Embryology Flashcards
What type of cell forms all embryonic and extra embryonic cell types?
Totipotent cells
What type of cell only forms embryonic cells types?
Pluripotent
What is a multipotent cell?
Forms multiple cell types typically from one germ layer
What are tri/bipotent cells?
Cell that forms 2/3 different cell types
What cells only form one type of cell?
Unipotent
What is the definition of specification?
Commitment to a particular fate that can be changed if those cells are moved to a new environment.
What is the definition of determination?
Commitment to forming a particular cell type.
What is the definition of differentiation?
When cells acquire their functional characteristics for that cell types.
What type of cell commitment is localised determinants?
Localised determinants are a type of cell that is made depending on the distribution of certain molecules.
Define regulative development
This is when a single embryo cell is capable to give rise to all cell types. This suggests that cells communicate with each other to restrict their potency.
Define embryonic induction
This is the process by which a cell’s fate is changed by signals from an adjacent group of cells. This requires a signalling and responding cell.
Describe competence
The ability of a cell to respond to a signal
How are embryonic signals transferred?
Embryonic signals are released from one cell population and they diffuse across the short space to act on nearby cells. The signals bind to cell receptors, and this causes a cascade of intracellular signalling which results in changes in gene expression (secondary messenger model).
Important signalling proteins include Transforming Growth Factor Beta, Fibroblast Growth Factor, Wingless/Int1 and Hedgehog.
What is epigenesis?
When organs and tissues are added progressively during development with complexity increasing over time.
What are morphogens?
Molecules that specify different cell types at different concentrations.
Outline the experiment carried out by Hans Dreisch that showed that cells are not filled with determinants but can form all cell types through regulation
Hans Dreisch isolated blastomeres from urchin embryos. He then divided then when they were in their cleavage stage. The embryos still went on to develop into larvae (although smaller) which shows that their development is conducted by regulation. eg. monozygotic twins are also an example of this
Overall, cells are divided and polarity determined by determinants, but is told what cell type to become by inductive proteins.
What is the function of the SHH (Sonic HedgeHog) gene?
The gene codes for the Sonic Hedgehog protein, which acts as a chemical signal essential for cell growth, specialisation, and normal shaping(patterning) of the body. It changes fate within cell types.
What can mutated versions of this gene cause?
They can cause congenital abnormalities and disrupt anterior/posterior polarity in each segment of the embryo. For example in Drosphila fruit flies it causes a shortened embryo with denticles (tooth projections) on the ventral (underside) surface.
What happens if there is not SHH?
The receptor PATCHED1 cannot bind to its ligand of SHH. This inhibits fusion of Smoothened receptor to the cell surface membrane (this receptor is needed for embryonic development and adult tissue homeostasis). The Gli transcription factors that activate and inhibit transcription by binding to Gli responsive genes that are important for growth, are turned into repressors so these are not transcribed. Leads to denticles.
What happens when SHH is present?
SHH can bind to the receptor PATCHED1. This stops inhibition of the Smoothened receptor so the Gli factors become transcriptional activators that can transcribe Gli proteins.
Outline the function of SHH in the development of the spine of the embryo (1)
In the ventral(lower) part of the neural tube there are many different types of neuronal cells. Their cell fate is specified by the notochord (embryonic midline structure that provides mechanical and signalling cues to pattern (shape) surrounding tissue in the developing embryo). The notochord induces ectopic floor plate (midline glial structure to provide structure and support) and motor neurones when grafted to the side of the neural tube. The floor plate induces motor neurone formation itself. They are both signalling centres that act on adjacent cells to change cell fate. Neither is formed if there is no notochord. The notochord expresses SHH and acts on the ventral neural plate, causing it to become the floor plate. Floor plate cells then begin producing SHH and acts on adjacent cells to change their cell fate. Without SHH certain neurones don’t form or are in the incorrect place.
Outline the function of SHH in the development of somites
In somites it induces the formation of sclerotome, a transient embryonic tissue made of pluripotent, mesenchymal stem cells located in the ventromedial region of the somite. This induces the formation of different cell types. The SHH and WNT signalling pathway induces the epaxial myotome (myotome above axis in the dorsal area ~from motor fibres). The WNT and BMP signalling induces the hypaxial myotome (myotome beneath the axis, ventral). WNT signalling pathway also induces dermatomes.
Outline the function of SHH as a morphogen
It can induce multiple different cell fates in a concentration dependant manner - there is a higher concentration ventrally and lower concentration dorsally. The varying concentration produces different cell types.
What are blastulae?
They are hollow spheres of cells, or blastomeres, produced during the development of an embryo by repeated cleavage of a fertilised egg. The cells of the blastula form an epithelial (covering) layer, called the blastoderm, enclosing a fluid-filled cavity, the blastocoel.
Outline the Animal Cap Assay experiment that can be carried out to see the effect of different signal inducers on mesoderm induction
Blastulae from organisms such as amphibians have vegetal and animal poles. Animal pole is where cells divide rapidly, and is above the vegetal pole which is not as active. Mesoderm can be formed when signals pass from the vegetal hemisphere to the animal hemisphere, and the mesoderm is only formed from the animal hemisphere. These signals can be detected by mis blastula fragments being incubated with candidate factors and assayed for mesoderm induction. If mesoderm inducing factors are present then the animal cap elongates and differentiates to mesodermal tissues.
This induction requires the NODAL gene. This was found out as a vegetal pole from an embryo that lacked a functional NODAL gene could not induce mesoderm in the animal cap - indicating that NODAL signalling is needed for mesoderm induction.
Same experiment can be repeated with other signal inducers to see their effect on mesoderm induction.
Define an oocyte
A cell capable of dividing by meiosis to form an ovum
How does the egg become fertilised and move into the uterus?
There are finger like projections called fimbriae at then ends of the fallopian tube that can sense when the egg is going to be released, so can capture the egg and pass it into the fallopian tube. The sperm will meet the egg in the upper region (ampulla) of the Fallopian tube. The fertilised egg then slowly passes down the oviduct. The first division will take place 1-2 days after fertilisation. The embryo is surrounded by the zone pellucida membrane, which stops the egg from implanting in the Fallopian tube. Division continues until a call of cells with a fluid filled cavity is formed = blastocyst. This will enter the uterus 4-5 days after fertilisation.
Define oogenesis
Production of the egg
How does oogenesis occur?
Firstly, the primordial germ cells (germline stem cells that give rise to gametes in vertebrates) move to the front of the gonad (organ that produces gametes) and divides to produce more cells. These cells are surrounded by a layer of granulosa cells (critical somatic component of the ovary that are essential for follicle development). Granulosa cells have LH receptors on their CSM, and aid production of progesterone and oestrogen - steroid hormones. Once entering the ovary it becomes an oogonium, which undergoes mitosis to increase cell number. Then it undergoes meiosis in the gonad, however this only occurs up to PROPHASE 1. After reaching puberty some primary oocytes have meiosis reinitiated and the first division is completed. This produces a large secondary oocyte and a small polar body which contains discarded chromosomes. The secondary oocyte will begin meiosis 2 but will only reach METAPHASE 2. This is when the egg is ovulated. Only after ovulation meiosis is completed and it is fertilised. This results in a fertilised egg and another polar body. Many primary oocytes are produced however only a fraction of these actually reach the stage of puberty.
How does spermatogenesis occur?
In the testes meiosis is not initiated until after puberty. The spermatogonium (early sperm cell) undergoes the first division of meiosis, which produces one primary spermatocyte. This then divides into two spermatocytes(same size). Then in meiosis 2, this becomes 4 spermatids are produced, which undergo differentiation to produce 4 spermatozoa.
Outline the process of fertilisation
Sperm cells become more active when they reach near an egg cell. There is a sperm receptor on the zone pellucida (ZP3) of the egg. After interacting with this receptor the acrosome of the sperm bursts, releasing enzymes that can digest the zone pellucida.
A specific protein on the head of the sperm interacts with a specific protein on the CSM of the egg, allowing the sperm to fuse with the egg and for the nucleus to pass into the egg.
This interaction also causes an increase in intracellular calcium levels, and this causes the egg nucleus to restart meiosis, and for the cortical granules to fuse with the plasma membrane. These release enzymes that interact and modify proteins on the zone pellucida so that no more sperm can fuse with the egg. When the male and female DNA comes together it is now called a zygote.
What is parthenogenesis?
This is when the mother produces clones of her off spring (doesn’t happen in mammals). If only female chromosomes present = small yolk and small trophoblast. If only male chromosomes = small yolk and large trophoblast.
The reason for this is genomic imprinting. 80 genes are methylated during oogenesis or spermatogenesis, which means that they are not transcribed due to being inaccessible to transcriptional factors.
However these genes are needed for embryonic development, and are only activated when both male and female copies of chromosome are present. (so if cannot be fertilised these genes are not activated).
What are cleavage divisions?
The mitotic divisions that the fertilised egg undergoes as it moves from Fallopian tube to the uterus. These are not accompanied by cell growth. The large egg is split into smaller cells of around 10-20 micrometres. This takes 12-24 hours, and will remain like this for 5 days.
These then undergo compaction, where cells maximise contact with each other. This creates 2 cell populations.
What is the outside population of cells of a blastocyst called?
Trophoblast
