Embryogenesis SDL Flashcards
Two man periods of human development
Embryonic period:
○ Up to the end of week 8
○ Most of the organogenesis occurs during this time
Foetal Period:
○ The remaining time in utero
○ Involves growth and modelling
Defects during embryogenesis results in congenital malformations
where does fertilisation occur?
ampulla region of the oviduct and it takes about a week to implant
Describe the cleavage stages
The zygote will then start to divide by a process called cleavage, into a 2-cell zygote and then a 4-cell zygote. At this stage, the nuclei, within the cells are now diploid. With cleavage divisions, the egg doesn’t get bigger, despite there being an increase in cells.
Describe the structure of a fertilised zygote
The fertilised zygote will contain two haploid nuclei, one from the male gamete, and the other from the female, there will also be two polar bodies produced as a result of the last stage of oogenesis when meiosis is complete. Surrounding the zygote is the zona pellucida, a non-cellular structure, like the shell of the egg.
What is compaction?
The egg is called a morula, when it reaches 12-15 cells, by day 3. The zona pellucida then starts to dissolve, in a process called compaction, where the shape of the egg changes internally. This is as the egg prepares to implant.
By now, the egg is called a blastocyst and will comprise 32-64 cells, by days 4/5. The blastocyst consists of two types of cells, it has a trophoblast cell layer. This is all to do with the implanting procedure. Then there is an inner cell mass, this will form the embryo body.
The rest of the egg is called the blastocoele and by days 6/7, the ICM will hatch out and egg will implant,
How will polarity give rise to the blastocyst structure?
- The outer cells will have a polarity
- The cells within do have a polarity and all their sites are in contact with other cells.
- This confers to them their difference and gives rise to the inner cell mass and trophoblast cell layer
- The outside cells will start pumping fluid inside the embryo. This forms the blastocoele, a fluid-filled cavity.
- This pushes the inner cell mass to one side of the. Cell. Now we call this embryo a blastocyst
What is the differentiation potential for a blastocyst
Early blastocysts are totipotent, meaning they can become any type of cell.
What moves the egg into the oviduct?
It is the fimbriae that waft the ovulated oocyte into the oviduct
what happens before the development of the embryo proper
the conceptus must first implant, then generate the “germ” disc. This takes ~10 days.
What is the difference between monozygotic and Dizygotic twins
Dizygotic – 2 oocytes (around 2/3 of twins)
Monozygotic – 1 oocyte, splitting at either:
Before the blastocyst (when a two-cell zygote) [before 5 days], there will be two of each, two amnions, two chorions, and two placentas. One for each blastocyst.
Describe week 2: implantation
The syncytiotrophoblast (outer layer of trophoblast) surrounds the embryo and invades the uterus (similar to metastasis). The trophoblast is the site of the synthesis of hCG (the basis for a pregnancy test).
The ICM then splits, into the epiblast (ectoderm, which will contribute to the human) and hypoblast (extraembryonic endoderm, which won’t contribute to human). Together, these form the bilaminar germ disk.
There are two cavities, amniotic and chorionic (not yet formed), now the egg is ready for gastrulation
What determines whether monozygotic twins share their amnion and chorion
- You have one morula, but ICM splits to form two ICM’s and therefore two bilaminar disks (two embryonic disks) [these are diamniotic, two amnions but shared chorion]
- You have one blastocyst and one bilaminar disk, but it forms two streaks and causes the amnion and chorion to be shared [rare]
What are the two ways you can get conjoined twins
In one morula/blastocyst, the ICM partially divides OR you get one germ disk (like normal) but you have a partially divided streak.
Gastrulation
A primitive groove forms and then a streak through the epiblast cells, in the midline of the bilaminar germ disc. It is at this stage, that you may get conjoined twins or get twins that share the same chorion and amnion.
What happens next is the formation of a third germ layer, arising from the epiblast, which is now known as the ectoderm, as its on the top. The mesoderm forms in the middle and the endoderm on the inside. This is referred to as the trilaminar disk.
The three germ layers and the main tissues that arise from them:
Endoderm:
-Linings: epithelium of GI, urinary, respiratory tracts, epithelial parts of liver, pancreas, thyroid
Ectoderm:
- Neuroectoderm: neural tube (CNS + spinal cord), neural crest
- Surface ectoderm: epidermis,hair, nails, internal ear, lens, enamel
Mesoderm: Head: Skull, dentine Paraxial: Somites–muscle, skeleton Intermediate:urogenital Lateral:Heart,spleen, blood
