Biology Chapter: Cell Division and Karyotypes

Karyotype

• A karyotype is an image displaying an individual's chromosomes lined up in pairs
• The 22 autosomes are numbered by size, and the sex chromosomes are X and Y
• Karyotypes are used to analyze the number and structure of chromosomes for abnormalities

Chromosomes

• Chromosomes are thread-like structures found in the nucleus of animal and plant cells
• Chromosomes are made of protein and DNA
• DNA contains the instructions that make each organism unique
• Each chromosome is made up of two chromatids connected by a centromere

Mitosis

• Mitosis is a type of cell division where one cell divides into two identical daughter cells
• Mitosis is essential for growth and replacing worn-out cells

Meiosis

• Meiosis is a process where a single cell divides twice to produce four cells with half the original amount of genetic information
• Meiosis produces our sex cells (sperm in males, eggs in females)
• These cells are called gametes and are haploid, meaning they have half the number of chromosomes as the parent cell

Spermatogenesis

• Spermatogenesis is the process of producing sperm from primitive sex cells called spermatogonia
• Spermatogenesis is a continuous process from puberty to old age
• Spermatogonia are the most primitive sex cells and divide by mitosis
• Type B spermatogonia mature into primary spermatocytes, which have 46 chromosomes
• Primary spermatocytes undergo meiosis I, resulting in two secondary spermatocytes (23 chromosomes each)
• Secondary spermatocytes undergo meiosis II, producing two spermatids (23 chromosomes each)
• Spermiogenesis is the transformation of spermatids into mature sperm by Sertoli cells

Oogenesis

• Oogenesis is the process of producing mature eggs (ova) from primitive sex cells called oogonia
• Oogenesis is a cyclic process occurring once every month, unlike the continuous process of spermatogenesis
• Oogonia are present in the ovary early in intrauterine life
• Most oogonia degenerate, and the surviving ones become primary oocytes
• Primary oocytes are arrested in the prophase stage of meiosis I until sexual maturity
• Each primary oocyte is surrounded by a layer of flat follicular cells, forming a primordial follicle
• Primordial follicles remain dormant until puberty when they start to mature
• The total number of primary oocytes decreases from about 1,000,000 at birth to roughly 40,000 at puberty
• Oogenesis takes place in the cortex of the ovary, and only one ovarian follicle matures each month

Steps of Oogenesis

• Maturation of the primordial follicle:
  • The primary oocyte undergoes meiosis I, resulting in a secondary oocyte (23 chromosomes) and a polar body (23 chromosomes)
  • The follicular cells surrounding the oocyte divide mitotically, forming multiple layers called granulosa cells
  • The granulosa cells are separated from the oocyte by a thick hyaline membrane called the zona pellucida
  • Cavities filled with fluid appear between granulosa cells, forming a single cavity called the follicular antrum
  • The antrum separates the granulosa cells into two layers: the stratum granulosum (outer layer) and the inner layer, which surrounds the secondary oocyte as the cumulus oophorus
• Formation of the mature (Graafian) follicle:
  • Pressure increases inside the antrum
  • A sheath of stroma cells forms around the follicle, called the theca follicle
  • The theca is divided into the theca externa (outer layer) and theca interna (inner layer)
  • The theca interna is the main source of estrogen hormone
• Rupture of Graafian follicle (ovulation):
  • The mature Graafian follicle ruptures through the thin and ischemic cortex of the ovary
  • This occurs around 14 days before the following menstrual cycle
  • The secondary oocyte, surrounded by the zona pellucida and corona radiata, is discharged from the ovary and enters the uterine tube
  • The rupture is facilitated by factors such as the increased pressure inside the antrum and the action of hormones

Phases of the Menstrual Cycle

• Follicular phase:
  • Starts on the first day of menstrual flow
  • Follicle-stimulating hormone (FSH) from the pituitary gland initiates primordial follicle maturation into the Graafian follicle
  • Estrogen hormone is secreted by the theca interna cells of the Graafian follicle
• Ovulation Day 14:
  • The Graafian follicle ruptures, and the ovum is released into the uterine tube
  • The wall of the Graafian follicle transforms into the corpus luteum
• Luteal phase:
  • Formation of the corpus luteum under the influence of luteinizing hormone (LH) from the pituitary gland
  • Progesterone hormone levels increase, while estrogen levels decrease
  • Estrogen and progesterone suppress FSH secretion, preventing further follicle maturation within the same cycle
  • They also induce proliferation and glandular activity in the endometrium of the uterus
  • At the end of the ovarian cycle, the corpus luteum degenerates, leading to decreased estrogen and progesterone levels
  • This allows FSH release, initiating a new ovarian cycle
  • If fertilization occurs, the corpus luteum remains active for a longer period, and the ovarian cycles stop until after childbirth

Menstrual Cycle

• The menstrual cycle is a series of changes in the endometrium of the uterus, occurring monthly under the influence of ovarian hormones

• It lasts for 28 days, beginning with menstrual blood flow

• It consists of three phases:

  • Menstrual flow phase:
    • Menstrual blood flow lasts for approximately 3-5 days
    • The blood flow is about 50 cc and contains:
      • Blood
      • Cells from the endometrium of the uterus
      • Mucus
      • Tissue fluids