WEEK 11 NOTES ANATOMY

Reproductive and Developmental Biology Study Guide (Week 12)

Mitosis vs. Meiosis

Mitosis

• Diploid → Diploid
• Results in 2 identical daughter cells
• Maintains chromosome number (46 chromosomes)

Meiosis

• Diploid → Haploid
• Results in 4 non-identical daughter cells
• Reduces chromosome number (from 46 to 23 chromosomes)

Female Reproductive Differentiation

• Genetic: XX at conception
• Gonadal: No SRY gene → ovaries form
• Phenotypic: Estrogen and progesterone guide the development of internal and external genitalia

Male Reproductive Differentiation

• Genetic: XY at conception
• Gonadal: SRY gene expressed → testes differentiate (TDF protein)
• Phenotypic: Testosterone promotes the development of male genitalia

Hormonal Regulation of Reproduction

Males

• GnRH: Pulsatile secretion from the hypothalamus stimulates the anterior pituitary
• FSH: Stimulates Sertoli cells which support sperm development and produce ABP and inhibin
• LH: Stimulates Leydig cells to produce testosterone
• Testosterone: Provides negative feedback on GnRH, FSH, and LH

Functions of Testosterone

• Develops reproductive organs via paracrine signaling
• Influences libido (sex drive)
• Induces secondary sexual characteristics:
  • Deep voice
  • Increased hair growth
  • Muscle and bone growth

Spermatogenesis

• Location: Occurs in seminiferous tubules
• Onset: Begins at puberty and continues throughout life
• Stages of Spermatogenesis:
  1. Mitosis: Replenishes spermatogonia
  2. Meiosis: Diploid spermatocytes transform into haploid spermatids
  3. Spermiogenesis: Involves morphological changes resulting in spermatozoa

Cell Types in Spermatogenesis

• Type A Daughter Cell: Remains as a stem cell

• Type B Daughter Cell: Develops into a primary spermatocyte

• Pathway: 1 primary spermatocyte → 2 secondary spermatocytes → 4 spermatids → mature sperm

Anatomy of Mature Sperm

• Head: Contains DNA + acrosome (enzymes to penetrate egg)
• Midpiece: Houses mitochondria for ATP production
• Tail: Flagellum facilitating movement

Developmental Aspects: Female vs. Male

• Female Development:
  • Breast development begins around age 11
  • Menarche (first period) occurs, followed by menopause (cessation of menstruation)
• Male Development:
  • Sperm development continues throughout life without a defined end.

Sperm Pathway

• Pathway: Testes → Epididymis → Ductus deferens → Ejaculatory duct → Urethra → Vagina → Uterus → Fallopian tube

Fertilization

• Union of oocyte (n) + sperm (n) forms a zygote (2n = 46 chromosomes)
• Oocyte viability: 12–24 hours post-ovulation
• Sperm viability: 2–5 days after ejaculation

Steps of Fertilization

1. Capacitation (~7–10 hrs)
2. Acrosomal reaction (enzymes penetrate the zona pellucida)
3. Sperm-egg membrane fusion
4. Cortical reaction prevents polyspermy
5. Zygote formation

Early Development: Pre-Embryonic Stage

Cleavage (Days 2–4)

• Rapid cell division occurs with no size change
• Development stages:
  • Zygote → Morula → Blastocyst → Gastrula

Cell Potency

• Totipotent cells: Up to the 8-cell stage (can form any cell type + placenta)
• Pluripotent cells: Inner mass cells (can form any body cell)
• Multipotent/unipotent cells: Found in adults

Blastocyst and Implantation (Days 6+)

• Trophoblast: Develops into chorion and amnion (support structures)
• hCG hormone secretion: Maintains corpus luteum
• Formation of cavities:
  • Amniotic cavity (epiblast)
  • Yolk sac (hypoblast)

Gastrulation (Day 15+)

• Formation of primitive streak
• Establishes body axes (bilateral symmetry)
• Formation of 3 germ layers:
  • Ectoderm: Skin, nervous system
  • Mesoderm: Muscle, skeleton, cardiovascular system
  • Endoderm: Gut lining, respiratory system

Human Development Stages

Development After Implantation

• Chorionic villi + uterine cells = placenta
• Amnion: Fluid-filled sac
• Umbilical cord: Connects embryo to placenta

Organogenesis (Weeks 4–8)

• Rudimentary organs develop
• Heart starts beating at ~4th week
• Limb movements begin by 6th week

Sex Determination

• Occurs between weeks 7–10

Trimesters

Premature Birth

Placenta Functions

• Nutrient and oxygen delivery
• Waste removal
• Endocrine function:
  • Secretes estrogen, progesterone, hCG
  • Takes over from the corpus luteum by the end of the 2nd month

Hormonal Changes During Pregnancy

• Progesterone: Plateaus then decreases before labor
• Estrogen: Increases steadily; ratio changes trigger contractions

Labor: Three Stages

1. Dilation: Cervix dilates (10 cm), contractions increase
2. Expulsion: Baby passes through cervix and vagina
3. Placental Stage: Delivery of placenta (~15 minutes post-birth)

Mammary Glands and Lactation

• Modified sweat glands present in both sexes
• Pathway: Alveoli → Lactiferous ducts → Nipple

Hormonal Regulation of Lactation

• Placental Estrogens and Progesterone: Stimulate prolactin release
• Prolactin: Induces milk production
• Oxytocin: Facilitates milk ejection ("let-down reflex")

Benefits of Lactation

For the Child:

• Better absorption of fats and iron
• Provides immunity (IgA, lysozymes, etc.)
• Enhances brain development (DHA)
• Reduces risk of infections and diseases

For the Mother:

• Faster uterine involution
• Lower risk of osteoporosis, obesity, and cancer
• Reduces postpartum depression
• Strengthens the mother-child bond