Coitus, Fertilisation and Preimplantation Flashcards
Male Sex Response
- Excitement - psychological and physical stimulation of the erogenous zones results in tumescence and erection of penis. Effect is due to stimulation of the parasympathetic nervous system. Relaxation of smooth muscle of corpora cavernosa, corpus spongiosum allows inflow of blood resulting in increase in volume and rigidity.
- Plateau (Emission) - Refers to movement of ejaculate into the prostatic of proximal part of the urethra, primarily under the synpathetic nervous system. Spinal reflex causes forceful expulsion if semen from the urethra. Reflex reaction is triggered by the entry of semen from the prostatic urethra into the bulbous urethra.
- Orgasm (Ejaculation) - Release of tension. Generalised, rhythmic contraction of the pelvic floor, ischiocavernous and bulbospongiosus muscles. Spasmodic contractions of muscles of hips and the anal sphincter.
- Resolution - Return to pre-excitement state, personal satisfaction and well-being. Refractory period.
Innervation of Male and Female Reproductive Organs
- Arousal can be produced by: erotic psychological stimuli (e.g. visual, olfactory), tactile stimuli at the level of the brain, tactile stimuli can also mediate local spinal reflexes.
- Sympathetic innervation via lower thoracic and lumbar spinal segments; hypogastric nerve, pelvic plexus and cavernous nerve. Maintains flaccidity.
- Parasympathetic innervation goes via a control centre in sacral spinal segments. Pelvic nerve, pelvic plexus and cavernous nerve carry out effects. Increased activity produces turgidity.
- Somatic - Pudendal nerve, contracts ischiocavernous muscle and increases engorgement of corpus spongiosum.
Tumescence of the Penis
The erection centre is in the sacral spinal cord and mediates erection reflex:
- the haemodynamic changes - involves corpus cavernosum (corpora cavernosa) and sinuses along the length of the penis.
- Dilatation of the arterioles to Corpora Cavernosa and Corpora Spongiosum.
- Closing of the arteriovenous shunts that normally bypass the Corpora Cavernosa.
- Probably also occlusion of veins draining the penis
Parasympathetic Relaxation of Vascular Smooth Muscle
Parasympathetic action involves Acetylcholine acting on the vascular endothelial cell. Acetylcholine binds to M3 muscarinic receptors via G-alpha-q, These receptors lead to stimulation of Phospholipase C, increased [Ca2+]i and activation of Nitric Oxide Synthase and local release of Nitric Oxide which causes relaxation of vascular smooth muscle.
Female Sex Response
- Excitement (Seduction) - psychological and physical stimulation of erogenous zones causes tumescence and erection of clitoris and engorgement of female tract. Effects carried out due to stimulation of parasympathetic innervation.
- Plateau - marked vasocongestion, “sex flush” (maculopapular rash on breasts, chest and epigastrium). Engorgement of the labia, engorgement of lower third of the vagina with narrowing of diameter, dilation of upper two thirds of vagina. Clitoral swelling and erection.
- Orgasm - release of tension. Generalised, rhythmic myotonic contractions of perivaginal muscles, anal sphincter and uterus.
- Resolution - return to pre-excitement state, personal satisfaction and well-being. New excitement cycles may be initiated.
Most Fertile Stage of Cycle
3 days before ovulation to one day after ovulation. Oocytes are viable for 12-24 hours. Sperm are viable for 24-72 hours. Ovulation is around day 14 of the cycle.
Embryonic Period
The first eight weeks. Includes preimplantation embryo, implantation and differentiation and development of organs. During this period the mother may not know she is pregnant and may not avoid things which be harmful and may affect the development of the embryo.
Foetal Period
Weeks 8-40. Differentiation and growth continue during this time.
Transport of Ovum
At ovulation, the egg is extruded onto surface of ovary. Fimbriae sweep over ovary surface and pick up ovum. Sticky cumulus cells cling to the ciliated surface of the fimbriae. Movement of the cilia and smooth muscle of the fallopian tube propels the ovum to the uterus. Ovum is transported by ciliary current to the ampulla of the fallopian tube.
Transport of Sperm
- Ejaculation deposits 150-600million sperm into the vagina. Sperm mortality from vagina to fallopian tubes is large, just 50 to 100 make it due to acidic environment (pH of 5.7), uterotubal junction being an anatomical barrier, phagocytosis by leukocytes in uterine lumen, and length/energy requirements of the trip. Passage into the cervical mucus dependent on oestrogen-induced changes in mucus consistency. Sperm can reach ampulla wihin 5-10 mins after ejaculation. Movement through uterus and fallopian tubes is via sperm’s own propulsions and vaginal, cervical, uterine contractions and ciliary beat movement in fallopian tubes.
- Head of sperm made of Acrosome Cap (contains enzymes to aid penetration of ovum) and Nucleus (contains genetic material). Midpiece contains mitochondria around a filamentous core and provides energy for the tail. The tail is a specialised flagellum that propels sperm forward.
Capacitation of Sperm
Freshly ejaculated sperm cannot immediately penetrate an ovum.
Capacitation (an irreversible process) occurs in the femal tract over 1-3hours
Results in: change from wave-like beats of sperm tail to whip-like action to propel sperm forward. Sperm’s plasma membrane is altered so it is capable of fusing with surface membrane of egg (acrosome reaction).
Capacitation involves: increase in Ca2+ permeability (rise in [Ca2+]i), removal of membrane proteins (glycoproteins), change in surface charge, and depletion of cholesterol.
Transport of Conceptus to Uterus
Ovum is released from ovary and wafted by fimbriae into fallopian tube. Fertilised in ampulla of fallopian tube. Conceptus held in fallopian tubes as oestrogen maintains the contraction of smooth muscle near where the fallopian tube enters wall of uterus. Conceptus undergoes mitotic cell divisions i.e. cleavage and morula is formed. Plasma progesterone levels rise 3-4 days after fertilisation, smooth muscle relaxes and conceptus passes into uterus. Approx. 4-5 days after fertilisation, cavities develop between the cells (blastocyst). For approx 3 days, conceptus/blastocyst lies free in the uterine cavit supported by uterine secretions.
Fertilisation
- Sperm cell weaves past follicular cells and binds to the zona pellucida.
- A rise in [Ca2+]i inside the sperm cell triggers the exocytosis of the acrosome (acrosomal reaction), which contains hydrolytic enzymes.
- Hydrolytic enzymes contained in the acrosomal cap are released. These enzymes locally dissolve the zona pellucida. The whip like action of the tail pushes the sperm head toward the oocyte membrane.
- With the head of the sperm now lying sideways, microvilli on the oocyte surround the sperm head. The two membranes fuse. The contents of the sperm cell enter the oocyte; the sperm-cell membrane remains behind.
- A rise in [Ca2+]i inside the oocyte triggers the cortical reaction, in which there is exocytosis of granules that previously lay immediately beneath the plasma membrane. The enzymes released lead to changes in the zona pellucida proteins, causing the zona pellucida to harden, preventing entry of other sperm cells.
- The rise in [Ca2+]i inside the oocyte induces the completion of the oocyte’s second meiotic division and the formation of the second polar body, which usually lies next to the first polar body.
- The head of the sperm enlarges to become the male pronucleus.
- The male and female pronuclei fuse.
How does Blastocyst avoid Maternal Rejection?
- Releases immunosuppressive agents e.g. interleukin 1alpha, 6 and 8. Also secres human chorionic gonadotropin (hCG) (closely related to LH, supports steroid synthesis of the corpus luterum, and therefore prevents menstruation and any further follicular development. Also promotes trophoblast and placental develop.
- In a non-fertile cycle, the corpus luteum will fail after 10 days and menstruation will occur.
- hCG stimulates the Leydig cells of male foetuses to produce testosterone - important for male duct system development.
Implantation
- Embryo hatching - day 6/7 after fertilisation, the zona pellucida degenerates and blastocyst is released. Lytic factors from endometrial cavity involved. Blastocyst probably produces factors too.
- Apposition - earliest contact between blastocyst wall, trophoectoderm, and endometrial epithelium. Occurs in crypt in endometrium.
- Adhesion - time limited window. Complex interactions between trophoblast and maternal epithelial tissue. Trophoblast attaches to uterine epithelium via microvilli of the trophoblast. Integrin receptor family thought to be involved.
- Invasion - Syncytiotrophoblast cells flow into the endometrium causing oedema, glycogen synthesis and increased vascularisation (decidualisation). Pregnant endometrium is now termed the decidua. Nutrition still depends on uterine secretion and tissues. Breakthrough bleeding may occur. Growth in the embryonic disk is slow and it remains very small (0.1-0.2mm).
