CONCEPTS IN BIOLOGY
PART VI. PHYSIOLOGICAL PROCESSES
27. Human Reproduction, Sex, and Sexuality
27.4. Sex Determination and Embryonic Sexual Development
In humans and some other organisms, the sex of an offspring is determined by the chromosomes they inherit from their parents.
Chromosomal Determination of Sex
Recall from chapter 10 that two of the 46 chromosomes are involved in determining sex and are called sex-determining chromosomes. The other 44 chromosomes are known as autosomes. There are two kinds of sex-determining chromosomes: the X chromosome and the Y chromosome that do not carry equivalent amounts of information, nor do they have equal functions (figure 27.4).
FIGURE 27.4. Human Male and Female Chromosomes
The chromosomes have been arranged into homologous pairs: (a) a male karyotype, with an X and a Y chromosome, and (b) a female karyotype, with two X chromosomes.
X chromosomes carry genetic information about the production of a variety of proteins, in addition to their function in determining sex. For example, the X chromosome carries information on blood clotting, color vision, and other characteristics. The Y chromosome, with about 80 genes, however, appears to be primarily concerned with determining male sexual differentiation.
When a human sperm, a haploid sex cell produced by sexually mature males, is produced, it carries 22 autosomes and a sex-determining chromosome. Unlike eggs, which always carry an X chromosome, half the sperm cells carry an X chromosome and the other half carry a Y chromosome. If an X-carrying sperm cell fertilizes an X-containing egg cell, the resultant embryo will develop into a female. If a Y-carrying sperm cell fertilizes the egg, a male embryo will develop. It is the presence or absence of the sex-determining region Y (SRY) gene located on the short arm of the Y chromosome that determines the sex of the developing individual. The SRY gene produces a chemical, called testes determining factor (TDF), which acts as a master switch that triggers the events that converts the embryo into a male. Without this gene, the embryo would become female.
The early embryo resulting from fertilization and cell division is not recognizable as either male or female. Sexual development begins when certain cells become specialized, forming the embryonic gonads known as the female ovaries and the male testes. This specialization of embryonic cells is called differentiation. If the SRY gene is present and functioning, the embryonic gonads begin to differentiate into testes 5 to 7 weeks after conception (fertilization).
Chromosomal Abnormalities and Sexual Development
Evidence that the Y chromosome and its SRY gene control male development comes from many kinds of studies, including research on individuals who have an abnormal number of chromosomes. An abnormal meiotic division that results in sex cells with too many or too few chromosomes is a form of nondisjunction (see chapter 9). If nondisjunction affects the X and Y chromosomes, a gamete might be produced that has only 22 chromosomes and lacks a sex-determining chromosome. On the other hand, it might have 24, with 2 sex-determining chromosomes. If a cell with too few or too many sex chromosomes is fertilized, sexual development is usually affected. If a normal egg cell is fertilized by a sperm cell with no sex chromosome, the offspring will have only 1 X chromosome. These people, always women, are designated as XO. They develop a collection of characteristics known as Turner’s syndrome (figure 27.5).
FIGURE 27.5. Turner's Syndrome
Individuals with Turner’s syndrome have 45 chromosomes. They have only 1 of the sex chromosomes, and it is an X chromosome. Individuals with this condition are female, have delayed growth, and fail to develop sexually. This woman is less than 150 cm (5 ft) tall and lacks typical secondary sexual development for her age. She also has the “webbed neck” that is common among individuals with Turner’s syndrome.
About 1 in 2,000 girls born has Turner’s syndrome. A female with this condition is short for her age and fails to mature sexually, resulting in sterility. In addition, she may have a thickened neck (termed webbing), hearing impairment, and some abnormalities in her cardiovascular system. When the condition is diagnosed, some of the physical conditions can be modified with treatment. Treatment involves the use of growth-stimulating hormone to increase her growth rate and female sex hormones to stimulate sexual development, although sterility is not corrected.
An individual who has XXY chromosomes is basically male (figure 27.6). This genetic condition is termed Klinefelter’s syndrome. It is one of the most common examples of abnormal chromosome number in humans. This condition is present in about 1 in 500 to 1,000 men. Most of these men lead healthy, normal lives and it is impossible to tell them apart from normal males. However, those with Klinefelter’s syndrome may be sterile and show breast enlargement, incomplete masculine body form, lack of facial hair, and some minor learning problems. These traits vary greatly in degree, and many men are diagnosed only after they undergo testing to determine why they are infertile. Treatments include breast-reduction surgery and testosterone therapy.
FIGURE 27.6. Klinefelter's Syndrome
Individuals with two X chromosomes and a Y chromosome are male, are sterile, and often show some degree of breast development and female body form. They are typically tall. The two photos show an individual with Klinefelter’s syndrome before and after receiving testosterone hormone therapy.
Fetal Sexual Development
The development of embryonic gonads begins very early during fetal growth. First, a group of cells begins to differentiate into primitive gonads at about week 5 (figure 27.7). By week 5 to 7, if a Y chromosome is present, the gene product (testes determining factor) from the chromosome begins the differentiation of these embryonic gonads into testes. They will develop into ovaries beginning about week 12 if 2 X chromosomes are present (and the Y chromosome is absent).
FIGURE 27.7. Differentiation of Sexual Characteristics
The early embryo grows without showing any sexual characteristics. The male and female sexual organs eventually develop from a common basic structure. (a) Shows the development of the internal anatomy. (b) Shows the development of the external anatomy.
As soon as the gonad has differentiated into an embryonic testis at about week 8, it begins to produce testosterone. The presence of testosterone results in the differentiation of male sexual anatomy, and the absence of testosterone results in the differentiation into female sexual anatomy in the developing embryo (Outlooks 27.1).
OUTLOOKS 27.1
Cryptorchidism—Hidden Testes
At about the seventh month of pregnancy (gestation), in normal males each testis moves from a position in the abdominal cavity to an external sac, called the scrotum. The testes pass through an opening called the inguinal canal. This canal closes off but continues to be a weak area in the abdominal wall, and it may rupture later in life. This can happen when strain (for example, from improperly lifting heavy objects) causes a portion of the intestine to push through the inguinal canal into the scrotum, a condition known as an inguinal hernia.
Occasionally, the testes do not descend, resulting in a condition known as cryptorchidism (crypt = hidden; orchidos = testes). Sometimes, the descent occurs normally during puberty; if not, there is a 25 to 50 times increased risk for testicular cancer. Because of this increased risk, surgery can be done to allow the undescended testes to be moved into the scrotum. Sterility will result if the testes remain in the abdomen. This happens because normal sperm cell development cannot occur in a very warm environment. The temperature in the abdomen is higher than the temperature in the scrotum. Normally, the temperature of the testes is very carefully regulated by muscles that control their distance from the body. Physicians have even diagnosed cases of male infertility as being caused by tight-fitting pants that hold the testes so close to the body that the temperature increase interferes with normal sperm development. Recent evidence has also suggested that teenage boys and young men working with computers in the laptop position for extended periods may also be at risk for lowered sperm counts.
27.4. CONCEPT REVIEW
7. Describe the processes that cause about 50% of babies to be born male and 50% to be born female.
8. Name two developmental abnormalities associated with nondisjunction of chromosomes.
9. When in embryological development does sexual differentiation begin?
10. What triggers sexual differentiation in embryos?