ORIGINAL ARTICLE Prenatal Development of the Adrenal Gland in the One-Humped Camel (Camelus dromedarius) S. M. El-Nahla 1 *, H. M. Imam 1 , E. A. Moussa 1 , A. K. Elsayed 1 and L. C. Abbott 2 Addresses of authors: 1 Department of Anatomy and Embryology, Faculty of Veterinary Medicine, Suez Canal University, Ismailia, Egypt; 2 Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX, USA Introduction The adrenal glands (Glandulae adrenales) are critically important endocrine organs that actually function as two different glands because each adrenal gland is divided into a medulla and cortex, which have divergent cellular and functional properties. The adrenal cortex is subdivided into three distinct layers that produce a complex array of steroid hormones, including glucocorticoids, mineralocor- ticoids and accessory sex hormones, while cells in the adrenal medulla are separated into two cell types, one that produces epinephrine and the other that produces norepinephrine (Bone, 1982; Vrezas et al., 2004; Kempna and Flu ¨ck, 2008). Lack of normal adrenal gland function will result in disruption of electrolyte levels and carbohy- drate metabolism, which may lead to circulatory collapse, hypoglycaemic coma and even death (Kempna and Flu ¨ck, 2008). Adrenalectomized animals usually do not live longer than 2 weeks (Frandson, 1970). The importance of adrenal gland function is also evi- dent in prenatal development as the steroid hormones produced by the fetal adrenal cortex promote maturation of many fetal organ systems, including lung, liver, thyroid and gastrointestinal tract (Liggins, 1976). In some species, including sheep, goat and rabbit, cortisol, which is pro- duced by the fetal adrenal glands, regulates the timing of parturition (Liggins et al., 1973; Liggins, 1980; Mesiano and Jaffe, 1997). Prenatal adrenal gland development has been described in numerous published reports covering a wide variety of species, including the ox (Katznelson, 1966; Wrobel and Suss, 1999), sheep (Davies, 1950; Wintour et al., 1975; Upadhyay and Zamboni, 1982; Naaman-Reperant and Durand, 1997; Grino, 2004), swine (Sokolov et al., 2006) *Correspondence: Tel: 002 0123600646; fax: 002 0643207052; e-mail: sanaaelnahla@yahoo.com With 14 figures and 3 tables Received March 2010; accepted for publication October 2010 doi: 10.1111/j.1439-0264.2010.01056.x Summary Each adrenal gland consisted of cortex and medulla that developed from differ- ent embryological origins and presented different cellular organization. One hundred male or female camel embryos or fetuses with crown vertebral rump lengths (CVRL) that ranged from 0.8 to 117 cm were examined. The adrenal cortex, which is derived from intermediate mesoderm, was first observed in the 0.8-cm CVRL camel embryo. The adrenal cortex initially was combined with the gonad as a thickened region of proliferating cells derived from splanchnic intermediate mesoderm. Adrenocortical tissue was first separated from the gonadal tissue in the 2-cm CVRL camel fetus and was observed as a separate dorso-medial mass of cells. At 2.5-cm CVRL, the adrenocortical tissue was sur- rounded by a capsule of undifferentiated mesenchymal cells, except at its proxi- mal pole, where an invagination was located through which chromaffinoblast cells entered the cortex. The chromaffinoblast cells migrated from the neural crest to form the medulla of the developing adrenal gland. In the 3.5-cm CVRL camel fetus, the adrenocortical cells differentiated into two layers: the inner fetal cortex and the outer definitive cortex. As development proceeded, the fetal cortex degenerated and the definitive cortex formed the zona glomerulosa and zona fasciculata. The zona reticularis did not form until the end of gestation. During prenatal life, the adrenal medulla was much thicker than the cortex. Anatomia Histologia Embryologia ª 2010 Blackwell Verlag GmbH • Anat. Histol. Embryol. 40 (2011) 169–186 169