HISTOLOGY ( first week of human development)

• Human development begins at fertilization when a sperm fuses with an oocyte to form a single cell, the zygote
• The epididymis is an elongated coiled duct that is continuous with the ductus deferens, which transports sperms to the urethra
• At ovulation, the nucleus of the secondary oocyte begins the second meiotic division, but it progresses only to metaphase when division is arrested
• Many genes and molecular factors are implicated in spermatogenesis, with the Y chromosome being essential for normal sperm production
• Proteins of the Bcl-2 family are involved in the maturation of germ cells and their survival at different stages
• HOX genes influence microtubule dynamics and the shaping of the head of the sperm and formation of the tail
• The oocyte is a massive cell compared to the sperm and is immotile, while the sperm is highly motile
• The oocyte is surrounded by the zona pellucida and a layer of follicular cells, the corona radiata
• Beginning during puberty, usually one ovarian follicle matures each month and ovulation occurs, except when oral contraceptives are used
• The uterus consists of the body, fundus, and cervix, with the walls containing perimetrium, myometrium, and endometrium layers
• The ovaries are almond-shaped reproductive glands producing oocytes and hormones responsible for female characteristics
• LH serves as the "trigger" for ovulation and stimulates the follicular cells and corpus luteum to produce progesterone
• A gonadotropin-releasing hormone is synthesized by neurosecretory cells in the hypothalamus
• Development of an ovarian follicle is characterized by growth and differentiation of a primary oocyte, proliferation of follicular cells, formation of the zona pellucida, and development of the theca folliculi
• Thecal cells produce an angiogenesis factor that promotes growth of blood vessels in the theca interna
• Growing follicles produce estrogen, a hormone that regulates development and function of the reproductive organs
• Ovulation is triggered by a surge of LH production
• The expelled secondary oocyte is surrounded by the zona pellucida and one or more layers of follicular cells, forming the oocyte - cumulus complex
• The LH surge seems to induce resumption of the first meiotic division of the primary oocyte
• Mature ovarian follicles contain secondary oocytes
• The zona pellucida is composed of three glycoproteins (ZPA, ZPB, and ZPC)
• Corpus luteum develops shortly after ovulation under LH influence
• Corpus luteum secretes progesterone and some estrogen
• Ovarian cycles terminate at menopause, usually between the ages of 48 and 55 years
• The average menstrual cycle is 28 days, with variations in length
• Ischemia occurs when the oocyte is not fertilized, leading to changes in the endometrium
• Luteal phase coincides with the formation and growth of the corpus luteum, stimulating glandular epithelium to secrete glycogen-rich material
• Transportation of Gametes
• Maturation of Sperms
• Viability of Gametes
• Male Fertility
• Sperms account for less than 10% of semen

  • Normal males have over 100 million sperms per milliliter of semen
  • Men with fewer than 10 million sperms per milliliter of semen are less likely to be fertile
  • Male infertility may result from various factors such as low sperm count, poor sperm motility, medications, and obstruction of genital ducts
• Vasectomy
• Vasectomy is the most effective method of permanent contraception in men

  • Reversal of vasectomy is technically feasible but success rates vary
• Dispermy and Triploidy
• Defects at any stage in the sequence of events may cause the death of the zygote
• The fertilization process takes approximately 24 hours
• Spontaneous abortions occur due to abnormalities, representing a natural screening of embryos to remove abnormal conceptuses that could not develop normally
• The syncytiotrophoblast at the embryonic pole differentiates into an outer syncytiotrophoblast and an inner cytotrophoblast