Maternal (pink) and paternal (blue) sets of chromosome doublets.
	Two pairs of homologous chromosome doublets.
	Maternal and paternal sets of single chromosomes.

A. Starfish embryo during the morula stage. It consists of a ball of actively dividing cells superficially resembling the multiple fruit of a mulberry (hence, the name morula). At this stage, each cell is unspecialized and can potentially develop into a separate organism. A human embryo is in the morula stage as it travels down the fallopian tube. At the time of implantation on the uterine wall (officially marking the onset of pregnancy), the embryo consists of a hollow sphere or blastocyst (blastula) consisting of approximately 100 cells roughly the size of a printed period. Multiple fruit of the black mulberry (Morus nigra).  The individual units are one-seeded drupelets rather than cells. B. Highly magnified view of a whitefish morula showing several stages of mitosis: 1 = prophase, 2 = metaphase, 3 = anaphase, 4 = telophase.

Note: The undifferentiated cells of human blastocysts are called embryonic stem cells. Blastocysts can be formed in vitro through test tube fertilizations. Undifferentiated stem cells are especially remarkable because they can give rise to different tissues and organs. Through complex gene interactions, these cells can literally develop into any number of cell types found in the human body. The controversy over the use of embryonic stem cells in research involves the question of what constitutes a human being and when does life officially begin. In a recent discussion by right wing conservatives on when life begins, the term oocyte was included. I'm not sure if they meant primary as well as secondary oocytes. If the cells of morulas and blastocysts are also considered human beings (or U.S. citizens as some religious conservatives propose), then so are diploid somatic cells in living humans, the nuclei of which can be placed in denucleated egg cells. Biologists in other countries must be laughing at this absolute nonsense. With the sophisticated techniques of modern biotechnology, the nucleus of any undifferentiated cell has the potential to grow into a clone if it is placed in a denucleated egg cell. The bottom line here is that the cells must be placed in a carefully controlled environment in order to grow into a human. The latter cells can be grown in vivo (within in living organism) or in vitro (in a vessel outside of a living organism). Stem cells cultured in vitro, provide an unprecedented opportunity for the study and understanding of human embryology and the generation of tissues and organs. This research could provide a remarkable potential for therapy and cures for many devastating human diseases, including various forms of diabetes, cancers of human tissues, organs and bone marrow, and diseases of the central nervous system (such as Parkinson's disease and Alzheimer's disease). Depending on how they are cultured, embryonic stem cells could potentially be grown into tissues and organs that could save the life of a child or an adult human. Some opponents of embryonic stem cell research consider human morulas and blastulas to be human beings and should not be harvested, not even to save the life of a loved one. Placental and amniotic tissue may provide an alternative and less controversial source of stem cells. A human morula composed of 16-32 cells.

When cells divide abnormally they often develop into tissue masses called tumors. Tumors can be produced throughout the body and they can be malignant or benign. Malignant tumors are often referred to as cancers. Some human cancers are caused by viruses, such as certain forms of the herpes virus that causes cervical cancer. Most cancers are neoplastic tumors caused by mutations in the DNA of cells. These mutations interfere with the cell's ability to regulate and limit cell division. Dormant cells enter the M-phase of the cell cycle and begin to divide out of control. Mutations that activate cancer-causing oncogenes or repress tumor-suppressor genes can eventually lead to tumors. Cells have mechanisms that repair mistakes in their DNA; however, mutations that affect repair enzymes may cause tumors to form. One of the best examples of the latter mechanism is a basal cell carcinoma.

Excessive exposure to UV radiation from the sun can cause mutations in undifferentiated basal keratinocytes (basal cells) of the epidermis. The specific mutation is called a thymine dimer within the DNA molecule. In normal DNA, the pyrimidine base thymine only pairs with the purine base adenine. When two adjacent thymine bases bond together this causes an abnormal configuration or "kink" in the DNA. Healthy cells can recognize and repair this mistake by excision repair enzymes. In some animals the mutation is repaired by DNA photolyase enzymes that clip out (cleave) the dimer. People with a genetic propensity for skin cancers may have insufficient repair enzymes due to mutations that repress the genes for these repair mechanisms. Although malignant basal cell carninomas generally do not metastasize, they may slowly invade deep layers of the skin and adjacent tissue and eventually be quite destructive. The following image shows the invasive growth of a basal cell carcinoma (technically a morpheaform bcc) that required the removal of about 1/3 of the author's nose. Unlike the nodule growth form of some basal cell carcinomas, the morpheaform bcc proliferates into deeper tissue with aggressive, tentacle-like branches. In addition to an increased number and density of dark-staining basal cells, the latter type of skin cancer produces a proliferation of fibroblasts within the dermis and an increased collagen deposition (sclerosis) that resembles a scar. The tumor appears as a whitish, waxy, sclerotic plaque that rarely ulcerates. It does not form noticeable scabs as in other skin cancers. On the surface of the author's ala (side of nose), this carcimoma resembled a small, concave scar; however, it had grown extensively into surrounding tissue. Although the sun is the vital energy source for all life on earth, it can also be a potent carcinogen.

On a positive note for sun exposure, synthesis of vitamin D, a vitamin essential to human biological function, begins with activation of a precursor molecule in the skin by UV rays. Enzymes in the liver and kidneys then modify the activated precursor and finally produce calcitrol, the most active form of vitamin D. During most of the year, a few hours per week of sun exposure to the face and arms is sufficient to meet the body's requirement for the activated calcitrol precursor. In general, fair-skinned people live in northern latitudes with lower light intensity compared with dark-skinned people of the tropical latitudes. Dark skinned people produce greater concentrations of melanin which protects their skin from harmful rays of the sun. Basal cell carcinomas are rare in Blacks and Asians, compared with fair-skinned Whites. It has been suggested that fair-skinned people of northern latitudes might have a slight advantage in synthesizing vitamin D, especially during months of the year in regions with reduced light intensity.

Dividing human cells can be photographed during prophase and metaphase, and all the 46 chromosome doublets can be arranged into 23 homologous pairs. A photographic or digital printed image called a karyotype is then made showing all the chromosomes neatly lined up in homologous pairs, from 1 through 23. Karotypes are very useful in determining chromosomal abnormalities, such as chromosomal deletions (missing genes) or incorrect numbers. For example, a person with Down's syndrome would have three number 21 chromosomes rather than two.

Karyotypes can also reveal the gender of a person. In addition to the 22 pairs of chromosomes (autosomes) in human somatic (body) cells, females have a 23rd pair consisting of two X chromosomes. The 23rd pair of males consists of an X and a Y chromosome. The smaller Y chromosome contains a region of DNA on the short arm of the Y responsible for masculinization of the fetus. In females one of the two X chromosomes appears as a condensed, dark-staining Barr body inside the nucleus of somatic cells, near the nuclear membrane. This structure is named after its discoverer, Murray Barr. Since Barr bodies only appear in nuclei with more than one X chromosome, they are not present in male cells. Up until the early 1990s, the lack of Barr bodies in nuclei from cheek epithelial cells of women could disqualify them for competition in the Olympic Games.

The calico cat is a sexual mosaic characterized by blotches of black, yellow and white fur. The genes (alleles) for black and yellow are linked to the same loci on two different X chromosomes. This is why calico cats are typically female because they have two X chromosomes, one with the black gene and one with the yellow gene. Since the black gene is dominant over yellow, how does the mosaic color pattern develop? The Barr body concept provides a nice cellular explanation for the patches of black and yellow fur. In regions with black fur, the black gene is active and the yellow gene is located on an inactive Barr body. In regions with yellow fur, the black gene is on the inactive Barr body while the yellow gene is on the active X chromosome. At an early stage in the cat's embryonic development, certain X chromosomes become inactive Barr bodies, apparantly at random. In the descendants of these cells, the same chromosomes are inactive, leaving the cells with only one functional allele for coat color. A rare calico male probably has an XXY karotype resulting in maleness, black fur and yellow fur. By the way, the white patches result from a gene interaction involving the "spotting gene," which blocks melanin synthesis entirely.

Gender verification in the Olympic Games now employs sophisticated DNA testing rather than counting Barr bodies within the nuclei of cells. The test is designed to detect the presence of the SRY gene (sex region Y chromosome), a region of DNA on the short arm of the Y chromosome responsible for masculinization of the fetus. Cells from the buccal mucosa (squamous epithelial cells), often called "cheek cells" in general biology classes, are obtained by gently scraping the inside of the mouth with a toothpick. The DNA in the nuclei of these cells is amplified using the PCR technique (polymerase chain reaction). If present, the SRY gene will show up as a unique banding pattern by electrophoresis on agar gels.

The following table shows different possible combinations of X and Y chromosomes in people. The gender of some of these chromosomal karyotypes and syndromes cannot be correctly identified using the Barr body technique:

The gender of the following chromosomal karyotypes and syndromes cannot be correctly identified using the Barr body technique. In addition the SRY test is not reliable in individuals with hormonal sex variations, such as androgen insensitivity and adrenogenital syndromes. For example, an XY person with androgen insensitivity has a Y chromosome with the SRY gene. Although they produce testosterone, they have a sex-linked gene on their X chromosome resulting in the lack of testosterone receptor proteins; therefore, they do not develop male characteristics. In other words, they produce androgens but do not respond to them.

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