Male Reproductive Control

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Male Reproductive Control

ScienceAide
23 Mar 2024
Biology
Male animals generally have different reproductive organs and gametes from females, but the hormones used to direct and regulate the reproductive system are by and large the same. Human reproductive organs begin developing after about seven weeks of gestation, determined by information carried by the sex determining chromosomes. In males, expression of a gene on the Y chromosome, as well as the production of testosterone at around week nine of gestation, causes the development of male reproductive organs, which have both internal components — the testes, epididymis, vas deferens and prostate — and the external structures — the scrotum and penis. These structures have many blood vessels and glands to boost the development, storage and ejaculation of sperm for fertilization.

Sperm is the male gamete in animals, and unlike eggs, they are continually made during the lifetime after puberty. They are about 10,000 times smaller than an egg and are haploid, containing 23 chromosomes. Unlike eggs, sperm have completed meiosis right off the bat, whereas eggs spend most of their lives as oocytes, having completed all but meiosis II. Sperm have a tail called a flagella for propulsion, a head called an acrosome containing enzymes to help them fertilize the egg, as well as their very own mitochondria to help power them on their journey (eggs have many more mitochondria — between 100 and 200,000 — to help the possible future zygote develop). Between 250 and 280 million sperm are contained in each ejaculation, all vying for the chance of fertilizing one egg. Sperm are ejaculated in a fluid called semen, made of sperm, sugar, enzymes, proteins and other ingredients.

Sperm are made in the testes, in long, coiled tubes called seminiferous tubules. They are then stored in a coiled tube called the epididymis at the top of each testicle. The testes are also the site of the manufacture of various androgens, or male sex hormones — most importantly, testosterone. Testosterone triggers spermatogenesis, the process of sperm production in the testes, and is responsible for the secondary sex characteristics in adolescence during puberty: the deepening of the voice, the growth of facial, and body hair, and the beginnings of a sex drive. In male humans, testosterone is produced from Leydig cells in the testes.

Sperm production starts at puberty, when gonadotropin-releasing hormone (GnRH) is released from the brain’s hypothalamus gland, signaling the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (sometimes called interstitial cell stimulating hormone [ICSH] in males, although they’re the same hormone), which then send instructions to women’s ovaries and men’s testes to produce sex hormones. When FSH reaches the testes, it triggers spermatogenesis, nourished and supported by Sertoli cells which line the seminiferous tubules. These cells also produce the hormone inhibin, which is released into the blood when the sperm count is too high. This inhibits the release of GnRH and FSH, causing spermatogenesis to slow. When the sperm count reaches 20 million/ml, the Sertoli cells cease the release of inhibin, allowing the sperm count to increase.
Functional sperm can normally be produced throughout a man’s lifespan, although the number can drop with age as testosterone levels decrease.

Fertilization and Development in Animals

It’s one thing to make gametes through spermatogenesis and oogenesis, but it’s quite another to get them together so fertilization can occur. But fertilization is required in order to produce offspring, so humans and other animals put in a lot of work to make it happen.
Fertilization occurs when a sperm finds and penetrates an egg. Sperm are thousands of times smaller than an egg and can be made relatively quickly and continuously. A female produces a limited number of eggs in her lifetime, and relatively immense amounts of energy and resources go into their creation and storage.
Sperm are also simple compared with eggs. They are comprised of a head, which contains the acrosome that holds enzymes that will first come in contact with the egg’s physical barriers; a nucleus, which contains one complete set of haploid DNA from the male parent; a midpiece, which contains mitochondria that will make ATP that will propel the sperm toward the egg; and the tail, or flagellum, that will allow the sperm to move. Flagella requires a lot of energy, so the tail of the sperm doesn’t become active until it enters the vagina.
Eggs are built to receive sperm, and even to attract them with chemical attractants. Eggs, unlike sperm, have not completed meiosis at the time of fertilization — they are stuck in metaphase II, wherein the sister chromatids of the haploid cell are still attached to each other. In order to penetrate, and thus fertilize, the egg, a sperm must contend with an egg’s external structures:
Corona radiata: a layer of cells that surrounds an egg made up of carbohydrates and protein. During fertilization, sperm cells use an enzyme called hyaluronidase to break down the corona radiata and enter the egg.
Zona pellucida: a thick outer coating made of carbohydrate-covered proteins surrounding the plasma membrane. Not only does the zona pellucida help protect the egg, it is essential for allowing the acrosome to bond with the egg. and is responsible for mediating the initial meeting of sperm and egg.
Vitelline membrane: similar function to zona pellucida, but in non-mammalian animals.

Fertilization

Sperm and egg most often meet in the female’s fallopian tubes. Only one in one million sperm will reach an egg, partly because the female reproductive system can make itself either hostile or welcoming to sperm with things like lowered pH meant to kill potential sexually transmitted infections (and sperm), or cervical mucus designed to help the little swimmers navigate the reproductive system. Because the vagina and uterus are susceptible to infection, the female reproductive system has to protect itself when fertilization isn’t likely, so sperm have an easier time closer to ovulation.
For the fortunate few sperm that make it to the egg, when the head of the sperm touches the egg’s exterior, it begins a process called sperm binding. An acrosome reaction unpacks the enzymes found in the head of the sperm, and these proteins then begin to break down the zona pellucida so the sperm can burrow down to the plasma membrane where it can release its genetic material, hereby fertilizing the egg.
To prevent two or more from fertilizing the egg at the same time, the instant of fertilization triggers the cortical reaction, also called a slow block. The fusion of egg and sperm releases calcium ions that cause the cortical granules inside the egg to fuse with the plasma membrane. As they fuse, these granules eject their contents from the cell, which digests the remainder of the zona pellucida, making it impossible for more sperm to bind to it. A new mechanical barrier is created around the sperm and egg.

Other animals– especially those that fertilize eggs externally in the ocean — utilize the fast block, wherein an electrical charge blocks further entry by other sperm in the second after fertilization. The temporary electrical depolarization lasts about a minute before fading.
Once the male set of haploid chromosomes — now called the male pronucleus — is inside the oocyte, the egg begins to complete its second meiotic division. While this is happening, the male pronucleus waits in the same cytoplasm with the female pronucleus. Here they both replicate their DNA and fuse to complete fertilization to form the diploid zygote.

Cleavage Divisions

Now that the genetic material is replicated and fused, we are left with one completely unique diploid eukaryotic cell called a zygote. From here, the business of cleavage, or cell division, begins. In animals, once the zygote is established it doesn’t remain one single, giant cell for long. Multiple rounds of cellular division happen in rapid succession, splitting the fertilized egg into many smaller cleavage cells called blastomeres. These cells are special because they have the potential to develop into many different cell types. At first this cell division occurs with no change to the overall mass of the fertilized egg, so with each division, the cells become smaller and smaller. It creates a solid ball of cells called a morula.

Once 100 blastomeres are generated, the morula reaches the status of blastula, a hollow ball of cells filled with fluid. The rapidity of cellular divisions slows at this point, and around day 5-6 after fertilization, once the cells implant in the wall of the uterus, it is referred to as an embryo. By this time, through a process called gastrulation, a tube called an archenteron is formed — a tunnel is cleaved through the blastula, creating a tube that looks a bit like a bead. From here, three germ layers of the embryo form: the ectoderm will become the nervous system, skin, and some bones, the mesoderm will become the muscles, bone, and connective tissues, and the endoderm will become the lining of the digestive and respiratory systems. These new organizing layers give rise to organs and tissues that begin to take shape through a process called morphogenesis.
The above description of early embryonic development is true for humans and many other mammals, but not for all animals. An embryo’s relationship to its parent varies widely in the animal kingdom. For instance, animals that practice oviparity lay fertilized eggs outside their bodies. This method is thought to be the way the ancestors of vertebrates reproduced. BIrds are famous for their oviparity, laying eggs with hard shells that continue to develop inside protective shells. Eggs are full of nutrients to feed the developing embryos until the time when they hatch, and the infant animal emerges.
Ovoviviparity is a reproductive strategy in which a female produces fertilized eggs, but does not “lay” them. The embryo develops inside the egg, inside the mother’s body until it’s ready to hatch. In this case, the mother only provides physical protection for the eggs — the embryos consume the nutrients that are inside the fertilized egg and take nothing additional from their mother. The embryo hatches from the egg while still inside its mother’s body, and exits during birth.
Viviparity is the name for the process humans and many other animals use for reproduction. A female produces eggs that are fertilized internally, and then nourished inside her uterus. The embryo eventually exits during birth. Variations of this strategy exist. For instance, in histotrophic viviparity, embryos develop within mother’s uterus, but instead of receiving nutrients directly from the mother, they obtain nourishment by consuming the eggs of less developed siblings or even other hatched (but yet unborn) siblings, which is the case in some shark and salamander species. This type of intrauterine cannibalism is called oophagy or ovophagy, which means “egg eating.” Most mammals display hemotrophic viviparity, in which embryos develop inside a uterus, connected by an umbilical cord to a placenta connected to the mother’s blood supply, which provides nutrients. The embryo can become much more developed within the womb in this system, but babies are usually born needing further nutrition, making it a very energy expensive style of reproduction. which puts a further strain on the mothers.

Human Embryology

All vertebrates look remarkably similar during an early embryonic stage called the phylotypic stage — they even share most of the same developmental instructions to a certain point.
After about 8 to 10 weeks, the body plan of the embryos begins to more closely resemble the form of the adult of its species. At this point, the developing embryo is called a fetus.

Human gestation – the development of the baby inside the mother – takes about 280 days, or nine months from the time of the beginning of the estrous cycle in which the mother’s ovulation occurred. This time period is divided into three trimesters. After gestation, birth, or parturition, separates the mother from the baby. This event can be divided into three major events in humans:
Dilation of cervix: During labor, the cervix, which is the bottom portion of the uterus, dilates in order to accommodate the circumference of the babies head.
Expulsion of fetus: under normal circumstances, the head passes through the cervix and vagina first. In the case of breech births, the baby is flipped and the opposite end comes out first. These births are often problematic, and may require surgical delivery via cesarean section, a surgical procedure to deliver a baby through an incision in the mother’s abdomen and uterus.
Expulsion of placenta, or after birth: the placenta, a temporary organ which has provided nutrients, oxygen, hormones and waste removal for the developing fetus is ejected from the endometrium after the birth of the baby. Like the baby before it, it passes through the cervix and the vagina around 30 minutes after the fetus.

Let’s help you remember the key topics with these memory tricks or mnemonic.

S.P.E.R.M.
Use the acronym S.P.E.R.M. to remember key components of the male reproductive system:
S – Spermatogenesis: The process of sperm production occurring in the seminiferous tubules of the testes.
P – Penis: The external organ responsible for delivering sperm during sexual intercourse.
E – Epididymis: A coiled tube where sperm mature and are stored.
R – Reproductive Hormones: Hormones like testosterone, FSH, and LH that regulate the development and function of male reproductive organs.
M – Mitochondria: Organelles in the sperm’s midpiece that provide energy for motility

Test your understanding

1. Where does spermatogenesis occur in the male reproductive system?
a) Epididymis
b) Vas deferens
c) Seminiferous tubules of the testes
d) Prostate gland
2. What is the primary function of the epididymis?
a) Production of testosterone
b) Storage and maturation of sperm
c) Secretion of seminal fluid
d) Filtration of urine
3. Which hormone is primarily responsible for stimulating testosterone production in males?
a) Follicle-stimulating hormone (FSH)
b) Luteinizing hormone (LH)
c) Gonadotropin-releasing hormone (GnRH)
d) Inhibin
4. What role does inhibin play in the male reproductive system?
a) Stimulates sperm production
b) Inhibits testosterone secretion
c) Regulates sperm production by inhibiting FSH release
d) Enhances libido
5. Which part of the sperm contains enzymes necessary for penetrating the egg during fertilization?
a) Midpiece
b) Tail
c) Acrosome
d) Nucleus

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References:

Male Reproduction:
https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.04%3A_Hormonal_Control_of_Human_Reproduction/43.4A%3A_Male_Hormones

https://www.sciencedirect.com/topics/immunology-and-microbiology/sertoli-cell#:~:text=Sertoli%20cells%20are%20somatic%20cells,science/article/pii/B9780128042397000597

https://www.ncbi.nlm.nih.gov/books/NBK222286/

Human Fertilization/Development:
https://www.merckmanuals.com/home/women-s-health-issues/normal-pregnancy/stages-of-fetal-development

https://www.khanacademy.org/test-prep/mcat/cells/embryology/a/egg-meets-sperm#:~:text=Unlike%20sperm%2C%20the%20egg%20has,only%20makes%20one%20viable%20egg.

https://pubmed.ncbi.nlm.nih.gov/34694475/

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.02%3A_Fertilization/43.2A%3A_External_and_Internal_Fertilization

https://theconversation.com/flipping-the-genetic-switch-that-makes-many-animals-look-alike-as-embryos-55631