Origins of Body Form

Date : xx / xx / xxxx

The First Month of Human Life (2.19)Lecture 2

Dr. Shahira SamirProfessor of Anatomy & Embryology

Objectives

By the end of this session (the 3 lectures) you should be able to describe the:

• Production of the gametes (eggs and sperm)

• Fertilization, implantation and some causes of infertility

• Methods of contraception

• Formation of the germ layers

• Formation of the body plan

• Origin of organs and tissues

Fertilization

What is fertilization?Where does it occur?How?Results….

Definition: The process of union between the male & female gametes to form the zygoteSite: Lateral 1/3 of the uterine tube (Ampulla)Steps….Results……

I. Preparatory steps:* Sperm→ vagina→ uterus→ uterine tube (by movement of tail + uterine contractions)* Around 300 million ejaculated sperms→ Around 300 reach site of fertilization & only 1 penetrate the 2ry oocyte

2 processes:

1. Capacitation (maturation or activation):

Removal of glycoprotein coat

2. Acrosomal reaction: Release of acrosomal

enzymes to penetrate the oocyte

II. Actual steps

Steps of Fertilization:

I. Preparatory steps:II. Actual steps:• Penetration of the head & neck of

the sperm into the oocyte→ rounded= Male pronucleus

• Male & Female pronuclei (each containing 23 (n) chr.) unite to form 1 nucleus (46 + 2n)

The results of fertilization:

• Stimulates the completion of the 2nd maturation division→ Mature ovum +2nd polar body

• Restores the normal diploid number of chromosomes (46). • Determination of the sex of the embryo. The sex chromosome

(Y or X) carried by the sperm determines embryonic sex.• Initiation of cleavage ( Segmentation). • Zona & cortical reactions occur to prevent fertilization of the

same ovum by more than one sperm.

Cleavage or segmentation of the zygote

Definition: Repeated mitosis so that a unicellular zygote → multicellular mass of embryonic cells (Blastomeres) It starts 30 hours after fertilizationP.S.

* The zona pellucida keep blastomeres adherent together & prevent the adhesion of blastomeres to the uterine tube

* Blastomeres are considered Totipotent

Cleavage or segmentation of the zygote

Steps:1. Repeated mitotic division of

Zygote + movement towards the uterine cavity by tubal cilia

2. 2- cell stage→ 4-cell….→ 12-16 cell stage= Morula after 72 hours

3. Morula reaches uterine cavity on the 4th day of fertilization, undergo accumulation of fluid: Blastocyst

Blastocyst formation involves fluid secreted within the morula that forms the

blastocyst cavity.The conceptus is now called a blastocyst.1. The inner cell mass is now called the embryoblast (becomes the embryo).2. The outer cell mass is now called the trophoblast (becomes the fetal portion of the placenta).

Results of cell cleavage:↗ in number of cells + ↘ in size of cells to reach the normal

cell size of the species

Implantation

Implantation

X

* Definition: The process of embedding of the free blastocyst into the uterine wall.

* Site: the normal site is the endometrium of the upper part of the body of the uterus mainly in the posterior wall, near the midline, near the fundus.

* Steps: 1. Starts at 6th day after fertilization the trophoblastic cells over the embryonic pole adheres to the uterine wall, erode the endometrium and the whole blastocyst progressively penetrates it

2. About the 9th day the blastocyst becomes completely implanted.

3. The trophoblastic cells rapidly proliferate, become arranged into 2 layers: a. An outer Syncytial trophoblast: a mass of protoplasm with randomly dispersed nuclei.b. An inner cytotrophoblast: formed of cells with well-defined walls.

4. The site of entry of the blastocyst is closed by a fibrin clot.

5. The blastocyst is getting its nutrition from the eroded maternal tissues.

Abnormal sites of implantation

*These are sites other than the normal site of implantation A. Intrauterine sites: (placenta praevia):* Implantation inside the endometrium very close to the cervix of the uterus (in the lower uterine segment). It includes:

a. Placenta praevia lateralis = just above internal osb. Placenta praevia marginalis = around internal os. c. Placenta praevia centralis = covers internal os.

B. Extra-uterine sites:

1. Tubal pregnancy: in the uterine tube.

2. Abdominal pregnancy: in the abdominal cavity.

3. Ovarian pregnancy: on the surface of the ovary.

** N.B.: Implantation outside the cavity of the uterus leads to Ectopic Pregnancy which cannot reach full-term due to inadequate nutrition.

Methods of Contraception

• Hormonal methods: to prevent ovulation, e.g. oral contraceptive pills, skin patch & injectable contraceptives

• Intrauterine device (IUD): releases hormones that inhibit sperm transport & capacitation also, it inhibits implantation

• Barrier methods: inhibits sperm transport, e.g. male condom, diaphragm & cervical cap

• Spermicides: kill sperms, they are in the form of cream, gel, suppository, foam)

• Natural methods: e.g. family planning (safe period), breast feeding

• Sterilization: inhibits fertilization by tubal ligation in females & vasectomy in males

The removal of glycoprotein over the head of the sperm is called:A. Cortical reactionB. Zona reactionC. Acrosome reactionD.CapacitationE. Fertilization

Morula is: A. One cell stageB. Two cell stageC. Four cell stageD.Eight cell stageE. Sixteen cell stage

Normal site of implantation is in:A. Posterior wall of the uterusB. Anterior wall of the uterusC. Uterine tubeD.Lower segment of uterusE. Ovary