CELL DIVISION & EPITHELIAL TISSUE

CELL DIVISION

One of the fundamental activities of all living organisms is reproduction. Reproduction is required for cellular replacement and is tissue repair, and it is necessary for the growth of an organism and essential for the continuity of life. In humans, this reproductive process is started by the duplication of DNA, followed by a partial duplication and then exact division of cellar contents. This kind of cell division is called mitosis. In observing the stages of mitosis, one is primarily concernest with the changes taking place in the nucleus. The changes in the cytoplasm are apparent only in the final phase (cytokinesis) upon cleavage and formation of the two daughter cells. The phases of cell division are:

•Interphase: Chromosomes are not visible but are dispersed as chromatin granules.

•Prophase & prometaphase: 46 chromosomes condense and appear. Centrioles appear, each with a set of starlike rays (asters). Connecting the two centrioles are strands of fibers-the spindle. The centrioles move to opposite poles of the cel, stretching the spindle between them. The nucleolus disappears and the nuclear membrane disintegrates.

•Metaphase: The 46 chromosomes line up along the middle of the cell in close relation to the spindle. Each chromosome is seen to split longitudinally, and each resultant half is called a chromatid, of which there are 92. The two halves of each chromosome attach to one another at the kinetochore, a disc-shaped protein structure that spindle fibers attach to.

•Anaphase: Each chromatid splits off from its sister chromatid and they head to opposite poles of the cell. Free chromatids now constitute new chromosomes, and there are 46 at each of the two poles of the cell. The cytoplasm begins to cleave in the midline.

•Telophase: A new nuclear membrane begins to appear around the chromosomes in each daughter cell. Chromosomes begin to disperse into chromatin. The two daughter cells mature into their final form, and the spindle fibers disappear. The two new daughter cells enter the interphase condition.

EPITHELIUM

Continuously growing or regenerating largely as a result of cell division, epithelium is one of the four basic types of tissue. It is a continuous, protective layer of densely packed cells, functioning as a barrier between the body and its environment. Epithelium is the free (unattached) surface of both closed and open cavities of the body, including all tubes, ducts, and vessels. In fact, there is an unbroken epithelial layer through your body.

It extends without interruption in this sequence:

skin → mouth → throat → esophagus → stomach → intestines → rectum → anal canal → anus → external skin

In a way, you're like a human doughnut! There is a continuity of epithelium within and to all parts of your body by virtue of the millions of interconnecting blood vessels. The kinds of epithelium found in all these areas will differ according to functional requirements.

Epithelium that lines the interior of the blood vessels and the heart is called endothelium, and the epithelium making up the cellular lining of serous membranes is called mesothelium.

Epithelial tissue is generally characterized by the following:

• Cells are situated next to one another.

• There is little intercellular material.

• A free surface.

• Cells supported by and bound to the underlying connective tissue by a basement membrane.

• Absence of blood vessels (avascularity).

• Nutrition received by diffusion.

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