Embryology an der Medical University Varna

The sequence of events in the organism from fertilization to birth

General embryology

• Gametogenesis: conversion of germ cells into male and female gametes
• 1st week of development: ovulation to implantation
• 2nd week of development: bilaminar germ disc
• 3rd week of development: trilaminar germ disc 
• 3rd to 8th week: embryonic period
• 3rd month to birth: fetus and placenta

 Special embryology

• Skeletal system 
• Cardiovascular system
• Respiratory system
• Nervous system, etc…. systems

A special form of nuclear division involved in sexual reproduction

• Meoisis - a diploid precursor cell gives rise to haploid progeny cells
•  Mitosis - a diploid precursor cell gives rise to diploid progeny cells;

 In vertebrates, only the diploid cells proliferate: the haploid gametes exist only briefly, do not divide at all, and are highly specialized for sexual fusion

• Each individual gamete contains either maternal or paternal version of each chromosome

→ the choice of maternal or paternal occurs independently and randomly for each pair of homologous chromosomes

• Crossing-over :  the maternal and paternal versions of each chromosome undergo genetic recombination during meiosis
• Sexual reproduction gives organisms a competitive advantage

•  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

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

•  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

• From ~300,000 primordial follicles at puberty, some start the process of follicular growth
• Oocyte diameter up to 120 µm;
• enlarged nucleus, mitochondria increase in number; RER hypertrophies, Golgi migrates beneath the cell surface

• 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

• Begins during embryonic life (1st month)
• Stop and start mechanisms

- arrested after diplotene of meiosis I

→ meiosis I is complete after puberty

> meiosis I is completed at the time of ovulation

> meiosis II is completed only if the released oocyte is fertilized

•  It takes nearly 40 years for some human oocytes to complete meiosis
• High error rates – 20% of aneuploidy → most of these result from nondisjunction in oocytes at meiosis I
• If homologous chromosome segregation fails to occur normally, the cells continue through meiosis and produce aneuploid eggs
• By the end of meiosis, a mammalian oocyte is fully mature

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