Cell Specialisation. The human body is comprised upward of 60,000,000,000 cells, all individually specialised (Dictionary, 2005). This essay will explore how their ultrastructure differentiates them, focusing on a few cells in particular.

Human Physiology Level 3 Elizabeth Chavez 18/10/2011

Cell Specialisation

After looking at a slice of cork under the microscope, Robert Hooke, in 1665, described the shapes he saw as ‘cells’ thinking of monks’ chambers (Iyer, 2009). The ‘cell’ is the most basic autonomous component of life. The human body is comprised upward of 60,000,000,000 cells, all individually specialised (Dictionary, 2005). This essay will explore how their ultrastructure differentiates them, focusing on a few cells in particular.

Animal cells are eukaryotic: they contain membrane-bound nuclei (Miller & White, 2004). This is one of many organelles; it coordinates cellular activity and processes. The nuclear membrane protects the genetic make-up of the cell: DNA (deoxyribonucleic acid) and is perforated by small nuclear pores which allow certain substances to pass in or out of the nucleus.

Information (in the form of molecules exiting the nucleus), then travels to the endoplasmic reticulum (ER); a structure continuous with the outer layer of the nuclear membrane. There are two subcategories of ER: rough and smooth. The difference is notable when observed under an electron microscope, in that rough ER appears ‘speckled’ due to the ribosomes covering its surface (Carter, 2004). Both are responsible for the synthesis of proteins and lipids, which are eventually excreted in the form of vesicles that travel freely through the cytoplasm; cytoplasm refers to cytosol and other organelles.

Vesicles then fuse with the Golgi body. Here, proteins are modified, for instance by adding a carbohydrate, during passage through then released either as secretory products or lysosomes. Lysosomes differ as they contain digestive enzymes to break down macromolecules, to smaller nutrients the cell can utilise, and expired organelles, such as mitochondria.

Mitochondria generate the energy required for metabolism in the form of ATP (adenosine triphosphate). They have a bilayer membrane; the outer being smooth and protective whilst the inner layer forms highly folded projections called cristae which increase surface area, facilitating maximum aerobic respiration. Glucose is combined with oxygen to form ATP.

Any waste products of the cell are excreted through the cell membrane (the thin protective lamina encapsulating the cell). It is selectively permeable meaning some substances may pass through while others are prohibited.

Animal cell:

One highly specialised cell is the erythrocyte; a biconcave disc about 7μm in diameter (Britanica, 2011). Its primary function is to transport oxygen from the pulmonary vasculature to tissues throughout the body.

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Animal cell:

It accomplishes this utilising haemoglobin; an iron-containing protein accounting for 30% of the cell’s weight (Slomianka, 2009). To procure maximum oxygen storage capacity the erythrocyte has evolved without organelles, as they would be an inefficient use of space; nuclei are replaced by haemoglobin during maturation in the bone marrow. This means the cells cannot replicate; the lifespan of a single erythrocyte is about 120 days (Waugh & Grant, 2010).

Its shape provides increased surface area through which diffusion can occur, the membrane is also highly permeable, virtually only impermeable to haemoglobin making it very efficient at transporting oxygen, waste products (carbon dioxide and urea), and other substances. Transmembrane proteins form a stabilizing cytoskeleton enabling the erythrocyte to retain its shape after passing though capillaries smaller than the cell itself with diameters of 4μm (Standring, 2008).

Erythrocyte:

7 μm

Adipocytes are difficult to identify in their early stages due to their apparent lack of specialisation, however upon maturation as lipids accumulate in the lipid vacuole(s) their identity becomes clear. There are two classifications of adipocyte: unilocular and multilocular. Multilocular brown fat tissue is found in neonates, later being replaced by unilocular white fat tissue (Albright & Stern, 1998).

The unilocular adipocyte has a limited number of organelles since the majority of space inside the cell is occupied by a single lipid globule. Its primary function being energy storage as lipids, however it also regulates satiety by the production and release of the hormone leptin. The multilocular adipocyte is more complex. It contains many smaller lipid vacuoles for lipid storage, but also a high abundance of specialised mitochondria for thermogenesis. These mitochondria contain larger and more numerous cristae increasing their metabolic surface area, and are modified to release energy not in the form of ATP but directly as heat (Standring, 2008). Neonates are less active and so less efficient at maintaining body heat.

White Adipocyte: Brown Adipocyte:

In contrast, neurones are recognisable by their microscopic projections: the dendrites and axon (developed for the transmission of electrical signals). The high level of specialisation prevents replication in most neurones, one exception being in the hippocampus (Holladay, 2004).

Within the soma an abundance of ER and ribosomes can be found pertaining to the high level of protein synthesis involved in neurotransmitter production (Standring, 2008). It is also teeming with mitochondria to capacitate the enormous energy requirements of the neurone.

Increased ribosomes found at the synaptic sites of dendrites moderate local protein synthesis. Both the dendrites and axon acquire their tensile strength from a cytoskeleton comprised of neurofilaments. The axon is ensheathed with insulating Schwann cells forming the myelin sheath, that combined with the increased density of sodium channels found at the Nodes of Ranvier increase the velocity of impulses. Electrical impulses are regulated by a fine balance between ionic charges across the cell membrane (Britannica, 2011).

Neurones are not physically connected at synapse points, neurotransmitters such as acetylcholine (produced at the end of the axon) aid the signals to ‘jump’ from one neurone to another (Britannica, n.d.).

Neurone:

Spermatozoa: an iconic cell easily identifiable by its oval shaped head and elongated flagellum. The mature spermatozoa exhibit hyperactivated motility facilitated by the flagellum (Bowen, 2000). It derives the incredible amount of energy needed to reach the oocyte from densely packed helical mitochondria found in the proximal end of the tail.

The head of the spermatozoa contains the acrosome and nucleus, both invaluable to fertilisation. The acrosome, essentially an oversized modified lysosome, contains zona-digesting enzymes. The acrosomal contents are released as the acrosomal membrane binds with the plasma membrane of the oocyte, disintegrating it forcing a path of entry for the successful spermatozoon.

The nucleus contains 23 chromosomes (half the normal amount); composed of DNA and proteins (Bailey, 2011). When combined with the 23 chromosomes of the oocyte during fertilisation the full amount of 46 is obtained producing the zygote.

From this evidence it can be concluded that even the smallest components of a cell can have a profound impact on its function and capabilities.

Spermatozoa:

Reference List

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Holladay, A. (2004). Wonder Quest - Few brain cells reproduce; 17-year cicadas once escaped ice. Retrieved October 11, 2011, from USA Today: http://www.usatoday.com/tech/columnist/aprilholladay/2004-08-13-wonderquest2_x.htm

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Slomianka, L. (2009). School of Anatomy and Human Biology - Blue Histology - Blood. Retrieved October 3, 2011, from The University of Western Australia: http://www.labanhb.uwa.edu.au/mb140/corepages/blood/blood.htm#Erythroctes

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