BioDigital Systems: Availble for free (individual) or for a fee (groups/businesses), this interactive system will first require you to sign in via your Facebook or Google account to gain access. Once inside the system, you can zoom and rotate your virtual skeleton. Eleven systems in total are able to view and examine. Note: this site allows you to repeatedly quiz yourself on all eleven systems that are covered within the site.
Anatomy Drill & Practice: This site covers the human body, the chemical, cellular, and tissue levels of organization, the integumentary system, skeletal system, muscular system, nervous system, cardiovascular system, respiratory system, digestive system, excretory system, and reproductive system.
This site not only includes images of and information on the above listed systems, but it also includes interactive drills and practice questions for students. NOTE: Flash required for the quizzes/practice questions.
The AK Lectures are a series of lectures from a (external) educational platform designed to "promote collaboration between our users and help spread knowledge to every part of the world."
These lectures vary in length, and will open in a new window when you click on the provided link.
Spermatogenesis: Spermatogenesis is the process by which sperm cells are produced in the seminiferous tubules of the testes (male gonads). Deep inside the wall of the seminiferous tubules are the diploid stem cells called spermatogonium. When Leydig cells release testosterone, this hormone goes on to stimulate spermatogonium to differentiate into the primary spermatocyte. The primary spermatocyte can then undergo meiosis I to produce two haploid cells called secondary spermatocytes. Each of these secondary spermatocytes then undergoes meiosis II to produce a total of four spermatids. With the help of Sertoli cells, the haploid spermatids then differentiate into sperm cells. Sertoli cells function to produce nutrients to the developing sperm cells as well as remove the cytoplasm. Once the sperm cells are differentiated, they swim up to the epididymus, where they mature and are stored until their release.
Oogenesis: The female gonads are called the ovaries and inside the ovaries oogenesis takes places. Oogenesis is the production of the female sex gametes called egg cells or ova (ovum for singular). During fetal development and before the birth of the female individual, all the oogonia (stem cells) differentiate into primary oocytes. Following birth, these primary oocytes remain in prophase I of meiosis until that individual reaches puberty and begins the menstrual cycle. Inside the ovaries, the primary oocyte does not exist by itself. Instead it is found inside a structure called an ovarian follicle. A follicle is a fluid-filled structure that contains the developing oocyte along with additional cells such as theca cells and granulosa cells that assist in the maturation process. During the menstrual cycle, the primary oocyte within the follicular structure undergoes meiosis I to produce the secondary oocyte. The follicle carrying this secondary oocyte is now called the secondary follicle and it eventually ruptures during ovulation and releases the secondary oocyte into the peritoneal cavity and from their it moves into the fallopian tube. This secondary oocyte is arrested in metaphase II of meiosis II and will only undergo the rest of meiosis if fertilization takes place. The remaining portion of what used to be the secondary follicle remains inside the ovaries and eventually becomes the corpus luteum. This functions as an endocrine gland during the menstrual cycle and pregnancy.
Fertilization: Fertilization is the process by which the sperm cell combines with the egg cell (also called the ovum) to form the zygote. This process (1) restores the diploid number of chromosomes and (2) stimulates metabolic processes such as protein synthesis that initiates embryological development. When the sperm cell contacts the egg cell, it first contacts a region of the egg cell called the zona pellucida (a glycoprotein layer found outside the plasma membrane). The tip of the sperm cell contains a structure called the acrosome that releases digestive enzymes which drill a hole in the zona pellucida. During this process, the plasma membrane of the egg cell depolarizes and changes polarity. The influx of calcium ions into the cell causes the release of cortical granules into the zone pellucida. This process reinforces the zona pellucida by changing its composition and hardening the glycoprotein membrane. The hardened membrane is called the fertilization membrane and it prevents and other sperm cells from entering this egg. This process is called the cortical reaction. Once the sperm cell reaches the plasma membrane of the egg cell, the membrane of the sperm fuses with the membrane of the egg and the sperm nucleus makes its way into the egg. The two nuclei then fuse, thereby restoring the diploid number of the organism and initiating embryological development of the zygote.
Anatomy of Reproductive Organs: The male gonads are called testes and they are enclosed in a sac-like structure called the scrotum. The scrotum functions to maintain a slightly lower temperature than the core temperature. This is done to ensure that the enzymes involved in spermatogenesis function effectively and efficiently. Inside the testes are the seminiferous tubules where sperm cells are actually formed. After being formed, the sperm cells migrate to the highly-convoluted tubule region called the epididymis. Inside the epididymis, the sperm cells mature and are stored until ejaculation. During ejaculation, the sperm cells move out of the epididymis and into the vas deferens. The vas deferens empties into the ejaculatory duct. An accessory gland called the seminal vesicle produces a fluid that contains nutrients such as fructose that is needed for the survival of the sperm cells. This fluid mixes with the sperm cells and it moves into the urethra. Two other glands found in close proximity are the prostate gland and the bulbourethral gland. They produce an alkaline (basic) fluid that functions to decrease the acidity of the vaginal tract and make it more hospitable for the sperm cells. The mixture of sperm cells and fluid is called semen and it now makes its way out of the urethra and to the outside environment. The female gonads are called the ovaries and inside the ovaries oogenesis takes place. During ovulation, one secondary oocyte is released into the fallopian tube. The fallopian tube (also called the uterine tube or oviduct) contains smooth muscle and cilia. The contraction of these smooth muscles (peristalsis) and the wave-like motion of the cilia help move the secondary oocyte towards the uterus. If sperm cells are present in the fallopian tube, fertilization can take place to form the zygote. The zygote will eventually make its way to the uterus, where it will implant itself into the endometrium. The endometrium is a mucous membrane that provides nutrients to the growing zygote. If no fertilization occurs, then the endometrium along with the secondary follicle will slough off and exit the uterus via the cervix and enter the vaginal tract. From their, it will exit the body altogether (a process known as menstruation).
Anatomy and Function of Female Breasts: The female breasts are an important organ involved not only in the human reproductive cycle but also in the development of the newborn following child birth. Female breasts contain mammary glands, which consists of lobules of grandular tissue as well as milk ducts. The lobules are grape-like structures (also called alveoli) that contain specialized gland cells that produce and secrete milk. The ducts carry that milk out of tiny holes located in the nipple. The primary function of the female breasts is to produce milk via a process called lactation. Milk provides the infant with nutrition as well as boosts their immunity. During pregnancy, estrogen and progesterone hormones are released at first by the corpus luteum and eventually by the placenta. They increase the size of the mammary glands and thereby enlarge the breasts. Before child birth and during the first few days after birth, the mammary glands produce and secrete a yellowish substance called colostrum. Colostrum, also called first milk, contains a high concentration of proteins and lactate but contains a low amount of fat. On about the third day after childbirth, the anterior pituitary gland releases a hormone called prolactin. Prolactin stimulates the mammary glands to begin producing milk, which unlike colostrum contains a high concentration of fat and carbohydrates. Both colostrum and milk contain antibodies that help boost the immune system of the infant (give the infant passive immunity against invading pathogens). So how exactly is the milk released from the mammary glands ? During the process of suckling, nerve cells in the nipple create an electrical signal that travels to the hypothalamus of the mother's brain. The hypothalamus then signals the posterior pituitary gland to secrete a hormone called oxytocin. Oxytocin travels through the blood and to the breasts, where it stimulates the muscles around the lobules to contract. As the muscles contract, they squeeze and force out the milk out of the gland cells and into the milk ducts. The milk eventually makes it way out of the tiny holes in the nipple.
The Menstrual Cycle: The menstrual cycle is a series of events that takes place in women who have reached puberty. It lasts approximately 28 days and its purpose is to prepare the woman for a possible pregnancy. If fertilization does not take place during this period, then the thickened endometrium will break down and slough off along with the secondary oocyte in a process called menstruation (characterized by bleeding and cramps). The menstrual cycle can be broken down into three stages - the follicular phase, ovulation and the luteal phase. During the follicular phase, the hypothalamus releases the gonadotropin releasing hormone (GnRH) that moves down to the anterior pituitary gland and stimulates it to release the luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Both FSH and LH move down to the ovaries and stimulate the development of the immature follicle into the secondary follicle. As the follicle develops, granulosa cells in the follicle begin to release estrogens, which initiates the thickening of the endometrium. The rise in estrogen also causes a sharp rise in LH (LH surge) as well as rise in FSH. This in turn causes ovulation to take place. During ovulation, the secondary follicle ruptures, thereby releasing the secondary oocyte into the peritoneal cavity and then into the fallopian tube. The LH rise also causes the remaining portion of the follicle inside the ovary to develop into the corpus luteum. The corpus luteum begins producing estrogen but also produces another hormone called progesterone. Progesterone maintains the thickening of the endometrium, inhibits the uterus from contracting and inhibits another follicle from maturing. The rise in progesterone and estrogen creates a negative feedback loop that causes a decrease in concentration of GnRH and therefore a decrease in the levels of LH and FSH. Since the LH is needed to maintain the corpus luteum, the decrease in LH causes the corpus luteum to deteriorate into the corpus albicans, which stops releasing progesterone. Less progesterone means that the endometrium will stop thickening and will begin to break down, initiating the process of menstruation. As the concentration of progesterone and estrogen fall, the cycle restarts itself and repeats as described above. This process will continue until the woman reaches menopause.
Hormones in Menstrual Cycle: The menstrual cycle can be broken down into the pre-ovulatory phase and the post-ovulatory phase. During the pre-ovulatory phase (before ovulation takes place), the gonadotropin releasing hormone (GnRH) stimulates the anterior pituitary gland to release the luteinizing hormone (LH) and the follicle stimulating hormone (FSH). LH stimulates theca cells in the immature follicle to differentiate and proliferate. Theca cells are responsible for producing androgens and releasing those androgens to another type of cell called granulosa cell. FSH on the other hand stimulates the proliferation of granulosa cells, which use the androgens to form estrogens. Estrogens initiate the thickening of the endometrium and create a positive feedback loop on GnRH, FSH and LH. This in turn causes the LH surge (as well as a rise in FSH), which leads to the process of ovulation. During the post-ovulatory phase (after ovulation takes place), the LH causes the formation of the corpus luteum, which begins producing estrogen as well as progesterone. Progesterone inhibits the contraction of the uterus, inhibits another follicle from maturing and maintains the thickening of the endometrium. Both progesterone and estrogen now create a negative feedback loop on GnRH, LH and FSH. This means that a rise in progesterone causes less LH to be produced and this is precisely why the corpus luteum begins to degenerate into the corpus albicans (remember LH is needed to form and maintain the corpus luteum). Therefore, the corpus luteum eventually breaks down and stops releasing progesterone and estrogen. As progesterone and estrogen levels fall, the endometrium can no longer be maintained and begins to break down in a process called menstruation. A fall in progesterone also causes uterine contractions, which is what women experiencing during menstrual cramps.
Asexual Reproduction (Budding, Fission, Regeneration and Parthenogenesis): Asexual reproduction is the process by which an organism is produced from a single parent cell. There are four major forms of asexual reproduction - budding, binary fission, regeneration and parthenogenesis. Budding is characterized by the formation of a daughter cell that has the same genetic information but is much smaller in size. Yeast cells (unicellular eukaryotes) and hydra (multicellular eukaryotes) are two organisms that undergo budding. Binary fission, a form of reproduction that bacterial cells undergo, is the process by which the cell divides into two equal daughter cells that have identical genetic information. Regeneration is a type of asexual reproduction in which the organism is capable of regrowing certain body parts. Regeneration occurs via mitosis. Lizards can regenerate their tails, star fish can regenerate their arms while humans have the ability to regenerate their liver to a certain extent. Parthenogenesis is the process by which an unfertilized egg transforms into a fully function organism. Since the egg is haploid, it produces organisms which are also haploid. In some cases, the organism can regain its diploid number of chromosomes. Ants and bees are two examples of organisms that undergo parthenogenesis.