In 1997, McPherron first identified MSTN (also known as growth differentiation factor-8) as a member of the TGF-β (transforming growth factor-β) superfamily (McPherron et al., 1997). MSTN is highly expressed in skeletal muscle, but lowly expressed in adipose tissue and cardiac muscle (McPherron et al., 1997; Sharma et al., 1999). MSTN strongly affects skeletal muscle development (McPherron and Lee, 1997; Zhao et al., 2005; Guo et al., 2009; Ahad et al., 2017). The MSTN mutation lead to a double muscling phenotype in cattle (Grobet et al., 1997; McPherron and Lee, 1997), mice (Bünger et al., 2004), sheep (Clop et al., 2006) and other animals (Schuelke et al., 2004; Mosher et al., 2007).

Since pigs are farmed as a major source of animal protein for humans, generation of pigs with MSTN loss-of-function mutations has become a research priority in order to obtain breeding stock with better meat quality and higher economic value. Previous studies have reported the successful production of healthy MSTN gene-inactivated Chinese Meishan pigs (Qian et al., 2015) and Erhualian pigs (Wang et al., 2017). In addition to phenotypic changes to muscle and fat tissues, organs weights were generally significantly reduced among MSTN ^−/− animals compared with the organs in their wild type (WT) counterparts (Bünger et al., 2004; Fiems, 2012; Luo et al., 2019). Bünger (2004) found that in MSTN knockout mice, the significant increase in muscle weight and decrease in body fat content were accompanied by significant reductions in the weights of heart, liver, kidney and digestive tract (Bünger et al., 2004). Luo (2019) examined the organ weights of MSTN ^−/− , MSTN ^+/− and WT piglets and found that the visceral weight of MSTN ^−/− homozygous piglets was significantly lower than that of MSTN ^+/− heterozygotes and WT piglets (Luo et al., 2019). These studies show that animals with double site knockout of MSTN have lighter internal organs than WT individuals. The development of organs has a direct impact on an individual’s health, and the weights of body tissues and organs directly affect the slaughter rate, an important index of pig slaughtering performance, which in turn affects economic benefits. However, the mechanism of how MSTN regulates visceral development remains unclear. These findings all raise concerns about the health and welfare of MSTN edited animals.

In our previous study, MSTN ^+/− Large White (LW) pigs underwent exon 3 editing were prepared. The lean meat production of the MSTN ^+/− LW pigs were significantly higher than that of the WT (75.50%: 69.44%, p < 0.0001). Conversely, the fat meat rates of MSTN ^+/− pigs were significantly lower than that of the WT pigs (5.11%: 10.36%, p = 0.0022). The majority of amino acids content in the MSTN-edited LW pork were significantly higher than that in the WT pork; and the levels of polyunsaturated fatty acids were enriched in the MSTN-edited LW lean meat (Fan et al., 2021). Pigs with a naturally occurring single copy MSTN mutation have higher muscle depth and lower fat depth compared to the WT (Matika et al., 2019). Therefore, the heterozygous of MSTN gene-edited pigs have excellent production performance and are likely to be used in market applications in the future. However, there has been no systematic study on the digestive tract organs and reproductive organs of MSTN ^+/− pigs. The structure and function of intestine can affect nutrient absorption from feed. And the function of reproductive system organs determines the reproductive efficiency of the breeding herd. These are important factors to the economic benefit of pig production. Therefore, we examined the histological features of major organs in digestive and reproductive system in adults, and also contrasted the intestine length of MSTN ^+/− LW pigs and their WT half-sibs. This work will provide an important reference for MSTN gene edited pigs that can be used in future pig breeding.

Many laboratories have encountered problems at different stages in the process of MSTN deletion or editing. For example, MSTN-knockout Large White/Landrace×Duroc piglets died within 24 h after birth, and weighed less at birth than WT piglets. The single surviving piglet was sacrificed at 29 days old because of weakness in its hindlimbs (Rao et al., 2016). Similarly, eight of the MSTN ^−/− Landrace newborns generated with CRISPR/Cas9 editing died within a week (Wang et al., 2015). Pigs with naturally occurring MSTN mutations were reported in 2019 (Matika et al., 2019). However, piglets that were homozygous for this mutant allele suffered from lameness syndrome and did not survive when growth with the live weight above 40 kg (Matika et al., 2019). MSTN homozygous knockout in LW pigs leads to lameness, in agreement with other studies. Subsequently, pigs used for this study carrying single, edited copies of MSTN were propagated to expand the population for up to 8 years, which was sufficient to show that these MSTN ^+/− swine exhibited no developmental defects related to the mutant allele. MSTN ^+/− heterozygotes had a significant decrease in fat content that was accompanied by an increase in lean meat content (Fan et al., 2021). Therefore, MSTN-edited pigs are most likely to appear as heterozygotes in the potential future market.

Several internal organs have a reduced size in MSTN ^−/− homozygous mutant animals compared with WT (Bünger et al., 2004; Fiems, 2012; Luo et al., 2019). The Belgian Blue double-muscled (BBDM) calves (11 base-pare deletion of the MSTN (Kambadur et al., 1997)) had a reduction of 18% for the digestive tract, and 14% for the heart (Fiems, 2012). Another study reported that mice carrying homozygous mutations in murine MSTN had significantly lower weights (12–20%) for organs including liver, kidney, heart and digestive tract compared to homozygous WT mice (Bünger et al., 2004). Similarly, neonatal MSTN ^−/− piglets had decreased heart, liver, lungs, kidney, and stomach weights relative to body weight by 21.4, 21.3, 29.8, 16.7, and 20.0%, respectively (Luo et al., 2019).

Cardiac performance, reserve, and capability of the smaller heart are reduced in MSTN ^−/− animals compared to WT (Amory et al., 1993). The lower cardiac performance of MSTN ^−/− animals leading more quickly to exhaustion after severe exercise (Monin et al., 1974), which may even terminate in sudden death (Holmes et al., 1973). Our results showed that there were no histological abnormalities of cardiac muscle structures, and no significant difference in weight between individuals from two genotypes adults. MSTN ^+/− LW pigs did not suffer from reduced heart function, so there is no need to take extra precautions for animals.

It is well known that the small intestine consists of duodenum, jejunum, and ileum, is responsible for digestion and absorption of nutrients, water, and electrolytes, while the large intestine dividing into cecum and colon, is responsible to absorb water. Notably, the digestive disorders are related to intestinal dysfunction (Johnson, 1985; Soybel, 2005; Iji, 2008). Besides, the length of the digestive tract is positively correlated with nutrient absorption, while the decreases in the digestive tract length may limit the basic metabolism and nutrient availability of the body (Ortigues and Doreau, 1995; Greig et al., 2019). As reported by several authors, a consequence of the smaller digestive tract is reduced feed intake capacity (Clinquart et al., 1995; Fiems et al., 1997). The decrease in feed intake means that MSTN ^−/− animals do not efficiently utilize low quality feed, and the costs of rearing will increase. According to our results, the histological structure and length of intestinal in MSTN ^+/− swine were not affected by single copy MSTN mutation. This coincides with our previous finding that no differences were detected in feed conversion ratio values between MSTN ^+/− and WT LW (Fan et al., 2021). Hence, there were no differences between MSTN ^+/− and WT LW pigs in the function of digestive tract.

Spermatogenesis occurs in the testis and completes in the epididymis (de Kretser et al., 1998). Meanwhile, the ovary, oviduct and uterus are critical organs responsible for the oogenesis and embryonic development (Bagnell et al., 2017). The BBDM cows have significant longer calving interval than dairy cows (averaging 435 days compared to 393 days) (Hanzen et al., 1994). That may be caused by underdeveloped maternal reproductive tract and increased birth weight of calves, which make cesarean section an elective operation (Kolkman et al., 2007). A longer calving interval can be affected by the cesarean section, which significantly reduces the subsequent pregnancy rate (Patterson et al., 1981), but this is not the sole cause. Compared with Holstein cows, BBDM cows have significantly poorer semen quality (Hoflack et al., 2006). Nevertheless, BBDM cows have better egg quality than Holstein cows (Leroy et al., 2005). Other studies have shown that compared with the control group, MSTN-immunized mice had reduced litter number, but normal embryo development in both groups (Liang et al., 2007). These results are consistent with those obtained with BBDM (Leroy et al., 2005). This indicates that the completely loss of MSTN function does not affect the function of the uterus, but reduces the quality of sperm and increases litter birth weight. Our results previous work showed that the total number born, the number born alive and litter birth weight of the MSTN ^+/− piglets did not differ from those of the WT (Fan et al., 2021). Here, results showed that there was no significant difference in the histomorphology of testis, epididymis, ovaries, oviducts and uterus in MSTN ^+/− LW compared with WT pigs. Additionally, our MSTN ^+/− LW pigs are now in their eight generation (Fan et al., 2021). Therefore, it can be seen that the characteristics of reproductive organs of MSTN ^+/− LW pigs are similar to WT pigs.

In summary, MSTN ^+/− LW pigs had no significant differences in the histomorphology of the heart, duodenum, jejunum, ileum, cecum, or colon, nor in the lengths of the large and small intestine compared to WT pigs. Similarly, no morphologic abnormality was evident in testis, epididymis, ovaries, oviducts and uterus. These results indicated that the characteristics of heart, digestive and reproductive organs of MSTN ^+/− heterozygotes are similar to that of WT LW pigs.