INTRODUCTION

The green turtle (Chelonia mydas) belongs to the Cheloniidae family and inhabits the tropical and subtropical oceans - Indian, Pacific and Atlantic (^{Ernest & Barbour, 1989}). It is the species that presents more coastal habits, including estuaries of rivers and lakes (^{Hirth, 1997}), make use of the Brazilian coast for food and spawned on the Red List of the International Union for Conservation of Nature (IUCN).

There are a number of threats to sea turtles, with human action being the main threat. Its name is due to the greenish coloration of its fat, but that is not related to the external appearance (^{Pritchard & Trebbau, 1984}; ^{Moreira et al., 1995}). They assume that this species in its pelagic phase is omnivorous with a strong carnivorous tendency, becoming basically herbivorous in its juvenile phase to adult (^{Chevalier & Lartiges, 2001}; ^{Fidelis et al., 2005}). From 25 to 35 cm of Curvilinear Length of Carapace (CCC) this species initiates a herbivorous feeding (^{Bjorndal, 1997}).

In chelonians, the gastrointestinal tract is anatomically diverse among the large variety of reptiles, and this fact requires further studies to understand their anatomical specificities (^{Costa et al., 2010}).

The esophagus is a muscular tube whose function is to transport food from the mouth to the stomach and basically contain the same layers as the rest of the digestive tract (^{Junqueira & Carneiro, 2013}). In the Cheloniidae family, the esophagus descends to only one position inside the plastron and leans to the left in an "S" shaped curve to join the stomach. In Dermochelydae, the esophagus is long and extends about half the length of the body before the curve to the left, returning almost to the level of the axilla, where it makes a new curvature and joins the stomach (^{Wyneken, 2001}).

The morphological knowledge of structures involved in basic physiological processes in organisms is extremely important in the acquisition of ecological and evolutionary knowledge about species (^{Calais Júnior et al., 2016}). The study was carried out for the anatomical description and morphological characterization of the esophagus of the green turtle (Chelonia mydas) in order to produce information that allows comparative analysis with other chelonians, as well as to subsidize new studies.

MATERIAL AND METHOD

Four marine green turtle (Chelonia mydas) specimens were used, found dead on Guaraú beach, Peruíbe - SP and obtained with authorization and licenses approved by ICMBio / SISBio: 50132-1. And approved by the Ethics Committee of the Faculty of Veterinary Medicine and Animal Science, under the protocol: 8336280317.

The animals were dissected according to the methodology used by Wyneken, allowing the analysis of external and internal morphological characteristics of the esophagus. Samples of the turtle's esophagus were processed for ultramicroscopy, light microscopy (ML) (with HE, Masson, Mallory's Tricomo and Toluidine Blue), and scanning electron microscopy (SEM) samples.

Microscopy of Light: The samples were fixed in 10 % formaldehyde, cut in the medial region of the esophagus, dehydrated in series of ethanols in increasing concentrations (70 to 100 %) and diaphanized in xylol, with subsequent inclusion in histological paraffin. Slices of 5 mm thickness were performed on the microtome (Leika, German) and stained with hematoxylin-eosin. And the images were obtained through the Nikon Eclipse E- 800 light microscope from the Advanced Center in Diagnostic Imaging - CADIFMVZ-USP.

Semi-Thin Cuts: The samples were fixed in 10 % formaldehyde, cut in the medial region of the esophagus, dehydrated in a series of ethanols in increasing concentrations (50 to 100 %), placed in propylene oxide, embedded in Spurr resin. The blocks were sectioned in semithin sections of 300 nm and stained with toluidine blue. Analyzed on the Nikon Eclipse E- 800 light microscope of the Advanced Center for Diagnostic Imaging - CADIFMVZ-USP.

Scanning Electron Microscopy: The 10 % formalin-fixed samples were dehydrated in increasing series of alcohols in concentrations of 70 %, 80 %, 90 % and 100 %, dried in a LEICA EM CPD 300 critical point apparatus, glued with carbon paste in metallic bases of (stub) and silver (sputtering) on the EMITECH K550 metallizer, analyzed and photographed in a scanning electron microscope LEO 435VP from the Advanced Center for Diagnostic Imaging (CADIFMVZ-USP).

RESULTS AND DISCUSSION

In the esophagus of sea turtles is a tubular organ with the presence of horny and keratinized papillae (Fig. 1A). The mucosal papillae are decreasing in size reaching the gastreophageal portion (Fig. 1B).

Fig. 1 Esophagus of the sea-turtle Chelonia mydas. A and B: Photographed open esophagus, conical and pointed keratinous papillae (arrows), facing. Bar 1cm. 

The digestive system of reptiles contains all structures present in other higher vertebrates (^{Putterill & Soley, 2003}); however, morphology is directly related to specific eating habits (^{Silva, 2004}). Corroborating with the morphology found in this study, the esophagus presents a tubular organ that shows papillary corneous papillae, whose main function is to transport the food content from the mouth to the stomach, facilitating swallowing and avoiding alimentary reflux, also to act as filtration devices (^{Porter, 1972}; ^{Parsons & Cameron, 1977}; ^{Work, 2000}; ^{Pressler et al., 2003}; Putterill, & Soley; Silva; Calais Júnior et al.). This characteristic present in the esophagus of sea turtles is unique among reptiles (^{Magalhães et al., 2010}).

The results confirm the presence of these papillae, disappearing at the transition from the esophagus to the stomach in all animals studied (^{Santos et al., 1998}; Putterill & Soley; ^{Elliott, 2007}).

Wyneken and Pressel et al. (2003) described that the esophagus of the green turtle is devoid of glands, indicating that this organ has only mechanical function in this species (Wyneken). Similar findings were described by another author, Santos et al. for the Amazonian turtle (Podocnemis expansa) and for species of the Pelomedusidae (^{George & Castro, 1998}). Against the chelonians later study, which characterizes the presence of glands with a probable muco production (Silva).

Light microscopy revealed that the esophagus is covered by a thick keratin layer of the keratinized pavement epithelial tissue (TEEPQ), where the keratinocytes, prickly stratum and germinative stratum are present. Below the TEEPQ is noted circular connective tissue and dense connective tissue not modeled with presence of blood vessels with nucleated erythrocytes. In the inferior part we noticed the presence of muscular bundles with slices in the transversal and muscular tissue wrapping longitudinally the organ, richly irrigated with blood vessels (Figs. 2A-E).

In the scanning electron microscopy confirmed the findings found in light microscopy observed in Figures 3A-F.

For reptiles, it has generally been described that the esophageal mucosa is lined by epithelium with one or two layers of columnar or cuboidal cells (George & Castro) however, other researchers have described that the Amazonian turtle (Podocnemis expansa) esophagus had coated mucosa by a prismatic stratified epithelium, (Santos et al.) whereas others, in a study carried out in the Pelomedusidae family, described that they had squamous stratified epithelium confirming our studies where the esophageal lining epithelial tissue is stratified, pavement and keratinized, acting to protect the esophageal mucosa against the friction of food passage and mechanical function and avoiding the return of the food while expelling the water (^{Vogt et al., 1998}), corroborating with our findings.

Fig. 2 Microscopy of esophagus of Chelonia mydas. Photomicrography (ML) A-HE, B-HE, C-Masson, D-Mallory, E-Blue Toluidine. A, B, C, D and E shows all the cell layers that make up esophageal tissue: Keratin layer (Q), keratinized pavement epithelial tissue (TEEPQ), spiny stratum (EE), germinative stratum (VC) and nucleated erythrocytes (Eri), transverse muscle (M), and longitudinal muscle (Mu), vasculature (VS). 

Fig. 3 Electron microscopy of the esophagus and papillas. Scanning electron microscopy photomicrograph, A: keratin (Q) layers, red arrow, keratinized pavement epithelial tissue (TEEPQ), stratum spinosus (EE), germinative stratum (EG), with presence of keratinocytes , circular connective tissue (CBT) and dense unshaped connective tissue (CT), blood vessel (VS) with nucleated erythrocytes (Eri), and collagen fibers (CF). B: longitudinal slice of the papilla, C and D: transversal slice of the papilla. E and F: (Arrows) surface of the projection of the dermis and epidermis, forming dermal papillae with conical and pointed, keratin coated. 

CONCLUSION

The esophagus of Chelonia mydas is covered by a layer of keratinized pointed conical dermal papillae important to exert the mechanical and protective function of the mucosa of this organ, considering that the feeding of these animals is extremely abrasive, besides serving as protection so that the food does not return.