15.1 Fertilization and Embryonic Development

Zygote

After 12 h of sperm entering egg, the sperm and egg nucleus membranes disappear and the chromosomes in the ovum and sperm join. Fertilization is complete and the single cell formed is called a zygote.

Heart

After neurulation, a reddish bulge that contains the heart forms. By the 18th day it starts beating.

Placenta

After the 2nd week, projections from the chorion extend into the uterine lining. Projections are called chorionic villi and make up the beginnings of the placenta. Placenta is fully developed by 10 weeks. Placenta does not filter out substances such as alcohol, drugs, and nicotine which can diffuse across membranes. Mother and fetus blood pool do not mix. Exchange occurs through diffusion. Allows exchange of nutrients from mother's blood to fetus's blood. Stores nutrients in early pregnancy and released them to fetus later. Transports wastes from fetal blood to mother's blood. Transports oxygen from mother to fetus and carbon dioxide from fetus to mother. Secretes hormones estrogen, progesterone, and hCG. Allows hormones from fetus to diffuse to mother's blood vice versa. Transports antibodies from the mothers into the fetus's blood to provide passive immunity.

Extra-embryonic membranes

Allantois, amnion, chorion and yolk sac. Extra-embryonic membranes, placenta, and umbilical cord are responsible for the protection, nutrition, respiration, and excretion of the embryo. These membranes, placenta, and umbilical cord are expelled from the uterus during birth. This event is referred to as the afterbirth.

Blastocyst

As the morula travels to the uterus for implantation, it is filled with fluid that diffuses from the uterus. The fluid filled space develops and two different groups of cells are formed called the trophoblast and the inner cell mass. The whole structure is called the blastocyst.

Gastrulation

At first, the embryonic disk has 2 layers; ectoderm (closer to amniotic cavity) and the endoderm. Shortly after, the mesoderm forms between the 2 layers. The process of forming these 3 layers is called gastrulation. The 3 layers are called the primary germ layers. Embryo is now called the gastrula. Marks the start of morphogenesis.

Umbilical cord

At the end of the 8th week the yolk sac shrinks and the amniotic sac enlarges, the umbilical cord forms. The umbilical cord leads from the navel area of the fetus to the centre of the placenta. Contains 2 arteries which transport oxygen depleted blood from fetus to placenta. Contains 1 vein which bring oxygen rich blood to fetus.

Implantation

By the 5th or 7th day the blastocyst attaches to the endometrium. The inner cell mass is positioned against the endometrium. The trophoblast cells secrete enyzymes that digest some of the endometrium. Implantation is complete by the 10th or 14th day. Woman is said to be pregnant now.

Inner cell mass

Develops into the embryo itself.

Neurulation

During the 3rd week, a thickened band of mesoderm cells develops on the back of the embryonic disk. They form a rod structure called the notochord which provides the basic framework for the skeleton. The nervous system develops from the ectoderm which is above the notochord. The cells above the notochord thicken and form the neural tube which develops into the brain and spinal cord. Formation of tube is called neurulation which marks the start of organ formation.

7th/8th week

Embryo has distinct human characteristics. Organs are formed and nervous system is starting to coordinate body activity. A skeleton of cartilage is formed. (Bone replaces cartilage at the 9th week) Eyes are well developed but eyes are closed. Nostrils are developed but plugged with mucus. External genitalia is still forming. From this time on, the developing life is now called a fetus.

5th week

Embryo's head is larger than body. Eyes are open but no eyelid or iris. Cells in brain are differentiating rapidly.

Ectoderm

Epidermis (skin, hair, etc.) and the Nervous System

Allantois

Foundation of umbilical cord. Provides blood vessels in placenta. During 2nd month, most of the allantois degenerates and remainder becomes part of urinary bladder.

Fertilization

Fusion of male and female gametes (sperm and egg) to form single cell with 46 chromosomes. Egg must be fertilized 12-24 hours after its release.

Corona Radiata

Layers of loose jelly follicle cells that surround Zona Pellucida. These cells were a source of nourishment for egg when it was an ovarian follicle.

Endoderm

Linings of digestive and respiratory tracts.

Mesoderm

Muscle, bone, and blood vessels.

Amniotic Cavity

Occurs during the second week as the blastocyst completes implantation. A space forms between the inner cell mass and trophoblast called the amniotic cavity which will fill with fluid and is the place where the baby will develop. As the amniotic cavity forms, the inner cell mass flattens into a disk shape and is called the embryonic disk. It is supported by a stalk connecting the blastocyst to the endometrium.

Organ formation

Organs form between 3rd and 8th weeks. Different rates of cell division in the primary germ layers cause tissues to fold into distinct patterns. The 3 layered embryo gradually forms into a body with organs and is recognizably human by the 8th week.

Trophoblast

Outer layer of blastocyst. Forms 2 membranes; amnion and chorion. Amnion develops into fluid filled sac which protects and insulates embryo. Chorion develops chorionic villi which grow into endometrium to absorb nutrients. Villi and part of endometrium form placenta.

Chorion

Outermost membrane. Encloses all other extra-embryonic membranes and the embryo. Forms the fetal portion of the placenta.

human chorionic gonadotropin (hCG)

Secreted by the trophoblast around the time implantation begins. Has the same effects as LH therefore maintaining the corpus luteum past the time it would degenerate. As a result, secretion of estrogen and progesterone continues and the endometrium is maintained. hCG is secreted at high levels for 2 months but declines at 4 months. Corpus luteum stays intact for whole pregnancy but is less important after first trimester as the placenta now secretes sufficient estrogen and progesterone to maintain endometrium.

Yolk Sac

Small sac that contributes to the formation of the digestive tract and produces the first blood cells and future egg or sperm cells. Unlike other animals, human yolk sac has no nutritive function.

6th week

The brain continues rapid development. Limbs lengthen and flex silightly. Gonads start to produce hormones that will influence the development of external genitalia.

Differentiation

The cellular process in which cells become specialized in structure and function. All cells, tissues, and organs of the body are derived from the 3 primary germ layers through differentiation.

Morphogenesis

The development of the form of an organism and its structures. Depends on the ability of early embryonic cells to differentiate.

Sperm entering into egg

The sperm's acrosome releases enzymes that digests its way through the corona radiata and zona pellucida. Sperm moves further by lashing its tail. Once sperm enters the egg's plasma membran depolarizes preventing other sperm from entering.

Zona Pellucida

Thin clear layer of protein and carbs surrounding the plasma membrane of the egg.

4th week

Time of rapid growth and differentiation. Blood cells form and fill developing blood vessels. Lungs and kidneys take shape. Small buds that will form into limbs appear. Distinct head is visible. Evidence of eyes, ears and nose.

Amnion

Transparent sac that develops from the cells of the embryonic disk. Grows to enclose embryo completely and is only penetrated by the umbilical cord. Filled with amniotic fluid which protects embryo from trauma and temperature fluctuations. Allows freedom of movement and prevents limbs from sticking to body.

Morula

When the zygote is a sphere of 16 cells. The morula reaches the uterus within 3-5 days after fertilization.

Cleavage

Within 30 h of fertilization the zygote divides by mitosis multiple times. This process occurs quickly with little time for individual cells to grow. The process of cell division without enlargement of the cells is called cleavage.