2 According to Hill (2), Mivart stated in 1873 the characteristics of primates: "unguiculate, claviculate placental mammals, with orbits encircled by bone; three kinds of teeth, at least, at one time in life; brain always a posterior lobe and calcarine fissure; innermost digit of at least one pair of extremities opposable; hallux with a flat nail or none; a well-defined caecum; penis pendulous, testes scrotal; always two pectoral mammary glands" (italics added).
usually scrotal, in the adults of some higher primates, e.g., Macaca mulatta, can be retractable into the lower portion of the inguinal canal because the lower ring of the canal does not fully close. Each testis is encapsulated in a tough white fibrous membrane, the tunica albuginea, with a large testicular artery coursing in a sinuous fashion along the anterior surface of the gonad. The parenchyma of the organ consists of many tortuous seminiferous tubules surrounded by intersitial tissue composed of Leydig cells, macrophages, nerves, blood vessels, and lymphatic vessels. In primates, the parenchyma is divided by fibrous tissue into lobules. Three types of germ cells, spermatogonia, spermatocytes, and spermatids, in addition to the somatic cells, named for Enrico Sertoli who identified their function 125 years ago (14), comprise the epithelium of the seminiferous tubules. The seminiferous epithelium is the site of spermato-genesis. Once spermatozoa are produced by the seminiferous epithelium they are released into the lumen of the tubule, and are transported through the tubulus rectus that forms when each seminiferous tubule loops around and rejoins itself to form this single straight tubule (15). The tubuli recti enter the anastomosing structure of the rete testis. The rete testis is embedded in dense connective tissue called the mediastinum, which, in most higher primates, runs through the long axis of the ovoid testis. The rete testis tubules join at the posterior surface of the testis into three to five ductuli efferen-tia that, in turn, form a single confluence that is the origin of the duct of the epididymis.
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