Gamete Transport & Fertilization
Oocyte Transport to uterine cavity:
- Takes ~3-4 days
Female Reproductive Tract
- Upper vagina, cervix, uterus, and uterine tubes
- Endometrial lining of the uterine cavity
- Key features of the uterine tube: the isthmus, ampulla (the widest part of the tube), and the finger-like fimbriae.
- Uterine tube wall comprises smooth skeletal muscle and mucosal projections, which feature ciliated epithelia.
Key events:
- Ovulation leads to two key events:
- Expulsion of the oocyte complex and formation of the corpus luteum.
- Oocyte complex: secondary oocyte, which is arrested in meiosis II; the zona pellucida, which is the oocyte's acellular protective coating; the cumulus oophorus, which comprises follicular cells.
- Uterine fimbriae "sweep" the oocyte complex into the uterine tube, where peristaltic smooth muscle contractions move the oocyte towards the uterine cavity.
- In the ampulla of the uterine tube, the oocyte might encounter sperm.
- If not, the oocyte complex degenerates within 24 hours.
- If the oocyte and sperm meet, fertilization may occur.
Sperm transport to Ampulla
- Semen comprises 100s of millions of sperm cells, which contribute less than 10% of total semen volume; the other 90% comprises protective seminal fluids and nutrients.
- Up to 10% of sperm are malformed; if malformation affects more than 20%, fertility may be impaired.
Transport through the male reproductive tract:
- Sperm form within seminiferous tubules of the testes.
- Enter epididymis, where sperm are stored for several weeks and acquire motility.
- During the emission phase, sperm and testicular fluids are transported to the ductus deferens (aka, vas deferens), which is a long muscular tube that, via peristaltic contractions, transports the sperm to the ejaculatory duct.
- Within ejaculatory duct, sperm are joined by secretions from the seminal vesicles and prostate gland before exiting the body via the urethra.
Transport through female reproductive tract:
Of the millions of sperm present in the initial ejaculate, only a portion of these will reach the ampulla; many will die within the female reproductive tract.
- Contractions of the vagina and uterus move the sperm towards the cervix.
- At the cervix, cervical mucus secreted by the cervical glands regulates passage into uterus:
- The composition of the mucus barrier changes according to ovulatory status, so that passage is only possible during the fertile window.
- In the days prior to ovulation, estrogen facilitates formation of watery mucus, which permits sperm passage;
- After ovulation, progesterone alters mucus composition: it becomes viscous and, thus, impenetrable.
- Within the uterine cavity, sperm are ushered towards the oocyte complex by uterine contractions and sperm "swimming" movements; interestingly, sperm tend to move towards the side where the oocyte was released, which may imply chemical signaling between the sperm and oocyte complex.
- In the isthmus region of the uterine tube, epithelial cells bind the sperm, holding them here for approximately 24 hours.
- During this time, tubal secretions induce sperm cell capacitation, which removes glycoproteins from the cell surface that would otherwise inhibit fertilization.
- Then, the sperm enters a period of hyperactivity, and frees itself from the tubal epithelia. Hyperactivity is characterized by asymmetrical flagellar motions that move the sperm cell nonlinearly.
- Finally, the sperm reach the ampulla, where fertilization may occur.
Fertilization
- Fusion of male and female gametes.
- Occurs within 24 hours of ovulation, after which the oocyte degenerates.
- Step 1: Sperm cells pass through the cumulus oophorus.
- Step 2: Glycoproteins on the zona pellucida bind receptors on the sperm cell that were exposed during capacitation.
- Binding initiates the cascade of events called the acrosome reaction, in which the acrosome of the sperm cell releases enzymes that dissolve the zona pellucida;
- Dissolution allows the hyperactive sperm cell to wiggle through the zona pellucida.
- Step 3: The sperm and oocyte cell membranes fuse, and are bound by oocyte microvilli;
- The contents of the sperm cell move into the oocyte (the sperm cell's plasma membrane remains outside of the oocyte).
- Step 4: The cortical reaction hardens the glycoproteins on the zona pellucida, which blocks additional sperm cells from binding with the oocyte.
- Step 5: The oocyte completes the second meiotic division, which produces the haploid oocyte and second polar body (both the cortical reaction and resumption of meiosis II are initiated by a rise in intercellular calcium, which was induced by fusion with the sperm cell).
- The oocyte chromosomes decondense and form the female haploid pronucleus.
- Step 6: The male pronucleus forms.
- Step 7: Fertilization is complete when the pronuclei merge; upon contact, their membranes breakdown and fuse, allowing the genetic content to intermingle.
- The result is a single diploid zygote.
Clinical correlations:
Multiple pregnancy is the production of two or more offspring in a single pregnancy.
Entails increased risk of serious complications (including preterm births, placental problems, preeclampsia, and gestational diabetes).
- Though rare in the past, multiple pregnancy has risen dramatically in places where the use of reproductive technologies and maternal age have increased. (For example, as of 2014, 34 twins were born per 1,000 births in the U.S. - CDC)
- Dizygotic (aka, fraternal, non-identical) twins are produced when two oocytes are ovulated and fertilized with two sperm cells; this is the most common type of twinning.
- Monozygotic (aka, identical) twins are produced when a single embryo splits after fertilization.
- Relatedly, polyspermy occurs when one oocyte fuses with two or more sperm; because this creates a zygote with an abnormal number of chromosomes, it is typically lethal.
- Recall that polyspermy is usually prevented by the cortical reaction of the zona pellucida.