L10-11. Embryology Flashcards
Describe the formation of the single cell that forms the human. What is the term for this cell?
ZYGOTE
The sperm and ova meet at the most distal part of the fallopian tube in a process called FERTILISATION.
This forms the zygote which moves into the uterus bounding along the ciliated wall of the fallopian tube.
Describe the process between the zygote and the cavitated blastocyst
The zygote undergoes numerous divisions:
1 division per 24 hrs initially and accelerating with time
By 3 days = 16 cells the embryo is known as the MORULA
By 4 days = The BLASTOCYST forms and begins to look asymmetric (cells take on different shapes) as cavitation forms (hole is called the blastocoele)
What is a congenital disorder?
What are the two major types?
A disorder that exists at birth or before birth
- Structural Deformities
- Functional disorders
What are some causes of congenital abnormalities?
Genes Chromosomes Intrauterine environment Teratogen exposure Metabolic requirements
Describe the structure of the blastocyst?
It has 2 cell types:
Outer ring layer = TROPHOBLAST = forms the extraembyronic structures (including the placenta)
INNER CELL MASS = holds the future embryo
When does the blastocyst implant into the uterine wall? Describe this process
At around day 5-10
It sinks into the wall and is enveloped by the uterine epithelium forming the interface between the mother (waste and nutrient transport)
Describe what happens to the inner cell mass as it implants into the uterine wall
It begins to compact (cells get closer together) and also cells begin to DIFFERENTIATE
they differentiate into a two layered structure called the epiblast on top and the hypoblast below
What is the primitive streak and where and how is it formed?
It is formed just after the differentiation of the two inner cell mass layers at the centre of the embryo (dividing the embryo in half in the saggital plane.
It is formed by cells of the epiblast migrating towards the centre and into one another forming a small bump
Describe the process of gastrulation
When the cells of the epiblast migrate to form the primitive streak, these cells eventually push against each other and burrow into the hypoblast and in between the two layers.
This forms a third layer in between the bilaminar disc.
As the cells migrate, they differentiate further and there is the formation of the 3 germ layers
What are the three germ layers?
Ectoderm
Mesoderm
Endoderm
Each layer gives rise to distinct lineages of tissues in the adult body
What is neurulation? What is the first structure to form to begin this process?
It is the formation of neural elements in early embryogenesis.
The first structure to appear is the NOTOCHORD - by the further differentiation of the cells in the centre of the mesoderm = a core just under the primitive streak
What is the purpose and fate of the notochord?
Its main purpose is in differentiation (doesn’t have much role or function in the adult except in some formation of the IV discs)
In embryogenesis: the notochord is important to organise the embryo into left, right, top and bottom
What occurs as a consequence of the formation of the notochord?
The notochord induces a change/differentiation in the ectodermal cells just above it (causes a ‘thickening’ of sorts) leading to the formation of a NEURAL PLATE
What happens to the neural plate cells of the ectoderm?
They dive in and burrow into the mesoderm and form a ting like structure called the neural tube along the length of the embryo
The neural tube zips up on itself to form a closed tube. As it does so what happens to some of the cells of the ectoderm?
They break off in the process and fly out into the mesoderm and away from the neural tube (acts like a fourth germ layer).
They are known as the neural crest cells and they form and differentiate into their own tissues.
What does the neural tube go on to eventually form?
The brain and the spinal chord
What are some derivatives of the neural crest?
Dorsal root ganglia Sympathetic and parasympathetic ganglia Enteric ganglia Schwann cells Melanocytes (pigment cells of the skin) Dentine (central part of the teeth) The MUSCLE, CARTILAGE and BONE of the SKULL, JAW, FACE and PHARYNX
Describe the origin and formation of the face
As the neural tube closes (from the top to bottom) the neural crest cells migrate from the back of the head to across the lateral sites to meet each other in the anterior midline.
Describe neural crest defects: cleft lip and palate
Occurs in 1:1000 live births
Migration of cells is very difficult (long distant migration that requires very tightly regulated signalling)
If something goes wrong then the cells are unable to meet in the midline and form an incomplete face
What are some derivatives of the mesodermal layer?
Dermis (not epidermis) Muscles Skeleton Urogenital Tract Heart and blood vessels Wall of the gut and respiratory tract Haemopoetic tissue (blood) Pleura, pericardium and peritoneum
What is the difference between the terms mesoderm and mesenchyme?
Mesoderm describe from which germ layer tissues are derived.
Mesenchymal refers to the shape and behaviour of cells - loose and independently moving, free with no attachments to its neighbouring cells (which is opposite to epithelial which means connected to one another)
Mesoderm cells often form mesenchymal tissue but they are not mutually exclusive terms
What are the three zones of the mesoderm?
They are distinct zones and they also behave differently to one another:
- Paraxial (medial) - close to midline
- Intermediate - narrow zones
- Lateral - on the edges
What structures arise from the paraxial mesoderm?
Dermis of the skin
Axial skeleton
Axial limb and muscles
What structures arise from the intermediate mesoderm?
Urogenital System
What structures arise from the paraxial mesoderm?
Ventrolateral body wall Limb skeleton Visceral pleura, peritoneum and pericardium Blood vessels and blood forming tissue Heart Wall of the gut and respiratory tube
What forms in the flat sheet of paraxial mesoderm on either side of the neural tube?
Bumps or swellings of cells called SOMITOMERES that appear progressively down the length of the mesoderm (from rostral to caudal end)
What happens at the 20 somitomere stage?
The 8th pair of somitomeres becomes larger and cells that make it up differentiate to the point that the somitomere separates itself for form a SOMITE
Once this occurs there is progressive replacement of the somitomeres into somites in the caudal direction
What is the importance of this somite formation only occuring from the 8th pair of somitomeres onwards?
Somitomeres form the head whilst the somitomeres (somites) from the 8th onwards form the neck and body (vertebral column)
There is a further differentiation of the somites into the three layers. Describe this zone formation
The somite splits in two: sclerotome medially and the dermomyotome laterally
The dermomyotome then splits into two zones: dermatome laterally and myotome medially (that separates it from the sclerotome)
What do each of the zones of the somite give rise to?
Sclerotome: axial skeleton (bone and cartilage)
Myotome: axial muscles from medial, appendicular (arm and leg) muscles and body wall musculature from the lateral
Dermotome: dermis of the skin
Describe limb development
Limbs grow out in a process similar to budding, they extend out from a bump of the myotome.
As cells bud out, the outer layer of surrounding ectoderm is pulled along with it (nervous structures)
What is the limb field?
A patch of mesoderm that has the capacity to form the limb.
It is SELF CONTAINED (highly specialised patch) meaning that transplantation of this whole patch of cells onto another embryo can cause a whole limb to arise from it. - removal of the free limb patch can be compensated for by surrounding tissue (peribrachial tissue) - removal of this means no compensation.
What happens as the limb starts to grow in terms of signalling processes?
The ectoderm begins to take over sending signals upwards to form the skin and downwards into the mesoderm - becomes the controlling factor
Describe digit development
The feet and hands start as flat, symmetrical discs
Digits are sculpted into this disc by a process of apoptosis to free out the finger
What is a consequence of dysfunction of the programmed apoptosis in digit development?
Webbing of syndactyl (fused fingers)
Describe limb abnormalities
Occur in 1:500 live births
Can be due to genetic problems or due to mechanical problems (eg. the limb protrudes out onto the wall of the uterus or the uterine wall impacts the ectoderm which is the control and thus control becomes haphazard).
Thalidomide drug
What are the derivatives of the endoderm?
Epithelial tissues:
GIT, RT, Tonsils, thyroid and parathyroid, Thymus, liver and pancreas
What is important about the relationship between the endoderm and mesoderm?
Often mesoderm is required to complete the tissues of the endoderm: very intermingled mixtures of tissue
Describe the formation of the coelum
The lateral mesoderm splits into two parts: the somatic which pairs with the ectoderm and the splanchnic which pairs with the endoderm.
The lateral sides of the embryo pinch in towards the centre and brings together the two sides of the splanchnic mesoderm to meet at the centre. When they meet they form a ‘purse string’ action that closes off most of the gut tube from the yolk sac below.
The two somatic mesoderm sides also move towards the centre and as they converge into the centre the splachnic cells meet the somatic cells and form the coelum (a cavity around the gut)
Prior to three weeks, does the gut tube have any openings? (with the exception of communication with the extraembryonic coelum/yolk sack?)
No - there is no moth or anus yet at this stage
Describe the formation of the mouth
The oral pit = stomadeum
the oral plate separating the foregut from the somadeum breaks forming the oral cavity
Why is the development and opening of the anus a little later in the process?
The anal pit = proctodeum
The anal end of the gut tube ends at a blind ending called the cloaca and the anus itself is sealed by the CLOACAL MEMBRANE (made of both endo and ectoderm).
Here the urogenital and the gut open together - there is a need to separate them first
Describe the formation of the anus
Coming off the gut is another branch called the ALLANTOIS.
Between the alllantois and the hind gut is the URORECTAL SEPTUM which is a piece of mesoderm that
The cells of this septum grow and extend back eventually connecting with the cloacal membrane that is blocking the proctodeum. Thus it divides the proctodeum into the anus and the opening of the urogenital tract.
What does the allantois give rise to?
The bladder and the urogenital tract
What are the pharangeal arches?
Gill-looking structures on either side of the pharynx. They are never gill-like in function.
They are modified to form the jaw, blood vessels, muscles of mastication and facial movement, middle ear, eustachian tube, hyoid, larynx, pharynx and the glands.
The pharynx is lined with endoderm and they give rise to strctures. Each structure is associated with a paryngeal pouch which sits between the pharyngeal arches. What are the distinct pouches?
Arch 1: eustachian tube and external ear canal
Arch 2: palatine tonsil
Arch 3: inferior parathyroid and epithelial thymus
Arch 4: superior parathyroid, thymus and postbranchial bodies
From where embroyonically is most of the heart from?
The lateral (splanchnic) mesoderm
Describe the formation of the heart
Initially two bilateral tubes (endocardial tubes) are formed by the splanchnic mesoderm and they are brought together in the midline.
They fuse together to form the heart
Describe the fusion of the endocardial tubes
Tubes connect in the midline and fuse. This fusion involves the breakdown of connections forming a single tube.
BUT the rostal and caudal ends keep their two separations (form the inflow and outflow tracts)
Describe the folding of the heart tube
It is an ASYMMETRICAL folding, the tube becomes a U shape and folds over itself forming the different chambers of the heart with coordinated fusion and breakdown of connections.
Describe the formation of the left and right ventricles
The intraventricular septum grows as a stalk up towards the base of the heart and eventually closes as membranous parts of the septum
Describe congenital heart defects
1:200 live births
Often of no consequence in the fetus: becomes an issue immediately after birth
Commonest 25% is ventricular septal defects: failure to divide the single ventrical into left and right
Why does failure of the neural crest cell migration affect development of the heart?
The neural crest cells contribute to structures of the vessels of the heart (they migrate in) - failure of migration results in malformed atrial outflow
At one month the gut tube consists of three major parts. What are they?
Foreget
Midgut
Hindgut
What is connected to the midgut?
The yolk stalk that connects the midgut to the yolk sac
Different parts of the gut are connected to (and eventually supplied by) different blood vessels, what are they?
Foregut: coeliac artery
Midgut: superior mesenteric artery
Hindgut: inferior mesneteric artery
These arteries continue to supply all derivatives off these part of the gut for the rest of life
Describe the growth and development of the GIT
It grows as a tube along the length of the fetus.
It begins to buckle and gold in the midgut around the stalk and rotates around the axis of the yolk stalk (and SMA) making the FIRST FOLD of the gut = folds the large intestine over the small intestine
Further elongation and further folding gives rise to the classic intestinal structures
What is meant by the ‘normal herniation’ of the GIT?
The gut tube grows at a very quick rate and often it does so too quickly and becomes to big for the body cavity (about week 6-7).
And so the gut pushes into the yolk stalk and spills gut and gut contents into the outside world (natural hernia)
By week 9, the body cavity grows large enough and the gut is retracted back
What happens when failure to retract occurs?
Called Omphalocele
often more due to failure to seal the abdominal wall properly
1: 25 000 live births
Describe the development of the viscera
The trachea, lungs, liver and pancreas appear as endoderm outpocketings that interact with the mesoderm
What is situs invertus?
1:10000 live births
The body organs are arranged as a mirror image: no deficits present
Caused by cilia generating opposite (left to right) spiral currents producing opposite changes that are otherwise completely normal
How does the embryo determine left from right?
Anterior-posterior axis is determined at the first cleavage of the zygote
Dorsal ventral axis is set by the blastocyst (where the inner cell mass is is the dorsal)
The left-right axis is set by the primitive groove
Describe the development of the urogenital system
Formed from the intermediate mesoderm (except the bladder and urethra which are from the alliantois off the gut tube)
Before the development of the definitive kidney, there is the development of structures to form the mesonephros (precursor to the definitive kidney) what is this process?
Pronephros
Forms nephrostomes and pronephric duct
Nephrostomes degenerate and the pronephric duct becomes the mesonephris duct with mesonephric tubes
Describe the development of the definitive kidney
The kidney is formed by mesoderm
Mesonephros degenerates leaving the mesonephric duct which grows and buds down the base forming separate kidney and ureters.
What is the connection made by the mesonephros and how is it different for males and females?
The mesonephros connects to the developing testes in males and makes the vas deferens. This disappears in females.
Describe the development of the external genitalia
Initally identical
The genital tubercle is present after the cloacal has split to form the urogenital opening. The tubercle becomes the clitoris in females and this is it.
In males it becomes the glands of the penis and the shaft is made by sealing together the edges of the urogenital membrane, as it happens the urogenital opening follows to the tip
What is Hypsopadiasis?
1:300 where the urogenital folds fuse incompletely to leave a urethral opening on the base of the penis instead of the tip
When is all major organogenesis completed by?
The first trimester (3 months)
When do the three germ layers appear?
2-3 weeks
