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Physiology

Structural Organization of Skeletal Muscle The principal function of skeletal muscles is to move the limbs, trunk, head, respiratory apparatus, and eyes. Most skeletal muscles are under voluntary control. They are composed of long multinucleated cells called muscle fibers, which are derived by fusion of many embryonic cells called myoblasts to form myotubes during development. The ends of the muscle fibers insert into tendons that, in…

Embryology

Development of Musculoskeletal System Evolution The development of the human musculoskeletal system is an interesting demonstration of ontogeny recapitulating phylogeny. The genetic code that guides the continually changing body plan of the developing human results in a résumé of body plans of the various forms of our vertebrate ancestors from which fish, amphibians, reptiles, and mammals evolved. In their adult state, a number of living animals…

GENETICS AND ENDOCRINE NEOPLASIA

MULTIPLE ENDOCRINE NEOPLASIA TYPE 1 Multiple endocrine neoplasia type 1 (MEN 1) is a rare (prevalence ∼two per 100,000) autosomal dominant endocrine disorder that is characterized by neoplasms of the pituitary, parathyroid, and pancreas. In addition, neoplasms may arise in the adrenal glands, duodenum (gastrinoma), lung (carcinoid tumor), thymus gland (carcinoid tumor), and esophagus (leiomyoma). An MEN1 mutation is highly probable in a patient with two…

LIPIDS AND NUTRITION

CHOLESTEROL SYNTHESIS AND METABOLISM Cholesterol is a 4-ring hydrocarbon structure with an 8-carbon side chain. Cholesterol serves as a key component of cell membranes, and it is the substrate for synthesis of steroid hormones and bile acids. Cholesterol is either synthesized endogenously or obtained exogenously by ingestion of animal fats (e.g., meat, eggs, and dairy products). The biosynthesis of cholesterol starts with three molecules of acetate…

BONE AND CALCIUM

HISTOLOGY OF THE NORMAL PARATHYROID GLANDS The parathyroid glands are derived from branchial pouches III and IV and number between two to six glands, although four is the usual number. In adults, each of these ovoid (bean-shaped) glands measures 4 to 6 mm × by 2 to 4 mm × 0.5 to 2 mm and weighs approximately 30 mg (the lower parathyroid glands are generally larger…

PANCREAS

PANCREAS ANATOMY AND HISTOLOGY The pancreas is a retroperitoneal organ that lies in an oblique position, where it slopes upward from the duodenum to the hilum of the spleen. The pancreas is 15 to 20 cm long and weighs 75 to 100 g. The four general regions of the pancreas are the head, neck, body, and tail. The head of the pancreas is located in the…

REPRODUCTION

DIFFERENTIATION OF GONADS FACTORS INFLUENCING NORMAL AND ABNORMAL GONADAL DIFFERENTIATION Whether the primordial gonad differentiates as a testis or as an ovary is determined by genetic information coded on the X and Y chromosomes. The differentiation of all the other anatomic and functional features that distinguish male from female stem secondarily from the effect of testicular or ovarian secretions on their respective primordial structures. The Y…

ADRENAL

Plate 3-1 Open full size image DEVELOPMENT OF THE ADRENAL GLANDS The detailed anatomy of the adrenal glands was first described by Bartholomeo Eustacius in 1563. Each adrenal gland consists of two parts—the cortex and medulla—that are enveloped in a common capsule. The cortex is derived from mesenchymal tissue and the medulla from ectodermal tissue. From the fifth to sixth week of embryogenesis, the cortical portion…

THYROID

ANATOMY OF THE THYROID AND PARATHYROID GLANDS Located between the larynx and the trachea medially and the carotid sheath and the sternomastoid muscles laterally, the thyroid gland weighs 15 to 25 g. The lateral thyroid lobes are 3 to 4 cm long and 1.5 to 2 cm wide; the isthmus is 1.2 to 2 cm long and 2 cm wide and crosses the trachea between the…

PITUITARY AND HYPOTHALAMUS

DEVELOPMENT OF THE PITUITARY GLAND The pituitary gland, also termed the hypophysis , consists of two major components, the adenohypophysis and the neurohypophysis. The adenohypophysis (anterior lobe) is derived from the oral ectoderm, and the neurohypophysis (posterior lobe) is derived from the neural ectoderm of the floor of the forebrain. A pouchlike recess—Rathke pouch—in the ectodermal lining of the roof of the stomodeum is formed by…

Stomach

Development of Stomach and Greater Omentum The foregut begins as a simple, midline, tubular structure lined by epithelium derived from endoderm. While the endoderm creates the lining of the stomach, the visceral mesoderm that surrounds it will form the muscles, connective tissues, and mesenteries that are associated with the organ. The portion of the foregut that will become the stomach first starts to expand in the…

Esophagus

Development of Esophagus The esophagus is the first section of the foregut and begins at the distal end of the pharynx. As with the rest of the digestive tract, the cells that line the lumen of the esophagus are derived from endoderm. The supporting structures of the esophagus come from two different sources, although they are all innervated by the vagus nerve. The muscles and connective…

Mouth and Pharynx

Development of Mouth and Pharynx The lining of the primitive gut tube is derived from the embryonic endoderm and the supporting tissues and mesentery from the visceral layer of lateral plate mesoderm. The amniotic cavity expands around the developing embryo to create the body wall but leaves the endoderm/ectoderm connection at the oropharyngeal membrane and caudally at the cloacal membrane. The early mouth, stomodeum, is formed…

Overview of Upper Digestive Tract

Development of Gastrointestinal Tract We will take a very short tour of early development prior to the trilaminar embryo stage, at which time we will follow the development of the gastrointestinal tract in detail. Thereafter, for each region of the gastrointestinal tract, we will begin with a short summary of the specific embryology relevant to the structures in that region. The single-celled zygote begins dividing roughly…

Colon

Development of Large Intestine The development of the large intestine is intimately tied to the development of other organs. The cecum, appendix, ascending colon, and transverse colon are part of the midgut and develop alongside the jejunum and ileum. The descending colon, sigmoid colon, and rectum are hindgut structures and develop in conjunction with the urogenital system. During normal development, the midgut elongates tremendously and herniates…

Small Bowel

Development of Small Intestine The small intestine includes the duodenum, jejunum, and ileum. During development of the gastrointestinal system, the duodenum comes from the distal portion of the foregut, whereas the jejunum and ileum come entirely from the midgut. The duodenum moves to the right of the midline as the stomach rotates and shifts to the left side of the abdomen during weeks 4 to 6…

Overview of Lower Digestive Tract

Blood Supply of Small and Large Intestines The blood supply to the small and large intestines is extremely variable and unpredictable. The variations concerning the origin, course, anastomoses, and distribution of the intestinal vessels are so frequent and so significant that conventional textbook descriptions are inadequate and, in many respects, even misleading, a situation much the same as that of the blood supply of the upper…

Pancreas

Development of Pancreas The foregut is the first segment of the gut tube within the abdomen. It is attached to the anterior body wall by a ventral/anterior mesentery and posteriorly by a dorsal/posterior mesentery, the latter distributing blood from the dorsal aorta via the celiac arterial trunk. Two diverticula extend from the foregut, one dorsally and the other ventrally. The dorsal pancreatic bud extends into the…

Gallbladder and Bile Ducts

Development of Gallbladder and Bile Ducts The foregut is the first segment of the gut tube within the abdomen. It is attached to the anterior body wall by a ventral/anterior mesentery and posteriorly by a dorsal/posterior mesentery, the latter supplying blood from the dorsal aorta via the celiac arterial trunk. Two diverticula extend from the foregut, one dorsally and the other ventrally. The dorsal pancreatic bud…

Liver

Development of Liver The foregut is the first segment of the gut tube within the abdomen. It is attached to the anterior body wall by a ventral/anterior mesentery and to the posterior wall by a dorsal/posterior mesentery, the latter supplying blood from the dorsal aorta via the celiac arterial trunk. Two diverticula extend from the foregut, one dorsally and the other ventrally. The dorsal pancreatic bud…