The formation and development of sperm in the seminiferous tubules of the testes.
In humans, spermatogenesis takes 65-75 days. It begins with the spermatogonia, which contain the diploid 46 number of chromosomes. When spermatogonia undergo mitosis, some spermatogonia remain near the basement membrane of the seminiferous tubule in an undifferentiated state to serve as a reservoir of cells for future sperm production. The rest of the spermatogonia lose contact with the basement membrane, squeeze through the blood-testis barrier, undergo developmental changes, and differentiate into primary spermatocytes. Primary spermatocytes, like spermatogonia, are diploid (2n); that is, they have 46 chromosomes. Shortly after it forms, each primary spermatocyte replicates its DNA and then meiosis begins. The two cells formed by meiosis I are called secondary spermatocytes. he four haploid cells resulting from meiosis II are called spermatids. The final stage of spermatogenesis, spermiogenesis, is the development of haploid spermatids into sperm. No cell division occurs in spermiogenesis; each spherical haploid spermatid becomes an elongated, slender sperm cell An acrosome forms atop the condensed, elongated nucleus, a flagellum develops, and mitochondria multiply. Sertoli cells dispose of the excess cytoplasm that sloughs off. Finally, sperm are released from their connections to Sertoli cells and enter the lumen of the seminiferous tubule. Fluid secreted by Sertoli cells pushes sperm along their way, toward the ducts of the testes.
In meiosis I, homologous pairs of chromosomes line up at the metaphase plate, and crossing-over occurs to rearrange genes between chromatids. Then, the meiotic spindle pulls one (duplicated) chromosome of each pair to an opposite pole of the dividing cell. The two cells formed by meiosis I are called secondary spermatocytes. Each secondary spermatocyte has 23 chromosomes, the haploid number (n). However, after meiosis I, each chromosome within a secondary spermatocyte is made up of two chromatids (two copies of the DNA) still attached by a centromere.
In meiosis II, the chromosomes line up in single file along the metaphase plate, and the two chromatids of each chromosome separate. The four haploid cells resulting from meiosis II are called spermatids. Therefore, a single primary spermatocyte with 46 chromosomes undergoes two rounds of cell division (in meiosis I and meiosis II) to produce four spermatids, each with 23 chromosomes.
Upon sexual stimulation (visual, tactile, auditory, olfactory, or imagined), parasympathetic fibers from the sacral portion of the spinal cord initiate and maintain an erection, the enlargement and stiffening of the penis. The parasympathetic fibers cause smooth muscle in the walls of arterioles supplying erectile tissue to relax, which allows these blood vessels to dilate. This in turn causes large amounts of blood to enter the erectile tissue of the penis. Parasympathetic fibers also cause the smooth muscle within the erectile tissue to relax, resulting in widening of the blood sinuses. The combination of increased blood flow and widening of the blood sinuses results in an erection.
the powerful release of semen from the urethra to the exterior, is a sympathetic reflex coordinated by the lumbar portion of the spinal cord. As part of the reflex, the smooth muscle sphincter at the base of the urinary bladder closes, preventing urine from being expelled during ejaculation, and semen from entering the urinary bladder. Even before ejaculation occurs, peristaltic contractions in the epididymis, ductus deferens, seminal vesicles, ejaculatory ducts, and prostate propel semen into the penile portion of the urethra (spongy urethra).
Once sexual stimulation of the penis has ended, the arterioles supplying the erectile tissue of the penis constrict and the smooth muscle within erectile tissue contracts, making the blood sinuses smaller. This relieves pressure on the veins supplying the penis and allows the blood to drain through them. Consequently, the penis returns to its flaccid (relaxed) state.
Michelle Provost-Craig, Susan J. Hall, William C. Rose 8th EditionElaine N. Marieb 12th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis 15th EditionDavid Shier, Jackie Butler, Ricki Lewis