THYROID GLAND Development of the thyroid gland The thyroid gland (Greek thyreos , shield; eidos , form) develops as a median endodermal downgrowth at the base of the tongue. A transient structure, the thyroglossal duct , connects the developing gland to its point of origin, the foramen cecum , at the back of the tongue. The thyroglossal duct disappears completely, leaving the thyroid to develop as…

HYPOPHYSIS ( 18-1 ) The hypophysis (Greek hypo , under; physis , growth), also known as the pituitary gland , consists of two embryologically distinct tissues: 1. The adenohypophysis , the glandular epithelial portion. 2. The neurohypophysis , the neural portion. The adenohypophysis is formed by three subdivisions or parts: 1. The pars distalis , or anterior lobe , is the main part of the gland.…

STRUCTURE OF A SALIVARY GLAND ( 17-1 and 17-2 ) We start the discussion by reviewing the general organization and function of a salivary gland, in particular its branching ducts (see Box 17-A ). An initial discussion concerning the general features of a compound or branched gland is included in Chapter 2, Epithelial Glands | Cell Biology. Box 17-A Classification of exocrine glands Depending on the…

SMALL INTESTINE ( 16-1 to 16-3 ) The 4- to 7-meter-long small intestine is divided into three consecutive segments: 1. Duodenum . 2. Jejunum . 3. Ileum . The duodenum is about 25 cm in length, is mainly retroperitoneal and surrounds the head of the pancreas. At its distal end, the duodenum is continuous with the jejunum, a movable intestinal segment suspended by a mesentery. The…

Mouth (oral cavity) The oral cavity is the entrance to the digestive tube. Ingestion, partial digestion and lubrication of the food, or bolus, are the main functions of the mouth and its associated salivary glands. We study the salivary glands in Chapter 17 , Digestive Glands. The mouth, or oral cavity, includes the lips, cheeks, teeth, gums (gingivae), tongue, uvula and palate. The various regions of…

THE KIDNEYS ( 14-1 ) The urinary system consists of paired kidneys and ureters and a single urinary bladder and urethra. Each kidneys has a cortex (subdivided into outer cortex and juxtamedullary cortex) and a medulla (subdivided into outer medulla and inner medulla) . The medulla is formed by conical masses, the renal medullary pyramids , with their bases located at the corticomedullary junction. A renal…

THE RESPIRATORY SYSTEM Nasal cavities and paranasal sinuses ( 13-1 ) The nasal cavities and paranasal sinuses provide an extensive surface area for: 1. Warming and moistening air. 2. Filtering dust particles present in the inspired air. In addition, the roof of each nasal cavity and part of the superior concha contain the specialized olfactory mucosa . Each nasal cavity, separated from the other by the…

THE CARDIOVASCULAR SYSTEM ( 12-1 ) The circulation is divided into the systemic or peripheral circulation and the pulmonary circulation . Arteries transport blood under high pressure and their muscular walls are thick. The veins are conduits for transport of blood from tissues back to the heart. The pressure in the venous system is very low and the walls of the veins are thin. There are…

ORGANIZATION AND TYPES OF SKIN ( 11-1 ) The integument is the largest organ of the body. It consists of two components: 1. The skin . 2. The epidermal derivatives , such as nails, hair and glands (sweat and sebaceous glands and the mammary gland). The skin has several functions: 1. Protection from injury (mechanical function). 2. As a water barrier to prevent dehydration. 3. Regulation…

COMPONENTS OF THE IMMUNE–LYMPHATIC SYSTEM ( 10-1 and 10-2 ) The lymphatic system includes primary and secondary lymphoid organs . The primary lymphoid organs produce the cell components of the immune system (see 10-1 ). They are: 1. The bone marrow . 2. The thymus . The secondary lymphoid organs are the sites where immune responses occur. They include: 1. The lymph nodes . 2. The…

EYE ( 9-1 ) The eyeball consists of three tunics or layers , which, from outside to inside, are: 1. The sclera and the cornea . 2. The uvea . 3. The retina . Three distinct and interconnected chambers are found inside the eyeball: the anterior chamber , the posterior chamber and the vitreous cavity. Aqueous humor circulates from the posterior to the anterior chamber. The…

Development of the nervous system ( 8-1 and 8-2 ) The central nervous system (CNS) develops from the primitive ectoderm (see 8-1 ; see Box 8-A and Box 8-B ). A simple epithelial disk, the neural plate , rapidly rolls into a hollow cylinder, the neural tube . This process is known as neurulation . Box 8-A Ectoderm germ cell layer The ectoderm germ cell layer…

SKELETAL MUSCLE ( 7-1 ) Muscle cells or fibers form a long multinucleated syncytium grouped in bundles surrounded by connective tissue sheaths and extending from the site of origin to their insertion. The epimysium is a dense connective tissue layer ensheathing the entire muscle . The perimysium derives from the epimysium and surrounds bundles or fascicles of muscle cells. The endomysium is a delicate layer of…

BLOOD Blood consists of cells and plasma . These components may be separated by centrifugation when blood is collected in the presence of anticoagulants. The sedimented red blood cells (RBCs) constitute about 42%–47% of blood volume. This percentage of erythrocyte volume is the hematocrit (Greek, haima , blood; krino , to separate). Sitting on top of the erythrocyte layer is the buffy coat layer, which contains…

OSTEOGENESIS (Bone development or ossification) Bone can develop either directly from an initial cell condensation of the mesenchyme (intramembranous ossification) or by gradual replacement of a pre-existing tissue, the cartilage, which acts as a template (endochondral ossification; bone formation inside cartilage). The initial mechanism of bone formation during intramembranous and endochondral ossification is essentially the same: An initial trabecular network, called primary spongiosa, is first laid…

Classification ( 4-1 to 4-3 ) Unlike epithelial cells, which are almost free of intercellular material, connective tissue cells are widely separated by components of the extracellular matrix (ECM). In addition, epithelial cells lack direct blood and lymphatic supply, whereas connective tissue is directly supplied by blood and lymphatic vessels and nerves. The classification of the connective tissue into specific types is based on the proportion…

CELL SIGNALING MECHANISMS An understanding of the molecular aspects of cell signaling is clinically relevant to uncover potential targets of novel therapeutics for the treatment of metabolic dysfunctions and diseases. Signaling molecules can transmit information by acting as ligands , binding to receptors expressed by their target cells. Some signaling molecules can act on the cell surface after binding to cell surface receptors. Others can cross…

EPITHELIAL GLANDS Types of epithelial glands ( 2-1 ) Most glands develop as an epithelial downgrowth extending into the underlying connective tissue. Exocrine glands remain connected to the surface of the epithelium by an excretory duct that transports the secretory product to the outside. Endocrine glands lack an excretory duct , and their product is released into the blood circulation. Endocrine glands are surrounded by fenestrated…

CLASSIFICATION OF EPITHELIA ( 1-1 to 1-4 ) The epithelium is a tightly cohesive sheet of cells that covers or lines body surfaces and forms the functional units of secretory glands . The main characteristics of epithelia are summarized in Box 1-A . Box 1-A General characteristics of epithelia Epithelia derive from the ectoderm, mesoderm and endoderm. Epithelia line and cover all body surfaces except the…