20.05.02 Oogenesis spermatogenesis, fertilisation and embryogenesis

Question 1

2 types of gametogenesis

Answer 1

1. females= oogenesis

2. males= spermatogenesis

Question 2

What happens during oogenesis

Answer 2

• 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

Question 3

What happens to primary oocytes at puberty

Answer 3

• 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

Question 4

What happens when a woman ovulates

Answer 4

Oocytes are in metaphase II, meiosis II is not completed until after fertilisation

Question 5

Structure of an oocyte

Answer 5

• 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

Question 6

What happens during spermatogenesis

Answer 6

• 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

Question 7

What happens to haploid spermatids

Answer 7

Undergo morphological differentiation into sperm. These then leave the lumen of the seminiferous tubule and stored in epididymus.

Question 8

How long does it take for a spermatocyte to develop into sperm

Answer 8

• 24 days for a spermatocyte to complete meiosis to become a spermatid
• 5 weeks for spermatid to develop into sperm

Question 9

Structure of a sperm cell

Answer 9

• 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

Question 10

What is fertilisation

Answer 10

The process by which an egg and sperm fuse to create a new individual

Question 11

Where does fertilisation usually occur

Answer 11

Ampulla of the fallopian tube

Question 12

What happens during fertilisation

Answer 12

1. Sperm attaches to outer layer of follicle cells of oocyte
2. Sperm reaches zona pellucida.
3. Acrosome bursts releasing enzymes (hyaluronidase) that digest the ZP
4. Sperm membrane comes into contact with oocyte membrane, fuse and sperm nucleus enters the cytoplasm of oocyte.
5. 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.
6. Oocyte undergoes 2nd meiotic division, producing a haploid mature ovum and a polar body.
7. Sperm tail and mitochondria degenerate (why mitochondria are exclusively maternally inherited).
8. Haploid oocyte and sperm nuclei form a pronuclei
9. First mitotic division occurs

Question 13

What is embryogenesis

Answer 13

Process of cell division and differentiation of the human embryo (classed as a fetus at 8th week of gestation)

Question 14

Steps of embryogenesis

Answer 14

1. Fertilisation
2. Cleavage
3. Compaction
4. Blastocyst
5. Implantation
6. Gastrulation

Question 15

What happens to embryo in cleavage step

Answer 15

• 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.

Question 16

What happens to embryo in compaction step

Answer 16

• 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

Question 17

What is a blastocyst

Answer 17

• 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)

Question 18

What is the chorion

Answer 18

An extraembryonic membrane which provides the fetal component of the placenta

Question 19

What does the inner cell mass (ICM) give rise to

Answer 19

• All cells of the organism

- Three extraembryonic membranes: yolk sac, amnion, allantois.

Question 20

When can monozygotic twins form

Answer 20

At any time until late blastocyst stage. Occurs when there is splitting.

Question 21

What happens to embryo in implantation step

Answer 21

• 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.

Question 22

Prior to implantation what do ICM cells differentiate into

Answer 22

• 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

Question 23

What is gastrulation

Answer 23

• 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

Question 24

How are ectoderm, endoderm and mesoderm layers formed

Answer 24

• 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

Question 25

What will the endoderm eventually form

Answer 25

Lung, liver, pancreas, thymus, endocrine glands

Question 26

What will the mesoderm eventually form

Answer 26

Blood, vessels, muscular tissue, connective tissue

Question 27

What will the ectoderm eventually form

Answer 27

Skin and derivatives, nervous system

Question 28

What epigentic reprogramming happens in embryos

Answer 28

• 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

Question 29

Three techniques for In vitro gametogenesis/ assisted reproductive technology (ART)

Answer 29

1. Intrauterine (artificial) insemination (IUI). Injection of sperm into the womb following stimulated ovulation.
2. 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
3. Intracytoplasmic sperm injection (ICSI). Microscopic injection of sperm directly into an egg nuclei in a petri dish.

Question 30

How is sperm taken from azoospermic males

Answer 30

Microsurgical epididymal sperm aspiration