General Properties of the BBB 7 страница

Large intestine

Figure 61. Photograph of the large intestine (left) outer surface; TC, teniae coli; HC, haustra coli, OA, omental appendices; (middle) internal (mucosal) surface; arrows, semilunar folds; (right) photomicrograph of the mucosa and part of the submucosa of the large intestine; arrows, openings of the glands at the intestinal surface

The large intestine is composed of the cecum, ascending colon, sigmoid colon, rectum, and anal canal.

The four layers characteristics of the alimentary canal are present throughout. There are, however several distinctive features at the gross level

-The mucosa has a smooth surface; neither plicae circulares nor villi are present.

-The outer longitudinal layer of the muscularis externa exhibits three equally spaced bands.

Mucosa

The mucosa of the large intestine contains numerous straight tubular glands

that extends through the full thickness of the mucosa. The glands consist of simple columnar epithelium.

The principal functions of the colon are reabsorption of electrolytes and water and elimination of undigested food and waste.

The absorptive cells have morphology essentially identical with that of the enterocytes of the small intestine. The reabsorption of water and electrolytes is the primary function of the columnar absorptive cells. Goblet cells produce mucin that is secreted continuously to lubricate the bowel, facilitating the passage of the increasingly more solid colonic contents. Goblet cells are more numerous in the large intestine than in the small intestine.

The mucosal epithelium of the large intestine contains the same cell types as the small intestine; Paneth cells are normally absent in the adult human.

Columnar absorptive cells predominate (4:1) over goblet cells in most of the colon, only near the rectum the number of goblet cells increasing (1:1).

The intercellular space is often dilated, indicating active transport of fluid.

In the luminal surface the secretion rate exceeds the synthesis rate, and “exhausted” goblet cells appear in the epithelium between crypts. The caveolated “tuft” cell, has also been described in the colonic epithelium, this cell may, however, be one form of exhausted goblet cell.

Epithelial cell renewal in the large intestine

As in the small, all of the mucosal epithelial cells of the colon arise from stem cells located at the bottom of the crypt or gland.

The lower third of the crypt constitutes the normal replicative zone.

Lamina propria

The structural features are:

-the collagen table, a thick layer of collagen and ground substance just below the free surface

-elaborate development of GALT

-a well developed pericryptal fibroblast sheath

-absence of lymphatic vessels in the lamina propria

Muscularis externa

In the colon, the outer layer of the muscularis externa is, in the part, condensed into prominent longitudinal bands of muscle that may be seen at the gross level; these are called the teniae coli. Between the bands, the longitudinal layer forms an extremely thin sheet.

In the rectum, the outer longitudinal layer of smooth muscle is a uniformly thick layer, as in the small intestine.

Bundles of muscle from the teniae coli penetrate the inner, circular layer of muscle at intervals along the length and circumference of the colon. These apparent discontinuities in the muscularis externa allow segments of the colon to contract independently, leading to the formation of saccules (haustra) in the colon wall.

The muscularis externa of the large intestine produces two major types of contraction: segmentation and peristalsis

Submucosa and serosa

The submucosa is not unique. Where the large intestine is directly in contact with other structures, its outer layer is adventitia; elsewhere, the outer layer is a typical serosa.

Cecum and appendix

Figure 63. Photomicrograph of a cross section through the vermiform appendix

The cecum forms a blind pouch just distal to the ileocecal valve; the appendix is a thin, finger-like extension of this pouch. The histology of cecum closely resembles that of the rest of the colon; the appendix differs from it in having a complete layer of longitudinal muscle in the muscularis externa. The most conspicuous feature of the appendix is the large number of lymphatic nodules that fuse and extend into the submucosa.

Rectum and anus

Figure 64. Drawing of the rectum and anal canal

The rectum is dilated distal portion of the alimentary tract. Its upper part is distinguished from the rest of the colon by the presence of folds called transverse rectal folds. The most distal portion of the alimentary canal is the anal canal. It extends from the anorectal junction to the anus. The upper part of the anal canal has longitudinal folds called anal columns. Depressions between the anal columns are called anal sinuses.

The mucosa of the rectum is similar to that of the rest of the distal colon, having straight, tubular intestinal glands with many goblet cells.

Anal canal

In the anal canal, anal glands extend into the submucosa and even into the muscularis externa. These are branched, straight tubular glands that secrete mucus onto the anal surface through ducts lined with stratified columnar epithelium.

Large apocrine glands, the circumanal glands, are found in the skin surrounding the anal orifice.

The muscularis mucosa disappears at about the level of the rectoanal margin, but at the same level, the circular layer of the muscularis externa thickness to form the internal anal sphincter. The external anal sphincter is formed by the striated muscles of the perineum.

Pancreas

The pancreas is an exocrine and endocrine gland.

Figure 65. Diagram of a pancreatic acinus and its duct system

The exocrine component is a serous gland that synthesizes and secretes, into the duodenum, enzymes that are essential for digestion in the intestine.

The endocrine component synthesizes and secretes, into the blood, insulin and glucagons, hormones that regulate glucose, lipid and protein metabolism in the whole body.

Exocrine pancreas

The secretory units are acinar or tubuloacinar in shape and are formed by a simple epithelium of pyramidal serous cells. The serous secretory cells of the acinus produce the digestive enzyme precursors secreted by the pancreas. Pancreatic acini are unique among glandular acini, in that the initial duct that leads from the acinus, the intercalated duct, actually begins within the acinus. The duct cells located inside the acinus are referred to as centroacinar cells.

The acinar cells are characterized by acidophilic zymogen granules.

Zymogen granules contain a variety of digestive enzymes in an inactive form.

Pancreatic enzymes are capable of digesting most food substances.

Figure 66. Photomicrograph of the pancreas; arrows islets of Langerhans

They include:

-trypsinogen, pepsinogen, and procarboxypeptidase digest proteins

-amylase digests carbohydrates

-lipase digests lipids

-deoxyribonuclease and ribonuclease digest nucleic acids.

The pancreatic digestive enzymes are activated only after they reach the lumen of the small intestine.

Duct system

The centroacinar cells are the beginning of the duct system of the exocrine pancreas.

Centroacinar cells are intercalated duct cells located in the acinus.

Centroacinar cells are continuous with the cells of the short intercalated duct that lies outside the acinus. The intercalated ducts are short and drain to intralobular collecting ducts. There are no striated (secretory) ducts in the pancreas.

The complex, branching network of intralobular ducts drains to the larger interlobular ducts, which are lined with a low columnar epithelium in which enteroendocrine cells and occasional goblet cells may be found. The interlobular ducts, in turn, drain directly into the main pancreatic duct. A second large duct, the ductus choledochus (accessory pancreatic duct) arises in the head of the pancreas.

The pancreas secretes about 1 liter per day. While the acini secrete a small volume of protein-rich fluid, the intercalated duct cells secrete a large volume of fluid rich in sodium and bicarbonate.

The bicarbonate serves to neutralize the acidity of the chyme that enters the duodenum from the stomach and to establish the optimum pH for the activity of the major pancreatic enzymes.

Hormonal control of exocrine secretion

Two hormones secreted by the enteroendocrine cells of the duodenum, secretin and CCK, also called pancreozymin, and are the principal regulators of the exocrine pancreas. The entry of the acidic chyme into the duodenum stimulates the release of these hormones into the blood. The coordinate action of the two hormones produces the secretion into the duodenum of a large volume of enzyme-rich, alkaline fluid.

Endocrine pancreas

The endocrine pancreas is a diffuse organ that secretes hormones that regulate blood glucose levels.

The islets of Langerhans, the endocrine component of the pancreas, are scattered throughout the organ. The islets constitute about 1-2% of the volume of the pancreas but are most numerous in the tail.

It is possible to identify three principal cell types designated A, B and D cells and three minor islet cell types designated PP, D-1 and EC cell.

Principal cell types of the islet

Figure 67. Diagram of an islet of Langerhans

The B cells constitute about 70% of the total islet cells in humans and are generally located in its central portion.

They secrete insulin. Its principal effects are on the liver, skeletal muscle, and adipose tissue. Insulin stimulates

-uptake of glucose from the circulation

-utilization and storage of glucose by all cells

-phosphorylation of glucose on the cells

-synthesis of glycogen from the phosphorylated glucose

Absence or inadequate amounts of insulin lead to elevated blood glucose levels and the presence of glucose in the urine, a condition known as diabetes mellitus.

In addition, insulin stimulates glycerol synthesis in adipose cells and inhibits lipase activity in these cells.

The A cells constitute about 15-20% of the human islet population and are generally located peripherally in the islets. They secrete glucagons. Glucagon raises blood glucose.

Glucagon stimulates release of glucose into the blood-stream, and stimulates gluconeogenesis (synthesis of glucose from metabolites of amino acids) and glycogenolysis (breakdown of glycogen) in the liver.

Glucagon also stimulates proteolysis to promote gluconeogenesis, mobilizes fats from adipose cells, and stimulates hepatic lipase.

The D cells constitute about 5-10% of the total pancreatic endocrine tissue and are also located peripherally in the islets. D cells secrete somatostatin. The precise role of somatostatin in the islets is unclear, but it has been shown to inhibit both insulin and glucagon secretion.

The minor islet cells constitute about 5% of the islet tissue.

The PP cells secrete pancreatic polypeptide. It stimulates gastric chief cells, inhibits bile secretion and intestinal motility, inhibits pancreatic enzymes and bicarbonate secretion.

The D-1 cells secrete vasoactive intestinal peptide. Its principal effects are similar to those of glucagon plus these cells stimulate pancreatic exocrine secretion.

The EC cells secrete secretin, motilin and substance P. Secretin- acts locally to stimulate bicarbonate secretion in pancreatic fluid and pancreatic enzyme secretion, motilin-increases gastric and intestinal motility, substance P-unclear.

Liver

The liver is the largest mass of glandular tissue in the body and is also the largest internal organ. The liver has both exocrine and endocrine functions. The liver is an organ engaged in numerous metabolic conversions involving substrates brought to and from the digestive tube, pancreas and spleen. Some of the products of these metabolic conversions are carried in the exocrine secretion of the liver, called bile.

The endocrine secretions of the liver are released directly into the blood that supplies the liver parenchyma. These secretions include substances synthesized by the liver cells, i.e.,

-albumin

-prothrombin

-nonimmune α and β globulins

-numerous glycoproteins, including fibronectin

-glycogen, and glucose released by hydrolysis of glycogen

Blood supply to the liver

The liver receives blood that initially supplied the small intestine, pancreas and spleen.

The hepatic portal vein carries about 75% of the blood supply to the liver. The hepatic portal vein (hpv) carries venous blood from the digestive tube, pancreas and spleen into liver.

The hepatic artery, a branch of the celiac trunk, carries oxygenated blood to the liver.

Within the liver, the distributing branches of the hpv and hepatic artery, which supply the sinusoidal capillaries (sinusoids) that bathe the hepatocytes (epithelial cells of the liver) and the draining branches of the bile duct system course together in a relationship termed the portal triad.

Figure 68. Blood supply to the liver, portal triad

Liver lobules

There are three ways of describing the structure of the liver in terms of a functional unit: the “classic” lobule, the portal lobule, and the liver acinus.

Classic lobule

Figure 69. Diagram of a classic liver lobule

The classic lobule is the traditional description of the organization of the liver parenchyma. The classic hepatic lobule is a roughly hexagonal block of tissue.

The classic lobule consists of stacks of anastomosing plates of hepatic cells, separated by the anastomosing system of sinusoids that perfuse the cells with the mixed portal and arterial blood. At the center of the lobule is the terminal hepatic venule (central vein) into which the sinusoids drain. The plates of cells radiate from the central vein to the periphery of the lobule, as do the sinusoids. At the angles of the hexagon are the portal areas (portal canals), loose stromal connective tissue characterized by the presence of the portal triads. At the edges of the portal canal between the connective tissue stroma and hepatocytes is a small space called the space of Mall. This is one of the sites where lymph originates in

the liver.

Liver acinus

Figure70. Diagram of a classic liver acinus

The liver acinus described as diamond shaped, is the smallest functional unit in the hepatic parenchyma. The short axis of the acinus is defined by the terminal branches of the portal triad that lie along the border between two “classic” lobules. The long axis is a line drawn between the two central veins closest to the short axis.

The hepatocytes in each liver acinus are described as being arranged in three concentric elliptical zones surrounding the short axis. Thus,

Zone 1 is closest to the axis. Zone 2 is farthest from the axis. Zone 3 lies between zones 1 and 3.

The cells in zone 1 are the first to receive both nutrients and toxins in the blood and are the first to show morphologic changes following bile stasis. These cells are also the last to die and the first to regenerate.

The cells in zone 3, on the other hand, are the first to show ischemic necrosis. They are last to respond to toxic substances and to bile stasis

The cells in zone 3 have all characteristic between zones 1 and 3.

The liver acinus provides the best correlation among blood perfusion, metabolic activity, and liver pathology.

Portal lobule

The portal lobule emphasizes the exocrine functions of the liver. The morphologic axis of the portal lobule is the interlobular duct of the portal triad of the “classic” lobule. Its outer margins are imaginary lines between the three central veins that are closest to that portal triad. This defines a roughly triangular block of tissue that includes those portions of three classic lobules that secrete the bile that drains into its axial bile duct.

Blood vessels of the parenchyma

Figure 71. Diagram of the blood and bile flow in the liver

The blood vessels that occupy the portal canals are called interlobular vessels. Only the interlobular vessels that form the smallest portal triads send blood into the sinusoids. The larger interlobular vessels branch into distributing vessels that are located at the periphery of the lobule. These distributing vessels send inlet vessels to the sinusoids. In the sinusoids the blood flows centripetally toward the central vein. The central vein courses through the central axis of the classic liver lobule and empties into a sublobular vein. Several sublobular veins converge to form larger hepatic veins that empty into the inferior vena cava.

Sinusoids

Hepatic sinusoids are lined with a thin discontinuous endothelium. Hepatic sinusoids differ from other sinusoids in that a second cell type, the stellate sinusoidal macrophage or Kupffer cell is a regular part of the vessel lining.

Kupffer cells belong to the mononuclear phagocytic system. Processes of Kupffer cells often seem to span the sinusoidal lumen and may even partially occlude it. The Kupffer cells may be involved in the final breakdown of some damaged or senile red blood cells that reach the liver from the spleen.

Figure 72. Schematic diagram of a plate of hepatocytes interposed between hepatic sinusoids

Perisinusoidal space (space of Disse)

The Perisinusoidal space is the site of exchange of materials between blood and liver cells. The perisinusoidal space lies between the basal surfaces of the hepatocytes and the basal surfaces of the endothelial cells and Kupffer cells that line the sinusoids.

Irregular microvillous processes project into this space from the basal plasma membrane of the hepatocytes. Because of the large gaps in the endothelial layer and the absence of a continuous basal lamina, there is no significant barrier between the blood plasma in the sinusoid and the hepatocyte plasma membrane that forms the parenchymal border of the perisinusoidal space. A third cell type, the lypocyte or adipose cell (commonly called an Ito cell) is found in the perisinusoidal space. These cells have been shown to be the primary storage site for vitamin A, the vitamin A is transported from the liver to the retina, where it is used in the synthesis of visual pigments.

Hepatocytes

Hepatocytes make up the anastomosing cell plates of the liver lobule. Hepatocytes

are large polygonal cells that constitute about 80% of the cell population of the liver. Nuclei of hepatocytes are large and spherical and occupy the center of the cell. Liver cells are capable of considerable regeneration when liver substance is lost to hepatotoxic processes, disease, or surgery. The hepatocyte cytoplasm is generally acidophilic. Specific cytoplasmic components include:

-rER and free ribosomes

-numerous mitochondria

-glycogen

-lipid droplets

-small Golgi complexes (elements of the Golgi complex concentrated near the bile canaliculus are believed to be associated with “exocrine” secretion of bile.

-a lot of peroxisomes

Peroxisomes have specific oxidative functions in:

-gluconeogenesis

-metabolism of purines

-metabolism of alcohol

-metabolism of lipids

In humans catalase and D-amino acid oxidase, as well as alcohol dehydrogenase are found in peroxisomes.

The sER contains enzymes involved in degradation and conjugation of toxins and drugs as well as enzymes responsible for synthesizing cholesterol and the lipid portion of lipoproteins.

Certain drugs and hormones stimulate increased activity in the smooth ER.

Lysosomes

Lysosomes concentrated near the bile canaliculus correspond to the peribiliary dense bodies. They have varied contents including:

-pigment granules (lipofuscin)

-myelin figures

-partially digested cytoplasmic organelles

Hepatocyte lysosomes may also be a normal storage site for iron.

Biliary tree

The bile canaliculus is small canal formed by grooves in neighboring cells. The canaliculus contains short, irregular microvilli from the hepatocyte. The bile canaliculi form a ring about the hepatocyte and constitute a network that drains into small bile ducts, the canals of Hering, these in turn, drain into the bile duct of the portal canals. Bile canaliculi are formed by apposed grooves in the surface of adjacent hepatocytes. In three dimensions bile canaliculi may form a complete loop around four sides of the idealized six-sided hepatocytes. Bile flows in the canaliculi centrifugal. The flow of bile is in a direction opposite to the flow of blood, i.e. from the region of the central vein toward the portal canal. Near the portal canal, but still within the lobule, bile canaliculi join to form small ductules, the canals of Hering that are lined with cuboidal cells that are not hepatocytes.

Intra hepatic bile ducts

The ductules carry the bile to the interlobar bile ducts that form part of the portal triad. Interlobular duct join to form the right and left lobar ducts that in turn join at the hilum to form the common hepatic duct.

Extra hepatic bile ducts

Extra hepatic bile ducts carry the bile to the gall bladder and intestine.

Bile

About 90% of the bile salts a component of bile is reabsorbed and resecreted by the hepatocytes. Bile also consists of cholesterol, lecithin, bile pigments, water and electrolytes.

Gall bladder

The gall bladder concentrates and stores bile.

Mucosa

The empty or partially filled gall bladder has numerous deep mucosal folds. The mucosal surface consists of a simple columnar epithelium. The tall epithelial cells exhibit the following features:

-numerous, apical microvilli

-apical junctional complexes that join adjacent cells

-complex lateral plications

-concentrations of mitochondria in the apical and basal cytoplasm

Lamina propria

-Mucin-secreting glands

-large number of lymphocyte and plasma cells

Muscularis externa, adventitia and serosa

The smooth muscle bundles are some what randomly oriented. Contraction of the smooth muscle reduces the volume of the bladder, forcing its contents out through the cystic duct.

External to the muscularis externa is a thick layer of dense connective tissue.

Where the gall bladder is attached to the liver surface, this layer is referred to as the adventitia. The unattached surface is covered by a serosa of visceral peritoneum consisting of a layer of mesothelium and a thin layer of loose connective tissue.

Respiratory system

Figure 73. Diagram of respiratory passages

The respiratory system consists of the paired lungs and a series of air passages that lead to and from the lungs. Three principal functions are performed by this system, namely, air con­duction, air filtration, and gas exchange (respiration). In addition, air passing through the larynx is used to produce speech, and air passing over the olfactory mucosain the nasal cavitiescarries the stim­uli for the senseof smell.

The Air Passages Consist of a Conducting Portion and a Respiratory Portion

The conducting portion of the respiratory system external to the lungs con­sists of

-Nasal cavities

-Nasopharynx and oropharynx

-Larynx

-Trachea

-Paired primary bronchi

Within the lungs are the internal bronchi, which undergo extensive branching to give rise to the distributing bron­chioles. Collectively, the internal bronchi and the bron­chioles constitute the bronchial tree.

The respiratory portion con­sists of

-Respiratory bronchioles

-Alveolar ducts

-Alveolar sacs

-Alveoli

Air passing through the respiratory passages must beconditioned before reaching the terminal respiratory units. Conditioning of the air occurs in the conducting portion of the system and consists of warming, moistening, and re moval of particulate materials.

NASAL CAVITIES

The nasal cavities are paired chambers separated by a bony and cartilaginous septum. Each chamber is divided into three regions:

-Vestibule (nostril)

-Respiratory segment

-Olfactory segment

Vestibule of the Nasal Cavity

The vestibule communicates anteriorly with the external environment. It is lined with stratified squamous epithe­lium, and contains the hairs that filter out large particulate matter before it is carried in the airstream to the rest of the cavity. Sebaceous glands are also present, and their secretions assist in the entrapment of particulate matter.

Respiratory Segment of the Nasal Cavity

The respiratory segment constitutes most of the volume of the nasal cavities. It is lined by a ciliated, pseudostrat­ified columnar epithelium.

It is composed of five cell types:

-Ciliated cells

-Goblet cells

-Brush cells, a general name for those cells in the respiratory tract that bear short, blunt microvilli

-Small granule cells, cells that resemble the basal cell but contain secretory granules

-Basal cells, stem cells from which the other cell types arise

The Mucosa of the Respiratory Segment Warms, Moistens, and Filters Inspired Air

The lamina propria of the respiratory segment has a rich, vascular network that includes a complex set of capillary loops. The lamina propria also contains mucous glands, many with serous demilunes. Their secretion supplements that of the goblet cells in the respiratory epithelium.

Olfactory Segment of the Nasal Cavity

Part of the dome of each nasal cavity form the olfactory segmentand are lined with olfactory mucosa.

Figure 74. Diagram of the olfactory mucosa of the nasal cavity

The olfactory epithelium, is also pseudostratified, but it contains very different cell types. The olfactory epithelium is composed of the follow­ing cell types:

-Olfactory cells, bipolar neurons that span the thickness of the epithelium

-Supporting or sustentacular cells, columnar cells that provide mechanical and metabolic support to the olfac­tory cells

-Basal cells, stem cells from which new olfactory cells and supporting cells differentiate

-Brush cells, the same cell type that occurs in the re­spiratory epithelium

Lamina propria

This connective tissue contains numerous blood and lymphatic vessels.

The olfactory glands, a characteristic feature of the mu­cosa, are branched tubuloalveolar serous glands

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