Sperm

Function

Sperm is the male reproductive cell, which is able to fertilize the counterpart female gamete, the oocyte, during conception to produce a single-celled individual known as a zygote. Sperm are smaller than the most cells in the body. A uniflagellar sperm cell that is motile is referred to as a spermatozoon, whereas a non-motile sperm cell is referred to as a spermatium. Mature sperm cells cannot divide and they have a limited life span, but after fusion with egg cells during fertilization, a new organism forms, beginning as a totipotent zygote. The human sperm cell is haploid, so that its 23 chromosomes can join the 23 chromosomes of the female egg to form a diploid cell. 

During fertilization, the sperm provides three essential parts to the oocyte:¨

1. a signaling or activating factor, which causes the metabolically dormant oocyte to activate 
2. the haploid paternal genome
3. the centrosome, which is responsible for maintaining the microtubule system

The sperm cells travel through the seminiferous tubules to the epididymis (located above the testicle). The epididymis is responsible for storing the sperm cells until they mature. The sperm cells then travel through the vas deferens where they pick up fluid from the seminal vesicles, prostate gland, and Cowper’s gland. This fluid and sperm cells are known as semen. The purpose of the fluid is to allow the sperm to move, give them nutrients and provide a comfortable environment to survive.

Structure

Sperm has a distinctive head, mid-piece, and tail region (Pic.1):

• The head contains the nucleus with densely coiled chromatin fibres (DNA), surrounded anteriorly by an acrosome. A structure called the acrosome covers most of the head of the sperm cell as a “cap” that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. The sperm head is composed of a nucleus and an acrosome. 

• The mid-piece has a central filamentous core with many mitochondria spiraled around it. Most of the energy (ATP) for sperm motility is derived from the metabolism of fructose carried in the seminal fluid. This takes place in the mitochondria located in the sperm's mid-piece. This energy is used for the journey through the female cervix, uterus, and uterine tubes.

• The tail or "flagellum" executes the lashing movements that propel the spermatocyte. The sperm tail is based on 9 + 2 microtubules (Pic. 2). The microtubule doublets are connected doublet- to-doublet by dynein arms. Motile sperm cells typically move via flagella and require a water medium in order to swim toward the egg for fertilization. These cells cannot swim backwards due to the nature of their propulsion.

Spermatogenesis

Spermatogenesis is the process in which spermatozoa are produced from male primordial germ cells by way of mitosis and meiosis (Pic. 3). The process begins at puberty, after which time sperm are produced constantly throughout a man’s life. Sperms develop in the testicles. One production cycle, from spermatogonia through formed sperm (Tab. 1), takes approximately 64 days. The initial cells in this pathway are called spermatogonia, which yield primary spermatocytes by mitosis. The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two spermatids by meiosis II. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa.