Gastrointestinal Anatomy - The Anatomy of Sea Turtles by Jeanette

The Anatomy of Sea TurtlesThe Anatomy of Sea Turtles

Jeanette Wyneken, Ph.D.Illustrated by Dawn Witherington

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The Anatomy of Sea Turtles108

GASTROINTESTINAL ANATOMY

Figs. 164a and 164b. The gastrointestinal tractwith digestive glands and the spleen. The GI tractfrom the esophagus to the rectum from a Kemp’sridley turtle shows the different regions as well asthe associated digestive glands, the liver andpancreas. The gall bladder stores bile, produced

by the liver, and releases it through the commonbile duct when food enters the duodenum. Thespleen, located at the distal end of the pancreas, isnot a digestive gland; rather it is a lymphoid organin turtles involved in immunological activity.

Gastrointestinal TractThe gastrointestinal tract (GI tract or gut)extends from the mouth to the cloaca (Fig. 164). Itis demarked by structural and functional divisions.The mouth captures and processes food. Theesophagus conveys food to the stomach andexpels excess water. It also works with the tonguein swallowing. The stomach starts the chemicaland physical process of digestion. In the smallintestines, digestive enzymes are added to food to

break down proteins and complex carbohydrates.The small intestines are regionally specialized toabsorb amino acids, carbohydrates, sugars, water,fatty acids, and minerals (particularly calcium andphosphorus). The large intestine (colon) typicallyreclaims water. The length of the gut is somewhatrelated to diet. It is proportionally longer in greenand leatherback turtles than in loggerheads,ridleys, and hawksbills.

ileum

colon

jejunum

pancreas

duodenum

liverright lobe

liverleft lobe

stomach

pylorus

spleen

gallbladder

esophagus

a b

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The mouth includes several GI, respiratory, and earstructures: the mandibles and the pharynxincludes the palate, esophagus, glottis,Eustachian tubes, and internal choanae (Fig.165). For convenience, these structures will bedescribed together here. The glottis and internalchoanae are part of the respiratory system and theEustation tube connects the pharynx with themiddle ear cavity. The tongue is fixed to the floorof the mouth and is not protrusible. The glottis islocated on the middle part of the tongue (see Sense

Organs, Fig. 209), just posterior and ventral to theinternal choanae (internal nares); it acts as a valveto open and close the airway. The esophagus startsat the back of the tongue; it is a muscular tube thatleads to the stomach. It passes slightly dorsal and tothe right of the trachea. The Eustachian tubes (oneon each side), are found in the posterolateralaspects of the mouth, medial to the jaw joint; theyfunction in maintaining normal pressure in themiddle ear (Fig. 165).

internalchoanae

palate

Eustachiantubes

esophagus(cut)

Figs. 165a and 165b. Ventral view of the palate with the tongue and hyoidapparatus cut away. The roof of the mouth has internal choanae (internal nares)that open above the glottis (removed in this picture). In the posterior lateral partsof the palate, near the jaw joint, are the openings to the Eustachian tubes, whichlead to the middle ear cavity.

a b

The Anatomy of Sea Turtles 109





The esophagus (Fig. 166) is lined with papillaethat are sharp and keratinized; they point inwardtowards the stomach. The papillae end where theesophagus joins the stomach (Fig. 166). Thepapillae are presumed to trap food while excesswater is expelled prior to swallowing. In Atlanticgreen turtles, the esophagus enters the stomach ina smooth transition. However, in Pacific greenturtles, there is a muscular specialization at thebase of the esophagus called a crop. Its function

is unclear. In cheloniids, the esophagus descendsto a position just inside the plastron and bends tothe left in an S-shaped curve to join the stomach.In Dermochelys, the esophagus is exceptionallylong and extends almost half the length of thebody before looping to the left, and returninganteriorly almost to the level of the axilla. There,the esophagus bends left again and joins thestomach (Fig. 167).

esophagus

transitionalpapillae

gastro-esophagealsphincter

stomach

rugae

}

Figs. 166a and 166b. The esophagus and anteriorstomach lining. The papillae that line theesophagus are keratinized for most of the length ofthe esophagus. They end abruptly; several flat,transitional papillae, lacking keratin line the

esophageal wall at the level of thegastroesophageal sphincter. Posterior to thissphincter, the stomach lining is very smooth andhas no papillae.

a b




Figs. 167a and 167b. Ventral view of aleatherback hatchling's viscera and heart. Thisdissection of a leatherback posthatchling showsthe extremely long esophagus, large stomach, andsmall intestines. On the animal's right is theremaining yolk sac. The yolk sac can persist wellafter the time that the animal has begun feeding.

In this animal, the coracoid processes were cutaway and the acromion processes reflectedanteriorly to provide a clear view of the heart andliver. The ventricle is pushed anteriorly exposingthe sinus venosus, which was injected with latexto provide contrast.

rightacromionprocess

gallbladder

cutpectoralmuscle

liverrightlobe

rightatrium

pubis

smallintestine

yolksac

colon

trachea

liverleft lobe

leftatrium

sinusvenosus

ventriclereflectedanteriorly

esophagus

stomach

duodenum

esophagus

pubiccartilage

a b

The stomach is on the animals left side andcurves around the more medially located liver andpericardium. It is attached to the liver's left lobeby a gastrohepatic ligament and the left lung bya gastropulmonary ligament. The stomach issmooth-walled along its length. It ends in a shortmuscular region, the pylorus (= pyloricsphincter or valve). The pylorus is usuallyconstricted and the intestinal lining on theduodenal side of the sphincter differs from that ofthe stomach (Fig. 169).

The pancreas runs distally along the duodenumfrom the pylorus to just past the common bileduct (Fig. 170). Both the pancreas and thecommon bile duct (from the gall bladder) deliverdigestive enzymes to the duodenum. The commonbile duct enters the duodenum via a small papilla,the ampulla of Vater, on the duodenum’s internalsurface. Its location can be identified from thegreen bile stain (Figs. 170-171). The pancreaticduct (not shown) is difficult to locate in all but thelargest turtles; it enters the duodenum near or in




common with the common bile duct. Theduodenum's lining is textured and, in somespecies, it is "honey-combed" in appearance (Fig.171). This textured lining is associated withincreased surface area and is well-developed in

green turtles and leatherbacks. It is not aspronounced in the carnivorous/omnivorous species(e.g., loggerheads, ridleys, and hawksbills).

The transitions from one type of small intestine tothe next (duodenum to jejunum to ileum) areoften difficult to identify. Gross differences areoften not obvious and are best confirmed by

histological examination for the functionalcharacteristics of the tissues. The transition fromileum to colon is clear. The ileum ends in amuscular sphincter, the iliocaecal valve. Theproximal end of the colon is a caecum (pouch) that

bulges somewhat more than the remaining largeintestine (Fig. 77). It is more prominent in greenturtles than other species. The colon narrowssomewhat past the caecum; it is constricted weaklyby segmentally arranged bands of muscle. Distally,the colon tapers to form a muscular rectum, whichis often pigmented; its muscular walls arethickened and folded (Fig. 172).


stomach

pyloricsphincter

duodenum

Figs. 169a and 169b. Linings of the stomach andduodenum. The stomach and duodenum areseparated by a short muscular sphincter, thepylorus. While stomach lining is generally smooth,

that of the duodenum is often textured. In theleatherback and green turtle, there are theoverlapping crypts containing mucus found alongthe length of the duodenum and into the jejunum.

a b





Fig. 171. Longitudinal section through the liverand gallbladder. The walls of the gallbladder areremoved dorsally to expose the common bile ductto the duodenum.

Fig. 170. Longitudinal section of the duodenum.The common bile duct opens into the duodenum ata papilla (termed the Ampulla of Vater). Thecommon bile duct extends from the gallbladder tothe duodenum.

liver

liver

gallbladder

gallbladder

bile duct

ampullaof Vater

(bile duct papilla)

duodenum

pylorus

duodenum






Figs. 172a and 172b. Ventralview of the posterior viscera.The rectum (collapsed here)narrows as it joins the cloaca.The urinary bladder, seenjust above the rectum, entersthe cloaca ventrally. Thedorsally-located kidneysproduce urine that travelsthough the ureters to enterthe dorsal cloaca. Severalrenal veins are exposedmedial to the kidney. Thetestis of this immature maleis still attached to theperitoneum (and is locatedanatomically ventral to thekidneys).

urinarybladder

rectumkidney

renal veins

cloaca

ureter

colon

kidney

testis

dorsal pelvicgirdle

reflectedposteriorly right coracoid

cartilageand deeppectoralmuscle

left coracoidcartilage

liverleft lobe

a

b

The rectum empties into the cloaca (Fig. 171), achamber that also receives the urine from thekidneys, eggs or sperm, and connects ventrally intothe urinary bladder. The cloaca empties to theoutside via the cloacal opening or vent. Eachfunction of the cloaca is associated with a region into

which the products empty. The coprodeum receivedfeces from the rectum. The urodeum is associatedwith the urinary papillae and the opening of theurinary bladder. The proctodeum is the most distalregion and is associated functionally with copulationand structurally with proximity to the genital ducts.



Gastrointestinal Anatomy - The Anatomy of Sea Turtles by Jeanette

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