Embryology Flashcards
How long is in-utero human development?
9 months (38-40 weeks) split into 3 trimesters
What is the name given to the first three weeks of in-utero development?
First trimester is called conceptus/embryo
What is the name given to the second trimester (weeks 3-8)?
The embryonic period
What is the name given to weeks 9-40 (third trimester) of in-utero development?
Foetal period
Describe the first trimester/conceptus
The embryo forms when mitosis of a fertilised egg begins, so even at the 2 cell stage it is technically classed as an embryo
What happens during the embryonic period?
Where differentiation starts to happen, cells start establish their future function and organ systems are established.
Why is the embryonic period crucial?
The potential for abnormalities or malformation is much higher. Embryo is most vulnerable to environmental agents such as viruses and tetarogens (any agent or factor that causes an abnormality following foetal exposure during pregnancy).
Define embryogenesis
The formation and development of an embryo.
What is gametogenesis?
Formation of a gamete- ova and sperm
What is a gamete?
Gametes are sex cells. A gamete is a cell with a compliment of 23 chromosomes (half a typical somatic cell).
Gametogenesis consists of mitosis and meiosis. Define these two processes and state what they lead to.
Mitosis- division of one cell into two identical cells
meiosis- division of a cell into two cells with half the genetic component and half the no.of chromosomes of the original cell.
Leads to genetic variation
Define oogenesis
Formation of female gametes
When does oogenesis start and finish?
Starts in development of the embryo- primordial germ cells originate in the yolk sac of the embryo.
Ends in menopause
What starts to happen to some primary oocytes by week 28-30 as they develop in the embryo?
Some cells have begun dying after week 20.
Primary oocytes begin meiosis but arrest in prophase until puberty.
When is reproductive life span determined?
In fetal life
Define spermatogenesis
Formation of male gametes- spermatozoa
When does spermatogenesis first occur?
At puberty and continues throughout life.
- each cycle takes approx. 64-74days
What is the production rate of sperm?
Approx. 6.5 million sperm per gram of testicular tissue/day
This decreases with age
What is the genetic outcome of oogenesis?
22+X
What are the possible genetic outcomes of spermatogenesis?
22+X or 22+Y
What is trisomy 21?
An additional copy of chromosome 21.
What is the typical cause of trisomy 21?
nondisjunction
Where does most nondisjunction occur?
75% of nondisjunction cases occur during oocyte formation
What does trisomy 21 lead to?
Down syndrome
Describe in detail the 4 stages of fertilization
- sperm binds in a human specific interaction with the zona pellucida glycoprotein (ZP3)
- Acrosomal enzymes released from the sperm head; the sperm digests its way into the egg.
- Egg and sperm plasma membrane fuse and sperm contents enter the egg
- sperm entry triggers:
- completion of meisos 2
- release of cortical granules by oocyte
How does the release of cortical granules help to prevent polyspermy?
Renders the zona pellucida impermeable to others
What is the zona pellucida? Describe it
- An extracellular matrix
- Transparent layer surrounding the oocytee
- specialized
- thought to be formed by oocyte secretions
- contains a no. of glycoproteins
What are some of the roles of the Zona Pellucida?
- protection during growth
- helping binding of sperm
- preventing polyspermy
- assists with blastocyst development
- helps prevent ectopic pregnancy
Where does fertilization typically occur?
In the ampulla of the uterine tube
What is cleavage?
The first cell divisions after fertilization.
After fertilization, the zygote undergoes a series of __________ divisions which subdivide the fertilised egg into many smaller daughter cells called _____________ .
- mitotic
- blastomeres
What are blastomeres thought to be up to the 8 cell stage?
Totipotent
What does it mean if a cell is totipotent?
Capable of forming or becoming all of the different types of cells in the body as well as becoming embryonic and placental cells.
Why are pluripotent cells different to totipotent?
Pluripotent can become any type of cell except for embryonic or placental cells.
Define mosaicism
Individuals with two or more cell lines with different chromosome compliments
Define mitotic nondijunction and state when it can occur
When chromosomes fail to separate equally
Can occur in early division of the zygote from mitosis I
What is the cause of mosaicism?
Error in mitosis
What is a morula?
16 + blastomeres
During cleavage, the overall size of the zygote __________ increase so the cells are getting _________ as they divide.
- doesn’t
- smaller
Describe compaction of the morula
- cells confined within the zona pellucida
- maximise available space by coming into closer contact with eachother and begin to form cell junctions
- outer cells (in contact with the zona pellucida) form extensive gap junctions
- fluid enters through ZP
what is the trophoblast of the blastocyst?
The embryonic part of the placenta
What is the inner cell mass in the blastocyst?
Embryo proper (embryoblast) also amnion and yolk sac.
What does the trophoblast eventually become?
The foetal or embryonic part of the placenta
What does the inner cell mass do?
Moves to one poll and forms the embryo itself
When is the blastocyst fully formed and ready to move to the uterus for implantation?
By days 4-5
Describe the process at round about day 5 known as blastocyst hatching
- Blastocyst hatches from the xona pellucida
- There is a series of associated contractions supported by enzymes dissolving the ZP
- Allows increased growth and allows blastocyst to access nutrient secretion from the uterus
Why is blastocyst hatching important?
Allows the embryo to interact directly with the endometrial lining of the uterus for implantation.
Where and when does implantation typically occur?
- Typically by day 6
- on the posterior or anterior uterine wall
When does the process of implantation begin?
When the trophoblast attaches to the endometrial epithelium, usually on the posterior wall but sometimes the anterior.
It needs to invade the endometrial, epithelial and connective tissue to implant.
Why is the corpus luteum important? What does it do?
- Structure important in the maintenance of a uterine environment that allows implantation and pregnancy. It does this by releasing a no. of pregnancy related hormones, the primary being progesterone
- Every time a woman ovulates a corpus luteum develops
- presence of progesterone is critical in the maintenance of the lining of the endometrium
What is formed at day 7.5?
Bilaminar embryo
Formation of the bilaminar embryo is an active process driven by ______________.
trophoblast
What does the trophoblast divide into?
- Cytotrophoblast
- Syncytiotrophoblast
Define cyto
cell-made up of individual cells
cytotrophioblast is a single cell layer
Define syncytium
A single, multinucleated cell
syncytiotrophoblast is a much more extensive, invasive layer which invades into the endometrium to help implantation.
What happens during formation of the bilaminar embryo?
The embryo organises itself into 2 layers
- epiblast and hypoblast
Amniotic cavity begins to form
What is the epiblast?
The dorsal surface of the embryo which forms the majority of the embryo.
The amnniotic cavity forms the epiblast layer.
What is the hypoblast?
The hypoblast is the ventral surface of the embryo
In which layer does the amniotix cavity form?
Epiblast layer
What does the syncytiotrophoblast do?
Produces Human chorionic gonadotropin (hCG)
Why is hCG important?
- Basis for pregnancy tests
- stimulates the corpus luteum which makes it vital for progesterone production.
On day 9 of the bilaminar embryo which two cavities are formed from the epiblast and the hypoblast?
Epiblast- amniotic cavity
hypoblast- primative yolk sack
What else happens on day 9 of the bilaminar embryo besides the formation of two cavities?
- Implantation is complete and closure by fibrin coagulum.
(made a slight defect in the wall of the uterine cavity to implant. Little defect is essentially plugged by fibrin coagulum (a blood clot))
- Vacuoles appear in the syncytiotrophoblast and unite to form lacunae
What do lacunae do?
Communicate with some of the small blood vessels in the uterine wall allowing nutrition to be provided to the embryo by maternal blood.
What happens on day 12 of the bilaminar embryo?
- establishment of the uroplacental circulation
- maternal blood flows into lacunae to diffuse to the embryoblast
- extraembryonic mesoderm develops and eventually degenerates
- forming the chorionic cavity
What happens on day 13 of the bilaminar embryo?
- Further development of the chorionic cavity and presence of connecting stalk (eventually umbilical cord)
- second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac.
Describe the connecting stalk
- from the embryo to the cytotrophoblast
- made up of extra-embryonic mesoderm
- forerunner to the umbilical cord
Describe what happens with the implanted embryo
- uteroplacental circulation forms as syncytiotrophoblast extends and contacts maternal blood supply.
- the amniotic cavity is present above the bilaminar embryonic disc and the definitive yolk sac below.
- the chorionic cavity has developed and encircles the entire embryo.
What is an ectopic pregnancy?
Where implantation occurs in a location outside of the uterus.
Why are ectopic pregnancies dangerous and potentially life threatening?
Danger of rupture and haemorrhage, even leading to maternal death.
What are the 3 abnormal sites where ectopic pregnancy may occur?
- ovary
- uterine tube (various locations)
- peritoneum
What is the potentially dangerous condition called if you get an implantation lower towards the cervix (internal os)?
Placenta previa
What happens at the beginning of week 3?
Primative streak appears in midline at caudal end of epiblast.
What two structures are at the cranial end of the primative streak?
- Primative pit
- Primative node
What is gastrulation?
Movement of epiblast cells through the primative streak to form the three primary germ layers.
What is established by the presence of the primative streak?
The body axes
During gastrulation what are hypoblast cells replaced by?
Definitive endoderm
What happens during gastrulation at day 16- what do the hypoblast and epiblast become?
- Cells migrate through the primative streak between the epiblast and the definitive endoderm.
- Epiblast becomes the ectoderm
- Hypoblast becomes the endoderm
- To develop a third layer the ectoderm cells start to move and migrate.
- 2 germ layers become 3 (trilaminar embryo)
When does gastrulation begin?
Day 15
What happens as the mesoderm is formed by the migration of epiblast cells through the primative streak?
Mesoderm spreads out laterally from the streak in all directions to lie between the endoderm and ectoderm except for in 2 specific locations- the oropharyngeal membrane (temporary seal for the oral cavity) and the cloacal membrane. At these locations the original ectoderm and endoderm remain in contact.
What is the fate of the ectoderm?
Epidermis of skin
- hair, nails, associated glands
Nervous system
- brain, spinal cord, peripheral nerves
Describe the location and structure of the notochord
- Little midline structure
- forms early in the trilaminar embryo
- initally ventral ectoderm and then ventral to the neural plate and neural tube
- required for pattern and differentiation of surrounding tissue
- secretes patterning signal called SHH.
- Secreted protein binds to target cell and activates a signalling pathway which helps tissues develop.
What is the fate of the mesoderm?
Becomes divided into 3 regions either side of the notochord.
What are the 3 regions the mesoderm divides into?
- Paraxial mesoderm
- intermediate mesoderm
- lateral plate mesoderm
- somatic layer
- visceral layer
What is the fate of the paraxial mesoderm?
- Axial skeleton
- Voluntary (skeletal) muscle
- Parts of dermis (back)
What is the fate of the intermediate mesoderm?
Urogenital systems
- kidney and ureter
- gonads and associated structures
What is the fate of the somatic part of the lateral plate mesoderm?
- Most of the dermis
- Lining of body wall
- Parts of limbs
What is the fate of the visceral part of the lateral plate mesoderm?
- Cardiovascular system
- mesothelial covering of organs
- smooth muscle
What is the fate of the endoderm?
- Lining of the gut tube
- lining of the respiratory tract
- lining of the bladder and urethra
