Ultrastructural Mapping of the Zebrafish Gastrointestinal System as a Basis for Experimental Drug Studies

Table 1

Comparison between the zebrafish and the rat digestive systems, including the GIT and the accessory organs. The information on the ZF digestive system presented in the table below mainly result from the data analysis obtained in this study, complemented by information providing various literature sources [20, 24, 40, 54–56, 60–63]. The information on the rat ZF are mainly compiled from a review of the literature, including the following references [64–70].

(a)

Zebrafish liver		Zebrafish pancreas		Zebrafish gut
Size	12 dpf: volume = 5 061 840 µm^3. Transverse diameter = 349 µm. Anterior-posterior diameter = 520 µm; Adult: volume = 0.535 mm^3. Transverse diameter = 2.08 mm. Anterior-posterior diameter = 1.64 mm. (In males, ∼2.10% of total body weight. In females, ∼4.51% of total body weight)	Size	12 dpf: Volume = 1 738 670 µm^3. Transverse diameter = 290 µm. Anterior-posterior diameter = 500 µm; Adult: volume = 0.3 mm^3. Transverse diameter = 1.9 mm. Anterior-posterior diameter = 2.5 mm	Size	Long tube of about 400 µm. The interior is also folded and lined with a layer of simple columnar epithelium
Location	Posterior to inner ears and pharyngeal pad, anterior to intestinal bulb	Location	Between the intestine and the swim bladder. The head starts right posterior to the liver	Location	From esophagus to anus
Shape	3 lobes, boomerang shape. The left lobe, the largest, crosses the midline, under the swim bladder and the right lobe extends ventrally towards the head of the pancreas	Organisation	The pancreas is very diffused and acinar cells are scattered	Oesophagus	Located under the trachea. Short and muscular
Organisation of hepatocytes	Tubular	Endocrine tissue	One principal islet (or Brockman body) in the head (50 μm diameter). 2-3 secondary islets surround the principal islet	Composition	Mostly goblet cells and a few enterocytes
Composition	Hepatocytes, endothelial cells, bile duct epithelial cells. Kupffer cells seem to be absent	Beta cells	Insulin granules (160 nm)	Role	First place of enzymatic digestion
Hepatocytes	65% of total liver volume	Alpha cells	Glucagon granules (182 nm)	Intestinal bulb	Dilated, up to 80 µm in diameter
Size	Polygonal, 14–17 µm in size. Hepatocytes form plates, lined with sinusoids and biliary ducts	Other-cells	Delta-cells produce somatostatin	Composition	Enterocytes with very long microvilli at the beginning (up to 7.5 µm long and 115 nm in diameter) and then shortened to 2-3 µm long
Organelles	Nucleus, rER, mitochondria, Golgi apparatus, glycogen, lipid droplets, lysosomes	Vascular system	Rich vasculature (12% of the islet)	Role	Fat absorption
Biliary system	18% of liver volume	Exocrine tissue	Production of pancreatic digestive enzymes	Midintestine	Narrow, folded 3 times on itself and measures 120–140 µm in diameter
Bile canaliculi	Presence of microvilli on the surface. Diameter = 2.22 µm	Acinar cell	Polyhedral, 8–10 µm in size, surrounding a central acinar duct	Composition	More goblet cells than in the intestinal bulb. Enterocytes have shorter microvilli (2-3 µm long, 115 nm in diameter)
Bile ducts	Intrahepatic, and extrahepatic ducts	Organelles	Zymogen granules (2.30 µm), nucleus, ER, mitochondria	Role	Proteins absorption
Vascular system	17% of liver volume	Pancreatic system	Main pancreatic ducts through the middle of the pancreas head	Posterior intestine	Narrow and measures 80–90 µm in diameter
Endothelial cells	Fenestrated, average diameter = 130 nm.			Composition	Rare or no microvilli
				Role	Osmoregulation

(b)

Rat liver		Rat pancreas		Rat gut
Size	Transverse diameter = 7.5–8 cm, superior-inferior diameter = 2.8–4.2 cm, anterior-posterior diameter = 2.2–2.5 cm. 5% of total body weight, mean weight = 13.6 g	Size	Weight = 804 mg	Size	The long tube measures up to 2 m in length
Location	Upper right portion of the abdomen, beneath the diaphragm and above the stomach. A small portion extends into the upper left quadrant	Location	In the cranial abdominal cavity, between the stomach and the small intestine	Location	From mouth to anus
Shape	4 lobes: left, middle, right, and caudate	Organisation	Diffused, lobulated in appearance and divided into 3 parts: biliary, duodenal, and gastrosplenic	Oesophagus	Long tube from mouth to stomach, approx. 20 cm long
Organisation of hepatocytes	Lobular	Endocrine tissue	2–3.5% of the pancreas. 5000 islets of Langerhans scattered within the organ	Composition	Stratified squamous epithelium
Composition	Hepatocytes, endothelial cells, bile duct epithelial cells, Kupffer cells	Beta cells	65–80% of the islet. Production of insulin granules (207 nm)	Role	Transport food from mouth to stomach
Hepatocytes	80.6% of total liver volume	Alpha cells	15–20% of the islet. Production of glucagon granules (172 nm)	Stomach	Functions are comparable to the ZF intestinal bulb, approx. 6 cm long
Size	Polyhedral, 27 µm in diameter. Overlapping plate-like sheets (trabeculae) form the three-dimensional structure of the liver lobule	Other-cells	Delta-cells (3–10% of the islet) produce somatostatin. PP-cells (3–5% of the islet) produce pancreatic polypeptides	Composition	Enterocytes are the main cell types (90% of the villi surface)
Organelles	Nucleus (25% are binucleate), mitochondria, rER, sER, lysosomes, Golgi apparatus, peroxisomes, lipid droplets, free ribosomes, lipoproteins, glycogen, polyosomes	Vascular system	Rich vasculature (10% of the islet)	Role	Storage of food, start of enzymatic digestion
Biliary system	0.2% of liver	Exocrine tissue	95% of the pancreas	Small intestine	Functions are comparable to the midintestine, approx. 1–1.5 m long
Bile canaliculi	Presence of microvilli on the surface. Diameter = 1.5 µm	Acinar cell	Polyhedral, 10 µm in diameter, surrounding a central acinar duct.	Composition	Divided into 3 parts: duodenum, jejunum, and ileum. Enterocytes are the main cell types
Bile ducts	Intrahepatic, interlobular, and extrahepatic bile ducts	Organelles	Zymogen granules (500–800 nm), nucleus, ER, mitochondria	Role	Absorption of nutrients
Vascular system	19.2% of liver volume	Pancreatic system	Anterior pancreatic duct (main duct) occupies 50–60% of the pancreas. Draining the pancreatic juice to the stomach and small intestine	Large intestine	Functions are comparable to the posterior intestine, approx. 22–26 cm long
Endothelial cells	Fenestrated, average diameter = 6.5 µm			Composition	Enterocytes
				Role	Absorption of water and left over digested nutrients