development & sex determination. learning objectives by the end of this class you should...

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!