H.A. 101: Membranes

Membranes are a vital component in protecting the body from its surrounding environment. In the human body, membranes are composed of Epithelial and Connective tissue. Each one of these membranes will consist of a sheet of epithelial cells and an underlying connective tissue layer. Membranes can be divided into four main categories:

  1. Mucous Membranes
  2. Serous Membranes
  3. Cutaneous Membranes
  4. Synovial Membranes
Mucous Membranes

Mucous membranes can be found in the digestive, urinary, respiratory, and reproductive tracts due to their ability to secrete a barrier of protection. This barrier of protection is responsible for resisting pathogen entry into the body and deeper tissues. The layer of secretion is created by mucous glands and it also helps maintain the moisture of epithelial cell surfaces.

In mucous membranes, the tissue that connects the epithelium to the underlying tissue is called the lamina propria. This is an areolar tissue that allows the epithelium to move free when compared to the deeper tissue.

Serous Membranes

Serous membranes are located in the lining of body cavities. Within these membranes, there are two major layers, the visceral and parietal layers. The visceral layer is responsible for covering the organ of the cavity. The parietal layer can be found lining the walls of the cavity the organ is located in. It is also important to know that within serous membranes there is a fluid called transudate. This serous fluid is responsible for reducing friction between layers. There are three types of serous membranes:

  1. Pleura
  2. Peritoneum
  3. Pericardium

Pleura serous membranes are responsible for lining the lungs. The parietal layer of this membrane is attached to the chest wall while the visceral layer is attached to the lungs. The fluid between these two layers is called the pleural fluid.

Peritoneum serous membranes can be found lining the peritoneal cavity (abdominal cavity). In this membrane, the visceral layer attaches itself to the organs of the peritoneal cavity such as the intestines, and helps hold it in place. This membrane helps reduce friction and maintain the organization of the organs during bodily processes and mechanical movements.

The pericardium serous membranes are found lining the heart. The main function of this membrane is to hold the heart in place and help it function properly.

Cutaneous Membranes

The cutaneous membrane is one of the largest membranes in your body and the most recognizable. This layer of epithelial cells and connective tissue in your skin consists of keratinized stratified squamous epithelium. The keratinization of the epithelium results in a waterproof and thick characteristic. The deeper cells are connected to areolar tissue and dense irregular tissue that helps maintain and secure the epithelial cells.

Synovial Membranes

Synovial membranes can be found lining the joint cavities of bones. These membranes are different from the other ones because it has no basal lamina or reticular lamina, the cells are created from fibroblasts and macrophages, and there are gaps between the cells.

Within this membrane, there is a fluid produced called synovial fluid. This functions to reduce friction between the joints to reduce damage to the bones and cartilage.

H.A. 101: Connective Tissue

Classification Of Connective Tissue

Connective tissue is one of the major tissue groups and is the most abundant tissue in the entire body. Within the category of connective tissue, there are three main components. Connective tissue has specialized cells, extracellular protein fibers, and a matrix.

The matrix of Connective tissue is the term used to describe the extracellular component of all connective tissue. This matrix can be made of fibrous proteins and a ground substance. The ground substance is the fluid portion of the matrix. In bone the matrix is calcified. This is one of the reasons it has a solid characteristic.

So, what is the function of Connective tissue? Connective tissue has many functions ranging from defense against pathogens and microorganisms to transporting fluid and materials. Other functions of Connective tissue are creating the structural framework of the body, protecting, supporting, and connecting tissues, and storing energy. Based on the location and structure of the Connective tissue it will have a different function.

To classify Connective tissue to understand its functions and characteristics we can categorize them into three groups. These three groups are:

  1. Connective Tissue Proper
  2. Fluid Connective Tissue
  3. Supporting Connective Tissue

Connective tissue proper is defined as tissues that have a matrix of fibers. These include loose and dense fibers. Loose connective tissue proper creates a loose framework and leads to tissue-like Areolar tissue, Adipose tissue, and Reticular tissue. Dense connective tissue proper is a connective tissue that has tightly packed fibers. These tissues include dense regular, dense irregular, and elastic tissues.

Fluid connective tissue is a connective tissue that has a fluid matrix. Many people think of tissue being solid, but this is highly untrue. This category of Connective tissue includes Blood and Lymph.

Supporting Connective tissue is similar to connective tissue proper but, it has a matrix of fibers and in some tissues, it has insoluble calcium salts. Supporting connective tissue that does not have insoluble calcium salts can be classified as Cartilage while those that do can be called Bone.

Connective Tissue Proper Cells and Fibers

Connective tissue proper can be divided into two classes of cells.

  1. Fixed Cells
  2. Wandering Cells

Fixed cells are cells that remain within connective tissue proper. These cells include:

  • Fibroblasts- contribute to the production of connective tissue
  • Fibrocytes- Maintain the fibers of connective tissue and matrix
  • Fixed macrophages- destroy pathogens and damaged cells using phagocytosis
  • Adipocytes- Stores fats within connective tissue
  • Mesenchymal cells- Stem cells within connective tissue that differentiates into other cell types
  • Melanocytes- Found in the skin and eyes and produces as well as contains pigment called melanin.

Wandering cells are cells that do not remain within connective tissue proper. This means that they are not fixed to a specific tissue and can travel between tissues. These include:

  • Free macrophages- Mobile defense cells, derived from monocytes, that use phagocytosis to engulf damaged cells and pathogens.
  • Mast cells- Stimulate inflammation
  • Lymphocytes- White blood cells that are a vital part in the immune response
  • Neutrophils- Another types of white blood cell that mobilize during infection

Connective tissue proper has three different fibers that are associated with it. These are:

  1. Collagen fibers
  2. Reticular fibers
  3. Elastic fibers

Collagen fibers can be defined as flexible yet strong fibers. Although it is highly flexible it also has great tensile strength. Tensile strength is the ability to resist breaking under tension. Its flexibility does not mean it’s stretchable, however. For this reason, Collagen fibers can be found in tissues like tendons and ligaments. They function to hold tissue together even during normal body movements.

Reticular fibers are a network of randomly branched collagenous fibrils. The goal of random branching is to increase surface area. This helps achieve its function of supporting organs and many other tissues.

Elastic fibers contain a specific type of protein called elastin. Elastin is an extracellular protein that specializes in creating recoil and extensibility in many tissues. Things such as the heart, large arteries, vessels, and the lungs have this protein due to their need to expand and recoil. Elastic fiber’s primary needs are elasticity and resilience.

Connective Tissue Proper Loose Connective Tissues

Within loose Connective tissues there are two categories

  1. Loose fibers (Areolar Tissue, Adipose Tissue, Reticular Tissue)
  2. Dense fibers (Dense Regular, Dense Irregular, Elastic)

Loose Connective Tissue Proper is defined as having a more complex extracellular matrix that consists of mainly ground substance and two main types of protein fibers (Elastic and Reticular).

Areolar Tissue is located deep within the dermis of the skin. It can be found around vessels, nerves, and between muscles. The function of this tissue is to connect the skin to the muscle and provide some support and independent movement. It also helps cushion organs and acts as a defense mechanism for pathogens. Its matrix consists mainly of fibers (elastic and collagen)

Adipose tissue is located in your buttocks, hypodermis, and around many of your organs. The goal of this very unique type of tissue is to provide insulation and cushion for the body. This tissue can be categorized into two different groups based on its color. Brown adipose tissue is usually activated when you become cold. It burns the fat and creates heat for your body to stay warm. White adipose tissue is the predominant type of fat found in the body and it functions to insulate and protect the body.

Reticular tissue can be found in the kidneys, appendix, bone marrow, liver, spleen, tonsils, and many more. The matrix of this tissue consists of reticular fibers and its function is to support organs and the framework of the body.

Dense connective tissue can be defined as fibers that are densely packed. This usually means that there is less ground substance than fibers or that the fibers are very close in proximity. There are two types of dense connective tissue. The first is Dense Regular and the second is Dense Irregular.

Dense Regular connective tissue can be found in the ligaments, tendons, aponeuroses, and elastic tissue. The function of this tissue is to provide attachment to muscles and other tissues. The tendon functions to connect muscle to bone. The aponeuroses function to connect muscle to muscle or connect an entire muscle by covering it. Ligaments connect bone to bone and finally, elastic tissue stabilizes the vertebrae of your spine.

Dense Irregular connective tissue is found near nerves and muscle sheaths. Here they provide the strength these areas need and form a fibrous capsule around the organs. This capsule helps prohibit overexpansion of the organs like in the bladder.

Fluid Connective Tissues

Fluid connective tissue can be categorized as Blood and Lymph. This tissue has a watery/syrupy matrix that suspends its cells within.

Blood can be found within the circulatory system within blood vessels. Blood contains a mixture of many different cells that are vital to keeping the body alive. Erythrocytes (Red Blood Cells) are responsible for transporting oxygen as well as carbon dioxide around the body and through blood vessels. Leukocytes (White Blood Cells) are a category of cells that help fight infection or disease. They consist of Monocytes, Lymphocytes, Eosinophils, Neutrophils, and Basophils. Thrombocytes or Platelets are membrane-enclosed packets of cytoplasm. They function in the process of blood clotting and coagulation to fix broken blood vessels. Finally, the liquid matrix all of these cells float in is called Plasma. It makes up 55% of blood and contains water, salts, proteins, antibodies, and more.

Lymph is located within the Lymphatic system where it travels through vessels into lymph nodes, the spleen, and more. The function of lymph is to house cells that can detect and fight infection and disease. It is a critical part of the immune response. Within lymph, there are cells called Lymphocytes. These cells develop into T cells and B cells.

Supporting Connective Tissue

Supporting connective tissues functions to support the body by creating a strong framework of tissue. Within this tissue, there is cartilage and bone.

Cartilage is a white flexible supportive connective tissue found in the trachea, ear, respiratory tract, and articulating surfaces of joints. The cells of this tissue are called chondrocytes. The matrix of this tissue is considered a gel and is made of chondroitin sulfate. It also has a fibrous perichondrium that consists of a cellular layer and a fibrous layer. This tissue is avascular and the cells reside in an area called the lacunae.

Cartilage is more common in the developing human because it is involved in appositional and interstitial growth. Appositional growth helps increase the cellular dimensions of the body while interstitial growth creates additional matrix material.

There are many different types of cartilage tissue as well. Since it is so important in the structure of the body, this tissue has adapted to meet the needs of different locations.

Hyaline cartilage is specialized for flexible support and reducing the friction movement may cause in the body. It can be found in areas such as the ribs, sternum, connections within the joints of the elbow and knee, and trachea. This cartilage provides a flexible and stiff support on bony surfaces.

Elastic cartilage is usually found in the auricle of the ear, epiglottis, and cuneiform of the larynx. It functions for flexible support while withstanding distortion and recoil.

Fibrous cartilage can be found in the pads of the knee joints, between the spinal vertebrae, and the pubic symphysis. Here, it functions to resist compression and absorb the shock that the body might experience. Its goal is to prevent bone-to-bone contact that might result in damage.

Bone is a solid matrix made of calcium phosphate. This tissue makes up the skeletal system and its function is to support the body and provide strength. It has a sold matrix called the lamellae. Everybody is covered with a layer of protection called the periosteum. The cells of the bone are called osteocytes. They reside in a circular region within the bone called the Osteon. This area also contains the central canal, lacunae, canaliculi, and lamellae.

There are two types of bone, compact bone, and Spongy bone. Compact bones have trapped blood vessels within the matrix while Spongy bones do not.

H.A. 101: Epithelial Cell Classification

Classification of Layers

Classification of Epithelial cells is critical due to their vital and diverse role in the human body. Found from your head to your toes, these cells come in numerous shapes, sizes, and populations. Thus, Epithelial tissue has been divided into two major types of layering patterns and three main shapes. The two layering patterns are:

  1. Simple Epithelium
  2. Stratified Epithelium

Simple Epithelium is defined as having a single layer of cells and the nuclei of these cells being present on the same level within each cell. Simple Epithelium is found in areas that are usually protected from the external environment and areas of diffusion. Places such as internal compartments of the body are a good example of this

Stratified Epithelium is defined as having two or more layers or almost anything that is not simple epithelium. With more cells come more protection so these areas of epithelium are often found in places of chemical and mechanical stress.

Classification of Shape

The next way to classify Epithelium is by its shape. There are three major shapes that they can be classified into. These are:

  1. Squamous Epithelium- Flat in appearance
  2. Cuboidal Epithelium- Cube-shaped
  3. Columnar Epithelium- Cells are longer than they are wide (like Columns)

Simple Squamous Epithelium is considered extremely delicate and is usually located in the mesothelium (lining of cavities) and endothelium (lining of heart and blood vessels). Their main function is often to reduce friction as well as absorb or secrete materials such as nutrients and water.

Stratified Squamous Epithelium is often found in places of high mechanical, chemical, and even microbial stress (Pathogens). Some areas that include Stratified Squamous Epithelium are the surface of the skin, cheeks of the mouth, esophagus, vagina, anus, and more. You may be asking how places like the cheeks of your mouth and the skin are the same types of cells. Though they both are Stratified Squamous Epithelium, they have a significant difference. The difference is that one group of cells may produce keratin while others do not. Cells that produce keratin are called keratinized epithelium and ones that do not are called nonkeratinized epithelium.

Simple Cuboidal Epithelium functions to secrete and absorb materials. They have very limited protection due to the amount of them. They are located in the thyroid gland, kidney tubules, and ducts. These epithelia are vital for the function of the tubes of the kidney in regards to maintaining homeostasis.

Stratified Cuboidal Epithelium is extremely rare. When found they often function in secretion and absorption. The places this type of cell is found are in the sweat glands. As well as absorption and secretion, these cells provide more protection than their simple counterpart.

Simple Columnar Epithelium functions to protect, absorb, and secrete. They are often found in the linings of the intestinal tract (intestines, gallbladder, uterine tubes, and collecting ducts)

Stratified Columnar Epithelium functions to protect areas of the body from mechanical, chemical, and microbial stress. Located in the Pharynx, epiglottis, anus, salivary glands, mammary glands, and urethra these cells tend to deform after the second layer. The first layer of Columnar Epithelium is usually perfect in shape. As you move deeper into the tissue the cells become deformed in shape. This does not mean that these cells are not columnar, however.

Pseudostratified and Transitional Epithelium

With most things in Biology and science, there are exceptions and outliers to the normal circumstances. Epithelial tissue has multiple outliers. The two major ones are Transitional Epithelium and Pseudostratified Epithelium.

Transitional Epithelium is located in areas that permit expansion and recoil after stretching. This includes places like the bladder and ureters. This type of epithelium consists of many different layers as well as an oddly shaped cuboidal cell type. When this layer of cell expands the cells layer will become stretched out and look as if the cells got wider and shortened in height.

Pseudostratified Epithelium is mainly ciliated columnar epithelium. The nuclei of these cells are located at different levels than each other, unlike normal epithelial cells. These cells maintain the function of protection and secretion. They are located in the lining of nasal cavities, the male reproductive system, and the trachea.

Glandular Epithelia

One of the many functions of epithelia is the ability to create secretions. Cells that can produce secretions are also known as Glandular Epithelia and contain gland cells. Glands can be classified using three main criteria:

  1. Type of Secretion
    • Exocrine glands (Serous, Mucous, Mixed)
    • Endocrine glands (Hormones)
  2. Structure of the gland
    • Number of cells and their shape
    • Strucutre of the duct
  3. Mode of Secretion
    • Eccrine secretion
    • Apocrine secretion
    • Holocrine secretion
Types of Secretion

Within the types of secretions, there are two main categories. The first is the Exocrine glands. Exocrine glands are glands that create secretions that pass through ducts and to the epithelial surface. Exocrine glands can further be categorized into three types of secretions/products made by the gland. These are:

  1. Serous Glands
  2. Mucous Glands
  3. Mixed Exocrine glands

Serous glands produce a watery fluid that contains enzymes. This fluid is also called serous fluid and contains enzymes like lysozymes and alpha-amylase. These glands can usually be found in the mouth to secrete fluid and help break down nutrients.

Mucous glands produce a special type of glycoprotein called mucin (mucus). The goal of mucins is to create a gel-like formation that can lead to numerous effects. Things such as lubricating the cell and forming chemical barriers are the main goals.

Mixed Exocrine Glands secrete both mucins and serous fluid.

The next type of secretion is Endocrine secretion. These glands’ secretions enter the lymphatic fluid or blood. Secretions that enter bodily fluids are typically known as hormones. These glands can be found all over your body and function to help maintain homeostasis and many other vital functions.

Structure Of The Gland

The Structure of a Gland can come in almost any shape, form, and the amount you can think of. For this reason, the structure of a gland is classified based on:

  1. The number of cells and their shape
  2. The structure of the ducts

If a gland is composed of a single cell, this gland is known as a Unicellular Gland. These cells typically secrete mucins and based on their location they are called one of two things. If this cell is found in the trachea it is known as a Goblet cell. If it is found in the salivary glands, it is known as a Mucous Cell. These cells can be found between epithelial cells.

If a gland is composed of multiple cells, these are known as Multicellular glands. These cells also secrete mucins and often secrete secretory sheets that cover a larger surface area. A common place to find multicellular glands are within the stomach. In the stomach, multicellular glands produce the lining using mucin to help protect against harsh enzymes. These cells can be found lined up next to each other on the apical surface.

Glandular Epithelia have three other major structures that are based on the shape of the tube the cells make. All of these glands are multicellular but, they form deeper into the tissue creating a duct where secretions are produced. This is not like Goblet cells or Mucous cells that produce secretions directly into the lumen or cavity. The three major shapes include:

  1. Tubular Glands
  2. Alveolar (acinar) Glands
  3. Tubuloalveolar (tubuloacinar) Glands

Tubular glands are cells arranged in a straight or coiled tube. These glands are extremely basic and usually secrete enzymes and materials for the digestive system and sweat. Specifically, Intestinal glands are a good example of straight tubular glands. Eccrine sweat glands are a good example of a coiled tubular gland.

Alveolar or Acinar glands form blind pockets. There is usually a duct that extends into the tissue and a group of glandular cells creates a bulb-shaped blind pocket. Sebaceous glands are examples of this type of gland.

The final type of gland is the Tubuloalveolar or Tubuloacinar gland. This gland combines the structure of a tubular and alveolar gland into one. They only come in compound forms and form glands of the respiratory passages, pancreas, and salivary glands.

Structure Of The Ducts

The ducts of a gland are the passageways that carry secretory products from the cells to the location it needs to go. Based on the structure of these passageways, you can categorize glands into two groups, Simple glands, and Compound glands.

A Simple gland is a gland that has no branching. This means that the ducts have only one way to travel. It is almost like having a town with one road. A Compound gland is a gland with repeated branches. This means that the duct diverges into different ducts creating a network. This would be like having a network of roads in a neighborhood.

Methods Of Secretion

Glandular Epithelia have many different ways of how they secrete their products. Based on where the cell is in the body and what it is producing can have a major role in the method of secretion. There are three main methods:

  1. Eccrine secretion
  2. Apocrine secretion
  3. Holocrine secretion

Eccrine secretion uses the process of Exocytosis to release its products. It will first package its product into a secondary vesicle. This secondary vesicle will carry the product to the apical surface and release the package through Exocytosis. This form of secretion is the most common in all cells and is used to produce things like your saliva and mucins from Goblet cells.

Apocrine secretion is the process of secreting products via the apical portion of the cell by shedding. The secretory vesicles will travel to the apical portion of the cytoplasm. Here, the cytoplasm will be shed from the rest of the cell and broken down in the lumen or cavity. This breakdown results in the release of secretory products from the vesicles. After, the gland cells will go through a period of regrowth before they can shed again. Lactiferous cells in the Mammary gland are good examples of this.

Holocrine secretion is the process of releasing the secretory product via bursting cells. During this secretory process, the entire cell will be packaged with secretory products. The cell will then burst and release the products. This process usually occurs in stratified epithelium because new cells need to replace the cells that have burst. Sebaceous glands near your hair follicles produce oil using this method.