Here we will learn the physiology of spermatogenesis, which results in the production of sperm cells; though we’ll mention mitosis and meiosis in this tutorial, the details of cell division are discussed separately.
Spermatogenesis comprises cell division and differentiation processes that produce sperm cells; it takes place in the seminiferous tubules of the testes.
As we learn
elsewhere, spermatogenesis requires sustentacular cell stimulation from FSH and testosterone.
The process of spermatogenesis takes approximately 64 days, in which germ cells pass through the following stages:
First, in spermatocytogenesis, mitotic divisions create spermatocytes.
Then, meiotic divisions create spermatids.
Next, in spermiogenesis, spermatids differentiate to become sperm (aka, spermatozoa).
During spermatocytogenesis and spermiogenesis, the developing sperm cells are enveloped within the cytoplasm of the sustentacular cell.
Finally, spermiation occurs when the sperm disassociate from the sustentacular cell and enter the lumen of the seminiferous tubule.
From here, sperm cells travel to the epididymis for storage and maturation.
Because spermatogenesis occurs continuously, each of the morphological stages are visible in any given seminiferous tubule sample.
We draw a section of a seminiferous tubule and indicate its lumen, with two sustentacular cells (aka, Sertoli cells), which extend from the wall of the seminiferous tubule to the lumen. Notice that they are columnar in shape and have extensive cytoplasmic processes, which are connected via tight junctions (not shown for simplicity).
Recall that sustentacular cells secrete substances that facilitate spermatogenesis and nurture the developing sperms; thus, they are sometimes referred to as “nurse” cells.
Spermatocytogenesis
Spermatocytogenesis begins with a single spermatogonium and ends with four spermatids. Spermatogonium are primordial stem cells from which the sperms originate.
Spermatogonium 1B undergoes mitosis and creates two cell types:
The first, we’ll call “1A,” because it’s simply a replication of 1B.
The second, we’ll call “2” because it represents the second morphological stage in the process and is “committed” to undergoing the full process of spermatogenesis.
In brief, spermatogonium 1B went through the process of mitosis to create two daughter versions of itself: 1A, which replenishes the spermatogonia population, and spermatogonium 2, which directly produces sperm cells
From this point on, we’ll be focused on the processes that spermatogonium 2 and its descendants pass through.
First, indicate that spermatogonium 2 is now called a primary spermatocyte;
The primary spermatocyte undergoes meiosis I to create two daughter cells: secondary spermatocytes.
As cell division continues, the cell products move closer to the lumen of the seminiferous tubule, but are still embedded in the sustentacular cell cytoplasmic processes.
The secondary spermatocytes undergo meiosis II, which produces two spermatids per spermatocyte.
This concludes the spermatocytogenesis phase of spermatogenesis.
Next, indicate that the spermatids undergo spermiogenesis to form immature sperm; during this time, the cells develop but no longer divide.
Key characteristics of the initial sperms:
They are still enveloped in the cytoplasmic processes of the sustentacular cells;
They have tails, which extend towards the lumen,
And, their cytoplasm has become concentrated towards one end, the head.
Spermiation
Finally, in spermiation, the sperm are released from the sustentacular cells into the lumen of the seminiferous tubule.
Maturation & Motility
During ejaculation, sperm join with seminal fluids from the seminal vesicle and other glands to form semen.
Secretions in the uterine tubes promote capacitation of the sperm cells, which is necessary for fertilization.
Let’s take a closer look at the anatomy of a sperm cell after it is released from
the sustentacular cell:
Draw the head, midpiece, and tail.
The head comprises the cell nucleus covered by the acrosome; the acrosome contains enzymes that facilitate joining of sperm and ovum at fertilization.
The midpiece comprises mitochondria, which provide the energy for sperm motility.
The tail produces flagellar movements (“back and forth” movements) that enable the sperm to “swim” upon ejaculation.
Sperm Cell Motility & Fertility
Sperm cells must be able to swim through the vagina, uterus, and uterine tubes to reach the ovum for fertilization; if motility is impaired, infertility can result.
Write that inadequate sperm motility can be caused by stress, heat, drugs, and dietary insufficiencies.
