How do exocrine glands differ from endocrine glands

Summary

Exocrine glands secrete their products directly into ducts, whereas endocrine glands release their products (hormones) into the bloodstream. Specific hormones influence the growth and function of certain target tissues. Hormones can be proteins or smaller polypeptides, amines, steroids, or fatty acid derivatives. Methods used in the science of endocrinology include bioassay, radioimmunoassay, nonradioactive methods such as ELISA, and molecular techniques. Paracrines are local chemical messengers that are not transported in the blood.

The pituitary (hypophysis) has two major parts: the neurohypophysis and the adenohypophysis. Neurosecretory neurons in the hypothalamus synthesize oxytocin and vasopressin, which travel to the neurohypophysis in neurosecretory cell axons. The adenohypophysis contains three regions: the pars distalis, pars tuberalis, and pars intermedia (reduced or absent in humans). Cells in the pars distalis secrete follicle-stimulating hormone, luteinizing hormone, prolactin, corticotropin, growth hormone, thyrotropin, lipotropin, endorphins, and enkephalins. Other pituitary cells secrete melanophore-stimulating hormone, and the pars tuberalis could also secrete FSH and LH.

Neurosecretory neurons in the hypophysiotropic area of the hypothalamus secrete releasing hormones or release-inhibiting hormones into the median eminence region at the base of the hypothalamus. Here, capillaries receive these hormones, which then travel in the blood of the hypothalamo–hypophysial portal system to the endocrine cells of the adenohypophysis. The releasing hormones then increase the secretion of specific adenohypophysial hormones, whereas the release-inhibiting hormones have the opposite effect.

Because one of these hypothalamic-releasing hormones increases the secretion of both FSH and LH, it is called a gonadotropin-releasing hormone. Because GnRH regulates the release of gonadotropic hormones, which themselves control gamete production and hormone release from the gonads (ovaries and testes), GnRH plays a central role in human reproduction. The surge center of the hypothalamus causes a surge of LH secretion just before ovulation by increasing GnRH secretion from the HTA. The pineal gland secretes the hormone melatonin, which exerts inhibitory effects on gonadotropin secretion.

Feedback systems control FSH and LH secretion from the adenohypophysis. FSH and LH cause the gonads to secrete gonadal hormones (estrogens, progestins, androgens, glycoproteins), that can decrease (by negative feedback) further secretion of FSH and LH. Estrogen also can have a positive feedback effect on LH secretion in women. Prolactin secretion from the pars distalis is controlled by a prolactin release-inhibiting factor (dopamine) from the hypothalamus, along with possible prolactin-releasing factors.

Reproduction

The prostate is an exocrine gland (excretes fluid) and has no known endocrine (hormonal) function. The exocrine secretions make up 15% of the ejaculate volume, but this fluid in itself is not necessary for fertility. The prostate along with the bladder neck and prostatic urethra play a critical role in coordinated antegrade sperm delivery during ejaculation. At the time of ejaculation, sympathetic nerve fibers innervating the α1 receptors in the smooth muscle in the prostate, bladder neck, and vas deferens cause emission of seminal fluid and sperm into the prostatic urethra, and closure of the bladder neck and prostatic urethra. This closure prevents the sperm and seminal fluid from going into the bladder during ejaculation and is necessary for normal ejaculation.

The Renal Lobule

The kidney is an exocrine gland that secretes urine. As in all exocrine glands the functional unit of the kidney is the lobule, but unlike many other exocrine glands, the lobules are not separated by connective tissue septae. Combined with the fact that the lobule is not a discrete mass of secretory cells pouring their secretions into an intralobular duct, this makes the renal lobule singularly difficult to visualise and describe. The three-dimensional structure of the renal lobule is not much discussed in textbook accounts of the kidney.

A renal lobule may be defined as a number of nephrons that discharge urine into a single collecting duct. This duct begins near the surface of the cortex, runs through the cortex in a medullary ray and continues into the medulla where it joins other collecting ducts to form the large papillary ducts that open into the pelvis of the kidney at the surface of the papilla. Prior to this merging, the collecting duct (or tubule) may be regarded as the intralobular duct of the renal lobule, and as such can be considered to define the axis of the lobule.

Each lobule has a cortical and a medullary component. The cortical component comprises the glomeruli and their tubules (proximal convoluted tubules, loops of Henle and distal convoluted tubules) that feed into the collecting duct. These components form a transected cone of cortical tissue (or a frustum, in geometrical terms: a cone without its pointed end) that sits with its large end towards the surface of the cortex and its smaller end at the cortico-medullary junction. The surfaces of the frusta (or frustums) are defined by the interlobular vessels that supply blood to, and collect blood from, the nephrons of the lobule. The long axis of the frustum is defined by the medullary ray, which is conical with its point towards the surface of the kidney, and its base at the cortico-medullary junction. The medullary ray widens as it approaches the cortico-medullary junction because it contains, in addition to the widening collecting duct, straight parts of the proximal tubules of nephrons lying deep in the cortex, and straight parts of distal tubules returning from the medulla.

The medullary part of the lobule comprises the loops of Henle. The lobule also contains the blood vessels that the supply tissue within it. We must therefore include the dense capillary plexuses of the cortex and part of the medulla and the bundles of vessels that dip deep into the medulla and then return to the arcuate vessels at the cortico-medullary junction.

Although the cortical part of the lobule is tolerably well defined by the interlobular vessels and the medullary rays, the lobular structure in the medulla is impossible to distinguish. Little attention seems to have been paid to the three-dimensional shape of the renal lobule: the three-dimensional geometry of the distribution of interlobular vessels at the periphery of the cortical parts of the lobule seems not to have been explored in detail, at least not with the enthusiasm that has been lavished on the lobules of the liver (see Chapter 14: Liver). Fig. 17.1 shows the arrangement of the blood vessels and tubules in the several zones of the rat kidney.

Function

The mammary gland is an exocrine gland in mammals—e.g., cows—that produce milk to nourish their young and for milk production (e.g., dairy herds). A mamma (L. the breast, mammary gland, lactiferous gland, udder) is called the breast in primates and udder in ruminants—e.g., cows, goats, and sheep. The mammary gland is considered part of the common integument system and is a modified cutaneous glandular structure with each gland producing milk in the alveoli. Lactation is the production of milk for nursing that occurs following a gestation/pregnancy of 9 months in cows. Lactation occurs under the hormonal influence of estrogen, progesterone, prolactin, and oxytocin.

In a few mammalian species, male lactation can also occur. Lactorrhea or galactorrhea (Gr.

milk + Gr.

flow) is the spontaneous flow of milk by the glands that can occur in any mammal.

3.2.2.3 Sebaceous glands

Sebaceous glands originated from epidermal cells are microscopic exocrine glands which secret an oily or waxy substance (sebum) to lubricate skin and hair and can be found with the hair follicles. Sebum is produced by holocrine secretion and releases from their fatty cytoplasm by the cells break down. Although the full function of these glands is unknown, but they are important in permeability of the epidermal barrier and hormonal signaling of the skin. They can transport antioxidants to skin and protect skin surface from UV radiation. They are inactive in childhood and will activate by puberty and become sensitive to androgens [34].

Determining the Source and Behavioral Effects of Trail Pheromones

Understanding the glandular sources and chemistry of trail substances is at the heart of the study of recruitment communication. The nature of the bioassay, the behavioral test used to determine the effectiveness of different substances as recruitment or trail pheromones, is critically important. The bioassays used in trail pheromone isolation and identification must distinguish among the range of behavioral responses involved in trail communication. It must be noted that trail pheromones can have both recruitment and orientation effects. A recruitment pheromone induces nestmates to leave the nest to travel to a work site. An orientation pheromone has no such stimulatory effect, but it can serve as a chemical “guide” for worker traffic. Trail substances, if they have a recruitment effect, will stimulate nestmates to leave the nest or otherwise alter their task performance in the context of a current need. In some ants, nestmates are recruited with a motor display delivered in the nest by a recruiting worker. The trail substance in this case does not have the ability to draw ants out from the nest; rather, it is used as an orientation cue by nestmates that have contacted a recruiting ant. Some trail pheromones can alone elicit both excitation and orientation in the absence of any other behavioral display or stimulus. If an artificial trail (one prepared from a solvent extract of the appropriate exocrine gland) is drawn out from the nest entrance and ants leave the nest to follow it, a recruitment effect has been demonstrated. If the artificial trail cannot induce inactive workers to leave the nest, yet the trail is able to orient workers alerted by either a motor display or some other trail chemical that has an alerting property, an orientation effect is occurring. Careful dissection of the kinds and sequences of behaviors in the recruitment process and detailed chemical analyses have revealed that several pheromone constituents may control a number of behaviors associated with recruitment and trail following.

3.4 The role of TRPM8-expressing neurons as humidity detectors

Changes in the continuous aqueous fluid secretion by exocrine glands that maintain the wetness of ocular surface can lead to eye dryness syndrome. The neural mechanisms underlying basal tearing that keeps the humidity of the cornea were described few years ago, unveiling a critical role of TRPM8 channel in this phenomenon (Parra et al., 2010).

Among the preparations used to record the electrical activity of primary sensory neurons, the cornea constitutes one of the most useful and powerful tools (Brock, McLachlan, & Belmonte, 1998; Parra et al., 2010) (Figure 11.1(B)). Trigeminal sensory neurons densely innervate this structurally simple tissue, where nerve endings of cold thermoreceptors, mechanoreceptors, and polymodal nociceptors are largely accessible to electrophysiological recording in vitro. Using focal recording of the electrical activity of unitary cold-sensitive nerve endings in the mouse cornea, Parra and coworkers demonstrated that not only the responses to cold but also the ongoing electrical activity of these exquisitely cold-sensitive sensory neurons were proportional to the expression level of TRPM8 channels (Parra et al., 2010) (Figure 11.1(C)). Basal tearing varies with environmental conditions, and blinking rate produces small changes in the temperature of the ocular surface. Parra and coworkers hypothesized that cold thermoreceptor neurons innervating the cornea serve as humidity detectors, sensing the small temperature drops of the corneal surface caused by the evaporation of the tear film during interblink periods. Thus, almost 80% of the cold-sensitive corneal nerve fibers respond to temperature decreases of 2 °C or less, a change equivalent to interblink temperature drops. Basal firing of corneal cold thermoreceptors and their responses to cold and menthol were absent in TRPM8 knockout mice, and proportionally reduced in heterozygous animals. Interestingly, mice lacking TRPM8 showed a largely reduced basal tearing compared to wild-type animals, and a normal nociceptor-mediated irritative tearing in response to activators of TRPV1 and TRPA1 channels. In wild-type animals, basal tearing is strongly reduced by heating the corneal surface, a maneuver that reduces the firing of cold thermoreceptors and also reduces the basal tearing in humans. Thus, TRPM8-expressing neurons innervating the cornea work as humidity detectors, a critical neural component of the physiological machinery that maintains the basal wetness of the ocular surface and, probably, other exposed mucosae.

Definition and Etiology

In CF, there is widespread dysfunction of exocrine glands that causes chronic pulmonary disease; pancreatic enzyme deficiency; intestinal obstruction in the neonate (distal intestinal obstruction syndrome); liver disease; infertility, especially in males; and abnormally high concentrations of electrolytes in sweat, resulting from the failure of salt reabsorption in the sweat gland ducts. This is the most common inherited disease in Caucasian populations. A gene located on chromosome 7, coding for the protein called cystic fibrosis transmembrane regulator (CFTR), is defective. CFTR acts as a cyclic-AMP-activated chloride channel blocker. More than 800 mutations of the gene have been identified, and they are categorized into five classes on the basis of CTFR alterations. The most predominant mutation, which accounts for approximately 70% of all the CTFR genes worldwide, is Δ508, but there is geographical variation and it is less common in non-white races.

Although previously this disease was considered lethal in childhood, the median survival for newborns in the 1990s is predicted to be 40 years. Survival is largely dependent on the severity and progression of lung disease, and more than 90% of mortality is due to chronic bronchial infections and their complications. Patients with pancreatic insufficiency have a worse prognosis in terms of growth, pulmonary function, and long-term survival. The mortality of females is generally greater than that of males.

Anatomical features in equids

Mammary gland

The equine mammary gland lies between 2 folds of abdominal skin. The gland is divided into symmetrical left and right mammae by a septum along the longitudinal intermammary groove. Each mamma consists of a glandular portion and a teat and is flattened laterally. The skin is sparsely haired, with numerous sebaceous and sweat glands. A layer of dark-colored sebum covers the skin of the intermammary groove and protects the mammae and reduces friction during locomotion.

The glandular portion of each mamma is organized into cranial and caudal lobes (Fig. 16.2-3). Within the lobes, secretory epithelial cells are organized in alveoli, which open into small ducts. The alveoli and ducts are grouped into clusters called lobules and are surrounded by myoepithelial cells, important for milk ejection. The lactiferous ducts converge into a milk cistern, one for each lobe. The cistern has a glandular portion or glandular cistern and a teat portion or papillary cistern. The papillary cistern opens into an orifice at the teat through the papillary duct. There are 2 orifices, one for each lobe, that open into a depression at the apex of the teat.

Figure 16.2-3. Anatomy of the equine mammary gland. Cranial is to the left.

This anatomical disposition has implications for local treatment of mastitis. Since the anterior and posterior lobes, and their respective cisterns, do not communicate with each other, intramammary antibiotics should be infused through both orifices of the teat to reach therapeutic drug concentrations within each cistern and lobe.

Support for each mamma is provided by the lateral and medial suspensory ligaments (see Case 23.2, Fig. 23.2-4). Supportive trabeculae project inward from the ligaments, enclose the lobes, and branch into finer lobular connective tissue that surrounds ducts and alveoli.

Blood supply, lymphatics, and innervation

Hormones inducing mammary development, lactation, and milk ejection arrive to the mammary gland via its blood supply. The blood supply is also important for milk production, since milk is essentially a blood filtrate. The main blood supply and drainage are provided by the external pudendal artery and vein. The external pudendal artery descends through the inguinal canal and enters the caudal part of the gland where it branches into the cranial and caudal mammary arteries. A venous plexus is present at the base of each mamma, which drains to the contralateral external pudendal veins. Blood also drains from the plexus cranially into the caudal superficial epigastric vein and then drains into the superficial vein of the thoracic wall. The plexus also drains caudally to the obturator vein or branches of the internal pudendal vein.

Premature or Inappropriate Mammary Gland Development

Some conditions associated with placental insufficiency and chronic fetal stress may induce premature mammary gland development or lactation (<  300 d of gestation). The most common cause of premature lactation in the mare is placentitis. Other differential diagnoses are twin pregnancy, uterine body pregnancy, or placental separation, and pituitary pars intermedia dysfunction. On the other hand, agalactia is a failure to produce milk or colostrum, most commonly caused by fescue toxicosis. Tall fescue can be infested with an endophyte that produces ergopeptines. These indole alkaloids can interfere with prolactin secretion and can constrict blood vessels. Other causes of agalactia or hypogalactia (not enough milk production) include advancing age, debilitation, malnutrition, or first parity. Foals from mares with inappropriate lactation (both premature lactation and agalactia) may fail to receive adequate amounts of protective antibodies from colostrum, which increases the risk for neonatal septicemia.

Lymphatic drainage is provided by lymphatic vessels that converge into superficial inguinal lymph nodes at the base of each mamma.

The main innervation is by the genitofemoral nerve coursing through the inguinal canal. The skin is supplied by nerves of the flank (presumably the iliohypogastric and ilioinguinal nerves, similar to what is reported in the cow) and a descending branch from the pudendal nerve.

The superficial inguinal lymph nodes can be enlarged in cases of mastitis.

Mammary gland changes with gestation and lactation

Major changes in mammary gland size and secretions occur at the end of gestation. Initiation of mammary gland development and lactation requires the coordinated action of several hormones. Prolactin is produced in the anterior pituitary and acts synergistically with estrogens and progestogens to promote development of ducts and alveoli, respectively. Prolactin also stimulates milk production and secretion. While the mammary gland is exposed to high concentrations of estrogens and progestogens during gestation, prolactin concentrations increase during the last week of gestation in the mare and remain high after foaling in support of lactation. Mammary gland development can first be seen 2–4 weeks prepartum. As parturition approaches, a small amount of gray watery secretions can be expressed from the mammary glands. These secretions become progressively whiter and thicker to become the thick immunoglobulin-rich colostrum at foaling.

Oxytocin also plays an important role in milk ejection, since it stimulates contraction of the myoepithelial cells that surround the alveoli and ducts.

The changes in prepartum mammary gland secretion electrolytes can be monitored and used to predict labor/foaling. Calcium concentration increases in mammary gland secretions in the last 72 h before foaling. This rise is associated with fetal readiness for birth.

Ultrasound of the Mammary Gland

As demonstrated in this case, mammary gland can be evaluated with ultrasound (Fig. 16.2-4). The intermammary septum appears as a hyperechoic line between the 2 mammae. Normal parenchyma is homogeneous and hyperechoic. The lactiferous ducts and milk cistern are identified as anechoic or hypoechoic structures and are more prominent in lactating or late-gestation mares. However, the duct system may be visualized in some mares that chronically lactate or accumulate small amounts of secretions in the milk cistern. The blood vessels are also more prominent in lactating mares and can be distinguished from the duct system with color flow Doppler (Fig. 16.2-4C). In cases of mastitis (Fig. 16.2-2), the mammary gland secretions may become more cellular due to the presence of neutrophils, yielding a more echogenic and heterogeneous image of the ducts and cistern. The glandular tissue may also appear more echogenic and heterogeneous due to the presence of fibrosis or inflammatory cell infiltration. Abscesses may be identified as focal cystic areas, while neoplasia usually appears as a focal mass with loss of normal architecture of the parenchyma.

Figure 16.2-4. (A) Ultrasonographic image of the glandular portion of the 2 mammae. Key: (1) Glandular portion of the milk cisterns of the left and right mammae, (2) septum at the intermammary groove. (B) Ultrasonographic image of one teat showing cranial and caudal papillary cisterns (3). (C) Color Doppler image of a blood vessel within the mammary gland parenchyma.

The Exocrine Pancreas Secretes Digestive Enzymes and HCO3−

The pancreas has both exocrine and endocrine functions. Exocrine glands are glands with ducts that secrete materials onto some surface—generally the skin, the gastrointestinal tract, or respiratory epithelium. Endocrine glands are ductless and secrete hormones into the blood. The pancreas is both of these. Clusters of cells called the islets of Langerhans, distributed throughout the pancreas, secrete the hormones insulin, glucagon, and somatostatin, which regulate metabolism and the fate of absorbed nutrients. These endocrine functions are discussed in Chapter 9.4. The exocrine pancreas consists of clusters of acini, hollow spheroids of some 20–50 pyramidal cells arranged around a central lumen. The acini form lobules that are separated by loose connective tissue. Each acinus is drained by an intralobular duct which joins other ducts to form interlobular ducts and then progressively larger ducts until they reach the main pancreatic duct. The acinar cells secrete protein enzymes that digest food. The ductal cells secrete a watery, alkaline solution that neutralizes stomach acid and carries the enzymes forward (see Figure 8.4.1).