The Adrenal Gland of Squamata (Reptilia): A Comparative Overview

SIMPLE SUMMARY: The adrenal gland plays a fundamental role in the physiology of vertebrates, regulating hydrosaline balance, carbohydrate and lipid metabolism, and stress response. This enables living vertebrates to deal with both internal and environmental stress stimuli. The adrenal gland consists of steroidogenic tissue, which synthesizes steroid hormones, and chromaffin tissue, which mainly produces catecholamines, norepinephrine, and epinephrine. The degree of separation between the two types of tissue, the compactness of the adrenal gland, and its topographical relationships with the kidney have changed over the course of evolutionary history. In most reptiles, the glands have close relationships with the gonads and genital ducts. In the main order of Reptilia called Squamata (e.g., lizards and snakes), the gland shows extreme variability in the organization between chromaffin and steroidogenic tissues and in the numerical ratio between norepinephrine and epinephrine cells. This variability reflects the relationship between the two adrenal tissues, which, in turn, could be related to the phylogenetic history of the species. This overview examines the general characteristics of the adrenal glands of the squamates and the different structural patterns in the different species belonging to this order. ABSTRACT: The adrenal gland is a complex endocrine organ composed of two components: a steroidogenic tissue, which produces steroid hormones, and a chromaffin tissue, which mainly produces norepinephrine and epinephrine. Through evolution, their relationships with each other changed. They begin as isolated chromaffin and steroidogenic cell aggregates, typical of fish, and end with the advanced compact gland, typical of mammals, which consists of an external steroidogenic cortical zone and an internal chromaffin medullary zone. The adrenal gland of reptiles is unique because, with few exceptions, it is near the gonads and genital ducts, and the chromaffin and steroidogenic tissues are closely associated. However, the degree of mixing is variable. For example, in Squamata, the mixing degree of chromaffin and steroidogenic tissues, their reciprocal position in the gland, and the relative quantities of norepinephrine and epinephrine secreted by the chromaffin cells are extremely variable. This variability could be related to the phylogenetic history of the species. After a brief discussion of the adrenal gland and its main functions in vertebrates, this overview will examine the general characteristics of the adrenal gland of squamates, the differences in morphology of the gland, and the possible relationships with the phylogeny of the different species.