Note: I am going to upload this blog in 2 parts as I think it is too long and your time is valuable to me too. Enjoy!
Hey! Ever given any thought to what's holding you together? What is found between your organs and skin? Are the organs just floating there? Or what makes up organs? I am going to answer every one of these brain-tickling questions with intriguing answers. As you must have read in the title of the blog, the topic we are going to explore today is TISSUES! The Foundation of Human Anatomy. Let's do this!
What are Tissues?
Cells which are the building blocks of every living organism, are divided into different types and shapes. Each cell's function is based upon its shape and structure. Different types of cells combine to make up different types of tissues. So, a group of cells combined together gives us tissues! Tissues are the building blocks of every organ in the human body. Their function depends on their shape and structure. Different parts of the body are made up of different tissues. Organs are made up of more than one tissue.
What are Stem Cells?
You can think of stem cells are cells which are still growing and dividing rapidly to eventually combine and form tissues, because that is what every cell does. When a cell differentiates, it means that it dedicates itself to one specific function, for example- muscle, neuron etc. But stem cells unlike other cells, are the undifferentiated cells, which have the ability to choose what type of cell it will become, ex-muscle cell, neuron etc.
History of Tissues
People couldn't really see tissues in the old times because there was no microscope, but the great scientist, who is also considered as the Father of Microbiology, Anton Van Leeuwenhoek created a microscope which was powerful enough to observe tissues.
He also kind of invented dyes and stains which are applied to tissues before looking under them with a microscope to enhance contrast. The study of tissues was launched in 1850's and was called Histology.
Different Types of Tissues
There are 4 primary types of tissues are as follows:
1. Muscular Tissue
2. Nervous Tissue
3. Epithelial Tissue
4. Connective Tissue
All of these tissue types have subtypes which we are going to discuss further.
Nervous Tissue
While it is one of the 4 main types of tissue, I won’t be diving in too deep about it since we covered it in The Nervous System Blog, so feel free to check that out if you need a refresher or a deep-dive. Let's touch on some major concepts about nervous tissue.
Nervous tissue helps in communication all throughout the body. It contains cells called Neurons, which can generate electricity. A long chain of neurons makes up a Nerve. All functions from moving our hand, to lifting our leg are done with the help of neurons. The major parts of a neuron are as follows:
Dendrites- These branch-like structures receive electrochemical signals from other neurons and send them to the cyton (cell body). Telodendria are smaller branches present at the end of dendrites.
Cyton (Cell Body)- The cell body of a neuron contains mitochondria, nucleus, DNA and all the essential parts for a cell to survive.
Axon- The axon is like the transmission cable and can be covered by Glial Cells, which speed up the process of transmission.
If you want more detail about neurons, check out my Nervous System Blog.
Muscular Tissue
This tissue, as you may have guessed by its name, makes up the muscles of the body. These tissues can contract and move. These tissues are classified into 3 types, based on their shape, structure and function.
1. Skeletal (Voluntary) Muscular Tissue-
This tissue is present in skeletal muscles, which we have voluntary control over. It attaches to all the bones in the body. The cells have striations, meaning those pin stripe running over them. The cells of this tissue are Multinucleated. The cells run parallel to each other.
2. Smooth (Involuntary) Muscular Tissue-
This tissue is present in places where contraction and movement are needed, but not voluntarily. Ex- They are present in the walls of the digestive system, so it can stimulate peristalsis. This muscle tissue, as the name suggests is smooth and has no striations. It is Uninucleated meaning its cells have a single nucleus. The cells of this tissue are short and tapered and are tightly bind.
3. Cardiac Muscular Tissue-
This tissue is present only in the walls of the heart, particularly the Myocardium (middle layer out of the 3 layers of the heart). This cells has obvious striations, the cells are uninucleate and they has a branched shape. This tissue is also involuntary, meaning its not under our control, as it would be really distracting to remind our heart to beat every second. On the ends of its branch-like cells, are Intercalated Disks which are like glues attaching cells together. The disks also contain pores, which help electric signals run through them.
Epithelial Tissue
This tissue lines and protects all of the organs in our body. It also forms the Epidermis of the skin, when it combines with Connective Tissue. It seperates organs and creates a sort of order inside our body. One important thing to remember is that Epithelial tissue is Avascular (it has no blood supply). The tissues are very close together, so the nerve fibers and blood vessels are not able to penetrate it. Epithelial Tissue has 2 main types:
1. Proper Epithelium- It is made up of specialized cells and covers the outer and inner linings of our body.
2. Glandular Epithelium- This epithelium is made up of specialized cells and forms the glands of the body. It helps in secreting hormones, sweat and other substances.
Epithelial Tissue is selectively permeable depending upon its cell's shape and structure. The shape of each epithelial cell correlates with its function. Epithelial Tissue is divided based upon Shape and Layering. Let’s talk about shape first.
Based upon shape, there are 3 types of Epithelial tissues:
1. Squamous- These are flat cells with a flat nucleus. Present in places where processes like diffusion and active transport are needed. Ex- Buccal (Oral) Cavity, Skin etc.
2. Cuboidal- These cells are cube shaped, about as wide as they are tall and have a circular nucleus. Present in places where secretion and absorption is needed, as they can do both. Ex- Kidney tubules, Ovary etc.
3. Columnar- These cells are arranged in columns and they are long. They have an elongated nucleus, as if its been stretched. They, like cuboidal cells can also secrete and absorb substances, and their primary function is to cushion the underlying tissues. Ex- Lining of the stomach etc.
Squamous cells are also present in the alveoli in the lungs and capillary walls, so it is easy for them to diffuse substances such as oxygen, carbon dioxide etc.
Cuboidal cells need to be cuboidal because they also secrete substances, so to occupy the internal machinery which takes to make up all of the secretions, they need to be bigger.
Columnar cells cushion the underlying tissues and again, secrete substances, so they need to be big though.
The squamous cells regenerate pretty quickly, as they are cheap to make but the cuboidal or columnar cells are expensive to make and regenerate quite slowly. That is why squamous cells are present on the outer layers of the skin, and then cuboidal and columnar cells are usually present deep inside. If you lose some squamous epithelial layers, no big deal. But if you lose cuboidal or columnar epithelial layers, you are going to need emergent medical care.
Now, based on layering. There are 3 types of Epithelial tissues:
1. Stratified- In this type, cells are arranged in layers, stacked on top of each other. Ex- Stratified Squamous Epithelium is present in the buccal cavity.
2. Pseudostratified- In this type, different shapes of cells are arranged together in columns, and look like they are stratified but actually they are not. Ex- Pseudostratified columnar epithelium is found in parts of the male reproductive system.
3. Simple- This is just a single layer of cells. Ex- Simple squamous epithelium is found in the alveoli of our lungs.
When we name an epithelium, we cite both its name and its layering. You can think of the first name as the layer and the last name as the shape of the cells. Ex- Stratified Cuboidal Epithelium is found in the ducts of salivary glands.
Glandular Epithelium
This type of epithelium is specialized to form your exocrine and endocrine glands. It specializes in secretion of substances such as hormones, mucus etc. Ex- Stratified Cuboidal Epithelium forms the salivary glands in the body.
One important thing to note is that simple columnar epithelium usually contains cilia which are little hairs present on its apical (upper) side which help in secretions. Ciliated epithelium is normally not classified as a primary type but rather a variation. Ex-Simple ciliated cuboidal epithelium is present in the bronchioles of the lungs which are branches of the Secondary Bronchus. Pseudostratified Ciliated Columnar Epithelium is found in the fallopian tubes and also facilitates in the movement of the egg down the uterus.
Epithelial Anatomy
The Epithelial tissue's Apical or upper side is exposed to whatever the tissue is lining and it might contain cilia based upon the type of tissue. The Basal side is connected to a tightly stretched membrane known as the Basement Membrane, which is a thin layer of mostly collagen fibers connected to the underlying Connective Tissue.
We will cover the next part, which is connective tissues in part 2 of the blog. Until then, you can check out my other blogs if you haven't already.
Regards,
Deeparsh Bhanot
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