BL Session 6 - Epithelia, Glands and Skin Flashcards
What are mucuous membranes and what do they consist of?
- Mucous membranes: line certain internal tubes which open to the exterior and bear mucus-secreting cells to varying degrees i.e. the alimentary tract, the respiratory tract, the urinary tract.
- Mucous membranes consist of:
I. an epithelium lining the lumen of the tube
II. An adjacent layer of connective tissue often referred to as lamina propria
III. A third layer consisting of a smooth muscle cells, referred to as muscularis mucosae (alimentary tract)
What are serous membranes and what are their properties ?
- Serous membranes: are thin, two-part membranes which line certain closed body cavities (spaces which do not open to the exterior). They envelope the viscera.
- These are their properties:
I. They line the peritoneum (envelopes many abdominal organs)
II. They line the pleural sacs (envelopes the lungs)
III. The pericardial sac (envelopes the heart)
Describe the pericardial sac.
- Visceral serosa of pericardium.
- Parietal serosa of pericardium.
Describe the pleurae of the lungs.
Describe the peritoneum.
What are epithelia?
- Epithelia: are sheets of contiguous cells, of varied embryonic origin, that cover the external surface of the body and line internal surfaces.
- There are three principal cell shapes associated with epithelial cells: squamous epithelium, cuboidal epithelium and columnar epithelium.
- There are three ways of describing the layering of epithelium: simple, stratified and pseudostratified.
Outline surfaces with epithelial linings.
- Exterior surface: Skin
- Interior spaces which open to the exterior
I. Gastrointestinal tract
II. Respiratory tract
III. Genitourinary tract
- Interior spaces which don’t open to the exterior
I. Pericardial sac
II. Pleural sacs
III. Peritoneum
IV. Blood vessels
V. Lymphatic vessels
What is the basement membrane?
- Basement membrane: is the name given to the thin, flexible, acellular layer which lies between epithelial cells and the subtending connective tissue.
- It consists of a basal lamina which is laid down by the epithelial cells and therefore lies closest to them.
- The basement membrane serves as a strong flexible layer to which epithelial cells adhere. It also serves as a cellular and molecular filter.
N.B. In literature, basement membrane and basal lamina are used inconsistently.
Outline, briefly, simple epithelia in terms of its structure, location and four major classes.
- Simple epithelia – consists of a single layer of cells.
- They are typically where absorption, secretion and filtration occur.
- The thinness of the epithelial barrier facilitates these processes.
- Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are:
I. Simple squamous
II. Simple cuboidal
III. Simple columnar
IV. Pseudostratified
Outline simple squamous epithelia in terms of its structure, location and functions.
- Epithelium cells are flat in shape and arranged in a single layer.
- It’s thinness facilitates passive diffusion.
- Location: This epithelial type is found in the lining of blood and lymph vessels (endothelium), the lining of pulmonary alveoli and the lining of closed body cavities such as the pericardium, pleura and peritoneum (mesothelium).
- This epithelium has the following functions:
I. Lubrication (pericardium, pleura, peritoneum)
II. Gas exchange (pulmonary alveoli)
III. Barrier (Bowman’s capsule)
IV. Active transport by pinocytosis (mesothelium and endothelium)
Outline simple cuboidal epithelia in terms of its structure, location and functions.
- Simple cuboidal – epithelium consists of a single layer cells that are as tall as they are wide.
- This epithelial type is found in the small ducts of many exocrine glands (pancreas and salivary glands), kidney tubules and the germinal epithelium of the ovary.
- This epithelium has the following functions:
I. Absorption and conduit (exocrine glands)
II. Absorption and secretion (kidney tubules)
III. A barrier/covering (ovary)
Outline simple columnar epithelia in terms of its structure, location and functions.
- Simple columnar – epithelium is a single row of tall, closely packed cells, aligned in a row.
- These cells are found in areas with high secretory function (stomach lining and gastric glands), or absorptive areas (small intestine and colon) as well as the oviducts.
- This epithelium has the following functions:
I. Absorption
II. Secretion
III. Lubrication (small intestine and colon)
IV. Transport (oviduct)
Outline pseudostratified epithelia in terms of its structure, location and functions.
- Pseudostratified – these are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading (hence pseudo) impression that the epithelium is stratified when the cells are viewed in cross section.
- Pseudostratified epithelium can also possess cilia. In this case, the epithelium is described as ciliated pseudostratified epithelium.
- Location: Ciliated epithelium is found in the lining of the nasal cavity, trachea and bronchi as well as the epididymis and ductus deferens.
- This epithelium has the following functions:
I. Secretion and conduit (respiratory tract, ductus deferens)
II. Absorption (epididymis)
III. Mucus secretion
IV. Particle trapping and removal (respiratory tract).
Outline, briefly, stratified epithelia in terms of its structure, location and three major types
- Stratified epithelia – stratified epithelium differs from simple epithelium by being multi-layered.
- It is therefore found where body linings have to withstand mechanical or chemical insults.
- Stratified epithelia are more durable and protection is one their major functions.
- Since stratified epithelium consists of two or more layers, the basal cells divide and push towards the apex, and in the process flatten the apical cells.
- Stratified epithelia can be columnar, cuboidal, or squamous type.
Outline stratified squamous non-keratinised epithelium in terms of its structure, location and functions.
- Stratified squamous non-keratinised epithelium – this epithelium can be found in the oral cavity, the oesophagus, the vagina and the anal canal
- This epithelium has the following functions:
I. Protection against abrasion (all sites)
II. Reducing water loss but maintaining moisture (all sites).
Outline keratinised epithelia in terms of its structure, location and functions.
- Keratinised epithelia – the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm.
- They contain a tough, resistant protein called keratin.
- This specialisation makes the epithelium waterproof, and it is abundant in the surface of mammalian skin.
- The functions of this epithelium include:
I. The prevention of water loss
II. The prevention of ingress of microbes
III. The protection against abrasion and physical trauma, shielding against UV light damage.
Outline transitional epithelia in terms of its structure, location and functions.
- Transitional epithelia – are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched, or stratified squamous when the organ is distended and the tissue stretches.
- Hence, surface cells vary in shape from columnar/cuboidal to stretched.
- Location: It is sometimes called the urothelium since it is almost exclusively found in the renal calyces, bladder, ureters, and urethra.
- The functions of this epithelium are:
I. Distensibility
II. The protection of underlying tissue from toxic chemicals.
What are the four layers of the epidermis?
- The horny layer (stratum corneum)
- The granular layer (stratum granulosum)
- The prickle cell layer (stratum spinosum)
- The basal layer (stratum basale)
Other cells in the epidermis are melanocytes. What are they, where are they found and what is their function?
- Dendritic cells of neural crest origin
- Occur at intervals along the basal layer of the epidermis
- Difficult to see histologically without special stains
- Produce melanin, the main pigment that gives skin its colour
- Produce more melanin but are not increased in number in black or tanned skin
Other cells in the epidermis are langerhans cells. What are they?
- Dendritic cells of bone marrow origin
- Scattered throughout the prickle cell layer
- Difficult to see histologically without special stains
- Highly specialised capacity to present antigens to T lymphocytes
- Mediate immune reactions
Explain the classification of the glands.
- By destination of secretion:
I. Exocrine glands – have ducts
II. Endocrine glands – ductless, hence secrete into the bloodstream
- By the structure of the gland:
I. Unicellular / Multicellular
II. Acinar / Tubular
III. Coiled / branched
- By the method of secretion
I. Merocrine (exocytosis - vast majority of glands)
II. Apocrine (secreted droplets covered by plasma lemma - e.g. fat droplets in milk)
III. Holocrine (whole cell breaks down - e.g. sebaceous gland of skin)
