Ross & Wilson Anatomy and Physiology in Health and Illness (202 page)

Gene mutation

Many diseases, such as cystic fibrosis (
p. 258
) and haemophilia (
p. 72
), are passed directly from parent to child via a faulty gene. Many of these genes have been located by mapping of the human genome, e.g. the gene for cystic fibrosis is carried on chromosome 7. Other diseases, e.g. asthma, some cancers and cardiovascular disease, have a genetic component (run in the family). In these cases, a single faulty gene has not been identified, and inheritance is not as predictable as when a single gene is responsible. The likelihood of an individual developing the disease depends not only on their genetic make-up, but also on the influence of other factors, such as lifestyle and environment.

Phenylketonuria

In this disorder, which is an example of an
inborn error of metabolism
, the gene responsible for producing the enzyme phenylalanine hydroxylase is faulty, and the enzyme is absent. This enzyme normally converts phenylalanine to tyrosine in the liver, but in its absence phenylalanine accumulates in the liver and overflows into the blood (
Fig. 17.11
). In high quantities, phenylalanine is toxic to the central nervous system and, if untreated, results in brain damage and mental retardation within a few months. Because there are low levels of tyrosine, which is needed to make melanin, depigmentation occurs and children are fair skinned and blonde. The incidence of this disease is now low in developed countries because screening of newborn babies detects the condition and treatment is provided.

Figure 17.11 
Phenylketonuria. A.
Gene function normal.
B.
Abnormal gene.

Chromosomal abnormalities

Sometimes during meiosis, a fault in the process produces a gamete carrying abnormal chromosomes – too many, too few, abnormally shaped, or with segments missing. Often, these aberrations are lethal and a pregnancy involving such a gamete miscarries in the early stages. Non-lethal conditions include Down syndrome and cri-du-chat syndrome.

Down syndrome

In this disorder, there are three copies of chromosome 21 (trisomy 21), meaning that an extra chromosome is present, caused by failure of chromosomes to separate normally during meiosis. People with Down syndrome are usually short of stature, with pronounced eyelid folds and flat, round faces. The tongue may be too large for the mouth and habitually protrudes. Learning disability is present, ranging from mild to severe. Life expectancy is shorter than normal, with a higher than average incidence of cardiovascular and respiratory disease. Down syndrome is associated with increasing maternal age, especially over 35 years.

Cri-du-chat syndrome

Cri-du-chat (cat’s cry) refers to the characteristic meowing cry of an affected child. This syndrome is caused when part of chromosome 5 is missing, and is associated with learning disabilities and anatomical abnormalities, including gastrointestinal and cardiovascular problems.

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.

CHAPTER 18

The reproductive systems

Female reproductive system
438

External genitalia (vulva)
438

Internal genitalia
439

Vagina
439

Uterus
440

Uterine tubes
442

Ovaries
442

Puberty in the female
444

The reproductive cycle
444

Menopause
445

Breasts
446

Male reproductive system
447

Scrotum
447

Testes
448

Seminal vesicles
449

Ejaculatory ducts
449

Prostate gland
449

Urethra and penis
449

Ejaculation
450

Puberty in the male
450

Sexually transmitted infections
452

Diseases of the female reproductive system
453

Pelvic inflammatory disease (PID)
453

Vulvar dystrophies
453

Imperforate hymen
453

Disorders of the cervix
453

Disorders of the uterine body
453

Disorders of the uterine tubes and ovaries
454

Female infertility
454

Disorders of the breast
455

Diseases of the male reproductive system
455

Infections of the penis
455

Infections of the urethra
455

Epididymis and testes
456

Prostate gland
456

Breast
456

Male infertility
457

ANIMATIONS

18.1
Female external genitalia
438

18.2
Vestibular glands
439

18.3
Female reproductive ducts
439

18.4
Ovaries
442

18.5
Menstrual cycle
445

18.6
Ovarian cycle
445

18.7
Breasts
446

18.8
Testes
448

18.9
Spermatozoa
448

18.10
Male accessory sex glands
449

18.11
Male reproductive ducts
449

18.12
Male external genitalia
450

18.13
Pathway of sperm
450

The ability to reproduce is one of the properties distinguishing living from non-living matter. The more primitive the animal, the simpler the process of reproduction. In human beings the process is one of sexual reproduction, in which the male and female organs differ anatomically and physiologically, and the new individual develops from the fusion of two different sex cells (gametes).

Adult males and females produce specialised reproductive germ cells, called
gametes
. The male gametes are called
spermatozoa
and the female gametes are called
ova
. They contain the genetic material, or
genes
, on
chromosomes
, which pass inherited characteristics on to the next generation. Other body cells possess 46 chromosomes arranged in 23 pairs but the gametes contain only 23, one from each pair. Gametes are formed by
meiosis
(
p. 432
). At
fertilisation
, the fusion of an ovum and a spermatozoon, the resulting cell is called a
zygote
, and now possesses the full complement of 46 chromosomes.

The zygote embeds itself in the wall of the uterus where it grows and develops during the 40-week
gestation period
before birth.

The functions of the female reproductive system are:

•
formation of ova

•
reception of spermatozoa

•
provision of suitable environments for fertilisation and fetal development

•
parturition (childbirth)

•
lactation, the production of breast milk, which provides complete nourishment for the baby in its early life.

The functions of the male reproductive system are:

•
production of spermatozoa

•
transmission of spermatozoa to the female.

The later sections of this chapter describe disorders of the reproductive systems.

Female reproductive system