Body Tissues

Cell Junctions, Glands, and Membranes

Cell Junctions

Cell junctions are sites of attachment between adjacent cells or tissues and are important for cellular communication, cellular transport, and cellular anchoring.

Cell junctions are specialized regions in the membrane important for anchoring to adjacent cells, resisting tears, regulating cellular transport, and allowing cytoplasm to pass between cells. These junctions involve interlocking proteins and protein channels. There are four types of cell junctions: gap junctions, tight junctions, desmosomes, and hemidesmosomes.

A gap junction connects adjacent cells via protein channels, allowing the direct movement of small molecules and ions from the cytoplasm of one cell to the cytoplasm of the next cell. Gap junctions can also open and close in response to electrical or metabolic signals from nearby cells, which coordinates the functions of tissues. For example, gap junctions in the intercalated discs of cardiac muscle facilitate the rapid movement of ions between cells, allowing heart cells to contract almost simultaneously.

As the name implies, a tight junction is a close connection between cells. The tightness of this cell-cell interaction forms an impermeable barrier that prevents or reduces the movement of substances between cells. Tight junctions play an important role in the blood-brain barrier, which is critical for preventing harmful pathogens in the bloodstream from reaching the brain.

A desmosome is a strong cell junction that reduces mechanical stress by connecting cells via specialized proteins at anchoring junctions and helping maintain tissue strength. These junctions penetrate the cell membrane and attach to the cell's cytoskeleton, a network of protein filaments throughout the cytoplasm that gives the cell its shape. Desmosomes are particularly important in epithelial and cardiac muscle tissues where stress is encountered regularly. A hemidesmosome (or half-desmosome) is mechanistically the same as a desmosome except it is a connection between a cell and the extracellular matrix.

Types of Cell Junctions

Different types of cell junctions are specialized to allow the passage of certain substances between cells and for cell adhesion.

Glands

Glandular tissue, derived from epithelial tissue, secretes substances in the bloodstream or into a duct that leads out of the body or into an organ.

Some epithelial tissues produce glands. A gland is an organ or cell that produces and secretes substances. Two main types of glands are endocrine and exocrine, distinguished by where the secreted product is released. Endocrine glands release hormones directly into the bloodstream. These hormones then travel through the body to eventually reach target cells with specific hormone receptors. Endocrine glands include the thyroid, ovaries, and testes. Exocrine glands secrete substances such as sweat and saliva into a duct that empties out of the body or into an organ.

Exocrine glands can be classified into merocrine glands, apocrine glands, and holocrine glands based on the mechanism of product secretion. Merocrine glands release substances that are packaged in vesicles. These vesicles fuse with the cell membrane and release the product into a duct via exocytosis, leaving the cell intact. An example of a merocrine gland is the type of sweat gland involved in thermoregulation. Apocrine glands secrete substances by the budding off of the apical side of the cell, releasing cell fragments into the duct. These fragments include the cellular product that was in the cell cytoplasm. Apocrine glands include milk-producing mammary glands. Holocrine glands accumulate a product within the cell, and eventually the cell ruptures and the product is released into ducts. These glands are shedding the entire cells along with the product. The oil glands, or sebaceous glands, of the skin are an example of holocrine glands.

Mechanisms of Glandular Secretion

Endocrine glands secrete substances directly into bloodstream and exocrine glands secrete substances into ducts. Exocrine glands are classified into merocrine, apocrine, and holocrine based on their mechanisms of secretion.

Membranes

Membranes are sheets of epithelial and connective tissues that serve as organ coverings and cavity linings.

A membrane is a sheet of tissue that covers organs and lines cavities, and also includes the skin. There are three types of membranes in the body that contain both epithelial and connective tissues: cutaneous, mucous, and serous.

The cutaneous membrane (skin) is the outer covering of the body, made up of sheets of stratified squamous epithelial tissue and connective tissue. Unlike the other two membrane types, the apical surface of the epithelial layer is dry. The cutaneous membrane serves to protect the internal organs from pathogens such as bacteria and viruses and from environmental insults.

A mucous membrane, also called mucosa, lines internal body cavities and organs that are open to the external environment, such as the digestive and respiratory tracts and parts of the eye and ear. These membranes are composed of stratified squamous or simple columnar epithelial tissue and an underlying loose connective tissue. Many mucous membranes secrete mucus, used to trap foreign substances for removal and to lubricate and protect organs. Mucous membranes in the urinary tract do not secrete mucus, but all mucous membrane secrete some fluids to provide a moist environment. Mucous membranes function in secretion, absorption, and protection.

A serous membrane lines body cavities not directly open to the outside, such as the abdominal and thoracic cavities. They also cover the organs located in those cavities. The serous membrane has distinct layers: an inner visceral layer and an outer parietal layer. A thin layer of fluid called serous fluid is found between these two layers, which keeps the membrane lubricated and minimizes friction from movement.

Type of Membrane Function Examples
Cutaneous membrane Provides protection Skin: covers the body externally
Mucous membrane Secretion and absorption in cavities open to the external environment; provides protection Mucosa of digestive system: lines the mouth, tongue, esophagus, stomach, and intestines Mucosa of respiratory system: lines the nasal cavity, trachea, bronchi, bronchioles, alveoli Mucosa of reproductive system: lines the vagina and uterus in females; found in foreskin and the duct system in males Mucosa of urinary system: lines the urethra, ureters, and urinary bladder
Serous membrane Provides lubrication to reduce friction during organ movement Pleura: covers the lungs Pericardium: covers the heart Peritoneum: lines abdominal cavity, covers abdominal organs