The Gonads 1 Testes Introduction In vertebrates the process of reproduction involves the fusion of two haploid gametes In humans the normal chromosome number is 46 22 somatic autosomal pairs and the sex chromosomes The male and female haploid gametes are the spermatozoa and the ova respectively Spermatozoa are produced by the testes and ova are produced by the ovaries The embryonic development of the testes Primordial germ cells migrate from the yolk sac to the genital ridge of mesoderm on the dorsal wall of the developing abdominal cavity Proliferation of the cells of the genital ridge results in the formation of undifferentiated gonads Consist of mesenchymal cells which will form the matrix of the gonadal tissue and primordial germ cells At this stage primitive male and female reproductive tracts are both present in the fetus Wolffian and Mullerian ducts respectively At around 6 weeks the paired indifferent gonads in a male fetus develop into testes due to the sex determining region on the Y chromosome SRY If this gene is present and if it is activated then the cell becomes a Sertoli cell cid 127 Other cells in the early testes develop into Leydig cells These begin to secrete androgens including testosterone Stimulate the development of the Wolffian ducts into the male reproductive system Long coiled seminiferous tubules lined by Sertoli cells come together to form the rete testis from which the vasa efferentia arise These then merge into another long coiled section called the epididymus which finally becomes the smooth muscle lined vas deferens cid 127 Once the indifferent gonads have been converted to testes the SRY gene is no longer expressed The genetic basis of spermatozoon production In males the germ cells remain quiescent until puberty when they are activated A1 spermatogonia Undergo a number of mitotic divisions the final of these produces B spermatogonia become primary spermatocytes Formed in the basal fluid space surrounding the seminiferous tubules Undergo meiotic divisions until they reach the final spermatozoon stage Produce either 22Y or 22X chromosomes Haploid cells secondary spermatocytes become attached to Sertoli cells and become spermatids which develop rudimentary tails Continue to develop into spermatozoa which enter the seminiferous fluid secreted by Sertoli cells 64 day cycle in humans cid 127 Once the SRY gene has been expressed and the gonads have become testes subsequent maleness is due to the hormones produced by the developing cid 127 MIH from Sertoli cells inhibits further development of the Mullerian ducts which subsequently regress Androgens from the interstitial Leydig cells stimulate the further development of the Wolffian ducts and also the development of the external genitalia along Complete androgen insensitivity syndrome androgen sensitive tissues fail to respond to the rising androgen production The endocrine basis for phenotype differentiation testes MIH and the androgens the male line penis and scrotum Both reproductive tracts fail to develop normally External genitalia remain female A partial insensitivity to androgen can also develop The anatomy and structure of the testes In the developing male fetus the two testes initially lie within the abdominal cavity but they then migrate downwards and out of the body to end inside the As the fetus elongate between weeks 10 and 15 the gubernaculum is pulled downwards together with the attached testes until they pass through the In man the surrounding temperature is 4 7 C lower than the inner core temperature Seems to be optimal for spermatogenesis cid 127 Within each testis the arterial blood passes through a coiled internal spermatic arterial system and leaves via the closely associated venous pampiniform scrotal sac inguinal canal plexus The close proximity of arterial and venous systems within the testis provides a heat exchange system which is beneficial in keeping the temperature of the incoming blood lower and the departing blood higher than would otherwise be the case rete Scrotal epithelium has a high density of sweat glands which also function to cool the testes The spermatic cord is the name given to the bundle of arteries veins and vas deferens from each testis passing through the inguinal canal The basic structure of the testis is the seminiferous tubule which consists essentially of Sertoli cells Continuously secrete seminiferous fluid which enters into the lumen of tubule and towards the rete testis and vas deferens cid 127 Multiple tight and gap junctions completely surround the Sertoli cells linking them all together cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Prevent the movement of spermatozoa into the blood may cause immune response Prevent the non selective passage of molecules into the seminiferous fluid Sertoli cells synthesize receptors for FSH also produce inhibin and are able to convert testosterone into DHT and estrogen 17beta estradiol Leydig cells are located within the interstitial fluid compartment between seminiferous tubules The testicular hormones and their receptors Androgens determine endocrine function of testes Activins and inhibins are associated particularly with the control of the hypothalamo adenohypophysial gonadal axis The androgens Family of hormones that are considered to represent the male or maleness Synthesized in the ovaries of women and also to a limited extent in the adrenal glands Synthesis storage and release Androgens are steroids the initial precursor being cholesterol In the testes the cells that synthesize androgens are the Leydig cells Progesterone is synthesized from cholesterol early on in the pathway and acts as a precursor to all other steroids The androgens themselves act as precursors in the synthesis of estrogens The first androgens to be synthesized are DHEA and androstenedione testosterone precursors In an adult man 4 10 mg of testosterone is secreted into the blood each day Also reaches the lymph male accessory sex glands such as the seminal vesicles and the prostate Testosterone can readily penetrate into the Sertoli cells forming the seminiferous tubules and fluid Testosterone can be converted by 5alpha reductase into an even more powerful androgen called dihydrotestosterone DHT Key