Double Fertilisation

Development of angiosperm gametophytes and the process of pollination

Within the microsporangia (pollen sacs) of an anther are many diploid cells called microsporocytes, also known as microspore mother cells.

Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually give rise to a haploid male gametophyte.

A microspore undergoes mitosis and cytokinesis, producing two separate cells called the generative cell and tube cell. Together, these two cells and the spore wall constitute a pollen grain, which at this stage of its development is an immature male gametophyte. The spore wall usually exhibits an elaborate pattern unique to the particular plant species. During maturation of the male gametophyte, the generative cell passes into the tube cell. The tube cell now has a completely free–standing cell inside it (the generative cell). The tube cell produces the pollen tube, a structure essential for sperm delivery to the egg. During elongation of the pollen tube, the generative cell usually divides and produces two sperm cells, which remain inside the tube cell. The pollen tube grows through the long style of the carpel and into the ovary, where it then releases the sperm cells in the vicinity of an embryo sac.

One or more ovules, each containing a megasporangium, form within the chambers of the ovary. One cell in the megasporangium of each ovule, the megasporocyte (or megaspore mother cell), grows and then goes through meiosis, producing four haploid megaspores.

The details of the next steps vary extensively, depending on the species. In most angiosperm species, only one megaspore survives. This megaspore continues to grow, and its nucleus divides by mitosis three times without cytokinesis, resulting in one large cell with eight haploid nuclei. Membranes then partition this mass into a multicellular female gametophyte—the embryo sac. At one end of the embryo sac are three cells: the egg cell and two cells called synergids. The synergids flank the egg cell and function in the attraction and guidance of the pollen tube to the embryo sac. At the opposite end of the embryo sac are three antipodal cells of unknown function. The remaining two nuclei, called polar nuclei, are not partitioned into separate cells but instead share the cytoplasm of the large central cell of the embryo sac. The ovule, which will eventually become a seed, now consists of the embryo sac and two surrounding integuments (layers of protective sporophytic tissue that eventually develop into the seed coat).

Pollination, the transfer of pollen from anther to stigma, is the first step in a chain of events that can lead to fertilisation. This step is accomplished in various ways. In some angiosperms, including grasses and many trees, wind is a pollinating agent. In such plants, the release of enormous quantities of pollen compensates for the randomness of this dispersal mechanism. At certain times of the year, the air is loaded with pollen grains, as anyone plagued with pollen allergies can attest. Some aquatic plants rely on water to disperse pollen. Most angiosperms, however, depend on insects, birds, or other animals to transfer pollen directly to other flowers.

The reproductive cycle of the human female

The Ovarian Cycle. 1 The cycle begins with the release from the hypothalamus of GnRH, which 2 stimulates the pituitary to secrete small amounts of FSH and LH. 3 The FSH (true to its name) stimulates follicle growth, aided by LH, and 4 the cells of the growing follicles start to make oestrogen. Notice in Figure(d) that there is a slow rise in the amount of oestrogen secreted during most of the follicular phase, the part of the ovarian cycle during which follicles are growing and oocytes maturing. (Several follicles begin to grow with each cycle, but usually only one matures; the others disintegrate.) The low levels of oestrogen inhibit secretion of the pituitary hormones, keeping the levels of FSH and LH relatively low.

The levels of FSH and LH, however, shoot up sharply when 5 the secretion of oestrogen by the growing follicle begins to rise steeply. Whereas a low level of oestrogen inhibits the secretion of pituitary gonadotropins, a high concentration has the opposite effect: It stimulates the secretion of gonadotropins by acting on the hypothalamus to increase its output of GnRH. 6 You can see this response in Figure (b) as steep increases in FSH and LH levels that occur soon after the increase in the concentration of oestrogen, indicated in Figure (d). The effect is greater for LH because the high concentration of oestrogen also increases the sensitivity of LH–releasing cells in the pituitary to GnRH. By now, the follicles can respond more strongly to LH because more of their cells have receptors for this hormone. The increase in LH concentration caused by increased oestrogen secretion from the growing follicle is an example of positive feedback. The LH induces final maturation of the follicle. 7 The maturing follicle develops an internal fluid–filled cavity and grows very large, forming a bulge near the surface of the ovary. The follicular phase ends, about a day after the LH surge, with ovulation: The follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte.

8 Following ovulation, during the luteal phase of the ovarian cycle, LH stimulates the transformation of the follicular tissue left behind in the ovary to form the corpus luteum, a glandular structure (c). (LH is named for this “luteinising” function.) Under continued stimulation by LH during this phase of the ovarian cycle, the corpus luteum secretes progesterone and estrogen (see Figure(d). As the levels of progesterone and oestrogen rise, the combination of these hormones exerts negative feedback on the hypothalamus and pituitary, inhibiting the secretion of LH and FSH. Near the end of the luteal phase, the corpus luteum disintegrates, causing concentrations of estrogen and progesterone to decline sharply. The dropping levels of ovarian hormones liberate the hypothalamus and pituitary from the inhibitory effects of these hormones. The pituitary can then begin to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle.

The Uterine (Menstrual) Cycle. The hormones secreted by the ovaries—oestrogen and progesterone—have a major effect on the uterus. Oestrogen secreted in increasing amounts by growing follicles signals the endometrium to thicken. In this way, the follicular phase of the ovarian cycle is coordinated with the proliferative phase of the uterine cycle (see Figure (e ). Before ovulation, the uterus is already being prepared for a possible embryo. After ovulation, 9 oestrogen and progesterone secreted by the corpus luteum stimulate continued development and maintenance of the endometrium, including enlargement of arteries and growth of endometrial glands. These glands secrete a nutrient fluid that can sustain an early embryo even before it actually implants in the uterine lining. Thus, the luteal phase of the ovarian cycle is coordinated with what is called the secretory phase of the uterine cycle.

10 The rapid drop in the level of ovarian hormones when the corpus luteum disintegrates causes spasms of the arteries in the uterine lining that deprive it of blood. The upper two–thirds of the endometrium disintegrates, resulting in menstruation—the menstrual flow phase of the uterine cycle—and the beginning of a new cycle. By convention, the first day of menstruation is designated day 1 of the uterine (and ovarian) cycle. Menstrual bleeding usually persists for a few days. During menstruation, a fresh batch of ovarian follicles are just beginning to grow.

Cycle after cycle, the maturation and release of egg cells from the ovary are integrated with changes in the uterus, the organ that must accommodate an embryo if the egg cell is fertilised. If an embryo has not implanted in the endometrium by the end of the secretory phase of the uterine cycle, a new menstrual flow commences, marking day 1 of the next cycle.

In addition to the roles of oestrogen in coordinating the female reproductive cycle, this hormone family is responsible for the secondary sex characteristics of the female. Oestrogen induces deposition of fat in the breasts and hips, increases water retention, affects calcium metabolism, stimulates breast development, and influences female sexual behaviour.

Menopause. After about 450 cycles, human females undergo menopause, the cessation of ovulation and menstruation. Menopause usually occurs between the ages of 46 and 54. Apparently, during these years the ovaries lose their responsiveness to gonadotropins from the pituitary (FSH and LH), and menopause results from a decline in estrogen production by the ovary. Menopause is an unusual phenomenon; in most species, females as well as males retain their reproductive capacity throughout life. Is there an evolutionary explanation for menopause? Why might natural selection have favoured females who had stopped reproducing? One intriguing hypothesis proposes that during early human evolution, undergoing menopause after having some children actually increased a woman′s fitness; losing the ability to reproduce allowed her to provide better care for her children and grandchildren, thereby increasing the survival of individuals bearing her genes.

Spermatogenesis and Oogenesis

In humans and other mammals, a complex interplay of hormones regulates gametogenesis

How exactly are gametes produced in the mammalian body? The process, gametogenesis, is based on meiosis, but details differ in females and males. Oogenesis, the development of mature ova (egg cells). Spermatogenesis, the production of mature sperm cells, is a continuous and prolific process in the adult male. Each ejaculation of a human male contains 100 to 650 million sperm cells, and males can ejaculate daily with little loss of fertilising capacity. Spermatogenesis occurs in the seminiferous tubules of the testes.

Oogenesis differs from spermatogenesis in three major ways. First, during the meiotic divisions of oogenesis, cytokinesis is unequal, with almost all the cytoplasm monopolized by a single daughter cell, the secondary oocyte. This large cell can go on to become the ovum; the other products of meiosis, smaller cells called polar bodies, degenerate. By contrast, in spermatogenesis, all four products of meiosis develop into mature sperms. Second, although the cells from which sperm develop continue to divide by mitosis throughout the male′s life, this is thought not to be the case for oogenesis in the human female. Third, oogenesis has long “resting” periods, in contrast to spermatogenesis, which produces mature sperm from precursor cells in an uninterrupted sequence.

Genetic (Introduction)