Human Embryology and Assisted Human Embryology and Assisted Reproductive Technology (ART)Reproductive Technology (ART)

• Introduction of Human Embryology;• Assisted Reproductive Technology (ART);• Chromosome in eggs and embryos;• Screening of embryos for genetic disease;

Experimental EmbryologyCourse: ANAT-381

Introduction of Human Embryology

Life cycle

Endocrine System

Hypothalamus and pituitary glands

Ovarian Cycle

• At puberty the female begins to undergo regular monthly cycle;

• These ovarian cycles are controlled by the hypothalamus;

• Gonadotropin-releasing hormone (GnRH) produced by the hypothalamus acts on cell of the anterior pituitary gland, which in turn secretes gonadotropins;

Ovarian cycle

Gonadotropins

• Gonadotropins are hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate and control cyclic changes in the ovary;

• FSH stimulates follicular development, and LH surge triggers follicular rupture and ovulation.

Follicular development

                                                                                                      

The adult ovary can be subdivided into three regions: the cortex, medulla, and hilus regions. The cortex consists of the surface epithelium (se), tunica albuginea (ta), ovarian follicles (primordial, primary (pf), secondary (sf), small, medium, large Graafian follicle (gf)) and corpora lutea (cl). The medulla consists of large blood vessels and nerves. The hilus contains large spiral arteries and the hilus or ovary Leydig cells. (Bloom W, Fawcett DW: A Textbook of Histology. Philadelphia, WB Saunders Company, 1975)

                                                                                

                      

In human females, all primordial follicles are formed in the fetus between 6 and 9 months' gestation. During this period, there occurs a marked loss of oocytes due to apoptosis. The number of primordial follicles decreases progressively as a consequence of recruitment, until very few if any are present after the menopause at ~50 years of age. (Baker TG: Radiosensitivity of mammalian oocytes with particular reference to the human female. Am J Obstet Gynecol 110:746, 1971. Reproduced with permission from Mosby, Inc.)

                                                                         

                             Diagram illustrating the major histological changes that accompany the gonadotropin-independent period of preantral folliculogenesis (Erickson, Gregory F. Normal ovarian function. Clin Obstet Gynecol 21:31, 1978. Reproduced with permission from Lippincott-Raven

Publishers.)

                                                                                      

                Photomicrographs of the early stages of human preantral folliculogenesis. A) Primordial follicle; arrow, squamous granulosa cell. B) Recruitment showing the primordial-to-primary transition; arrow, cuboidal granulosa cell. C) Primary follicle with multiple cuboidal granulosa cells. D) fully grown primary follicle at the primary-to-secondary transition stage; arrow, formation of a secondary layer of granulosa cells. All photos are 40x.

                                                                                      

                A typical healthy secondary follicle. It contains a fully grown oocyte surrounded by the zona pellucida, 5 to 8 layers of granulosa cells, a basal lamina, a theca interna and externa with numerous blood vessels. (Bloom W, Fawcett DW In A Textbook of Histology. WB Saunders Company, Philadelphia 1975. With permission from Arnold.)

                                                                                          

            Photomicrograph of an early tertiary follicle 0.4 mm in diameter at the cavitation or early antrum stage. zona pellucida (ZP); granulosa cells (GC); basal lamina (BL); theca interna (TI); theca externa (TE); granulosa mitosis (arrowheads). (Bloom W, Fawcett DW In A Textbook of Histology. Philadelphia, WB Saunders Company, Philadelphia 1975. With permission from Arnold.)

                                                                                                      

Diagram of the architecture of a typical Graafian follicle. (Erickson GF: Primary cultures of ovarian cells in serum-free medium as models of hormone-dependent differentiation. Mol Cell Endocrinol 29:21, 1983. Reprinted with permission from Elsevier)

                                                                                   

                   Photomicrograph of ovulation of a mature egg-cumulus complex through the stigma. (Hartman CG, Leathem JH In Conference on Physiological Mechanisms Concerned with Conception. Pergamon Press, New York 1959)

                                                                                                      

Diagram of the cellular mechanisms by which the preovulatory surge of FSH and LH causes ovulation. (Erickson GF: The Ovary: Basic Principles and Concepts: In Felig P, Baxter JD, Broadus AE, Froman LA, (eds): Endocrinology and Metabolism, 3rd edition. New York, McGraw Hill, 1995)

                                                                                       

               Photomicrograph of a human corpus luteum. (Bloom W, Fawcett DW (eds) in A Textbook of Histology. WB Saunders Co., Philadelphia 1975 with permission from Arnold)

First week of development:from ovulation to implantation

Human embryo development(5 days)

        

1 2 3

4 5 6

7 8 9

Spermatogenesis

Spermatogenesis

Sperm morphology

SpermUnder light microscope

Insemination (In vivo=1:300-500 / in vitro=1:1x106)

In vitro In vitro

Scanning electron micrograph (SEM)of sperm binding to the zona pellucida

In vitro

Fertilization

• The process by which male and female gamete fuse;

• It occurs in the ampullary region of uterine tube;

• Sperm may remain viable in the female reproductive tract for several days;

• The isthmus of the uterine tube serves as a sperm reservoir;

Capacitation• It is a period of conditioning in the female

reproductive tract that in the human lasts approximately 7 hours;

• During this time a glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the sperm;

• Only capacitated sperm can pass through the corona and undergo the acrosome reaction;

Acrosome reaction

• Which occurs after binding to the zona pellucida;

• It is induced by zona proteins;

• This reaction culminates in the release of enzymes needed to penetrate the zona pellucida, including acrosin;

Acrosome reaction

The three phases of sperm penetration

Penetration reactions• Cortical and zona reactions;

• Resumption of the second meiotic division;

• Activation of the oocyte;

Cortical and zona reactions:

• As a result of the release of cortical oocyte granules, which contain lysosomal enzymes

• The oocyte membrane becomes impenetrable to other sperm;

• The zona pellucida alters its structure and composition to prevent sperm binding and penetration;

• These reactions prevent polyspermy (penetration of more than one sperm into the oocyte)

Resumption of the second meiotic division:

• The oocyte finishes its second meiotic division immediately after entry of the sperm;

• The second polar body forms;

Activation of the oocytes

• The activating factor is probably carried by the sperm;

• Formation of female and male pronuclei;

• Each pronucleus is haploid;

Main results of fertilization

• Restoration of the diploid number of chromosomes, half from the father and half from the mother;

• Determination of the sex of new individual;

Early embryonic development

• After fertilization, the zygote undergoes a series of mitotic divisions, increasing the numbers of cells;

• These cells become smaller with each cleavage division;

• The cleaved cells are known as blastomeres;

Compact morula

• After 8-cell stage, blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions;

• This process calls compaction, it occurs in 16 or 32-cell stage embryo called morula;

Blastocyst formation

• Approximately 5 days after fertilization, the compacted morula divide to form blastocyst (>32-cells);

• The cleaved cells constitute the inner cell mass (or embryoblast), and surrounding cells compose trophoblast;

Blastocyst hatching

Inner cell mass (ICM) develops to fetus,and trophoblast contributes to the placenta.

Implantation

Changes of endometrium during menstrual cycle

Events during the first week of human development

Clinical correlations

• Assisted Reproductive Technology (ART):First week of embryonic development;

• Contraception methods:1. Barrier techniques;

2. Contraceptive pill;

3. Vasectomy and tubal ligation;

• What kind of hormones produced from hypothalamus?

• What kind of hormones secreted from pituitary gland?

• Where fertilization occurred in vivo?• What kind of hormones secreted from corpus

luteum?• In the human, after fertilization, how long the

embryo takes to migrate into uterine for implantation?

Questions for Introduction of Human Embryology:

1. Langman’s Medical Embryology (9th Edition); ISBN: 0-7817-4310-9;Author: T. W. Sadler, Ph.D.Publisher: Lippincott Williams & Wilkins.

2. Human Reproductive Biology (3rd Edition); ISBN 13:978-0-12-088465-0Authors: Richard E. Jones & Kristin H. Lopez

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