II. Fertilization

III. Reproductive technologies

Terms

A. Anatomy of the male reproductive system.
1. The gonads are considered the primary sexual organs. In the case of males, the gonad is a testis (testicle). The testes have two primary functions: production of sperm and secretion of two substances (testosterone and Mullerian inhibitor, to be discussed in a later lecture).

2. A testis consists of two principal structures: seminiferous tubules and interstitial cells.

3. The seminiferous tubules are long tube-like structures that empty into the epididymis. Surrounding the central open space (lumen) are Sertoli cells. Imbedded within the Sertoli cells are the germ cells. The spermatogonial cells are at the periphery. As meiosis and spermiogenesis occur, the germ cells move toward the lumen. Mature sperm are released into the lumen and move into the epididymis, where they collect and mature further.

4. The interstitial cells fill the spaces between the seminiferous tubules.

5. During ejaculation, sperm pass through the ductus deferens (vas deferens), where they are mixed with fluid from the seminal vesicles, bulbourethral glands, and prostate gland to form semen, which passes out through the urethra.

6. It is possible to freeze semen and store at very low temperatures (liquid nitrogen) for later use. This is commonly done with domestic animals, allowing a prize bull, for example, to sire large numbers of offspring without the bother of conventional mating. The same technology can be used with humans. The idea of sperm banks has been around a number of years but has never caught on. It is common for a young male who is to be treated for cancer with agents that might cause birth defects or sterility to bank sperm for possible future use.

B. Anatomy of the female reproductive system.
1. The female gonad is the ovary. This is the site of germ cells as well as tissues that secrete female sex hormones.

2. The germ cells are in the form of primary oocytes, each located in a follicle, which is a mass of cells that surround the oocyte. Most of the follicles are small, primary follicles. At puberty, a few of the follicles begin to develop, becoming very large. The germ cell within the follicle completes meiosis I and becomes a secondary oocyte. Each four weeks, (usually) one of the follicles erupts, releasing the germ cell surrounded by a layer of protective cells, into the fallopian tubes (referred to in the textbook as oviducts), where they risk being fertilized.

A. The mature ovum is released from the ovary and enters the fallopian tube, where it may encounter sperm.

B. The acrosomal body of a sperm penetrates the corona of cells and the zona pellucida that surround the ovum.

C. The sperm, including the tail, enters the ovum. The nucleus of the sperm forms a pronucleus similar to that from the female. This fertilized egg is known as a zygote.

D. Once a sperm has penetrated the ovum, other sperm are blocked from entering.

E. The pronuclei fuse, restoring the diploid state. Mitosis begins; embryonic development has started.

F. After approximately seven days in humans, the embryo reaches the uterus and implants in the uterine wall.

A. Artificial insemination involves using sperm from a male who is known not to transmit a gene that may be of concern. For example, if a couple is at risk of having a child with a dominant disorder transmitted by the male, use of semen from another male can avoid that disorder. In the case of recessive disorders, where both parents must be heterozygous, use of semen from a male who is not heterozygous reduces the risk of the recessive disorder to zero.

B. In vitro fertilization (IVF) allows both marriage partners to be the biological parents of a child. Although first used in cases where maldevelopment of the fallopian tubes prevented an otherwise fertile woman to become pregnant, the procedure is increasingly used for other reasons, especially genetic. For this purpose, the zygote is allowed to develop into an 8-cell embryo in vitro. One of the cells can then be removed for genetic analysis for conditions for which the embryo is at risk. If the analysis is negative, the embryo is then implanted into the mother's uterus for a normal pregnancy. If the analysis shows the embryo to have a detrimental genetic trait, then the embryo is discarded. Embryos produced by IVF can be frozen and used later, a process that has generated some interesting legal and social issues.

C. A problem in some matings is infertility due to low sperm count or to defective sperm, i.e. low motility. In these instances, the haploid nucleus from a single sperm can be extracted and injected mechanically directly into the cytoplasm of the ovum. The fertilized egg is then treated as in IVF.

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