Identification of cells migrating from the thecal layer of ovarian follicles

In the mammalian ovary, oocytes are contained within ovarian follicles. These consist in an oocyte surrounded by supporting cells: an inner layer of granulosa cells and an outer layer of thecal cells separated by a basal lamina. At any one time, a developing cohort of follicles exists, from which only a small species-specific number are selected for continued development towards ovulation, with the remainder dying by follicular atresia. Here, we use in vitro methods to study interactions between two follicles in culture (follicle co-cultures). We show that, when two individual follicles are grown together in culture, cells and cellular processes migrate from the outer thecal layer of one follicle to the thecal layer of the other co-cultured follicle. These cells are identified as a mixed population containing primarily endothelial but also neuronal cells. Both are able to migrate through the ovarian interstitum, making contact with the basal lamina of other follicles and with similar cells from these other follicles. Networks of such cells might be involved in interfollicular communication and in the coordination of follicle selection for ovulation.     

Structure of the ovotestis and evidence for heterosynthetic incorporation of yolk precursors in the oocytes of the nudibranch mollusc,Spurilla neapolitana

The ovotestis of Spurilla neapolitana consists of a series of spherical lobes, each of which is composed of radially arranged, sac-like acini or follicles. The male and female portions of each acinus are separated by ovarian follicle cells and testicular accessory cells. A thick basal lamina serves as a barrier between adjacent acini. The surface of each ovotestis lobe is covered by several layers of myoepithelial cells resting on a connective tissue layer. Developing oocytes are intimately associated with follicle cells except in the last stages of vitellogenesis. Follicle cells are characterized by the presence of extensive arrays of rough endoplasmic reticulum (RER) and Golgi complexes and may play a role in vitellogenesis. An ultrastructural analysis of vitellogenesis suggests that oocytes utilize both auto- and heterosynthetic mechanisms of yolk formation. Autosynthetsis is suggested by the activity of the Golgi complex and RER, while heterosynthesis is indicated by high levels of endocytotic activity by the oocyte. Follicle cell development and high endocytotic activity in the oocytes may be a reproductive adaptation to accelerate yolk synthesis, resulting in more rapid egg production.     

Changes with starvation in the rat of the lipoprotein lipase activity and hydrolysis of triacylglycerols from triacylglycerol-rich lipoproteins in adipose tissue preparations.

Lipoprotein lipase activity was higher in fat-pad pieces than in isolated adipocytes from the same fed rats, whereas hydrolysis of triacylglycerols from triacylglycerol-rich lipoproteins was similar in the two preparations when incubated either in basal conditions or in the presence of heparin. In both preparations there was a similar release of lipoprotein lipase activity into the medium during basal incubation, enhanced by the presence of heparin. In fat-pad pieces, but not in isolated adipocytes, incubation with heparin produced a decrease in the lipoprotein lipase activity measured in the tissue preparation. In fat-pad pieces from 24 h-starved rats, lipoprotein lipase activity was the same as in isolated adipocytes from the same animals and incubation with heparin did not affect the appearance of lipoprotein lipase in the medium or the utilization of triacylglycerols from triacylglycerol-rich lipoproteins. These results support the following conclusions. (1) The effectiveness of lipoprotein lipase in adipose tissue preparations in vitro depends more on its availability to the substrate than on its total activity. (2) Heparin acts on adipose tissue preparations from fed animals both by enhancing the release of pre-existing extracellular enzyme (which is absent in isolated adipocytes) and by enhancing the transfer outside the cells of the intracellular (and mainly undetectable) enzyme that is activated in the secretion process. (3) In adipose tissue from starved animals there is not only a decrease in the active extracellular form of lipoprotein lipase activity but also a reduction in the intracellular (and mainly undetectable) pool of the enzyme.     

Basal lamina of avian ovarian follicle: influence on morphology of granulosa cells in-vitro

Experiments were conducted to determine the influence of basal lamina on the morphology of ovarian granulosa cells in vitro. Pure and intact basal lamina was isolated from the large preovulatory follicles of the chicken ovary and designated basal lamina of avian ovarian follicle (BLAOF). Examination of the isolated basal lamina with electron microscope revealed an ultrastructure that is similar to that of basal lamina in the intact ovarian follicle. Pieces of the intact basal lamina were attached to the bottom of 32 mm culture dishes (BLAOF-coated dishes) in which differentiated granulosa cells isolated from the largest preovulatory follicle or undifferentiated granulosa cells isolated from immature small yellow chicken ovarian follicles were cultured; uncoated dishes served as controls. Granulosa cells incubated on intact basal lamina assumed spherical shape, whereas granulosa cells incubated directly on plastic in control dishes became highly flattened. Interestingly, granulosa cells that attached to plastic close to BLAOF (in BLAOF-containing dishes) became rounded. The storage of BLAOF-coated culture dishes at 4°C for 2 years had no apparent effect on its ability of the matrix material to induce changes in granulosa cell shape. Some components of the basal lamina could be solubilized with guanidine–HCl alone (fraction 1; 90–95% of total protein in BLAOF) with the remaining components solubilized with β-mercaptoethanol containing guanidine–HCl (fraction 2; 5–10% of total protein in BLAOF). Differentiated and undifferentiated chicken granulosa cells became rounded when incubated in fraction 1-pre-coated wells; whereas those incubated directly on plastic in control wells were flattened. Similarly, when fraction 1 of solubilized basal lamina was added as liquid to incubation mixture, it caused both differentiated and undifferentiated granulosa cells to assume spherical shapes. The storage of fraction 1-coated culture dishes at 4°C for 12 or more months had no apparent effect on its ability to influence granulosa cell shape. Fraction 1-induced changes in granulosa cell shape were similar to those observed for complete and intact basal lamina (BLAOF). These findings demonstrate that intact homologous basal lamina (BLAOF) or its solubilized (fluidized) form can induce normal (in vivo) morphology in granulosa cells. It is suggested that BLAOF or its solubilized form can be used to culture cells in experiments designed to examine the influence of the natural basal lamina microenvironment on cellular behavior and function.     

Ultrastructural studies on developing follicles of the spotted ray Torpedo marmorata.

Light and ultrastructural investigations on sub-adult and adult sexually mature females, demonstrates that in Torpedo marmorata folliculogenesis starts in the early embryo and that the two ovaries in the adult contain developing follicles of various sizes and morphology. Initially, the follicle is constituted by a small oocyte, surrounded by a single layer of squamous follicle cells. The organization is completed by a basal lamina and, more externally, by a theca, that at this stage is composed by a network of collagen fibers. As the oocyte growth goes on, during previtellogenesis and vitellogenesis, the organization of the basal lamina and of the oocyte nucleus does not change significantly. The basal lamina, in fact, remains acellular and constituted by fibrils intermingled in an amorphous matrix; the nucleus always shows an extended network of chromatin due to the lampbrush chromosomes, and one or two large nucleoli. By contrast, the granulosa (or follicular epithelium), the ooplasm, and the theca cells significantly change. The granulosa shows the most relevant modifications becoming multi-layered and polymorphic for the progressive appearance of intermediate and pyriform-like cells, located respectively next to the vitelline envelope, or spanning the whole granulosa. The appearance of intermediate cells follows that of intercellular bridges between small follicle cells and the oocyte so that one can postulate that, as in other vertebrates, small cells differentiate into intermediate, and then pyriform-like cells, once they have fused their plasma membrane with that of the oocyte. Regarding the ooplasm, one can observe as in previtellogenic follicles, it is characterized by the presence of intermediate vacuoles containing glycogen, while in vitellogenic follicles by an increasing number of yolk globules. The theca also undergoes significant changes: initially, it is constituted by a network of collagen fibers, but later, an outermost theca esterna containing cuboidal cells and an interna, with flattened cells, can be recognized. The role of the different constituents of the ovarian follicle in the oocyte growth is discussed.     

Lipoprotein lipase of ovarian follicles in the domestic chicken (Gallus domesticus) (38537).

A lipase, bearing the characteristics of adipose tissue lipoprotein lipase (LPL) has been characterized in avian ovarian granulosa cells. The activity is low in cells from follicles weighing less than 0.5 g; in heavier follicles which have entered the rapid growth phase, significant activity (1 mumole fatty acid/mg protein/hr) could be identified. Granulosa LPL provides follicular tissues with the requisite enzyme system to hydrolyze very low density lipoprotein triglyceride en route to the oocyte.     

Microscopic and molecular characterization of ovarian follicle atresia in Rhodnius prolixus Stahl under immune challenge

In this work we characterized the degenerative process of ovarian follicles of the bug Rhodnius prolixus challenged with the non-entomopathogenic fungus Aspergillus niger. An injection of A. niger conidia directly into the hemocoel of adult R. prolixus females at the onset of vitellogenesis caused no effect on host lifespan but elicited a net reduction in egg batch size. Direct inspection of ovaries from the mycosed insects revealed that fungal challenge led to atresia of the vitellogenic follicles. Light microscopy and DAPI staining showed follicle shrinkage, ooplasm alteration and disorganization of the monolayer of follicle cells in the atretic follicles. Transmission electron microscopy of thin sections of follicle epithelium also showed nuclei with condensed chromatin, electron dense mitochondria and large autophagic vacuoles. Occurrence of apoptosis of follicle cells in these follicles was visualized by TUNEL labeling. Resorption of the yolk involved an increase in protease activities (aspartyl and cysteinyl proteases) which were associated with precocious acidification of yolk granules and degradation of yolk protein content. The role of follicle atresia in nonspecific host-pathogen associations and the origin of protease activity that led to yolk resorption are discussed.     

Zinc deficiency and the activities of lipoprotein lipase in plasma and tissues of rats force-fed diets with coconut oil or fish oil

The present study was performed to investigate the effect of zinc deficiency on the activities of lipoprotein lipase in postheparin serum and tissues of rats fed diets containing either coconut oil or fish oil as dietary fat, using a bifactorial experimental design. To ensure an adequate food intake, all the rats were force-fed by gastric tube. Experimental diets contained either 0.8 mg zinc/kg (zinc-deficient diets) or 40 mg zinc/kg (zinc-adequate diets). The effects of zinc deficiency on the activities of lipoprotein lipase in postheparin serum and postprandial triglyceride concentrations and distribution of apolipoproteins in serum lipoproteins depended on the type of dietary fat. Zinc-deficient rats fed the coconut oil diet exhibited a reduced activity of lipoprotein lipase in postheparin serum and adipose tissue, markedly increased concentrations of triglycerides in serum, and a markedly reduced content of apolipoprotein C in triglyceride-rich lipoproteins and high density lipoproteins compared with zinc-adequate rats fed coconut oil. By contrast, zinc-deficient rats fed the fish oil diet did not exhibit reduced activities of lipoprotein lipase in postheparin serum and adipose tissue and increased concentrations of serum lipids compared with zinc-adequate rats fed the fish oil diet. This study suggests that a reduced activity of lipoprotein lipase might contribute to increased postprandial concentrations of serum triglycerides observed in zinc-deficient animals. However, it also demonstrates that the effects of zinc deficiency on lipoprotein metabolism are influenced by dietary fatty acids.     

The binding of very low density and low density lipoproteins to the plasma membrane of the hen's oocyte : A morphological study

The binding of lipoproteins to the oocyte plasma membrane of the domestic fowl (Gallus domesticus) was examined by electron microscopy in preparations of the ovarian follicle in the main phase of yolk formation. Numerous particles, 26 nm in diameter, were present on the untreated membrane. They were dissociated from the membrane by incubation at 4 °C in buffer at pH 6.2 and with heparin at pH 7.4. Added calcium was not required for binding, though the number of bound particles was reduced by treatment with EDTA. Very low density (VLD) lipoproteins from laying hen's plasma were found to bind to the denuded membrane and to correspond in size to the native particles. The results suggest that the binding characteristics are similar in quality to those determined for the binding of low density (LD) lipoproteins to mammalian cells. The oocytes, however, bound 100-fold more particles per unit length of membrane. VLD and LD lipoproteins from immature hens also adhered to the denuded membrane, although their apoprotein composition was very different from that of laying hen VLD lipoproteins. LD lipoproteins from immature hens and VLD lipoproteins from laying hens both contained apo-B, which formed about 80 and 35%, respectively, of the total apolipoproteins. Apo-VLDL-II is the other major apoprotein identified in laying-hen VLD lipoproteins. Apo-VLDL-II was not positively identified as a component of immature-hen LD lipoproteins and could only have been present as a minor component. Despite their great difference in apo-VLDL-II content, immature-hen LD lipoproteins and laying-hen VLD lipoproteins showed similar dissociation constants for binding to the oocyte plasma membrane. This evidence strongly suggests that the cell surface receptors recognize the B apoprotein of avian VLD lipoproteins.     

Expression of lipoprotein lipase in ovaries of the guinea pig

Guinea pig ovaries were found to have significant lipoprotein lipase (LPL) activity, corresponding to almost one-tenth the activity in paraovarian adipose tissue and in heart per gram of tissue. Northern blot analysis demonstrated the same three species of LPL mRNA in ovaries (1.8, 3.1, and 3.5 kb) as in adipose tissue. In situ hybridization showed LPL mRNA in cells of the follicular wall, and in granulosa and theca lutein cells of the mature corpus luteum. By immunolocalization, LPL was visualized in the vascular endothelium throughout the ovary, but with highest concentration in the endothelium of capillaries and large vessels of the cortical region and capillaries in the stroma of the corpus luteum. These results suggest that in the guinea pig LPL may have a function for the delivery of lipids from lipoproteins to ovarian cells.     

Factors promoting vitellogenic competence and yolk deposition in the cockroach ovary: The post-ecdysis female

Incorporation of labelled histidine into a follicle cell product was shown to be juvenile hormone-dependent in Periplaneta americana. After 30 min the labelling was localized in the follicle cell cytoplasm, followed by labelling of intercellular spaces between the follicle cells after 60 min, and finally labelling of cortical yolk spheres after 120 min. Synthesis of the follicle cell product was observed exclusively in ovarian follicles participating in yolk formation. DNA synthesis was observed in follicle cell nuclei of ovarian follicles from the early stages of follicle differentiation to chorion formation. The evidence indicates that, although ovarian follicles exhibit protein synthetic responses to juvenile hormone in adult females, incorporation of thymidine into follicular nuclei suggests that changes in the cell cycle after adult ecdysis do not necessarily affect sensitivity to juvenile hormone.     

Immunohistochemical localization of epidermal growth factor receptor (EGF-r) and transforming growth factor alpha (TGF-alpha) in developing human ovarian follicles

In this study, we aimed to detect the distribution of epidermal growth factor receptor (EGF-r) and transforming growth factor alpha in ovarian follicles at different stages. Indirect immunohistochemical methods and EGF-r polyclonal and TGF-alpha monoclonal antibodies were used; tissues were examined with light microscope. While dense collection of both growth factors were observed in primordial follicles, there was a strong reaction especially for EGF-r in follicles. Strong reactivity for EGF-r and moderate reactivity for TGF-alpha were observed in the nearby connective tissue. In examinations of primary follicles for EGF-r presence only, dye uptake was moderate in oocytes and dense in apical and basal cytoplasm of follicle cells. Reactivity was moderate in the nearby connective tissue. In the corpus luteum, there was weak reaction for both growth factors. But in stromal cells, reaction was strong. In degenerated follicle cells and in stroma of atretic follicles, reaction was positive for both growth factors; but EGF-r reactivity was more obvious. While strong staining was observed for both factors especially in granulosa cells surrounding the oocyte in Graafian follicle, moderate TGF-alpha reactivity was determined in oocyte cytoplasm. In conclusion, it is possible that EGF-r and TGF-alpha have ortocrine and paracrine effects on development and regression of human ovarian follicles.     

A scanning electron microscopic study of the luteo-follicular complex. II. Events leading to ovulation.

Morphological changes on the ovarian surface of different mammals both before and during ovulation have been examined by scanning electron microscopy. Preovulatory follicles were blisterlike structures that protruded markedly from the ovarian surface. Basal areas of preovulatory follicles were covered with polyhedral cells containing numerous microvilli, whereas on the lateral surfaces, superficial cells were elongated and possessed few microvilli. At the apex of the follicle, cells were very flattened and possessed few microvilli, which were present only in regions of intercellular contact. In some apical areas, cells appeared to be degenerating, whereas in other regions, groups of cells had "sloughed off." In addition, a fluidlike material was observed to exude from intercellular spaces of the superficial epithelium and to cover some apical cells. By transmission electron microscopy, the same fluidlike material was observed to (1) infiltrate the connective tissue of the tunica albuginea, (2) accumulate under the basal lamina, and (3) distend intercellular spaces of the superficial epithelium. Just prior to ovulation, large, irregular areas of the apex were ruptured and the oocyte, covered with a large amount of fluid, appeared to emerge from the follicle. At ovulation, the oocyte was not completely covered with follicle cells and the zona pellucida was clearly evident. The surface of the zona was quite irregular and contained numerous infoldings, channels and crypts. Follicle cells had polyhedral or star shapes and possessed large cytoplasmic evaginations that obliquely penetrated the zona. Both the zona pellucida and corona cells were covered with a fine layer of granular material. The SEM results and parallel TEM observations suggest that a local increase of fluids (edema) may be an important factor in the final decomposition of the distended and weakened apex of the preovulatory follicle. In addition, the participation of follicle cells, smooth muscle cells and the oviduct in the escape of the oocyte from the ruptured follicle is discussed.     

Localization of follicle regulatory protein in the porcine ovary

The granulosa cell secretes a protein (follicle regulatory protein: FRP) that affects the responsiveness of other follicles to gonadotropin stimulation. This protein was purified, partially characterized, and rabbit antisera as well as monoclonal antibodies were prepared against FRP. Fixed sections of porcine ovaries were prepared on slides and then incubated with the monoclonal antibody or polyclonal antisera and then incubated with either biotinylated mouse IgM or rabbit IgG antisera, respectively. These sections were then incubated with avidin conjugated to horseradish peroxidase, followed by substrate. Staining with both the monoclonal antibody and the antisera was present in the cytoplasm of granulosa cells of small- or medium-sized antral follicles. Staining distribution was localized preferentially to cells near the basal lamina; the antral granulosa cells of viable follicles did not stain. Neither primordial follicles nor pre-antral follicles (less than 300 microns in diameter) showed any positive staining. Thecal cells were not stained in follicles less than 5 mm in diameter, whereas some large follicles (greater than 5 mm) contained staining in the theca. In the latter, specific granulosa staining was only weakly positive with the polyclonal antibody and negative with the monoclonal antibody. Atretic follicles contained significant staining of all epithelial cells adjacent to the basal lamina by both the monoclonal and polyclonal antibody preparations. Staining of the luteal ovary by the monoclonal antibody was limited to the large luteal cells. These findings suggest that FRP is produced by the granulosa cells of porcine follicles at the stage of maturation corresponding to 0.5 mm in diameter. As the viable follicle increases in size, production of FRP in the granulosa is reduced below the detectable level when the follicle exceeds 5 mm in diameter. The main source of FRP during the luteal phase is the large cell of the corpus luteum.     

Ultrastructure of the basal lamina of bovine ovarian follicles and its relationship to the membrana granulosa

Different morphological phenotypes of follicular basal lamina and of membrana granulosa have been observed. Ten preantral follicles (< 0. 1 mm), and 17 healthy and six atretic antral follicles (0.5-12 mm in diameter) were processed for light and electron microscopy to investigate the relationship the between follicular basal lamina and membrana granulosa. Within each antral follicle, the shape of the basal cells of the membrana granulosa was uniform, and either rounded or columnar. There were equal proportions of follicles 相似文献   

Morphology and ultrastructure of the somatic cells in Astacus leptodactylus ovary

We defined the somatic environment in which female germinal cells develop, and performed ultrastructural analyses of various somatic cell types, with particular reference to muscle cells and follicle cells, that reside within the ovary at different stages of oogenesis. Our findings show that ovarian wall of the crayfish is composed of long muscle cells, blood cells, blood vessels and hemal sinuses. The follicle and germinal cells lie within a common compartment of ovarian follicles that is defined by a continuous basal matrix. The follicle cells form branching cords and migrate to surround the developing oocytes. A thick basal matrix separates the ovarian interstitium from ovarian follicles compartment. Transmission electron microscopy shows that inner layer of basal matrix invaginates deeply into the ovarian compartment. Our results suggest that before being surrounded by follicle cells to form follicles, oogonia and early previtellogenic oocytes reside within a niche surrounded by a basal matrix that separates them from ovarian interstitium. We found coated pits and coated vesicles in the cortical cytoplasm of previtellogenic and vitellogenic oocytes, suggesting the receptor mediated endocytosis for transfer of material from the outside of the oocytes, via follicle cells. The interstitial compartment between the inner muscular layer of the ovarian wall and the basal matrix of the ovarian follicle compartment contains muscle cells, hemal sinuses, blood vessels and blood cells. Granular hemocytes, within and outside the vessels, were the most abundant cell population in the ovarian interstitium of crayfish after spawning and in the immature ovary. J. Morphol. 277:118–127, 2016. © 2015 Wiley Periodicals, Inc.     

The fine structure of the cumulus oophorus during follicular development in sheep

Summary The cumulus and membrana granulosa of non-atretic ovarian follicles from primordial up to a stage shortly before ovulation were studied by electron microscopy.The follicular cells of primordial follicles were undifferentiated and rested on a thick basal lamina. In secondary follicles the endoplasmic reticulum had proliferated forming an anastomosing network. In early antral and antral follicles (0.5–2.0 mm dia.) the ER was composed of short cisternae, the mitochondria had elongated and gap junctions were first observed. In late antral follicles (3.0–5.9 mm dia.) gap junctions were frequent. In the cumulus the glycogen was associated with electron lucent areas whereas in the granulosa it was invariably associated with membranes. In large antral follicles large membrane bound bodies were present in the basal cells of the cumulus. At early oestrus a distinctive mitochondrial morphology was noted in the granulosa but not elsewhere in the follicles. At mid oestrus numerous annular nexuses were present in the granulosa but not in the cumulus. At late oestrus numerous lipid droplets were formed in both cumulus and granulosa, the boundary with theca interna became indistinct and the basal lamina became incomplete.Deceased     

Ultrastructure and composition of Call-Exner bodies in bovine follicles

Call-Exner bodies are present in ovarian follicles of a range of species including human and rabbit, and in a range of human ovarian tumors. We have also found structures resembling Call-Exner bodies in bovine preantral and small antral follicles. Hematoxylin and eosin staining of single sections of bovine ovaries has shown that 30% of preantral follicles with more than one layer of granulosa cells and 45% of small (less than 650 microns) antral follicles have at least one Call-Exner body composed of a spherical eosinophilic region surrounded by a rosette of granulosa cells. Alcian blue stains the spherical eosinophilic region of the Call-Exner bodies. Electron microscopy has demonstrated that some Call-Exner bodies contain large aggregates of convoluted basal lamina, whereas others also contain regions of unassembled basal-lamina-like material. Individual chains of the basal lamina components type IV collagen (alpha 1 to alpha 5) and laminin (alpha 1, beta 2 and delta 1) have been immunolocalized to Call-Exner bodies in sections of fresh-frozen ovaries. Bovine Call-Exner bodies are presumably analogous to Call-Exner bodies in other species but are predominantly found in preantral and small antral follicles, rather than large antral follicles. With follicular development, the basal laminae of Call-Exner bodies change in their apparent ratio of type IV collagen to laminin, similar to changes observed in the follicular basal lamina, suggesting that these structures have a common cellular origin.     

High-cholesterol diet-induced lipoproteins stimulate lipoprotein lipase secretion in cultured rat alveolar macrophages

We have previously shown that cultured rat alveolar macrophages synthesize and secrete lipoprotein lipase into the medium. The purpose of the present experiments is to examine whether cholesterol-enriched lipoproteins from cholesterol-fed animals have any effects on the lipoprotein lipase secretion and the lipid accumulation in macrophages. Macrophages incubated with the VLDL obtained from rats fed a normal diet secreted 2-fold higher amounts of lipoprotein lipase than those without lipoproteins. Intermediate-, low- and very-low-density lipoproteins from rats fed a high-cholesterol diet also enhanced the lipoprotein lipase secretion. Normal high- and low-density lipoproteins, and high-density lipoproteins from hypercholesterolemic animals did not cause any increase in the lipoprotein lipase secretion. The lipoproteins which stimulated the lipoprotein lipase secretion caused intracellular accumulation of both triacylglycerol and cholesterol. It is speculated that macrophages residing in the environment rich in lipoproteins, especially hypercholesterolemic lipoproteins, take them up and accumulate lipids intracellularly, and that this process links with the lipoprotein lipase secretion. The secreted lipoprotein lipase could facilitate, by degrading lipoproteins, the uptake of lipoprotein lipase-modified lipoproteins. Probably such a series of events is of importance in the foam cell formation of macrophages.     

Very low density lipoproteins and lipoprotein lipase in serum of rats deficient in essential fatty acids

Rats fed a diet deficient in essential fatty acids have a low level of serum very low density lipoproteins (VLDL). It was found that after intraperitoneal injection of heparin, deficient rats had a higher level of lipoprotein lipase activity in their plasma than did normal rats. VLDL isolated from serum of normal and deficient rats were compared as substrates for postheparin lipase of rat plasma. There was no significant difference in V(max) between the two preparations of lipoproteins, but the apparent K(m) for lipoproteins from deficient animals was significantly less than that for normal animals. These observations suggest that the low concentration of VLDL in deficient rats may be explained (a) by an increased activity of lipoprotein lipase in the tissues of these animals and (b) by the VLDL of deficient rats being more rapidly hydrolyzed at low concentrations by lipoprotein lipase than VLDL from normal rats.