Chapter 25. Chapter 25: Reproduction and Development

Many legal and ethical complications can arise with assisted reproductive technologies. Just one example: children conceived by sperm or egg donation may wish to make contact with and even seek a relationship with the sperm or egg donor.

The genetically identical offspring resulting from asexual reproduction are less likely to produce a population with the genetic capability to adapt to, and to produce offspring adapted to, a changing environment.

External fertilization doesn’t work well on land because gametes cannot be moved around (and thus toward each other) without water and they quickly dry out.

In adult men, sperm are continuously produced in the testes by meiosis. Diploid cells called spermatogonia in the seminiferous tubules of the testes divide by mitosis, each spermatogonium producing two cells. One of these cells remains a spermatogonium, so the male never runs out of sperm-producing cells; the other is a primary spermatocyte, which undergoes meiosis. Each primary spermatocyte produces two cells in the first meiotic division—the secondary spermatocytes. These then complete the second meiotic division, each producing two spermatids, which mature into sperm.

There are several adaptations resulting from sperm competition. (a) Physical barriers to copulation, such as mating plugs, prevent other males’ sperm from entering the female’s reproductive tract. (b) Toxic semen components can incapacitate the sperm of other males. (c) Genital morphology, such as claspers and scrapers, allow males to dislodge the sperm of other males that have already mated with the female.

Genetically, the production of eggs barely differs from sperm production. The key differences are: Meiosis begins in the ovaries before birth, producing primary oocytes, then stops at prophase I. Beginning at puberty, periodic release of hormones stimulates completion of meiosis I in several oocytes. A secondary oocyte is released at ovulation, but completion of meiosis II is triggered only after fertilization, forming the ovum (egg). A much smaller number of eggs than sperm are produced, and each egg is considerably larger than a sperm cell.

If the egg is not fertilized, it disintegrates, and the corpus luteum in the ovary degenerates, abruptly removing this source of estrogen and progesterone. This drop in hormone levels causes the lining of the uterus to slough off, and it is shed, along with the egg, as menstruation begins.

When the plasma membranes of the sperm and egg fuse (activation), the membrane of the egg changes in such a way that it is impossible for any other sperm to fuse with and fertilize the egg. In addition, this fusion induces the egg to complete its second meiotic division. On doing so, the ovum is formed, along with a (much smaller) polar body, which disintegrates or is ejected from the egg. Finally the haploid egg nucleus fuses with the haploid sperm nucleus, creating a diploid cell—the zygote.

The estrogen-progesterone pill prevents ovulation by keeping estrogen levels just high enough that the release of follicle-stimulating hormone (FSH) by the pituitary gland is never triggered. As long as FSH is not released, eggs do not develop and ovulation does not occur. The progesterone component of the pill causes just enough development of the lining of the uterus to allow a mucous plug to form at the connection between the vagina and uterus, blocking sperm from getting through.

STDs are difficult to eradicate for several reasons: (a) Their symptoms may be mild or completely absent, causing many people to unwittingly pass on an infection to their partners. (b) To prevent reinfection, both partners must be treated simultaneously. (c) Populations of a microbe can evolve quickly and become resistant to existing drugs, reducing the long-term effectiveness of treatments.

Three distinct layers of tissue, called germ layers, form during gastrulation: ectoderm, endoderm, and mesoderm. The cells in these layers have not yet differentiated (that is, they look similar to one another), but they have become “determined,” which means that the type of tissue they will become is already decided. The ectoderm becomes the outer layer of skin, hair, the lining of the nose and mouth, and the nervous system. The endoderm becomes the digestive tract, respiratory tract, liver, and pancreas. The mesoderm becomes the muscles and skeleton.

The presence of high levels of testosterone causes ducts that connect the gonads and the outside of the body to develop into the male internal reproductive structures, including the vas deferens (on each side), ejaculatory duct, and prostate. In the absence of testosterone, adjoining ducts become the female reproductive structures, including the uterus, cervix, and Fallopian tubes.

The first trimester of pregnancy is primarily a time of development and differentiation of cells into specialized types of tissues, as the embryo implants in the uterus and the placenta forms. The second and third trimesters are characterized mostly by significant growth and rapid development of the fetus’s nervous system.

Ovulation is suppressed during pregnancy because of a hormone- mediated chain of events. During pregnancy, human chorionic gonadotropin (hCG) prevents degradation of the corpus luteum, which continues to secrete progesterone, thereby maintaining the lining of the uterus (the endometrium). With the endometrium intact, the pituitary gland does not release follicle-stimulating hormone (FSH) and follicles do not grow and develop, and no luteinizing hormone (LH) is released.

During lactation, suckling by the infant prevents the pituitary gland from releasing a sufficient surge of LH to cause ovulation. For this reason, during lactation, a woman’s fertility is significantly reduced.

In each type of assisted reproductive technology, several secondary oocytes (eggs) are collected from a woman’s ovaries. In vitro fertilization-embryo transfer (IVF-ET) involves combining eggs with sperm in a Petri dish, where fertilization occurs. Once fertilized cells are at the 8-cell stage, they are inserted into the uterus, where development continues. Zygote intra-Fallopian tube transfer (ZIFT) also involves combining eggs with sperm in a Petri dish, where fertilization occurs. Unlike IVF-ET, however, the fertilized eggs are inserted into the Fallopian tube at the 1-cell stage. Gamete intra-Fallopian tube transfer (GIFT) involves immediately mixing eggs with sperm and inserting the mixture into the Fallopian tube, where fertilization occurs. Typically, women have numerous transferred embryos implanted into their uteruses. This, in turn, increases the likelihood of pregnancy as well as the risk of multiple births.