J. Anim. Sci. 2006. 84:579-587
© 2006 American Society of Animal Science
ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
Characterization of mammary gland development in pregnant gilts1
F. Ji*,
W. L. Hurley
and
S. W. Kim*,
,2
* Department of Animal and Food Sciences and
and
Institute for Pig Research and Education, Texas Tech University, Lubbock, TX 79409; and
and
Department of Animal Sciences, University of Illinois, Urbana 61801
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Abstract
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The purpose of this study was to quantify mammary gland (MG) growth during pregnancy in gilts and to determine the effect of anatomical location on gland growth. Size, composition, and histomorphology of MG were determined during gestation in 29 primigravid gilts. Gilts were allotted randomly to 6 slaughter groups: d 45 (n = 6), 60 (n = 4), 75 (n = 5), 90 (n = 4), 102 (n = 5), and 112 (n = 5) of gestation. Mammary glands were obtained at slaughter, and skin and extraneous fat pad were removed to obtain parenchymal MG tissue. Mammary glands were further separated into individual MG, and their locations were recorded. Individual MG were weighed and bisected in an approximate midsagittal section to measure cross-sectional area. Mammary glands were ground individually and pooled according to anatomical region: the first and second pairs of MG = anterior MG; the third, fourth, and fifth pairs of MG = middle MG; the sixth, seventh, and eighth pairs of MG = posterior MG. Contents of DM, CP, ether extract, and crude ash were measured. Wet weight, DM, CP, and ash content of total and individual MG increased (P < 0.01) between d 45 and 112 of gestation. Cross-sectional area of individual MG increased (P < 0.01) as gestation progressed. Percentage of CP and ash increased (P < 0.01), whereas percentage of ether extract decreased (P < 0.01) as gestation progressed. This inverse relationship between percentages of CP and ether extract (r = –0.999; P < 0.0001) was consistent with the histological shift from primarily an adipose tissue in early gestation to one containing extensive lobuloalveolar tissue in late gestation. Wet weight of middle MG was greater (P < 0.05) than that of posterior MG at d 102 and 112 of gestation, and amount of CP in middle MG was greater (P < 0.05) than that in anterior and posterior MG at d 102 and 112 of gestation, indicating that middle MG grow faster than other MG during late gestation. Rates of wet weight gain and protein accretion were accelerated (P < 0.01) after d 74 and 75 of gestation, respectively, indicating the importance of MG growth during the last trimester of gestation. The increase in rate of protein accretion after d 75 indicates a greater protein requirement for MG growth during later gestation.
Key Words: anatomical location • gilt • mammary gland • pregnancy • swine
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INTRODUCTION
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Sow milk production is one of the most important factors limiting neonatal piglet growth and survival. Increased litter weight gains during lactation are mainly the result of greater milk production from the sow (Hartman and Pond, 1960
; Noblet and Etienne, 1987; King et al., 1993). Conversely, suboptimal lactation function of the mammary gland (MG) is limiting to piglet growth (Boyd and Kensinger, 1998). A major determinant of the yield of milk from a gland is the number of secretory cells present (Knight et al., 1984; Knight and Peaker, 1984; Tucker, 1987). Growth of suckling piglets is correlated with the content of protein and DNA in the suckled MG (Kim et al., 2000; Nielsen et al., 2001). Characterization of MG growth, therefore, provides a basis for understanding how milk production is regulated in swine and is an essential step to increase milk production, and therefore litter growth.
Periods of extensive MG development in swine occur during pregnancy and lactation. Mammary growth during pregnancy is slow during the initial two-thirds of pregnancy and then accelerates during the final third (Hacker and Hill, 1972; Sørensen et al., 2002). Mammary tissue DNA, DNA concentration, and parenchymal volume dramatically increase during the latter third of pregnancy in gilts (Kensinger et al., 1982; Hurley et al., 1991; Sørensen et al., 2002). After parturition, substantial MG growth continues in primiparous gilts (Kim et al., 1999), to the extent that total mammary DNA doubles during a 21-d lactation.
Although the overall pattern of MG growth during pregnancy has been characterized with respect to tissue mass and DNA content, only limited information is available on changes in gross tissue composition during pregnancy or on growth patterns of glands from different locations. The objectives of this study were to characterize the patterns of tissue growth of MG throughout pregnancy and to evaluate the impact of MG anatomical region on gland growth.
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MATERIALS AND METHODS
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Animals, Experimental Design, and Diet
The 29 gilts (159.3 ± 3.4 kg, Camborough-22, PIC) were housed in individual gestation crates at Texas Tech University Swine Research Farm (New Deal, TX). The animal use and care protocol was approved by Animal Care and Use Committee of Texas Tech University. Gilts were randomly allotted to 1 of 6 slaughter groups by d of pregnancy: d 45 (n = 6), d 60 (n = 4), d 75 (n = 5), d 90 (n = 4), d 102 (n = 5), and d 112 (n = 5). Gilts had a single daily meal of 2.0 kg of a gestation diet during pregnancy. The gestation diet contained 3.1 Mcal of ME/kg, 12.2% CP, and 10.2 g of true ileal digestible lysine/d, as previously described (Ji et al., 2005). The ME and true ileal digestible lysine allowances, as well as other nutrients, met the recommendations by the NRC (1998).
Slaughter and Sampling of Mammary Glands
Pregnant gilts were transported to the Texas Tech University Meat Laboratory at 1700 of the day before assigned slaughter day. Gilts had access to water, but their feed was withheld overnight. Gilts were electrically stunned and killed by exsanguination. After slaughter, whole MG were separated from the body. Skin and extraneous fat pad were removed from MG to obtain mammary tissues. Total MG tissues were further separated into individual MG as described in Kim et al. (1999). The location of individual MG was recorded. Individual MG were weighed and bisected in an approximate midsagittal section to measure cross-sectional area as described in Kim et al. (1999). Half of each individual MG was collected and ground with a commercial grinder (Applics Consumer Products; Miami Lakes, FL). Ground individual MG were pooled according to anatomical region on the underline of the gilt. Pooling of glands was based on regional differences among individual glands observed at farrowing, as previously described (Kim et al., 2000). The number of glands per gilt ranged from 13 to 16. The first and second pairs of MG were pooled and named anterior MG. The third, fourth, and fifth pairs of MG were pooled and named middle MG. The sixth, seventh, and eighth pairs (when present) of MG were pooled and named posterior MG. All samples were stored at –20°C for further analysis.
Chemical Analyses
Frozen samples were freeze-dried (DURA-Top; FTS Systems, Chatswood, NSW, Australia) after weighing. Freeze-dried samples were reweighed to calculate the water loss and then ground again with a commercial blender (Waring Products, New Hartford, CT). Freeze-dried samples (1 g) were further desiccated to measure DM at 105°C for 8 h in a forced-air oven. Crude protein content (N x 6.25) was determined using LECO FP-2000 (LECO Corp., St. Joseph, MI). Ether extract was determined from dried tissue according to AOAC (1995). Ash was measured by the combustion of dried tissue at 500°C for 8 h.
Histological Analyses
Mammary tissue samples were collected from the central portion of the fourth MG. Samples were initially fixed in 10% neutral buffered formalin (10% formalin, 75 mM of sodium phosphate, pH 7.2) for 6 to 8 h at room temperature, after which the formalin was replaced with fresh formalin and the samples were stored in fix until further processing. Prior to processing, tissues were transferred to 70% ethanol. Tissue samples were dehydrated using a Tissue-Tek Vacuum Infiltration Processor Model E150 (Sakura Finetek U.S.A., Inc., Torrance, CA) and embedded in paraffin. Histological sections (5 µm) were prepared (American Optical Microtome, Buffalo, NY) and were dried overnight on a slide warmer (40°C). Tissues were de-paraffinized in xylene, and sections were rehydrated in a series of descending ethanol baths and, finally, water. Mammary sections (minimum of 4 sections per gilt) were then stained with hematoxylin and eosin. Representative digital images were obtained with a Spot RT digital camera (Diagnostic Instruments, Sterling Heights, MI) using an Olympus BH-2 microscope (Melville, NY) with brightfield optics.
Statistical Analyses
Data were analyzed as a complete randomized design, using the GLM procedure (SAS Inst. Inc., Cary, NC). Gilt was the experimental unit. Residual standard deviation was used to evaluate the variability of a data set. Data were further analyzed using the REG procedure of SAS with the forward option to obtain regressions between the variables and day of gestation. The model was
in which y is a variable, x is a day of gestation, ß0 is an intercept, ß1 is a coefficient for linear regression, ß2 is a coefficient for quadratic regression, ß3 is a coefficient for cubic regression, and e is an experimental error. Regressions described the changes of all MG, as well as MG by their location.
Variables that could be explained by quadratic or cubic regressions were further analyzed to find the breakpoint (day of gestation) where the rate of accretion changed at alpha = 0.05. The NLREG software (Sherrod, 1992) was used to obtain the multiphasic regressions and breakpoints as described by McPherson et al. (2004) and Ji et al. (2005).
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RESULTS
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Total and Individual Mammary Glands
In preliminary studies, MG parenchymal tissue at the beginning of gestation was found to be visually indistinguishable from the surrounding fat pad to the extent that samples necessary for analysis of gross composition could not be dissected from the tissue. Samples of MG tissue, therefore, were obtained beginning at d 45 of gestation. Adipose tissue (outer fat pad) distinct from the mammary parenchymal tissue was trimmed away and excluded from the analyses. Mass of parenchymal tissue and its components provide an indication of the rate and nature of tissue development during gestation.
Wet weight of total MG and average individual MG increased (cubic; P < 0.01) approximately 10 and 7.5 fold, respectively, between d 45 and 112 of gestation (Tables 1 and 2). Dry tissue weight and amounts of CP and crude ash in both total MG and average individual MG increased (cubic; P < 0.01) as gestation progressed (Tables 1 and 2). Cross-sectional area of a midsagittal section of average individual MG increased approximately 3.5 fold over the period from d 45 to 112 of gestation (cubic; P < 0.01; Table 2). Ether extract, a measure of tissue lipid, did not change in either total MG (P = 0.112; Table 1) or average individual MG (P = 0.172; Table 2) during this period of gestation.
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Table 2. Morphology and composition of individual mammary glands (MG) on different days of gestation and by different anatomical regions1
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Percentage of DM in average individual MG decreased (linear; P < 0.01) between d 45 and 112 of gestation (Table 3). This decline in DM percentage primarily was accounted for by a decreased percentage of ether extract (DM basis) in average individual MG (cubic; P < 0.01). In contrast, percentage of CP (DM basis; cubic; P < 0.01) and crude ash (DM basis; quadradic; P < 0.01) increased during this period of gestation. Percentages of CP and ether extract in average individual MG were negatively correlated (r = –0.999; P < 0.001).
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Table 3. Percentage composition of individual mammary glands (MG) on different days of gestation and by different anatomical regions1
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Mammary Glands by Anatomical Region
Tissues from the individual MG were pooled to evaluate tissue mass and composition among three defined anatomical regions on the gilt, the anterior (first and second pairs), middle (third, fourth, and fifth pairs), and posterior (sixth, seventh, and eighth pairs) glands. Wet weight, cross-sectional area, dry weight, and amounts of ash and protein increased (cubic; P < 0.01; Table 2
) between d 45 and 112 of gestation for MG in each of the anatomical regions. In contrast, amount of ether extract did not change (P 
0.111) as gestation progressed for any of the regions. Differences among anatomical regions in wet weight, dry weight, ash, and CP occurred only at the end of gestation, specifically at d 102 and 112 (Table 2). In general, the largest glands were the middle MG and the smallest were the posterior MG (P < 0.05).
Percentage of DM in anterior, middle, and posterior MG decreased (linear; P < 0.01) between d 45 and 112 of gestation (Table 3). Percentage of crude ash (DM basis) increased in the middle MG (quadratic; P < 0.01), as well as in anterior and posterior MG (linear; P < 0.01), as gestation progressed. Percentage of CP (DM basis) in anterior, middle, and posterior MG increased (cubic; P < 0.01), whereas percentage of ether extract (DM basis) in anterior, middle, and posterior MG decreased (cubic; P < 0.01), as gestation progressed. Differences in MG composition among the anatomical regions were observed only at d 102 of gestation. The average anterior MG had the least ash and protein percentage and greatest ether extract percentage at d 102 of gestation (Table 3). The average middle MG had the greatest protein percentage and least ether extract percentage at d 102 of gestation.
Mammary Glands by Stage of Gestation
Wet weight and CP amount in the average individual MG increased especially during the late gestation (cubic; P < 0.01) and thus the data were reanalyzed by NLREG software (Sherrod, 1992) to obtain multiphasic regressions and estimate the break point where 2 regression equations meet. Wet weight and CP amount were fitted with 2 linear regressions to separately describe the slow rate of increase during early gestation and rapid rate of increase during late gestation. A break point (day of gestation) when the rates of accretion from both linear regressions changed at alpha = 0.05 also was identified.
From this analysis, the day of gestation (break point) when the rate of accretion changed (P < 0.05) was d 74 ± 7 for the average individual MG wet weight and d 75 ± 3 for individual CP amount. Wet weight in the average individual MG was estimated to increase by 1.4 g/d until d 74 of gestation [g = 103.66 + 1.4035 x (d – 73.86); d = day of gestation] and 4.8 g/d after d 74 of gestation [g = 103.66 + 4.8204 x (d – 73.86)]. Protein accretion in the average individual MG was estimated to increase by 0.08 g/d until d 75 of gestation [g = 5.89 + 0.0787 x (d – 74.83)] and 1.05 g/d after d 75 of gestation [g = 5.89 + 1.053 x (d – 74.83)]. This regression analysis indicated that the average individual MG would have gained a total of 103.9 g prior to d 74 of gestation and then 193.3 g after d 74 of gestation, and would reach an estimated 297.3 g at d 114 of gestation.
Histomorphology of Mammary Glands at Stages of Gestation
The negative relationship between proportion of CP and ether extract in the MG tissue during gestation was consistent with the qualitative changes in histomorphology observed in the tissues. Qualitatively, a greater proportion of the tissue was composed of adipocytes at d 45 and 60 (Figure 1) compared with d 102 and 112 (Figure 2). Duct branching was apparent by d 45 of gestation, with some lobular formation; ducts were primarily associated with thick tracts of connective tissue. A terminal ductule lobular unit, the characteristic structural unit of the developing MG in many species (Hovey et al., 1999), is shown in Figure 1D from d 60. By d 75 of gestation, extensive lobule formation was present, although the diameter of lobular ductules was smaller than during late gestation. Ducts and lobular ductules contained stained luminal material at all stages of gestation. Some alveoli were distended with luminal contents by d 102 of gestation, and most were fully distended at d 112 (Figure 2).
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Figure 1. Histomorphology of mammary gland tissue from gilts during midpregnancy. Panels are from gestational days: A and B = 45, C and D = 60, and E and F = 75. Panels A, C, and E are from lower magnification; B, D, and F are from higher magnification; all bars are 50 µm. Where present, a = adipocyte; c = connective tissue tract; d = duct; dl = ductule in lobule; ic = interlobular connective tissue; and t = central duct of a terminal ductule lobular unit.
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Figure 2. Histomorphology of mammary gland tissue from gilts during later pregnancy. Panels are from gestational days: A and B = 90, C and D = 102, and E and F = 112. Panels A, C, and E are from lower magnification; B, D, and F are from higher magnification; all bars are 50 µm. Where present, a = adipocyte; al = alveolar epithelial cell; l = lumen of alveolus; and ic = interlobular connective tissue.
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DISCUSSION
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Growth of the MG in the primigravid gilt is slow during the middle third of gestation, but the growth rate increased by more than 3-fold during the last third of gestation. The composition of MG tissue shifts from one of a high lipid content to one of a high protein content. Consistent with these composition changes, MG histological structure indicates a shift from an immature gland composed primarily of MG adipose in early gestation to one with extensive development of ductal and lobular structures in later gestation. The sow has multiple MG that may provide for the newborn litter. However, these glands are not equivalent in their rate of growth, especially in later gestation when the middle MG increase in mass fastest compared with anterior and posterior glands. The variation in size, milk yield, and growth rate among the MG of the sow during lactation is well documented (Pluske and Dong, 1998; Kim et al., 2000).
The use of multiphase regression and break point analysis on the data from the current study identifies an early phase of MG development up to around d 75, followed by a more rapid development. This break point estimate is consistent with the findings of other reports that most mammary tissue accumulation takes place in the last third period of gestation (Weldon et al., 1991; Sørensen et al., 2002). Hacker and Hill (1972) also found that the total amount of dried fat-free tissue, DNA, and RNA in MG of pregnant gilts did not change from d 25 to 50 of gestation, but increased 6-fold from d 50 to 100 of gestation. The estimated MG weight of 297 g at d 114 of gestation is somewhat less than the 373 g at d 112 of gestation reported by Sørensen et al. (2002) or the 431.5 g within 12 h postpartum from primiparous sows reported by Kim et al. (1999).
Nevertheless, the rapid acceleration of MG growth after about d 75 coincides with the period when blood concentrations of several mammogenic hormones begin to increase, including estrogen, relaxin, and prolactin. Concentrations of estradiol begin increasing between d 60 and 80 of gestation in pregnant gilts; concentrations of estradiol peak at d 112 (Kensinger et al., 1986; Eldridge-White et al., 1989). Relaxin is another important mammogenic hormone in swine (Hurley et al., 1991). Relaxin concentrations are low until about d 80 and then begin increasing slowly to d 100, followed by an abrupt increase in late gestation (Eldridge-White et al., 1989). Prolactin concentrations in the blood begin increasing during the last trimester of gestation in the gilt (Farmer et al., 2000). Inhibition of prolactin secretion by administration of bromocriptine in the last third of pregnancy results in decreased weight of MG parenchymal tissue and MG cell numbers by d 110 (Farmer et al., 2000).
It is interesting to note the apparent rapid increase in MG tissue that occurred between d 45 and 60. Development of MG parenchymal tissue through early gestation is diffuse in nature, making it difficult to delineate a visible boundary between parenchymal-free fat pad and parenchymal tissue. Sørensen et al. (2002) described tissue mass of fat pad plus parenchymal tissue during early gestation; however, only individual gilts were evaluated on any day of gestation, and it is difficult to draw specific conclusions about this midgestation period. At this time, we are not aware of any hormonal changes that may stimulate such a rapid growth phase of MG tissue during the d 45- to 60-period. The period of MG development during early to middle gestation deserves a more intensive evaluation than that provided by this or previous studies.
There is a dramatic change in overall structure of the mammary tissue during gestation. The tissue histology changes from one of primarily adipose and fibrous connective tissue in early gestation, with minimal ductal development, to one of minimal numbers of adipose cells and extensive lobular development with extended alveoli by the end of gestation. This change in histological structure is consistent with the overall change from a mammary parenchymal tissue with a high fat composition (92% ether extract) and low protein composition (7% CP) on d 45 of gestation to one of a protein composition increased to 38% and fat composition reduced to 59% in mammary parenchymal tissue by d 112 of gestation. These gross histological and tissue composition changes are indicative of the significant interaction that occurs between the developing epithelial structures and the stromal and adipose structures that define MG growth in postpubertal animals and during gestation (Hovey et al., 1999). In a species such as the pig, the major ducts appear to elongate within the interconnected network of collagenous tissue that defines the lobules of adipocytes that make up the fat pad. Most ducts in the early stages of gestation are surrounded by thick collagenous tissue. Epithelial lobules, identified as terminal ductule lobular units, often are observed to branch out into the spaces occupied by the adipose lobules.
The current study indicates that there are differences in the rate of development of the MG in later gestation dependent on gland location; the middle glands develop more rapidly than the anterior or posterior glands during the period after d 90. This latter observation is consistent with previously reported differences in MG size among anatomical location at or shortly after farrowing (Kim et al., 2000). In the latter case, the fourth and fifth glands had the greatest development of parenchymal tissue. The enhanced size of these glands relative to the anterior and posterior glands becomes apparent during the last 2 to 3 wk of gestation when MG development is regulated by the mammogenic hormones associated with pregnancy. Further development of the glands postfarrowing occurs in response to suckling. The effect of suckling includes a combination of milk removal and suckling-induced prolactin secretion (Wilde and Peaker, 1990; Hurley, 2001). During lactation, the anterior MG grow rapidly, even though they are smaller than middle MG at farrowing (Kim et al., 1999). At the end of a 28-d lactation, anterior MG, as well middle MG, are larger and produce more milk than posterior MG (Kim et al., 2000). Findings from the current study and from Kim et al. (1999) do not support observations from older studies of postpartum mammary development in swine that suggested that the anterior MG might be larger or produce more milk (Donald, 1937; Gill and Thomson, 1956). The increased milk yield observed in modern sows has been discussed by King (2000).
The growth of the MG during pregnancy includes substantial increases in tissue protein components. Protein accretion in average individual MG is 0.08 g/d before d 75 of gestation and 1.05 g/d after d 75 of gestation. If a gilt has 16 MG, then she will have approximately 1.3 g of CP/d for mammary tissue accretion before and 16.8 g of CP/d after d 75 of gestation. Considering the nutrient requirements for a 150 kg of BW gilt at breeding, as suggested by NRC (1998), a gilt under a conventional feeding program may receive 235 g of total CP/d from the diet. The amount of CP accreted for fetal tissues in a sow is 56 g/d after d 70 of gestation (McPherson et al., 2004). Combined results from this study and from McPherson et al. (2004) indicate that the amount of CP accreted in MG and fetal tissues is approximately 73 g/d during late gestation. Considering protein digestibility (75 to 95% for the major feed ingredients; Stein et al., 2001), protein needs for maternal maintenance (NRC, 1998), protein (lysine) needs for maternal growth for young sows and gilts (Ji et al., 2005), and amino acid oxidation during metabolism (Moehn et al., 2004; Wu et al., 2004; Schaart et al., 2005), this study indicates that the CP requirement for pregnant pigs during the late gestation (after d 70 of gestation) may be underestimated. That conclusion supports the need for phase feeding during gestation as proposed by Ji et al. (2005).
In conclusion, growth of MG parenchymal tissue is most rapid during approximately the last 6 wk of gestation in the primigravid gilt. The extent of MG development is variable by anatomical region, with the middle MG growing greatest by parturition. The MG tissue growth during gestation establishes the framework for the rapid growth that occurs in response to suckling during lactation. The rapid growth of the MG during late gestation also means that the nutrient requirements for MG tissue growth are rapidly changing during that period. Current estimates of dietary protein requirements for gilts in late pregnancy may be underestimated.
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Footnotes
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1 The authors acknowledge the financial support of CJ Corp. (Seoul, Korea), Texas Tech University, and the Illinois Agric. Exp. Sta. (Hatch Project 538-327), and the technical help of H. Adams and D. Gronlund. 
2 Corresponding author: sungwoo.kim{at}ttu.edu
Received for publication August 6, 2005.
Accepted for publication October 12, 2005.
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Abstract
INTRODUCTION
