Exercise 18

The Endocrine System

laboratory Objedives On completion of the activities in this exercise, you will be able to:

Describe the difference between an endocrine gland and an exocrine gland.

• Discuss how a hormone affects a target cell. Explain the functional relationship between the endocrine system and the nervous system. Identify the locations of the endocrine glands in the hu­man body.

• Describe the anatomical relations of the endocrine glands to adjacent structures. List the hormones produced by the various endocrine glands and describe their functions. Describe the microscopic structure of the endocrine glands.

Materials Anatomical models: • Human brain • Human torso Human sklills Compound light microscopes

• Prepared microscope slides: • Pituitary gland • Thyroid gland • Parathyroid gland • Thymus • Pancreas • Adrenal gland • Ovary • Testis

The endocrine system consists of a diverse collection of organs and tissues that contain endocrine glands. These glands secrete chemicals known as hormones

into nearby blood capillaries. Once in the circulatory system, hormones can be transported to target cells at some distant location. At the target cell, a hormone binds to a specific re­ceptor. Once this occurs, the target cell will respond to the hormone's chemical message. Hormones can in!1uence a target cell's metabolic activities by regulating the production of spe­cific enzymes or critical structural proteins. This can be ac­complished by promoting or inhibiting specific genes in the nucleus or by controlling the rate of protein synthesis. Hor­mones can also activate or deactivate an enzyme's activity by altering its three-dimensional structure.

Endocrine glands represent one of the two types of glands found in the body. The second type, exocrine glands, secrete substances into ducts , which transport the secretions into the

lumina (internal cavities) of organs, into body cavities, or to the surface of the skin. Sweat glands and sebaceous glands, which you studied in Exercise 6, are examples of exocrine glands.

The endocrine system operates in conjunction with the nerv­ous system to maintain homeostasis and to ensure that bodily functions are carried out efficient'ly. This functional relationship is sometimes expressed as a neuroendocrine effect in which nerve impulses can affect the release of hormones and, in turn, hormones can regulate the !1ow of nerve impulses.

As you learned earlier, the nervous system performs its functions by conducting electric impulses along nerve fibers and releasing neurotransmitters across synapses to nearby tar­get cells. Neural responses occur quickly but do not last for long periods of time. Endocrine responses are not as rapid as neural responses but can persist for several hours to several days , and are equally effective in regulating physio llogical activities.

WHAT"S IN A WORD The word hormone is derived from the Greek word hormao, which means "to provoke" or "set in mo­tion. " Hormones released by endocrine glands influence their target organs by "setting in motion" or promoting a specific function.

Gross Anatomy of the Endocrine System An overview of the endocrine system is illustrated in Figure IS.1. Some structures have an exclUSively endocrine function. They in­clude the pituitary gland, pineal gland, thyroid gland, parathyroid glands, and adrenal glands. In addition, the en­docrine system includes several other organs that produce hor­mones but also perform nonendocrine functions. They include the hypothalamus, thymus gland, pancreas, testes, ovaries, heart, stomach, small intestine, and kidneys.

ACTIVITY 18.1 Examining Gross Anatomy of the Endocrine System

Endocrine Organs Located in the Head

1. Obtain a model of a midsagittal section of a human brain.

2. Identify the pineal gland, located along the roof of the third ventricle (Figure 18.2). Melatonin, the hormone se­creted by this gland , is believed to control daily sleep­ing-waking patterns and other cyclical phYSiological processes (circadian rhythms).

327

---- --

EXER C ISE E IGHTEEN

Figure 18.1 Overview of the endocrine system. The human endocrine system contains a diverse array of organs scattered throughout the body.

Figure 18.2 Endocrine structures in the brain. Cranial structures that produce hormones include the hypo­

HYPOTHALAMUS PINEAL GLAND

Production of ADH, Melatonin oxytocin, and regulatory hormones

PARATHYROID GLANDS (on posterior surface of

PITUITARY GLAND thyroid gland) Anterior lobe: Parathyroid hormone (PTH)ACTH, TSH, GH, PRL, FSH, LH, and MSH

Posterior lobe: Release of oxytocin andADH

THYROID GLAND

Thyroxine (T4) ADIPOSE TISSUE Triiodothyronine (T3)

Calcitonin (CT) Leptin Resistin

PANCREATIC ISLETS

Insulin, glucagon ADRENAL GLANDS

Each adrenal gland is subdivided into: GONADS

Adrenal medulla: Testes (male): Epinephrine (E) Androgens (especially Norepinephrine (NE) testosterone), inhibin

Adrenal cortex: Ovaries (female): Cort isol , corticosterone, Estrogens, progestins, aldosterone, androgens inhibin

thalamus, pi tu itary gland, and pineal gland.

Cerebral hemisphere ~------;-,,=f-~-------':k-----.lb,.----::l;.-----~,--,.L-

Corpus callosum

Thalamus -r:::::t-t-tk~~s:=---I-Hypothalamus

Infundibulum

Pituitary ­ --­-----­+

=-='::--=~---=~~--=:=..J,- Pineal gland

_~...,!~~...,L:q~=d::=--__ Midbrain

--=:~~:i_-- Cerebellum

Medulla oblongata

CLINICAL CORRELATION

Melatonin production and secretion increase during periods of darkness and decrease during periods of light. This fluctuati on in activity is believed to be the underlying cause of seasonal af­fective disorder. This condition brings about unusual changes in mood, sleeping pattern, and appetite in some people living at high latitudes where periods of darkness are quite long during winter months.

3. Locate the hypothalamus, just inferior to the thalamus (Figure IS.2). It produces a number of releasing hormones that increase, and inhibiting hormones that reduce , the pro­duction and secretion of hormones in the anterior pituitary (Table IS. 1). The hypothalamus also releases antidiuretic hormone (ADH), which acts on the kidneys to reduce water loss, and oxytocin, which stimulates smooth muscle con­tractions, most notably labor contractions in the uterus and contractions in reproductive glands and ducts of both sexes during intercourse. ADH and oxytocin are transported along axons to the posterior lobe of the pituitary gland, where they are stored and eventually released.

4. Identify the pituitary gland, or hypophysis, which is di­rectly connected to the hypothalamus by a stalk of tissue called the infundibulum (Figure lS.2). It lies within the sella turcica of the sphenoid bone (F igure lS.3a). Obtain a human skull and identify the sella turcica along the floor of the cranial cavity.

The pituitary gland is divided into two distinct re­gions: the anterior lobe and the posterior lobe

Table 18.1 Hormones Produced by the Anterior Pituitary

Hormone Target Effect

TI-I E1\ OOCRI N E SYSTEM

(Figure lS.3). The anterior lobe produces several hor­mones (Table lS.l ) that regulate the activities of other structures, including other endocrine glands, through­out the body. The posterior lobe, as described earlier, stores and secretes the two hypothalamic hormones, ADH and oxytocin (Table lS.2) .

WHAT' S IN A WORD The term pituitary is derived from the Latin word pitllita, which means phlegm or thick mucous secre­tion. The Renaissance anatomist , Andreas Vesalius, gave the pi­tuitary its name because he mistakenly thought that it produced a mucous secretion related to the throat. When the true function of the pituitary was determined , some 200 years later, it was given a ne,v name, the hypophysis, which is the Greek word for "undergrowth". This is probably a be tter name for the gland since it describes its position suspended from the inferior surface of the hypothalamus.

Endocrine Organs Located in the Neck

1. Obtain a torso or head and neck model.

2. Locate the thyroid gland, which is composed of two elon­gated lobes located on each side of the trachea, just infe­rior to the thyroid cartilage. The isthmus of the thyroid travels across the anterior surface of the trachea and con­nects the two lobes (Figure lS.4a). The thyroid gland pro­duces the hormones thyroxine (T4 ) and triiodothyronine (T3), which regulate cell metabolism, general growth and development, and the normal development and matura­tion of the nervous system. It also produces calcitonin that reduces the levels of calcium ions in body fluids.

Hypothalamic regulatory hormone

Pars distalis

Thyroid stimulating hormone (TSH)

Adrenocorticotropic hormone (ACTH)

Gonadotropins

a. Follicle stimulating hormone (FSH)

b. luteinizing hormone (lH)

Prolaction

Growth hormone

Pars intermedia

Melanocyte stimulating hormone (MSH)

Thyroid gland

Adrenal cortex

Follicle cells in ovaries

Sustentacular cells in testes Follicle cells in ovaries

Interstitial cells in testes

Mammary glands

All cells

Melanocytes

Promotes secretion of thyroid hormones

Promotes secretion of glucocorticoids

Promotes estrogen secretion and follicle development Promotes sperm maturation Promotes ovulation, corpus luteum formation, and progesterone secretion Promotes testosterone secretion

Stimulates milk production

Growth, protein synthesis, lipid mobilization, and catabolism

Increased melanin synthesis

Thyrotropin-releasing hormone (TRH)

Corticotropin-releasing hormone (CRH)

Gonadotropin-releasing hormone (GnRH)

Prolactin-releasing factor (PRF); prolactin-inhibiting hormone (PIH)

Growth hormone-releasing hormone (GH-RH); Growth hormone-inhibiting hormone (GH-IH)

Melanocyte stimulating hormone-inhibiting hormone (MSH-IH)

EXERCISE EIGHTEEN

Anterior lobe

Infundibulum

Diaphragma -i;;;~~s~~~ sellae

Median eminence

Posterior lobe

-----sPhenoid

Anterior lobe

Mamillary body

Secretes other Secretes Releases ADH (sella turcica) pituitary hormones MSH and oxytocin

(a) (b)

Figure 18.3 Anatomy of the pituitary gland. a) Diagram of the anterior and posterior lobes of the pituitary gland. Note that it is connected to the hypothalamus by the infundibulum. b) light micrograph of the anterior and posterior lobes of the pituitary (lM x 100).

Table 18.2 Honnones Secreted by the Posterior Pituitary

Hormone Target Effect Hypothalamic regulatory hormone

Antidiuretic Kidneys Reabsorption of water; elevation of blood volume None; transported along axons from hypothalamus hormone (ADH) and pressure to posterior pituitary

Oxytocin (OT) Uterus, mammary glands labor contractions; milk ejection Same as above

Ductus deferens, Contractions of ductus deferens and prostate prostate gland gland

3. Typically there are two pairs of parathyroid glands em­bedded on the posterior surfaces of the thyrOid gland lobes (Figure IS.Sa). These glands may not be illus­trated on the models in your lab. If not, locate them in an illustration. They produce parathyroid hormone (PTH), which opposes the action of calcitonin by in­creasing the concentration of calcium ions in body fluids.

Endocrine Organs Located in the Thoracic Cavity

1. Remove the anterior body wall from a torso model so that the contents of the thoracic cavity are exposed (Figure IS.6).

2. Note that the heart is located in the central region of the tho­racic cavity, known as the mediastinum. If blood volume is

elevated above normal , cardiac muscle cells in the heart wall secrete natriuretic peptides. These hormones act on the kidneys to promote the loss of sodium ions and water.

WHAT' S IN A WORD Natriuretic peptides promote natriuresis, or the excretion of sodium in the urine. The term nau"iuresis is de­rived from two Greek words: natrium, meaning "sodium" (the chemical s)'lnbol for sodium is Na) , and Ollron, meaning "urine".

3. The thymus gland is located just posterior to the sternum. If it is present on the models in your lab, observe how it covers the superior portion of the heart and extends supe­riorly into the base of the neck. The thymus produces a group of hormones called thymosins that promote the maturation of T-lymphocytes , a type of white blood cell

THE EN DOCR t N E SYSTEM

Thyroid cartilage of larynx ll~fS~~~~ulr---- Hyoid bone

Superior ~~ ....WH"'---~· Superiorthyroid vein thyroid artery

Right lobe Internal of thyroid jugular vein

gland Cricoid cartilage

Middle of larynx thyroid vein

Left lobe of thyroid gland

Common carotid artery -------,~-4~ Isthmus of

thyroid gland Thyrocervical-- --------"""""..

Inferior thyroid artery trunk ---j~~~~~~~J~

Trachea

Inferior thyroid

clavicle veins

OUtiineOf~ CuboidalsternumFollicle epithelium

cells of follicle C cell

Follicle cavities

C cell Tlhyroglobulin stored in

colloid of follicle

(e)

Figure 18.4 Anatomy of the thyroid gland. a) Diagram of the thyroid gland, illustrating its relationship with neighboring structures; b) diagram; and c) light micrograph, illustrating the microscopic structure of the thyroid gland (lM x 200).

(a)

Thyroid follicle

(b)

that coordinates the body's immune response. The thymus is relatively large in newborns and young children. After puberty, the size of the thymus is gradually reduced, and the glandular tissue is replaced by fat and fibrous connec­tive tissue . Exercise 23 presents the structure and function of the thymus in greater detail.

Endocrine Organs Located in the Abdominopelvic Cavity

1. On a torso model, remove the digestive organs from the abdominopelvic cavity to expose the structures along the posterior wall. The stomach and small intestine produce several hormones that are important for regulating diges­

tive activities, which will be discussed later when you study the digestive system.

2. Locate the elongated pancreas that stretches across the pos­terior bod wall between the duodenum (first part of the small intestine) and the spleen. The head of the pancreas is that portion which is nestled within the C-shaped curvature of the duodenum on the right side (Figure 18.7a). Moving to the left , the body of the pancreas is the main portion of the organ. It. gives rise to an elongated tail that extends to the left toward the spleen. Although the pancreas is largely composed of exocrine glands that produce digestive en­zymes , scattered throughout are regions of endocrine tissue

EXERCISE EIGHT EEN

Figure 18.5 Anatomy of the parathyroid glands. a) Diagram showing the position of the four parathyroid glands along the posterior wall of the thyroid gland; b) light mi­crograph of a region similar to the area enclosed by the rectangular box in (a). Portions of both the parathy­roid and thyroid glands are shown (lM x 100); c) light micrograph of a region of the parathyroid gland simi­lar to the area enclosed by the rec­tangular box in (b). Principal (chief) cells and oxyphil cells are present (lM x 400).

lobe

Trachea ------"'-..,.;,,::~_\I+.:....

Right ------;,,<------IJ-. .......,.------+----Left lobe

--T-----~-----THYMUS

Figure 18:6 Endocrine structures in the thoracic cavity. The heart pro­duces natriu re tic peptldes and the thymus produces thymosins.

known as the pancreatic islets (islets of Langerhans; Figures 18.7b and c) . The two main hormones produced by the islet cells are glucagon and insulin , which regulate blood glucose levels. Glucagon elevates blood glucose levels by promoting the breakdown of glycogen, the synthesis of glucose from fats and proteins, and the release of glucose into the blood. Insulin lowers blood glucose levels by pro­moting glucose uptake into most cells. Additionally, in skeletal muscles and in the liver, insulin increases glucose storage by stimulating the production of glycogen. Two other hormones, somatostatin and pancreatic polypep­tide (PP), are also produced by the pancreatic islets. 50­

(a) Thyroid gland, posterior view

(e) Principal Oxyphil (chief) cells cells

matostatin regulates the secretion of both insulin and glucagon. Pancreatic polypeptide inhibits muscular contrac­tions in the wall of the gallbladder and controls the pancre­atic production of digestive enzymes.

CLINICAL CORRELATION

Normally, any glucose that is filtered out of the blood by the kid­neys is reabsorbed back into the blood. Thus, glucose is usually not present in urine. However, an individual with diabetes mel­litus has glucose levels that are well above normal, a condition called hyperglycemia, and the kidneys cannot reabsorb the ex­cess. As a result, glucose will be present in the urine. There are two main types of diabetes mellitus. Type I diabetes accounts for 5% to 10% of all cases in the United States. It usually develops in children or young adults and destroys the pancreatic cells that produce insulin. It can be treated by daily administration of in­sulin, supplemented by a carefully monitored dietary plan. Type II diabetes is far more common, making up 90% to 95% of all cases. In addition, a strong correlation exists between type II dia­betes and obesity. People with type II dia betes produce normal amounts of insulin but cannot utilize the hormone effectively. This could be due to the production of defective insulin mole­cules or the lack of insulin receptors on target cells. Careful di­etary control, weight reduction, and other lifestyle changes (e.g., regular exercise) are the best treatments for this form of the dis­ease. Diabetes is a long-term, progressive disorder that has po­tentially serious systemic effects. It can contribute to blindness heart disease, stroke, kidney failure, circulatory problems resul~ing in limb amputations, and nerve damage. It is also one of the leading causes of death in the United States.

Ducts --+=;---~

~--i-'\-=,,;"""":-="':'--:~~

tissue septum

Exocrine cells in pancreatic

Endocrine cells .".---!=-­

Pancreatic duct

Lobules

1\ Body o f

pancreas

in pancreatic islet

~Tailofr p~ncreas o

(endocrine)

Pancreatic acini

THE EN DOCRINE SYSTEM

Duct-~~~~~~II~~~~~

Connective -+--=0-­

acini

I

(exocrine)

Pancreatic islet -~~~~~4'~

(h)

3. The adrenal (suprarenal) glands are pyramid-shaped struc­tures resting on the superior margins of the kidneys (Figure IS.Sa). Fibrous connective tissue attaches the ad­rena'! glands to the connective tissue capsule that sur­rounds the kidneys. If possible, remove the anterior portion of one adrenal gland and observe its internal structure. Identify the inner adrenal medulla and the outer adrenal cortex (F igure IS.Sb). The adrenal cortex produces three categories of hormones.

• Mineralcorticoids, such as aldosterone, act on the kid­neys to conserve water and sodium ions , and to secrete potassium ions.

• Glucocorticoids, such as cortisol, act on many cells to conserve glucose by utilizing fatty acids and proteins as an energy source (glucose-sparing effect), and they function as anti-inflammatory agents by inhibiting ce\1ls in the immune system.

• Androgens (gonadocorticoids) are male sex hormones that are produced in small quantities and converted to estrogens (female sex hormones) when they enter the blood. The function of adrenal androgens is not clear.

The adrenal medulla releases two hormones , epinephrine and norepinephrine, in response to sympa­thetic nervous system activation, contributing to the fight­or-flight response. The effects include increased hekrt rate, blood pressure, and respiratory rate , and decreased diges­tive activity.

(e)

CLINICAL CORRELATION

Glucocorticoids are steroid hormones. Because of their inflam­matory effects, these chemicals, or derivatives of them, are used in prescription and over-the-counter "steroid creams" to treat skin rashes such as poison ivy. In addition, many college and professional athletes are given cortisone injections to reduce the inflammation that occurs at an injured joint. These injections are effective in reducing injury-related pain, but they do little in repairing damaged tissue. Thus, an athlete who re­ceives a series of cortisone injections might misinterpret a re­duction in pain for complete recovery, return to normal activity prematurely, and possibly cause a more serious injury.

4. Locate the kidneys (Figure l S.Sa) . Although they are mostly involved with waste removal, they also have en­docrine functions. Under the influence of parathyroid hor­mone, the kidneys release a hormone called calcitriol, which acts on the small intestine to increase absorption of calcium and phosphate. The kidneys also release erythropoietin (EPO) , which stimulates red blood cell production in bone marrow.

5. The gonads include the ovaries in females and the testes in males . On a female model, locate the ovaries along the lat­eral wall of the pelvic cavity (Figure IS.9a). They produce the female sex hormones called estrogens. On a male model , locate the testes. They originate in the abdominal cavity near the kidneys, but descend into the scrotum ,

Figure 18.7 Anatomy of the pancreas. a) Diagram showing the relationship of the pancreas to the duodenum; b) diagram; and c) light micrograph illustrating the light mi­croscopic strudure of the pancreas. The pancreatic islets are the en­docrine portions of the pancreas (lM X 100).

--- - -

Superior --..... ___• mesenteric

artery

Abdominal aorta

Inferior vena cava

--"..---\----- Left adrenal (suprarenal) gland

Suprarenal arteries

Left suprarenal vein

__JJIIII.. "~-----"""r Left renal artery ~....~-:::--"""""!~+- Left renal vein

--~- Left kidney

- -'-:::----Cortex

~....,,--~-Medulla

(b)

(a)

Cells of zona

reticularis

EXERC ISE EIGHT EEN

Figure 18.8 Anatomy of the ad­renal gland. a) Diagram showing the relationship of the adrenal gland to the kidney and neighboring blood ves­sels; b) diagram showing the two re­gions of the adrenal gland-the inner ad renal medulla and the outer adre­na l cortex; c) light micrograph illustrat­ing the microscopic strudure of the adrenal gland. Notice that the ad renal cortex is divided into three distind zones, which are illustrated in the three insets at higher magnification (lM x 400) .

Adrenal cortex

Zona Zona Zona Adrenal medulla reticu laris fasciculata glomerulosa

Cells of Cells of zona zona

(e) fasciculata glomerulos

-

Ovary

Urinary bladder

Vagina

THE EN DOCRINE SYSTEM

Figure 18.9 Anatomy of the ovary. a) Midsagittal section of the female pelvic cavity, showing the re­lationship of the ovary to neighboring structures; b) diagram illustrating the structure of the ovary in cross sec­tion; c) light micrograph illustrating the microscopic structure of the ovary. Notice the developing follicles in the cortex of the ovary. The follicu­lar cells that surround the egg pro­duce female sex hormones (lM x 40).

(a)

Egg cells """i~~I~!im~~I~11(oocytes)

Corpus luteum

Cortex

Medulla

(c)

(b)

which is outside the body cavity (Figure 18.10a) . The Endocrine glands are surrounded by an exten­testes produce male sex hormones (androgens), of which sive network of blood capillaries. Suggest a rea­testosterone is the most important. The sex hormones

son why this anatomical relationship is significant.control the development and maturation of sex cells (egg and sperm) , maintain accessory sex organs, and support secondary sex characteristics.

EXERCISE EIGHTEEN

Figure 18.10 Anatomy of the testis. a) Midsagittal section of the male pelvic cavity. In the adult male, the testes are located in the scrotal sac, outside the body cavity. b) low­power light micrograph showing cross sections of seminiferous tubules in a testis (lM x 100) . c) High-power light micrograph and corresponding diagram of a single seminiferous tubule, similar to the tubule enclosed by the box In (b). The interstitial cells produce testos­terone (lM x 200).

Pubic ----7.-:---;-;..:;:,;;:.­ fj{-7;ti---­ -"-'-=.-;---7--Seminal vesicle symphysis

Spongy urethra --:i-.~-f. ':

External urethral orifice

+T.r---'-=;.s~=-:-:--'----;'=--:-'------ Prostate gland

Ductus deferens

EpididymiS

Scrotum

Ejaculatory duct Bulbourethral gland Anus

(b)

(a)

c:'!!~-'-t-;? Developing sperm cells

Microscopic Anatomy of the Endocrine System The cells of endocrine glands possess the following common features.

• The cells are usually cuboidal or polyhedral (many sides) with a large, spherical nuclei.

• With the exception of the hypothalamus , all endocrine cel'ls are derived from epithelial tissue.

• The cells are typically arranged in clusters, small islands (islets) , or cords.

• Endocrine cells form glands that lack a system of ducts . Hormones are secreted directly into the surrounding tissue spaces and eventually gain entry into the blood circulation.

• Endocrine cells have an extensive blood supply, and all of them have at least one surface that is directly adjacent to a capillary.

As you study the microscopic anatomy of the various en­docrine organs, be aware of these similarities as well as the unique features that characterize each structure.

ACTIVITY 18.2 Examining Microscopic Anatomy of Endocrine Organs

Pituitary Gland

1. Obtain a slide of the pituitary gland (hypophysis).

2. View the slide with the scanning or low-power objective lens. Depending on your slide preparation, adjacent brain and bone tissue may also be present.

TH E EN DOCRI N E SYSTEM

• Vl1hat region of the brain would you expect to see on lengthen, but instead become thicker and denser, particularly in your slide 7 _________________ the face, hands, and feet. Both gigantism and acromegaly are

• What skull bone would you expect to be present? usually caused by a tumor in the pars distalis and can be treated by its surgical removal.

3. Move the slide so that the pituitary gland is centered. Identify the darker staining anterior lobe (adenohypoph­ysis) and the lighter staining posterior lobe (neurohy­pophysis or pars nervosa). If possible , identify the infundibulum that connects the pituitary gland to the hy­pothalamus (Figure IS.3a).

4. Observe the anterior lobe of the pituitary with high power. The anterior lobe is a true endocrine gland because it con­tains several types of endocrine cells that produce and se­crete hormones (Table IS. 1). Identify the following regions of the anterior lobe (Figure lS.3b).

• The pars distalis consists of glandular epithelial cells arranged in cords or clusters. Notice that these ceUs have a cuboidal shape and possess well-defined nuclei. As you scan the slide , you will see that the cells vary conSiderably in their staining properties. The different colors that you observe in these cells depend on the staining technique used to prepare your slide. Neverthe­less, this variability reflects the fact that the pars distalis contains several cell types, each responsible for produc­ing a specific hormone (Table IS. I).

• The pars intermedia is a narrow band of tissue between the pars distalis and the posterior lobe. In the fetus , young children, and pregnant women, the cells in this region produce melanocyte-stimulating hormone (Table IS. I). In most adults, this region of the anterior pituitary is normally inactive.

• The pars tuberalis is an extension of the anterior lobe that wraps around the infundibulum , then spreads along the inferior margin of the hypothalamus. If the in­fundibulum and hypothalamus are present on your slide, • look for the pars tuberalis hugging their outside borders.

CLINICAL CORRElATION

Growth hormone (GH), secreted by the pars distalis, pro­motes protein synthesis in virtually all cells. It is particularly im­portant for the growth and development of muscle, cartilage, and bone. Inadequate production (hyposecretion) of GH before puberty leads to a condition called pituitary dwarfism. Peo­ple with this disorder have normal body proportions, but abnor­mally short bones due to reduced activity at the epiphyseal plates. Pituitary dwarfism can be successfully treated before pu­berty by administering synthetic GH.

Two other abnormalities are caused by excessive secretion (hypersecretion) of GH. Gigantism is the overproduction of GH before bone fusion. Individuals with this disorder have normal body proportions but excessively long limbs and can reach heights up to 8.5 ft. Acromegaly is caused by excessive GH production after bone fusion . In this condition, bones cannot

5. The posterior lobe of the pituitary gland, or pars nervosa, is actually an extension of the brain. It is not a true endocrine gland because it does not produce its own hormones. View the posterior lobe with high power (Figure 18.3b) and notice that most of this structure con­tains axons, which originate from neuron cell bodies in the hypothalamus. Antidiuretic hormone and oxytocin, produced in the hypothalamus, travel along these axons and are released from axon terminals in the posterior lobe.

Thyroid Gland

1. Obtain a slide of the thyroid gland.

2. View the slide "vith the scanning or low-power objective lens. Notice that the thyroid has a distinctive structure , consisting of numerous thyroid follicles of various sizes (Figures 18.4b and c) .

3. Use the high-power objective lens to examine the thyroid follicles more closely. Notice that each follicle consists of a central follicle cavity surrounded by a single layer of cuboidal follicle cells (Figures 18.4b and c).

4. Inside the follicle cavities, identify a lightly staining mate­rial known as colloid. Follicle cells produce a globular protein known as thyroglobulin and secrete it into the colloid for storage. Thyroglobulin is later used to synthe­size the thyroid hormones thyroxin (T4) and triiodothyro­nine (T )).

5. In the regions of connective tissue between the follicles, identify the parafollicular cells (C cells), which produce calcitonin. They usually appear in small clusters and are characterized by their pale or lightly stained cytoplasm and large nuclei (Figures lS.4b and c).

Parathyroid Gland

1. Obtain a slide of the parathyroid gland.

2. View the slide with the scanning or low-power objective lens. Since the parathyroid glands are embedded in the posterior wall of the thyroid gland, your slide may display tissue from both structures (Figure lS.5b).

3. Center an area of parathyroid tissue and switch to high power. The darkly stained cells that fill the field are principal (chieO cells, which produce parathyroid hormone. If you look carefully, you should notice that these cells are arranged in a curvilinear fashion (Figure lS.5c).

4. A second cell type, the oxyphil cells are found only in human parathyroid glands. If you are viewing a human parathyroid, attempt to locate these cells (Figure lS.5c). They are larger, stain lighter, and are far fewer than the principal cells. The function of the oxyphil cells is un­known.

I--~---------~--~----==,.".....,..".".,..""""-;=-"""'=-~=-=---~==========O::::-=-,,==-= .,==~-

EXERCIS E EIGHTEEN

Pancreas

1. Obtain a slide of the pancreas .

.., View the slide with the low-power objective lens and iden­tify the pancreatic acini. Each acinus contains a cluster of cuboidal cells (pancreatic acinar cells), arranged around a central lumen (Figures lS.7b and c). The acinar cells are the exocrine portion of the pancreas, and produce diges­tive enzymes. Observe that the darkly stained acinar cells comprise the vast majority of the pancreas.

3. Scattered among the pancreatic acini, identify the islands of lighter staining cells. These are the pancreatic islets or islets of Langerhans (Figures lS.7b and c), which are the endocrine portion of the pancreas. The pancreatic islets contain four ce ll types. On your slides, you will probably be unable to identify the different cell types.

However, each type is responsible for producing a spe­cific hormone, as follows:

• Alpha cells produce glucagon.

• Beta cells produce insulin.

• Delta cells produce somatostatin.

• F cells produce pancreatic polypeptide (PP).

Adrenal Gland

1. Obtain a slide of the adrenal gland.

2. View the slide with the scanning or low-power objective lens and identify the outer adrenal cortex and the inner adrenal medulla (Figure lS.Sb).

3. Center the adrenal cortex and switch to high power. Iden­tify the following three cellular layers (Figure lS.Sc ).

• The zona gomerulosa is the outermost layer and is cov­

ered by a connective tissue capsule (the capsule may not be present on your slide). It comprises 10% to 15% of adrenal cortical volume. Notice how the cells in this layer are arranged in small circular clusters. These cells produce mineralcorticoids.

• The zona fasciculata is the middle layer and makes up 75% to 7S% of the volume of the adrenal cortex. As you move the slide into this region, notice that the cells are larger and more lightly stained than those in the previ­ous layer. The lighter staining is due to the large supply of lipids in the cytoplasm. Observe that the cells in this layer are organized into stacks or columns, rather than clusters. These cells manufacture glucocorticoids.

• The zona reticularis is the smallest (7% to 10% of the total cortical volume) and innermost layer of the adrenal cortex . As you move into this layer, notice that the cells are more deeply stained and form an irregular, intersect­ing network. The cells in this layer produce a small amount of androgens.

WHAT'S IN A WORD The three zones of the adrenal cortex are named according to the organization of the cells in each layer. The term glomerulosa is derived from the Latin word glomus,

which means "a baH". The cells in the zona glomerulosa are arranged in a spherical fashion . The term fasciclIlata has its ori­gins from the Latin word fasciculus, which refers to "a bundle or corel". The name describes the columns of cells in the zona fas­ciculata. The word reticularis comes from the Latin term reticulatus, which means "netlike", ancl is suggestive of cell ar­rangment in the zona reticularis.

4. Switch back to low power, locate the adrenal medulla , and center this region in the field of view.

5. With high power, observe the cells in the adrenal medulla (Figure lS.Sc). This region consists of loosely arranged polyhedral cells with large round nuclei. An extensive net­work of capillaries travels between the cells and a large medullary vein (or veins), which drains the entire adre­nal gland. may be identified. The endocrine cells in the adrenal medulla resemble cells found in sympathetiC gan­glia, and their secretory activity is promoted by pregan­glionic sympathetic nerve fibers . The majority of the cells produce epinephrine, and a smaller number synthesize norepinephrine.

Ovary

1. Obtain a slide of the ovaries from a human or another mammalian species. The ovaries are the primary sex or­gans in the female.

2. View the slide with the scanning or low-power objective lens. Identify the two regions of the ovary (Figures lS.9b and c).

• The outer cortex contains the ovarian follicles at vari­ous stages of development. Each follicle contains a de­veloping egg cell, known as an oO(:yte.

• The inner medulla is a region of loose connective tis­sue with numerous blood vessels, nerves, and lym­phatics.

3. Scan the cortex and identify follicles at various stages of development (Figure lS.9c). In each developing follicle, identify the egg cell and the multiple layers of follicular cells that surround it. The follicular cells produce the fe­male sex hormones known as estrogens.

Testis

1. Obtain a slide of the testes from a human or another mam­malian species. The testes are the primary sex organs in the male.

2. View the slide with the scanning or low-power objective lens. Scan along the edge of the section and observe the fi­brous connective tissue covering called the tunica albug­inea. Connective tissue partitions derived from the tunica albuginea divide the testes into lobules.

3. Within each lobule of the testes are three or four seminiferous tubules. As you scan the slide under low power, the tubules can be observed throughout the field of view. Each tubule is surrounded by connective tissue and contains several layers of cells surrounding a central lu­

men (Figure IS. lOb) . Because of the plane of section, the lumen may not be evident in some tubule profiles.

4. Observe a seminiferous tubule under high power (Figure lS.10c). Most of the cells in the walls of the tubules are sperm cells in various stages of development. Collectively, these cells are called spermatogenic cells. As the sperm cells form , they move from the base to the lumen of the seminiferous tubules. Observe these various cells on the slide. Note that as you view the cells in the tubule walls, from base to lumen, their appearance changes progressively.

5. Scan the slide under high power and observe areas of connective tissue between seminiferous tubules. These in­terstitial areas contain the interstitial (Leydig) cells (Figure IS.10c), which produce the male sex hormone , testosterone.

THE ENDOCRI N E SYSTEM

Based on your microscopic observations in the previous activity, identify structural similarities

and differences in the various endocrine organs. Focus your at­tention on the arrangement and structure of the glandular cells in each structure.

Similarities:

Differences:

Name ____________________________________ ___ Exercise 18 Review Sheet LabSectio" ________ ----_______________________

The Endocrine System Date ______________________________________ __

1. Discuss the differences between an endocrine gland and an exocrine gland.

2. Target cells respond to inputs from both the nervous system and the endocrine system. In general , how does a neural response differ from an endocrine response?

3. What is meant by a neuroendocrine effect?

4. Explain how the hypothalamus influences the function of the anterior lobe of the pitu­itary gland.

5. Describe the functional relationship between the hypothalamus and the posterior lobe of the pituitary.

Questions 6-12: Match the hormone in column A with its function in column B.

A B

6. Insulin a. Regulates cell metabolism

7. Oxytocin b. Lowers blood glucose levels

8. Aldosterone c. Promotes sperm development

9. Epinephrine d. Elevates b'lood calcium levels

10. Testosterone e. Promotes uterine contractions during labor

II. Thyroxine f. Promotes egg development

12. Parathyroid hormone g. Acts on the kidneys to conserve water and sodium

h. Lowers blood calcium levels

i. Promotes the fight-or-flight response

j. Elevates blood glucose levels