20.05.02 Oogenesis spermatogenesis, fertilisation and embryogenesis Flashcards
2 types of gametogenesis
- females= oogenesis
2. males= spermatogenesis
What happens during oogenesis
- Starts in embryonic development
- diploid primordial germ cells migrate into the embryonic ovary and proliferate. Stops at 7th month, giving rise to 7 million oogonia
- Many oognoia subsequently die. Remaining oogonia undergo first meiotic division to become primary oocytes.
- Primary oocytes stop at meiosis 1 prophase diplotene stage. Maintained at this stage until puberty
What happens to primary oocytes at puberty
- Groups of oocytes periodically resume meiosis
- Of the millions of primary oocytes present at birth, many continue to die. Only ~400 mature in a woman’s lifetime
What happens when a woman ovulates
Oocytes are in metaphase II, meiosis II is not completed until after fertilisation
Structure of an oocyte
- Surrounded by a layer of follicle cells (cumulus cells), which nurture oocytes before and after ovulation
- Cytoplasm containing large numbers of mitochondria, ribosomes, DNA/RNA polymerases, protective chemicals and morphogenetic factors.
- Thick extra cellular matrix: zona pellucida
What happens during spermatogenesis
- Diploid primordial germ cells migrate to embryonic testis and undergo rapid proliferation generating spermatogonia.
- Some daughter cells stop proliferating and differentiate into primary spermatocytes, which then undergo meiosis.
- In meiosis 1 division is symmetrical (2 diploid secondary spermatocytes), then meiosis 2 produces 4 haploid spermatids
What happens to haploid spermatids
Undergo morphological differentiation into sperm. These then leave the lumen of the seminiferous tubule and stored in epididymus.
How long does it take for a spermatocyte to develop into sperm
- 24 days for a spermatocyte to complete meiosis to become a spermatid
- 5 weeks for spermatid to develop into sperm
Structure of a sperm cell
- Small reduced cytoplasm.
- Haploid nucleus (transcriptionally inactive, histones replaced with protamines)
- Acrosomal vesicle at head of sperm containing digestive enzymes.
- Mid piece contains many mitochondria
- Flagellum for propulsion
What is fertilisation
The process by which an egg and sperm fuse to create a new individual
Where does fertilisation usually occur
Ampulla of the fallopian tube
What happens during fertilisation
- Sperm attaches to outer layer of follicle cells of oocyte
- Sperm reaches zona pellucida.
- Acrosome bursts releasing enzymes (hyaluronidase) that digest the ZP
- Sperm membrane comes into contact with oocyte membrane, fuse and sperm nucleus enters the cytoplasm of oocyte.
- Cortical reaction occurs where cortical granules inside oocyte fuse with plasma membrane releasing enzymes into ZP. Causes glycoproteins of ZP to cross link, so ZP is impermeable to sperm.
- Oocyte undergoes 2nd meiotic division, producing a haploid mature ovum and a polar body.
- Sperm tail and mitochondria degenerate (why mitochondria are exclusively maternally inherited).
- Haploid oocyte and sperm nuclei form a pronuclei
- First mitotic division occurs
What is embryogenesis
Process of cell division and differentiation of the human embryo (classed as a fetus at 8th week of gestation)
Steps of embryogenesis
- Fertilisation
- Cleavage
- Compaction
- Blastocyst
- Implantation
- Gastrulation
What happens to embryo in cleavage step
- Zygote repeatedly divides to form a number of smaller cells (blastomeres)
- asynchronous cleavage. Not all blastomeres divide at the same time. The first cleavage is meridional, and the second cleavage is rotational.
What happens to embryo in compaction step
- Loosely associated blastomeres of an eight cell embryo, flatten against each other to form a morula.
- Purpose: to introduce a degree of cell polarity.
- Tight junctions form to stabilise
- Outer cells secrete fluid, inner cells for gap junctions so small molecules and ions pass between them
What is a blastocyst
- 16 cell stage. Two types of cells: external polarised, internal non-polar cells
- Outer layer of cells (tophoblast) give rise to the chorion
- Inner cells congregate at one end of the blastocele (fluid filled cavity) to form the inner cell mass (ICM)
What is the chorion
An extraembryonic membrane which provides the fetal component of the placenta
What does the inner cell mass (ICM) give rise to
- All cells of the organism
- Three extraembryonic membranes: yolk sac, amnion, allantois.
When can monozygotic twins form
At any time until late blastocyst stage. Occurs when there is splitting.
What happens to embryo in implantation step
- Enzyme is released to make a hole through zona pellucia, releasing the blastocyst.
- Blastocyst then attaches to uterine epithelium
- Trophoblast cells rapidly proliferate and differentiate into an inner (cytotrophoblast) and outer (syncytiotrophoblast) layer. Outer layer invades connective tissue of uterus.
Prior to implantation what do ICM cells differentiate into
- Inner layer: hypoblast (primitive endoderm). Gives rise to extraembryonic mesoderm
- Outer layer: epiblast (primitive ectoderm). Gives rise to all embryonic tissue, amnion, yolk sac and allantois.
- Epiblast and hypoblast form a bilaminar germ disk
- 2 fluid filled cavities: amniotic cavity and yolk sac
What is gastrulation
- Process where the orientation of the body is established. Primitive streak forms along bilaminar germ disk.
- Embryo is converted from bilaminar structure to three layers: ectoderm, endoderm and mesoderm
How are ectoderm, endoderm and mesoderm layers formed
- Primitive streak forms a groove that develops into a depression (primitive pit) near germ disk.
- Epiblast cells near primitive streak proliferate and flatten and develop pseudopodia to allow them to migrate to the space between epiblast and hypoblast
- Eventually hypoblast is replaced by epiblast derived cells forming the endoderm
- Epiblast cells diverge between epiblast and endoderm to form intraembryonic mesoderm
- Residual epiblast is now the ectoderm
- Now a trilaminar germ disk
What will the endoderm eventually form
Lung, liver, pancreas, thymus, endocrine glands
What will the mesoderm eventually form
Blood, vessels, muscular tissue, connective tissue
What will the ectoderm eventually form
Skin and derivatives, nervous system
What epigentic reprogramming happens in embryos
- In the fetal germline, all DNA methylation patterns are erased, and then paternal and maternal methylation imprints are established during gametogenesis.
- The two germline genomes that are combined at fertilization undergo parent-specific genome reprogramming in the early embryo, during which most germline patterns are erased again and somatic patterns (green) are established.
- Only imprinted genes maintain their germline patterns during development of the new organism
Three techniques for In vitro gametogenesis/ assisted reproductive technology (ART)
- Intrauterine (artificial) insemination (IUI). Injection of sperm into the womb following stimulated ovulation.
- In vitro fertilisation (IVF). Sperm added to extracted eggs in a petri dish. Incubated and monitored for successful fertilisation. Developing embryos are transferred back to womb
- Intracytoplasmic sperm injection (ICSI). Microscopic injection of sperm directly into an egg nuclei in a petri dish.
How is sperm taken from azoospermic males
Microsurgical epididymal sperm aspiration
