Development

1
Development

• Chapter 29

2
Human Development

• Fertilization and Pre-embryonic Stage
• Embryonic and Fetal Stages
• Neonate

3
Sperm Migration

• Few sperm get to egg
• destroyed by vaginal acid
• stuck in cervical mucus
• go wrong way
• Assisted by female physiology
• strands of cervical mucus
• uterine contractions
• chemical attraction

4
Capacitation

• Spermatozoa reach uterine tube in 10 min
• Capacitation
• takes 10 hours
• female fluids wash away inhibitory factors
• Sperm fertile for 48 hours
• Conception optimal if deposited 48 hrs before
  ovulation to 14 hrs after

5
Fertilization

• Acrosome
• releases enzymes
• penetrates granulosa cells, zona pellucida
  surrounding egg
• membranes of gametes fuse and sperm enters
• Prevention of polyspermy
• fertilization membrane

6
Fertilization

• Secondary oocyte completes meiosis if fertilized
• sperm and egg pronuclei
• Chromosomes of 2 gametes mix
• Fertilized egg zygote

7
Pre-embryonic Stage - First 2 Weeks

• Cleavage
• mitotic divisions
• 30 hrs 2 cell stage (blastomeres)
• 72 hrs morula stage (solid ball of cells)
• Morula enters uterine cavity 6-7 days
• nourished by endometrial secretion (uterine milk)
• Zona pellucida disintegrates releases blastocyst
• outer cells (trophoblast) helps to form placenta
• inner cell mass (embryoblast) develops into embryo

8
Implantation of Blastocyst

• Attaches to uterine wall 6 days after ovulation
• Syncytiotrophoblast - multinucleate mass grows
  roots and digests its way into endometrium
• secretes human chorionic gonadotropin (HCG)
• becomes chorion
• Endometrium completely encloses embryo

9
Ectopic Pregnancy

• Blastocyst implants outside uterus
• 1 out of 300 pregnancies
• most cases in uterine tube (tubal pregnancy)
• tubal obstruction from previous pelvic
  inflammations, repeated abortions or tubal
  surgery
• Tube ruptures by 12 weeks
• conceptus may reimplant in abdominopelvic cavity
• Where ever adequate blood supply
• usually requires an abortion
• 9 of abdominal pregnancies result in live birth
  by cesarian section

10
Embryonic Stage or Weeks 2 to 9

• Begins when all 3 primary germ layers present
• Embryo receives nutrients from placenta
• Germ layers differentiate into organs and organ
  systems
• presence of organs marks the beginning of fetal
  stage

11
Placentation

• Placenta forms 11 days to 12 weeks
• Chorionic villi
• extensions into endometrium
• digestion and growth of roots of tissue
• Placental sinus
• pools of maternal blood surround villi
• stimulates growth of chorionic villi

12
Organogenesis

• Formation of organs from primary germ layers
• 8 weeks, all organs present in 3 cm long fetus
• heart is beating and muscles contract
• Derivatives of ectoderm
• epidermis, nervous system, lens and cornea, inner
  ear
• Derivatives of mesoderm
• skeleton, muscle, cartilage, blood, lymphoid
  tissue, gonads and ducts, kidneys and ureters
• Derivatives of endoderm
• gut and respiratory epithelium and glands,
  bladder and urethra

13
Premature Infants

• Infants born weighing under 5.5 lb.
• Infants born before 7 months suffer from
• respiratory distress syndrome
• insufficient surfactant ? alveolar collapse
• thermoregulatory problems
• digestive system not well developed low-fat
  formula instead of breast milk
• immature liver fails to synthesize plasma
  proteins
• edema, deficiency of clotting and jaundice from
  bile

14
Nondisjunction and Aneuploidy

• Nondisjunction of sex chromosomes
• Triplo-X syndrome (XXX)
• infertile female with mild intellectual
  impairment
• Klinefelter syndrome (XXY)
• sterile males with average intelligence
  (undeveloped testes)
• Turner syndrome (XO)
• sterile, webbed neck, female with no 2nd sexual
  features
• Nondisjunction of autosomes often lethal
• Most survivable type is Down syndrome
  (trisomy-21)
• short stature, flat face with epicanthal folds on
  eyes, enlarged tongue, stubby fingers and mental
  retardation
• Occurs in proportion to age of mother