Lacrimal Gland
The lacrimal gland is anterior in the superolateral region of the orbit, and is divided into two parts by the levator palpebrae superioris muscle. The lacrimal gland consists of acini that are built of a luminar lining of columnar epithelial cells that are surrounded by a basal layer of myoepithelial cells and an enclosing basement membrane . The human lacrimal gland is a tubuloalveolar gland of serous type. Intercalated and interlobular ducts drain the secretions into the conjunctival fornix beneath the temporal bone. The tubules discharge without any characteristic excretory duct system (histologic distinction from serous salivary glands) into the interlobular ducts. The connective tissue between the acini contains accumulations of lymphocytes as well as many plasma cells mainly secreting IgA and being part of the eye-associated lym¬phoid tissue (EALT). As already mentioned, the lacrimal gland produces elec¬trolytes, water, and a large variety of proteins, peptides and glycopeptides. Of these, recent research regarding tear film rheology and innate immunity focus on production of different constitutively and inducible antimicrobial peptides, such as (3-defensins , surfactant proteins A and D as well as MUCs 4, 5AC, 5B and 7 that are secreted into the tear film.
Eyelid
The 'skeleton' of the eyelid is a collagen plate called the tarsus (fig. 1a). It contains a row of branched alveolar sebaceous glands, unrelated to the eye¬lashes. These tarsal or Meibomian glands have punctate openings along the free edge of the eyelid close to its posterior margin . They produce a lipid material whose synthesis is dependent on neuronal, hormonal, and vascular fac¬tors. This lipid material is fluid, spreads easily, is a surfactant as well as an aque¬ous barrier and must remain functional after a blink. To satisfy these requirements, the Meibomian lipids have a specific composition. Even after delivery it may be modified by lipases produced by ocular bacteria, and modi¬fications in the lipid components can lead to unique disease states . Sexual hormones, especially androgens, seem to play a decisive role in Meibomian physiology .
Near the anterior margin of the eyelids there are two or three rows of stiff hairs - the eyelashes (fig. 1a). In the middle of the lid is the cross-striated orbic- ularis oculi muscle, the fiber bundles of its palpebral part overlapping one another like tiles on a roof (fig. 1a). The tendon of the cross-striated levator palpebral muscle is inserted into the tarsus; beneath it is the smooth tarsalis muscle (figs 1a, d). The tone of the latter is determined by autonomic nervous impulses and is supposed to adjust the width of the palpebral opening. The apocrine ciliary glands (Moll's glands) open close to the eyelashes (fig. 1a). These apocrine glands are active from birth in producing agents against patho¬genic microorganisms in the eyelid shaft and on the ocular surface, i.e. lysozyme, P-defensin-2, adrenomedullin, lactoferrin, and IgA . In the con- junctival fornix the eyelid also contains small accessory lacrimal glands (Krause's glands, Wolfring's glands). Although much smaller, these glands are histologically comparable to the main lacrimal gland (figs 1a, d). However, only less is known about the secretions of these small glands and their contribution to tear film physiology.
Nasolacrimal Ducts
A lacrimal system consists of the upper and the lower lacrimal canaliculus, the lacrimal sac and the nasolacrimal duct. The structures are surrounded by a wide ranging cavernous system and are embedded in the osseous canal between the maxilla and the lacrimal bone . The internal wall of each lacrimal canaliculus is lined by a thick non-cornified epithelium resting on a basement membrane. The lacrimal sac and the nasolacrimal duct are lined by a double-layered epithelium with integrated goblet cells sometimes forming characteristic mucous glands (fig. 2e). As a draining and secretory system, the nasolacrimal ducts play a role in tear transport by production of MUCs 2, 4, 5AC, 5B, and 7 , TFF peptides TFF1 and TFF3 and non-specific immune defense . Moreover, components of tear fluid are absorbed in the nasolacrimal passage and are transported into the surrounding vascular system . This system is similar to a cavernous body that is subject to autonomic control and regulates tear outflow . Tear duct-associated lymphoid tissue (TALT) is present in the efferent tear ducts . Under normal conditions, tear fluid components are constantly absorbed into the blood vessels of the sur¬rounding cavernous body. These vessels are connected to the blood vessels of the outer eye and could act as a feedback signal for tear fluid production, which ceases if these tear components are not absorbed .
The lacrimal gland is anterior in the superolateral region of the orbit, and is divided into two parts by the levator palpebrae superioris muscle. The lacrimal gland consists of acini that are built of a luminar lining of columnar epithelial cells that are surrounded by a basal layer of myoepithelial cells and an enclosing basement membrane . The human lacrimal gland is a tubuloalveolar gland of serous type. Intercalated and interlobular ducts drain the secretions into the conjunctival fornix beneath the temporal bone. The tubules discharge without any characteristic excretory duct system (histologic distinction from serous salivary glands) into the interlobular ducts. The connective tissue between the acini contains accumulations of lymphocytes as well as many plasma cells mainly secreting IgA and being part of the eye-associated lym¬phoid tissue (EALT). As already mentioned, the lacrimal gland produces elec¬trolytes, water, and a large variety of proteins, peptides and glycopeptides. Of these, recent research regarding tear film rheology and innate immunity focus on production of different constitutively and inducible antimicrobial peptides, such as (3-defensins , surfactant proteins A and D as well as MUCs 4, 5AC, 5B and 7 that are secreted into the tear film.
Eyelid
The 'skeleton' of the eyelid is a collagen plate called the tarsus (fig. 1a). It contains a row of branched alveolar sebaceous glands, unrelated to the eye¬lashes. These tarsal or Meibomian glands have punctate openings along the free edge of the eyelid close to its posterior margin . They produce a lipid material whose synthesis is dependent on neuronal, hormonal, and vascular fac¬tors. This lipid material is fluid, spreads easily, is a surfactant as well as an aque¬ous barrier and must remain functional after a blink. To satisfy these requirements, the Meibomian lipids have a specific composition. Even after delivery it may be modified by lipases produced by ocular bacteria, and modi¬fications in the lipid components can lead to unique disease states . Sexual hormones, especially androgens, seem to play a decisive role in Meibomian physiology .
Near the anterior margin of the eyelids there are two or three rows of stiff hairs - the eyelashes (fig. 1a). In the middle of the lid is the cross-striated orbic- ularis oculi muscle, the fiber bundles of its palpebral part overlapping one another like tiles on a roof (fig. 1a). The tendon of the cross-striated levator palpebral muscle is inserted into the tarsus; beneath it is the smooth tarsalis muscle (figs 1a, d). The tone of the latter is determined by autonomic nervous impulses and is supposed to adjust the width of the palpebral opening. The apocrine ciliary glands (Moll's glands) open close to the eyelashes (fig. 1a). These apocrine glands are active from birth in producing agents against patho¬genic microorganisms in the eyelid shaft and on the ocular surface, i.e. lysozyme, P-defensin-2, adrenomedullin, lactoferrin, and IgA . In the con- junctival fornix the eyelid also contains small accessory lacrimal glands (Krause's glands, Wolfring's glands). Although much smaller, these glands are histologically comparable to the main lacrimal gland (figs 1a, d). However, only less is known about the secretions of these small glands and their contribution to tear film physiology.
Nasolacrimal Ducts
A lacrimal system consists of the upper and the lower lacrimal canaliculus, the lacrimal sac and the nasolacrimal duct. The structures are surrounded by a wide ranging cavernous system and are embedded in the osseous canal between the maxilla and the lacrimal bone . The internal wall of each lacrimal canaliculus is lined by a thick non-cornified epithelium resting on a basement membrane. The lacrimal sac and the nasolacrimal duct are lined by a double-layered epithelium with integrated goblet cells sometimes forming characteristic mucous glands (fig. 2e). As a draining and secretory system, the nasolacrimal ducts play a role in tear transport by production of MUCs 2, 4, 5AC, 5B, and 7 , TFF peptides TFF1 and TFF3 and non-specific immune defense . Moreover, components of tear fluid are absorbed in the nasolacrimal passage and are transported into the surrounding vascular system . This system is similar to a cavernous body that is subject to autonomic control and regulates tear outflow . Tear duct-associated lymphoid tissue (TALT) is present in the efferent tear ducts . Under normal conditions, tear fluid components are constantly absorbed into the blood vessels of the sur¬rounding cavernous body. These vessels are connected to the blood vessels of the outer eye and could act as a feedback signal for tear fluid production, which ceases if these tear components are not absorbed .
Posted in: Ophthalmology Articles
0 comments:
Post a Comment