Introduction to Embryology 1st and 2nd week of development Prof. Abdulameer Al-Nuaimi E-mail: a.al-nuaimi@sheffield.ac.uk abdulameerh@yahoo.com

Embryology (from Greek embryon, "the unborn, embryo"; and  logia) Embryology is the science of the development of an embryo from the fertilization of the ovum to the foetus stage At the end of week 8, the embryo becomes a foetus. The kidneys, liver, brain, and lungs are all beginning to function. The fingers and toes are separate and the external genitalia are formed. At 12 weeks, the foetus is about three inches long and weighs about one ounce = 28.349 gm

History Hippocrates (460 BC–370 BC) is the first true embryologist, he believed that the embryo begins development by extracting moisture and breath from the mother. He also supported the view that the human foetus gained nourishment by sucking blood from the placenta. Aristotle (384 BC–322 BC) studied embryos of different organisms by opening up bird eggs at different stages of development and dissecting mammalian and cold-blooded embryos. He proposed that the embryo develops through Epigenesis which means through cell division and the formation of organs.

Until the 18th century, preformation concept was predominant. It was believed that, an embryo exists as a pre-formed entity contained inside semen; a pre-formed, miniature human is planted in the female during intercourse. With advancements in microscopy during the 19th century, biologists could examine the embryos at different stages of development. Eventually the theory of preformation was abandoned in favour of the epigenesis concept. After the 1950s when the DNA helical structure was discovered, an increased knowledge in the area of molecular biology was established. This led to the emergence of developmental biology. Developmental biology: examines the correlations between genes and morphological changes that occur in the embryo.

Anatomy of the male and female genital organs

Anatomy of the female genital organs www.google.co.uk/search? Intramural part Anatomy of the female genital organs tubal ostium

Ampulla Isthmus Infundibulum Intramural part Ovary and ovum www.google.co.uk/search?

Fallopian tube = Uterine tube Fallopian tube allows passage of the egg from the ovary to the uterus. Its different segments are: The infundibulum with its associated fimbriae are near the ovary The ampulla is the widest part of the tube, it represents the major portion of the lateral tube The isthmus is the narrower part of the tube that links to the uterus The intramural part (Interstitial part) is that transverses the uterine musculature. The tubal ostium is the point where the tubal canal meets the peritoneal cavity The uterine opening is the entrance into the uterine cavity, the utero-tubal junction

Ovulation and fertilization With each ovarian cycle, a number of primary follicles begin to grow, but usually only one reaches full maturity, and only one oocyte is discharged at ovulation. Ovulation is inducted through a surge in the Leutenizing Hormone(LH)from the pituitary gland. Human egg is 0.1 mm. in diameter The tubal fimbriae , carries the oocyte into the uterine tube through Its sweeping action. Ovary F. T. www.google.co.uk/search? Fertilization takes place in the fallopian tube, at the junction of the ampulla and isthmus (ampullary-isthmic junction).

Embryonic Period (8 weeks) The length of pregnancy is 280 days (40 weeks or 9 months) after the onset of the last normal menstrual period. More accurately, 266 days (38 weeks) after fertilization. (Fertilization takes place on day 15 following the onset of the last normal menstrual cycle) 1 15 28 P Embryonic Period (8 weeks) O From time of fertilization to the end of the 8th week is called the embryonic period. From the beginning of the 9th week to birth is known the foetal period.

(280 days) o

Changes during Embryonic period The 1st week—Fertilization and formation of the blastocyst  F F The 2nd week—Implantation and formation of bilaminar embryonic disc  2 2 The 3rd week—Further development of the embryo and formation of trilaminar embryonic disc  3 3 The 4th week—Folding of the embryo  F F The 5th – end of 8th week — Development of all organs At the end of 8th week — the embryo looks like an adult and Is called foetus

Growth in length of the foetus is greatest during the 3rd, 4th and 5th months of pregnancy. Increase in weight of the foetus is most striking during the last two months of pregnancy. Measurement of foetal length in centimeters (through an Ultrasound) is correlated with the age of the foetus in weeks. Length of the foetus is measured as Crown-rump length (CRL) (sitting height), between the top of the head to the area above where the legs begin and as Crown-heel length (CHL) (standing height). The measurement is from the vertex of the skull to the heel.

Male reproductive system The function of the male reproductive organs : 1- To produce, maintain, and transport the male reproductive cells (sperms) and nourishing fluid (semen). 2- To discharge sperms within the female reproductive tract during sex. 3- To produce and secrete male sex hormones.

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Testis Testis and Sperm Cross section in the Testis Sperm www.google.co.uk/search?

Fertilization Is the union of male and female reproductive cells (gametes) to produce a fertilized reproductive cell (zygote)

ovum Ovum Sperm Ovum Fertilization www.google.co.uk/search?

Results of fertilization 1-Restoration of the diploid number of chromosomes. 2-Determination of chromosomal sex. 3-Initiation of cleavage. The fertilized ovum, a zygote, will undergo rapid mitotic divisions, i.e. the 2-cell, 8-cell, and, a 16-cell Morula. As the morula enters the uterus on the third or fourth day after fertilization , a cavity begins to appear and the Blastocyst forms. Implantation of the blastocyst in the endometrial stroma of uterine wall occurs at the end of the first week. By the end of the second week, the blastocyst is completely embedded, and the surface defect in the mucosa of uterus has healed.

The blastocyst implants in the endometrium along the anterior or posterior wall (the uterus at the time of implantation is in the secretary phase). Morula, enters the uterine cavity on day 3 or 4 Blastocyst penetrates uterine wall. On day 8. Cells of the blastocyst differentiate into two cell masses 1-Inner cell mass, called the Embryoblast at one pole, it develops into the embryo proper. 2- Outer cell mass called Trophoblast, the cells surround the embryoblast and the Blastocyst cavity.

Fertilization www.google.co.uk/search?

Developing Embryo Blastocyst (Embryoblast) Trophoblast (Blastocyst cavity) Developing Embryo Blastocyst www.google.co.uk/search?

Embryoblast differentiates into two layers 1- Epiblast 2- Hypoblast These two layers (together) form a flat disc, the bilaminar disc Epiblast cells give rise to Amnioblasts that line the amniotic cavity superior to the Epiblast layer. Hypoblast gives rise to cells that line the blastocyst cavity and the inner surface of the trophoblast.

Epiblast Blastocyst cavity www.google.co.uk/search? Amnioblasts Syncytiotrophoblast Cytotrophoblast 9 day Embryo Amniotic Cavity Epiblast Pprimit. Yolk S. Hypoblast Exocoelomic membrane (migrated cells)

Trophoblast differentiates into 1- Cytotrophoblast Cells originate from the hypoblast, line the blastocyst cavity and the inner surface of the trophoblast, form the Exocoelomic membrane. The cavity is now known as the, Exocoelomic cavity (primitive Yolk sac). Trophoblast differentiates into 1- Cytotrophoblast 2- Syncytiotrophoblast Syncytiotrophoblast is the outer layer, it erodes maternal tissue. www.google.co.uk/search? Syncytiotroblast Cytotrophoblast 9 day Embryo Amniotic Cavity Epiblast Pprimit. Yolk S. Hypoblast Exocoelomic membrane

Placental circulation is established By day 9, lacunae develop in the Syncytiotrophoblast. Maternal sinusoids are eroded, subsequently, Maternal blood enters the lacunar Network and a primitive utero- Placental circulation is established Lacunae Maternal Sinusoid 9 days Embryo www.google.co.uk/search?

Extraembryonic Mesoderm New population of cells appear between the inner surface of the cytotrophoblast and the outer surface of the Exocoelomic membrane of the primitive yolk sac. These cells, derived from yolk sac cells, they form a fine, loose connective tissue, the Extraembryonic mesoderm www.google.co.uk/search? Extraembryonic Mesoderm Exocoelomic membrane Primitve Youlk sac Development of Extraembryonic Mesoderm

Extraembryonic mesoderm (chorionic plate) eventually fills all of the space between the Cytotrophoblast and Exocoelomic membrane. Large cavities develop in the Extraembryonic mesoderm, these cavities become confluent, they form a new space called the Extraembryonic cavity , or Chorionic cavity. The Extraembryonic mesoderm = (chorionic plate) forms two layers, Splanchnopleuric (splanchnic or visceral) and Somatopleuric (somatic or parietal) layers. extra-embryonic mesoderm is responsible for the formation of the blood vessels that will connect the embryo to the placenta. This extra-embryonic mesoderm looks and acts just like embryonic mesoderm, but it is found outside of the developing embryo.

Extraembryonic Somatopleuric Trophoblastic Lacunae Syncytiotrophoblast Cytotrophoblast Exocoelomic membrane Extraembryonic Splanchnopleuric Mesodermr (Visceral)m Epiblast A. C hypoblast Extraembryonic (Chorionic) cavity Primary Yolk sac Extraembryonic Somatopleuric Mesoderm (Somatic) Endometrium Site of penetration 13 day human blastocyst Formation of Extraembryonic mesoderm www.google.co.uk/search?

The next important stage The Hypoblast forms additional new cells that migrate along the inside of the Exocoelomic membrane, they proliferate and gradually form a new cavity within the primitive yolk sac. These cells are endodermal cells. This new cavity is known as the secondary yolk sac (definitive yolk sac), it replaces the primitive yolk sac.

Formation of bilaminar disc amniotic cavity Amnioblasts Epiblast bilaminar disc Hypoblast Secondary Yolk sac Primary yolk sac Exocoelomic membrane Newly growing cells from hypoblast Formation of bilaminar disc www.google.co.uk/search?

chorion (Langman’s Medical Embryology) Prim Y. S. Sec y. s. Buccopharyngeal membrane (Langman’s Medical Embryology) Prim Y. S. Sec y. s. (new cells that Migrate) chorion  Extraembryonic Splanchnopleuric Mesoderm (visceral) Cytotrophoblast Syncytiotrophoblast (primitive yolk sac)

Summary Embryonic period starts at time of fertilization, it lasts for 8 weeks. From beginning of the 9th week till birth is known as the foetal period. Fertilized ovum is implanted in the wall of uterus and Passes in many stages of development.

Pelargonium graveolens Thank You