Human Reproduction: Anatomy, Processes, and Development
Sexual Reproduction
We humans have a sexual reproduction, which means that we need two people to generate the sexual cells, called gametes (sperm and ovum), for them to take part in a process called fertilization, fusing into the zygote (which will eventually become the new individual). The gametes are produced in special organs called gonads (testicles and ovaries).
Sexuality
The way each of us feel as being sexed is called sexuality, and includes gender identity, physical attraction, romantic trends, and any other kind of behavior regarding sex. Sexuality, in everyone’s own way, is essential for humans, and it’s not only related to reproduction, but to a healthy lifestyle, and mental wellbeing, as long as each of us participates of it freely and safely. The most important condition for it is to respect ourselves and others, allowing society members to develop and express themselves in an open environment.
Changes in the Reproductive Life
Start being sexually functional is called puberty, and it starts with the spermarche in males (1 ejaculation) and menarche in females (menstruation) from 10 to 15. Secondary Sexual characteristics include body hair, menopause, and andropause.
Male Gonads
The testicles are essential in producing both hormones (androgens, like testosterone) and gametes. Their inner structure is made up of a network of rolled seminiferous tubes, whose walls are responsible for producing sperm. Due to the optimal temperature for this process, testicles are lodged outside the abdomen, protected by a layer of skin called scrotum.
Male Reproductive Tract
The sperm released by the testicles need to reach the outside, going through a set of tubes. The first one is the epididymis, wrapped next to the testicle in a sponge-like mass, where sperm mature and get ready. Then we have the ductus deferens, which stores the sperm and leads it into the abdomen. Both ductus converge into the urethra (with a common use with the urinary system), traversing the penis up to the outside.
Penis
The penis is a copulatory organ, which means its main biological function is to favor the introduction of semen into the female body; that’s why, in erection, it enlarges and gets rigid, thanks to the corpora cavernosa within it, two spongy masses located on top of the urethra which get filled with blood pressure. There’s another erectile mass, the corpus spongiosum, that surrounds the urethra and widens at the end of the penis, forming the glans, a quite sensitive part protected by a fold of skin called foreskin.
Accessory Glands
The semen is made up of four ingredients: the sperm (only 10% of it), and secretions released into the urethra during ejaculation, produced by the following glands. The seminal vesicles (behind the bladder) secrete the spermatic fluid, with nutrients for sperm to survive and move. The prostate (surrounding the urethra) secretes prostatic fluid, with substances to stimulate sperm activity and protect them from the acidity in the vagina. The Cowper glands, also called bulbourethral glands (very small, at the base of the penis) secrete a lubricating fluid that eases sexual intercourse.
Female Gonads
The ovaries are essential in producing both hormones (estrogen and progesterone) and gametes. Found within the abdomen, their external layer is made up of a set of circular cavities called ovarian follicles, each of them containing a possible future ovum. The ones that develop and get bigger, with ova ready to be released, are called Graafian follicles.
Female Reproductive Tract
When an ovum is released from an ovary, it goes into the Fallopian tube (or oviduct) through its first part (with a funnel-shaped opening called infundibulum) and is led thanks to cilia into the uterus. The uterus is a hollow organ where the baby-to-be is lodged, with two important layers in its walls: the myometrium (the muscular layer) and the endometrium (the mucous surface where the embryo attaches itself). At the base of the uterus, the tract narrows forming the cervix, working as a kind of ‘gate’. Then we find the vagina, an elastic tube whose walls produce lubricating secretions to smooth penetration. In the opening of the vagina, there’s an incomplete membrane, the hymen, which is typically said to break during the first penetration (but it doesn’t always happen like that).
Vulva and Glands
The external female genital organ is the vulva, and it comprises skin folds (labia majora and minora) and the clitoris, a very sensitive organ made of erectile tissue (just like the penis) which extends into the abdomen and is mainly involved in female sexual pleasure. Inside the vulva, besides the openings to the urethra and the vagina, we find openings to two kinds of glands: Bartholin’s glands (they lubricate the external genitals to facilitate intercourse) and Skene’s glands (their functions are still under study, but they may be related to female ejaculation and sperm stimulation).
Chromosomes
We humans have 46 chromosomes in each of our cell nuclei, containing all the genes, i.e., the instructions to make us the way we biologically are, with two lungs, one stomach, etc. That means, we have 23 ‘types’ of chromosomes with different genes, but we have two copies of each, one belonging to the biological mother, and one belonging to the biological father (they have the information for the same characteristics, but maybe slightly different, producing different traits, such as blue or brown eyes). When the gametes fuse during fertilization, they merge all their genetic material. If they put all the mother and father chromosomes into the zygote, we would duplicate the number of chromosomes in each generation, so that’s why we need a strategy to avoid this.
Meiosis
During gametogenesis, the cells need to undergo a process called meiosis, a sort of cellular division in which each of the resulting cells contain only one of the two copies of each type of chromosome. That means that, in human species, we start with a regular cell (such any of the body), with 46 chromosomes, and end up with cells with only 23 chromosomes (the gametes). That way, when the sperm and the ovum merge, we get a zygote with 46 chromosomes again, forming a whole new person.
Female Cycles
We start from a menstruation, which is the destruction of endometrium and consequent bleeding. The hypophysis, in the brain, secretes FSH (follicle stimulating hormone), which gets to the ovary and causes follicle development. The maturing Graafian follicle also secretes estrogens, which get to the uterus and causes endometrial reconstruction. After a while, the hypophysis secretes LH (luteinizing hormone), which gets to the ovary and causes ovulation (the release of the mature ovum out of the ovary into the Fallopian tubes). The broken follicle then becomes the corpus luteum, which in turns secretes progesterone, which gets to the uterus and causes maximal endometrial development, preparing the tissue to host an embryo. The story may continue two possible ways now, depending on whether the released ovum is fertilized; since most times it is not, we’ll assume that’s the case, and check the alternative later. After a while (unless there’s a pregnancy), the corpus luteum degenerates, so it stops secreting progesterone, and the endometrium reacts to it by breaking off, since it’s not going to be used for its intended purpose anytime soon, and we have menstruation again.
Fertilization and Nesting
For a natural conception to occur, sexual intercourse must take place between five days prior to ovulation (the amount of time sperm survive in the female tract) to two days after (the time it takes the ovum to go to waste). Most times, fertilization (the union of both gametes, that recognize each other thanks to molecular markers) take place in the Fallopian tubes, and then we have a zygote, which will start dividing forming the embryo (known specifically as morula in the first stages of development, due to its resemblance in shape to a blackberry). This embryo goes on through the oviducts until it reaches the uterus, and attaches to the endometrium that will nourish it, a process called nesting or implantation, moment from which it is considered a pregnancy.
Gestation and Labor
The word gestation refers to all the time the new individual is developing, from conception to birth, and in humans, it takes around 280 days = 40 weeks = 9 months. After the first stages of embryo development, the mass of cells start having quite a resemblance to a human being, and from the third month on, it is called a fetus. The embryo (or fetus) lives within a membrane called amnion, filled with amniotic fluid to protect it from bumps. The placenta is the organ in charge of the exchange of nutrients and residues between fetus and mother’s blood, and it connects to the fetus through the umbilical cord. When delivery comes, the cervix must dilate to allow the baby to pass through (which usually takes the longest time, up to 12 h), while contractions get more intense and frequent, and the amnion breaks, releasing its fluid (process known as breaking Water). Then the baby is released (or extracted by a cesarean section if needed) and the cord is pinched and cut. Finally, the placenta must be expulsed too, and labor concludes.
Multiples Pregnancies
Humans mostly have one baby per pregnancy, but sometimes, one zygote can split into separate embryos, generating monozygotic or identical twins, triplets, etc. (in Spanish, ‘gemelos, trillizos…’), since they all come from the same chromosomes and genetical instructions (they are natural clones). They usually share only one placenta, and if the division is late, it can be incomplete and lead to Siamese twins. Another rare possibility is that, for any reason, two ova are released during a small window of time, and both are fertilized with respective sperm, giving place to dizygotic or fraternal twins (in Spanish, ‘mellizos’). Since they come from different conceptions and have different genes, they are no more alike than any other pair of siblings (they can even belong to different fathers); it’s like having two pregnancies at the same time.
