Coitus, Fertilisation and Preimplantation Flashcards
Describe what occurs in the excitement stage of the male sex response
- Psychological and physical stimulation of erogenous zones,
- Swelling and erection of penis due to PSNS,
- Relaxation of SM of corpora cavernosa, corpus spongiosum which allows increase in blood flow = increased volume and ridigity.
Describe the plateau stage of the male sex response
- Movement of ejaculate into prostatic or proximal part of urethra, under SNS control,
- Spinal reflex - forced expulsion of semen from urethra,
- Reflex reaction triggered by entry of semen from prostatic urethra into bulbous urethra
Describe the orgasm stage of the male sex response
- Release of tension,
- Generalised, rhythmic contraction of pelvic floor, ischiocavernosus and bulbospngiosus muscles,
- Spasmodic contractions of muscles of hip and anal sphincter.
Describe the resolution phase of the male sex response
- Return to pre-excitement state, personal satisfaction and well-being.
- Refractory period
Describe the excitement phase of the female sex response
- Psychological and physical stimulation of erogenous zones,
- Tumescence and erection of clitoris, engorgement of female tract
Describe the Plateau phase of the female sex response
- Marked vasocongestion,
- “Sex flush” - maculopapular rash on breasts, chest and epigastrium.
- Engorgement of labia, lower third of the vagina (with narrowing of diameter) and dilation of upper two thirds of vagina,
- Clitoral swelling and erection.
Describe the orgasm phase of the female sex response
- Release of tension,
- Generalised, rhythmic myotonic contractions of perivaginal muscles, anal sphincter and uterus
Describe the resolution phase of the female sex response
- Return to pre-excitement state, personal satisfaction and well-being.
- New excitement cycles may be initiated.
What is the most fertile stage of the cycle?
No more than 3 days before ovulation and no more than 1 day after ovulation. This is because sperm are viable for 24-72 hours and oocytes 12-24hours
What is the embryonic period?
The first 8 weeks where there is the preimplantation embryo, implantation and then differentiation and development of the organ systems
What is the foetal period?
8-40 weeks where differentiation continues and growth
Describe the transport of the ovum
- Fimbriae sweep over ovary surface and picks up the ovum.
- Sticky cumulus cells cling to the ciliated surface of the fimbriae.
- The movement of cilia and smooth muscle of fallopian tubule propels ovum to uterus.
- Fertilised in the ampulla
Describe the transport of sperm
- Ejaculation deposits 150-600 million sperm however mortality is high so only 50-100 make it due to acidic environment of vagina, uterotubal junction, phagocytosis by leukocytes and the length and energy requirements. Passage into cervical mucus depends on oestrogen-induced changes
Briefly describe the anatomy of a sperm cell
Head - Acrosome which contains enzymes to aid penetration of ovum and nucleus.
Midpiece - Contains mitochondria which provide energy for tail.
Tail - Specialised flagellum that propels sperm forward
What is capacitation of sperm?
Freshly ejaculated sperm cannot immediately penetrate ovum so capacitation occurs which results in;
- Change from wave-like beats to whip-like action of sperm tail,
- Sperm’s plasma membrane is altered so it is able to fuse with membrane of ovum.
On a cellular level, what does capacitation involve?
- Increase in calcium permeability,
- Removal of membrane proteins,
- Change in surface charge,
- Depletion of cholesterol
Summarise the process of fertilisation?
1) Sperm binds to zona pellucida,
2) Rise in calcium triggers exocytosis of acrosome.
3) Hydrolytic enzymes in acrosome dissolve zona pellucida and the whip like tail of sperm pushes head towards oocyte membrane.
4) Two membranes fuse and contents of sperm head enter oocyte.
5) Rise in Ca in oocyte causes release of enzymes that harden the zona pellucida and completion of oocyte second meiotic division,
6) Sperm head becomes male perinuclei and then male and female perinuclei fuse
Describe how the blastocyst avoids maternal rejection
- Release of immunosupressive agents,
- Secretion of human chorionic gonadotrophin (hCG)
What is the function of the human chorionic gonadotrophin?
It supports steroid synthesis of the corpus luteum and therefore prevents menstruation and any further follicular development. It promotes trophoblast growth and placental development. Stimulates leydig cells of male foetuses to produce testosterone.
Explain the embryo hatching stage of implantation
Occurs on day 6/7 after fertilisation, the zona pellucida degenerates and blastocyst is released. Lytic factors from endometrial cavity involved.
Explain the apposition stage of implantation
Earliest contact between blastocyst wall and trophectoderm and endometrial epithelium. Occurs in crypts in endometrium.
Explain the adhesion stage of implantation
- Time limited window where there is complex interactions between trophoblast and maternal epithelial tissue. The trophoblast attaches to uterine epithelium via microvilli of trophoblast. Integrin receptors involved
Describe the invasion stage of implantation
- Syncytiotrophoblast cells flow into endometrium causing oedema, glycogen synthesis and increased vascularisation.
- Pregnant endometrium is now termed decidua.
- Nutrition still depends on uterine secretions.
- Breakthrough bleeding may occur
Describe the levels of hCG
The syncytiotrophoblasts secrete hCG soon after implantation. It is measurable 7/8 days post conception.
