S1: Early Human Devlopment Flashcards
Where does fertilisation usually occur?
Ampulla region of the oviduct
How long does implantation usually take?
A week
What wafts the ovulated oocyte into the oviduct?
Fimbriae
What does the fertilised zygote contain?
- 2 haploid nuclei (one from the male gamete, one from the female gamete)
- 2 polar bodies produced as a result of the large stage of oogenesis when meiosis is complete
What surrounds the fertilised zygote?
The zona pellucida (a non cellular structure) like the shell of the egg
What is cleavage?
The zygote dividing into a 2 cell zygote and then a 4 cell zygote.
What is the ploidy of the cell during cleavage?
Diploid
Does the zygote increase in size during cleavage?
No, with cleavage divisions the egg doesn’t get bigger despite there being an increase in cells.
What is the egg called when it reached 12-15 cells?
Morula
What is compaction?
The zona pellucida dissolves and shape of egg changes internally. This is as the egg prepared to implant.
What is the egg called when it is 32-64 cells?
Blastocyst
How many days does it take for the egg to become a blastocyst?
Days 4/5
What are two types of cells in the blastocyst?
What is the rest of the egg called?
- Trophoblast cell layer on the outside (of the internal enviroment of egg). This is all to do with the implanting procedure.
- Inner cell mass which sticks into the middle slightly and will form the embryo body
The rest of the egg is called the blastocoele.
What does totipotent mean?
They can become any type of cell
What determines a cell’s fate?
- Cell fate (specialisation) depends on the genes expressed in that cell
- All cells contain the same genomic DNA, but only a subset of genes are expressed in any one cell
- Gene expression is controlled by transcription factors (and modification of DNA/histones)
- Cell fate in the embryo is determined by signals from surrounding cells
- These signals involve a cascade of events from receptor to transduction pathway to transcription factor
- The first cell two cell fate decisions set aside cells that form the placenta and extraembryonic membranes (eg amnion)
List some preimplantation applications
- Animals (e.g. Dolly) are cloned from a somatic nucleus and put into an enucleated unfertilised egg
- ICM cells can be removed and cultured as embryonic stem (ES) cells
What is an ecotopic pregnancy?
It is when the blastocyst implants in a place other than the uterus. It could be:
- Oviduct
- Ovary
- Peritoneal surfaces
- Lower part of the uterine or cervix
What is the outer layer of trophoblast called?
The syncytiotrophoblast
What does the syncytiotrophoblast do?
It surrounds the embryo and invades into uterus during implantation
What is the site of synthesis of hCG (basis for pregnancy test)?
The tropoblast
What does the inner cell mass split into at implantation?
- Epiplast (ectoderm which will contribute to the human)
- Hypoblast (extra embryonic endoderm, which won’t contribute to human
What does the epiblast and hypoblast together form?
Bilaminar germ disk
What week does implantation occur in?
Week 2
How many oocytes do dizygotic and monozygotic twins have?
Monozygotic - 1 oocyte , splitting at either
Dizygotic - 2 oocytes
What type of twins are identical?
Dizygotic are non identical
Monozygotic are identical
How does the development of dizygotic and monozygotic twins differ?
Dyzygotic: Two oocytes are fertilised and each one implants seperately
Monzygotic: A two cell zygote splits and two blastocysts implant serperately
How many chorions and amnions do twins usually have?
2 chorions
2 placentas
Describe the development of twins who have two amnions but a shared chorion
One morula but inner cell mass splits to form 2 inner cells masses. There are therefore 2 bilaminar disks (2 embryonic disks). These are diamniotic (2 amnions but shared chorion)
Describe the development of twins who have a shared amnions and a shared chorion
One blastocyst and one bilaminar disk but it forms two streaks and causes amnion and chorion to be shared.
List the two ways conjoined twins arise
- One morula/blastocyst partially divides
2. One germ disk as normal but a partially divided streak is formed
What happens in gastrulation?
A primitive groove forms and then a streak through the epiblast cells in the midline of the bilaminar germ disc.
Describe the formation of a third germ layer
The third germ layer arises from the epiblast which is now known as ectoderm as it is on top. The mesoderm forms in the middle and the endoderm is on the inside. This is called the trilaminar disc.
What main tissues arise from the endoderm?
Linings of epithelium on the: GI Urinary Respiratory Tracts Epithelial parts of the liver Pancreas Thyroid
What main tissues arise from the mesoderm?
Head: Skull, dentine
Paraxial: Somites- muscle, skeleton
Intermediate: Urogenital
Lateral: Heart, Spleen, Blood
What main tissues arise from the ectoderm?
Neuroectoderm: Neural tube (CNS+spinal cord), neural crest
Surface ectoderm: Epidermis, hair, nails, internal ear, lens, enamel
What is neurulation?
The formation of the brain, spinal cord, cranial and spinal nerves from the neural tube.
The cranial and spinal ganglia are from the neural crest
What happens to the neural plate in neurulation?
The neural place (which is part of the ectoderm- neuroectoderm) becomes groove. It then folds and invades the mesoderm and pinches off the ectoderm so that it runs through the mesoderm. This will form structures such as the CNS and brain/spinal nerves.
What does non closure of neural tube (caudal neuropore) lead to?
Spina Bifida
What does non closure of neural tube (cranial neuropore) lead to?
Anencephaly
What is somitogenesis?
The process by which somites (divisions of our body) form.
What do epithelial somites diffrentiate into?
Sclerotome: Vertebrae and ribs
Mytotome: Epimere, hypomere, limb muscle
Dermatome: Dorsal dermis
