Embryology Flashcards
What is gametogenesis?
Formation of the gametes in the gonads where germ cells first under go mitosis to increase cell numbers to form:
Speratogonium
Oogonium
Both 46 chromosome
Explain process of spermatogenesis
After birth, the testes of newborns males haven’t progressed passed mitosis. After birth, the gonads are inactive until puberty where the spermatogonia undergo meiosis to form 4 haploid gametes
Explain process of oogenesis
At birth, oogonia have complete oogenesis and meiosis to produce 3 degenerate ova, polar bodies and one haploid ovum
Explain process of fertilisation
- Ovum swept into Fallopian tube by beating cilia
- Sperm surrounds ovum, but only one penetrated
- The (pro)nucleus of sperm enters and fuses with the (pro)nucleus of the ovum
- Forms a diploid zygote
What is a morula?
A solid mass of cells formed when the zygote divides by mitosis
What does the morula become and why?
Becomes a blastocyst as it must alter structure as the large number of cells in the morula mass means it is hard to get nutrition to the centre
How does the blastocyst form from the morula?
- A blastocystic cavity forms as cells accumulate to one end
- Accumulation of cells = Inner cell mass
- the outer lining of cells around perimeter = trophoblast
How long does the first division of the zygote take in becoming a blastocyst?
36 hours
2nd - 24hr
3rd - 12 hours
Name a maternally inherited mitochondrial disease which causes blindness
Leber’s hereditary Optic Neuropathy
What is a chorion?
Developed from the trophoblast, of the blastocyst, which decided into two layers which form the chorion
The chorion develops process; chorionic villi
What is an ectopic pregnancy?
When the embryo implants abnormally outside the womb, usually in the fallopian tubes. Can be dangerous, and usually due to abnormal cilia function
What is the chorions function?
Implantation process
Forms part of the placenta
Secretes Human Chorionic Gonadotropin (HCG) - used to detect pregnancy
What does human chorionic gonadotropin (HCG) do?
It’s a hormone which stimulate the ovaries to produce oestrogen and progesterone, to keep the endometrium intact
What is the decidua basalis?
Part of endometrium deep to the implanted conceptus, where the maternal side of the placenta is found
Chorion roll in the implantation process
- Implants blastocyst into endometrium
- enzymes from villi breakdown maternal blood vessels
- Forms part of the placenta
- Secrete HCG to maintain endometrium
What two cavities are formed following the formation of the bilaminar disc in the embryo?
Amniotic sac (above epiblast) Yolk sac (below hypoblast)
What is the function of the allantoic cavity?
Gas exchange and removal of liquid waste - later forms the urinary bladder
What are the 3 main functions of the placenta?
Foetal nutrition
Transport of waste and gases
Immune protection
Describe the foetal part and surface of the placenta
Smooth with foetal blood vessels and end of umbilical cord
Describe the maternal part and surface of the placenta
Decidua basalis of endometrium - rough and has maternal blood vessels
When does the placenta become fully mature?
18-20weeks
Describe fraternal/dizygotic multiple births
Comes from two separate zygote after two ova are released - genetically unique and have own placenta
Describe identical/monozygotic twins
One zygote is formed which divides into two and each cells develops into a different embryo - same genetic material and share one placenta
What occurs in week 3?
Gastrulation
Neurulation
Development of smites
Early CVS
What is gastrulation?
Formation of germ layers
What is neurulation?
Formation of neural tube
Where does the primitive streak form?
Formed in the midline of the epiblast by the dipping in of cells (invagination)
What happens in gastrulation?
Cells of the epiblast migrate down into the space between the epiblast and the hypoblast, and keeps dividing until they displace the hypoblast cells
This forms 3 germ layers and the trilaminar disc:
- Ectoderm
- Mesoderm
- Endoderm
What are the cells in the trilaminar disc specialised to eventually become?
Ectoderm - nervous tissue
Mesoderm - bone, cartilage and skeletal muscle
Endoderm - epithelial lining
What is the notochord?
Sends out signals which are picked up by receptors to initiate cell differentiation to form he neural tube
How is the notochord formed?
Cells in the primitive streak sink down to in between the mesoderm and the endoderm
Describe stages in neurulation
- Notochord signal ectoderm to form neural plate
- Dip forms in the plate which deepens
- Two end of dip join to form a tube and loses connection with ectoderm
What will the neural tube eventually form?
The brain and spinal cord at the dorsal end of body
What is the neural tubes affect on the mesoderm?
It induces the mesoderm to thicken
What are the 3 parts that the mesoderm separate into?
Paraxial
intermediate plate
Lateral plate: this divides further into somatic and splanchnic with the space in between called the intraembryonic coelom
What are the 3 mesoderm parts specialised to form?
Paraxial -> somites
Intermediate plate -> urogenital system (kidneys + reproductive)
Lateral -> body cavity and coverings
How many pairs of somites are there?
43
Describe folding of the embryo
Folding occurs in the transverse plane and sagittal plane due to differing rate of growth of structures - occur simultaneously
Describe lateral folding
The two ends join
Endoderm becomes innermost tube (gut tube which has connection to yolk sac)
Ectoderm surrounds outside
What happens to each somite and the fate of each?
Each somites separate further into three:
Dermatome -> dermis of skin
Myotome -> muscles
Sclerotome -> bones including vertebrae
What are teratogens?
Environmental factors that cause abnormal development as they’re able to cross the placenta
Name 2 examples of teratogens
Maternal measles -> congenital rubella syndrome
Thalidomide use -> malformed limbs
Give examples of environmental factors with can teratogens
Drugs
Alcohol/tobacco
Infectious agents: toxoplasma, rubella, cytomegalovirus, herpes (ToRCH)
Others - radiation
Give examples of genetic factors which cause abnormal development of embryo
Too many/few - turner’s, Down’s syndrome
Structural - deletions of genes
What period of embryo development is most sensitive to teratogens?
Weeks 3-8
What does the risk of teratogens depend on?
Critical periods of development
Dosage of drug/chemical/factor
Genetic constitution of embryo (some more susceptible than others)
What are the three parts to the gut tube formed by the endothelium?
Foregut
Midgut
Hindgut
What do the trachea and lungs form from?
The ventral wall of the foregut
At what week of development do the trachea and lungs begin to form?
4 weeks of gestation
Describe first stages of the respiratory primordium
Starts as a median outgrowth as the laryngotracheal groove (diverticum) form the ventral part of the foregut
What happens to the diverticulitis after its outgrowth?
Become invested with mesoderm and enlarges to form trachea and lung buds
What is the oesophagotracheal septum?
The septum that develops and separate the oesophagus and trachea
What is a fistula?
An abnormal or surgically made passage between a hollow tube and the body surface, or between two hollow tubes
I.e. between two epithelialised surfaces
What is atresia?
A condition in which an orifice or passage in the body is closed or absent
What is a tracheoesophageal fistula (TOF)?
The abnormal connection between the trachea and oesophagus
Where can atresia occur?
Can occur in the oesophagus, where the tube doesn’t lead to stomach, but ends, and there is a connection between the trachea and rest of oesophageal tube to stomach (fistula)
Where do the somatic and splanchnic mesoderm originate from?
Lateral plate mesoderm
What mesoderm does the visceral pleura develop form?
Splanchnic mesoderm
What mesoderm does the parietal pleura develop from?
Somatic mesoderm
What is the space in-between the pleura called initially?
Pleuroperitoneal canals
What develops to separate the pleural cavity from the pericardial cavity?
Pleuropericardial folds
What are the 4 key stages during the differentiation of the lung buds?
- Pseudoglandular
- Canalicular
- Saccular
- Alveolar
When does the pseudoglandar stage occur and what happens?
1-17weeks
Set up of all pulmonary structures except elements needed for gas exchange
When does the canalicular stage occur and what happens?
17-27 weeks
The terminal bronchioles gives rise to tubes that make up the respiratory part of the lung
Differentiation of epithelium into cuboidal secretory cells (Type II penumocytes) which will produce surfactant and flat cells (Type I)
When does the saccular stage occur and what happens?
27-40 weeks
Formation of alveolar sacs
When does the alveolar stage occur and what happens?
Post-natal: 32 - 8 years Formation of alveoli from terminal sacs 95% of alveoli formed postnatally Alveoli enlarge Number of respiratory bronchioles (and alveolar ducts and alveoli) increased as lungs increase, and adult size around 8yrs
What is Respiratory Distress Syndrome (RDS)?
Lungs are deficient in surfactant, which prevents their proper expansion and causes the formation of hyaline material in the lung spaces.
What is a major cause of RDS?
Surfactant deficiency
What are other congenital conditions of the lung?
Presence of accessory lobe
Lobe of azygous vein
Agenesis of lung
What is he lobe of the azygous vein?
Congenital condition where the azygous vein embeds itself into the R Lung, making it look like there’s an extra lobe
What is anagenesis of the lung?
Congenital condition where the lung didn’t develop
What are the four embryonic components that form the diaphragm?
- Septum transverse
- Pleuroperitoneal membrane
- Dorsal mesentery of oesophagus
- Muscular growth from lateral body walls
What does the septum transversum form in the diaphragm?
Central tendon
What does the pleuroperitoneal membrane form in the diaphragm?
Primitive diaphragm
What does the dorsal mesentery of oesophagus form in the diaphragm?
Median portion and crura of diaphragm
What does the muscular ingrowth from the lateral walls form in the diaphragm?
Peripheral parts of the diaphragm
Why is the diaphragm innervated by spinal nerves C3, 4, 5 when it is located lower in the adult body?
As the septum transverse was initially located opposite C3-5 somites then migrated down caudally during diaphragm development, bringing C3, 4, 5 with it
What are the congenital abnormalities associated with diaphragm development?
Failure of the diaphragm to completely close during development Herniation of the abdominal contents into the chest Pulmonary hypoplasia (incomplete development of the lungs)
What is a hernia?
Term used to describe a bulge or protrusion of an organ through the structure or muscle that usually contains it
What are three different types of diaphragmatic herniae?
Posterolateral
Anterior
Central
What are the two types of hiatal herniae?
Sliding: stomach through to oesophagus
Paraoesophageal: stomach slides up parallel to oesophagus
