Early human development Flashcards
What is the site of fertilisation?
- Ampulla region of uterine (fallopian) tube
What are some of the most common Ectopic (abnormal) sites of implantation?
- Ovary
- Ampulla
- Uterine tube
- Outside of uterine tube, within peritoneum
- Lower part of uterine body or cervix
Where should implantation normally occur?
- Posterior uterine wall
What is the basis for several assisted reproduction techniques?
The ability to manipulate gametes and embryos in culture
Describe the term inner cell mass
- Refers to one of the 2 types of cells that form the blastocyst
- Inner cell mass are a group of 20-30 pluripotent cells located one side of the blastocyst, and this is what gives rise to the definitive structures of the foetus
- The inner cell mass forms the epiblast and amnionic ectoderm during implantation
Describe the term trophoblast
- Single layer of flattened cells within the blastocyst
- Stems from morula’s outer mass
- Gives rise to the placenta
- Forms extra-embryonic tissues
Describe the process of formation of oocytes and spermatozoa
- Germ cells begin meiosis - Diploid genome duplication results in 2 sets of homologous chromosomes (the duplicate versions of the paternal and maternal chromosomes are called sister chromatids).
- During meiosis I, homologous chromosomes undergo recombination, or crossing over.
- Homologous chromosomes are then separated by cellular and nuclear division and another round of division and another round of division separates the sister chromatids from one another to give rise to 4 daughter cells, each now containing a haploid genome (1 copy of each chromosome). At this point, each of those nuclei is packaged into 4 spermatids that then differentiate into 4 spermatozoa
- However, the oocytes arrest during meiosis I, and meiosis won’t resume until a single oocyte has been selected to mature within an ovarian follicle.
- As the first meiotic division proceeds, one set of homologous chromosomes is excluded from the oocyte and remains within the zona pellucida - this is the first polar body
- The oocyte is paused in meiosis again and the second meiotic division won’t resume until the oocyte has been fertilised. One set of genetic material (this time, one set of sister chromatids) is excluded from the oocyte and this is the second polar body
Describe the cleavage stage of fertilisation
- Haploid pronuclei of sperm and oocyte fuse, forms a zygote containing both maternal and paternal DNA
- Zygote undergoes mitotic division forming a 2-cell zygote, and then the 2-cell zygote divided to form a 4-cell zygote
Describe morula and blastocyst formation
- 4- cell zygote undergoes mitotic division to form 8-cell zygote
- 3 days after fertilisation, cells of compacted embryo divide again forming a cluster of cells - This is a mulberry-shaped 16-cell morula (can range from 12-16 cells) - It’s composed of 2 zones: inner, outer cell mass
- After the third cleavage, blastomeres (cells outside of the morula) form compact balls of cells, connected by tight junctions (compaction) - This causes fluid to accumulate within the internal cavity (this is called the blastocoele - a fluid-filled cavity) of the zygote, and the morula becomes a blastocyst
- The zona pellucida begins dissolving
- 4-5 days, embryo consists of approx 100 cells
Describe the structure of the blastocyst
Fluid filled hollow cell, 2 zones:
- Trophoblast - single layer of large flattened cells, stemming from morula’s outer cell mass; gives rise to placenta
- Embryoblast (inner cell mass) - 20-30 pluripotent cells located on one side stemming from inner cell mass; gives rise to embryo
Describe hatching
Enzymes within the embryo and uterine environment are secreted to break down the zone pellucida protein coat surrounding the blastocyst so that it’s able to implant within the posterior uterine wall
The blastocyst has 2 poles, the embryonic pole (this is the side of the inner cell mass) and the abembryonic pole
The embryonic pole is the side that fuses with the posterior uterine wall
Describe the process of implantation on Day 7
- Trophoblast binds to uterine wall with L-selectin, integral receptors- penetrates between epithelial cells
- High progesterone released from corpus luteum, develops endometrium for implantation
- Blastocyst implants into decidua basalis, along superior posterior wall of uterus
What happens to the trophoblast on Day 8?
- Proliferates, forms 2 layers
- Cytotrophoblast (cellular trophoblast):
- Inner layer of mononucleated cells
- Produces primary chorionic villi, protrudes into syncytiotrophoblast
- Syncytiotrophoblast:
- Outer multinucleated mass of cells (without distinct cell boundaries)
- Invades decidua basalis with finger-like processes, make enzymes that erode uterine cells; blastocyst burrows into decidua basalis surrounded by pool of blood leaked from degraded blood vessels
- Secretes hCG protein- maintains viability of copus luteum, increases secretion of progesterone and oestrogen
- Cytotrophoblast (cellular trophoblast):
What happens to the inner cell mass (embryoblast) on day 8?
- Differentiates into 2 layers, forms flat disc:
- Hypoblast - small cuboidal cells adjacent to blastocyst
- Epiblast - columnar cells
- Cavity forms inside → amniotic cavity
- Lined with amnioblasts
What do the syncytiotrophoblast cells do?
- Finger-like projections that fuse blastocyst to posterior uterine wall
- Multinucleated cells, degrade extracellular matrix and tissue of uterine wall, allow embryo to implant
- Implantation site then sealed off by a fibrin plug
