Membranes are sheets of tissue that cover or line surfaces or separate organs or parts (lobes) of organs from each other. Many membranes provide specialized secretions.
The two major categories of membranes are epithelial (serous and mucus) and connective tissue.
Membrane is a term that has more than one meaning. For example, at the cellular level, membranes are lipid bilayers that restrict the passage of ions and other solutes.
At the tissue level, membranes form a barrier or interface (basement membranes that separate epithelia from connective tissues).
On another level, epithelia and connective tissues combine to form membranes that cover and protect other structures and tissues.
Types of membranes:
- Mucous membranes line cavities that connect with the exterior, including the digestive, respiratory, reproductive, and urinary tracts. The epithelial surfaces are kept moist at all times. The connective tissue portion of a mucous membrane is called the lamina propria. The mucous membranes are usually lined with simple epithelia that perform absorptive or secretory functions (eg. the simple columnar epithelium of the digestive tract). Examples of others are the stratified squamous epithelium, that covers the mucous membranes of the mouth, and the transitional epithelium, covering the mucous membranes of the urinary tract.
- Serous membranes line the sealed, internal cavities of the body. There are three such membranes with each consisting of a simple epithelium supported by loose connective tissue:
- the pleura lines the pleural cavities and covers the lungs
- the peritoneum lines the peritoneal cavity and covers the surfaces of enclosed organs such as the liver and stomach
- the pericardium lines the pericardial cavity and covers the heart.
A serous membrane also has parietal and visceral portions. Parietal lines the outer wall of the internal chamber. The visceral portion covers organs within the body cavity. Serous fluid covers the surfaces to minimize friction between opposing surfaces.
- Cutaneous membranes of the skin cover the surface of the body. They consist of stratified squamous epithelium and the underlying connective tissues. Cutaneous membranes are thick, relatively waterproof, and dry.
- Synovial membranes produce synovial fluid which helps lubricate joints permitting smooth movement. The synovial membrane consists mainly of loose connective tissue with the epithelial layer being incomplete.
Connective Tissue Membranes: (membrane – location – function)
- Superficial fascia – between the skin and muscles – adipose tissue stores fat
- Periosteum – covers each bone – contains blood vessels that enter the bone
- Perichondrium – covers cartilage – contains capillaries and is the only blood supply for cartilage
- Synovial – lines joint cavities – secretes synovial fluid to prevent friction
- Deep fascia – covers each skeletal muscle – anchors tendons
- Meninges – covers the brain and spinal cord – contains cerebrospinal fluid
- Fibrous pericardium – forms a sac around the heart – protection