Development of epithelial glands

Most glands develop as epithelial outgrowths into the underlying connective tissue (Figure 2-1). Exocrine glands remain connected to the surface of the epithelium by an excretory duct that transports the secretory product to the outside. Endocrine glands lack an excretory duct, and their product is released into the blood circulation.

Figure 2-1 Development of exocrine and endocrine glands

Endocrine glands are surrounded by fenestrated capillaries and commonly store the secretions they synthesize and release after stimulation by chemical or electrical signals. Exocrine and endocrine glands can be found together (for example, in the pancreas), as separate structures in endocrine organs (thyroid, parathyroid), or as single cells (enteroendocrine cells). Endocrine glands will be studied later in Chapter 18, Neuroendocrine System, and Chapter 19, Endocrine System.

Classification of epithelial glands

Glands are classified according to the type of excretory duct into simple and branched (also called compound) glands. The gland can be simple (Figure 2-2) when the excretory duct is unbranched. The gland can be branched when the excretory duct subdivides (Figure 2-3).

Figure 2-2 Simple glands

Figure 2-3 Glands with branched ducts

The secretory portion can be unicellular or multicellular

An exocrine gland has two components: a secretory portion and an excretory duct. The secretory portion of a gland may be composed of one cell type (unicellular, for example, goblet cells in the respiratory epithelium and intestine) or many cells (multicellular).

According to the shape of the secretory portion (see Figures 2-2 and 2-3), glands can be tubular, coiled, or alveolar (Latin alveolus, small hollow sac; plural alveoli), also called acinar (Latin acinus, grape; plural acini).

Tubular glands are found in the large intestine. The sweat glands of the skin are typical coiled glands. The sebaceous gland of the skin is an example of an alveolar gland.

Shape of the secretory portion

Glands can be classified as simple tubular or simple alveolar (or acinar) according to the shape of the secretory portion. In addition, tubular and alveolar secretory portions can coexist with branching excretory ducts; the gland is called a branched (or compound) tubulo-alveolar (or acinar) gland (for example, the salivary glands). The mammary gland is an example of a branched alveolar gland.

A branched gland (Figure 2-4) is surrounded by a capsule. Septa or trabeculae extend from the capsule into the glandular tissue. Large septa divide the gland into a number of lobes. Branches from the septa separating adjacent lobes divide the lobes into smaller compartments called lobules.

Figure 2-4 Histologic overview of a compound salivary gland

During development, a main excretory duct gives rise to branches that lie either between (interlobar) or within lobes (intralobar). Small branches derived from each of these ducts generate small subdivisions that constitute the lobule of a gland. These branches can be found first between lobules (interlobular) and within lobules (intralobular). Additional details are presented in Chapter 17, Digestive Glands.

Types of secretion

Based on the type of secretion, exocrine glands can be classified as mucous glands, when their products are rich in glycoproteins and water; serous glands, with secretions enriched with proteins and water; and mixed glands, which contain both mucous and serous cells (Figure 2-5).

Figure 2-5 Histologic differences between submandibular, sublingual, and parotid glands

Mechanisms of secretion

Exocrine glands can also be classified on the basis of how the secretory product is released (Figure 2-6).

Figure 2-6 Mechanisms of glandular secretion

In merocrine secretion (Greek meros, part; krinein, to separate), the product is released by exocytosis. Secretory granules are enclosed by a membrane that fuses with the apical plasma membrane during discharge or exocytosis. An example is the secretion of zymogen granules by the pancreas.

In apocrine secretion (Greek apoknino, to separate), the release of the secretory product involves partial loss of the apical portion of the cell. An example is the secretion of lipids by epithelial cells of the mammary gland. Proteins secreted by epithelial cells of the mammary gland follow the merocrine pathway (exocytosis).

In holocrine secretion (Greek holos, all), the secretory product constitutes the entire cell and its product. An example is the sebaceous glands of the skin, which produce a secretion called sebum.

Plasma membrane

A review of major concepts of cytomembranes and organelles and their clinical relevance is presented in this chapter. Epithelial glands are a convenient topic for this integration. We initiate the review by addressing the structural and biochemical characteristics of the plasma membrane. Additional information related to plasma membrane–mediated cell signaling is presented in Chapter 3, Cell Signaling.

The plasma membrane determines the structural and functional boundaries of a cell. Intracellular membranes, called cytomembranes, separate diverse cellular processes into compartments known as organelles. The nucleus, mitochondria, peroxisomes, and lysosomes are membrane-bound organelles; lipids and glycogen are not membrane-bound and are known as inclusions.

The plasma membrane consists of both lipids and proteins. The phospholipid bilayer is the fundamental structure of the membrane and forms a bilayer barrier between two aqueous compartments: the extracellular and intracellular compartments. Proteins are embedded within the phospholipid bilayer and carry out specific functions of the plasma membrane such as cell-cell recognition and selective transport of molecules (see Box 2-A).

Phospholipid bilayer

The four major phospholipids of plasma membranes are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin (Figure 2-7). They represent more than one half the lipid of most membranes. A fifth phospholipid, phosphatidylinositol, is localized to the inner leaflet of the plasma membrane.

Figure 2-7 Structure of the plasma membrane

In addition to phospholipids, the plasma membrane of animal cells contains glycolipids and cholesterol. Glycolipids, a minor membrane component, are found in the outer leaflet, with the carbohydrate moieties exposed on the cell surface.

Cholesterol, a major membrane constituent, is present in about the same amounts as are phospholipids. Cholesterol, a rigid ring structure, does not form a membrane but is inserted into the phospholipid bilayer to modulate membrane fluidity by restricting the movement of phospholipid fatty acid chains at high temperatures. Cholesterol is not present in bacteria.

Two general aspects of the phospholipid bilayer are important to remember:

Membrane proteins

Most plasma membranes consist of about 50% lipid and 50% protein (Figure 2-8). The carbohydrate component of glycolipids and glycoproteins represents 5% to 10% of the membrane mass. The surface of a plasma membrane is coated by a glycocalyx (see Box 2-B).

Figure 2-8 Peripheral and integral proteins of the plasma membrane

According to the fluid mosaic model of the membrane structure, membranes are two-dimensional fluids in which proteins are inserted into lipid bilayers. It is difficult for membrane proteins and phospholipids to switch back and forth between the inner and outer leaflets of the membrane. However, because they exist in a fluid environment, both proteins and lipids are able to diffuse laterally through the plane of the membrane. However, not all proteins can diffuse freely; the mobility of membrane proteins is limited by their association with the cytoskeleton.

Restrictions in the mobility of membrane proteins are responsible for the polarized nature of epithelial cells, divided into distinct apical and basolateral domains that differ in protein composition and function. Tight junctions between adjacent epithelial cells (discussed in Chapter 1, Epithelium) not only seal the space between cells but also serve as barriers to the diffusion of proteins and lipids between the apical and basolateral domains.

Two major classes of membrane-associated proteins are recognized: peripheral proteins and integral membrane proteins.

Peripheral membrane proteins are not inserted into the hydrophobic interior of the membrane but are, instead, indirectly associated with membranes through protein-protein ionic bond interactions, which are disrupted by solutions of high salt concentration or extreme pH.

Portions of integral membrane proteins are inserted into the lipid bilayer. They can only be released by solubilization using detergents. Detergents are chemical agents that contain both hydrophobic and hydrophilic groups. The hydrophobic domains of the detergent penetrate the membrane lipids and bind to the membrane-inserted hydrophobic portion of the protein. The hydrophilic domains combine with the protein, forming aqueous-soluble detergent-protein complexes.

Numerous integral proteins are transmembrane proteins, spanning the lipid bilayer, with segments exposed on both sides of the membrane. Transmembrane proteins can be visualized by the freeze-fracture technique.

Freeze-fracture: Differences between a surface and a face

The freeze-fracture technique is valuable for the visualization of intramembranous proteins with the electron microscope. This technique provided the first evidence for the presence of transmembrane proteins in the plasma membrane and cytomembranes.

Specimens are frozen at liquid nitrogen temperature (−196°C) and “split” with a knife (under high vacuum) along the hydrophobic core of the membrane. As a result, two complementary halves, corresponding to each membrane bilayer, are produced. Each membrane half has a surface and a face. The face is artificially produced during membrane splitting.

A replica of the specimen is generated by evaporating a very thin layer of a heavy metal (generally platinum with a thickness of 1.0 to 1.5 nm) at a 45° angle to produce a contrasting shadowing effect. The platinum replica is then detached from the real specimen by floating it on a water surface, mounted on a metal grid, and examined under the electron microscope.

Figure 2-9 indicates the nomenclature for the identification of surfaces and faces in electron micrographs of freeze-fracture preparations.

Figure 2-9 Freeze-fracture: Differences between surface and face

The surface of the plasma membrane exposed to the extracellular space is labeled ES, for extracellular surface. The surface of the plasma membrane exposed to the cytoplasm (also called protoplasm) is labeled PS, for protoplasmic surface.

The face of the membrane leaflet looking to the extracellular space (the exocytoplasmic leaflet in the illustration) is labeled EF, for extracellular face. Similarly, the face of the leaflet facing the protoplasmic space (identified as a protoplasmic leaflet) is PF, for protoplasmic face.

Now that we have an understanding of what surface and face represent, remember that faces are chemically hydrophobic and surfaces are chemically hydrophilic. One last point: Note that a transmembrane protein stays with the protoplasmic leaflet, leaving a complementary pit in the opposite exocytoplasmic leaflet. Why? Cytoskeletal components may be directly or indirectly attached to the tip of the protein exposed to the cytoplasmic side and will not let go.

Transporter and channel proteins

Most biological molecules cannot diffuse through the phospholipid bilayer. Specific transport proteins, such as carrier proteins and channel proteins, mediate the selective passage of molecules across the membrane, thus allowing the cell to control its internal composition.

Molecules (such as oxygen and carbon dioxide) can cross the plasma membrane down their concentration gradient by dissolving first in the phospholipid bilayer and then in the aqueous environment at the cytosolic or extracellular side of the membrane. This mechanism, known as passive diffusion, does not involve membrane proteins. Lipid substances can also cross the bilayer.

Other biological molecules (such as glucose, charged molecules, and small ions—H⁺, Na⁺, K⁺, and Cl⁻) are unable to dissolve in the hydrophobic interior of the phospholipid bilayer. They require the help of specific transport proteins (Figure 2-10) and channel proteins, which facilitate the diffusion of most biological molecules.

Figure 2-10 Transporters

Similar to passive diffusion, facilitated diffusion of biological molecules is determined by concentration and electrical gradients across the membrane. However, facilitated diffusion requires one of the following:

Carrier proteins transport sugars, amino acids, and nucleosides. Channel proteins are ion channels involved in the rapid transport of ions (faster transport than carrier proteins), are highly selective of molecular size and electrical charge, and are not continuously open.

Some channels open in response to the binding of a signaling molecule and are called ligand-gated channels. Other channels open in response to changes in electric potential across the membrane and are called voltage-gated channels.

Endoplasmic reticulum

The endoplasmic reticulum is an interconnected network of membrane-bound channels within the cytoplasm, part of the cytomembrane system and distinct from the plasma membrane.

The endoplasmic reticulum system, consisting of cisternae (flat sacs), tubules, and vesicles, divides the cytoplasm into two compartments:

The smooth endoplasmic reticulum lacks ribosomes and is generally in proximity to deposits of glycogen and lipids in the cytoplasm. The smooth endoplasmic reticulum has an important role in detoxification reactions required for the conversion of harmful lipid-soluble or water-insoluble substances into water-soluble compounds more convenient for discharge by the kidneys. It also participates in steroidogenesis (see Chapter 19, Endocrine System).

Products released into the luminal compartment of the endoplasmic reticulum are transported to the Golgi apparatus by a transporting vesicle and eventually to the exterior of the cell by exocytosis. One can visualize the sequence in which the lumen of the cytomembrane system is interconnected and remains as such in an imaginary stage; you can visualize that the luminal compartment of a secretory cell is continuous with the exterior of the cell (Figure 2-11). The surrounding space is the cytosolic compartment in which soluble proteins, cytoskeletal components, and organelles are present.

Figure 2-11 Intracellular compartments

Now, let us visualize the membrane of each component of the cytomembrane system as consisting of two leaflets (Figure 2-12):

Figure 2-12 Leaflets of cytomembranes and plasma membrane

Let us imagine that exocytoplasmic and protoplasmic leaflets form a continuum. During the freeze-fracturing process, the knife fractures the membrane as it jumps from one fracture plane to the other across the hydrophobic core and splits membranes into two leaflets. The knife cannot stay with a single membrane because cytomembrane-bound organelles occupy different levels and have random orientations within the cell. This randomness will be apparent during the examination of the replica.

The sample may contain a combination of exocytoplasmic and protoplasmic leaflets which, in turn, can expose surfaces and faces. Membrane proteins tend to remain associated with the cytoplasmic (protoplasmic) leaflet and appear as particles on the PF (protoplasmic face). A shallow complementary pit is visualized in the EF (exocytoplasmic face).

Rough endoplasmic reticulum

The rough endoplasmic reticulum is recognized under the light microscope as a diffuse basophilic cytoplasmic structure called ergastoplasm.

The rough endoplasmic reticulum is involved in the synthesis of proteins, carried out by their attached ribosomes (Figure 2-13). In contrast, the membranes of the smooth endoplasmic reticulum lack attached ribosomes (see Figure 2-13). Most proteins exit the rough endoplasmic reticulum in vesicles transported to the cis portion of the Golgi apparatus (see Figures 2-16 and 2-17). Other proteins are retained by the rough endoplasmic reticulum to participate in the initial steps of protein synthesis (see Figure 2-15). The retained proteins contain the targeting sequence Lys-Asp-Glu-Leu (KDEL) at the C-terminal. A lack of the KDEL sequence marks proteins for transport to the Golgi apparatus.

Figure 2-13 The endoplasmic reticulum

Figure 2-14 Protein synthesis, transport, and secretion by exocrine pancreatic cells

Figure 2-15 Protein synthesis: Signal hypothesis

Figure 2-16 Secretory and lysosomal sorting pathways

Figure 2-17 Compartments of the Golgi apparatus

Protein synthesis and sorting

The role of the endoplasmic reticulum in protein synthesis and sorting was demonstrated by incubating pancreatic acinar cells in a medium containing radiolabeled amino acids and localizing radiolabeled proteins by autoradiography. The secretory pathway taken by secretory proteins includes the following sequence: rough endoplasmic reticulum, to Golgi apparatus, to secretory vesicles, to the extracellular space or lumen (Figure 2-14). Plasma membrane and lysosomal proteins also follow the sequence of rough endoplasmic reticulum to Golgi apparatus but are retained within the cell.

Proteins targeted to the nucleus, mitochondria, or peroxisomes are synthesized on free ribosomes and then released into the cytosol. In contrast, proteins for secretion or targeted to the endoplasmic reticulum, Golgi apparatus, lysosomes, or plasma membrane are synthesized by membrane-bound ribosomes and then transferred to the endoplasmic reticulum as protein synthesis progresses.

Ribosomes attach to the endoplasmic reticulum under the guidance of the amino acid sequence of the polypeptide chain being synthesized. Ribosomes synthesizing proteins for secretion are directed to the endoplasmic reticulum by a signal sequence at the growing end of the polypeptide chain.

The mechanism by which secretory proteins are directed to the endoplasmic reticulum is explained by the signal hypothesis (Figure 2-15).

Functions of the Golgi apparatus

Three specific functions are carried by the Golgi apparatus: (1) Modification of carbohydrates attached to glycoproteins and proteoglycans received from the endoplasmic reticulum. This process is called glycosylation. A characteristic glycosylation event within the Golgi is the modification of N-linked oligosaccharides on glycoproteins. More than 200 enzymes participate in the biosynthesis of glycoproteins and glycolipids in the Golgi apparatus. The enzymes called glycosyltransferases add specific sugar residues; the enzymes called glycosidases remove specific sugar residues. (2) Sorting of cargos to several destinations within the cell. We discuss in another section of this chapter how the Golgi apparatus marks specific proteins for sorting to lysosomes. (3) The synthesis of sphingomyelin and glycosphingolipids.

Once processed, cargos bud off from the Golgi apparatus and are either sorted to the secretory of lysosomal sorting pathway (anterograde traffic) or back to the endoplasmic reticulum (retrograde traffic) (see Figure 2-16).

Certain classes of cargos are stored into secretory granules for later release in response to an extracellular signal. This mechanism is called facultative or regulated secretion. Other cargos can be secreted continuously without a need of a stimulus. This mechanism is called constitutive secretion; it supplies newly synthesized lipids and proteins to the plasma membrane or proteins to be released outside the cell such as proteins of the extracellular matrix or immunoglobulins during immune reactions.

Cargo sorting occurs along microtubules or actin filaments with the help of motor proteins. The presence of specific lipids domains in the membrane of a vesicle cargo recruit coating proteins and tethering factors to sort the cargo in the direction of an acceptor membrane site. Essentially, the sorting and transport of cargos depend on specialized coats preparing the cargo to be moved along the cytoskeleton by molecular motor proteins. Tethering factors (rod-shaped proteins) attach the cargo to the cytoskeleton. When the vesicle cargo reaches an acceptor membrane, it fuses with the help of fusion proteins.

Exocytosis or secretory pathway and endocytic pathway

Vesicle transport involves the mobilization of proteins and membrane between cytomembrane compartments. The exocytosis or secretory pathway starts in the endoplasmic reticulum, continues through the Golgi apparatus, and ends on the cell surface. The endocytic pathway consists in the internalization and degradation of extracellular material from the plasma membrane, through endosomes to lysosomes.

These two events depend on distinctive proteins coating the cytosolic side of the membrane of the transport vesicle that becomes a coated vesicle. The coat helps the recruitment of molecules for transport. Before fusion with the acceptor membrane, the vesicle sheds its coat, allowing the membranes to interact directly and fuse.

Transport vesicles are coated by the protein clathrin. Clathrin-coated vesicles are seen in the exocytosis/secretory and endocytosis pathway. In the endocytosis pathway (Figure 2-18), vesicles start at the plasma membrane as clathrin-coated pits. Clathrin molecules assemble in a basket-like arrangement on the cytosolic face of the plasma membrane and the pit shape changes into a vesicle.

Figure 2-18 Endocytosis pathway: cholesterol uptake

Dynamin, a small GTP-binding protein, surrounds the neck of the invaginated coated pit, causing the neck of the vesicle to pinch off from the plasma membrane. A second class of coat proteins are the adaptins. Adaptins stabilize the clathrin coat to the vesicle membrane and assist in the selection of cargos for transport by binding to cargo receptors on the vesicle membrane. When the cargo reaches the target acceptor membrane, the coat proteins are shed and the membranes can fuse.

Sorting of clathrin-coated vesicles and COP-coated vesicles

A continual process of budding and fusion of transport vesicles mobilizes products from the endoplasmic reticulum to the Golgi apparatus (anterograde traffic), between membranous stacks of the Golgi apparatus, and from the Golgi apparatus to the endoplasmic reticulum (retrograde traffic) (see Figure 2-16).

The vesicular transport mechanism involves two types of coated vesicles (Figure 2-19):

Figure 2-19 Clathrin- and COP-mediated vesicle transport

We have already seen that adaptins mediate the binding of clathrin to the vesicular membrane as well as select specific molecules to be trapped in a vesicle. What about COP-coated vesicles?

A guanosine triphosphate (GTP)-binding protein called ARF (for adenosine diphosphate [ADP]-ribosylation factor), is required for the assembly of COPI and COPII molecules to form a protein coat called a coatomer on the cytosolic side of a transporting vesicle. When GTP is converted to guanosine diphosphate (GDP) by hydrolysis, the coatomer dissociates from the vesicle just before the vesicle fuses with a target membrane. ARF is related to Ras proteins, a group of oncogene proteins also regulated by the alternate binding of GTP and GDP (see the MAP kinase pathway in Chapter 3, Cell Signaling).

Vesicle fusion to a target membrane requires NSF and SNARE proteins

The fusion of a transporting vesicle to a target membrane (Figure 2-20) requires the recognition of the specific target membrane so the vesicle and target membranes can fuse to deliver the transported cargo.

Figure 2-20 Vesicle fusion

Vesicle fusion is mediated by two interacting cytosolic proteins: NSF (for N-ethylmaleimide-sensitive fusion) and SNAPs (for soluble NSF attachment proteins). NSF and SNAP bind to specific membrane receptors called SNARE (for SNAP receptors). SNAREs are present on the transporting vesicle (v-SNARE) and target membranes (t-SNARE) and represent docking proteins. Following docking, the SNARE complex recruits NSF and SNAPs to produce the fusion of the vesicle and target membranes.

Lysosomal sorting pathway: significance of M6P and its receptor

Lysosomal hydrolases are synthesized in the endoplasmic reticulum, transported to the cis-Golgi, and finally sorted to lysosomes. This sorting mechanism involves two important steps (Figure 2-21):

Figure 2-21 Golgi apparatus: Lysosomal sorting pathways

By this mechanism, M6P-containing lysosomal enzymes are separated from other glycoproteins in vesicles with the M6P receptor. After being transported to a clathrin-coated transporting vesicle, lysosomal enzymes dissociate from the M6P receptor and become surrounded by a membrane to form a primary lysosome. Membranes containing free M6P receptor are returned to the Golgi apparatus for recycling.

Receptor-mediated endocytosis: Uptake of cholesterol

Receptor-mediated endocytosis increases the capacity of the cell to internalize specific macromolecules with great efficiency and in large amount. A classic example is the uptake of cholesterol used to make new cell membranes. As you recall from your biochemistry course, cholesterol is highly insoluble and is mobilized in the bloodstream bound to protein as low-density lipoprotein (LDL) particles. LDL carries about 75% of the cholesterol and circulates in blood for about 2 to 3 days. Approximately 70% of LDL is cleared from blood by cells containing LDL receptors; the remainder is removed by a scavenger pathway using a receptor-independent mechanism.

The internalization of a ligand (such as LDL, transferrin, polypeptide hormones, or growth factors) by a cell requires a specific membrane receptor. The LDL receptor–LDL complex is internalized by receptor-mediated endocytosis. We have seen that this process involves the assembly of the protein clathrin on the cytosolic side of the plasma membrane, which forms a coated pit (see Figure 2-16).

The function of clathrin, together with adaptin, is to concentrate receptor-ligand complexes in a small surface area of the plasma membrane. Receptors with their bound ligands move by lateral diffusion in the plane of the lipid bilayer. The coated pit invaginates to form a coated vesicle, which pinches off from the plasma membrane to transport receptor-ligand complexes to a specific intracellular pathway, usually an endosome. Recall that dynamin assembles around the neck of the budding coated vesicle to pinch off the vesicle from the plasma membrane with the help of other proteins recruited to the neck site.

After internalization, clathrin of the coated vesicle is removed and the uncoated vesicle fuses with the endosome, with an internal low pH. In this acidic environment, LDL detaches from the receptor and is delivered to a primary lysosome, which changes into a secondary lysosome. LDL is broken down by lysosomal hydrolytic enzymes and is released into the cytosol as free cholesterol, where it can be used for new membrane synthesis.

The LDL receptor, in turn, is continuously recycled back to the plasma membrane to be used again. The LDL receptor can recycle every 10 minutes and can make several hundred cycles in its 20-hour life span.

Cholesterol is required for the synthesis of steroid hormones, the production of bile acids in liver hepatocytes, and the synthesis of cell membranes. Receptor-mediated endocytosis is also used for the uptake of vitamin B₁₂ and iron.

Clinical significance: Familial hypercholesterolemia

The mechanism of cholesterol uptake is disrupted in familial hypercholesterolemia, characterized by an elevation of LDL, the predominant cholesterol transport protein in the plasma. The primary defect is a mutation in the gene encoding the LDL receptor, required for the internalization of dietary cholesterol by most cells. High levels of LDL cholesterol in blood plasma lead to the formation of atherosclerotic plaques in the coronary vessels, a common cause of myocardial infarction.

Patients with familial hypercholesterolemia have three types of defective receptors: (1) LDL receptors incapable of binding LDL; (2) LDL receptors that bind LDL but at a reduced capacity; and (3) LDL receptors that can bind LDL normally but are incapable of internalization.

Lysosomes and intracellular digestion

Two types of lysosomes are recognized: primary lysosomes (Figure 2-22), defined as the primary storage site of lysosomal hydrolases, and secondary lysosomes, regarded as lysosomes engaged in a catalytic process.

Figure 2-22 Types of lysosomes

As discussed, the plasma membrane can internalize extracellular particles and fluids using vesicles resulting from the invagination of the membrane by a process called endocytosis. Endocytosis has two important goals: to bring material into the cell, and to recycle the plasma membrane. The reverse process, called exocytosis, is the transport outside the cell of products processed or synthesized by the cell.

Endocytosis involves three major types of vesicles: (1) clathrin-free phagosomes used to internalize large particles (for example, virus, bacteria, or cell debris); (2) clathrin-coated vesicles, to take in small macromolecules; and (3) pinocytosis (cellular drinking), to internalize fluids in a vesicle called caveola coated by the protein caveolin.

Most cells take in fluid by pinocytosis but phagocytosis is the function of specialized cells, including macrophages. We study them in Chapter 4, Connective Tissue (macrophages), Chapter 6, Blood and Hematopoiesis (white blood cells), and Chapter 10, Immune-Lymphatic System (macrophages and antigen-presenting cells). Phagocytic cells scavenge cell remnants during apoptosis and aging blood cells in spleen.

The lysosome is a membrane-bound organelle that, in addition to hydrolytic enzymes, has membrane-bound transporters that allow digested products, such as amino acids, sugars, and nucleotides, to reach the cytosol for reuse or excretion. The lysosomal membrane also contains an ATP-dependent pump that provides H⁺ into the lysosome to maintain an acidic environment (see Figure 2-22).

We review the lysosomal sorting pathway (see Figure 2-21) to highlight important steps: (1) Lysosomal enzymes and lysosomal membrane proteins are synthesized in the endoplasmic reticulum and transported through the Golgi apparatus to the trans-Golgi network; (2) an important event in the cis-Golgi is the tagging of lysosomal enzymes with a specific phosphorylated sugar group, M6P, that is recognized in the trans-Golgi by the corresponding receptor, M6P receptor; and (3) tagging enables enzymes to be sorted and packaged into transport vesicles that leave the trans-Golgi network toward the lysosomes.

The different paths of various materials to lysosomes are described in Figure 2-22. Note some important terminology: phagosomes fuse with lysosomes and endocytic vesicles fuse with an endosome before delivery to a lysosome. Also, autophagy signifies the degradation of disposable components of the cell itself with the help of an autophagosome (see Box 2-C).

Clinical significance: Lysosomal storage disorders

Lysosomal storage disorders or diseases are caused by the progressive accumulation of cell membrane components within cells because of a hereditary deficiency of enzymes required for their breakdown. An example is Tay-Sachs disease. Additional details on the mechanism leading to lysosomal storage diseases are presented in Figure 2-23.

Figure 2-23 Lysosomal storage disorders: Tay-Sachs disease

Mitochondria participate in apoptosis, steroidogenesis, and thermogenesis

Mitochondria participate in three additional functions: programmed cell death or apoptosis, stereidogenesis (production of steroid hormones), and thermogenesis. Enzyme proteins transported to the matrix must cross the outer and inner mitochondrial membranes.

Concerning apoptosis, mitochondria contain procaspases-2, -3, and -9 (precursors of proteolytic enzymes), apoptosis initiation factor (AIF), and cytochrome c. The release of these proteins in the cytosol initiates apoptosis. We come back to mitochondria and apoptosis in Chapter 3, Cell Signaling.

Concerning steroidogenesis, mitochondrial membranes contain enzymes involved in the synthesis of the steroids aldosterone, cortisol, and androgens. We discuss the participation of mitochondria in steroid production in Chapter 19, Endocrine System, and Chapter 20, Spermatogenesis.

Concerning thermogenesis, most of the energy from oxidation is dissipated as heat rather than converted to ATP. Uncoupling proteins (UCPs), members of the superfamily of mitochondrial anion-carrier proteins present in the mitochondrial inner membrane, mediate the regulated discharge of H⁺ (called proton leak), resulting in the release of heat. Proton leak across the mitochondrial inner membrane is mediated by UCP–1. UCP-1 is present in the mitochondrial inner membrane of brown adipocytes. Its role is to mediate regulated thermogenesis in response to cold exposure (see section on adipose tissue in Chapter 4, Connective Tissue).

Clinical significance: Mitochondrial inheritance

Mitochondria are transmitted by the mother (maternal inheritance). Both males and females can be affected by mitochondrial diseases, but males never transmit the disorder. Males do not transmit mitochondria at fertilization.

Myoclonic epilepsy with ragged red fibers (MERRF) is characterized by generalized muscle weakness, loss of coordination (ataxia), and multiple seizures. The major complications are respiratory and cardiac failure because the respiratory and cardiac muscles are affected. Muscle cells and neurons are the most affected because of their need for significant amounts of ATP to function.

Histologic preparations of muscle biopsies of individuals with MERRF display a peripheral red-stained material corresponding to aggregates of abnormal mitochondria, giving a ragged appearance to red muscle fibers. MERRF is caused by a point mutation in a mitochondrial DNA gene encoding tRNA for lysine. An abnormal tRNA causes a deficiency in the synthesis of proteins required for electron transport and ATP production.

Three maternally inherited mitochondrial diseases affect males more severely than females:

Clinical significance: Zellweger syndrome

Zellweger syndrome (see Figure 2-26) is a rare, congenital disease, fatal within the first year of life. It belongs to the group of leukodystrophies, affecting myelin formation in axons of the brain. Zellweger syndrome is characterized by a reduction or absence of peroxisomes in hepatocytes, and cells of the kidney and brain. Multiple peroxisomal proteins, peroxins, fail to be imported into peroxisomes.

The primary defect is the mutation of the PEXR1 (peroxisome receptor 1) gene encoding the receptor on the surface of peroxisomes for peroxisome-targeted enzymes that are required for cellular lipid metabolism and metabolic oxidations.

The clinical characteristics include hepatomegalia (enlargement of the liver), high levels of iron and copper in blood, and defective vision. Affected children may show at birth muscle hypotonicity, an inability to move, and a failure to suck or swallow. A diagnostic test is the measurement of plasma very-long-chain fatty acid (VLCFA) concentrations, regarded as an indicator of defects in the peroxisomal fatty acid metabolism.

Concept mapping

Epithelial Glands