Spermatogenesis in pigs; neonate to puberty.
Keywords: pig, spermatogenesis, porcine, testicle
A series of four cross sections of testicles from young boars: a neonate and at three, four and five months of age. In the neonate, spermatogenesis is totally absent and the predominant cell types are Leydig cells (Lc) and spermatogonia (Sg). At three months of age it can be seen that the onset of puberty has begun and primary spermatogonia (Ps) are seen for the first time in this series of images.
Notice that primary spermatogonia have many different appearances, some with tightly packed DNA and others with loose granular DNA and light nuclei. These differences are noticeable between the images but are also obvious between individual seminiferous tubules in the same image. For example, notice the difference in appearance of the primary spermatogonia between one tubule and another in the cross-section from a four months old boar. To understand why this is the case, one must first appreciate that primary spermatogonia are the "showcase" cells for demonstrating most of the stages of meiosis (leptotene, zygotene, pachytene, diplotene, diakinesis); each stage having a distinctly different appearance in these images.
Before meiosis begins there is an "interphase"(a phase between the normal multiplication of spermatogonia and the onset of reduction division of spermatozocytes). During that time, the amount of DNA in the spermatogonia doubles so that this lengthly process is not required in the later stages of sperm formation when secondary (haploid) spermatozocytes are formed. This is important to remember because it explains why secondary spermatozocytes are so seldom visible in cross sections of seminiferous tubules i.e. immediately after secondary spermatozocytes have been formed, they divide into spermatids which are also haploid but have half the amount of DNA present in secondary spermatozocytes. In these cross sections one can see spermatids such as those in the Golgi phase (G) of spermiogenesis, but no secondary spermatozocytes are visible anywhere.
Understanding the difference in appearance of primary spermatocytes as they pass through the stages of the prophase of meiosis (leptotene, zygotene, pachytene, diplotene, diakinesis) one can appreciate that a sections of tubules adjacent to one another will look different if they are in different stages of meiosis. For example, one cross-section may contain primary spermatozocytes that are in leptotene while an adjacent tubule may contain primary spermatozocytes in pachytene, the cells having a very different appearance to those in leptotene.
Within a particular tubule, meiosis starts at one point in the tubule and adjacent to it, other cells are triggered to start meiosis as well. The biochemical signal that stimulates this is not understood but it is akin to a domino effect with one set of basal cells stimulating those adjacent to it. This progresses along the tubule so that all the stages of meiosis and spermiogenesis (the later stages of sperm formation) could be seen adjacent to one another if one was able to traveled along the "line of dominos" in the tubule. Every "domino" is different and is akin to a set of different cell relationships from basement membrane as meiosis progresses. These cell relationships are called the stages of spermatogenesis. Typically there would be a dozen stages; some more, some less depending on the species In an animal with 12 stages, this would be akin to 12 dominos. Once the set of 12 dominos have all been knocked down, the next set, further along the tube begins to be knocked over. By analogy, spermatogenesis occurs in waves along the seminiferous tubules and there are many waves of spermatogenesis along the entire length of any given tubule. With this physiology in mind, one can then understand why one tubule in a cross-section can appear so different to its neighbor; they are in different stages of spermatogenesis.
The other cells that are visible here are Sertoli cells (Sc). They occur along the basement membrane of the seminiferous tubules and are the only somatic cells to be found in the tubules. They are essentially the interface between the chromosomal variation in the tubules and the rest of the body; the blood;testis barrier. They support the growth and division of spermatogonia, spermatozocytes and spermatids in the process of becoming spermatozoa. This supporting role is both anatomical and physiological, providing FSH and androgens to cells that will become spermatozoa. They are also responsible for production of inhibin and anti-müllerian hormone (AMH), both of which suppresses the development of the female reproductive tract in male animals. Sertoli cells also have numerous other functions Among these are the formation of androgen binding protein to make androgens available for spermatogenesis and remarkably, aromatization of androgens to estrogens so that testicles produce substantial amounts of estrogen. In fact, in stallions (where mole for mole, more estrogens are produced than androgens) estrone sulphate is often used in stallions to diagnose cryptorchidism.
The stain used in these preparation was Periodic Acid Schiff (PAS) a particularly useful method of demonstrating acrosomes and basement membranes in seminiferous tubules.
The entire process of spermatogenesis usually takes between 45 and 60 days depending on the species. In pigs, it is close to 45 days.
