Table of Contents
Inquiry Question
How does reproduction ensure the continuity of a species?
Content description
- Analyse the features of fertilisation, implantation and hormonal control of pregnancy and birth in mammals
General themes in mammalian reproductive biology
- Mammals are classified into three groups: monotremes, marsupials and placentals (also called eutherians).
- Most mammals (marsupials and placentals) are viviparous (they give birth to live young). Monotremes are the only group of oviparous mammals (egg-laying).
- The mammary glands of viviparous mammals produce milk, which is a nutrient source for their newborns.
- In mammals, sex is determined genetically.
Sex determination in mammals
In most mammals, sex is determined by the XY system of chromosomes. This pair of chromosomes, XX or XY, is referred to as the sex chromosomes. A Y chromosome makes the individual a biological male, while its absence renders it a female. The X chromosome carries the genes that encode the hormone estrogen. This hormone promotes the development of the female reproductive system. The Y chromosome carries a gene known as SRY (Sex-determining Region Y). This gene encodes a protein called the Testis-Determining Factor (TDF). During early development, TDF stimulates the expression of other genes that induce the primordial reproductive tissues to develop into cells that become part of the male reproductive system (Sertoli and Leydig cells). These cells secrete testosterone and the anti-Mullerian hormone, which then promote the formation of the testis.
The sex-determination system of the platypus is unusual in that ten chromosomes are involved in that process. As a result, males are XYXYXYXYXY, while females are XXXXXXXXXX.
During early embryonic development, a group of cells become the ‘presumptive gonad‘. These cells will eventually give rise to the tissues and organs of the reproductive system. This presumptive gonad is neither male nor female. The genes that determine the sex of a mammal are carried on the X and Y chromosomes. When only X chromosomes are present (XX), some of the genes on those chromosomes will cause the presumptive gonad to develop into female reproductive tissues (for example, the uterus, Fallopian tubes and the ovaries). However, if the embryo contains a Y chromosome, the SRY gene on that chromosome directs the presumptive gonad to develop into male reproductive tissues (testis). Subsequently, the release of estrogen by the ovaries and testosterone by the testis will direct the development of the embryos as female and male individuals, respectively.
Reproductive maturity
Young mammals cannot reproduce (bear young). The ability to reproduce occurs later in life, and this ability is referred to as reproductive maturity (also referred to as sexual maturity). Reproductive maturity occurs at a specific stage in an individual’s growth and development. In humans, the onset of reproductive maturity is accompanied by bodily changes, referred to as puberty. During puberty, individuals develop secondary sexual characteristics, such as developing breasts in human females and the growth of facial hairs and deeper voice in human males. In addition, both males and females start to produce gametes (sperm and eggs) for reproduction.
Fertilisation, implantation, hormonal control of pregnancy and birth
While sex determination is genetically controlled, the regulation of reproduction in mammals is largely coordinated by hormones. Both the brain and the reproductive system are involved in this regulation.
Spermatogenesis
In humans, spermatogenesis (the production of sperm) starts at puberty and continues until death. Two endocrine organs located in the brain produce hormones that induce spermatogenesis in the testis. In the brain, the hypothalamus produces Gonadotropin-Releasing Hormone (GnRH). GnRH travels to the anterior pituitary gland and stimulates that organ to produce two hormones, Leuteinising Hormone (LH) and Follicle Stimulating Hormone (FSH). The FSH and LH travel via the bloodstream to the testis. In the testis, LH stimulates the Leydig cells to produce testosterone. On the other hand, FSH stimulates the Sertoli cells to form mature sperm cells (the seminiferous tubules produce the sperm cells, but the Sertoli cells are involved in their maturation before release). Thus, in addition to the FSH, the Sertoli cells require the testosterone produced by the Leydig cells.
Oogenesis
Oogenesis is the process of forming egg cells in the ovaries. As in human males, the regulation of oogenesis in human females is coordinated by hormones produced in the brain and the ovaries. The hormones produced by the female brain are similar to the ones produced by the male brain: GnRH is produced by the hypothalamus. In turn, GnRH stimulates the production of LH and FSH in the anterior pituitary gland.
In females, FSH and LH act on the ovaries. In response, the ovaries will recruit a follicle for maturation (the follicle is a structure that contains an egg cell). This period of follicular maturation is known as the follicular phase. Although each ovary has millions of immature follicles, only one will be recruited to develop a menstrual cycle. In addition to follicle recruitment, the ovaries will release two hormones: oestradiol and progesterone. As the follicle matures, the concentration of these two hormones in the blood will continue to rise – oestradiol increases more rapidly than progesterone. One of the roles of oestradiol and progesterone is to increase the thickness of the epithelial lining of the uterus (endometrium). As a result, the endometrium also becomes vascularised (filled with blood vessels). This process prepares the uterus for implantation by the embryo.
The oestradiol level will continue to rise during the follicular phase and reach a maximum when the follicle matures. This causes the pituitary gland to release large amounts of LH (known as the LH surge). In turn, the follicle ruptures, releasing the egg cell into the oviduct (fallopian tube). This process is called ovulation. The egg begins its journey through the fallopian tube, being pushed along by the delicate cilia that line the fallopian tube. The remnant of the follicle in the ovary, now called the corpus luteum, begins to secrete large amounts of progesterone, causing its concentration in the blood to rise. This enhances the thickening of the endometrium.
A few days after ovulation, the corpus luteum degenerates. As a result, the level of progesterone in the blood will fall. This triggers the next phase of the menstrual cycle in human females, called the menstrual phase. In this phase, the endometrium breaks down and is discharged through the vagina. This discharge is known as menstruation and signifies that pregnancy has not occurred.
The menstrual cycle is uncommon in mammals – only human females and the females of some primates appear to go through menstruation. The females of other mammalian species have the estrous cycle. The estrous cycle is similar to the menstrual cycle, except that the menstrual phase is replaced with the estrous phase. During estrous, the endometrium is resorbed rather than being discharged.
Fertilisation and pregnancy
The egg cell is non-motile (after ovulation, the cilia that line the Fallopian tube push the egg cell towards the uterus), while the sperm cells are motile. Fertilisation is the fusion of a single egg cell with a single sperm cell. Through copulation, semen is introduced into a female’s reproductive tract. The sperm cells swim through the layer of mucus that covers the surfaces of the female reproductive tract. The chemical composition of the mucus (for example, the bicarbonate ions) activate the sperm cells. Only those cells that are structurally intact begin the long journey to the Fallopian tubes. It is unclear how the sperm cells are guided to the egg cell in the Fallopian tube, but chemoattraction (chemotaxis) and temperature (thermotaxis) may be involved.
When the sperm cells encounter the egg cell, they attach to its surface. Although many sperm cells may attach to the surface of an egg cell, only one of those sperm cells will successfully fertilise the egg. The egg cell surface has proteins called the zona pellucida proteins (ZP1, ZP2 and ZP3). The sperm cell has proteins that recognise and bind to the ZP proteins. It is only when those proteins interact that a strong binding occurs. The first sperm to successfully attach to the egg cell initiates the fertilisation process and prevents other sperm from fertilising the egg. This process is called the block to polyspermy (polyspermy means an egg cell fertilised by multiple sperm cells). After attachment, the sperm cell undergoes the acrosome reaction, in which the head of the sperm cell (which contains the nucleus) penetrates the egg cell.
After fertilisation, the egg cell is called a zygote. The zygote initially has two nuclei: the haploid egg nucleus and the haploid sperm nucleus. Soon, the two haploid nuclei will fuse into a single diploid nucleus.
The zygote will undergo a series of mitotic divisions (each cell division is called a cleavage). The multicellular structure is called an embryo and continue to travel towards the uterus. Soon, the embryo becomes a solid spherical mass of cells called a morula. Then, some of the cells in the interior of the embryo die and that space becomes filled with fluid. The embryo is called a blastula. In the uterus, the embryo (blastula) attaches to the uterine wall. By this stage, the embryo reaches the uterus. It attaches to the wall (endometrium) of the uterus. A group of cells in the embryo called the inner cell mass divides and invades the endometrium. Eventually, they will develop into the placenta. The placenta is the interface between the embryo and maternal blood circulation. The transfer of gases, nutrients and wastes occur across the placenta.
The placenta also functions as an endocrine organ and produces the hormone, human chorionic gonadotropin (hCG). hCG ensures that pregnancy continues until the birth of the baby. In addition, it prevents further cycles of ovulation.
| Mammal | Gestation (days) |
|---|---|
| Opossum | 12 |
| Mouse | 19 |
| Kangaroo | 42 |
| Dog | 61 |
| Tiger | 109 |
| Human | 270 |
| Cow | 283 |
| Giraffe | 430 |
| Sperm whale | 535 |
| Asian Elephant | 617 |
Birth
In placental mammals (including humans), after the placenta is formed, the mother’s biology becomes adapted to maintaining the pregnancy. Two hormones are essential to this: estrogen and progesterone. Estrogen is produced initially by the corpus luteum in the ovary and later by the fetus, while progesterone is produced by the placenta. Both of these hormones work in concert to maintain the pregnancy (e.g. strengthening the uterine walls to support the growing fetus; preventing the contraction of the uterus; developing the mammary glands for lactation).
Birth is the process of externalising (bringing forth) the young animal. Scientifically, birthing is referred to as parturition. The process of birthing is complex and is coordinated by various systems, such as the nervous and endocrine systems. The endocrine control of birth involves both positive and negative feedback. The following description provides an overview of some events that occur during labour and birth in human females (not that the description is incomplete).
- As the fetus inceases in size, it causes the uterine muscles to stretch. This causes the placenta to release the hormone, estriol.
- Estriol inhibits the release of progrsterone from the placenta. Therefore, the block to quiescence (i.e. preventing the contraction of the uterine muscles) is lifted.
- Estriol also stimulates the release of oxytocin (a hormone) from the pituitary gland. The oxytocin hormone bind to receptors on uterine muscles, promoting muscle contractions.
- The contraction of the uterine muscles stimulates the estriol-oxytocin pathway. This is a postive feedback pathway that increases the intensity and frequency of contractions, leading to the birth of the child. The release of chemicals called prostaglandins by the fetus further promote uterine contractions.
- Oxytocin and prolactin (another hormone) stimulates milk production in the mammary glands.
Understanding hormonal control of pregnancy and birth
Students learning this topic may find it useful to consider the various stages of mammalian reproduction as a series of developmental events. For example, gamete production, pregnancy and birth represent specific developmental events that are regulated by hormones. In females, the reproductive system serves three distinct events associated with reproduction: ovulation, pregnancy and birth. The brain regulates these events by secreting hormones, as well as responding to hormones produced by the reproductive organs and the fetus. This two-way communication is important for a successful pregnancy and birth, including post-natal care (production of milk). The hormones involved in this complex communication between the brain and the reproductive system form the brain-gonad axis – more specifically, the hypothalamo-pituitary-gonadal axis. As with all hormonal systems, both positive and negative feedback loops regulate the biological processes. For example, in both males and females, LH and FSH from the pituitary gland stimulate the production of gametes in the gonads. In females, progesterone prepares the uterus for the implantation of the embryo, and the maintenance of the pregnancy. During birth and post-natally, oxytocin causes the expulsion of the fetus during birth (parturition) and milk production.
Perspectives
Hormonal control of mammalian reproduction is complex – the descriptions provided in this webpage cover only some aspects of that system. The hormones ensure that the organs of the reproductive system function as an orchestrated sequence of events, where the hormones themselves are the conductors of the symphony. In all non-oviparous mammals, maternal and fetal hormones regulate pregnancy and gestation so that the fetus develops in an optimal environment before birth. This includes the regulation of metabolic, respiratory and other physiological systems. Similarly, the interplay of material and fetal hormones stimulate birth.
It is important for students to note that the testis and ovaries are both reproductive and endocrine organs.