General embryology Langman’s Medical Embryology, 13th Ed Jun Zhou (周俊） School of Medicine, Zhejiang University 20171226

OBJECTIVES The definition of Embryology: 3 periods, total 38/40 weeks 3 phases of fertilization, normal site Normal sites of implantation and the most abnormal sites The formation of 2 cells stage, morula, blastocyst 5. Bilaminar germ discs- epiblast & hypoblast Trilaminar germ discs The formation and derivatives of ectoderm, mesoderm and endoderm. The components of fetal membrane and their functions. Structure and function of placenta. Birth defects

a science which study the processes and regulations of development of human fetus.(from 1 cell (zygote) to (5-7) x1012 cells) In general the length of pregnancy is considered to be 280 days or 40 weeks after the onset of the last menstruation, or more accurately, 266 days or 38 weeks after fertilization. Onset of last M.C. Fertilization 266 days 14 days Labor 280 days

Onset of M.C. Ovulation

Total 38 weeks: Preembryonic period: before 2 weeks Embryonic period: 3-8 weeks Fetal period: after 9 weeks

Preembryonic period: before 2 weeks Embryonic period: 3-8 weeks Fetal period: after 9 weeks

Fertilization Definition: the process by which the male and female gametes(sperm and ovum) unite to give rise to zygote place: in the ampullary region of the oviduct

Prerequisite for fertilization: 1)  maturation of oocyte 2)  maturation of spermatozoa 3)  capacitation and acrosome reaction of spermatozoa: capacitation factor: secreted by epi. cells of oviduct, capacity of releasing acrosome enzymes 4) quality and quantity of spermatozoa: 5)  meeting of sperm and ovum within 24 hrs

Oocyte maturation Primordial  primary Zona pellucida Antrum – secondary Cumulus oophorus Secondary oocyte – 36-48 h before ovulation, polar body

Spermatogenesis Germ cells dormant until puberty At puberty become mitotically active sertoli cells, nutrition, protection and mechanical support Unlike oogenesis, 4 sperms per clone

Processes of fertilization: Phase I penetrate the corona radiata, acrosomal release Phase II ZP with receptors – zona pellucida spongy – the release of local enzymes Phase III Success formation of zygote

Entry – response Results of fertilization Cortical or zona reactions -Compaction - impermeable to more sperm Oocyte finishes 2nd meiotic division Female /Male pronucleus Metabolic activation of the egg Results of fertilization Diploid Genetic sex Cleavage

Cleavage Mitosis and cytoplasmic division 2 cells stage (30 hr) Morula (72 hr) 12-16 cells Blastocyst ( Day 4-5)(No ZP) Blastocoele Inner cell mass Trophoblasts

Blastocyst trophoblasts blastocoele inner cell mass

Uterus at Time of Implantation Puberty – menstruation (28 days) Hormonal regulation GnRH FSH LH Granulosa/theca cells Estrogens Progesterones The endometrium of the uterus is in the secretory phase

-Passage through the uterine tube Implantation -Passage through the uterine tube Day 5 –Day 11 Uterus Orientation of implantation - anterior or posterior, fundus Trophoblasts (overlying the inner cell mass) attach to the endometrium, begin to invade.

Processes zona pellucide disappear → polar trophoblast touch the endometrium→secrete enzymes →dissolve the endometrium → embedded into endometrium → coagulation plug seal the space

Implantation site Normal: anterior or posterior wall of the uterus Abnormal Tubal (most common) Ovarian Abdominal Internal os of the cervix

Endometrium →decidua: -Decidua basalis -Decidua capsularis -Decidua parietalis Prerequisite: -Endometrium is in secretory phase -Morula/blastocyst reach the cavity of uterus on time -Zona pellucide disappears in time

(Second week of development) Bilaminar Germ Disc (Second week of development) Implantation - sinks deeper The outer cell layer Trophoblasts Cytotrophoblasts Syncytiotrophoblasts (Syncytium) Inner cell mass Fluid collection – amnion

Week two – Day 9-12 Implantation Complete – a slight bulge Trophoblasts Vacuoles within syncytiotrophoblasts- fuse and form lacunar networks Establish Uteroplacental circulation Primary yolk sac Extra-Embryonic Mesoderm btwn troph & cavity

Week 2 – Day 13-14 Yolk sac Primary to seconday Connecting stalk

The Bilaminar Embryo By the end of week 2 Disc btwn Amnion Yolk sac Bilaminar germ disc Epiblasts (bottom of amnion) Hypoblasts (roof of yolk sac)

End of the Week of Twos 2 major cell masses Embryonic Cavities Trophoblasts Inner cell mass Cytotrophoblasts Syncytiotrophoblasts Embryonic Cavities Amniotic cavity Yolk sac Germ Disc Epiblasts Hypoblasts

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