilaterally symmetrical but is remodelled so that all the systemic venous blood drains into the right side of the heart via the superior and inferior vena cava.
