4.3 Reproduction in Humans

Human reproduction is a complex process involving the interaction of male and female reproductive systems to produce offspring. The development of reproductive tissues of male and female humans are similar in utero until about the seventh week of gestation during when a low level of testosterone is released from the gonads of the developing male.

Under the influence of testosterone the primitive gonads differentiate into male sexual organs.

In the absence of testosterone  the primitive gonads develop into ovaries.

Under the hormonal influence the tissues that produce a penis in males produce a clitoris in females. The tissue that will become the scrotum in a male becomes the labia in a female.

Thus from the  common embryonic structures ,the male and female anatomies arise from the different developmental pathways  .

Male Reproductive System

The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction and includes:

1. Testes: The testes are a pair of male gonads . The testes produce sperm and reproductive hormones ( Testosterone). Each testis is approximately 2.5 by 3.8 cm (1.5 by 1 inch) in size .It is  divided into wedge-shaped lobes by septa.  These lobes houses the coiled seminiferous tubules that produce sperm.
2. Epididymis: This is a narrow ,tightly coiled tube attached to the testicles that  stores sperm as they mature. The Sperm cells finish maturing in the epididymis and stored. 
3. Vas Deferens:  Vas deferens is a long , muscular tube that carries mature sperm from the epididymis to the urethra prior to ejaculation
4. Seminal Vesicles and : The seminal vesicles  are also called vesicular glands or seminal glands .These are a pair of convoluted glands that lie  behind the urinary bladder of males . They secrete seminal fluid that nourishes and transports sperm.
5. Prostate Gland :  The  prostrate gland is located  just below the bladder and in front of the rectum in men. It is the largest accessory gland of the male reproductive system. The prostatic fluid comprise about one-third volume of the ejaculate .The main constituents of the prostatic fluid include Zinc, citric acid, choline , acid phosphatase ( enzyme), seminin, plaminogen activator and Prostate Specific Antigen (PSA).PSA is an important blood marker of prostate size and proliferation and its concentration increases in benign prostate hypertrophy, inflammation and prostatic cancer.
6. Penis:  The penis is located at the front of the body at the base of pelvis .The scrotum containing the testes is situated beneath the penis.  The penis delivers sperm into the female reproductive tract. The penis drains urine from the urinary bladder and is a copulatory organ during intercourse.

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Female Reproductive System

The female reproductive system consists of :

1. Ovaries:  The ovaries are a pair of egg shaped  female glands ,on each side of the uterus , that produce eggs (ova) and hormones like estrogen and progesterone. These hormones play an important role in the development of female traits, such as breast development, body shape, and body hair and  are also involved in the menstrual cycle, fertility, and pregnancy. Each of the egg produced by the ovary is located inside a structure called a follicle. The cells in the follicle protect the egg and help it mature.
2. Fallopian Tubes:  These extend from the upper corners of the uterus and connect the ovaries to the uterus . The ovary end of the tube has a fringelike structure that moves in waves. The motion sweeps eggs from the ovary into the tube. The fallopian tubes are the site of fertilization.
3. Uterus:  The uterus is a muscular organ shaped like an inverted pear. The uterus has a thick lining of tissues called the endometrium. The uterus houses and nourishes the developing fetus until birth. During pregnancy the uterus can expand and can accommodate the developing fetus The contractions of the muscular walls of the uterus push the baby through the cervix and out of the body during birth . 
4. Cervix: The lower part of the uterus that opens into the vagina. It is a muscular tunnel like organ. The cervix is about 4 cm long and 3 cm in diameter, and its size varies according to a woman’s age and reproductive history.The sperm travel from the vagina up the cervical canal into the uterine cavity, then into the fallopian tubes to fertilize the egg. Fluids such as the menstrual blood, to pass from the uterus into the vagina through the cervix.  It also widens during the birth of a baby. The cervix has two main parts, the ectocervix  ( outer part of the cervix ) and the endocervix ( inner part of the cervix that forms a canal that connects the vagina to the uterus). The area between the endocervix and ectocervix  is called the squamocolumnar junction (also called the transformation zone). Most abnormal cell changes and cervical cancers begin in the squamous cells (thin, flat-looking cells under a microscope) of the squamocolumnar junction.
5. Vagina:  This is a tube-like structure about 9 centimeters (3.5 inches) long. The  vagina begins at the vulva  and extends upward to the uterus. It has muscular walls lined with mucous membranes. It Receives sperm during intercourse and serves as the birth canal.
6. Vulva :  The external female reproductive structures are referred to collectively as the vulva and include the labia , which are the “lips” of the vulva. The labia protect the vagina and urethra.  Both of vagina and urethra have openings in the vulva .

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The Process of Human Reproduction

1. Ovulation: Ovulation is the release of egg cells from the ovaries as part of the ovarian cycle. Women are born with about 1 million to 2 million eggs of which  only 300 to 400 are released  through ovulation during their lifetimes. Ovulation is a phase in the menstrual cycle of a women. On average, it happens on day 14 of a 28-day menstrual cycle. The process of ovulation begins when the hypothalamus of the  brain releases the  Gonado tropin releasing hormone  (GnRH). GnRH causes your pituitary gland of the brain to secrete Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH). FSH stimulates the maturation of the follicles in the ovary and at around the 14th day of the mensural cycle the LH surges causing the ovary to release the egg which is referred to as ovulation.. After ovulation, the hormone progesterone rises and prepares the uterus for pregnancy.
2. Fertilization: Fertilization is a complex process that refers to the process of union of the sperm and ova to form a zygote ( fertilized egg). The zygote which is a single cell, contains all the genetic information ( half from mother, half from father) needed to become a baby and initiates embryonic development. Fertilization marks the beginning of pregnancy. Of the millions of sperm released in to the vagina during ejaculation many are overcome by the acidity of the vagina ( pH 3.8). Further the thick  cervical mucus prevent their entry in to the uterus. In addition to all these ,  of the sperms that enter , thousands are destroyed by the phagocytic uterine leukocytes. The journey of the sperm is thought to be facilitated by uterine contractions and usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. On the other hand  oocyte can survive independently for only approximately 24 hours following ovulation.

“Sperm and the Process of Fertilization” by Rice University is licensed under CC BY 4.0

3. Implantation: The cilia ( hair-like structures)  in the cells lining the fallopian tube help to sweep the fertilized egg (zygote) through the tube and into the uterine cavity. As the zygote moves down the cells of the zygote divide (split into 2 cells) repeatedly The zygote enters the uterus within 3 to 5 days. The cells continue to divide, becoming a hollow ball of cells called a blastocyst. The blastocyst implants in the wall of the uterus about 6 days after fertilization. The blastocyst connects to the uterine wall approximately six days after fertilization, generally near the top. By day nine or ten, the implantation process is completed. The blastocyst wall is one cell thick. The placenta develops from the outside cells in the thickened area of the blastocyst, which burrow into the uterine wall, while the inner cells in the thickened area become the embryo.  The placenta produces numerous hormones that aid in maintaining the pregnancy . Human chorionic gonadotropin, for instance, is a hormone produced by the placenta that stimulates the ovaries to continually generate progesterone and estrogen while inhibiting the ovaries from releasing eggs. Additionally, the placenta transfers nutrients and oxygen from the mother to fetus

4. Gestation: The embryo develops within the amniotic sac, under the lining of the uterus on one side. The embryo grows and develops into a fetus over approximately nine months. Most of the internal organs and external body structures are formed during this stage. At about 16 days after fertilization, the heart and major blood vessels develop. Almost all organs are completely formed by about 12 weeks of pregnancy. The brain and spinal cord are exceptions—they continue to form and develop throughout pregnancy. By about 14 weeks of pregnancy the sex can be identified on ultrasound. By about 16 to 20 weeks the pregnant woman can feel the fetus moving. The brain accumulates new cells throughout pregnancy and the first year of life after birth. The lungs continue to mature until near the time of delivery.

5. Birth: The fully developed baby is delivered through the birth canal.

Watch the video from Khan Academy on Female reproductive system

For further reading visit the chapter on human reproduction from Concepts of Biology – 1st Canadian Edition

and also the chapter 43.3 Human Reproductive Anatomy and Gametogenesis from Biology from openstax