Conception Cairns
Conception Cairns
conception cairns
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CONCEPTION
Of the millions of sperm deposited in the vagina during sex, perhaps fewer than 200 reach the fallopian tube. There might not be more than 10 sperm in the ampulla at the time the the fallopian tube. There might not be more than 10 sperm in the ampulla at the time the close contact for fertilization to become probable, but we don’t know how it does fertilization needs to take place within about 10 hours of ovulation for the embryo later to develop normally* so it’s best if the first sperm can get to the tube before the egg arrives. Ejaculated sperm are motile (they swim), but they re inhibited from displaying their peak performance by a surface layer of protein molecules (see the box, post-production in the peak performance by a surface layer of protein molecules (see the box, post’production I n
The effect of taking off these brakes is to power up the sperms swimming speed and to give the cell membrane over the sperm head the stickiness the sperm needs to cling to the egg’s outermost membrane-in all, a process called capacitation. Sperm that are being stored in receptive mucus in the cervix are usually not yet in a capacitated state. Capacitation can happen naturally, by having the sperm swim up through the uterus and out along the tube, or it can be made to happen in the laboratory, by spinning and washing the sperm through a series of solutions (colloquially called sperm washing). Conception Fertilization Cairns
Fertilization
The ovulated egg has several layers that the capacitated sperm needs to power through; these are shown in Figure 3.6 and in Figure 3.7 first there is the jelly-like cumulus, which the sperm likes to get its tail into (the beating sperm tail gets a better grip in the cumulus, propelling it forwards more effectively). Second is a tough, glassy-looking membrane, the zona pellucida (Latin for’clear layer’), to which the sperm must successfully attach itself if it’s to get through it. What happens next is complicated but important.
The sperm cell up to this point has been wearing a crash-helmet, the acrosome. Imagine pushing your fist into a soft balloon: the fist is the sperm head’s nucleus (where its genetic material is packed densely into the 23 chromosomes); the balloon is the acrosome.The outside of the sperm head, provided the acrosome is intact (that is, the balloon hasn’t popped), is specifically sticky for the zona pellucida-the way Velcro is for its matching surface.
The successful sperm binds tightly to the zona (see Figure 3.7).This causes the acrosome to break open, (as a balloon would) a potent mixture of enzymes, which digest a path through the zona; these are helped by the capacitated sperm’s forcefully beating tail, itself with a good grip in what’s left of the cumulus.
The process of the balloon bursting is called the acrosome reaction. And so it is that the sperm, now bereft of its acrosome, finds itself next to the within the perivitelline space\ all it now needs to do is drift up to the vitellus-ihz egg itself. Covering the acrosome-reacted sperm head now is the inner layer of the acrosome, which, unlike the outer layers (now gone), is specifically sticky for the egg membrane.
Contact is made and the trap is sprung (see Figure 3.7 again). Like nerve cells and muscle cells, the egg is electrically charged.The moment the sperm fuses with it, an electrical signal causes several reactions inside the egg. Almost immediately fuses with it, an electrical signal causes several reactions inside tne tfgg.nunosi immediate!^ a mucus that fills the perivitelline space, smothering the egg membrane, and stopping any more sperm that might be on their way in from attaching to the egg. (If another sperm chances to reach the egg during this 15 seconds, more than one sperm will fertilize it and the embryo will be ruined by having at least 23 too many chromosomes-a process called polyspermia fertilization, or simply “polyspermy m) At the same time, the egg finishes expelling its own extra chromosomes, reducing the number from 46 to 23, to match those of the sperm. Third, the machinery of the egg starts preparing for it to become an embryo. In about 24 hours it will divide into two cells, then four cells, and so on.
The sperm head, now inside the egg, falls apart, revealing all as the egg strips down to its chromosomes.This is no time for men to be bashful! At no other point are the chromosomes to match its own: each set forms a watery blob within the substance of the ess (visible with the help of a microscope in Figure 3.6c)-the male and the female pronwcIei.
This is the first definite sign of fertilization for the watching scientist, and is looked for in IVF laboratories between 16 and 20 hours after eggs are inseminated vitro. The male and female pronuclei then join together so that the two sets of chromosomes can mingle and match, a process called syngarrty-^he chromosomes now number 4-frthe normal complement again. If the sperm had an X<hromosome to pair with the egg’s X-chromosome the resultant chromosome count, or karyotype, will be 46,XX (female); if the sperm hadaY-chromosome the resultant karyotype will be 46,XY (male). The fertilized egg then divides several times without increasing its overall size (see a process termed cleavage. Upon reaching about 16 cells, the cells themselves, till now just a loosely aggregated ball within the zona pellucida, for the first time begin to bind strongly together, in effect forming a tissue.
We call this compaction. As a result, the embryo now has a homogeneous appearance, and is termed a morula The developing embryo enters the endometrial cavity three days after ovulation and fertilization. It is, or very soon will be, a blastocyst, a hollow ball of cells surrounding a cystlike cavity, and soon to number a hundred or more cells (see Figure 3.6g and h). The blastocyst’s cells have taken their first step towards differentiation, or specialization. A rim of outer cells is called the trophectoderm, and will give rise to the membranes and placenta. The inner cells, which form a ball off to one side, called the inner cell mass, -will, after implantation, give rise to the embryo proper, or fetus. These structures are more obvious when the blastocyst has expanded (Figure 3.6h) . Finally, the expanded blastocyst eases its way out of the now very thin and stretched zona pellucida (we do cali it ‘hatching’-see Figure 3.6i).The naked blastocyst is now ready to adhere to the surface of the endometrium. Already it is furiously signaling its presence by producing many crucial substances, among which is that most ‘primitive of the body’s proteins, human chorionic gonadotropin(ViCGV hcG is the signal that tells the corpus luteum to keep producing progesterone and is, a little later, measurable in blood or urine—the usual chemical basis for pregnancy tests. Conception Fertilization Cairns
Implantation
The blastocyst attaches to the endometrium on about dav ? 1 of the menstrual cycle, or seven days after fertilization. It squeezes through the surface layer of the endometrium to lie within the endometrial stroma, the watery connecting substance between the endomc trial glands—which is what we mean by implantation. Nutrition comes from secretions the endometrial glands. The outer layer of the implanted blastocyst, the trophoblast k soon in potential touch with the mother’s blood, so oxygen diffuses through to the developing embryo. Sometimes as the blood first bathes the embryo (on about day 26 of the cycle), a small amount of blood leaks out of the uterus into the vagina-this is implantation bleeding. If it occurs it’s nearly always much lighter than a menstrual period, so there should not be much confusion. Conception Fertilization Cairns
A blood pregnancy test, set to measure serum hCG levels greater than 50 units, will be positive.
Conception Fertilization Cairns / Conception Fertilization Cairns / Conception Fertilization Cairns/ Conception Fertilization Cairns / Conception Fertilization Cairns
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