Cytoplasmic filaments in fetal and neonatal pig testis.

Leydig cells in developing fetal pig testis contained during the fetal regressive phase large accumulations of intermediate filaments. Before and after this period these filaments were arranged in a criss-cross fashion. In the pig as well as in the dog testis these filaments have been characterized as vimentin. Within the vimentin aggregates occasionally a weak positive actin reaction was seen in pig but not in dog Leydig cells. Microfilaments were hardly observed. Most Sertoli cells contained a layer of actin microfilaments close to the basal cell membrane. In the lower cell compartment and around the nucleus (intermediate) vimentin filaments could be observed in a criss-cross configuration.

In vitro model of the first phase of testicular descent: identification of a low molecular weight factor from fetal testis involved in proliferation of gubernaculum testis cells and distinct from specified polypeptide growth factors and fetal gonadal hormones.

The gubernaculum testis is the connective tissue organ that causes the testis to descend. How the process of testicular descent is regulated is not fully understood. Current hypotheses postulate that a nonandrogenic fetal testicular factor controls the first phase of descent, that is characterized by growth of the gubernaculum and transabdominal migration of the testis. When gonadal extracts from fetuses with ages corresponding to the first phase of testicular descent (50, 60, and 75 days) were tested on gubernacular cells, the growth stimulatory effect of testicular extracts exceeded the effect of both ovarian extract and fetal calf serum. Gonadal extracts from 80-, 90-, and 100-day-old fetuses showed only a minor sex difference. No sex difference or age-dependent changes were detected when fetal gonadal extracts were tested on murine 3T3 cells. Polypeptide growth factors (epidermal growth factor, insulin, fibroblast growth factor, platelet-derived growth factor, and transforming growth factor-beta) were tested for growth stimulatory activity and had only minor effects on gubernaculum cells. Fetal testicular hormones (anti-Müllerian hormone, inhibin, and androgenic steroids) did not induce initiation of DNA synthesis at concentrations that are highly bioactive in typical target systems. When testicular samples were dialyzed, the high mol wt fraction (greater than 3500) had lower growth stimulatory activity in gubernaculum cells, but not 3T3 cells. Bioactivity of ovarian extracts and fetal calf serum was not diminished after dialysis. The low mol wt fraction (less than 3500) of testicular extract was distinctly stimulatory to gubernaculum cells but not 3T3 cells, and the low mol wt fraction of ovarian extracts did not stimulate growth in either cell type. It was concluded that the fetal porcine testis during the first phase of testicular descent contains low mol wt factor(s) to which gubernaculum cells and not 3T3 cells are responsive. The bioactive fraction probably contains the factor(s) that initiate testicular descent. We suggest the name descendin for this new activity.

Lesions in the hypothalamus after active immunisation against GnRH in the pig.

The terminals of the hypothalamic gonadotrophin hormone-releasing hormone (GnRH) neurons are located within the median eminence and thereby extend beyond the protection of the blood-brain barrier. Thus, these terminals may be subjected to direct autoimmune action in animals that are actively immunised against GnRH. Boars (male pigs) (n = 108) were actively immunised against GnRH by two successive injections with synthetic GnRH, covalently coupled to KLH and dissolved in CFA or IFA. They were killed at 26 weeks of age. Immunised boars were selected on the basis of the resultant testes size, which indicates the effectiveness of the immunisation. The hypothalami of 25 selected animals were studied by histological and immunocytochemical techniques and compared with the hypothalami of three sham- and nine control animals. In the immunised animals, changes in the GnRH system had taken place. These comprised dystrophy of the perikarya and a sharp decrease of the GnRH immunocytochemical reactivity in the terminals within the median eminence. In addition, various degrees of inflammatory reactions were present, particularly within the median eminence. These consisted of tissue disruption by edema, collapse of the capillaries, fibrosis and infiltration with fibroblasts. In addition, accumulations of neurosecretum within the median eminence in combination with hypertrophy of magnocellular neurons within the hypothalamus were present. The reactions were restricted to the median eminence and did not involve other neurohemal organs or other parts of the hypothalamus. A correlation could be established between the incidence of the lesions and the effectiveness of the GnRH autoimmunity (as indicated by the size and endocrine function of the gonads and the anti-GnRH titres). Changes in extra- and intracellular IgG immunocytochemical reactivity within the median eminence indicated the involvement of IgG. The effects were absent from control and sham vaccinated animals and after vaccinations with other compositions of the vaccine. Thus, hypothalamic lesions have been observed in this selected group of animals, vaccinated against GnRH with this particular vaccine.

Development of a new Leydig cell population after the destruction of existing Leydig cells by ethane dimethane sulphonate in rats: an autoradiographic study.

The formation of new Leydig cells in adult male rats was studied after the complete destruction of the original population by ethane dimethane sulphonate (EDS). Following administration of EDS, proliferating interstitial cells were labelled in a pulse-chase experiment by way of three [3H]thymidine injections on days 2, 3 and 4 after EDS administration. Some of the newly formed Leydig cells found 14 days after EDS administration were labelled with [3H]thymidine, indicating that these Leydig cells were derived from precursor cells, most likely mesenchymal cells, that had incorporated [3H]thymidine at days 2, 3 or 4 after EDS administration. At 21 days after EDS administration, the total number of Leydig cells (labelled plus unlabelled) had increased 7- to 16-fold compared with the number of cells that were present 14 days after EDS had been administered. In a second series of experiments, [3H]thymidine was given 2 h before the rats were killed (short-term labelling experiment). In this experiment it was shown that the proliferative activity of the mesenchymal cells, which are presumed to be the precursors of the Leydig cells, after a considerable increase at day 2 after EDS administration, had returned to the control level at day 7. However, the total number of mesenchymal cells (labelled plus unlabelled) remained increased from 2 to 49 days after EDS administration. This indicated that the majority of the new Leydig cells which were formed from day 14 onwards probably did not derive from differentiating mesenchymal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of testicular blood flow and focal degeneration of tissue in the rat after administration of human chorionic gonadotrophin.

The effect of 100 IU human chorionic gonadotrophin (hCG) on testicular capillary blood flow was studied in adult male rats using a 133Xe clearance method and a radioactive microsphere technique. To investigate the role of Leydig cells in regulation of testicular blood flow after treatment with hCG, rats were pretreated with ethane dimethylsulphonate (EDS) which selectively destroys mature Leydig cells. Six hours after treatment with hCG, testicular blood flow decreased in control and hypophysectomized rats to 25-50% of normal values, but not in EDS-pretreated animals. Prostaglandin E2 levels were also determined 6 h after an injection of hCG. A 300-fold increase in the concentration of prostaglandin E2 occurred in normal testis tissue. This rise was markedly inhibited if EDS was given 3 days before administration of hCG. Furthermore, 6 h after administration of hCG, the filling of the testicular capillary bed with methylacrylate was decreased, while in control rats and rats treated with EDS and hCG, complete filling of the capillaries was seen. Cell degeneration in some subcapsular seminiferous tubules was observed 6-10 days after treatment with hCG. The results suggest that the hCG-induced precapillary vasoconstriction, probably mediated (in part) by prostaglandins, causes reduction in testicular blood flow 6 h after administration of hCG, and may result in cell damage.

Hormone-induced resistance of rat Leydig cells to the cytotoxic effects of ethane-1,2-dimethane sulphonate.

Several studies have shown that the cytotoxic agent ethane-1,2-dimethane sulphonate (EDS) specifically destroys Leydig cells in the adult rat testis. It has also been reported that when rats are pretreated with human chorionic gonadotrophin (hCG), administration of EDS does not result in the complete destruction of the Leydig cell population. It has been suggested that hCG pretreatment 'protects' Leydig cells against the cytotoxic action of EDS. In the present study the underlying principles for this resistance to the cytotoxic effects of EDS have been investigated. Within 48 h of the start of daily hCG treatment the number of nuclear profiles of Leydig cells (henceforth called relative number of Leydig cells) had increased from 1014 +/- 40 to 1368 +/- 30 cells per 1000 Sertoli cell nuclei. Previous experiments have indicated that these newly formed Leydig cells probably develop from differentiating Leydig cell precursors. When EDS is administered concomitantly with the third injection of hCG (2 days after the start of hCG treatment), the relative number of Leydig cells surviving EDS treatment was 388 +/- 52 per 1000 Sertoli cells. Hence, there is a similarity between the increase in the relative number of Leydig cells after 2 days of hCG treatment and the relative number of EDS-resistant Leydig cells. The Leydig cells that survived EDS administration showed characteristics which also occur in developing Leydig cells in the immature testis. It is concluded that, in rats pretreated with hCG for 2 days before EDS administration, new Leydig cells with some immature characteristics are formed. One of these characteristics is that these cells are insensitive to EDS.

Effects of pure FSH and LH preparations on the number and function of Leydig cells in immature hypophysectomized rats.

The effects of pure FSH and/or LH preparations on the number of Leydig cells and their function in immature hypophysectomized rats have been investigated. As a result of hypophysectomy at the age of 17-18 days, the number of recognizable Leydig cells per testis decreased, as did the steroidogenic capacity in vivo and in vitro. Treatment with 64 micrograms FSH on both 22 and 23 days of age, did not affect the number of recognizable Leydig cells. In contrast, two injections of LH (10 micrograms) caused a sixfold increase in the number of Leydig cells, but had a negative effect on spermatogenesis. These stimulatory and inhibitory effects of LH diminished when FSH was added. Treatment with FSH for 7 days caused a twofold increase in the number of Leydig cells when compared with hypophysectomized controls. 3 beta-Hydroxysteroid dehydrogenase (3 beta-HSD) and esterase activity in Leydig cells also increased under the influence of FSH. The pregnenolone production per Leydig cell in the presence of 5-cholesten-3 beta,22(R)-diol (22R-hydroxycholesterol) as substrate showed a sevenfold increase. Plasma testosterone levels 2 h after injection of human chorionic gonadotrophin in intact rats and hypophysectomized FSH-treated rats were the same. Following LH treatment for 7 days, the number of Leydig cells proved to be 11 times higher, and 3 beta-HSD and esterase activity were not different from intact controls. The testicular pregnenolone production was four- to fivefold higher when compared with untreated hypophysectomized rats. However, pregnenolone production per Leydig cell in LH-treated rats was only slightly different from the hypophysectomized controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Proliferation and differentiation of possible Leydig cell precursors after destruction of the existing Leydig cells with ethane dimethyl sulphonate: the role of LH/human chorionic gonadotrophin.

The influence of LH levels on the proliferation and differentiation of possible Leydig cell precursors was investigated in adult rats, after the destruction of the existing Leydig cells with the cytotoxic drug ethane dimethyl sulphonate (EDS). In rats bearing a testosterone implant which prevented the rise in plasma LH levels and kept them within the normal range after the destruction of the Leydig cells, the proliferative activity of possible Leydig cell precursors still increased seven- to eightfold 2 days after EDS administration. Apparently, in this situation, locally produced factors, and not LH, may play a role in the stimulation of proliferation. The proliferative activity of the possible precursor cells could be further stimulated by treating rats with daily injections of human chorionic gonadotrophin (hCG) following EDS administration. It was concluded that the proliferative activity of possible Leydig cell precursors is probably regulated by both paracrine and endocrine factors. Almost no Leydig cells were formed in the rats bearing a testosterone implant during the first 4 weeks after EDS administration. When these rats were treated with hCG, starting 28 days after administration of EDS, a substantial number of Leydig cells was found after 2 days, and these cells also showed 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and alpha-naphtyl esterase (alpha-NE) activity. When hCG treatment was started at 14 or 21 days after EDS administration, some cells with the nuclear characteristics of Leydig cells were present after 2 days, but no 3 beta-HSD or alpha-NE activity could be detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of hypoxia in the ram testis during single and split-dose X-irradiation.

Spermatogonial stem-cell survival in the ram was studied after single (6 Gy) and split-dose (2 x 3 Gy, interval 21-24 h) X-irradiation both under normal and hypoxic conditions. Hypoxia was induced by inflation of an occluder implanted around the testicular artery. The occluders were inflated about 10 min before irradiation and deflated immediately after. Stem-cell survival was measured at 5 or 7 weeks after irradiation by determination of the Repopulation Index (RI) in histological testis sections. The RI-values after fractionated irradiation were only half those after single dose irradiation. Hypoxia had a protective effect on the stem-cell survival. After split-dose irradiation under hypoxic conditions two times more stem cells survived than under normal oxic conditions; the RI-values increased from 34% (oxic) to 68% (hypoxic). This effect of hypoxia was also found after single dose irradiation where the RI-values increased from 68% (oxic) to 84% (hypoxic). The development of the epithelium in repopulated tubules was also studied. Under hypoxia, a significantly higher fraction of tubules with complete epithelium was found after single (38 vs. 4%) as well as after split-dose irradiation (12 vs. 0%).

Induction of spermatogenesis in the naturally cryptorchid pig.

Abdominal testes of adult, naturally unilaterally cryptorchid boars were subjected to continuous artificial cooling for 5, 15, 25, or 45 days. This treatment initiated development of the spermatogenic epithelium. After a cooling period of 45 days there was complete differentiation in many seminiferous tubules. The results indicate that the spermatogenic arrest in abdominally located testes is not due to an inborn defect in the testis, but is caused by the maintenance of the testis at the abdominal temperature.

The origin of orchiopexy-induced testicular lesions in the pig.

Cooling experiments of abdominal testes in adult, naturally cryptorchid pigs indicate that spermatogenic arrest in abdominal testes is not due to an inborn defect, but is caused solely by maintenance of the testis at abdominal temperature. It is postulated that failure of spermatogenic cells to differentiate after orchiopexy results from surgical trauma. Evaluation of the orchiopexy procedure revealed that simple manipulation of a normally descended testis may give rise to damage to the spermatogenic epithelium. Furthermore, it appeared that, in normally descended testes of naturally unilaterally cryptorchid pigs subjected to orchiopexy, the spermatogenic epithelium was poorly developed as compared with that of scrotal testes of unilaterally cryptorchid pigs that had not undergone a surgical procedure.

Testis volumes, semen quality, and hormonal patterns in adolescents with and without a varicocele.

OBJECTIVE: To study the effects of varicocele on testicular function in adolescents. DESIGN: A prospective controlled study in 88 randomly selected adolescents. SETTING: All participants were referred to the fertility outpatient clinic of our University Hospital. PARTICIPANTS: All participants with a grade II varicocele (group 2) or a grade III varicocele (group 3) were selected at a district military medical council, whereas a similar group of healthy volunteers without a varicocele served as controls (group 3). INTERVENTIONS: Testis volumes were measured using an orchiometer. Semen analysis was performed according to standard procedures, and serum hormone levels were determined using a radioimmunoassay. MAIN OUTCOME MEASURE(S): Testis volumes, semen quality, and hormonal parameters in adolescents with and without a varicocele were compared. RESULTS: In group 1 (n = 21), the mean left testis volume (24.5 mL; 95% confidence interval [CI]: 22.8 to 26.2) was significantly (P less than 0.05) different from group 2 (n = 15) (20.9 mL: 95% CI: 18.5 to 23.4) and group 3 (n = 52) (20.7 mL; 95% CI: 19.2 to 22.2) (P less than 0.01) adolescents. In adolescents with a pronounced varicocele-associated left testicular growth failure, the total sperm number was reduced. However, sperm concentration, motility, and morphology were not altered. Luteinizing hormone, follicle-stimulating hormone, testosterone, and prolactin levels were all within the normal ranges in the three groups. CONCLUSIONS: Left testicular growth failure in adolescents with a varicocele is only associated with a decrease in total sperm number.

Effects of varicocele treatment in adolescents: a randomized study.

OBJECTIVE: To study the effects of varicocele treatment on testicular function in adolescents. DESIGN: A prospective controlled study in 88 randomly selected adolescents. SETTING: All participants were referred to the fertility outpatient clinic of our university hospital. PARTICIPANTS: All participants with a varicocele were randomly assigned into two groups. Group 1 (n = 33) was not treated, whereas group 2 (n = 34) was treated. A similar group of healthy volunteers without a varicocele served as a control group (group 3, n = 21). INTERVENTIONS: Testes volumes were measured at intake and during follow-up using an orchiometer. Semen analysis was performed according to standard procedures both at intake and after 1 year of follow-up. Serum hormone levels were determined at intake using a radioimmunoassay. Treatment was performed by means of transcatheter embolization of the left testicular vein. MAIN OUTCOME MEASURES: Testes volumes and semen quality at intake and after 1 year of follow-up were compared within and between the three groups. Hormonal parameters were determined at intake only. RESULTS: Before treatment, the mean left testis volume in groups 1 (n = 26) and 2 (n = 27) (20.0 mL; 95% confidence interval [CI]: 18.2 to 21.8 and 21.6 mL; 95% CI: 19.4 to 23.8, respectively) were significantly smaller than those in the control group (n = 19) (24.5 mL; 95% CI: 22.7 to 26.4). During follow-up, left testis volumes of the treated group were comparable with those in the control group (24.2 mL; 95% CI: 22.2 to 26.1 and 24.8 mL; 95% CI: 23.0 to 26.7 respectively) and significantly (P < 0.001) different from the untreated group (20.3 mL; 95% CI: 18.8 to 21.8). A significant increase in left (P < 0.01) as well as right (P < 0.05) testis volume was observed after treatment. Semen parameters before treatment were not significantly different between the three groups. Sperm concentration increased significantly (P < 0.01) from 47.4 x 10(6)/mL (95% CI: 42.5 to 53.3) to 68.9 x 10(6)/mL (95% CI: 50.6 to 87.2) in the treated group, whereas semen quality in the untreated and control groups did not change. Although both testes volumes and sperm concentration improved in the treated group, these phenomena were not consistently correlated to each other. CONCLUSIONS: Although not apparent in all adolescents, varicocele correction resulted in an increase in left testis volume and sperm concentration. At this moment, it is not clear if early preventive treatment of varicocele in adolescents, in time, will have a positive effect on testicular function.

Induction of varicocele in the dog: I. Partial ligation of the left renal vein does not induce a varicocele in the dog.

Induction of varicocele was attempted by partial ligation of the left renal vein in 10 male dogs. The effects on sperm count, sperm motility, and sperm morphology, as well as on hemodynamics, were assessed. Furthermore, testicular, vascular, and kidney morphology was studied. Changes in the diameter and consistency of the left spermatic cord were found to be temporary. Total sperm count, sperm motility, and the total number of oval forms were not significantly altered. Hemodynamic studies revealed a renocaval pressure gradient, but retrograde flow in the distal part of the left testicular vein could not be observed by arteriography. A collateral network was found to compensate for the restricted left renal vein. Histologic examination revealed no damage to the seminiferous epithelium. Changes were not found in the kidney and left pampiniform plexus. Although some temporary changes induced by the partial ligation of the left renal vein are suggestive of varicocele, this hemodynamic study shows that the presented dog model does not mimic varicocele as encountered in man.

The regulation of the proliferation and differentiation of rat Leydig cell precursor cells after EDS administration or daily HCG treatment.

The proliferation and differentiation of possible Leydig cell precursors in adult rats were studied after destruction of the existing Leydig cells with EDS or after daily treatment with hCG. After 2 days with either treatment, a 12- to 16-fold increase in the number of [3H]thymidine-incorporating interstitial cells was found. In the case of hCG treatment, this was probably due to the high plasma hCG levels. However, after EDS treatment, LH levels start to rise between days 1 and 3, suggesting a paracrine stimulation of the proliferation of interstitial cells. After hCG treatment, a substantial increase in the numbers of Leydig cells was already found at day 2. It was concluded that hCG induced a rapid differentiation, without cell division, of existing precursor cells into recognizable Leydig cells. In rats treated with both EDS and hCG, new Leydig cells were not formed during the first 10 days. This indicates that EDS destroys not only mature Leydig cells but also those Leydig cell precursors that are able to differentiate rapidly into recognizable Leydig cells.

Stimulation of the proliferation and differentiation of Leydig cell precursors after the destruction of existing Leydig cells with ethane dimethyl sulphonate (EDS) can take place in the absence of LH.

In hypophysectomized rats, 2 days after the administration of the cytotoxic drug ethane dimethyl sulphonate (EDS), the proliferative activity of Leydig cell precursors increased six-fold. Thus, factors other than LH act locally to stimulate the proliferation of precursor cells after EDS. Twenty-six days after EDS administration, neither cells with the morphological characteristics of Leydig cells nor histochemical enzyme activities, such as 3 beta-HSD and alpha-naphtyl esterase, could be detected in testis tissue. In hypophysectomized rats treated daily with hCG (100 iu) for 7 days, starting at 26 days after EDS, the number of Leydig cells was increased to 48 +/- 11 cells (per 1000 Sertoli cells), which is approximately 4.5% of the intact control level. 3 beta-HSD and alpha-naphtyl esterase activity could be detected, and plasma testosterone levels had increased 15-fold compared with the hypophysectomized controls. These results show that proliferation and some differentiation of precursor cells along the Leydig cell lineage can occur independent of LH, but the final stages of the differentiation process require hCG stimulation.

Reversible harmless interruption of testicular blood supply in the ram.

An effective method of interrupting testicular blood flow temporarily and repeatedly in the ram has been developed. Blockade of flow has been achieved mechanically by an inflatable occluder placed around the testicular artery at the level of the spermatic cord. The effect of the blockade on total testicular blood supply was investigated using Doppler flowmetry and a percutaneous Xenon-133 injection method. With both approaches, the blood flow changes after inflation or deflation of the occluders could be estimated satisfactorily. A substantial decrease of testicular blood flow was achieved in eight of the 10 testes with inflated occluders. However, there were indications that in the remaining two testes blockade of the arterial flow was not complete. After deflation of the occluders, blood flow was restored rapidly and completely in all testes. Macro- and microscopic examinations revealed no long-term damage to the testis after blood flow interruptions lasting 30 or 60 minutes.

Nerve endings in the pulmonary trunk, ductus arteriosus and aorta of intact and decapitated pig fetuses.

Nerve fibres reactive to acetyl-thiocholine, and tissues showing catecholamine fluorescence were examined in the pulmonary trunk, ductus arteriosus and aorta of 28 pig fetuses between 31 and 113 days of gestation (term = 114 +/- 1 days). Eight additional fetuses, which had been decapitated in utero at 40-43 days, were also studied at ages between 51 and 114 days of gestation. Spherical micro-networks of nervous tissue reactive to acetyl-thiocholine are present in the adventitia on the cranial aspect of the pulmonary trunk and ductus arteriosus, between the aorta and pulmonary trunk, and on the caudal aspects of the pulmonary trunk and the pulmonary arteries. These fibres invest spherical clusters of catecholamine containing cells which are well supplied with blood vessels. Nerve fibres which fluoresce are also found in association with these cells. Decapitation in utero does not appear to affect the distribution of morphology of these structures. The observations show that structures are present in the major arteries of the fetal pig which may act as sensory receptors, and that these structures are unaffected by chronic vagotomy of the fetus produced by decapitation early in gestation.

Development of nervous tissue in the heart of the fetal and neonatal pig and the effect of decapitation in utero.

The development and distribution of the nerves in the heart of the pig was studied macroscopically and by light microscopy. Hearts were collected from 86 fetuses between 31 and 114 days of gestation (term = 114 days), from 12 neonatal pigs aged 9 and 20 days and from 6 adult sows of the Dutch Landrace breed. The effect of vagotomy produced by decapitation in utero at 40-43 days was studied in an additional 24 hearts from fetuses aged between 51 and 114 days of gestation. The amount of acetyl-thiocholine reactive fibres increases in the atria, A-V node and ventricles throughout gestation. At every age the amount of nervous tissue is highest in the A-V node and lowest in the ventricles. Hearts from decapitated fetuses have smaller amounts of nerve tissue than those from intact fetuses at every age studied. Ganglia are present in both intact and decapitated fetuses. Fluorescent cells containing catecholamines are observed in hearts from fetuses as young as 35 days gestation. Although fluorescent nerve fibres are rarely seen in hearts at 70 days gestation, more fibres are present near birth and thereafter there appears to be a considerable increase in the number of fibres and in the intensity with which they fluoresce. These results show that there is substantial nerve growth into the heart of the pig during gestation and that catecholamine containing nerve fibres develop later than those reactive to acetyl-thiocholine.

The effects of gestational age and chronic fetal decapitation on arterial blood pressure in the pig fetus.

Blood pressure was measured in anaesthetized pig fetuses decapitated at 40-43 days of gestation and in intact fetuses between 35 and 112 days of gestation (term is 114 days). In the intact fetuses arterial blood pressure increased significantly from 0.8 +/- 0.1 kPa (mean +/- SEM) at 35 days to 5.8 +/- 0.2 kPa at 112 days (P less than 0.05). The arterial blood pressure of decapitated fetuses was similar to that of intact fetuses at 70 days of gestation (2.7 +/- 0.4 kPa vs. 2.5 +/- 0.1 kPa, respectively) but did not change with increasing gestational age thereafter. Hence in late gestation (greater than 90-100 days) the arterial blood pressure of the decapitated fetuses was significantly less than that of intact fetuses (P less than 0.05). These observations demonstrate that the control of blood pressure in the pig varies with gestational age and suggest that the developmental changes occurring after about 100 days gestation require tissues within the head.