Lab # 13: Human Development

It completes meiosis II

Zygote

Embryo

Oocyte dies

Secondary oocyte(arrested in metaphase

of meiosis II)

Oocyte at OvulationOvulation of mature

(graafian) follicle

Egg

It is a layer of glyco-protein gel secreted by granulosa cells around the oocyte

It is composed of several layers of granulosa cells

Zona pellucida

If fertilized

First polar body

Corona radiata

Egg membrane

If not fertilized

Second polarbody (dies)

Corona radiata

Zona pellucidaEgg

Female pronucleus

It is the exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg

Hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together

When a path has been cleared, a sperm binds to the zona pellucida

Two acrosomal enzymes are released:

Acrosin, a protease similar to trypsin

Acrosomal reaction1Cortical reaction2

Fertilization membrane3

Rejected sperm4

Cortical granules

Spermpronucleus

Eggpronucleus

Zygote2-celled stage

(30 hours)

Cleavage:It refers to the mitotic divisions that occurs in the first 3 days, while the conceptus migrates down the uterine tubes

Blastomeres It is a solid ball of 16 cells that resemble a mulberryThe morula lies free in uterine cavity for 4-5 days and divides into a 100 cells or so

The zona pellucida dis-integrates and releases conceptus: blastocyst

Migration of the Conceptus

The Preembryogenic Stage It comprises the first 16 days of development, culminating with the existence of an embryo.

1- Cleavage 2- Implantation3- Embryogenesis

4-celled stage

8-celled stage

Morula(72 hours)

Blastocyst

Implanted blastocyst(6 days)

Fertilization(0 hours)

2- Implantation (about day 6)

Endometrium:

Blastocyst:

(6-7 days)

Blastocoel

TrophoblastInner cell mass or Embryoblast

EpitheliumEndometrial gland

The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterusImplantation: It is the process of attachment to uterine wall that begins when blastocyst adheres to the endometrium

The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation)

3- Embryogenesis (by the end of 2nd week)

It is the arrangement of the embryoblast into three primary germ layers: ectoderm, mesoderm, and endoderm

Embryogenesis:

Endoderm

Mesoderm

Ectoderm

The embryoblast separates slightly from the trophoblast and creates a narrow space between them: the amniotic cavity

Amniotic cavity

Yolk sac

Once the three primary germ layers are formed, embryogenesis is complete and the individual is considered an embryo. It is about 2 mm long and 16 days old.

The mesoderm is a more loosely organized tissue which differentiates into a loose fetal connective tissue called mesenchyme

The ectoderm and endoderm are epithelia composed of tightly joined cells.

16 days

Yolk sac

Amniotic cavity

Amnion

Chorion

Placental sinus

They are extensions of syncytiotrophoblast into endometrium by digestion and growth of “roots” of tissue

They are lacunae filled with maternal blood that merge and surround villi

Chorionic villi

Formation of Embryonic Membranes and Placenta

As the placenta grows:

Placenta and Umbilical Cord

The villi grow and branch and their surface area increasesThe membrane becomes thinner and more permeableThe placental conductivity (the rate at which substances diffuse through the membrane) increases

Materials diffuse from the side of the membrane where they are more concentrated to the side where they are less concentratedOxygen and nutrients pass to the fetal blood. Fetal wastes pass the other way and are eliminated by the motherThe placenta is also permeable to nicotine, alcohol, and most other drugs that may be present in the maternal blood stream