33) Morphological and functional features of gametes - eggs and sperm

Size and shape

The egg cell (or ovum, or oocyte) is the largest human cell. She measures 0.15 to 0.2 mm and is just visible to the naked eye. She is also the roundest cell, she is almost perfectly round (Fig. 4). She therefore has the largest volume in relation to her surface. The cell consists of a large amount of cytoplasm (= cell fluid) in which the nucleus is dissolved (and therefore invisible) until just before conception. 

Sperm cells are the smallest human cells. They are no more than a nucleus with a small amount of cytoplasm, some mitochondria (the energy suppliers of the cell) and a long tail. They have hardly any content and are the straightest cells. Egg cell and sperm are each others opposite. Large versus small, round versus straight, cytoplasm versus nucleus. The differences are great, at the same time they belong together if we perceive the ovum as a sphere and the straight sperm as the corresponding radius.

Mobility The cytoplasm of a normal body-cell is in movement, the nucleus is not. The two gametes (= germ cells) show different features. The egg cell consists primarily of cytoplasm, she is internally mobile. The nucleus is outspread, the chromosomes are unwound (not folded up). The cell is internally active and mobile. The sperm cells have hardly any cytoplasm and are concentrated in their nuclear DNA. They have a crystalline structure. These cells are internally structured and rigid.

In contrast, the ovum is externally not active. After her release, she is passively moved by the fluid-flow in the oviduct (uterine tube), while the sperm cells are active, using their tails to swim against the stream of fluid in the oviduct. They are externally active and mobile.

The ovum is internally mobile and externally passive, this is a polarity. The sperm shows the opposite: internally passive and externally mobile. Egg cell and sperm have a polarity and are opposite to each other, we see a double polarity. Metabolism An egg cell is a metabolically active cell; substances are absorbed and released. E.g. nutrients are absorbed, substances that affect the uterus and substances that attract the sperm are released. An egg cell lives only 12 to 24 hours in her own environment and cannot be preserved. The egg cell can easily be destroyed. She is an active cell and open to the environment. 

Sperm cells do not absorb or release substances. There is no interaction with the environment. They live about 3 to 5 days in the womb and can be preserved and frozen at temperatures below 60 °C. They are not easy to destroy. They are closed off from the environment and metabolically passive.

The open and vulnerable state of the egg cell is polar to the closed and robust state of the sperm cells.  Number For a conception one ovum and millions of sperm are required. The one ovum is worth as much as all those millions of sperm. A man with less than 20-40 million sperm in an ejaculation is barren. Such great numbers are necessary because most sperm do not reach the ovum. Also, for a conception more than one spermatozoon is necessary. See the page Conception. 

The ovum is alone and the sperm are with millions. One sperm cell is nothing, one ovum determines everything. One is polar to millions. One comprises everything, it is all there is, whereas the millions of sperm cells are infinitive, have no importance on their own.

Location The egg cell develops in one of the two ovaries in the warm abdominal cavity, the sperm develop in the testicles just outside the body in a relatively cold environment. 

The ovum develops in warm- and sperm in relative cold conditions.

Development Egg cells are produced well before birth in a huge number of so called primordial egg cells (primordial oocytes). From the beginning on, there is a continuous process of dying, so that at birth 2 million (!) are left. That process of dying goes on after birth. At the onset of puberty there remain about 40,000 ova. Then every four weeks a number of them begin a process of maturation. Of these, only one (sometimes two or three) ovum matures, the rest dies. In total about 400 ova mature (13 per year for 30 years). At menopause, no primordial egg cells are left.  In men, a very different process is going on. The first sperm cells are formed only from puberty on, before that they are not produced. Then the production goes on and on and never stops, hundreds per second, millions each day. Sperm cells are constantly being newly formed. 

Egg cells are old cells that became mature. Primordial oocytes are in a process of dying. Sperm cells are newly formed and are young. The maturation process of ova is an expiring process, it stops. The formation of the sperm is a vital process, it never stops.

37 Cell markers. Types of selective markers. Selectable marker genes, also known as marker genes or selectable markers, are genesequences which are intentionally introduced into cells as developmental tools inrecombinant DNA technology. This technology is used to transfer genes for desired traits between organisms. Selectable marker genes are important for the production of novel or new organisms. They are used as a tool to identify those cells which have successfully incorporated genes for a desired trait during transformation. Since the percentage of cells that will incorporate genetic material during transformation is low, selectable markers are extremely important because they act as "genetic tags" to easily identify successfully modified cells. However, they have no actual function in the novel organism. The Science of Selectable Marker Genes. Selectable markers can be used in bacterial, plant, animal or other cells. The following is a summary of how selectable markers are incorporated into novel organisms:1.Gene Construct - includes the genetic material for the desired trait and may include selectable markers, genetic promoters (genes which may aid the expression of the trait) and vectors (the instrument- e.g. a virus or plasmid, which carries new DNA into a cell). 2. Agent - the lethal substance to which the selectable marker is tolerant/resistant (e.g., antibiotics, herbicides, etc.). 3.Transformed Cells - those cells that have incorporated the gene construct. Types of Selectable Marker Genes. There are four categories of selectable marker genes presently used as tools to develop novel products. The different types of selectable markers and their respective modes of action are described below. Antibiotic Resistant Marker Genes. Antibiotic resistant marker genes confer the trait of resistance to a specific antibiotic. For example, the neomycin phosphotransferase-II (NPT-II) gene, is a selectable marker gene that is the blueprint for resistance to the antibiotics neomycin and kanamycin. While they are useful in the early development of transgenic cells, antibiotic resistant marker genes are engineered to be expressed at minimal and often undetectable levels in the final product. Herbicide Tolerant Marker Genes. Herbicide tolerant marker genes confer the trait of tolerance to the application of a specific herbicide. For instance, the gene conferring tolerance to the herbicide glufosinate ammonium is often used as a selectable marker in plant biotechnology. Those cells which survive the exposure to the herbicide are selected and regenerated into whole organisms. Metabolic/Auxotrophic Marker Genes. Metabolic or auxotrophic marker genes enable transformed cells to synthesize an essential component, usually an amino acid, which the cells cannot otherwise produce. The surrounding medium is made to intentionally lack the essential component, which cells require to grow. Cells that have successfully incorporated the selectable marker and the rest of the gene construct will produce the essential components within the cells, and thereby survive. These cells are selected and regenerated into whole organisms. Screenable Marker Genes. Screenable markers, also known as assayable markers, are genes which encode for a protein that can be identified through various laboratory assessments. The presence of the protein confirms that transformation has taken place. Screenable markers are not commonly used because their use is very time consuming, expensive equipment and supplies are required to perform the assessments and the final tissue product has to be destroyed to determine if transformation has taken place. Some of these selectable marker genes are more commonly used than others because they are better understood and are more compatible with the tissues/cells under transformation. For example, herbicide tolerance is a selectable marker commonly used in the development of plants with novel traits because the mode of action of herbicide tolerance has been extensively studied and it is compatible with plant tissue and cells. Removing Selectable Marker Genes. Selective breeding and cellular and molecular biology techniques are used to remove the selectable marker genes from novel cells and organisms while retaining the genes of commercial interest. Of these techniques, the selective breeding of novel organisms to breed out the selectable marker is the most common.