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A woman can conceive only during a short window of time each month. About every 28 days, an ovum bursts from one of the ovaries into the long, thin fallopian tube that leads to the uterus; this event is known as ovulation (see Figure 3.1). The ovum is the largest cell in the human body, yet it is only 1/175th of an inch in diameter (about the size of the period at the end of this sentence). Over several days, the ovum travels down the fallopian tube, which connects the ovaries to the uterus, while the corpus luteum, the spot on the ovary from which the ovum was released, secretes hormones that cause the lining of the uterus to thicken in preparation for the fertilized ovum (Sadler, 2018). If fertilization does not occur, the lining of the uterus is shed through menstruation about 2 weeks after ovulation.

A tiny sperm is fertilizing the much larger ovum.

©iStockphoto.com/Dr_Microbe

Conception, of course, also involves the male. Each day, a man’s testes produce millions of sperm, which are composed of a pointed head packed with 23 chromosomes’ worth of genetic material and a long tail. During ejaculation, about 360 million, and as many as 500 million, sperm are released, bathed in a protective fluid called semen (Moore & Persaud, 2016). After entering the female’s vagina, sperm travel through the cervix into the uterus and onward toward the ovum. After about 6 hours the sperm reach the fallopian tube, where an ovum may—or may not—be present. The journey is difficult: Some sperm get tangled up with other sperm, some travel up the wrong fallopian tube, and others do not swim vigorously enough to reach the ovum. On average, about 300 sperm reach the ovum, if one is present (Webster, Morris, & Kevelighan, 2018). Those that travel up the fallopian tube can live up to 6 days, able to fertilize a yet unreleased ovum. The ovum, however, remains viable for only about a day after being released into the fallopian tube.

Description

Figure 3.1 Female Reproductive System

Source: Levine and Munsch (2010, p. 102).

Both sperm and the woman’s reproductive tract play a role in fertilization (Suarez, 2016). Sperm are guided by temperature, tracking the heat of an expectant ovum, as well as by chemical signal (Lottero-Leconte, Isidro Alonso, Castellano, & Perez Martinez, 2017). In the presence of an ovum, sperm become hyperactivated, they swim even more vigorously, and the sperm’s head releases enzymes to help it penetrate the protective layers of the ovum (Bianchi & Wright, 2016). As soon as one sperm penetrates the ovum, a chemical reaction makes the ovum’s membrane impermeable to other sperm. The sperm’s tail falls off, and the sperm’s genetic contents merge with that of the ovum.

At the moment of conception, the zygote contains 46 chromosomes, half from the ovum and half from the sperm. After fertilization, the zygote rapidly transforms into a multicelled organism. Prenatal development takes place over three developmental periods: (1) the germinal period, (2) the embryonic period, and (3) the fetal period.

This ball of cells, known as a morula, is formed at about 3 days after conception. Each of these cells is identical. Differentiation has not yet begun.

Pascal Goetgheluck/Science Source