Chapter Summary

Lymphoid tissue consists of diffuse and dense lymphoid tissue. The principal cell of lymphoid tissue is the lymphocyte, of which there are three categories: null cells, B lymphocytes and T lymphocytes. Additionally, macrophages, reticular cells, plasma cells, dendritic cells, and antigen-presenting cells perform important functions in lymphatic tissue.

I. LYMPH NODE

A. Capsule

The capsule, usually surrounded by adipose tissue, is composed of dense irregular collagenous connective tissue containing some elastic fibers and smooth muscle. Afferent lymphatic vessels enter the convex aspect; efferent lymphatics and blood vessels pierce the hilum.

B. Cortex

The cortex of a lymph node is characterized by the presence of lymphatic nodules, which have a dark corona, predominantly occupied by B lymphocytes, and lighter staining germinal centers, housing activated B lymphoblasts, macrophages, and dendritic reticular cells. Connective tissue trabeculae subdivide the cortex into incomplete compartments. Subcapsular and cortical sinuses possess lymphocytes, reticular cells, and macrophages.

C. Paracortex

The paracortex is the zone between the cortex and medulla, composed of T lymphocytes. Postcapillary venules, with their characteristic cuboidal endothelium, are present.

D. Medulla

The medulla displays connective tissue trabeculae, medullary cords (composed of macrophages, plasma cells, and lymphocytes), and medullary sinusoids lined by discontinuous endothelial cells. Lymphocytes, plasma cells, and macrophages are the common cell types in the lumina of sinusoids. The region of the hilum is distinguished by the thickened capsule and lack of lymphatic nodules.

E. Reticular Fibers

With the use of special stains, such as silver stains, an extensive network of reticular fibers may be demonstrated to constitute the framework of lymph nodes.

II. TONSILS

A. Palatine Tonsils

1. Epithelium

Covered by stratified squamous nonkeratinized epithelium that extends into the tonsillar crypts. Lymphocytes may migrate through the epithelium.

2. Lymphatic Nodules

Surround crypts and frequently display germinal centers.

3. Capsule

Dense, irregular collagenous connective tissue capsule separates the tonsil from the underlying pharyngeal wall musculature. Septa, derived from the capsule, extend into the tonsil.

4. Glands

Not present.

B. Pharyngeal Tonsils

1. Epithelium

For the most part, pseudostratified ciliated columnar epithelium (infiltrated by lymphocytes) covers the free surface as well as the folds that resemble crypts.

2. Lymphatic Nodules

Most lymphatic nodules possess germinal centers.

3. Capsule

The thin capsule, situated deep to the tonsil, provides septa for the tonsil.

4. Glands

Ducts of the seromucous glands,beneath the capsule,pierce the tonsil to open onto the epithelially covered surface.

C. Lingual Tonsils

1. Epithelium

Stratified squamous nonkeratinized epithelium covers the tonsil and extends into the shallow crypts.

2. Lymphatic Nodules

Most lymphatic nodules present germinal centers.

3. Capsule

The capsule is thin and ill defined.

4. Glands

Seromucous glands open into the base of crypts.

III. SPLEEN

A. Capsule

The capsule, composed predominantly of dense irregular collagenous connective tissue, is significantly thickened at the hilum.The capsule also possesses a small amount of elastic fibers and some smooth muscle cells. It is covered by mesothelium (simple squamous epithelium) but is not surrounded by adipose tissue. Trabeculae, bearing blood vessels, extend from the capsule into the substance of the spleen.

B. White Pulp

White pulp is composed of periarterial lymphatic sheaths and lymphatic nodules with germinal centers. Both periarterial lymphatic sheaths (predominantly T lymphocytes) and lymphatic nodules (predominantly B lymphocytes) surround the acentrically located central artery.

C. Marginal Zone

A looser accumulation of lymphocytes, macrophages, and plasma cells are located between white and red pulps. The vascular supply of this zone is provided by capillary loops derived from the central artery.

D. Red Pulp

Red pulp is composed of pulp cords and sinusoids. Pulp cords are composed of delicate reticular fibers, stellateshaped reticular cells, plasma cells, macrophages, and cells of the circulating blood. Sinusoids are lined by elongated discontinuous endothelial cells surrounded by thickened hoop-like basement membrane in association with reticular fibers. The various regions of penicilli are evident in the red pulp. These are pulp arterioles, sheathed arterioles, and terminal arterial capillaries. Convincing evidence to determine whether circulation in the red pulp is open or closed is not available, although, in humans, the open circulation is believed to be the most prevalent.

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E. Reticular Fibers

With the use of special stains, an extensive network of reticular fibers, which constitute the framework of the spleen, can be demonstrated.

IV. THYMUS

A. Capsule

The thin capsule is composed of dense irregular collagenous connective tissue (with some elastic fibers).

Interlobular trabeculae extending from the capsule incompletely subdivide the thymus into lobules.

B. Cortex

Typically, the cortex is devoid of lymphatic nodules or plasma cells. It is composed of lightly staining epithelial reticular cells, macrophages, and densely packed, darkly staining, small T lymphocytes (thymocytes) responsible for the dark appearance of the cortex. Epithelial reticular cells also surround capillaries, the only blood vessels present in the cortex.

C. Medulla

The lightly staining medulla is continuous from lobule to lobule. It is occupied by plasma cells, lymphocytes, macrophages, and epithelial reticular cells. Moreover, thymic

(Hassall’s) corpuscles, concentrically arranged epithelial reticular cells, are characteristic features of the thymic medulla.

D. Involution

The thymus begins to involute subsequent to puberty. The cortex becomes less dense because its population of lymphocytes and epithelial reticular cells is, to some extent, replaced by fat. In the medulla, thymic corpuscles increase in number and size.

E. Reticular Fibers and Sinusoids

The thymus possesses neither reticular fibers nor sinusoids.

CHAPTER OUTLINE

Graphics

Graphic 10-1 Pituitary Gland and Its Hormones

p. 237

Graphic 10-2 Endocrine Glands p. 238

Graphic 10-3 Sympathetic Innervation of the Viscera and the Medulla of the Suprarenal Gland p. 239

Tables

Table 10-1 Pituitary Gland Hormones

Table 10-2 Hormones of the Thyroid, Parathyroid, Adrenal, and Pineal Glands

Plates

The endocrine system, in cooperation with the nervous system, orchestrates homeostasis by influencing, coordinating, and integrating the physiological func-

tions of the body. The endocrine system consists of several glands, isolated groups of cells within certain organs, and individual cells scattered among parenchymal cells of the body.This chapter considers only that part of the endocrine system that is composed of glands. Islets of Langerhans, interstitial cells of Leydig, cells responsible for ovarian hormone production, and DNES (diffuse neuroendocrine) cells are treated in more appropriate chapters.

The endocrine glands to be discussed here are the

•pituitary,

•thyroid,

•parathyroid,

•suprarenal glands, and

•pineal body.

All of these glands produce hormones that they secrete into the connective tissue spaces. There are three types of hormones, depending on how far they act from their site of secretion:

•those that act on the cell, which releases them (autocrine hormones)

•those that act in the immediate vicinity of their secretion (paracrine hormones), and

•those that enter the vascular system and find their target cells at a distance from their site of origin (endocrine hormones).

This chapter details endocrine hormones (see Tables 10-1 and 10-2), whereas other chapters (nervous tissue, respiratory system, and digestive system) discuss autocrine and paracrine hormones.

Some hormones (e.g., thyroid hormone) have a generalized effect, in that most cells are affected by them; other hormones (e.g., aldosterone) affect only certain cells.

•Receptors located either on the cell membrane or within the cell are specific for a particular hormone.

•The binding of a hormone initiates a sequence of reactions that results in a particular response.

•Because of the specificity of the reaction, only a minute quantity of the hormone is required.

•Some hormones elicit and others inhibit a particular response.

Hormones, based on their chemical nature, are of three types, nonsteroid, steroid based, and amino acid derivatives. Nonsteroid-based hormones (proteins and polypeptides) are small peptides (antidiuretic hormone [ADH] and oxytocin) or small proteins (glucagon, insulin, anterior pituitary proteins, and parathormone). Amino acid derivatives include insulin, norepinephrine, and thyroid hormone. Steroid-based hormones and those of fatty acid

E N D O C R I N E S Y S T E M 229

derivates are cholesterol derivatives (aldosterone, cortisol, estrogen, progesterone, and testosterone).

Nonsteroid-Based Hormones and Amino

Acid Derivatives

Nonsteroid-based endocrine hormones and amino acid derivatives bind to receptors (some are G protein linked, and some are catalytic) located on the target cell membrane, activate them, and thus initiate a sequence of intracellular reactions. These may act by

•altering the state of an ion channel (opening or closing) or

•by activating (or inhibiting) an enzyme or group of enzymes associated with the cytoplasmic aspect of the cell membrane.

Opening or closing an ion channel will permit the particular ion to traverse or inhibit the particular ion from traversing the cell membrane, thus altering the membrane potential. Neurotransmitters and catecholamines act on ion channels.

•The binding of most hormones to their receptor will have only a single effect, which is the activation of adenylate cyclase.

•This enzyme functions in the transformation of ATP to cAMP (cyclic adenosine monophosphate), the major second messenger of the cell. cAMP then activates a specific sequence of enzymes that are necessary to accomplish the desired result.

•There are a few hormones that activate a similar compound, cyclic guanosine monophosphate (cGMP), which functions in a comparable fashion.

Some hormones facilitate the opening of calcium channels;

•calcium enters the cell, and three or four calcium ions bind to the protein calmodulin, altering its conformation.

•The altered calmodulin is a second messenger that activates a sequence of enzymes, causing a specific response.

Thyroid hormones are unusual among the amino acid derivative and nonsteroid-based hormones, in that they directly enter the nucleus, where they bind with receptor molecules. The hormone-receptor complexes control the activities of operators and/or promoters, resulting in mRNA transcription. The newly formed mRNAs enter the cytoplasm, where they are translated into proteins that elevate the cell’s metabolic activity.

Steroid-Based Hormones

Steroid-based endocrine hormones diffuse into the target cell through the plasma membrane and, once inside the cell, bind to a receptor molecule.