Development & Sex Determination

Learning Objectives By the end of this class you should understand: The key components of the male and female reproductive systems The general processes of fertilization, gestation, and birth The effects of teratogens and two commonly used teratogens The process of sex determination in humans Reasons a human's chromosomal sex may not match their physical sex Effects of X inactivation

Reproduction The process of reproduction involves a male gamete (sperm) fertilizing a female gamete (egg) This chapter will cover this process in more detail Will also cover the development of the resulting zygote to a baby

Male Reproductive System The male reproductive system is designed to constantly produce large amounts of sperm  Sperm are the gametes produced in the testes  Semen is the fluid that contains sperm as well as sugars, mucus, and alkaline solution  Semen is expelled (ejaculation) through the penis

Male Reproductive System

Spermatogenesis

Female Reproductive System Female system hinges on ovaries releasing one egg per month into the oviduct  AKA fallopian tube  This is the site of fertilization  If fertilized, zygote enters uterus and implants  If unfertilized, ultimately ejected along with uterine lining during menstruation

Female Reproductive System

Oogenesis

Gamete Formation Production of sperm is called spermatogenesis and proceeds constantly in the seminiferous tubules of the testes Production of eggs is called oogenesis, where meiosis is initiated in the ovaries when the female is still a fetus, but does not complete until fertilization

Fertilization When sperm enter the vagina, they cross the cervix and uterus and enter both oviducts If they encounter an egg they will initiate fertilization Multiple sperm aid in breaching the egg barrier but only one sperm nucleus enters

Embryo Formation The instant the sperm and egg fuse they cease to be gametes and are termed a zygote  You were a zygote once! The zygote is nourished by the fluid of the uterus and eventually implants in the uterine wall  Becomes an embryo once it begins mitosis

Implantation

Placenta The uterine wall creates the placenta, which provides nutrients to the embryo, including oxygen  Fetuses do not need to breathe since the mother is delivering oxygen through the placenta to the baby!

Embryo The embryo undergoes mitosis many times to form all the starter tissues of the body early

Fetal Development The embryo is redefined as a fetus at week 9  End of first trimester  Organs have already formed  Sex is still undefined Sex determination is initiated during second trimester  If a Y chromosome is present, the SRY gene activates and causes the gonads to become testes  If no Y chromosome is present, the gonads default to become ovaries

What Can Go Wrong? Exposure to certain chemicals or other conditions may alter and/or damage fetus development These environmental factors are called teratogens A wide variety are known and have different deleterious effects on the fetus What are some well-known teratogens?

Teratogens Alcohol Cigarettes X-rays Viruses  Rubella, HIV, Herpes Parasites  Toxoplasma gondii

Development

Sex Determination As previously mentioned, the SRY gene on the Y chromosome causes the gonads to become testes  Any fetus with a Y chromosome will become male Initiates production of male hormones called androgens  Testosterone is the most famous

Sexual Development The analogous body parts develop under the influence of SRY or lack thereof A rare mutation causes the body to fail to detect the SRY signal and the fetus will become female despite being XY This is called Complete Androgen Insensitivity (CAIS)

Dosage Compensation Many critical genes are on the X chromosome including Factor XIII and Retinal red cones Women have two X chromosomes yet they do not express more than men  Even women with three or more X chromosomes still have the same level of expression as a woman with Turner Syndrome (monosomy X) The level of X expression being equal is called dosage compensation

Mechanism of Dosage Compensation The nucleus of women's cells contains a strange dense structure called a Barr body Barr bodies are inactivated X chromosomes and men do not have one  Exception: men with Klinefelter's Syndrome! Women with three X chromosomes have two Barr bodies per cell!

Results of X Inactivation At the embryonic stage, one of the X chromosomes in each stem cell inactivates Typically it is random which one is inactivated  In cats, the X chromosome has a color gene, so female cats can be a swirl of colors but male cats cannot!

X Inactivation Since it is random which X chromosome inactivates, women may sometimes express X-linked recessive disorders  Example: colorblindness if the cells that form the eye all inactivated their nonfaulty X chromosome

X-linked Recessive Expression In this pedigree, identical female twins have discordant expression of colorblindness due to X inactivation in the eyes of one girl  Note this is NOT an example of multifactorial expression

See you tomorrow!