The electron microscopic autoradiographic studies described here revealed the presence of specific silver grains over nuclei, lysosomal vesicles, rough endoplasmic reticulum and Golgi apparatus after incubation of placental tissue for 2h at 38°C with 1 nM-[125I]EGF. Three-step mask analysis, which corrects for radiation spread, showed that the relative grain density was the highest in nuclei, followed by lysosomal vesicles, then Golgi and rough endoplasmic reticulum, equally. The nuclear grain density, however, was lower than that in microvillus plasma membranes. There were very few grains in basolateral plasma membranes, none in the basement membrane area and a considerable number in capillary endothelial cells. The present results demonstrating the association of internalized [125I]EGF with a variety of intracellular organelles raise the possibility of EGF acting on the intracellular sites in addition to cell surface sites.

We have shown in the preceding paper that lysosomes, rough and smooth endoplasmic reticulum and Golgi apparatus as well as outer cell membranes of term human placenta contain binding sites for epidermal growth factor (EGF) and EGF-stimulated kinase activity (Ramani et al. 1986). The extensive characterization of the sites revealed numerous biochemical differences as well as similarities between the sites in various organelles (Ramani et al. 1986). The differences suggested that the binding sites in different organelles have different biochemical properties. These findings led us to the present studies in which we investigated whether internalized [125I]EGF might associate with the intracellular organelles that contain the binding sites.

The details on collection of term human placentae, their processing and the radio-iodination of mouse EGF are presented in the preceding paper (Ramani et al. 1986). Most of the chemicals and supplies were purchased from Polysciences, Warrington, PA.

Small tissue pieces cut from various cotyledons of placenta were washed five times with ice-cold Krebs-Ringer bicarbonate (KRB) buffer at pH 7 · 0 and incubated under room atmosphere for 2h at 38°C in KRB with 1 nM-[125I]EGF in the presence and absence of 100 nM unlabelled EGF. After the incubation, the tissue pieces were washed six times with ice-cold KRB to remove free [125I]EGF. The tissue pieces were then fixed in 2·5% glutaraldehyde/2% paraformaldehyde buffered with 100mM-sodium cacodylate containing 250 mM-sucrose (pH 7·3), washed three times with cacodylate buffer, post-fixed in osmium tetroxide, dehydrated, embedded, cut on an LKB microtome and processed for autoradiography at the light (Rao et al. 1984) and electron microscopic levels (Chegini et al. 1984a).

A total of 436 grains that could be assigned to different organelles with a high degree of confidence were counted in about 40 autoradiograms and subjected to three-step mask analysis according to Salpeter et al. (1978). This analysis, which corrects for radiation spread, was performed on a computer using a ‘Stepit’ program (Chandler, 1965). The initial selection of organelles as grain and source compartments was based on our biochemical data in the preceding paper (Ramani et al. 1986) with the following exceptions: (1) smooth endoplasmic reticulum was not included in the present studies as the grains could not be assigned to this organelle with any reasonable degree of confidence; (2) the nuclei and capillary endothelial cells, which were not dealt with in the preceding paper, were included as grains could be assigned to them with a high degree of confidence. Initial analysis helped us to choose the final grain and source compartments. The final source compartments are the same as the final grain compartments except that organelle in and out distinctions were not made. It was assumed that 1000 Å section coated with Ilford L4 emulsion has an HD value (half-distance) of 900 Å in radius according to Salpeter et al. (1978). The legend to Fig. 6 contains further details on this analysis.

Light microscopic autoradiography was first carried out to determine the association of silver grains with specific regions of placenta. Fig. 1A shows the association of numerous grains with syncytiotrophoblasts and very few or no grains over the interior regions of placenta. Some areas of syncytium were more heavily labelled than others and perinuclear arrangement of grains can be seen. The average total number of grains counted in a 100^m2 area was 17·1 ±0·3 (mean count of 20 areas of 100 Mm2). Coincubation of labelled with excess unlabelled EGF (Fig. IB) resulted in a reduction of grains to 2·5 ± 0·2 in 100 μm2 (mean of 20 areas of 100 μm2). Thus, the grains observed in Fig. 1A represent substantially specific grain distribution.

Fig. 1.

A. Light microscopic autoradiograph of term human placenta incubated with [125I]EGF. Note the localization of silver grains over syncytiotrophoblasts (arrowhead). ×600; bar, 10μm. B. Light microscopic autoradiograph of term human placenta coincubated with [125I]EGF and excess unlabelled EGF. Note a considerable reduction of silver grains over the cellular areas. ×1100; bar, 10 μm. C. Electron microscopic autoradiograph of term human placenta incubated with [125I]EGF. Note the presence of silver grains (arrowheads) over microvilli (mv), nuclei (n) and nuclear membranes. ×8800; bar, 1 μm.

Fig. 1.

A. Light microscopic autoradiograph of term human placenta incubated with [125I]EGF. Note the localization of silver grains over syncytiotrophoblasts (arrowhead). ×600; bar, 10μm. B. Light microscopic autoradiograph of term human placenta coincubated with [125I]EGF and excess unlabelled EGF. Note a considerable reduction of silver grains over the cellular areas. ×1100; bar, 10 μm. C. Electron microscopic autoradiograph of term human placenta incubated with [125I]EGF. Note the presence of silver grains (arrowheads) over microvilli (mv), nuclei (n) and nuclear membranes. ×8800; bar, 1 μm.

Electron microscopic autoradiographic studies revealed the association of silver grains with various intracellular organelles. Figs 1C and 2 show the association of silver grains with microvillus plasma membranes, nuclei, nuclear membranes, rough endoplasmic reticulum and lysosomal vesicles. The association of grains with these organelles as well as Golgi apparatus, basal plasma membranes and capillary endothelial cells is shown in Figs 3, 4. A grain over a lysosomal vesicle of a capillary endothelial cell can be seen in Fig. 4C. In none of the autoradiographs were the grains observed in the basement membrane area. Placental tissue incubated at 4°C for 2h with [125I]EGF shows a total number of grains of 5·4 ± 0·2 in a 100μm2 area (mean of 20 areas of 100 μm2) by light microscopic autoradiography. Electron microscopic examination revealed that all of the grains were located over microvillus plasma membranes (data not shown). Fig. 5 shows a considerable reduction of grains following coincubation with excess unlabelled EGF.

Fig. 2.

Electron microscopic autoradiographs of term human placenta incubated with [125I]EGF. Note the grains (arrowheads) in association with microvilli rough endoplasmic reticulum (rer), lysosomal vesicles (ly), nuclei (n) and nuclear membrane; bs, basement membrane; bm, basal plasma membrane. A, ×11200; B,C, ×7830; bar, 1 μm.

Fig. 2.

Electron microscopic autoradiographs of term human placenta incubated with [125I]EGF. Note the grains (arrowheads) in association with microvilli rough endoplasmic reticulum (rer), lysosomal vesicles (ly), nuclei (n) and nuclear membrane; bs, basement membrane; bm, basal plasma membrane. A, ×11200; B,C, ×7830; bar, 1 μm.

Fig. 3.

Electron microscopic autoradiographs of term human placenta incubated with [125I]EGF. A and B show association of grains with the Golgi (G) region; C and D show grains over microvilli (mu) and basal plasma membranes; and D shows grains over capillary endothelial cells (ce). Arrowheads indicate basement membrane. ×8320; bar, 1 μm.

Fig. 3.

Electron microscopic autoradiographs of term human placenta incubated with [125I]EGF. A and B show association of grains with the Golgi (G) region; C and D show grains over microvilli (mu) and basal plasma membranes; and D shows grains over capillary endothelial cells (ce). Arrowheads indicate basement membrane. ×8320; bar, 1 μm.

Fig. 4.

Electron microscopic autoradiographs of term human placenta incubated with [125I]EGF. Grains are seen in association with microvillus plasma membranes (mr), basal plasma membrane ibm) and capillary endothelial cell (ce). Note the presence of a grain over a lysosomal vesicle of a capillary endothelial cell in C. Arrowheads indicate basement membrane. ×8320; bar, 1 μm.

Fig. 4.

Electron microscopic autoradiographs of term human placenta incubated with [125I]EGF. Grains are seen in association with microvillus plasma membranes (mr), basal plasma membrane ibm) and capillary endothelial cell (ce). Note the presence of a grain over a lysosomal vesicle of a capillary endothelial cell in C. Arrowheads indicate basement membrane. ×8320; bar, 1 μm.

Fig. 5.

Electron microscopic autoradiograph of term human placenta coincubated with [125I]EGF and excess unlabelled EGF. The arrowheads indicate two non-specific silver grains in association with microvilli (mv). ×ll 000; bar, 1μm.

Fig. 5.

Electron microscopic autoradiograph of term human placenta coincubated with [125I]EGF and excess unlabelled EGF. The arrowheads indicate two non-specific silver grains in association with microvilli (mv). ×ll 000; bar, 1μm.

Fig. 6.

Computer printout of observed and expected grain analysis in term human placenta incubated with [125I]EGF. YOBS value = observed grain distribution. The following abbreviations are used: microvillus plasma membranes (PM)/IN = 2 HD when the grains were outside but in association with PM and 1 HD if the grains were inside the PM but not overlapping the other organelles; PM/OUT = grains within 2HD from the tips of microvilli; rough endoplasmic reticulum (RER)/IN = grains within 1 HD if the RER profiles are elongated and 2HD if the profiles are round; RER/OUT = grains within 2HD around all RER profiles; lysosomes (LY) = 1 HD if the grain compartment was inside the LY and 2HD if the grains were at the limiting membrane; GOLGI = 2 HD if the grains were on the Golgi profiles and 2 HD if grains were overlapping with the secretory granules; nuclei (N)/IN = 1 HD if the grains were in association with condensed and dispersed chromatin; N/OUT = 1 HD if the grains were inside or outside the nuclear membrane; basal plasma membrane (B) = 1 HD if the grains were inside or outside B; capillary endothelial cell (CE) = 1 HD if the grains were associated with capillary endothelial cells.

Fig. 6.

Computer printout of observed and expected grain analysis in term human placenta incubated with [125I]EGF. YOBS value = observed grain distribution. The following abbreviations are used: microvillus plasma membranes (PM)/IN = 2 HD when the grains were outside but in association with PM and 1 HD if the grains were inside the PM but not overlapping the other organelles; PM/OUT = grains within 2HD from the tips of microvilli; rough endoplasmic reticulum (RER)/IN = grains within 1 HD if the RER profiles are elongated and 2HD if the profiles are round; RER/OUT = grains within 2HD around all RER profiles; lysosomes (LY) = 1 HD if the grain compartment was inside the LY and 2HD if the grains were at the limiting membrane; GOLGI = 2 HD if the grains were on the Golgi profiles and 2 HD if grains were overlapping with the secretory granules; nuclei (N)/IN = 1 HD if the grains were in association with condensed and dispersed chromatin; N/OUT = 1 HD if the grains were inside or outside the nuclear membrane; basal plasma membrane (B) = 1 HD if the grains were inside or outside B; capillary endothelial cell (CE) = 1 HD if the grains were associated with capillary endothelial cells.

The meaningful assessment of grain distribution over subcellular organelles requires correction for radiation spread. There are several methods for this (Salpeter & McHenry, 1973; Blackett & Parry, 1977) and the one used in the present studies,i.e. three-step mask analysis, was developed by Salpeter et al. (1978). This method was previously used to elucidate the prolactin secretory pathway in pituitary mam-motrophs (Salpeter & Farquhar, 1981), specific intracellular organelle association of internalized gonadotropin (Chegini et al. 1984b) and prostaglandins (Chegini et al. 1984c).

Fig. 6 shows the computer printout of the observed grain distribution and generated source-to-grain matrix. The final X2 value was non-significant, indicating that the observed grain distribution was similar to the expected grain distribution. The optimized source density is the final grain distribution after correction for radiation spread. These values show that the relative grain density was the highest in microvillus plasma membranes of syncytiotrophoblasts followed by nuclei > lysosomes > Golgi apparatus = basal plasma membranes = rough endoplasmic reticulum. The grain density in capillary endothelial cells was similar to that of microvillus plasma membranes of syncytiotrophoblasts.

The objective of the present study was to investigate whether internalized [125I]EGF might associate with the intracellular organelles that have been shown in the preceding paper (Ramani et al. 1986) to contain EGF-binding sites. Incubation conditions that result in minimal (i.e. 4°C, 2h) and considerable (38°C, 2h) internalization of [125I]EGF were used for autoradiographic experiments. Obviously, autoradiography does not lend itself to examining the nature of intracellular organelle-associated radioactivity. However, the findings that eluted intracellular organelle-bound radioactivity can rebind to fresh organelles (Ramani et al. 1986) and coincubation with excess unlabelled EGF resulted in a considerable reduction of grains, suggested that grains associated with intracellular organelles represent substantially intact [125I]EGF molecules. Consistent with this suggestion are the recent findings of Lai et al. (1986), which showed that internalized [125I]EGF was intact. Authors of previous autoradiographic studies on EGF (Gorden et al. 1978a), insulin (Gorden et al. 1978b; Carpentier et al. 1978), gonadotropin (Chegini et al. 1984a) and prostaglandins (Chegini et al. 1984b) have also concluded that internalized hormones are intact.

In agreement with Lai et al. (1986) we have found that internalization was temperature dependent, i.e. not seen at 4°C and was readily observed at 38°C. After correction for radiation spread, the internalized [125I]EGF was found associated with various intracellular organelles. Nuclei had the highest grain density followed by lysosomes, Golgi apparatus = rough endoplasmic reticulum. Using primary culture of human placental syncytiotrophoblasts, Lai et al. (1986) have found an association of internalized [125I]EGF with many of the same intracellular organelles as we did.

The possibility that the association of internalized [125I]EGF with the intracellular organelles represents part of the phenomenon of transplacental movement of molecules was considered. As seen in Results, there were very few grains in basolateral plasma membranes, none in the basement membrane area and a considerable number in capillary endothelial cells. Capillary endothelial cell grains may have come from transplacental movement of [125I]EGF and, or, from medium [125I]EGF entering capillary spaces from the cut ends of the capillaries in the tissue pieces. If there is transplacental movement, we should have seen grains in the basement membrane, an area between basolateral plasma membranes and capillary endothelial cells. For this reason we favour the second possibility, unless grains can be demonstrated in capillary endothelial cells under conditions in which vascular spaces are inaccessible to external [125I]EGF. Such an experiment, for example in vivo perfusion, could not be done on humans.

The intracellular organelle-associated grains may represent [125I]EGF-receptor complexes internalized from the cell surface and, or, [125I]EGF dissociated from its receptors during internalization and then reassociated with the receptors in various organelles. The latter scenario is possible because acidic pH, which promotes dissociation of receptor-bound [125I]EGF, does exist in coated vesicles and endosomes of cells (Anderson et al. 1984).

There are at least two possibilities (not exclusive of each other) to explain the association of internalized [125I]EGF with various intracellular organelles. (1) The [125I]EGF in various intracellular organelles, with the possible exception of nuclei, represents [125I]EGF and, or, its receptors en route to lysosomes for degradation (degradative pathway). (2) [125I]EGF associated with the intracellular organelles may serve some function inside the placental cells (non-degradative pathway). This is a provocative possibility and there is no definitive evidence for excluding it. Moreover, consistent with this possibility, we have found that the intracellular organelles also contain EGF-responsive kinase (Ramani et al. 1986) and increased phosphorylation is one of the first detectable events to occur following EGF binding to its receptors (Carpentered al. 1978; King et al. 1980). Coexistence of degradative and non-degradative pathways of [125I]EGF internalization have previously been described by Miskimins & Shimizu (1982). Previous biochemical and morphological studies, mostly on cultured cells, have shown that internalized [125I]EGF associates either only with lysosomes (Carpenter & Cohen, 1976; Gorden et al. 1978a; Willingham et al. 1979; Haigler et al. 1979; Carpentier et al. 1982) or also with rough endoplasmic reticulum, Golgi elements (Miskimins & Shimizu, 1980; Willingham & Pastan, 1982; Matrisian et al. 1984) and nuclei (Johnson et al. 1980a; Savion et al. 1981). The nuclear association required the use of lysosomotropic agents (Johnson et al. 1980a; Savion et al. 1981). In a more recent study, however, nuclear translocation of intact [125I]EGF in liver, after partial hepatectomy, was demonstrated by Raper et al. (1985).

Concerning the possible intracellular sites of EGF action, there have been reports in the literature showing that the biological effects of EGF may require its internalization (Aharonov et al. 1978; Fox & Das, 1979; King et al. 1981). Regenerating rat liver cells accumulate intact EGF in their nuclei (Raper et al. 1985). GH3 cells accumulate EGF in their nuclei in the presence of lysosomotropic agents (Johnson et al. 1980a; Savion et al. 1981). EGF increases the phosphorylation of specific nuclear proteins in GHj cells, and this effect is augmented under conditions that increase nuclear EGF accumulation (Johnson et al. 1980b). EGF induces perturbation of nuclear chromatin structure (Johnson et al. 1982). The Golgi uptake of [125I]EGF was enhanced under conditions that can fully support cell growth (Miskimins & Shimizu, 1982), and the internalized EGF-receptor complex is biologically active and catalyses phosphorylation of soluble proteins (Cohen & Fava, 1985). Although some of these data may not be considered as unequivocal proof of intracellular EGF involvement in generation of biological responses, all of these studies nevertheless are of sufficient importance not to exclude such a possibility.

The donation of unlabelled and labelled EGF by Dr Gregory Schultz and the help of Dr Gary Cobbs in the three-step mask analysis of the data by computer are gratefully acknowledged.

Aharonov
,
A.
,
Pruss
,
R. M.
&
Hershman
,
H. R.
(
1978
).
Epidermal growth factor: Relationship between receptor regulation and mitogenesis in 3T3 cells
.
J. biol. Chem
.
253
,
3970
3977
.
Anderson
,
R. G. W.
,
Falck
,
J. R.
,
Goldstein
,
J. L.
&
Brown
,
M. S.
(
1984
).
Visualization of acidic organelles in intact cells by electron microscopy
.
Proc. natn. Acad. Sci. U.SA
.
81
,
4838
4842
.
Blackett
,
N. M.
&
Parry
,
D. M.
(
1977
).
A simplified method of “Hypothetical Grain” analysis of electron microscope autoradiographs
.
J. Histochem. Cytochem
.
25
,
206
214
.
Carpenter
,
G.
&
Cohen
,
S.
(
1976
).
125I-Labeled human epidermal growth factor binding, internalization and degradation in human fibroblasts
.
j. Cell Biol
.
71
,
159
171
.
Carpenter
,
G.
,
King
,
L.
&
Cohen
,
S.
(
1978
).
Epidermal growth factor stimulates phosphorylation in membrane preparations in vitro
.
Nature, Lord
.,
276
,
409
410
.
Carpentier
,
J.-L.
,
Gorden
,
P.
,
Amherdt
,
M.
,
Van Obberghen
,
E.
,
Kahn
,
C. R.
&
Orci
,
L. J.
(
1978
).
12SI-insulin binding to cultured human lymphocytes: Initial localization and fate of hormone determined by quantitative electron microscope autoradiography
.
J1, elm. Invest
.
61
,
1057
1070
.
Carpentier
,
J.-L.
,
Gordon
,
P.
,
Anderson
,
R. G. W.
,
Goldstein
,
J. L.
,
Brown
,
M.
,
Cohen
,
S.
&
Orci
,
L. J.
(
1982
).
Co-localization of 125I-epidermal growth factor and ferritin-low density lipoprotein in coated pits: A quantitative electron microscopic study in normal and mutant human fibroblasts.^
.
Cell Biol
.
95
,
73
77
.
Chandler
,
J. P.
(
1965
).
“Stepit” is a phlegmatic method of solving a problem. Program available from quantum chemistry program exchange
.
Department of Chemistry
,
Indiana University, Bloomington, IN 47401
,
USA
.
Chegini
,
N.
,
Rao
,
Ch. V.
&
Cobbs
,
G.
(
1984a
).
A quantitative electron microscope autoradiographic study on 125I-human choriogonadotropin internalization in bovine luteal slices
.
Expl Cell Res
.
151
,
483
493
.
Chegini
,
N.
,
Rao
,
Ch. V.
&
Cobbs
,
G.
(
1984b
).
A quantitative electron microscope autoradiographic study on 3H-prostaglandin E, and internalization in bovine luteal slices. Molec. cell
.
Endocr
.
38
,
117
129
.
Cohen
,
S.
&
Fava
,
R. A.
(
1985
).
Internalization of functional epidermal growth factor: receptor/kinase complexes in A-431 cells. J’, biol
.
Chetn
.
260
,
12351
12358
.
Fox
,
C. F.
&
Das
,
M. J.
(
1979
).
Internalization and processing of the EGF receptor in the induction of DNA synthesis in cultured fibroblasts: The endocytic activation hypothesis
.
J. supramolec. Struct
.
10
,
199
214
.
Gorden
,
P.
,
Carpentier
,
J.-L.
,
Cohen
,
S.
&
Orci
,
J.
(
1978a
).
Epidermal growth factor: morphological demonstration of binding, internalization and lysosomal association in human fibroblasts
.
Proc. natn. Acad. Sci. U.SA
.
75
,
5025
5029
.
Gorden
,
P.
,
Carpentier
,
J.-L.
,
Lecam
,
A.
,
Freychet
,
P.
&
Orci
,
L. J.
(
1978b
).
Limited intracellular translocation of 125I-insulin: direct demonstration in isolated hepatocytes
.
Science
200
,
782
785
.
Haigler
,
H. T.
,
Mckanna
,
J. A.
&
Cohen
,
S.
(
1979
).
Direct visualization of the binding and internalization of a ferritin conjugate of epidermal growth factor in human carcinoma cells A 431
.
J. Cell Biol
.
81
,
382
395
.
Johnson
,
L. K.
,
Baxter
,
J. D.
,
Vlodavsky
,
I.
&
Gospodarowicz
,
D.
(
1980b
).
Epidermal growth factor and expression of specific genes: effect on cultured rat pituitary cells are dissociable from the mitogenic response
.
Proc. natn. Acad. Sci. U.SA
.
11
,
394
398
.
Johnson
,
L. K.
,
Vlodavsky
,
L
,
Baxter
,
J. D.
&
Gospodarowicz
,
D.
(
1980a
).
Nuclear accumulation of epidermal growth factor in cultured rat pituitary cells
.
Nature, Bond
.
287
,
340
343
.
Johnson
,
L. K.
,
Vlodavsky
,
I.
&
Eberhardt
,
N. L.
(
1982
).
Nuclear actions of epidermal growth factor in rat pituitary tumor cells. In Evolution of Hormone Receptor Systems
, vol.
6
(ed.
R. A.
Bradshaw
&
G. N.
Gill
), pp.
397
411
.
New York
:
Alan R. Liss
.
King
,
A. C.
,
Hernaez-Davis
,
L.
&
Cuatrecasas
,
P.
(
1981
).
Lysosomotropic amines inhibit mitogenesis induced by growth factors
.
Proc. natn. Acad. Sci. U.SA
.
78
,
717
721
.
King
,
L. E.
,
Carpenter
,
G.
&
Cohen
,
S.
(
1980
).
Characterization by electrophoresis of epidermal growth factor stimulated phosphorylation using A-431 membranes
.
Biochemistry
19
,
1524
1528
.
Lai
,
W. H.
,
Guyda
,
H. J.
&
Bergeron
,
J. J. M.
(
1986
).
Binding and internalization of epidermal growth factor in human term placental cells in culture
.
Endocrinology
118
,
413
423
.
Matrisian
,
L. M.
,
Planck
,
S. R.
&
Magun
,
B. E.
(
1984
).
Intracellular processing of epidermal growth factor. I. Acidification of 125I-epidermal growth factor in intracellular organelles
.
J. biol. Chem
.
259
,
3047
3052
.
Miskimins
,
W. K.
&
Shimizu
,
N.
(
1982
).
Involvement of multiple subcellular compartments in intracellular processing of epidermal growth factor
. In
Evolution of Hormone Receptor Systems
, vol.
6
(ed.
R. A.
Bradshaw
&
G. N.
Gill
), pp.
105
114
.
New York
:
Alan R. Liss
.
Ramani
,
N.
,
Chegini
,
N.
,
Rao
,
Ch. V.
,
Woost
,
P. G.
&
Schultz
,
G. S.
(
1986
).
The presence of epidermal growth factor binding sites in the intracellular organelles of term human placenta
.
J. Cell Sei
.
84
,
19
40
.
Rao
,
Ch. V.
,
Carman
,
F. R.
Jr.
,
Chegini
,
N.
&
Schultz
,
G. S.
(
1984
).
Binding sites for epidermal growth factor in human fetal membranes
.
J. din. Endocr. Metab
.
58
,
1034
1042
.
Raper
,
S. E.
,
Burwen
,
S. J.
,
Barker
,
M. E.
&
Jones
,
A. L.
(
1985
).
Intact epidermal growth factor is translocated to the nucleus during liver regeneration
.
J. Cell Biol
.
101
, no.
5
, part 2, Abst. no. 1327.
Salpeter
,
M. M.
&
Farquhar
,
M. A.
(
1981
).
High resolution analysis of the secretory pathway in mammotrophs of the rat anterior pituitary
.
J. Cell Biol
.
91
,
240
246
.
Salpeter
,
M. M.
&
Mchenry
,
F. A.
(
1973
).
Electron microscope autoradiography. Analysis of autoradiograms
. In
Advanced Techniques in Biological Electron Microscopy
(ed.
J. K.
Koehler
), pp.
113
152
.
New York
:
Springer-Verlag
.
Salpeter
,
M. M.
,
Mchenry
,
F. A.
&
Salpeter
,
E. E.
(
1978
).
Resolution in electron microscope autoradiography: IV. Application to analysis of autoradiographs
.
J. Cell Biol
.
76
,
127
145
.
Savion
,
N.
,
Vlodavsky
,
I.
&
Gospodarowicz
,
D.
(
1981
).
Nuclear accumulation of epidermal growth factor in cultured bovine corneal endothelial and granulosa cells
.
J. Cell Biol
.
256
,
1149
1154
.
Willingham
,
M. C.
,
Maxfield
,
F. R.
&
Pastan
,
I. H.
(
1979
).
az-macroglobulin binding to the plasma membrane of cultured fibroblast. Diffuse binding followed by clustering in coated regions
.
J. Cell Biol
.
82
,
614
625
.
Willingham
,
M. C.
&
Pastan
,
I. H.
(
1982
).
Transit of epidermal growth factor through coated pits of the Golgi system
.
J. Cell Biol
.
94
,
207
212
.