Chapter 19: The Endocrine System
Chapter 19: Endocrine System Developmentby John F. Neas
Endocrine glands are ductless glands that secrete their products into the extracellular space around the secretory cells. The hormones pass into capillaries for transport in the bloodstream. The secretions of endocrine glands are hormones, chemicals that regulate various physiological activities. The hormonal messengers circulate throughout the body, passing unnoticed through most tissue but causing dramatic changes when they encounter specific target tissues.
Anatomically, the human endocrine system consists of several distinct glands and less organized tissues in various organs that function within a particular body system. The endocrine system is the only body system whose organs do not have structural connections. Embryologically, the isolated endocrine organs have a separate and independent development in widely separated parts of the embryo. During the prenatal development of epithelial tissue, certain epithelial cells invade the underlying connective tissue and form the specialized secretory exocrine glands. The embryonic development of the endocrine system establishes the anatomical relationships between the endocrine glands and associated structures. Knowledge of the origin and nature of these structural relationships helps in the understanding of endocrine function.
All three embryonic germ layers (endoderm, mesoderm, and ectoderm) contribute to the development of the endocrine glands. All secretory glands, whether exocrine or endocrine, develop from epithelia. Endocrine organs develop from the epithelia that cover the outside of the embryo, that lines the digestive tract, and that lines the coelomic cavity. The hypothalamus, pituitary, and pineal gland develop from ectoderm. (The pineal gland, not discussed below, develops from the ectoderm of the diencephalon as an outgrowth between the thalamus and the superior and inferior colliculi.) The adrenal cortex develops from mesoderm. (The adrenal medulla develops from neural crest). The thyroid gland, parathyroid glands, pancreas, and thymus develop from endoderm. Certain gastrointestinal cells that develop from endoderm secrete hormones (e.g., stomach and intestinal gastrin, secretin, cholecystokinin (CCK), enterocrinin, and gastric inhibitory peptide (GIP) that aid digestion.
The ovaries are the female gonads that produce estrogen within the ovarian follicles and corpus luteum; estrogen is related to development and maintenance of female sexual characteristics, menstrual cycle, pregnancy, and lactation. The testes are the male gonads that produce the male sex hormone testosterone within the interstitial cells; testosterone is related to the development and maintenance of male sexual characteristics. The origin of the ovaries and testes is discussed in Chapter 27 (Reproductive System).
The sections that follow will describe the development of the pituitary, thyroid, parathyroids, thymus, pancreas, and adrenal glands. The development of the other endocrine organs is described in other chapters. That is, the development of the (1) skin, (2) atria, (3) stomach, intestines, liver, and pancreas, (4) kidneys, and (5) placenta is described in Chapters 4, 21, 25, 26, and 28, respectively.
Pituitary Gland (Hypophysis)
The pituitary gland, or hypophysis, is a single organ located in the sella turcica, but it consists of two distinct types of tissues that have different embryonic origins from different regions of ectoderm, that secrete different hormones, and that are under different control systems. The anterior glandular portion, or adenohypophysis, develops from the ectoderm that lines the primitive oral cavity. The posterior nervous portion, or neurohypophysis, develops from the neuroectoderm of the developing brain. The pars intermedia is the avascular zone between the anterior and posterior lobes.
The neurohypophysis is an extension of the brain and, like the brain, meninges surround the entire pituitary gland. The adenohypophysis, in contrast, develops from the same non-neural embryonic tissue that will form the epithelium over the roof of the mouth. These different embyrologies have important consequences concerning the function of the two parts of the pituitary gland.
The neurohypophysis (posterior lobe) develops as a neurohypophyseal bud, a depression in the neuroectoderm of the floor of the hypothalamus. The infundibulum, another diverticulum from the inferior aspect of the diencephalon, develops as an outgrowth of the neurohypophyseal bud and connects the neurohypophysis to the hypothalamus.
The neurohypophysis develops as the infundibulum grows inferiorly from the diencephalon to meet the developing adenohypophysis. The fully developed neurohypophysis consists of the infundibulum and the pars nervosa. The neural connection between the hypothalamus and pars nervosa is through the hypothalamohypophyseal tract that develops within the infundibulum
Hormones synthesized by the hypothalamus and stored in the neurohypophysis include oxytocin (OT) and antidiuretic hormone (ADH). Injections of oxytocin may be given to a woman during labor if she is having difficulty in parturition. Increased amounts of oxytocin assist uterine contractions and generally accelerate delivery. Oxytocin administration after parturition causes the uterus to regress in size and constricts the blood vessels, thus minimizing the danger of hemorrhage.
The adenohypophysis (anterior lobe) begins to develop during the third week as a pouch-like outgrowth of ectoderm, called the hypophyseal (Rathkes) pouch, from the dorsal midline roof of the stomodeum (primitive oral cavity). The hypophyseal pouch grows toward the brain and the neurohypophyseal bud. As the hypophyseal pouch and the infundibulum make contact, the hypophyseal pouch loses its connection with the pharynx, creating a hollow ball of cells that lies inferior to the floor of the diencephalon posterior to the optic chiasm. These cells undergo division, the central chamber gradually disappears, and this endocrine mass becomes the anterior pituitary gland. The fully developed adenohypophysis has a glandular pars distalis, a thin proximal extension called the pars tuberalis, and a narrow pars intermedia.
The border in the mouth between ectoderm and endoderm lies along the line formed by the circumvallate papillae of the tongue. This boundary approximately corresponds to the middle of the mandibular (first) arch, or at the level of the second pair of pharyngeal pouches. The thyroid gland develops here in the ventral midline, and the pituitary gland forms in the dorsal midline.
The thyroid gland begins its development during the third week as a thickening in endoderm in the midventral floor of the primitive pharynx. The thickening soon invaginates inferiorly as the thyroid diverticulum and differentiates into the right and left lateral lobes and the isthmus of the gland. As the embryo grows and changes shape, the thyroid shifts caudally to a position near the thyroid cartilage of the larynx. On its way, the thyroid gland incorporates C cells from the walls of the fifth pharyngeal pouch. As the thyroid diverticulum branches slightly, its walls thicken, and the paired masses lose their connection with the surface.
A narrow thyroglossal duct connects the descending primordial thyroid gland to the pharynx. As descent continues, the tongue begins its development, and the opening into the thyroglossal duct, called the foramen cecum, penetrates the base of the tongue. By the seventh week, the thyroid gland lies immediately inferior to the larynx and around the front and lateral sides of the trachea. Once the thyroid gland is in its final position, the thyroglossal duct disappears and the foramen cecum regresses to a vestigial pit that persists throughout life.
Parathyroid Glands and Thymus
The pharyngeal region of the embryo plays a particularly important role in the development of the endocrine glands. The pharyngeal arches are well formed after four to five weeks of development. Human embryos develop five or six pharyngeal arches, but the fifth arch may not appear or may develop and degenerate almost immediately. Deep ectodermal grooves, or pharyngeal clefts, separate the five major arches (I-IV, VI).
Five pharyngeal pouches extend laterally toward the pharyngeal grooves. The first pharyngeal pouch lies caudal to the first (mandibular) arch. The fifth and sixth pharyngeal pouches are very small and interconnected. The endoderm that lines the third, fourth, and fifth pairs of pharyngeal pouches forms dorsal and ventral masses of cells that migrate beneath the endodermal epithelium. The dorsal masses of the third and fourth pharyngeal pouches become the parathyroid glands. There are usually four small parathyroid glands embedded on the posterior surface of the thyroid gland. Cells that originate in the walls of the small fifth pharyngeal pouch become incorporated into the thyroid gland, where they differentiate into C cells.
The ventral masses from the third pharyngeal pouches move toward the midline and fuse to produce the thymus gland.
The pancreas begins development during the fifth week from dorsal and ventral outgrowths of endoderm that arise from the caudal portion of the foregut, the region that later becomes the duodenum. These primordial pancreatic buds grow independently until the ventral bud is carried dorsally and fuses with the dorsal bud as the duodenum rotates to the right. The fusion of the two portions to form the pancreas occurs during the seventh week.
The pancreas thus develops from and maintains connections with the intestine through the pancreatic duct. The exocrine products of the pancreas, carried in pancreatic juice, flow through the pancreatic duct to the intestine. The islets of Langerhans, the endocrine parts of the pancreas, excrete their products (insulin and glucagon) into the blood. The exact origin of the endocrine cells of the pancreas is uncertain. Some of the endocrine cells possibly develop as buds from pancreatic ductules, whereas others arise from neuroectodermal cells that migrate to the pancreas to form these endocrine cells and the autonomic innervation of the pancreas.
The adrenal glands, positioned along the superior border of the kidneys, begin development during the fifth week. Like the pituitary, the adrenal glands consist of two different endocrine tissues that are located in the same organ but secrete different hormones and are regulated by different control systems; part is neural and part is not. Each adrenal gland has an outer cortex that develops from mesoderm, and an inner medulla that develops from neuroectoderm. The mesodermal ridge that produces the adrenal cortex is in the same region that produces the gonads.
Shortly after formation of the neural tube, neural crest cells migrate from the central nervous system. This migration leads to the formation of the dorsal root ganglia and sympathetic ganglia. On each side of the coelomic cavity, neural crest cells aggregate in a mass that will become the adrenal medulla. The overlying intermediate mesoderm from the same mesodermal ridge that produces the gonads responds by undergoing division, and the daughter cells surround the neural crest cells to form the thick adrenal cortex. The developing adrenal cortex gradually encapsulates the adrenal medulla, a process that continues into the fetal stage. The formation of the adrenal gland is not completed until the end of the third year of age.