43.3 Human Reproduction and Gametogenesis

Learning Objectives

  • Describe spermatogenesis and oogenesis, in mammals, and discuss their differences and similarities

As animals became more complex, specific organs and organ systems developed to support specific functions for the organism. The reproductive structures that evolved in land animals allow males and females to mate, fertilize internally, and support the growth and development of offspring.

Mammal Reproductive Anatomy

The reproductive tissues of male and female humans develop similarly in utero until a low level of the hormone testosterone is released from male gonads. Testosterone causes the undeveloped tissues to differentiate into male sexual organs. When testosterone is absent, the tissues develop into female sexual tissues. Primitive gonads become testes or ovaries.

Male Reproductive Anatomy

In the male reproductive system, the scrotum houses the testicles or testes (singular: testis), including providing passage for blood vessels, nerves, and muscles related to testicular function. The testes are a pair of male reproductive organs that produce sperm and some reproductive hormones.

Visual Connection

Illustration shows a cross section of the penis and testes. The penis widens at the end, into the glans, which is surrounded by the foreskin. The urethra is an opening that runs through the middle of the penis to the bladder. The tissue surrounding the urethra is the Corpus spongiosum, and above the Corpus spongiosum is the Corpus cavernosum. The testes, located immediately behind the penis, are covered by the scrotum. Seminiferous tubules are located in the testes. The epididymis partly surrounds the sac containing the seminiferous tubules. The Vas deferens is a tube connecting the seminiferous tubules to the ejaculatory duct, which begins in the prostate gland. The prostate gland is located behind and below the bladder. The seminal vesicle, located above the prostate, also connects to the seminal vesicle. The bulbourethral gland connects to the ejaculatory duct where the ejaculatory duct enters the penis.
Figure 43.8 The reproductive structures of the human male are shown.

Female Reproductive Anatomy

Side and front views of female reproductive organs are shown. The vagina is wide at the bottom, and narrows into the cervix. Above the cervix is the uterus, which is shaped like a triangle pointing down. Fallopian tubes extend from the top sides of the uterus. The Fallopian tubes curve back in toward the uterus, and end in fingerlike appendages called fimbriae. The ovaries are located between the fimbriae and the uterus. The urethra is located in front of the vagina, and the rectum is located behind. The clitoris is a structure located in front of the urethra. The labia minora and labia majora are folds of tissue on either side of the vagina.
Figure 43.10 The reproductive structures of the human female are shown. (credit a: modification of work by Gray’s Anatomy; credit b: modification of work by CDC)

Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina, shown in Figure 43.10.  Ovaries consist of a medulla and cortex: the medulla contains nerves and blood vessels to supply the cortex with nutrients and remove waste. The outer layers of cells of the cortex are the functional parts of the ovaries. The cortex is made up of follicular cells that surround eggs that develop during fetal development in utero. During the menstrual period, a batch of follicular cells develops and prepares the eggs for release. At ovulation, one follicle ruptures and one egg is released, as illustrated in Figure 43.11a.

Illustration A shows a cross section of a human ovary, which is oval with a stem-like structure at one end that anchors it to the uterus. The central part of the ovary is the medulla, and the outer part is the cortex. Follicles exist in the cortex. Small, immature follicles are located near this stem-like structure. As a follicle matures, it grows and moves toward the edge of the ovary opposite the stem, it ruptures, releasing the egg. The follicle is now called a corpus luteum. The corpus luteum matures and moves back toward the stem, along the opposite edge of the ovary from which the follicle matured. The corpus luteum shrinks and eventually disintegrates. The light micrograph shows an oval follicle with a large oocyte located at the center. Around the oocyte are much smaller cells.
Figure 43.11 Oocytes develop in (a) follicles, located in the ovary. At the beginning of the menstrual cycle, the follicle matures. At ovulation, the follicle ruptures, releasing the egg. The follicle becomes a corpus luteum, which eventually degenerates. The (b) follicle in this light micrograph has an oocyte at its center. (credit a: modification of work by NIH; scale-bar data from Matt Russell)

Gametogenesis (Spermatogenesis and Oogenesis)

Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis. During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an earlier stage of the cell’s life cycle. Meiosis produces haploid cells with one member of each pair of chromosomes normally found in diploid cells. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis.

Spermatogenesis

Spermatogenesis begins when the 2 n spermatogonium undergoes mitosis, producing more spermatagonia. The spermatogonia undergo meiosis I, producing haploid 1 n secondary spermatocytes, and meiosis I I, producing spermatids. Differentiation of the spermatids results in mature sperm.
Figure 43.12 During spermatogenesis, four sperm result from each primary spermatocyte.

Spermatogenesis, illustrated in Figure 43.12, occurs in the wall of the seminiferous tubules (Figure 43.8), with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell and the other giving rise to the next generation of sperm.

Meiosis starts with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This cell is haploid and must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid and when it reaches the lumen of the tubule and grows a flagellum, it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis.

Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age.

Oogenesis

Oogenesis, illustrated in Figure 43.13, occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to about one to two million cells in the embryo.

Oogenesis begins when the 2 n oogonium undergoes mitosis, producing a primary oocyte. The primary oocytes arrest in prophase I before birth. After puberty, meiosis of one oocyte per menstrual cycle continues, resulting in a 1 n secondary oocyte that arrests in metaphase I I and a polar body. Upon ovulation and sperm entry, meiosis is completed and fertilization occurs, resulting in a polar body, shown as 1 n, and a fertilized egg, 2 n.
Figure 43.13 The process of oogenesis occurs in the ovary’s outermost layer.

The cell starting meiosis is called a primary oocyte, as shown in Figure 43.13. This cell will start the first meiotic division and be arrested in its progress in the prophase I stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones, follicle stimulating hormone (FSH) cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.

Egg production begins before birth, is arrested during meiosis until puberty, and then individual cells continue through at each menstrual cycle. One egg is produced from each meiotic process, with the extra chromosomes and chromatids going into polar bodies that degenerate and are reabsorbed by the body.

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