The Digestive System
The Embryology of the Digestive System
Fore gut: Celiac artery, stomach, liver, pancreas, gall bladder & part od duodenum
Mid gut: Superior mesenteric artery, rest of duodenum, ileum, jejunum, ascending and transverse colon
Hind gut: Inferior mesenteric artery, descending colon, sigmoid colon, rectum
Ascending colon of the large intestin
Left lobe of liver
Tail of pancreas
Ascending part of duodenum
what are these called??
Supporting the abdominal viscera
Upper picture serosa
Lower picture adventitia
The basic organisation of the GIT is of a hollow muscular tube with regional histological and therefore functional differences.
The basic layout is of 4 layers:-
1. Mucous membrane
3. Muscularis externa
4. Adventitia (retroperitoneal) or Serosa (intraperitoneal)
The mucous membrane consists of three layers.
1. A surface epithelium which rests on a basal lamina
2. A lamina propria of supporting tissues with neurovascular and immune system cells .
3. The muscularis mucosae, a thin, double layer of smooth muscle.
The Submucosa ??
The submucosa consists of loose connective tissue with vessels and nerves which form Meissner’s plexus. It also contains leukocytes and a variable amount of fat.
The Muscularis Externa
The muscularis externa consists of two thick layers of smooth muscle with the inner layer being circular and the outer longitudinal . Between the two is a plexus of nerves.
The General Layout of the Gastrointestinal System
The Oral Cavity
Food is :
Taken into the oral cavity in suitably sized pieces.
Tasted to check that it is suitable to be eaten.
Broken down into small enough for the GIT to deal with
Moistened to aid its passage.
Pushed onto the next phase of the system.
The mouth, the first part of the digestive system, is divided into two parts; a smaller external part the vestibule and a larger internal part the oral cavity proper. The vestibule is bounded by the lips anteriorly, the cheeks laterally and the teeth. Posteriorly it is continuous with the oral cavity behind the third molar teeth.
The lips consists of a sheet of skeletal muscle covered by skin externally and mucous membrane internally. The epithelium of the skin is keratinised stratified squamous and the mucous membrane non keratinised stratified squamous. The vermillion border has many blood vessels but no glands.
The cheeks are directly continuous with the lips and have the same general structure.
The gingiva (gums) are composed of dense fibrous tissue covered with mucous membrane and attached to the teeth. It is continuous with the periodontal ligament which anchors the teeth to the alveolar bone of the mandible and maxilla.
The oral cavity proper is bounded anteriorly and laterally by the teeth and posteriorly it communicates with the pharynx. The floor of the mouth consists of mucous membrane covered muscle and the roof by the hard and soft palates. The mouth is filled by the tongue .
function and properties??
The tongue is a muscular organ covered in mucous membrane which inferiorly is smooth but roughened on its upper surface by the papillae anteriorly and is smooth posteriorly but nodular due to lymphatic tissue.
The vallate papillae, 7 to 12 in number, are the largest and separate the anterior 2/3rds of the tongue which lies in the oral cavity from the posterior 1/3 which lies in the oral part of the pharynx. They contain taste buds and the openings of the von Ebner glands.
The fungiform papillae are the bright red spots seen mostly on the tip and margins of the tongue and usually contain taste buds.
The filiform papillae are the most numerous and are pointed projection with the tips being keratinised.
Taste buds are also found on the soft palate.
what is this??
Shapes of glandular tissue??
Cell Types in the Salivary Glands
Serous acinar cell
Mucous acinar cell
Numerous small mucous and serous glands are scattered throughout the lamina propria of the oral cavity
SA Serous Acini
MA Mucous Acini
SD Serous demilunes
StD Striated ducts
What is this?
The parotid gland
What is this??
The Sublingual Gland
Mucous acini predominate in the sublingual gland.
What is this??
The Submandibular Gland
A mixture of serous and mucous acini
Intercalated ducts are lined by cuboidal epithelium. These ducts are longest in the parotid gland and join within the lobule to form the striated duct. They have been shown to secrete HCO3- and absorb Cl-
Striated Ducts ?
Striated ducts are lined by simple cuboidal epithelium which gradually becomes columnar and have numerous infoldings of their basal surface (striations).The folds are lined by elongated mitochondria and Na+ is actively transported out of the secretion and K+ and HCO3- into the secretion. More Na+ is absorbed than K+ secreted and therefore the so the secretion becomes hypotonic.
Serous cells forming a demilune cap the mucous acini, although this appearance is now known to be an artefact due to the swelling of the mucous cells during fixation.
The striated ducts have basal infoldings containing mitochondria.
Exocrine glands in the mouth produce saliva which lubricates the food, begins the digestive process and has protective functions. There are three main pairs of salivary glands which excrete in the presence of food in the mouth but there are numerous small gland which are constantly secreting to keep the mouth moist and prevent the build up of bacteria. Each major gland in surrounded by a fibrous capsule which infiltrates the gland dividing it into lobules for support and passage for the ducts, blood vessels and nerves. The secretion of the gland may be serous, mucous or a mixture of the two. The parotid gland secretes serous (watery) fluid, the sublingual mucous and the submandibular a mixture of the two. Thus there are two types of secretory cells which form acini with a small lumen at the centre. Myoepithelial cells are found within the basal lamina and their contractile properties prevent distension of the lumen and accelerates the secretion of the product. A series of duct systems bring the product to the main duct and then to the mouth with the submandibular and sublingual opening into the floor of the mouth while the parotid opens into the vestibule by the upper second molar tooth.
Much of the duct system is a passive root for the product except intercalated duct which have a small effect and the striate duct which have a profound affect on the final product.
How does the salivary gland work??
Passage of the secretion from acini to duct
The blue represents the cells that prodce the initial products. The product contains: proline-rich proteins, sialic acid andsulfacte rich mucins and different enzymes.
Saliva therefore contains:
Water, Mucins, calcium phosphate, lysozyme, thiocyanate, sialoperoxidase, secretory Immunoglobulin, A- amylase, lingual lipase, R-proteins, Cells (human and bacteria) and Hypotonic to plasma
Saliva- electrolyte composition
Compared to plasma saliva has a higher potassium (X7) and bicarbonate (X3) concentration; a lower sodium concentration (0.1). Saliva also contains a high calcium , phosphorus, chloride, thiocyanate and urea concentration
Functions of Salvia
The salivary glands produce approximately 1,200ml of fluid per day and it has both protective and digestive functions.
1. Moistens oral mucosa
2. Moistens dry food
3. Medium for dissolving food that then stimulate the taste buds
4. Bicarbonate ions buffer the oral cavity 5. α-amylasedigestscarbohydrates
6. Controls bacterial flora by lysozymes
Saliva contain IgA which protects against the proliferation of microorganisms, microbes and foreign molecules.
Control of Salivary Flow (neural)