Humans have many options to achieve fulfillment in their personal lives, including the free choice of whether or not to reproduce. For the discussion that follows, reference to male and female is made within the context of their biological roles in reproduction.
Gametogenesis
Primordial Germ Cells
Fig 1. Sagittal section of an early embryo showing primordial germ cells in the posterior wall of the yolk sac.
During the second week of development cells from the embryonic disc migrate to the wall of the yolk sac where they commit to become primordial germ cells. Later in development these same primordial germ cells migrate into the body of the embryo to seed the developing gonads.
There, they wait, until this yet unborn individual reaches puberty, at which time they continue with development and maturation to form special reproductive cells known as gametes.
This ensures that early in development, a population of cells is committed to become reproductive cells even though they will not be needed by this individual until many years in the future.
It also safeguards a pristine copy of the individual's genetic code until needed for reproduction and underscores the notion that preservation of the species is one of the most important purposes of life.
Gametogenesis
Gametogenesis is the process by which primordial germ cells, that are diploid, become converted to special reproductive cells called gametes, that are haploid.
The process by which this is possible is called meiosis and involves two successive cell divisions with only one replication of the chromosomes.
Gametes are special reproductive cells that possess only one set of chromosomes. Since they have half as many sets of chromosomes as a diploid cell they are said to be haploid.
Haploid gametes are necessary in sexual reproduction to maintain a constant number of chromosomes in each generation of the species.
The production of haploid gametes from diploid body cells is called gametogenesis a process that relies on meiosis.
Standard notation for illustrations is lower case 'n' to indicate number of sets of chromosomes and upper case "N" to indicate amount of DNA.
Primordial germ cells undergo mitosis to form primary gametocytes that are diploid, possessing 2 sets of chromosomes (2n) and 2N DNA. During prophase of meiosis I, primary gametocytes replicate their entire complement of DNA and though still diploid (2n), are now 4N in DNA.
Each chromosome pair consists of four strands of DNA called chromatids. When homologous chromosomes align during metaphase of meiosis I, a process called synapsis, they form tetrads. Synapsis permits chromatids to exchange genetic material through crossovers. This is how variation in the species comes about.
Completion of meiosis I produces two secondary gametocytes, each having one set of double stranded chromosomes (1n). Since they have only one set of chromosomes these cells are haploid, but because each chromosome consists of two chromatids, the are (2N DNA).
Completion of meiosis II results in the production of haploid gametes (1n), each having one set of single stranded chromosomes possessing 1N DNA.
- Male gametes are spermatozoa and are produced by spermatogenesis within the seminiferous tubules of the testes.
- Female gametes are ova and are produced by oogenesis within the follicles of the ovaries.
Fig 2. Diagram showing the stages of meiosis, in one pair of 23 pairs of chromosomes. The white chromosomes is from one of the individual's parents and the blue chromosome is from the other parent. Changes in chromosome number (n) and amount of DNA (N) in each stage is indicated.
As a result of this interaction, the DNA an individual passes on to their offspring is a blend of the DNA they were given by their male and female parents. It's how genetic variation comes about in nature, and why, in most cultures, it is taboo for siblings to marry.
Clinical Correlates
Normal Gametogenesis
The full complement of chromosomes in human somatic cells is the diploid number of 46 or two sets of 23 chromosomes.
Normal gametes have one complete set of 23 chromosomes and so are haploid.
After replication of the chromosomes during meiosis I, tetrad formation and synapsis leads to crossovers and a redistribution of genetic material.
Completion of meiosis I and II results in redistribution of chromosomes into haploid gametes.
Fig 3. Animated gif image showing normal distribution of chromosomes in meiosis I and meiosis II.
Chromosomal Abnormalities
Chromosomal abnormalities fall into two general categories: numerical or structural. They are important in accounting for a large number of spontaneous abortions, known to occur in the early weeks of pregnancy.
Numerical Abnormalities
Nondisjunction is the failure of homologous chromosomes to separate from each other during meiosis. Nondisjunction can occur in either meiosis I or meiosis II, resulting in abnormal gametes possessing an extra copy of one chromosome, trisomy or the absence of a chromosome, monosomy.
Down syndrome or Trisomy 21 is a well known example in which a gamete, possessing an extra copy of chromosome 21 is involved in fertilization, resulting in a zygote with three copies of chromosome 21. Although trisomy 21 has long been associated with advanced maternal age, other causes have also been identified.
Nondisjunction may also involve the sex chromosomes, again resulting in common syndromes such as Klinefelter syndrome in which nondisjunction of the X chromosomes during gametogenesis may result in a phenotypically male individual with an extra X chromosome (XXY) or Turner syndrome in which a phenotypical female lacks one X chromosome, (45, X). This is the only monosomy that is known to be compatible with life. 80% of the cases are related to nondisjunction of the X chromosome during spermatogenesis.
Nondisjunction Meiosis I
Fig 4. Animated gif showing how non disjunction in meiosis I can produce gametes with an abnormal number of chromosomes.
Nondisjunction Meiosis II
Fig 5. Animated gif showing how non disjunction in meiosis II can produce gametes with an abnormal number of chromosomes.
Structural Abnormalities
Long and short arms of chromosomes are determined by the position of the centromere, the short arm, designated by 'p' and the long arm designated by 'q'. Structural anomalies, that may involve one or more chromosomes are due to breakage usually resulting in a deletion or partial deletion of genes. Deletions can occur with either the male or female gamete.
Cri-du-chat syndrome is a well known condition related to deletion of the short arm of chromosome 5. It is so named because afflicted infants have a characteristic cat like cry.
Microdeletions occur by deletion of only a few contiguous genes on a chromosome. A fairly well known deletion involves genes on the long arm (q) of chromosome 15.
If this microdeletion occurs on chromosome 15 of the ovum, the maternal gamete, Angelman syndrome results, if on chromosome 15 of the spermatozoan, the paternal gamete, Prader-Will syndrome results.
Normal Chromosomes
Fig 6. Diagram of a chromosome pair joined by a centromere defining the short arm (p) from the long arm (q) of each chromosome. Deletions may involve either the short or the long arm of a chromosome.
Short Arm Deletion
Fig 7. Deletion of the short arm of the red chromosome results in one abnormal gamete at the end of meiosis.
The examples cited above are meant to illustrate conditions that are the result of abnormal numbers of chromosomes or partial deletions of chromosomes. These and other chromosomal anomalies will discussed in greater detail in Genetics.