reproductive system

Luteal phase

After ovulation, LH induces the ruptured follicle to develop into the corpus luteum, which secretes estrogen and progesterone. Progesterone causes the glands of the endometrium to mature and produce secretions that prepare it for the implantation of an embryo. Progesterone and estrogen are essential for the maintenance of the endometrium.

Gastrulation

After week two, the embryo is fully implanted in the uterus and cell migrations transform the single cell layer of the blastula into a three-layered structure called a gastrula. These three primary germ layers, listed below, are responsible for the differential development of the tissues, organs, and systems of the body at later stages of growth:

Fertilization

An egg can be fertilized during the 12-24 hours after ovulation. Fertilization occurs in the lateral, widest portion of the fallopian tube. Sperm must travel through the vaginal canal, cervix, uterus, and into the fallopian tubes to reach the ovum. Sperm remain viable and capable of fertilization for 1-2 days after intercourse. The first barrier that the sperm must penetrate is the corona radiata. Enzymes secreted by the sperm aid in penetration of the corona radiata. The acrosome is responsible for penetrating the zona pellucida; it releases enzymes that digest this layer, thereby allowing the sperm to come into direct contact with the ovum cell membrane. Once in contact with the membrane, the sperm forms a tubelike structure called the acrosomal process, which extends to the cell membrane and penetrates it, fusing the sperm cell membrane with that of the ovum. The sperm nucleus now enters the ovum's cytoplasm. It is at this stage of fertilization that the ovum completes meiosis II. The acrosomal reaction triggers a cortical reaction in the ovum, causing calcium ions to be released into the cytoplasm; this, in turn, initiates a series of reactions that result in the formation of the fertilization membrane. The fertilization membrane is a hard layer that surrounds the ovum cell membrane and prevents multiple fertilizations. The release of Ca2+ also stimulates metabolic changes within the ovum, greatly increasing its metabolic rate. This is followed by the fusion of the sperm nucleus with the ovum nucleus to form a diploid zygote. The first mitotic division of the zygote soon follows.

Neurulation

By the end of gastrulation, regions of the germ layers begin to develop into a rudimentary nervous system; this process is known as neurulation and starts before week three. A rod of mesodermal cells called the notochord develops along the longitudinal axis of the embryo just under the dorsal layer of ectoderm. The notochord has an inductive effect on the overlying ectoderm, causing it to bend inward and form a groove along the dorsal surface of the embryo. The dorsal ectoderm folds on either side of the groove; these neural folds grow upward and finally fuse, forming a closed tube. This is the neural tube, which gives rise to the brain and spinal cord (central formed, it detaches from the surface ectoderm.

Birth and maturation

Childbirth is accomplished by labor, a series of strong uterine contractions. Labor can be divided into three distinct stages. In the first stage, the cervix thins and dilates, and the amniotic sac ruptures, releasing its fluids. During this time contractions are relatively mild. The second stage is characterized by rapid contractions, resulting in the birth of the baby (the umbilical cord is usually cut after this stage). During the final stage, the uterus contracts, expelling the placenta and the umbilical cord. The embryo develops into the adult through the process of maturation, which involves cell division, growth, and differentiation. Differentiation of cells is complete when all organs reach adult form.

Dizygotic (fraternal) twins

Dizygotic twins result when two ova are released in one ovarian cycle and are fertilized by two different sperm. The two embryos implant in the uterine wall individually, and each develops its own placenta, amnion, and chorion (although the placentas may fuse if the embryos implant very close to each other). Fraternal twins share no more characteristics than any other siblings because they develop from two distinct zygotes.

First trimester

During the first weeks, the major organs begin to develop. The heart begins to beat at approximately 22 days, and soon afterward the eyes, gonads, limbs, and liver start to form. By five weeks the embryo is 10 mm in length; by six weeks the embryo has grown to 15 mm. The cartilaginous skeleton begins to turn into bone by the seventh week. By the end of eight weeks, most of the organs have formed, the brain is fairly developed, and the embryo is referred to as a fetus. At the end of the third month, the fetus is about 9 cm long.

Second trimester

During the second trimester, the fetus does a tremendous amount of growing. It begins to move around in the amniotic fluid, its face appears human, and its toes and fingers elongate. By the end of the sixth month, the fetus is 30-36 cm long.

Cleavage

Early embryonic development is characterized by a series of rapid mitotic divisions known as cleavage. These divisions lead to an increase in cell number without a corresponding growth in cell protoplasm (i.e., the total volume of cytoplasm remains constant). Thus, cleavage results in progressively smaller cells with an increasing ratio of nuclei to cytoplasm. Cleavage also increases the surface-to-volume ratio of each cell, thereby improving gas and nutrient exchange.

EMBRYOLOGY

Embryology is the study of the development of a unicellular zygote into a complete, multicellular organism. In the course of nine months, a unicellular human zygote undergoes cell division, cellular differentiation, and morphogenesis in preparation for life outside the uterus.

Endoderm:

Endoderm: gives rise to the epithelial linings of the digestive and respiratory tracts (including the lungs) and parts of the liver, pancreas, thyroid, and bladder lining

Estrogens

Estrogens are steroid hormones necessary for normal female maturation. They stimulate the development of the female reproductive tract and contribute to the development of secondary sexual characteristics and sex drive. Estrogens are also responsible for the thickening of the endometrium (the inner lining of the uterine wall). Estrogens are secreted by the ovarian follicles and the corpus luteum.

Fertilization

Fertilization is the union of the egg and sperm nuclei to form a zygote with a diploid number of chromosomes.

Gametogenesis:

Gametogenesis: Eggs develop in women and sperm develop in men, which permits reproduction to occur.

Gestation

Human pregnancy, or gestation, is approximately nine months (266 days) long and can be subdivided into three trimesters. The primary developments that occur during each trimester are described below.

Menstruation

If the ovum is not fertilized, the corpus luteum atrophies. The resulting drop in progesterone and estrogen levels causes the endometrium (with its superficial blood vessels) to slough off, giving rise to the menstrual flow (menses). If fertilization occurs, the developing placenta produces hCG (human chorionic gonadotropin), maintaining the corpus luteum and thus the supply of estrogen and progesterone that maintains the uterus. Eventually the placenta takes over production of these hormones and hCG is no longer produced.

In an ectopic pregnancy,

In a normal pregnancy the blastula implants in the uterus. In an ectopic pregnancy, the embryo implants outside the uterus, for example, in the fallopian tube. An embryo cannot be maintained for long outside of the uterus; it will abort spontaneously, and hemorrhaging will follow. Ectopic pregnancies can be fatal if not caught in time and managed appropriately.

Fertilization

In mammals, an egg can be fertilized within 12-24 hours after ovulation. Fertilization occurs in the lateral, widest portion of the oviduct where sperm traveling from the vagina encounter an egg. If more than one egg is fertilized, fraternal (dizygotic) twins may be conceived. Identical (monozygotic) twins, in contrast, occur when one zygote divides into two separate embryos

Internal fertilization

Internal fertilization is practiced by terrestrial vertebrates and provides a direct route for sperm to reach the egg cell. This increases the chance for fertilization success, and females produce fewer eggs. The number of eggs produced is affected by other factors as well. If the early development of the offspring occurs outside of the mother's body, more eggs will be laid to increase the chances of offspring survival. The amount of parental care after birth is also related to the number of eggs produced. Species that care for their young produce fewer eggs.

Ovulation

Midway through the cycle, ovulation occurs—a mature ovarian follicle bursts and releases an ovum. Ovulation is caused by a surge in LH that is preceded and in part caused by a peak in estrogen levels. Women ovulate approximately once every four weeks (except during pregnancy and, usually, lactation) until menopause, which typically occurs between the ages of 45 and 50. During menopause the ovaries become less sensitive to the hormones that stimulate follicle development (FSH and LH) and eventually atrophy. The remaining follicles disappear, estrogen and progesterone levels greatly decline, and ovulation stops. The profound changes in hormone levels are often accompanied by physiological and psychological changes that persist until a new balance is reached.

Monozygotic (identical) twins

Monozygotic twins result when a single zygote splits into two embryos. If the splitting occurs at the two-cell stage of development, the embryos will have separate chorions and separate placentas; if it occurs at the blastula stage, then the embryos will have only one chorionic sac and will therefore share a placenta and possibly an amnion. Occasionally the division is incomplete, resulting in the birth of "Siamese" (conjoined) twins, which are attached at some point on the body, often sharing limbs or organs. Monozygotic twins are genetically identical because they develop from the same zygote. Monozygotic twins are therefore of the same sex, blood type, and so on.

Oogenesis

Oogenesis, the production of female gametes, occurs in the ovarian follicles. At birth, most of the immature ova, known as primary oocytes, that a female will produce during her lifetime are already in her ovaries. Primary oocytes are diploid cells that form by mitosis in the ovary. After menarche (the first time a female menstruates), one primary oocyte per month completes meiosis I, yielding two daughter cells of unequal size—a secondary oocyte and a small cell known as a polar body. The secondary oocyte is expelled from the follicle during ovulation. Meiosis II does not occur until fertilization. The oocyte cell membrane is surrounded by two layers of cells; the inner layer is the zona pellucida, and the outer layer is the corona radiata. Meiosis II is triggered when these layers are penetrated by a sperm cell, yielding two haploid cells—a mature ovum and another polar body. The first polar body may also undergo meiosis II; either way, the polar bodies rapidly degenerate. The mature ovum is a large cell containing the cytoplasm, RNA, organelles, and nutrients needed by the developing embryo. When the polar bodies are formed, they lack these resources, having given them to the developing oocyte.

Progesterone

Progesterone is a steroid hormone secreted by the corpus luteum during the luteal phase of the menstrual cycle. Progesterone stimulates the development and maintenance of the endometrial walls in preparation for implantation.

SEXUAL REPRODUCTION

Sexual reproduction differs from asexual reproduction in that there are two parents involved and the end result is a genetically unique offspring. Sexual reproduction occurs via the fusion of two gametes, specialized sex cells produced by each parent. Sexual reproduction requires the following: • The production of functional sex cells or gametes by adult organisms • The union of these cells (fertilization or conjugation) to form a zygote • The development of the zygote into another adult, completing the cycle

spina bifida.

Sometimes during development the neural tube does not close properly, resulting in a condition in humans called spina bifida. This opening can be corrected surgically, but most patients have neurological impairments for the rest of their lives. The incidence of spina bifida can be decreased dramatically if the mother takes folic acid supplements during pregnancy.

Spermatogenesis

Spermatogenesis, or sperm production, occurs in the seminiferous tubules. Diploid cells called spermatogonia differentiate into diploid cells called primary spermatocytes, which undergo the first meiotic division to yield two haploid secondary spermatocytes of equal size; the second meiotic division produces four haploid spermatids of equal size. After meiosis the spermatids undergo a series of changes leading to the production of mature sperm, or spermatozoa, which are specialized for transporting the sperm nucleus to the egg, or ovum. The mature sperm is an elongated cell with a head, neck, body, and tail. The head consists almost entirely of the nucleus. The tail (flagellum) propels the sperm, while mitochondria in the neck and body provide energy for locomotion. A caplike structure called the acrosome, derived from the Golgi apparatus, develops over the anterior half of the head. The acrosome contains enzymes needed to penetrate the tough outer covering of the ovum. After a male has reached sexual maturity, approximately 3 million primary spermatocytes begin to undergo spermatogenesis per day, the maturation process taking a total of 65-75 days.

Organogenesis:

The body organs begin to form. In this process, the cells interact, differentiate, change physical shape, proliferate, and migrate.

the neural crest cells.

The cells at the tip of each neural fold are called the neural crest cells. These cells migrate laterally and give rise to many components of the peripheral nervous system, including the sensory ganglia, autonomic ganglia, adrenal medulla, and Schwann cells.

The first complete cleavage of the zygote occurs approximately

The first complete cleavage of the zygote occurs approximately 32 hours after fertilization. The second cleavage occurs after 60 hours, and the third cleavage after approximately 72 hours, at which point the eight-celled embryo reaches the uterus. As cell division continues, a solid ball of embryonic cells, known as the morula, is formed. Blastulation begins when the morula develops a fluid-filled cavity called the blastocoel, which by the fourth day becomes a hollow sphere of cells called the blastula. The blastula is the stage of the embryo that implants in the uterus. Emergency contraception works partially by inhibiting implantation of the blastula in the uterus, which is why it is most effective if taken within 72 hours of the time of potential fertilization.

Follicular phase

The follicular phase begins with the cessation of the menstrual flow from the previous cycle. During this phase, FSH from the anterior pituitary promotes the development of the follicle, which grows and begins secreting estrogen.

Gonads

The gametes are produced in specialized organs called the gonads. The male gonads, called testes, produce sperm in the tightly coiled seminiferous tubules. The female gonads, called ovaries, produce oocytes (eggs). Some species are hermaphrodites, which have both functional male and female gonads. These include the hydra and the earthworm.

Placental development

The growing fetus receives oxygen directly from its mother through a specialized circulatory system. This system not only supplies oxygen and nutrients to the fetus but removes carbon dioxide and metabolic wastes as well. The two components of this system are the placenta and the umbilical cord, which both develop in the first few weeks after fertilization.

The Menstrual Cycle

The hormonal secretions of the ovaries, hypothalamus, and anterior pituitary play important roles in the female reproductive cycle. From puberty through menopause, interactions between these hormones result in a monthly cyclical pattern known as the menstrual cycle. The menstrual cycle may be divided into the follicular phase, ovulation, luteal phase, and menstruation.

Growth:

The organs increase in size, which is a continual process from infancy to childhood to adulthood.

FEMALE REPRODUCTIVE SYSTEM

The ovaries are found in the abdominal cavity below the digestive system. The ovaries consist of thousands of follicles; a follicle is a multilayered sac of cells that contains, nourishes, and protects an immature ovum. It is actually the follicle cells that produce estrogen. Approximately once a month, an immature ovum is released from the ovary into the abdominal cavity and drawn by cilia into the nearby oviduct. The oviducts are also known as fallopian tubes. Each fallopian tube opens into the upper end of a muscular chamber called the uterus, the site of fetal development. The lower, narrow end of the uterus is called the cervix. The cervix connects with the vaginal canal, which is the site of sperm deposition during intercourse and is also the passageway through which a baby is expelled during childbirth.

Female Sex Hormones

The ovaries synthesize and secrete the female sex hormones, including estrogens and progesterone. The secretion of both estrogens and progesterone is regulated by luteinizing hormone (LH) and follicle stimulating hormone (FSH), which, in turn, are regulated by gonadotropin-releasing hormone (GnRH).

The placenta and the umbilical cord are outgrowths of the four extra-embryonic membranes formed during development:

The placenta and the umbilical cord are outgrowths of the four extra-embryonic membranes formed during development: the amnion, chorion, allantois, and yolk sac. The amnion is a thin, tough membrane containing a watery fluid called amniotic fluid. Amniotic fluid acts as a shock absorber of external pressure during gestation and localized pressure from uterine contractions during labor. Placenta formation begins with the chorion, a membrane that completely surrounds the amnion. The chorion assists with transfer of nutrients from the mother to the fetus. A third membrane, the allantois, develops as an outpocketing of the gut. The blood vessels of the allantoic wall enlarge and become the umbilical vessels, which will connect the fetus to the developing placenta. The yolk sac, the site of early development of blood vessels, becomes associated with the umbilical vessels.

Gametogenesis

The production of functional sex cells is called gametogenesis. In males, the process is more specifically called spermatogenesis, or sperm production, and occurs in the seminiferous tubules of the testes. In females, the process is more specifically called oogenesis, or egg production, and occurs in the ovaries. Spermatogenesis and oogenesis will be discussed in more detail in the sections on the male and female reproductive systems, respectively.

Third trimester

The seventh and eighth months are characterized by continued rapid growth and further brain development. During the ninth month, antibodies are transported by highly selective active transport from the mother to the fetus for protection against foreign matter. The growth rate slows and the fetus becomes less active as it has less room to move about.

MALE REPRODUCTIVE SYSTEM

The testes are located in an external pouch called the scrotum that maintains the testes' temperature at 2°C-4°C lower than body temperature, a condition essential for sperm survival. Sperm pass from the testes through the vas deferens to the ejaculatory duct and then to the urethra. The urethra passes through the penis and opens to the outside at its tip. In males, the urethra is a common passageway for both the reproductive and excretory systems. The testes are also the sites of production of testosterone. Testosterone regulates secondary male sex characteristics including facial and pubic hair and voice changes.

Ectoderm:

gives rise to the integument (including the epidermis, hair, nails, and epithelium of the nose, mouth, and anal canal), the lens of the eye, the retina, and the nervous system

Mesoderm:

gives rise to the musculoskeletal system, circulatory system, excretory system, gonads, connective tissue throughout the body, and portions of digestive and respiratory organs