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Ultrastructure of retinal capillaries of the rat
Exp. Eye Res. (1961) 1, 1-4
Ultrastructure of Retinal Capillaries of the Rat ABBOTT T. KrSSEN, PH.D.t .AND J. M. B. BLOODWORTH, JR., M.D. Departments oj Ophthalmology and Pathology, Oollege of Medicine, OhioState University, Oolumbus 10, Ohio (Received 26 September 1960) The retinal capillaries of normal, albino rats were studied with the electron microscope. The capillaries are lined by flat endothelial cells which rest upon a homogeneous basement membrane. Mitochondria were rare. Spherical vesicles, referred to as pinocytic vesicles by some authors, were found distributed throughout the endothelial cytoplasm but more often aligned along the free border of the oell. In addition to the vesicles, fine structures, appearing as disoontinuous pairs of parallel lines and interpreted as portions of endoplasmic retioulum, were also dispersed in the endothelial cytoplasm. Microvilli of various lengths projeoted from the endothelial cytoplasm into the lumen of the capillaries. There was considerable variation in the width of the basement membrane which encircles the endothelial layer of the vessel. Perioytes were embedded in the basement membrane and lay in a plane parallel to the longitudinal axis of the capillary. Although the eapillaries were not, as a rule, completely surrounded by perioytes, the latter cells were often superimposed upon one another, adding significantly to the over-all thickness of the capillary. In all specimens examined, the capillaries were invested by glial elements of the retina. There was no oontiguity of capillary and neuronal tissue.
1. Introduction
On the basis of studies in this (Bloodworth) and other laboratories (Friedenwald. 1950; Ashton, 1958) diabetic retinopathy appears to be, with rare exceptions, a specific disease of human retinal capillaries. Comparable lesions in other parts of the human body, or in animals, have not been observed. Experimental reproduction of diabetic retinopathy has not been successful (Wise, 1957). The recognition of these facts leads to the conclusion that a specific morphological arrangement of the human retinal capillary may be responsible for this disease. Retinal vessels have been studied extensively by gross and light microscopic examination. However, the only electron microscopic investigation which concerns itself exclusively with retinal capillaries is the study by Maeda (1959) of the human eye. The ultrastructure of the retinal capillaries of the normal rat and a comparison of these findings with prior reports on capillary ultrastructure will be presented in this paper. This is the first of a series of investigations of mammalian retinal capillaries. In subsequent studies the ultrastructure of retinal vessels of other experimental animals and of the human will be compared in an attempt to explain the failure of diabetic animals to develop diabetic retinopathy. Another objective of these studies is to determine the laboratory animal with retinal capillaries most similar to those of the human. Such information is needed in order to select the ideal laboratory animal for the study of diabetic retinopathy.
t Present address: Biothermal Section, Biomedical Laboratory, AerospaceMedicalDivision,'Vright .Air Development Division, Wright-Patterson Air Force Base, 1
2
ABBOTT T. KISS EN AND J. M. B. BLOODWORTH, JR.
2. Methods and Materials The eyes of four normal, adult albino rats wereused forthis study. The eyeswereremoved at autopsy and 2-5 mm triangles of retina were immersed in 4° veronal- acetate buffered 1 % osmium tetroxide at pH 7.4 for 35 to 50 min. After fixation, the tissues were washed briefly in distilled water and dehydrated with alcohols in the conventional manner. After dehydration, the tissues were cut into 1 to 2 mm 2 pieces and placed in a 1:1 mixture of absolute alcohol-methacrylate monomer (20% methyl methacrylate in n-butyl methacrylate) and then transferred through 3 changes of the pure monomer in 90 min. Finally each piece of tissue was embedded in catalysed (Luperco) monomer at 45° for 12 hours. Ultrathin sections, cut with a diamond knife on a Porter-Blum microtome, were mounted on collodion-carbon-coated copper grids and examined in an RCA, EMU-2B, or an Akashi, TRS-50EI electron microscope without removing the plastic. 3. Results Retinal capillaries are lined by flat endothelial cells which rest upon a basement membrane composed of a homogeneous substance. The endothelial cell membrane comprising the free border of the cell is continuous, with no :indication of fenestrations or pores. Microvilli of the endothelial cytoplasm extend into the capillary lumen, varying :in appearance and size from slender, filiform processes 500 to 600 m(J. :in length to stubby projections (Plates I-III). The cytoplasm of the endothelial cell is moderately osmiophilic and contains spherical, cytoplasmic vesicles, initially described by Palade (1953), and referred to as pinooytio vesicles by Moore and Ruska (1956) and Maeda (1959). These vesicles in the rat endothelial cells range from 40 to 60 mp, in diameter and are most often found close to the free surface of the endothelial cell. Plate II illustrates a linear grouping of the vesicles. Mitochondria were rare. Fine lines of high electron density were also noted within the endothelial cell cytoplasm. With higher magnification (Plate IV) these structures appear as discontinuous parallel lines, 13-14 mfL apart, and are interpreted as portions of endoplasmic reticulum. The endothelial cell nucleus, in cross-section, is ovoid and contains a nucleolus (Plate I). When sectioned obliquely (Plate II), the characteristically scalloped profile of the nucleus becomes apparent. The basement membrane is homogeneous in structure with none of the variations in density reported by Maynard, Schultz, and Pease (1957) for the same structure in vessels of the rat cerebral cortex, or by Maeda (1959) for the human retina. Variations in basement membrane thickness, illustrated in Plate IV, range from 12 to 57 mfL. Characteristically, the basement membrane is split in several places along its length. The areas within the split basement membrane are occupied by the processes of pericytes. These processes are, in effect, embedded in basement membrane and morphologically isolated from endothelium on one side and glial elements on the other. Occasionally there is multiple splitting of the basement membrane resulting in a stratification or at least an overlapping ofperieytes (Plate IV). The pericyte cytoplasm is of low electron density and includes vesicles similar to those described in the endothelial cytoplasm. Occasionally a pericyte nucleus is seen. In accommodating the pericyte nucleus there is an over-all increase of 40 to 45% in the vessel surface. area. This increase is accomplished by enlargement of the perivascular elements of the capillary wall, leaving the volume of the lumen unchanged. A cross-section of a pericyte nucleus is shown in Plate III. In this plane the nucleus is sphreical and smooth in contour. The pericyte nucleus contains a nucleolus and exhibits a more uniform concentration of nuclear material than does the endothelial nucleus.
P L ATE 1. A cross-sec t ion of a capillary. A portion of an astrocyt e (A ) with it s nucleus (AN) is at the upper left . A p rocess of t he nstrooytc extends t o th e left side of th e enpillary. Di visions of the baseme nt memb ran e (B) which surround peri cytcs (P) arc clearly evide n t . Th ere is no overlap of t he p oricy t es . The ovoid en dothelia l nucleus ( [~ N ) is sur rounded b y a th in rim of endot helial cyt oplasm. Micro vllli (M V ) project fr om end ot helia l cyto pla sm into th e lum en (L) of the. ca pillary ( X H),5 (]O).
PLA'J'E II. A slightly obliqu e sectio n thro ugh a capillary. Glial en dfeot (AE) completely surroun d the capillary. T he angle of sect ion demon strates the irregul ar profile of the endot helial nu cleu s (E N ). A lin e of en do thelial ve sicles (V ) occupi es a position dire ctly un der t he free surface of the endothelial cell ( X 22, 200 ).
A
j
v
L
B
A
PLATE III. A cross-section of 0. capillary an d a pericyte nucleus. The peri cyte wit h its nucleus (PN ) and thc encircling basement membrane ( B) occupy au area almost equal to thn.t of the capillary alone. Small vesicles (V) are p resent within the endothelial cytoplasm. Over half of the su rface area of the capillary is in con ta ct with a single a strocyte (A) ( X 21,500).
p
8
-
MV
L
PLATE: IV. An oblique section through 11 capillary. The Iuu1('Uis at tho right. Division of the bn.sement mombrruu- (B) into layers of varying thicknesses is illustrated. Pericyt os (P) lie between the sheets of bascmont membruno, Endoplasmic rot iculuni (Elt) of the endothelial cell (E) appears as a discontinuous paiL' of lines (X 82,140).
ULTRASTRUCTURE OF RETINAL CAPILLA .RIJ
3
In addition to features pertinentto the capillaries, Plates I -III illustrate the relationship of these vessels to surrounding neuroglial elements. The capillary in Plate I is at the lower right. At the upper left is a portion of the nucleus of an astrocyte whose cytoplasm presents a characteristic "watery" appearance. A cytoplasmic extension of this cell extends to the upper left aspect of the capillary. Other cytoplasmic areas of equally low electron density occupy positions adjacent to the basement membrane and are the glial end-feet ofthe same or neighbouring astrocytes. Plate II illustrates an oblique section through a capillary completely surrounded by glial end-feet. At least 50% of the surface area of the capillary in Plate III is bounded by the cytoplasm of a single astrocyte. 4. Discussion
Oapillaries of the rat retina were observed within the plexiform, the ganglion cell, and the nerve fibre layers. This distribution coincideswith that described by Maeda (1959) for similar vessels in the human retina. There appears to be no significant difference in the morphology of the capillary with respect to location within the retina. It is only coincidental that all of the vessels illustrated in this paper were found in the plexiform layers. The endothelial cells, as well as the pericytes, contain an abundance of spherical vesiclesin their cytoplasm . The presence of similar structures has been reported in the literature reviewed concerning the fine structure of capillaries. Although it is not within the scope of t his paper to discuss the role of these vesicles, Moore and Ruska (1956) have suggested that they constitute a means of active and selective transmission through capillary walls. The absence of endothelial fenestrations or pores is in agreement with observations by Maeda (1959) on the human retina as well as by Dempsey and Wislocki (1955), van Breeman and Clemente (1955), and Maynard et al (1957) on capillaries of the nervous system. In addition to vesicles, elongate channels, 13-14 mp, in width, were also seen within the endothelial cytoplasm, lying parallel to the long axis of the cell. Kisch (1957) also noted the presence of "fine longitudinally sectioned channels" in the endothelial cells of heart capillaries. We agree with his suggestion that these structures are analogous to the endoplasmic reticulum observed in the cytoplasm of other cells. The basement membrane is homogeneous in structure and completely surrounds the endothelial layer. It is difficult to be precise concerning the thickness of the basement membrane since virtually all of the capillary wall, exclusive of the endotheliallayer, consists of multiple layers of the membrane which envelop the pericytes. Therefore, even though the average thickness of the basement membrane, of the capillaries examined here may be given as about 30 mu, any section through the capillary wall is very likely to pass through multiple thicknesses of basement membrane. In view of its dense and imperforate nature, it is possible that alterations in the state of this membrane are associated with alterations in the permeability and stability of the capillary wall. One other structure that may significantly contribute to the function of the retinal capillaries is the pericyte. While occasional sections are devoid of pericytcs, they are prevalent enough to warrant consideration as a structural component of the capillary wall. They usually appear as narrow islets of cytoplasm surrounded by basement membrane, arranged either in a single plane or superimposed, forming 2 or 3 layers. Without benefit of serial sections, the question of whether these islets are
4
ABBOTT T. KISSEN AND J. M. B . BIJOODWORTH, JR.
the processes of separate pericytes or the dendriform or stellate processes of a single pericyte, as suggested by Maeda (1959), cannot be resolved. The capillaries of the central nervous system must receive special consideration because of glial elements located between the capillary and the parenchymal cells of the organ. The developmental relationship of retinal and central nervous tissue suggests anatomical similarities which, in fact, exist. All of the retinal capillaries of the rats examined were completely invested. in glial elements, and if these elements constitute, as van Breeman and Clemente (1955) and Dempsey and Wislocki (1955) maintain, a "blood-brain barrier", then it is reasonable to assume that there also exists a "blood-retina barrier". Problems involving capillary physiology have led to a reinvestigation of the basic anatomy of these vessels with the aid of the electron microscope. Consistent and significant differences exist between the capillary bed of one tissue and another, which warrant individual evaluations if peculiarities of function are to be analysed. In an attempt to codify the diverse presentations of capillary structure, Bennett, Luft and Hampton, (1959) selected anatomical features-the basement"membrane, endothelial intracellular fenestrations or pores, and pericapillary cellular investments -as classification criteria. On the basis of presence, absence, or modifications of these features, they have established a simple three-digit system for characterizing and designating the type of any capillary with respect to the classifications submitted. In accordance with the criteria established by Bennett et al, the vessels described here for the rat retina would be designated as type A-I-~.
REFERENOES Ashton, N . (1958). Advanc. Opllthalmol. 8, 1. Bennett, H. S., Luft, .T. H. and Hampton, J. C. (1959) . Amer. J . Physiol. 196,381. Bloodworth, J. M. B ., -Ir., Unpublished data. Dempsey, E. W. arid Wialocki, G. B. (1955). J . biophys. biochem, Gytal. 1, 245. Friedenwald, .T. S. (1950). A mer . J. Ophtllalmol. 33, U87. Kisch, :B. (1957). Exp. Med. Burg. 15, 89. Maeda, J. (1959). Jap . J . Ophthalmol. 3, 37. Maynard, E. A., Sohultz, R. L. and Pease, D. C. (1957). Amer. J. Anat. 100, 409. Moore, D. H. and Ruska, H . (1957). J. biophys. biochem, Oytol. 3, 457. Palade, G. E. (1953) . J. appl. Phys. 24, 1424. van Breemari, V. L. and Clemente, C. D. (1955). J. biophys. biochem: Gytol. 1, 161. Wise, G. N. (1957). Arch. Ophthal., Ohicago 5, 151.
Ultrastructure of Retinal Capillaries of the Rat ABBOTT T. KrSSEN, PH.D.t .AND J. M. B. BLOODWORTH, JR., M.D. Departments oj Ophthalmology and Pathology, Oollege of Medicine, OhioState University, Oolumbus 10, Ohio (Received 26 September 1960) The retinal capillaries of normal, albino rats were studied with the electron microscope. The capillaries are lined by flat endothelial cells which rest upon a homogeneous basement membrane. Mitochondria were rare. Spherical vesicles, referred to as pinocytic vesicles by some authors, were found distributed throughout the endothelial cytoplasm but more often aligned along the free border of the oell. In addition to the vesicles, fine structures, appearing as disoontinuous pairs of parallel lines and interpreted as portions of endoplasmic retioulum, were also dispersed in the endothelial cytoplasm. Microvilli of various lengths projeoted from the endothelial cytoplasm into the lumen of the capillaries. There was considerable variation in the width of the basement membrane which encircles the endothelial layer of the vessel. Perioytes were embedded in the basement membrane and lay in a plane parallel to the longitudinal axis of the capillary. Although the eapillaries were not, as a rule, completely surrounded by perioytes, the latter cells were often superimposed upon one another, adding significantly to the over-all thickness of the capillary. In all specimens examined, the capillaries were invested by glial elements of the retina. There was no oontiguity of capillary and neuronal tissue.
1. Introduction
On the basis of studies in this (Bloodworth) and other laboratories (Friedenwald. 1950; Ashton, 1958) diabetic retinopathy appears to be, with rare exceptions, a specific disease of human retinal capillaries. Comparable lesions in other parts of the human body, or in animals, have not been observed. Experimental reproduction of diabetic retinopathy has not been successful (Wise, 1957). The recognition of these facts leads to the conclusion that a specific morphological arrangement of the human retinal capillary may be responsible for this disease. Retinal vessels have been studied extensively by gross and light microscopic examination. However, the only electron microscopic investigation which concerns itself exclusively with retinal capillaries is the study by Maeda (1959) of the human eye. The ultrastructure of the retinal capillaries of the normal rat and a comparison of these findings with prior reports on capillary ultrastructure will be presented in this paper. This is the first of a series of investigations of mammalian retinal capillaries. In subsequent studies the ultrastructure of retinal vessels of other experimental animals and of the human will be compared in an attempt to explain the failure of diabetic animals to develop diabetic retinopathy. Another objective of these studies is to determine the laboratory animal with retinal capillaries most similar to those of the human. Such information is needed in order to select the ideal laboratory animal for the study of diabetic retinopathy.
t Present address: Biothermal Section, Biomedical Laboratory, AerospaceMedicalDivision,'Vright .Air Development Division, Wright-Patterson Air Force Base, 1
2
ABBOTT T. KISS EN AND J. M. B. BLOODWORTH, JR.
2. Methods and Materials The eyes of four normal, adult albino rats wereused forthis study. The eyeswereremoved at autopsy and 2-5 mm triangles of retina were immersed in 4° veronal- acetate buffered 1 % osmium tetroxide at pH 7.4 for 35 to 50 min. After fixation, the tissues were washed briefly in distilled water and dehydrated with alcohols in the conventional manner. After dehydration, the tissues were cut into 1 to 2 mm 2 pieces and placed in a 1:1 mixture of absolute alcohol-methacrylate monomer (20% methyl methacrylate in n-butyl methacrylate) and then transferred through 3 changes of the pure monomer in 90 min. Finally each piece of tissue was embedded in catalysed (Luperco) monomer at 45° for 12 hours. Ultrathin sections, cut with a diamond knife on a Porter-Blum microtome, were mounted on collodion-carbon-coated copper grids and examined in an RCA, EMU-2B, or an Akashi, TRS-50EI electron microscope without removing the plastic. 3. Results Retinal capillaries are lined by flat endothelial cells which rest upon a basement membrane composed of a homogeneous substance. The endothelial cell membrane comprising the free border of the cell is continuous, with no :indication of fenestrations or pores. Microvilli of the endothelial cytoplasm extend into the capillary lumen, varying :in appearance and size from slender, filiform processes 500 to 600 m(J. :in length to stubby projections (Plates I-III). The cytoplasm of the endothelial cell is moderately osmiophilic and contains spherical, cytoplasmic vesicles, initially described by Palade (1953), and referred to as pinooytio vesicles by Moore and Ruska (1956) and Maeda (1959). These vesicles in the rat endothelial cells range from 40 to 60 mp, in diameter and are most often found close to the free surface of the endothelial cell. Plate II illustrates a linear grouping of the vesicles. Mitochondria were rare. Fine lines of high electron density were also noted within the endothelial cell cytoplasm. With higher magnification (Plate IV) these structures appear as discontinuous parallel lines, 13-14 mfL apart, and are interpreted as portions of endoplasmic reticulum. The endothelial cell nucleus, in cross-section, is ovoid and contains a nucleolus (Plate I). When sectioned obliquely (Plate II), the characteristically scalloped profile of the nucleus becomes apparent. The basement membrane is homogeneous in structure with none of the variations in density reported by Maynard, Schultz, and Pease (1957) for the same structure in vessels of the rat cerebral cortex, or by Maeda (1959) for the human retina. Variations in basement membrane thickness, illustrated in Plate IV, range from 12 to 57 mfL. Characteristically, the basement membrane is split in several places along its length. The areas within the split basement membrane are occupied by the processes of pericytes. These processes are, in effect, embedded in basement membrane and morphologically isolated from endothelium on one side and glial elements on the other. Occasionally there is multiple splitting of the basement membrane resulting in a stratification or at least an overlapping ofperieytes (Plate IV). The pericyte cytoplasm is of low electron density and includes vesicles similar to those described in the endothelial cytoplasm. Occasionally a pericyte nucleus is seen. In accommodating the pericyte nucleus there is an over-all increase of 40 to 45% in the vessel surface. area. This increase is accomplished by enlargement of the perivascular elements of the capillary wall, leaving the volume of the lumen unchanged. A cross-section of a pericyte nucleus is shown in Plate III. In this plane the nucleus is sphreical and smooth in contour. The pericyte nucleus contains a nucleolus and exhibits a more uniform concentration of nuclear material than does the endothelial nucleus.
P L ATE 1. A cross-sec t ion of a capillary. A portion of an astrocyt e (A ) with it s nucleus (AN) is at the upper left . A p rocess of t he nstrooytc extends t o th e left side of th e enpillary. Di visions of the baseme nt memb ran e (B) which surround peri cytcs (P) arc clearly evide n t . Th ere is no overlap of t he p oricy t es . The ovoid en dothelia l nucleus ( [~ N ) is sur rounded b y a th in rim of endot helial cyt oplasm. Micro vllli (M V ) project fr om end ot helia l cyto pla sm into th e lum en (L) of the. ca pillary ( X H),5 (]O).
PLA'J'E II. A slightly obliqu e sectio n thro ugh a capillary. Glial en dfeot (AE) completely surroun d the capillary. T he angle of sect ion demon strates the irregul ar profile of the endot helial nu cleu s (E N ). A lin e of en do thelial ve sicles (V ) occupi es a position dire ctly un der t he free surface of the endothelial cell ( X 22, 200 ).
A
j
v
L
B
A
PLATE III. A cross-section of 0. capillary an d a pericyte nucleus. The peri cyte wit h its nucleus (PN ) and thc encircling basement membrane ( B) occupy au area almost equal to thn.t of the capillary alone. Small vesicles (V) are p resent within the endothelial cytoplasm. Over half of the su rface area of the capillary is in con ta ct with a single a strocyte (A) ( X 21,500).
p
8
-
MV
L
PLATE: IV. An oblique section through 11 capillary. The Iuu1('Uis at tho right. Division of the bn.sement mombrruu- (B) into layers of varying thicknesses is illustrated. Pericyt os (P) lie between the sheets of bascmont membruno, Endoplasmic rot iculuni (Elt) of the endothelial cell (E) appears as a discontinuous paiL' of lines (X 82,140).
ULTRASTRUCTURE OF RETINAL CAPILLA .RIJ
3
In addition to features pertinentto the capillaries, Plates I -III illustrate the relationship of these vessels to surrounding neuroglial elements. The capillary in Plate I is at the lower right. At the upper left is a portion of the nucleus of an astrocyte whose cytoplasm presents a characteristic "watery" appearance. A cytoplasmic extension of this cell extends to the upper left aspect of the capillary. Other cytoplasmic areas of equally low electron density occupy positions adjacent to the basement membrane and are the glial end-feet ofthe same or neighbouring astrocytes. Plate II illustrates an oblique section through a capillary completely surrounded by glial end-feet. At least 50% of the surface area of the capillary in Plate III is bounded by the cytoplasm of a single astrocyte. 4. Discussion
Oapillaries of the rat retina were observed within the plexiform, the ganglion cell, and the nerve fibre layers. This distribution coincideswith that described by Maeda (1959) for similar vessels in the human retina. There appears to be no significant difference in the morphology of the capillary with respect to location within the retina. It is only coincidental that all of the vessels illustrated in this paper were found in the plexiform layers. The endothelial cells, as well as the pericytes, contain an abundance of spherical vesiclesin their cytoplasm . The presence of similar structures has been reported in the literature reviewed concerning the fine structure of capillaries. Although it is not within the scope of t his paper to discuss the role of these vesicles, Moore and Ruska (1956) have suggested that they constitute a means of active and selective transmission through capillary walls. The absence of endothelial fenestrations or pores is in agreement with observations by Maeda (1959) on the human retina as well as by Dempsey and Wislocki (1955), van Breeman and Clemente (1955), and Maynard et al (1957) on capillaries of the nervous system. In addition to vesicles, elongate channels, 13-14 mp, in width, were also seen within the endothelial cytoplasm, lying parallel to the long axis of the cell. Kisch (1957) also noted the presence of "fine longitudinally sectioned channels" in the endothelial cells of heart capillaries. We agree with his suggestion that these structures are analogous to the endoplasmic reticulum observed in the cytoplasm of other cells. The basement membrane is homogeneous in structure and completely surrounds the endothelial layer. It is difficult to be precise concerning the thickness of the basement membrane since virtually all of the capillary wall, exclusive of the endotheliallayer, consists of multiple layers of the membrane which envelop the pericytes. Therefore, even though the average thickness of the basement membrane, of the capillaries examined here may be given as about 30 mu, any section through the capillary wall is very likely to pass through multiple thicknesses of basement membrane. In view of its dense and imperforate nature, it is possible that alterations in the state of this membrane are associated with alterations in the permeability and stability of the capillary wall. One other structure that may significantly contribute to the function of the retinal capillaries is the pericyte. While occasional sections are devoid of pericytcs, they are prevalent enough to warrant consideration as a structural component of the capillary wall. They usually appear as narrow islets of cytoplasm surrounded by basement membrane, arranged either in a single plane or superimposed, forming 2 or 3 layers. Without benefit of serial sections, the question of whether these islets are
4
ABBOTT T. KISSEN AND J. M. B . BIJOODWORTH, JR.
the processes of separate pericytes or the dendriform or stellate processes of a single pericyte, as suggested by Maeda (1959), cannot be resolved. The capillaries of the central nervous system must receive special consideration because of glial elements located between the capillary and the parenchymal cells of the organ. The developmental relationship of retinal and central nervous tissue suggests anatomical similarities which, in fact, exist. All of the retinal capillaries of the rats examined were completely invested. in glial elements, and if these elements constitute, as van Breeman and Clemente (1955) and Dempsey and Wislocki (1955) maintain, a "blood-brain barrier", then it is reasonable to assume that there also exists a "blood-retina barrier". Problems involving capillary physiology have led to a reinvestigation of the basic anatomy of these vessels with the aid of the electron microscope. Consistent and significant differences exist between the capillary bed of one tissue and another, which warrant individual evaluations if peculiarities of function are to be analysed. In an attempt to codify the diverse presentations of capillary structure, Bennett, Luft and Hampton, (1959) selected anatomical features-the basement"membrane, endothelial intracellular fenestrations or pores, and pericapillary cellular investments -as classification criteria. On the basis of presence, absence, or modifications of these features, they have established a simple three-digit system for characterizing and designating the type of any capillary with respect to the classifications submitted. In accordance with the criteria established by Bennett et al, the vessels described here for the rat retina would be designated as type A-I-~.
REFERENOES Ashton, N . (1958). Advanc. Opllthalmol. 8, 1. Bennett, H. S., Luft, .T. H. and Hampton, J. C. (1959) . Amer. J . Physiol. 196,381. Bloodworth, J. M. B ., -Ir., Unpublished data. Dempsey, E. W. arid Wialocki, G. B. (1955). J . biophys. biochem, Gytal. 1, 245. Friedenwald, .T. S. (1950). A mer . J. Ophtllalmol. 33, U87. Kisch, :B. (1957). Exp. Med. Burg. 15, 89. Maeda, J. (1959). Jap . J . Ophthalmol. 3, 37. Maynard, E. A., Sohultz, R. L. and Pease, D. C. (1957). Amer. J. Anat. 100, 409. Moore, D. H. and Ruska, H . (1957). J. biophys. biochem, Oytol. 3, 457. Palade, G. E. (1953) . J. appl. Phys. 24, 1424. van Breemari, V. L. and Clemente, C. D. (1955). J. biophys. biochem: Gytol. 1, 161. Wise, G. N. (1957). Arch. Ophthal., Ohicago 5, 151.