Embryology Flashcards
What takes place on day 0 of development?
Fertilisation
Where does fertilisation initially take place?
Ampulla
How long does fertilisation take?
Around 20 hours
What is a zygote?
A fertilised oocyte containing the DNA of the ovum and the spermatocyte
What happens around 24 hours after fertilisation?
The zygote begins to increase its number of cells by rapid mitosis, though the actual size doesn’t increase
What is a blastomere?
A type of cell produced by the cleavage (cell division) of the zygote after fertilisation
What is a morula?
An embryo at the early stage of development, consisting of cells (called blastomeres) in a solid ball
What is the morula contained within?
Zona pellucida
What are the cells inside the morula called?
The inner cell mass (embryoblast)
What are the cells on the outside of the morula called?
Outer cell mass (trophoblast)
What will the outer cell mass become?
Supporting structures for the embryo
What does the zone pellucida do?
Protects the zygote from any more sperm
What occurs around 4 days after fertilisation?
Morula passes into uterus
What do the trophoblast cells do to the luminal fluid in the uterine cavity?
Pull it into the centre of the morula
What is a blastocoele?
A fluid filled region of the blastocyst
What happens to the inner cell mass as the blastocoele forms and what is become known as?
Inner cell mass is pushed to one end
Becomes known as embryonic pole
What is a blastocyst?
Structure formed around 5 days after fertilisation and possesses an inner and outer cell mass and a blastocoele
What happens to the zone pellucida around 5 days after fertilisation?
What does this allow?
Blastocyst loses zona pellucida
Allows it to grow in size and interact with uterine wall
What is triggered when the blastocyst attaches to the endometrial epithelial lining of the uterus?
Changes in the trophoblast cells
Changes to the endometrium in preparation for implantation
What are the three phases of the menstrual cycle?
Proliferative phase
Secretory phase
Menstrual phase
When is the proliferative phase of the menstrual cycle and what happens during it?
Day 5-13 of menstruation, prior to ovulation.
Increased thickness of endometrium and increase in vascularisation
What happens in the secretary phase of the menstrual cycle?
Arteries and glands become coiled and secretions increase, helping to maintain thickness of endometrium
Why does implantation occur?
Allow developing embryo to take oxygen and nutrients from mother
When does implantation of the blastocyst begin?
6-7 days after fertilisation
When the blastocyst ‘sticks’ to the uterine wall, where is the embryonic pole?
Closest to endometrium
If it isn’t it rotates till aligned with the decidua
What is the entire surface of the endometrium covered with and what happens if fertilisation doesn’t occur?
It’s covered with decidual cells, which shed, along with the spongy and compact layers if fertilisation does not occur
What is the decidual reaction?
Endometrial decidua react to contact of blastocyst by increasing secretory functions of endometrium at area of implantation
At implantation what do the cells accumulate?
Fats and glycogen
What is a syncytium?
Multinucleated barrier that form the interface with the maternal blood stream
What is the syncytiotrophoblast?
Epithelial covering of highly vascular embryonic placental villi, which invade the wall of the uterus to establish nutrient circulation between the embryo and mother
What part of the trophoblast forms the syncytiotrophoblast?
Trophoblast cells that are in contact with the endometrium
What do the trophoblast cells that are not in contact with the endometrium become?
Cytotrophoblast
What happens to cells that become the cytotrophoblast?
Spread out and become thinner
What is the bilaminar disc?
Where cells of embryoblast have differentiated and become two layers: epiblast and hypoblast
What forms as a space above the epiblast?
Amniotic cavity
What are the cells that line the amniotic cavity called and what do they do?
Amnioblasts
Secrete amniotic fluid
What space appears below the hypoblast on day 9 and what does it do?
Primary yolk sac
Provides nutrients for the embryo until placenta is formed
On day 12 what begins to form from the syncytiotrophoblast?
Lacunae
What do the lacunae of the syncytiotrophoblast do?
Join up and meet with the maternal blood vessels to become the maternal/foetal blood interface
What does the chorionic cavity form from?
Extra-embryonic coelom
By which day post-fertilsation has the endothelium of the endometrium completely closed over the blastocyst?
Day 12
What happens to the primary yolk sac after day 13?
It becomes known as the secondary yolk sac
What is the connecting stalk?
A yolk sac diverticulum, derived from extra-embryonic mesoderm
Goes on to form umbilical cord
What do decidual cells have a high capacity to secrete?
What is their main functional quality?
Laminin and fibronectin
Have high adhesive qualities
What happens in gastrulation?
Single-layered blastula is reorganised into a trilaminar structure known as the gastrula
What are the three germ layers?
Ectoderm
Mesoderm
Endoderm
When is gastrulation initiated?
14 to 15 days after fertilisation
What is the primitive streak?
A depression that runs on the epiblastic surface of the bilaminar disc
Is restricted to caudal half of embryo
What is the primitive node?
A round mass of cells at the cephalic end surrounding the primitive pit
How are the germ layers formed in gastrulation?
Epiblastic cells migrate towards the primitive streak and slip under (invaginate) the epiblast later to form new layers
What happens to the first cells that invaginate in gastrulation?
They replace the hypoblast later and become the endodermal layer
From what is the mesodermal layer formed?
Epiblast cells that sit between the epiblast and endodermal layer
From what is the ectodermal layer formed?
Remaining epiblast layer
What will the ectoderm form? Give examples.
External layer of the embryo:
Epidermis of skin
Neural crest cells (go on to form nervous system)
What will the mesoderm form? Give examples.
Major contributor to embryo and cells: Bones Cartilage Muscle Cardiovascular and respiratory system Kidneys Spleen Dermis of skin
What will the endoderm form? Give examples.
Epithelia that line passages exposed to external substances: GI tract Lungs Epithelia of urethra and bladder Thymus Thyroid
What is the embryonic period?
First 8 weeks after fertilisation
What happens during the embryonic period?
Major structures of the embryo formed
What happens in the foetal period?
Rapidly grows in size, mass and complexity
What is the difference in embryologists and clinicians timings?
And why?
14 days
Embryologists measure from fertilisation
Clinicians measure from LMP
What is the gestational period according the embryologists and clinicians?
38 weeks or 9.5 lunar months to embryologists
40 weeks or 10 lunar months to clinicians (equal to 9 months and 7 days)
By what three ways can growth occur?
Proliferation
Hypertrophy
Accretion
What is proliferation?
An increase in cell number by cells dividing to produce daughter cells
What is hypertrophy?
An increase in cell size
What is accretion?
An increase in tissue by cells increasing the amount of extracellular matrix
What is differentiation?
Proces by which cells and tissues become specialised
How do cells know where and what part of the body they will become?
Through signalling proteins and connections between neighbouring cells
What is morphogenesis?
Formation of a shape during development
How does morphogenesis occur?
Through migration and adhesion of cells
When does the development of the digestive system occur?
During the fourth week of development
What is the primitive gut?
A cavity formed by the endodermal cells, which is the forerunner of the GI tract
What is the cranial end of the primitive gut covered by?
Bucopharyngeal membrane
What is the caudal end of the primitive gut covered by?
Cloacal membrane
What does the bucopharyngeal membrane separate?
Foregut from stomodeum
What will the stomodeum develop into?
Oral cavity
What does the cloacal membrane separate?
Hindgut from proctodeum
What will the proctodeum develop into?
Anus
How is the gut tube separated?
Into anterior foregut, intermediate midgut and posterior hindgut.
Divided by region of the rube that remains attached to the yolk sac and the the branches of the aorta that supply each part
What happens to the yolk sac at the 5th week?
Constricts and detaches from the midgut and the midgut seals
How does the stomach develop from the primitive gut tube?
A part of the foregut begins to dilate at week 4
Dorsal side grows faster than ventral
Then rotates to bring left side around to become ventral surface
What does the rotate of the stomach do to the duodenum?
Moves it into its adult C-shaped position
What does the rotation of the stomach do to the bile ducts?
Moves the entry of the bile ducts into the duodenum posteriorly
What happens to the midgut at approximately week 6?
Has grown so quickly it cannot be contained in abdominal cavity and so herniates into umbilical cord
How does the midgut rotate?
What does this do to the caecum?
270 degrees counter-clockwise
Brings caecum from inferior abdomen to left of developing small intestine, to top of abdomen and then descends into right lower quadrant
When does the midgut re-enter the abdomen?
About week 10
Name the factors that affect re-entry of the midgut into the abdomen.
Growth of the abdomen
Regression of the mesonephric kidney
Reduced rate of liver growth
What does the urorectal septum divide the cloaca into?
Primitive urogenital sinus anteriorly
Anorectal canal posteriorly
What is the ventral mesentery derived from?
Septum transversum
What will the dorsal mesentery form?
Greater omentum
Mesenteries of small and larger intestine
What will the ventral mesentery form?
Lesser omentum
Falciform ligament
What is the main embryological function of the liver?
Haematopoiesis
In what week does the liver start forming bile?
Week 12
What is the period of respiratory development?
Day 28 after fertilisation continuing to childhood
What are the five stages of lung development?
Embryonic stage Pseudoglandular stage Canalicular stage Saccular stage Alveolar stage
When and what happens in the embryonic stage of lung development?
Upto week 5
Initial bud and branching
When and what happens in the pseudoglandular stage of lung development?
Week 6-16
Complete branching
When and what happens in the canalicular stage of lung development?
Week 17-24
Terminal bronchioles
When and what happens in the saccular stage of lung development?
Week 25 to birth
Terminal sacs and capillaries come into close contact
When and what happens in the alveolar stage of lung development?
Week 36 onwards
Well-developed blood-air barrier
How does the development of the respiratory system begin?
Growth of an endodermal bud from the ventral wall of the developing gut tube in week 4
What splits the dorsal foregut and ventral lung bud?
Tracheoesophageal septum
Which cells produce surfactant?
Lung-specific type II alveolar cells (pneumocytes)
What are the cells of gaseous exchange?
Type I alveolar cells (pneumocytes)
What is respiratory distress syndrome caused by?
A lack of surfactant resulting in lung collapse
How many alveoli are there at birth compared to an adult lung?
Around 20-50 million at birth
Increasing to 400 million in an adult lung
What is neurulation?
Transformation of the neural tube into the neural plate
What starts off neurulation in the ectoderm?
Signals from the notochord
What are the primary brain vesicles?
Prosencephalon (forebrain)
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)
What happens in primary neurulation?
Neural plate creases inward until the edges come in contact and fuse
What signal induces formation of the floor plate of the incipient neural tube?
Sonic Hedgehog (SHH) signal
By what day after fertilisation has the notochord formed?
Day 22
At the beginning of neural tube formation, what are the openings at either end called?
Cranial and caudal neuropores
What day does the cranial neuropore close?
Day 24
What day does the caudal neuropore close?
Day 26
Where would neural crest cells be found on the neural plate?
At the border between the neural plate and the epidermis
As the neural tube forms, what happens to neural crest cells?
Leave neural tube and migrate to other parts of the embryo
Where do spinal defects in the embryo normally arise and why?
In the cranial or caudal end as the neuropores are the last bit to close
What are the subdivisions of the embryonic forebrain and what do they form?
Telencephalon - cerebral hemispheres
Diencephalon - Pituitary, hypothalamus, thalamus
What are the subdivisions of the embryonic hindbrain and what do they form?
Myelencephalon - medulla oblongata
Metencephalon - cerebellum, pons
