Animal Reproduction and Development

• Development - Overlaps with reproduction. It involves the formation fo sex cells, the growth of the zygote, and other stages during one's life span until it terminates in death.
• Gametogenesis - Process of producing gametes, one egg in females, and four sperms in males.
• Fertilization - Sperm when a sperm and an egg fuse into a single-celled, diploid zygote. A human zygote is a potent stem cell that can differentiate into any kind of cell called totipotent cell.
• Cleavage Stage - A series of mitosis forms a multicellular blastula.
• Gastrulation - During this stage, cells migrate to create the gastrula, which in vertebrates has three distinct germ layers, namely: endoderm, mesoderm, and ectoderm.
• Organogenesis - During this period, the germ layers differentiate into specific organ systems, followed by continuous growth and cell division.
• Endoderm (Internal Layer)

  • Lung cells (alveolar cell)
  • Thyroid cells
  • Digestive cells (pancreatic cell)
• Mesoderm (Middle Layer)

  • Cardiac muscle cells
  • Skeletal muscle cells
  • Tubule cells of the kidney
  • Red blood cells
  • Smooth muscle cells (in gut)
• Ectoderm (External Layer)

  • Skin cells of epidermis
  • Neuron on brain
  • Pigment cells
• Monozygotic - Zygote splits up during the first cleavage stage, resulting in identical twins. It also shares placenta.
• Dizygotic - If two or more fertilization events result in two zygotes rising at the same time, it is said to be fraternal twins. It has separate placentas.
• Sperm cells are produced in the testis, temporarily stored in the epididymis supported by the scrotum, and pass through the vas deferens before reaching the urethra, a passage for both urine and semen, that terminates in the opening at the tip of the penis.

  
• The seminal vesicle creates a part of the semen that counteracts the acidity of the vagina to protect the sperm, while the prostate adds enzymes, lipids, etc. to the semen. Bulbourethral glands secrete the pre-ejaculate to lubricate and clear urine before semen passes.

  
• Part of the Male Reproductive System

  • Testis
  • Epididymis
  • Scrotum
  • Vas Deferens
  • Seminal Vesicle
  • Prostate
  • Bulbourethral Gland
  • Urethra
• The homologue of the penis is the clitoris, near the external opening which is the labia. The vagina is the main entrance to the female reproductive tract and ends in the cervix, a small passage to the uterus.

  
• Ovaries make eggs which are released in ovulation and must be caught by the fallopian tube, the site of fertilization. A zygote grows to a solid blastula, called morula, then moved as a hollow blastula, or blastocyst, for implantation in the endometrial lining.
• Parts of the Female Reprductive System

  • Clitoris
  • Labia
  • Vagina
  • Cervix
  • Uterus
  • Ovary
  • Fallopian Tube
• Implantation - Signals the start of pregnancy, as the blastocyst grows into an embryo. After two months, the embryo becomes a fetus.
• Gestation - Stage of carrying the embryo/fetus inside the uterus. In humans, it takes nine months.
• Menstrual Cycle - Lasts for about 28 days in the ovary and uterus. In the ovary, it consists of the follicular phase and luteal phase. In the uterus, it consists of the menstrual phase, proliferative phase, and secretory phase.
• Follicular Phase - Follicles in the ovary mature.
• Luteal Phase - Matured oocytes (with egg) are released from follicles.
• Menstrual Phase - The "period" phase as the endometrium degenerates and sheds off.
• Proliferative Phase - When endometrium heals.
• Secretory Phase - Corresponds to the luteal phase to prepare for implantation.
• Contraception - Process blocking any of these stages of reproduction:

  • Release and transport of gametes
  • Fertilization
  • Implantation
  • Actual completion of development of the embryo/fetus
• Ovulation-Supressing Methods - Some contraceptives prevent the oocyte, or egg, to mature. Oral contraceptives like birth control pills prevent them from ovulating.
• Barrier and Surgical Methods Prevent Fertilization - To prevent fertilization, a condom (penis), a diaphragm (cervix), or a cervical cap (cervix) are forms of barriers. There are also chemical means such as spermicidal cream (vaginal canal), killing sperm on contact, or surgical, vasectomy for men or tubal ligation for women.
• Implantation-Suppressing Methods - Some contraceptives block the implantation of the blastocyst in the uterine wall, like the intra-uterine device (IUD), or prevent implantatior by preventing the action of hormone that prepare the uterus to receive the embryo, like the morning-after pill.
• Behavioral Methods

  • Abortion
  • Coitus Interruptus
  • Abstinence
• Abortion - Deliberate removal of the embryo or fetus before it completes gestation.
• Coitus Interruptus - Known as withdrawal method, is ending intercourse before ejaculation.
• Abstinence - The practice of not engaging in sexual intercourse.