Semen composition and analysis (animal, human), related testsWhat is semen?Semen is composed of spermatozoa (sperm), produced in the seminiferous epithelium of the testis, and seminal plasma, the components of which are secreted by the excurrent duct system and accessory sex glands. When a spermatozoon is released from the seminiferous epithelium the major structural elements are in place, but additional changes are induced by exposure to sequential milieus provided by the epididymis and mixture with fluids from the accessory sex glands at ejaculation. Typical spermatozoa of the rat, human and stallion are shown in Fig. 1 <p26fig1.asp> and the important elements of a spermatozoon are depicted in Fig. 2 <p26fig2.asp>. A partial list of spermatozoal attributes essential for fertility is presented in Table 1. Collectively, these attributes depend on the normal development and function of the genomic package, the mitochondria, dense fibers and microtubular elements of the axoneme, the acrosome and enzymes therein, and the multi-compartmentalized plasma membrane.

Seminal plasma is the fluid portion of an ejaculum, but only one of several distinctly different fluids to which sperm are exposed. Spermatozoa are transmitted from the seminiferous epithelium in a fluid milieu, and the solutes therein are removed and replaced within the efferent ducts and epididymis. Ultimately, sperm in cauda epididymal fluid are conveyed through the vas deferens at the time of ejaculation and mixed with fluids from the accessory sex glands, namely the prostate gland, vesicular glands (i.e., seminal vesicles), and bulbourethral glands. Some species have a complete array of accessory sex glands, including the above three types. Other species lack the bulbourethral glands or vesicular glands. Certain proteins and other molecules in the secretion of one or more accessory sex glands are identical to some components of cauda epididymal fluid, or even blood plasma, and others are unique products of that gland. Thus, seminal plasma includes a broad spectrum of chemical constituents contributed by the epididymis and the accessory sex glands.The relative contributions of the epididymis or different accessory sex glands to the seminal plasma of a given ejaculum are dependent on many factors including the interval of sexual abstinence, duration of foreplay, pathophysiological processes in the male, and the species. Because semen is a mixture of spermatozoa and fluids moved by emission from the cauda epididymidis and vas deferens with fluids from the accessory sex glands, the sperm to fluid ratio is quite variable.

The more important attribute is the total number of normal sperm in an ejaculum rather than the concentration of sperm per unit volume. For similar reasons, in analyses of constituents of seminal plasma, the total amount of a component should be considered in parallel with its concentration. A human might ejaculate 40-300 million spermatozoa, not greatly different from the number ejaculated by a rabbit (100-300 million), but substantially less than a dog (0.2-2 billion) or horse (5-25 billion).



What is the goal of seminal analysis?

For a clinician, evaluation of seminal quality is Iinked with a desire to predict potential fertility, identify causes of infertility, or detect changes in potential fertility. The clinician is concerned with minimal requirements to achieve fertilization or contraception. For an epidemiologist or toxicologist, seminal evaluations are the basis of assessing hazards in the workplace, environmental factors, or risk assessments relating to drugs and chemicals. Detection of a significant probability of reduced fertility in a population is more important than accurate prediction of fertility for an individual. For the animal breeder, the primary goal is to determine which male(s) will be the most fertile of genetically superior sires. Evaluations of sperm quality and estimation of potential fertility are the basis for management decisions which might lead to production of several hundred thousand offspring from an individual sire. For each application, the implied goal is to predict accurately the potential fertility of a seminal sample from an individual male.Unfortunately, this goal is not easily achieved. Success in predicting fertility is limited by features of spermatozoa, the process of fertilization, and approaches used for evaluation in vitro of seminal quality. Also, spermatozoal attributes necessary for fertilization will depend on the methodology used to join the gametes, i.e., copulation or in vitro fertilization; on prior history of the sperm, i.e., freshly ejaculated sperm or frozen-thawed sperm; and on female factors, i.e., age or uterine and tubal environments.Sometimes the conclusion from a seminal analysis is obvious. When the semen analysis reveals azoospermia, no progressively motile spermatozoa, or a high proportion of morphologically abnormal spermatozoa, the fertilizing potential of the individual is poor. In most cases the challenge is more complex. The goal of a clinician or animal breeder is to predict correctly that a given male probably will be infertile or will be reasonably fertile, relative to the average value for males of that race or species, or that a given seminal sample will provide fertility similar to that previously obtained with other samples from the same male. As contrasted to lack of fertilizing capability, considered above, accurate prediction of high fertilizing capability is extremely difficult, or impossible, because a spermatozoon must retain function of each of a number of essential attributes (see Table 1 <p27tb1.asp>) to be capable of fertilizing an oocyte. It follows that a number of spermatozoa in a sample could be incapable of fertilizing an oocyte, each for a different reason. Limitations of current approaches for evaluation of seminal quality provide great opportunity for individuals intrigued by investigating male reproductive function.