Lernmaterialien für Embryology an der Medical University Varna

The sequence of events in the organism from fertilization to birth

A special form of nuclear division involved in sexual reproduction

•  Golgi phase
• Cap phase
• Acrosomal phase
• Maturation phase

• Preembryonic stage – weeks 1 & 2
• Embryonic stage – weeks 3 ÷ 8
• Fetal stage - months 3 ÷ 9

•  Begins at puberty
• Goes on continuously, without stop and start mechanisms
• It takes about 24 days for a human spermatocyte to complete meiosis
• Very low error rates – 3-4% → if meiosis goes wrong, a a quality control system is activated, which arrests meiosis and leads to apoptosis
• By the end of meiosis, a sperm has only just begun its differentiation

•  Some of the haploid gametes produced > lack a particular chromosome, while others have more than one copy of it
• Cells with the correct number of chromosomes : euploid
• Cells with an abnormal number of chromosomes:  aneuploid  (most embryos die, … but some survive)
• Down syndrome – the leading single cause of mental retardation; caused by an extra copy of chromosome 21
• Klinefelter syndrome – nondisjunction of the XX homologues → extra X chromosome

• Human chorionic gonadotropin (hCG), secreted by the placenta, maintains the corpus luteum for 3 months → corpus luteum of pregnancy
• Grows to a diameter of 5 cm and continues to secrete hormones necessary for the maintenance of pregnancy
•  Placenta becomes the main site of production of the various hormones involved in maintaining pregnancy, but corpus luteum continues to form these hormones for several months

• Each of the 3 germ layers (ectoderm, mesoderm, and endoderm) gives rise to a number of specific tissues and organs
• By the end of the 2nd month, the main organ systems have been established → the major features of the external body form are recognizable