Embryology Flashcards
What is the embryonic period?
First 8 weeks after fertilisation
What is the correct term for a baby from fertilisation to the end of Week 3?
Conceptus
Rarely used, so embryo also works
What is the correct term for a baby from Week 4 to Week 8?
Embryo
What is the correct term for a baby from Week 9 to Birth?
Foetus
What are the six phases of embryogenesis?
Gametogenesis Fertilisation Cleavage Gastrulation Morphogenesis Organogenesis
What occurs during gametogenesis?
Germ cell formation
Oogenesis = Oocyte (egg)
Spermatogenesis = Spermatoza (sperm)
What occurs during fertilisation?
Formation of the zygote
What is cleavage?
Period of rapid cell division
Formation of morula then blastocyst inside the zone pellucida
What occurs during gastrulation?
Formation of the germ layers
Ectoderm, mesoderm and endoderm formed
Body axes established
What occurs during morphogenesis?
Formation of the body plan Embryonic folding (tube within a tube)
What occurs during organogenesis?
Primordial of all organ systems
All organs begin and continue to develop at the sam time
What occurs during the foetal period?
Growth and weight gain
Tissues mature and become functional
Overt sexual differentiation (at roughly 10 weeks)
Bone laid down, connections made in CNS
How does the zygote develop all the cells that make up the human body, despite using only a relatively small number of processes?
The processes interact with each other, producing more complex outcomes
What are the six Primary Processes?
Cell Division Apoptosis Differentiation Cell Attachment Induction Cell Migration
What occurs during cell division?
There is an increase in cell number
What occurs during apoptosis?
Programmed cell death
Allows cells to act as a scaffold for further growth before dying off when they are no longer needed (e.g. webbing between fingers)
What occurs differentiation?
Change in appearance/structure
Adoption of new functions (specialisation)
What occurs during cell attachment?
Physical/functional linkages between cells creating coherent assemblies (tissues)
What occurs during induction?
Ability of one cell type to cause another to differentiate
Some cells release proteins which react with the surrounding cells in a specific manner
What occurs during cell migration?
Movement form one location to another
What are the three Secondary Processes?
Axis formation/Polarity
Folding/Rotation
Changes in mass/dimension
Why is axis formation/polarity important?
Determines which way is up Cells need to know where they are in relation to each other and the embryo as a whole
Why is folding/rotation important?
Gives the embryo its 3D form
allows the formation of complex organ structures (e.g. heart, gut)
What changes in mass/dimension can occur?
Hypertrophy = Increase in dimension with no increase in cell number
Hyperplasia = Increase in cell number with an increase in dimension
Compaction =
Decrease in cell mass with decrease in dimension
OR
Increase in cel number with no increase in dimension
Which three factors control development?
Genetic
Epigenetic
Environment
How do genetics control development?
Through gene expression
Tightly regulated in time and space
How do epigenetic control development?
Some genes show preferential expression for either the maternal or paternal copy
How does environment control development?
Placenta will pass on anything from the mother to the embryo
(e.g. alcohol in mother may lead to foetal alcohol syndrome)
What occurs during Phase 1 of fertilisation?
Sperm travels through a layer of corona radiata cells which pose no barrier
Binds in a human specific interaction with the zone pellucida glycoproteins
What occurs during Phase 2 of fertilisation?
Acrosomal enzymes are released from the sperm head which break down the zone pellucida
The sperm digests its way through and into the egg
What occurs during Phase 3 of fertilisation?
Egg and sperm plasma membranes fuse, and the nucleus of the sperm is released into the oocyte
What two things does sperm entry into the oocyte trigger?
Completion of Meiosis II
Release of cortical granules by the oocyte, causing zone pellucida hardening and prevents further sperm from entering
Where does fertilisation occur?
Ampulla
What specifically happens to the zygote during cleavage?
Zygote undergoes a series of mitotic divisions which subdivide the fertilised egg into many smaller daughter cells called blastomeres
Up to what stage are blastomeres thought to be totipotent?
Up to the 8 cell stage
Why does compaction of the zygote occur during cleavage?
The cells are rapidly dividing in an enclosed space (due to the zone pellucida)
What are the four parts of the blastocyst and what do they give rise to?
Zona pellucida = Protective outer shell which degenerates
Trophoblast = Placenta
Inner cell mass (embryoblast) = Embryo proper
Blastocyst cavity
At which point does the blastocyst generally attach to the uterine cavity?
6 days
What is the zone pellucida’s role in blastocyst travel along the uterine tract?
Prevents attachment to the walls
Provides protection
Holds everything together
At what stage does the zoo pellucida break down?
After sperm implantation in the oocyte
Generally occurs once the blastocyst reaches the uterus
Allows the blastocyst to attach to the uterine cavity
Where are the normal sites for blastocyst implantation?
Middle or upper dorsal uterine wall
What is an eptopic pregnancy?
when the blastocyst implants in a region which is not designed to support growth
When a blastocyst implants, it will send out signals for a blood supply; in abnormal sites, there is not the muscle to support this
May result in rupturing of cells and sever blood loss if undiagnosed
Where are the abnormal sites for blastocyst implantation?
Ovary Ampulla Uterine tube Lower part of the uterus Cervix or peritoneum
How does blastocyst implantation occur (6 days)?
Trophoblast sends out signal to the uterine epithelium and begins to work its way in
How does blastocyst implantation occur (7.5 days)?
Trophoblast basically turns into a syncytiotrophoblast
The leading edge acts as a membrane for this cell, and the cells inside will loose their individual membranes
The embryo blast will develop in two: epiblast and hydroblast
How does blastocyst implantation occur (9 days)?
Complete
A coagulation plug forms, enclosing the embryo within the uterine wall
Syncytiotrophoblast covers the entire embryo
Production of cells occurs at the hypobast end, spreading out to form a layer of primary yolk sac around the cavity
What is the appearance of the implanted embryo (9-12 days)?
The syncytiotrophoblast has extended and by erosion contacts maternal blood supply
The amniotic cavity is present above the bilayers embryonic disc and the definitive yolk sac below
The chorionic cavity develops and encircles the entire embryo
What would you see in the dorsal view of the embryo at week 2?
Oropharyngeal membrane = Site of the future mouth
Primitive streak = Driving force behind gastrulation
How do the three layers form during gastrulation?
Movement of epiblast cells through the primitive streak
Hypoblast is replaced to become definitive endoderm
Intraembryonic mesoderm layer formed between endoderm and epiblast
Epiblast becomes ectoderm
What is the fate of ectoderm?
Epidermis of skin
Nervous system
What is the fate of the mesoderm?
Becomes sub divided into three regions either side of the notochord:
Paraxial = Axial skeleton, parts of dermis, voluntary muscle
Intermediate = Urogenital system
Lateral Plate = Somatic part becomes lining of body wall, most of dermis and parts of limbs. Visceral part becomes mesothelial covering of organs and the cardiovascular system
What is the fate of the endoderm?
Lining of gut tube
Lining of respiratory tract
What occurs during lateral folding?
Converts open coelom into a closed cavity, creating thoracic and abdominal cavities and closing fore, hind, then mid gut
What occurs during cranial folding?
Forms the foregut ad defines the thoracic cavities
Developing heart tube displace into thoracic cavity
What occurs during caudal folding?
Cloacal membrane and connecting stalk are displaces ventrally alongside the neck of the yolk sac, forming the hindgut, anus and umbilical cord
What occurs during the third month of foetal development?
Eyes and ears position and face becomes more human looking
Limbs lengthen
Primary ossification centres in long bones and skulls
Sex can be determined by external genitalia
What occurs during the fourth and fifth months of foetal development?
Rapid increase in length
What occurs between the six month of foetal development and birth?
Rapid increase in weight
What are the main causes of congenital abnormalities?
Unknown
Genetic factors
Environmental (inc. infectious agnates, radiation, deficiencies, maternal disease)
Multifactorial
When is the embryo most at risk of defects forming?
Weeks 1 -2 = Loss
Weeks 3 -7 = Major abnormalities
Weeks 8 - 36 = Functional and minor abnormalities
Generally, specific structures are at most risk when they are developing - the timing of this varies from structure to structure
What is an Atrial Septal Defect, and what are some examples?
Defects of one or both of the atrial septae
Excessive Resorption of Septum Primum = Opening is too large for septum scandium to fully cover
Absent Septum Secundum = Communication between the two atria is not blocked after birth
Probe Patent Formamen Ovale = Septum primum and secundum fail to fuse, but mixing of blood is prevented by pressure difference (increased RA pressure can result in blood mixing)
What two things can cause Ventricular Septal Defects?
Muscular defect = most common, least severe
Membranous defect = relatively uncommon, very severe
What is Transposition of the Great Vessels?
Aorta exits right ventricle
Pulmonary trunk exits left ventricle
Deoxygenated blood pumped around the body
Due to failure of the septum of the outflow tract to spiral as it grows
What are the four component of the Tetralogy of Fallot?
Pulmonary stenosis
Hypertrophy of right ventricle
VSD
Over-riding aorta
