Cells which make up the Human Body

For the functions of various cell structures, see this post.

In today’s post we will look at some of the different cells which make up the human body. They are:

• Epithelial cells
• Sperm cells
• Egg cells
• Nerve cells
• Muscle cells

Epithelial Cells

There are four major tissues in the human body: connective, muscle, nervous and epithelial. Of the four, epithelial is the most prolific.

Epithelial cells are bound together in sheets of tissue called epithelia are the safety shields of the body. They are a barrier between the inside and outside of your body and are often the first place attacked by viruses as they begin their invasion deeper into the body. Sheets of epithelia are held together through several types of interactions, including: tight junctions, adherens, desmosomes and gap junctions.

 Epithelial cells differ from other cells because they are:

• Avascular: capillaries do not reside in epithelial tissue.
• Sensory: Neuronal endings are present within epithelial tissue and they perceive a stimulus.
• Glide over each other: epithelial cells slough off and glide in order to replace dead cells. This function allows epithelial cells to maintain a closed barrier to the environment.
• Transitional: multi-layered epithelia are able to stretch.
• Tight barrier: this allows them to withstand mechanical stress.
• Different from endothelial cells: endothelial cells line the inside of body cavities.

Epithelial cells have many different functions that are not specific to every type of epithelial cell. They are listed here:

• Provide boundary and protection
• Sensory function
• Aid in transport of materials via active transport and other methods
• Absorption function
• Secretory and lubricant function
• Some have cilia which aids in movement.

The different types of epithelial cells are listed below.

Sperm Cells

A sperm cell is a gamete. Gametes are sex cells. Sperm cells are the smallest human cells. They consist of a nucleus, small amount of cytoplasm, some mitochondria and a long tail.

With regards to mobility, the cytoplasm of the sperm cells, is scanty as mentioned previously and concentrated in DNA. This prevents the usual movement of cytoplasm. However, the sperm cell on a whole is externally active and mobile. They use their tails to swim.

There is no interaction with the environment for sperm cells. They do not absorb or release substances. They are closed off from the environment and are metabolically passive. They can be preserved and frozen at temperatures below 60 degrees C and are not easy to destroy.

Sperm cells develop in the testicles, just outside the body in a relatively cold environment. The first sperm cells are formed at puberty, prior to that they are not produced. As of puberty, sperm cell production keeps going continuously; sperm cells are constantly being formed.

Egg Cells

The second human gamete is the egg. The egg, which is also referred to as the ovum or oocyte, is the largest cell in the human body. This cell has a large amount of cytoplasm in which the nucleus is dissolved until just before conception.

As a result of the large amount of cytoplasm on the cell, the egg is inwardly mobile. The nucleus is spread out and the chromosomes are unwound. The egg is inwardly active and outwardly passive.

The egg is metabolically active and open to the environment. An egg cell can only live for about 12 to 24 hours in its own environment. They can be easily destroyed.

The egg develops in one of two ovaries in the warm abdominal cavity. Egg cells are produced before birth in vast numbers. In this form they are referred to as primordial cells. Primordial cells, once they are formed, begin to die out. This begins prior to birth and continues after birth. At the onset of puberty, there are about 40,000 ova remaining. Once menstruation begins, every couple of weeks, some primordial cells mature into eggs while the rest die. This continues for about 30 years or more until menopause kicks in, at which point there should be no more primordial cells remaining.

Nerve Cells

The cells of the nervous system are called neurones and are specialised to carry messages through an electrochemical process from one part of your body to another.

Neurones are similar to other cells in the body because:

• they are surrounded by a cell membrane,
• they have a nucleus that contains genes,
• they contain cytoplasm, mitochondria and other organelles and,
• they carry out basic cellular processes such a protein synthesis and energy production.

With as many similarities that they have, there are differences as well:

• Neurones have specialised cell parts called dendrites and axons. Dendrites bring electrical signals to the cell body and axons take information away from the cell body.
• Neurones communicate with each other through an electrochemical process.
• Neurones contain specialised structures and chemicals.

Neurones can be divided into three types:

• Sensory neurones which pass information about stimuli such as light, heat or chemicals from both inside and outside your body to your central nervous system.
• Motor neurones, which pass instructions from your central nervous system to other parts of the body, eg. muscles or glands.
• Associated neurones, which connect your sensory and motor neurones.

Your neurones carry messages in the form of electrical signals called nerve impulses. To create a nerve impulse, your neurones have to be excited. Stimuli such as light, sound or pressure all excite your neurones, but in most cases, chemicals released by other neurones will trigger a nerve impulse.

Muscle Cells

There are three different types of muscles and so three different types of muscle cells:

• Skeletal: skeletal muscles are also termed striated as a result of their striped appearance. They are attached to your skeleton by strong, springy tendons or are directly connected to rough patches of bone. These muscles voluntary muscles; you control what they do. Skeletal muscles are also important for holding your bones in the correct position and preventing your joints from dislocating. Skeletal muscles generate heat as a by-product of activity. This heat is important for maintaining body temperature.
• Smooth muscle: This type of muscle is found in the walls of hollow organs, eg. stomach. They work automatically without your knowledge.
• Cardiac Muscle: Your heart is made of cardiac muscle and this is the only place in the body it exists. Unlike other muscles, cardiac muscles work automatically and constantly without ever pausing to rest.