From a surface (apical) view, squamous epithelial cells have an appearance that resembles ‘crazy paving’, and for this reason this tissue is also commonly referred to as pavement epithelium. Many of the body’s internal membranes, such as the peritoneal membrane which lines the abdominopelvic cavity, the pericardial membranes that surround the heart and the pleural membranes that surround the lungs, have a layer of squamous epithelial cells associated with them. These membranes are referred to as serous membranes since their resident squamous cells secrete a thin watery fluid called serous fluid.
This fluid is very slippery and acts as a natural internal lubricant within the body, reducing friction between the visceral organs during bodily movement. Many of the body’s internal organs such as the outer layers of the gut (serosa) and the uterus (perimetrium) also have a thin serous layer composed of squamous cells, which contribute further to the secretion of this internal lubricant. When major body cavities are opened during surgery this serous fluid is clearly visible as a glistening shiny surface coating the internal organs.
Simple cuboidal epithelium
This consists of a single layer of cube-shaped cells (Figure 1.8B) and is found forming the walls of the kidney tubules where it plays a key role in regulating the composition of the urine. Within the brain are specialised cuboidal epithelial cells called ependymal cells which produce cerebrospinal fluid which surrounds the brain and spinal cord.
Simple columnar epithelium
This consists of a single layer of tall, thin, column-shaped cells (Figure 1.8C) found forming the mucosal lining of many areas of the gastrointestinal tract. These relatively thick cells allow gradual absorption of nutrients across the gut into the blood. A specialised ciliated columnar epithelium (Figure 1.8D) is located in the lining of the fallopian tubes; here the cilia beat in coordinated waves, playing a key role in transporting ova from the ovaries to the uterus.
Stratified epithelium
Stratified squamous epithelium
This consists of several layers of thin, flat cells (Figure 1.8E). The epidermis which forms the outer layer of the skin is composed of a stratified squamous epithelium. The initial layer of cells that resides on the basement membrane is continually dividing by mitosis and is the origin of the cells above. As cells move up through the layers, they accumulate the tough, dense protein called keratin. This makes the epidermal layer very strong but also renders the cells near the surface impermeable to oxygen so that they gradually die.
This means that the outer layer of the epidermis is composed of entirely dead skin cells which gradually slough off and flake away from the surface. There are examples of stratified cuboidal epithelium found surrounding developing ovarian follicles and stratified columnar epithelium found lining the male urethra, and these are discussed further in subsequent chapters.
Pseudostratified epithelium
Lining the nasal cavity, trachea and upper portions of the bronchial tree is a specialised ciliated epithelium. This is composed of a combination of tall column-shaped cells and smaller cells squashed between. This gives the false appearance of a tissue that is stratified (multilayered). The term pseudostratified (which literally means falsely layered) is often used to describe this tissue. In reality, all the cells within this tissue are in contact with the basement membrane and so pseudostratified epithelium is actually a specialised simple epithelium (Figure 1.8F).
Transitional epithelium
The bladder is lined by an elastic stratified epithelium with cells that change shape according to the volume of urine currently stored. When the bladder is full, the pressure exerted by the urine compresses this tissue and the cells take on a thin, flat, squamous appearance (Figure 1.8G2). As the bladder empties, the cells become progressively less compressed, gradually adopting a cuboidal and then a columnar appearance (Figure 1.8G1).
Prokaryotic cells
Cells which contain their DNA within a nuclear membrane are referred to as eukaryotic cells. The cells which make up the bodies of animals (including humans), plants and fungi are all eukaryotic. Human erythrocytes (red blood cells) lose their nucleus as they mature; this loss of the nucleus allows more haemoglobin molecules to be packed into the cell, improving the efficiency of oxygen transport. However, since erythrocytes are derived from nucleated cells, they are still eukaryotic in origin.
Unlike eukaryotic cells, bacterial cells do not have their DNA enclosed within a nuclear envelope and are referred to as prokaryotic cells.
Figure 1.9 Structure of prokaryotic cells
Prokaryotic cells typically have other differences from eukaryotic cells; most are surrounded by a thick, robust cell wall which allows them to survive in fluctuating environmental extremes of temperature, pH and dryness (Figure 1.9). A key feature of bacteria is that they are able to replicate incredibly fast. This allows huge populations to be generated in relatively short periods of time; in the human body this can have disastrous consequences, particularly if bacteria gain access to the blood.
Sepsis is often defined as an overwhelming life-threatening infection. It is more common than myocardial infarction (heart attack) and kills more people in the UK than breast, bowel and prostate cancer combined. Sepsis is more common in the very old and very young whose immune systems are in decline or not fully developed, while patients on immunosuppressive medications such as certain steroids are also at increased risk (Knight and Hore, 2018).
Symptoms of sepsis vary according to the age of the patient. According to the UK Sepsis Trust, the signs of sepsis in an adult include:
1 ‘Slurred’ speech or confusion
2 Extreme shivering or muscle pain
3 Passing no urine (in a day)
4 Severe breathlessness
5 It feels like you’re going to die
6 Skin mottled or discoloured.
In children, sepsis should be suspected if the child:
1 Is breathing very fast
2 Has a ‘fit’ or convulsion
3 Looks mottled, bluish, or pale
4 Has a rash that does not fade when you press it
5 Is very lethargic or difficult to wake
6 Feels abnormally cold to touch.
A child under five may have sepsis if he or she:
1 Is not feeding
2 Is vomiting repeatedly
3 Has not passed urine for 12 hours.
(UK Sepsis Trust, 2019)
Since sepsis is life-threatening and so common, it is essential that nurses learn to recognise some of the key features of this medical emergency early in their training. To help develop your knowledge, read through Mary’s case study.
Case study: Mary – sepsis evidence-based practice
Mary is 72 and has become extremely unwell over the last 24 hours. She has become increasingly breathless and is expectorating green sputum. She was seen by her GP who sent her into her local hospital as an emergency admission. On admission, Mary was fully conscious and alert but slightly confused. Her temperature was 38°C, her heart rate was 125, her blood pressure 110/58, respiratory rate 26 breaths per minute and oxygen saturation 92 per cent on room air. Mary’s vital signs were recorded using the NEWS 2 (National