General Properties of the BBB 5 страница

In addition to the two cell types, the human pineal gland is characterized by the presence of calcified concretions, called brain sand.

ADRENAL GLANDS

The adrenal (suprarenal) glands secrete both steroid hormones and catecholamines. They have a flattened tri­angular shape and are embedded in the perirenal fat at the superior poles of the kidneys.

The glands are covered with a thick connective tissue capsule from which trabeculae extend into the parenchyma, carrying blood vessels and nerves. The secretory parenchymal tissue is organized in cortical and medullary regions:

The cortex is the steroid-secreting portion. It lies be­neath the capsule and constitutes nearly 90% of the gland by weight.

The medulla is the catecholamine-secreting portion. It lies deep to the cortex and forms the center of the gland.

Parenchymal Cells of the Cortex and Medulla Are of Different Embryologic Origin

Embryologically, the cortical cells originate from mesodermal mesenchyme, whereas the medulla originates from neural crest cells that migrate into the developing gland.

Blood Supply

The adrenal glands are supplied with blood by the su­perior, middle, and inferior adrenal arteries. In the capsule, the ar­teries branch to give rise to three principal patterns of blood distribution.

Zonation of the Adrenal Cortex

Figure 42. Diagram illustrating the organization of the cells within the adrenal gland and their relationship to the blood vessels

The adrenal cortex is divided into three zones on the ba­sis of the arrangement of its parenchymal cells. The zones are designated

Zona glomerulosa, the narrow outer zone

Zona fasciculata, the thick middle zone

Zona reticularis, the inner zone

Zona Glomerulosa

The cells are relatively small columnar or py­ramidal shaped. In the human, some areas of the cortex may lack a recognizable zona glomerulosa.

The Zona Glomerulosa Secretes Aldosterone That Functions in the Control of Blood Pressure

The cells of the zona glomerulosa secrete mineralocorticoids, compounds that function in the controlling electrolyte homeostasis (act in distal tubule cells of kidney to increase sodium resorption and decrease potassium resorption) and also maintaining the osmotic balance in the urine. The principal secretion is aldosterone (95% of mineralocorticoids).

Zona Fasciculata

The cells of the zona fasciculata are large and polyhedral. The cells of the zona fasciculata have a lightly staining spherical nucleus. The generally acidophilic cytoplasm contains numerous lipid droplets. The lipid droplets contain neutral fats, fatty acids, cholesterol, and phospholipids that are precursors for the steroid hormones secreted by these cells. The cells have highly developed sER and mitochondria with tubular cristae. They also have a well-developed Golgi complex and numerous profiles of rER.

The Principal Secretion of the Zona Fasciculata Is Glucocorticoids That Regulate Glucose and Fatty Acid Metabolism

The zona fasciculata secretes glucocorticoids, so called because of their role in regulating gluconeogenesis and glycogenesis. The most important of the glucocorticoids secreted by the zona fasciculata is hydrocortisone (cortisol). Also they secrete cortisone and corticosteron. This compound acts on many different cells and tissues to in­crease the metabolic availability of glucose and fatty acids.

Glucocorticoids depress the immune response and the in­flammatory response and, as a result of the latter, inhibit wound healing. They depress the inflammatory response by inhibiting macrophage recruitment and migration. They also stimulate destruction of lymphocytes in lymph nodes and inhibit mitosis by transformed lymphoblasts. They also provide resistance to stress and suppress some allergic reaction.

Figure 43. Photomicrograph of the adrenal gland

Zona Reticularis

The cells of the zona reticularis are noticeably smaller than those of the zona fasciculata, and their nuclei are more deeply stained. They are arranged in anastomosing cords, separated by fenestrated capillaries. The cells have relatively few lipid droplets. Both light and dark cells are seen. They have a well-developed sER and numer­ous mitochondria with tubular cristae, but they have little rER.

The Principal Secretions of the Zona Reticularis Are Weak Androgens and Glucocorticoids

The principal secretion of the cells in the zona reticularis consists of weak androgens, mostly dehydroepiandrosterone (DHA). The cells also secrete some glucocorticoids, in much smaller amounts than those of the zona fasciculata. Here, too, the principal glucocorticoid secreted is hydrocortisone. The zona reticularis is also under feedback control of the CRF-ACTH system.

Adrenal Medulla

Adrenal Medullary Cells Are Modified Postganglionic Neuronal Cells That Have a Secretory Function

The central portion of the adrenal gland, the medulla, is composed of a parenchyma of large, epithelioid cells, called chromaffin cells, connective tissue, nu­merous sinusoidal blood capillaries, and nerves.

The catecholamines epinephrine (80%) and nor-epinephrine (20%) secreted by the medullary cells are produced by different cell types. Two populations of cells are distinguished by the nature of the mem­brane-bounded granules.

One population of cells contains only large dense core granules. These cells secrete norepinephrine. The other population of cells contains granules that art smaller, more homogeneous, and less dense. These сеlls secrete epinephrine.

Catecholamines have sympathomimetic functions (increases heart rate, blood pressure, sweating, rate of respiration and decreases blood flow to viscera and skin, digestion, urine production and others.

Thyroid gland

Figure 44. Thyroid gland

The thyroid is a bilobed endocrine gland located in the neck, anterolateral to the larynx and upper trachea.

The lobes are connected by a thin band of thyroid tissue, the isthmus that crosses anterior to the upper part the trachea. A thin connective tissue capsule surrounds

the gland. It sends trabeculae into the parenchyma to partially outline irregular lobes and lobules. Secretory follicles constitute the functional units of the gland.

Thyroid gland function is essential to normal growth and development.

The thyroid gland produces three hormones, each of which is essential to normal metabolism and homeostasis.

-Thyroxin (tetroidothyronine T₄) and triidothyronin T₃ regulate cell and tissue metabolism, influences body and tissue growth and development of the nervous system in the fetus and young child.

-Calcitonin regulates blood calcium level.

Follicular cells

The follicle is the structural unit of the thyroid gland. A follicle is a spheroidal cyst-like compartment with a wall formed by a simple squamous or cuboidal epithelium, the follicular epithelium.

The lumina of the follicles are filled with a gel-like mass called colloid. The apical surfaces of the follicular epithelial cells are in contact with the colloid, and the basal surfaces rest on a typical basal lamina.

Two basic cell types are present in the follicles principal or follicular cells and parafollicular or C cells.

Follicular cells secrete T₄ and T₃. Parafollicular cells secrete calcitonin.

The follicles are surrounded by an extensive network of fenestrated capillaries.

Lymphatic capillaries are present in the interfollicular connective tissue and may also provide a second route for conveying the hormones from the gland.

The nuclei of follicular epithelial cells are spherical. In the basophilic cytoplasm presents Golgi complex and lipid droplets.

Figure 45. Diagram of steps in thyroid hormone synthesis

The active cuboidal or columnar cells reveal the presence of organelles commonly associated with both secretory and absorptive cells. The organelles include typical junctional complexes at the apical end of the lateral plasma membrane, short microvilli on the apical of the cells, rough surfaced endoplasmic reticulum in the basal region, supranuclear Golgi complex, numerous small vesicles in the apical cytoplasm, that are morphologically similar to the vesicles associated with the Golgi complex, abundant lysosomes and multivesicular bodies, and membrane limited vesicles, identified as colloidal resorption droplets in the apical region. Colloid contains thyroglobulin, the inactive storage form of the thyroid hormones.

The protein portion of thyroglobulin is synthesized in the rER of the follicular epithelial cells and glycosylated there and in the Golgi complex before it is secretes by exocytosis into the lumen of the follicle.

The follicular epithelial cells actively transport iodide from the blood into their cytoplasm. The iodide is oxidized to iodine by thyroperoxidase in the cytoplasm. It is then released into the follicular lumen where iodination of some tyrosine residues of the thyroglobulin occurs near the microvilli of the apical cell surface.

The synthesis, storage and release of thyroid hormones are controlled by thyroid-stimulating hormone from the adenohypophysis.

The thyroid hormones are formed by the coupling of two iodinated tyrosine residues to produce thyroxine (T₄ and T₃).

These active thyroid hormones (in T₄ : T₃ ratio of 20: 1) cross the basal membrane and enter blood and lymphatic capillaries.

Parafollicular cells

Parafollicular cells are present within the follicular epithelium or are scattered in the connective tissue.

The parafollicular cells (C cells, calcitonin cells) are the second type, of endocrine cell found in the thyroid. In the follicles they are located near the basal lamina and do not extend to the follicular lumen to the contact the colloid. They have numerous, small, membrane-bounded secretory granules.

Parafollicular cells produce calcitonin, a hormone that lowers blood calcium level.

Calcitonin (thyrocalcitonin) is the physiologic antagonist to parathyroid hormone.

Calcitonin lowers blood calcium levels by suppressing bone resorption and increasing the rate of osteoid calcification. Secretion of calcitonin is regulated directly by blood calcium levels. High levels of calcium stimulate secretion, low levels inhibit it.

Parathyroid glands

The parathyroid glands are small endocrine glands closely associated with the thyroid. They are ovoid, a few millimeters in diameter, and arranged in two pairs, to comprise the superior and inferior parathyroid glands. They are usually located in the connective tissue on the posterior sur­face of the thyroid.

Development and Structure

Embryologically, the inferior parathyroid glands (and the thymus) derive from the third branchial pouch; the superior glands, from the fourth branchial pouch.

Structurally, each parathyroid gland is surrounded by a thin connective tissue capsule that separates it from the thy­roid. Septa extend from the capsule into the gland to divide it into lobules and to separate the densely packed cords of cells.

Principal (Chief) Cells and Oxyphil Cells Constitute the Epithelial Cells of the Parathyroid Gland

Principal cells, the more numerous of the parenchymal cells of the parathyroid, are responsible for the secretion of parathyroid hormone (PTH). The cytoplasm contains small granules with PTH. They are small, polygonal cell.

Oxyphil cells constitute a minor portion of the parenchymal cells. They are more rounded and larger than the principal cells.

Function

The parathyroids function in the regulation of blood ion levels of calcium and phosphate. Parathyroid hormone is essential for life.

Parathyroid Hormone Regulates Calcium and Phosphate Levels in the Blood

The parathyroids produce PTH, also known as parathormone. Release of this hormone causes the level of calcium in the blood to in­crease. Simultaneously, it reduces the concentration of phosphate. Bone resorption is stimulated by PTH. During osteolysis, calcium and phosphate are both re­leased from calcified bone matrix into the extracellular fluid.

Parathyroid Hormone and Calcitonin Have Reciprocal Effects in the Regulation of Blood Calcium Levels

Digestive system

The digestive system consists of alimentary canal and its associated organs, namely, the tongue, teeth, salivary glands, pancreas, liver and gallbladder.

The alimentary mucosa is the surface across which most substances enter the body.

The alimentary mucosa has numerous functions in its role as an interface between the body and the environment. These include its

1. Barrier function: The mucosa serves as a barrier to the entry of noxious substances, antigens, and pathogenic organisms.

2. Immunologic function: Lymphatic tissue in the mucosa serves as a first line of defense of the body by the im­mune system.

3. Secretory function: The lining of the alimentary canal secretes, at specific sites, digestive enzymes, hydro­chloric acid, mucin, and antibodies.

4. Absorptive function: The epithelium of the mucosaabsorbs metabolic substrates, i.e., the breakdown products of digestion, as well as vitamins, water, electrolytes, and other substances.

ORAL CAVITY

The oral cavity consists of the mouth and its contents, i.e., the tongue, teeth, salivary glands, and tonsils.

Tongue

The tongue is a muscular organ. The striated muscles of the tongue are arranged in bundles that generally run in three planes and usually separated by connective tissue.

The dorsal surface of the tongue is divided into an anterior two-thirds and a posterior one-third by a V shaped depression, the sulcus terminalis.

Papillae cover the dorsal surface of the anterior portion of the tongue. There are four types:

-filiform

-fungiform

-circumvallate

-foliate

Filiform papillae are the most numerous in humans and the smallest. They are conical, elongated projections of connective tissue that are covered with partly keratinized stratified squamous epithelium.

Fungiform papillae are mushroom shaped projections located on the dorsal surface of the tongue. They are more numerous near the tip of the tongue. Taste buds are present on the dorsal surface of these papillae.

Circumvallate papillae are the large, dome-shaped structures that reside in the mucosa just anterior to the sulcus terminalis. Each papilla is surrounded by a moat-like invagination lined with stratified squamous epithelium that contains numerous taste buds.

Foliate papillae occur on the lateral edge of the tongue in aged humans; the foliate papillae may not be recognized. In younger individuals, they are easily found and contain many taste buds in the epithelium.

Lingual tonsils

Lingual tonsils are located in the lamina propria of the root or base of the tongue. They are found posterior to the sulcus terminalis. The lingual tonsils contain lymphatic nodules. Epithelial crypts usually invaginate into the lingual tonsil.

Gingiva

Figure 46. Schematic diagram of gingiva

The gingival is the part of the mucous membrane commonly called the gums. The gingival is firmly attached to the teeth and underlying bony tissue. It is composed of stratified squamous epithelium and numerous connective tissue papillae. This epithelium is bound to the tooth enamel by means of a cuticle that resembles a thick basal lamina and forms the epithelial attachment of Gottlieb. Between the enamel and the epithelium is the gingival crevice a small deepening surrounding the crown.

Teeth and supporting tissues

Teeth are a major component of the oral cavity and are essential for the digestive process. Teeth are embedded in and attached to the maxilla and mandible. A child has 10 deciduous teeth in each jaw, consisting, on each side of

• A central incisor

• A lateral incisor

• A canine tooth

• Two molar teeth

Figure 47. Diagram of a section through an incisor tooth and surrounding bony and mucosal structures

Over a period of years, usually beginning at about age 6, these are gradually replaced by 16 permanent teeth in each jaw, consisting, on each side, of

• A central incisor

• A lateral incisor

• A canine

• Two premolar teeth

• Three molar teeth

Incisors and canines have one root each, premolars usu­ally have two, and molars may have three or four roots. All teeth have the same basic structure.

The adult tooth consists of four distinct structural components, the enamel and the cementum on the outside, the dentin beneath them, and the pulp in a central pulp cavity.

Enamel

Enamel covers the crown of the tooth. That part of the crown that is exposed and visible above the gum line is called the clinical crown; the anatomical crown describes all of the tooth that is covered by enamel, some of which is below the gum line. Enamel varies in thickness over the crown and may be as thick as 2.5 mm on the cusps (biting and grinding surfaces) of some teeth. The enamel layer ends at the neck or cervix of the tooth at the cementoenamel junction; the root of the tooth is then covered by cementum.

Enamel Is the Hardest Substance in the Body;

Figure 48. Diagram showing the basic organization and structure of enamel rods

It Consists of 96-98% Hydroxyapatite. The hydroxyapatite of the enamel is ar­ranged in rods or prisms that vary in diameter from 4-8 µm. The structural and functional unit of the tooth is rods or prisms. Each enamel rod spans the full thickness of the enamel layer from the dentinoenamel junction to the enamel surface; the spaces between the rods are also filled with hydroxyapatite. Striations observed on enamel rods (contour lines of Retzius) may represent evidence of rhythmic growth of the enamel in the developing tooth. The enamel in a mature tooth is acellular and nonreplaceable.

Although the enamel of an erupted tooth is devoid of cells and cell processes, it is not a static tissue. It is under the influence of substances in saliva, the secretion of the salivary glands, that are essential to its maintenance. The substances in saliva that affect teeth include

• Digestive enzymes

• Antibacterial enzymes

• Antibodies

• Inorganic (mineral) components

Mature enamel contains very little organic material. De­spite its hardness, enamel can be damaged by decalcification by acid-producing bacteria acting on food products trapped on the surface of the enamel. This is the basis of the initiation of dental caries.

Enamel Formation (Amelogenesis)

Enamel is produced by ameloblasts with the close cooperation of other enamel organ cells

Figure 49. Photomicrograph showing the developing crown of an incisor

The major stages of amelogenesis are the period of matrix production, or the secretory stage, and the period of maturation. In the formation of mineralized tis­sues of the tooth, dentin is produced first. Then, partially mineralized enamel matrix is deposited di­rectly on the surface of the previously formed dentin. The cells producing this partially mineralized organic matrix are referred to as secretory ameloblasts. The major stages of amelogenesis are the period of matrix production, or the secretory stage, and the period of maturation. In the formation of mineralized tis­sues of the tooth, dentin is produced first. Then, partially mineralized enamel matrix is deposited di­rectly on the surface of the previously formed dentin. The cells producing this partially mineralized organic matrix are referred to as secretory ameloblasts. As do osteoblasts in bone, these cells produce an organic proteinaceous matrix by activity of the rough endoplasmic reticulum (rER), Golgi apparatus, and secretory granules.

Figure 50. Schematic diagram of a partially formed tooth showing details of amelogenesis

The major stages of amelogenesis are the period of matrix production, or the secretory stage, and the period of maturation. In the formation of mineralized tis­sues of the tooth, dentin is produced first. Then, partially mineralized enamel matrix is deposited di­rectly on the surface of the previously formed dentin. The cells producing this partially mineralized organic matrix are referred to as secretory ameloblasts. As do osteoblasts in bone, these cells produce an organic proteinaceous matrix by activity of the rough endoplasmic reticulum (rER), Golgi apparatus, and secretory granules. The secretory amelo­blasts continue to produce enamel matrix until the full thickness of the future enamel is achieved.

Maturation of the partially mineralized enamel matrix in­volves the removal of organic material as well as continued influx of calcium and phosphate into the maturing enamel. Cells involved in this second stage of enamel formation are referred to as maturation ameloblasts. Maturation amelo­blasts differentiate from secretory ameloblasts and function primarily as a transport epithelium, moving substances into and out of the maturing enamel.

Secretory ameloblasts are narrow, highly polarized, co­lumnar cells. They are directly adjacent to the developing enamel. At the apical pole of the cell is a pro­cess, Tomes' process, that is surrounded by the developing enamel. Adjacent to the mitochondria is the nucleus; in the main column of cytoplasm are the rER, Golgi, secretory gran­ules, and other cell elements. Junctional complexes are present at both apical and basal extremities. Contractile fil­aments joined to these junctional complexes may be in­volved in moving the secretory ameloblast over the developing enamel. The rod produced by the ameloblast follows in the wake of the cell. Thus, in mature enamel, the di­rection of the enamel rod is a record of the path taken ear­lier by the secretory ameloblast.

At their basal poles, the secretory ameloblasts are ad­jacent to a layer of enamel organ cells called the stratum intermedium. The plasma membrane of these cells and that of the basal pole of the ameloblasts is positive for alkaline phosphatase, an enzyme active in calcification. Stellate enamel organ cells are external to the stratum intermedium and are separated from the adjacent blood vessels by a basal lamina.

The histologic feature that marks the cycles of matura­tion ameloblasts is the presence of a striated or ruffled bor­der. Maturation ameloblasts with a striated border occupy about 70% of a specific cycle, and those that are smooth ended are about 30% of a specific cycle. There is no stratum intermedium in the enamel organ during enamel maturation; stellate papillary cells are adjacent to the mat­uration ameloblasts.

The maturation ameloblasts and the adjacent papillary cells are characterized by the presence of numerous mito­chondria. This is indicative of cellular activity that requires large amounts of energy and is a reflection of the fact that the maturation ameloblasts and the adjacent papillary cells function as a transporting epithelium.

The matrix of developing enamel contains three major proteins:

• Amelogenins

• Enamelins

• Tuft protein

Mature enamel contains only enamelins and tuft protein.

Amelogenins are removed during enamel maturation. Tuft protein is located near the dentinoenamel junction. It is present in enamel tufts and accounts for the fact that the enamel tufts are hypomineralized; i.e., they have a higher percentage of organic material than the remainder of the mature enamel. The maturation of the developing enamel results in the continued mineralization of enamel, so that it becomes the hardest substance in the body. The amelo­blasts degenerate after the enamel is fully formed, at about the time of tooth eruption from the gum.

Cementum

Cementum covers the root of the tooth. The root is that part of the tooth that fits into its socket or alveolus in the maxilla or mandible. Cementum is a thin layer of bone-like material that is secreted by cementocytes, cells that closely resemble osteocytes. Like bone, ce­mentum has a mineral content of 45-50%. The lacunae and canaliculi in the cementum contain the cementocytes and their processes, respectively. They resemble those struc­tures in bone that contain osteocytes and osteocyte pro­cesses.

Unlike bone, cementum is avascular. Also, the canalic­uli in cementum do not appear to form an interconnecting network. A layer of cementoblasts (cells that resemble the osteoblasts of the surface of growing bone) is seen on the outer surface of the cementum, adjacent to the periodontal ligament.

Figure 51. EM of Sharpey’s fibers

Collagen fibers that project out of the matrix of the ce­mentum and embed in the bony matrix of the socket wall form the bulk of the periodontal ligament. These fibers are another example of Sharpey's fibers. Oxytalan fibers that resemble developing elastic fibers and stain with elastic stains are also a component of the periodontal ligament. This mode of attachment of the tooth in its socket allows slight movement of the tooth to occur naturally and forms the basis of the orthodontic procedures that are used to straighten teeth and reduce malocclusion of the biting and grinding surfaces of the maxillary and mandibular teeth. During such corrective tooth movement, the alveolar bone of the¹ socket is resorbed and resynthesized, but the ce­mentum is not.

Dentin

Figure 51.Photomicrograph of a decalcified tooth showing dental pulp and structure of dentin

Dentin is calcified material that forms most of the tooth substance Dentin lies deep to the enamel and cementum. It contains less hydroxyapatite than does enamel, about 70%, but more than is found in bone and cementum. Dentin is secreted by odontoblasts that form an epithelial layer over the inner surface of the dentin, i.e., that surface that is in contact with the pulp. Odontoblasts, too, are elongated cylindrical cells that contain a well-developed rER, a large Golgi apparatus, and other organelles associated with the synthesis and secretion of large amounts of protein. The apical surface of the odontoblast is in contact with the forming dentin; junctional complexes between the odontoblasts at the level separate the dentinal compartment from the pulp compartment.

The layer of odontoblasts retreats as the dentin is laid down, leaving odontoblasts processes embedded in the dentin is narrow channels called dentinal tubules. The tubules and processes continue to elongate as the dentin continues to thicken by rhythmic growth. The rhythmic growth of dentin produces certain "growth lines" in the dentin (incremental lines of von Ebner and the thicker lines of Owen) that can identify significant developmental times such as birth (neonatal line) and may identify when unusual substances such as lead were incorporated into the growing tooth. Study of growth lines has proved useful in forensic medicine.

Predentin is newly secreted organic matrix, closest to the cell body of the odontoblasts that has yet to be mineralized. An unusual feature of the secretion of collagen and hydroxyapatite by odontoblasts is the presence, in Golgi vesicles, of arrays of a formed filamentous collagen precursor to which granules believed to contain calcium attach. This gives rise to structures called abacus bodies. The abacus bodies become more condensed as they mature into secretory granules.

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