W6 - FERTILISATION AND EMBRYO DEVELOPMENT Flashcards
1
Q
What are the two phases of the development of the foetus? Explain embryology
A
- The development of the foetus is divided into two phases
- Prenatal
- Before birth
- Postnatal
- After birth
- Prenatal
- Embryology is the study of embryos and foetuses
- Prenatal development
- Embryonic period - fertilisation until the end of the 8th week
- Placenta develops
- All major adult organs are formed (gastrulation and organogenesis)
- Foetal period - 9th week until birth
- Growth and development
- Embryonic period - fertilisation until the end of the 8th week
- Prenatal development
2
Q
How long is an oocyte viable for? What is the duration of pregnancy?
A
- An oocyte is viable for 12-24 hours after ovulation
- Most sperm retain fertilisation power for 24-48 hours after ejaculation
- Thus, coitus must occur no more than 2 days before ovulation and not later than 24 hours after for fertilisation to occur
- Need to allow 4-6 hours for sperm transport and capacitation
- At the optimal time, the oocyte is about 1/3 of the way down the length of the uterine tube
- Ovum retained in the ampulla of the uterine tube - allows time for uterine lining to be adequately prepared by progesterone from the luteal phase of the ovarian cycle
- Pregnancy will take approximately 38 weeks (266 days) form zygote to birth - spread over three trimesters
- The due date for birth is calculated from 4 weeks from the first day of the last normal menstrual period (LNMP)
- 40 weeks (280 days or 9 months plus 7 days)
3
Q
What is the clinical significance of embryology?
A
- Clinical significance
- Pre-implantation developmental abnormalities
- Failure to implant within the uterus - failed or ectopic pregnancy
- Reason for most problems in pregnancy
- Severe embryonic period abnormalities
- Teratogen exposure and infection during week 1-8 - neural tube and other defects
- Often quite severe - disrupts organ system development
- Teratogen exposure and infection during week 1-8 - neural tube and other defects
- Foetal period developmental abnormalities
- Malformation or mechanical abnormalities - talipes
- Pre-implantation developmental abnormalities
- Important to know the developmental point of the embryo/foetus so that the effects of the trauma can be more accurately identified
4
Q
What are some reasons why sperm may not reach the oocyte?
A
- Main reasons
- Fall out of vagina
- Trapped in crypts, glands and mucus
- Run out of energy
- Destroyed by environment (Acidic nature of the vagina)
- Incapable of fertilisation due to abnormal shape/reduced motility
- Do not pass down the correct fallopian tube
- Only a few thousand make it into the uterine tube
5
Q
Explain the week 1 events of embryo development
A
- Fertilisation
- Cleavage and blastocyst formation
- Blastocyst begins to implant in endometrium
6
Q
Explain the process of sperm capacitation
A
- Freshly ejaculated sperm cannot immediately fertilise the oocyte
- They need to spend several hours in the female reproductive tract to attain a capacity to fertilise
- Their motility must be enhanced through the cervical mucus, uterus and uterine tubes
- Oestrogen and vaginal mucus destabilise the sperm plasma membrane and trigger hyperactive motility
- The process involves
- Removal of the protein coating acquired in the epididymis
- Reorganisation of plasma membrane to expose binding sites
- This prevents the spilling of acrosomal enzymes
- Fragile acrosomal membranes could rupture prematurely in the male reproductive tract, causing some degree of autolysis of male reproductive organs
- The sperm appear to “sniff” their way to the oocyte - olfactory receptors that respond to chemical stimuli
- Oocyte and surrounding cells release signalling molecules that direct the sperm
- Capacitation - secretions from the female tract and oestrogens weaken the sperm’s protective protein coat acquired in the epididymis
- Reorganisation of plasma membrane to expose binding sites and hypermotility is triggered
7
Q
Explain the acrosome reaction
A
- Swelling of the acrosome and fusion of the outer acrosomal membrane with overlying plasma membrane
- Calcium-dependent event
- Increase intracellular Ca2+ and cAMP
- Stimulated by progesterone and a protein (ZP3) on the zona pellucida that surrounds the oocyte
- Release of digestive enzymes from the acrosome
- Hyaluronidase - cumulus cell penetration
- Acrosin - zona pellucida digestion
- Acrosome-reacted sperm have a very short lifespan
- Process
- Once in the vicinity of the oocyte, sperm weaves its way through the cells of the corona radiata, assisted by cell-surface hyaluronidase on the sperm - digests the cement between the granulosa cells in the area
- After breaching the corona, the sperm head binds to ZP3 of the zona pellucida (functions as a sperm receptor)
- The binding, and progesterone, opens Ca2+ channels, leading to a rise in Ca2+ inside the sperm which triggers the acrosomal reaction
- Acrosomal reaction - release of acrosomal enzymes that digests holes in the zona pellucida
- Single sperm can fertilise the egg and penetrate the zona pellucida
- Once the path is cleared, the sperms tail gyrates, forcing the sperm’s head to move towards the oocyte membrane
- Sperm’s post-acrosomal “collar” binds the oocyte’s plasma membrane receptors causing
- Oocyte to form microvilli that surround the sperm head, and the sperm and oocyte membranes fuse
- Cytoplasmic contents of the sperm enter the oocyte, leaving the sperm’s plasma membrane behind
- Gametes fuse together with perfect contact, causing the contents of both cells to combine within a single membrane - all without spilling
8
Q
Explain the process of fertilisation
A
- Capacitation
- Oestrogen/mucus destabilise sperm plasma membrane and cause hyperactive motility
- Acrosome reaction
- Progesterone and binding to ZP3 causes increase Ca2+ and release of digestive enzymes from sperm
- Fertilisation
- Sperm digests zona, binds to sperm-binding receptor and fuses with oocyte plasma membrane
- Polyspermy block
- Fusion causes increase Ca2+ and electrical block of oocyte plasma membrane (fast block), then cortical granule release which hardens the zona and removes sperm-binding receptors (slow block)
9
Q
Explain the polyspermy block
A
- Once sperm has entered the oocyte, Ca2+ is released in waves by the endoplasmic reticulum into the cytoplasm
- Activated oocyte to prepare for 2nd meiotic division
- Cortical reaction - granules located just inside the plasma membrane spill their enzymes into the extracellular space beneath the zona pellucida - zonal inhibiting proteins
- Destroys the sperm receptors, preventing any more sperm from entering
- Spilled material binds to water, and as the material swells and hardens, it detaches all sperm still bound to the receptors on the oocyte membrane, completing the slow block
- Electrical block - attachment of a sperm to ovum surface induces membrane depolarisation and increased permeability to Ca2+, triggering an electrical block on the surface of the ovum, preventing the fusion of other sperm
10
Q
Explain the completion of meiosis II and fertilisation
A
- Sperm loses its plasma membrane as the cytoplasmic contents of the sperm enter the oocyte
- The centrosome elaborates microtubules from its midpiece, which are used by the sperm to locomote its DNA-rich nucleus toward the oocyte nucleus
- During this, the nucleus swells to about 5 times the normal size to form the male pronucleus
- The secondary oocyte (activated from its semidormant state by the Ca2+ influx) completes meiosis II, forming the ovum nucleus and 2nd polar body
- Accomplished ovum nucleus swells, becoming the female pronucleus and the two pronuclei approach each other
- Syngamy - the two pronuclei come together to form the zygote
- As the mitotic spindle develops between them, the pronuclei membranes rupture, releasing their chromosomes together into the immediate vicinity of the newly formed spindle
- Almost as soon as the male and female pronuclei come together, their chromosomes replicate
- Zygote is now ready to undergo the first mitotic division of the conceptus
11
Q
Explain the pre-implantation development process
A
- Zygote - the fertilised egg
- 4-cell stage - day 2
- Morula - solid ball of blastomeres - day 3
- Early blastocyst - morula hollows out, fills with fluid and “hatches” from the zona pellucida - day 4-5
- Implanting blastocyst - consists of a sphere of trophoblast cells and an eccentric cell cluster called the inner cell mass - day 7
12
Q
What are the stages of pre-implantation?
A
- Cleavage stage - 2-8 cells
- Totipotent blastomeres (has not undergone differentiation; generate complete individual including placenta)
- Period of fairly rapid mitotic divisions of the zygote without intervening growth (increase in cell number without increase in cell size)
- Goal - to produce small cells with a high surface-to-volume ratio - enhances their uptake of nutrients and oxygen and disposal of wastes
- Provides a large number of cells to serve as building blocks for constructing the embryo
- About 36 hours after fertilisation - the first cleavage division of the zygote produces two identical daughter cells - blastomeres
- Morula stage - 16-32 cells
- Inner and outer cell populations formed (beginning of differentiation)
- Determines the future fate of the cells
- Gap junctions between inner cells
- Tight junctions between outer cells
- By 72 hours, a loose collection of cells that form a berry-shaped cluster of 16-32 cells called the morula has been formed
- Transport of the embryo continues towards the uterus
- Na+ pumped into the morula - influx of water
- Inner and outer cell populations formed (beginning of differentiation)
- Blastocyst stage - >64 cells
- First cell differentiation event (trophectoderm and inner cell mass formation)
- Pluripotent inner cell mass (generate complete individual excluding placenta)
- Accumulating fluid, floating free in the uterus
- Rapid growth and enzymes - hatching from the zona pellucida - zona pellucida starts to crack due to the swelling blastocyst, blastocyst “hatches” from it
- Blastocyst now a fluid-filled hollow space composed of a single layer of large, flattened cells called trophoblast cells and a small cluster of 20-30 rounded cells called the inner cell mass
- Increase in CDX2 in outer cells - trophectoderm differentiation
- Increase in OCT4 in inner cells - inner cell mass differentiation
13
Q
Explain the week 2 events of embryo development
A
- Implantation completed
- Placenta and extraembryonic membranes begin development
- Development of inner cell mass proceeds
14
Q
Explain the process of implantation
A
- Adherence (day 6)
- Protein-mediated binding of trophoblasts to endometrium
- Outer trophoblasts mediate the process
- Digestive enzymes from trophoblasts erode the endometrial layer - it embeds itself
- Implants high on the uterine wall - if the endometrium is not optimally mature, the blastocyst detaches and floats to a lower level with proper receptors and chemical signals
- Higher = better (thick endometrium and myometrium)
- Secretion of hormones and proteolytic enzymes from blastocyst causing the neighbouring endometrial cells to degenerate
- Implants high on the uterine wall - if the endometrium is not optimally mature, the blastocyst detaches and floats to a lower level with proper receptors and chemical signals
- Protein-mediated binding of trophoblasts to endometrium
- Endometrium quickly thickens at the point of contact and takes on characteristics of acute inflammatory response
- Invasion (day 7)
- Trophoblast grows to form cytotrophoblast and syncytiotrophoblast
- Cytotrophoblast - cell in inner layer retain their cell boundaries
- Syncytiotrophoblast - loose plasma membranes and form multinuclear cytoplasmic mass - these cells digest and invade the endometrium
- Human chorionic gonadotropin (hCG) is produced for the first time
- Trophoblast grows to form cytotrophoblast and syncytiotrophoblast
- Completion (day 12)
- Blastocyst enveloped by endometrium
- Endometrium is eroded so the blastocyst can burrow in
- Endometrial cells proliferate to cover and seal the implanted blastocyst
- Syncytiotrophoblast secretes hCG - directly stimulates the corpus luteum to produce progesterone (maintains the pregnancy for 8-12 weeks)
- Maintains secretion of oestrogens and progesterone
- Oestrogen stimulates prostaglandins from the endometrium of the uterus - increases vascular permeability and swelling of the endometrium
- Blastocyst enveloped by endometrium
15
Q
What is the bilaminar embryonic disc and extraembryonic membranes?
A
- Inner cell mass differentiates into an epiblast and hypoblast
- Fluid-filled cavities form in epiblast (amnion) and hypoblast (yolk sac) - bilaminar embryonic disc (contains epiblast on the top and hypoblast on the bottom)
- Out-pocket hypoblast cells form allantois (early base of the umbilical cord)
- Beginning of the placenta
- Extra-embryonic mesoderm, cytotrophoblasts and syncytiotrophoblasts form chorion and chorionic villi - beginning of foetal placenta
