Page 112 - 86 human physiology part-2

Pregnancy and Birth

Implantation

The blastocyst preserves itself by secreting a hormone that indirectly stops menstruation. The
trophoblast cells secrete hCG hormones that help maintain the corpus luteum that would normally
regress. In turn, the corpus luteum continues to secrete progesterone, which maintains the endometrium
of the uterus in the secretory phase. This helps the blastocyst to continue to grow and stay embedded
within the endometrium. The fetal life support system and the placenta begin to form, and eventually
the placenta will take over the job of producing progesterone.

• Gastrulation and Formation

The embryoblast within the blastocyst forms 3 primary germs layers: ectoderm, mesoderm, and
endoderm.

Ectoderm

This forms the nervous tissue and the epithelium covering the outer body surface. Epidermis of
skin, including hair and nails, glands of skin, linings of oral cavity, nasal cavity, anal canal, vagina,
brain, spinal cord, sensory organs, lens of eye and epithelium of conjunctiva (a membrane that covers
the sclera and lines the inside of the eyelids), pituitary gland, adrenal medulla, and enamel of teeth.

Mesoderm

This forms all of the muscle tissue and the connective tissue of the body, as well as the kidneys
and the epithelium of the serous membranes and blood vessels. All muscle tissue (skeletal, smooth,
cardiac), all connective tissue (fibrous connective tissue, bone, blood, cartilage), dentin of teeth,
adrenal cortex, kidneys and ureters, internal reproductive viscera, epithelium lining vessels, joint
cavities, and the serous body cavities.

Endoderm

Forms the lining epithelium and glands of the visceral body systems. Lining epithelium and glands
of digestive, respiratory, and parts of urogenital systems, thyroid and parathyroid glands, and thymus.

Formation of Placenta & Amniotic Sac

The endometrium makes changes. Cellular growth and accumulation of glycogen occur causing
fetal tissue and maternal tissue to come together. This formation makes the functional unit called the
placenta. The placenta envelops the entire fetus. It provides protection from harmful substances. The
amniotic sac contains amniotic fluid. Amniotic fluid is the watery liquid surrounding and cushioning a
growing fetus within the amnion. It allows the fetus to move freely without the walls of the uterus
being too tight against its body. Buoyancy is also provided here for comfort.

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