Embryology Ⅰ Yanqiu Hu

Today’s contents Introduction of embryology Gametogenesis 1st week, ovulation to implantation ★ 2nd week, bilaminar germ disc ★

Ⅰ. Introduction What is embryology? Embryology is a science that studies the normal development as well as the birth defects of a human being in the maternal uterus.

Normal development Takes 38 weeks from fertilization to parturition of a mature fetus. Three periods are divided: 1) Preembryonic period: First 2 weeks, Fertilization to formation of the bilaminar germ disc.

2) Embryonic period: Weeks 3 – 8, Primordial of all major organs develop from the three germ layers.

3) Fetal period: Week 9 – birth. Growth of the organ systems

Birth defects The study of the congenital malformations is the Teratology Causes 1) Genetic factors (25%) 2) Environmental factors (teratogens) (10%) 3) Interaction of genetic and environmental factors (65%)

The genetic control of early embryonic development

Chapter 1. Gametogenesis A process of formation and maturation of the gametes (sperm and ovum)

1.1 At the end of 3rd week, PGCs in the wall of the yolk sac and start to migrate to the developing gonad through the hindgut.

Spermatogenesis Oogenesis 2n, 46 2n, 46 n, 23 n, 23 n, 23 n, 23

1.2 The purpose of gametogenesis Reduction of the number of chromosomes to half by meiosis, which occurs only in germ cell. Alteration of the shape of germ cells for fertilization

1.3 Differences between male and female Male Female Has stem cells No stem cells Continuous after puberty Pause twice 4 sperm from a spermatocyte Only 1 ovum fromed Equal division Unequal division Functional mature and No capacitation

1.5 Functional mature and capacitation of sperm Functional mature: getting ability of forward movement and potential of fertilizing eggs in the epididymis Capacitation: obtaining capacity of fertilizing an egg in the female reproductive tract through removal of a glycoprotein coat from the plasma membrane of the acrosomal region by enzyme (b-amylase).

Abnormal gametes For fertilization to occur, egg and sperm must meet between 2 – 48 hrs after sex. Egg is only fertilizable for 48 hrs.

Chapter 2. First week of development Ovulation to implantation

2.1 Fertilization -- union of a sperm and an oocyte to form a fertilized egg (zygote) Time: 12 - 24 hours after ovulation. Site: ampulla of uterine tube.

2.1.1. Fertilization precursors Women: Ovulation  Secondary oocyte released. Accompanying the oocyte: Corona radiata Zona pellucida

2.1.1. Fertilization precursors Men: ~10 hour journey for sperm Some sperm are gradually released into the cervical canal; which increases the chances of fertilization. 200~300 million sperm  3 million make it to uterus  300~500 sperm reach the fertilization site.

2.1.2. Important events in fertilization Before: Capacitation Acrosome reaction: release of enzymes from acrosome induced by, and needed for sperm penetration of, the zona pellucida. removal of a glycoprotein coat from the plasma membrane of the acrosomal region by enzyme (amylase).

Capacitation Takes 5-7 hours. This maturation of sperm is called capacitation Capacitation is facilitated by substances in the female tract. Removal of a glycoprotein coat from the plasma membrane of the acrosomal region by enzyme (β-amylase). Capacitation is required for the acrosomal reaction to take place.

The acrosome reaction Sperm bind to zona pellucida The acrosome reaction provides the sperm with an enzymatic drill to get through zona pellucida red blue pink

2.1.2. Important events in fertilization During: Penetration of corona radiata, zona pellucida, oocyte cell membrane. Recognition: a zona protein ZP3 is responsible for species-specific fertilization.

After entry: Zona reaction: zona becomes impenetrable to other sperm (monospermy), through enzyme release from cortical granules of oocyte to change structure and composition of the zona. Oocyte finishes 2nd meiosis. Formation and merge of male and female pronuclei.

2.2. Cleavage 24 hr after fertilization, the mammalian zygote (1 cell) begins cleavage. a series of mitotic divisions of fertilized egg It is difficult to count the cells in a morula; the embryo shown here probably has between 20 and 30 cells.

Morula solid ball of cells Zygote Blastocyst with blastocoele cavity

2.3 Blastocyst formation Formation: morula → cell number↑→ spaces appearing →blastocyst

3) Blastocele: will form exocoelomic cavity 1) Inner cell mass (embryoblast): develops into embryo proper. 2) Trophoblast: absorbs nutrients. Will go on to contribute to fetal membrane systems. 3) Blastocele: will form exocoelomic cavity

Trophoblast forms syncytiotrophoblast- erodes into endometrium Implantation - embedding of blastocyst into endometrium begins at day 5~6 Trophoblast forms syncytiotrophoblast- erodes into endometrium Cytotrophoblast - carries nutrients to inner cell mass Lacunae and primary villi formed by trophoblast All of these form placental tissues Fig 28-3

2.4. Implantation Blastocyst “floats” in the uterus for 2-3 days Blastocyst implants 5-6 days after ovulation

hCG Implantation completed by 14 days after ovulation hCG from the placental chorion signals the hypothalamus, pituitary, and corpus luteum that implantation has occurred Hormone levels are maintained which prevents uterine sloughing (menses) hCG

Endometrial epithelium grows around implanted blostocyst Chorion – developed from trophoblasts after implantation, continues this hormonal stimulus

The chorion develops fingerlike villi, which: Become vascularized Extend to the embryo as umbilical arteries and veins Lie immersed in maternal blood

Ectopic implantation

Chapter 3. Second week of development Bilaminar Germ Disc

Day 7

Day 8

Day 9

Day 12

3.5 Extraembryonic mesoderm extraembryonic coelom somatopleuric splanchnopleuric chorionic cavirty connecting (body) stalk