Wednesday March 24, 1999
Announcements:

Lecture notes:

Gametogenesis includes both gamete formation and meiosis.

Gametogenesis means formation of gametes.

In males it is called spermatogenesis

In females, it is called oogenesis.

Both of these processes are under hormonal control.

Both occur in the gonads: The ovary in females, the testis in males.

Results: haploid gametes sperm and egg.

Spermatogenesis

The "Germ cell" is called a spermatogonium (diploid) is located in the wall of the seminiferous tubule in the testis. (About 140 yards of tubule in each testis).

Male primates produce sperm continuously after puberty.

Spermatogonia become activated and divide by mitosis to become primary spermatocytes. (Still diploid)

Meiosis I occurs to form secondary spermatocytes (haploid with duplicated DNA)

Meiosis II occurs to form spermatids (haploid unduplicated DNA)

The process by which spermatids become sperm (mature and mobile) is called spermiogensis.

Oogenesis occurs in the ovaries.

"Germ cell" is the oogonium (diploid)

They divide during fetal development. Estimates range from 250,00-1,000,000 per ovary.

At birth all potential ova are present as primary oocytes - arrested in meiosis I where the homologues are lined up at the equator and ready to divide.

This situation continues until puberty when menstrual cycles begin and follicles start to mature.

About 36 hours before ovulation, meiosis I goes to completion forming 1 secondary oocyte and 1 polar body (haploid duplicated DNA).

The "chosen" follicle continues to ripen while others undergo atresia (withering)

When the follicle ruptures "ovulation" occurs.

Fertilization of ovum by sperm occurs in the oviduct (fallopian tubes)

There are about 350,000,000 sperm in the ejaculate.

Only a few hundred make it to the ovum. (7.9" trip) (think 6 mile swim)

Time estimates are 30 minutes to 6 hours.

The digestive enzymes of all of the sperm present at egg help to dissolve layers.

One sperm fertilizes egg.

When the sperm reach the ovum, the secondary oocyte undergoes meiosis II and becomes the "mature ovum". (haploid, unduplicated DNA)

When the sperm penetrates the ovum, it dissolves and the 2 nuclei unite to form the zygote (diploid chromosomes)

At meiosis II the second polar body is formed. The first polar body may or may not divide at this time.

The polar bodies degenerate.

Physicians consider that couples have a 20% chance to conceive each menstrual cycle.

For people who use no birth control, there is a 90% chance that they will conceive a child within a year.

The standard estimate is the 155 of couples are infertile.

Causes of infertility: 20% unknown or trouble with both partners.
40% males: blocked tubes, high testicular temperature, varicocele, low sperm count, low motility of
sperm.
40% females: blocked tubes, infrequent ovulation, highly acid secretions in vagina, failure to implant.