8. Cell Type Development

In this section, we are going to talk mostly about animals since we are starting to zoom in on our particular story. Multicellular plants and fungi also adapted differentiated cell types in order to perform specific functions, but we’ll stick with relatable animals here.

First, some vocabulary! A group of similarly structured cells within an organism is called tissue. Groups of often different types of tissues serving an overall function make up an organ. Multiple organs that serve a specific purpose in the organism is called an organ system. Okay, let’s jump in!

Single celled organisms have a lipid bilayer in order to separate the inside and outside of the cell as well as protect the cell from possible invaders. Multicellular organisms have specialized cells whose only purpose is protection and separation of organs. These cells adapted to grow large and flat in shape. Flatter objects cover more surface area so it is an advantageous shape for this type of cell. They also specialized in manufacturing and secreting molecules that could be used by the organism in some way. We call these cells epithelial and they make up the skin, hair and membranes of multicellular organisms.Every organ is lined with epithelial cells in order to protect it and keep what is inside in and what is outside out. These cells secrete the chemicals that allow for more functions than just separation. The primary purpose of epithelial cells is protection and their secretions can solidify to form scales on some animals or acidic chemicals to kill microbes landing on the organism.

Microscopic image of hair. Strikingly similar to tree bark on this scale
Microscopic image of hair. Strikingly similar to tree bark on this scale

Secreted chemicals can aid in the breakdown of food, saliva and stomach acid are two examples. They can also maintain body temperature by secreting water to absorb heat, known as “sweating,” or form long towers of cells called hairs to trap warm air. Most of the horns, spikes and  antlers seen on animals are epithelial cells with dense material within each cell.

Single celled organisms needed to build external structures, like cilia or flagella, in order to move around. Multicellular organisms could assign the task of moving to specialized cells that contracted in a single direction to pull the cell in on itself.

Rope-like proteins were attached to smaller proteins with movable “heads.” When activated, the heads bend, pulling the rope with it. The rope would be pulled from opposite directions and the cell would shrink in one direction, growing in another direction to displace its volume.

Myosin heads pulling actin strands
Protein “heads” pulling on other protein strands to cause movement

You might be thinking, “A cell is really small, how can it shrinking to a slightly smaller size actually cause an effect?” This is where strength in numbers plays a part. If many many cells all pull the same direction and pull on each other, the movement will be compounded. These types of cells are called muscle cells and they eventually adapted into the shape of long strands within larger strands.

The muscle strand, with most of its volume in a single direction, was ideal for maximizing movement. The strand shape also allowed for many more strands to fit in a smaller area, thus allowing for greater pulling power in a small space. The most important thing to realize is that muscle cells never adapted a method of pushing, only pulling. In order to reverse the pulling effect, a group of muscles had to pull on the other side so the original muscles could relax. Take a moment and move around a bit. Notice which muscle strands are pulling with each movement as well as which strands pull in order to relax the first muscle.

Single celled organisms are small enough that signals can be transferred by transporting molecules within the cell. Multicellular organisms are too large to communicate this way. Instead, specialized cells have adapted to receive and transmit signals throughout the organism.

These cells adapted to have many branches on one end in order to maximize the chance that a signal is received. On the other end of the cell, a long stem allows the signal to travel within the cell until it reaches the end where it will pop a bubble of molecules to transfer the signal to the next cell. These communicative cells are called nervous cells, the receivers are called dendrites and the stem is called an axon.Nervous cells are able to send and receive signals from all types of cells in the organism and they can even send and receive signals from other nervous cells. Organisms developed regions where many, many nervous cells could connect with each other; we call this a brain. Because the nervous cells adapt to serve specific communicative functions, it would be detrimental to the organism if they reproduced on their own; this is why people say brain cells don’t grow back. Some actually do, but it’s probably not something for you to test in yourself.

Many, many other types of cells diversified in order to fulfill a need of the organism. The physical shape and structure of the cell give good indication at what their purpose is.

Lymph tissue
Lymph tissue

Lymph cells have a web-like structure and serve the purpose of collecting material that does not help the organism, they often swell when full of invaders and dead protector cells.

Blood cells
Blood cells

Blood cells are mainly used for transportation of resources throughout the body, but don’t use the resources for themselves.

Cross section of bone tissue
Bone tissue

Bone cells are filled with dense atoms, usually calcium, and serve the purpose of supporting the organism and protecting nervous cells. There are about 200 types of differentiated cells in humans, each with a specialized purpose adapted over millions of years!

Never has the phrase “divide and conquer” sounded so effective as when looking at the incredible functions to sustain multicellular life. Don’t ever forget that even when you think you’re doing nothing at all, a vast community of specialized cells is working within you at all times.

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