Spermatozoa

Spermatozoa Anatomy

A fully mature sperm cell is uniquely designed for its job. Its tail, called a flagellum, is the only true tail in the human body. The tail doesn’t move in a whipping motion like we see in textbook cartoons. Instead, it spins like a little propeller. Around the baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). of the tail, mitochondria are tightly wound in a spiral pattern to produce ATPThe energy currency of cells used for muscle contraction.. This energyThe capacity to do work or cause change. is essential for the long swim the sperm must make through the female reproductive tract.

At the front of the sperm, in the headRounded proximal end that fits into the acetabulum of the hip bone., we find the acrosome. This is a specialized structure. It acts like an enzyme-filled cap. This cap helps the sperm break through the outer layers of the egg during fertilization. The rest of the head contains the tightly packed DNA that will combine with the DNA in the oocyte.  This nucleusThe control center of the cell that contains DNA and directs cellular activities. contains only 23 chromosomes. It will combine with the 23 in the oocyte. This combination gives the zygote the 46 chromosomes average for a human.

SpermATOgenesis

Spermatogenesis is the process of creating sperm cellsThe basic structural and functional units of life.. It makes 300,000 sperm per minute, or about 400 million per day. That’s a lot of sperm.

When we talk about making sperm, we actually use two different terms, and it’s important not to mix them up. Spermatogenesis is the process of creating spermatozoa — that’s the complete process of forming sperm cells through mitosis and meiosis. Then we have spermiogenesis, which is the final stage when those spermatozoa become fully mature and capable of fertilization.

Spermatogenesis begins with a round of mitosis, followed by two rounds of meiosis. As these divisions happen, sperm cells migrate from the outer edge of the seminiferous tubule inward toward the lumenThe inside space of a hollow organ or structure.. While this process unfolds, the developing spermatocytes are surrounded and protected by those nurse cells.  These stem cells divide through mitosis. One daughter cell stays behind to continue the stem cell population. The other goes forward into meiosis and gradually turns into a gamete A haploid reproductive cell (sperm or egg)..

Once meiosis is complete, we have four cells called spermatids. These spermatids are not yet mature sperm. They typically don’t have a fully formed tail. They also lack the right structure for fertilization. They’ll need to mature further before they can function. A fully developed sperm cell is called a spermatozoon, and the plural form is spermatozoa.

Sperm-cell Names

Knowing that spermatogenesis is a series of cell divisions is the first step to understanding it.  Remember that mitosis makes clones. Meiosis takes a cell and cuts the chromosome number in half. This process creates cells called gametes. 

Cells called spermatogonia are positioned in the basal compartment of the seminiferous tubule.  It divides every 16 days or so using mitosis.  Remember that mitosis is about making 1 clone from the original.  Mitosis destroys the original cell, replacing it with one of the new clones.  The other clone made here goes on to do meiosis.  This clone, at first is called a primary spermatocyte. 

This primary spermatocyte them begins to divide via meiosis.  In the first round of meiosis, two cells are created called secondary spermatocytes.  These have half the number of chromosomes as the original primary spermatocyte.  The secondary spermatocytes divide in the second round of meiosis, aptly named meiosis II. Each of them divides into cells called spermatids.  There are four spermatids that remain embedded in the luminal compartment of the nurse cell.  They break free. Then they start to move through the seminiferous tubules. Next, they advance to the straight tubules that lead to the rete testis.

Spermiogenesis

While waiting in the epididymis, spermiogenesis occurs.  This is not a cell division, but more of a maturation of the sperm cell. At first, a small flagellaLong, whip-like structures used by some cells (e.g., sperm) for movement. develops, anchored to the centriolesCylindrical structures involved in organizing microtubules and cell division..  The organellestructures within a cell that perform specialized functions. involved in protein making are all broken down except for the endoplasmic reticulum.  The ER becomes this sac of enzymesProteins that speed up chemical reactions in the body. called the acrosome.  It positions itself on the opposite side of the nucleus from the flagellum attachment. Its purpose is to release all those enzymes to penetrate the ovumA mature egg cell, if encountered.  The mitochondria start to arrange themselves in the midpiece of the spermatozoon and take on this spiral sort of arrangement.  The final spermatozoon has a head, midpiece, and tail.  This vessel is the perfect vessel for ovum seeking.