Embroylogy

Embryology

GENERAL EMBRYOLOGY

EMBRYOLOGY:

• Embryology – study of the origin and development of single individual

• Prenatal period• Embryonic period – first 8 weeks• Fetal period – remaining 30 weeks

• Embryogenesis starts with fertilization

• Fertilization: process of fusion of male gamete(germ cell) sperm with female gamete(germ cell) to form zygote

Fertilization:

• Two sex cells fuse to form a new cell containing genetic material derived from both parents.

• Restores the diploid number of chromosomes.• Determines the sex of the organism• Initiates cleavage.• Occurs in the widest part of the uterine tube (the

ampulla).

Fertilization:

• Millions of sperm cells are deposited in the female reproductive tract during intercourse.

• Only a few hundred have a chance at fertilization.• Only the first sperm to enter the secondary oocyte

is able to fertilize it.• The remaining sperm are prevented from

penetrating the oocyte.

The Stages of Embryogenesis

• Cleavage. The zygote divides by mitosis to form a multicellular structure called blastocyst.

• Gastrulation. The blastocyst cells form three primary germ layers, which are the basic cellular structures from which all body tissues develop.

• Organogenesis. The three primary germlayers arrange themselves in ways that give rise to all the organs within the body.

The Embryonic Period

• Week 1 – from zygote to blastocyst• Conception – in lateral third of uterine tube• Zygote (fertilized oocyte) moves toward the uterus• Blastomeres – daughter cells formed from zygote• Morula – at about 12–16 cell (blastomeres)stage• Blastocyst –further division, fluid-filled structure –

about 60 cells

Fertilization and the Events of the First 6 Days of Development

Figure 3.3

Blastocyst:

Formation of blastocyst: by 5th day, blastomere reach to 107, small space appear between them

--• The blastocyst is a sphere of about 150 cells, with

an outer layer (the trophoblast), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass).

Implantation:

• By the end of the first week after fertilization,

the blastocyst enters the lumen of the uterus.

• Implantation is the process by which the blastocyst burrows into and embeds within the endometrium.

Week 2 – The Two-Layered Embryo

• Bilaminar embryonic disc – inner cell mass divided into two sheets• Epiblast and the hypoblast• At the site of implantation,the outer cell mass of

blastocyst start proliferating and give rise to trophboblast

• Placenta: is made up of finger like process known as villi and inetvillious space. An organ that joins mother and off spring.

Placenta:

• Functions in exchange of nutrients, waste products, and respiratory gases between the maternal and fetal bloodstreams.

• Transmission of maternal antibodies to the developing embryo or fetus.

• Production of hormones to maintain and build the uterine lining.

Week 2 – The Two-Layered Embryo

• Amniotic sac – formed by an extension of epiblast• Outer membrane – forms the amnion• Inner membrane – forms the amniotic sac cavity

• Filled with amniotic fluid

Week 2 – The Two-Layered Embryo

• Yolk sac – formed by an extension of hypoblast• Digestive tube forms from yolk sac• Tissues around yolk sac

• Gives rise to earliest blood cells and blood vessels

Implantation of the Blastocyst

Figure 3.4

Week 3 – The Three-Layered Embryo

• Primitive streak –the embryonic disc become elongated.The primitve streak is a midline proliferative region of epiblast cells which may break free from the epithelium and migrate beneath the epiblast.

• Ingression: it’s a process by which cell become part of the streak and then migrate away from it

• Gastrulation –formation of primitive streak is the beginning of gastrulation, a process of where epiblast give rise to a trilaminar structure with a define cranio-caudal axis and formation of embryonic shape

The Primitive Streak

Figure 3.5e-h

Gastrutlation:

• Occurs during the third week of development immediately after implantation.

• One of the most critical periods in the development of the embryo.

• Cells of the epiblast migrate and form the three primary germ layers which are the cells from which all body tissues develop.

• The three primary germ layers are called ectoderm, mesoderm, and endoderm