Lesson # 12

1. Lesson # 12 Early Development

2. Oogenesis Between the third and seventh month of fetal life: Oogonia Oogonia undergo mitosis and produce primary oocytes (diploid). OOGENESIS Diploid MITOSIS Primary oocytes (diploid) begin MEIOSIS I but it is stopped in prophase I. First Primary Second Secondary Secondary Diploid At birth: oocyte polar polar oocyte oocyte Primordial follicles MEIOSIS I Primary oocytes (diploid) in prophase I of MEIOSIS I. Before birth It stops in It stops in metaphase prophase At puberty: After puberty Primary follicles MEIOSIS II MEIOSIS I FSH triggers the start of the ovarian cycle. Completed Completed Secondary follicles MEIOSIS I is completed to form one secondary oocyte (haploid)and the first polar body. Haploid Ovulation Tertiary follicles body body MEIOSIS II Before ovulation During reproductive life: After ovulation Every month one secondary oocyte begins MEIOSIS II that is stopped in metaphase II. If fertilization Haploid Ovulation occurs, and if the secondary oocyte is fertilized, MEIOSIS II is completed to form the ovum and the second polar body. occurs

3. Oogenesis The follicle is transformed in the corpus luteum (yellow body), which for a time produces progesterone that facilitate pregnancy 3 6 7 1 2 5 4 If fertilization does not occur, the corpus degenerates into the corpus albicans Ovulation Corpus luteum Corpus albicans Primordial follicles Secondary follicle Primary oocytes (diploid) joint with nurturing complexes of follicle cells FSH Tertiary or Mature Graffian follicle Primary follicle The mature follicle contains the secondary oocyte (in metaphase II), which is expelled at the end of the process and then begins the journey through the uterine tubes One of the primordial follicles matures (once every 28 days) to primary follicle. Meiosis I is completed (haploid secondary oocytes)

4. Once it has been released, the oocyte begins a slow journey to the uterus through the uterine tubes (fallopian tubes) Ovulation While within the tube, the oocyte may encounter sperm and become fertilized prior to entering the uterus.

5. Oocyte Activation 1 2 Rejected sperm Fertilization membrane • It prevents the polyspermy (fertilization by two or more sperm). Cortical reaction It is the exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg Acrosomal reaction • Two acrosomal enzymes are released: • Hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together. Cortical granules • Acrosin, a protease similar to trypsin. • When a path has been cleared, a sperm binds to the zona pellucida.

6. Fertilization and Oocyte Activation

9. Amphimixis: It is the process by which the male and female pronucleus fuse to form a zygote that contains 46 chromosomes. The formation of the zygote is the “moment of conception”. Cleavage: It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells. The final product of the cleavage is the formation of the morula.

11. The Preembryogenic Stage 1- Cleavage 2- Implantation It comprises the first 16 days of development, culminating with the existence of an embryo. 3- Placentation 3- Embryogenesis It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells. 1-Cleavage: Advanced Morula DAY 4 It is a solid ball of 16 cells that resemble a mulberry. Blastomeres Zygote • The morula lies free in uterine cavity for 4-5 days and divides into a 100 cells or so. 2-celled stage DAY 1 4-celled stage DAY 2 Early morula DAY 3 Egg pronucleus Hatching Blastocyst DAY 6 Sperm pronucleus • The zona pellucida dis-integrates and releases conceptus: blastocyst. 2- Implantation DAYS 7-10 Fertilization (0 hours) Implanted blastocyst

12. Cleavage and Blastocyst Formation Zygote

15. Implantation Inner cell mass (or embryoblast) Blastocyst: Blastocoel Trophoblast Trophoblast: Inner cell mass Cellular trophoblast Syncytial trophoblast The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation). Endometrium: Epithelium Endometrial gland (DAY 8) (DAY 7) The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterus. The trophoblast on the attachment side separates into two layers: • The superficial layer in contact with the endometrium. The plasma membranes break down and trophoblastic cells fuse into a multinucleate mass called syncytial trophoblast. Implantation: It is the process of attachment to uterine wall that begins when blastocyst adheres to the endometrium. • The deep layer, close to embryoblast, retains the individual cells divided by membranes and is called cellular trophoblast.

16. Stages of Implantation

17. Formation of the Amniotic Cavity The inner cell mass separates slightly from the trophoblast and creates a narrow space between them: the amniotic cavity Inner cell mass Cellular trophoblast Syncytial trophoblast Gastrulation and Germ Layer Formation Amniotic cavity Ectoderm Gastrulation: Mesoderm It is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm Endoderm Inner cell mass Yolk sac Once the three primary germ layers are formed, embryogenesis is complete and the individual is considered an embryo. It is about 2 mm long and 16 days old.

18. Gastrulation and Germ Layer Formation Inner cell mass

22. The First Trimester Figure 29–5 Extraembryonic Membranes and Placenta Formation.

24. Placenta Myometrium Amniotic cavity Umbilical cord Endometrium