Gastrulation: Meaning, Formation and Types

Ans. This is the process when the blastocyst implants in the uterus. The blastocyst is embedded in the endometrium layer of the uterus. In the region of the embryonic knob, the blastocyst comes into contact with the endometrium. Corrosion of the endometrium lining is because of the surface cells of the trophoblast. These surface cells secrete lytic enzymes. They also help give rise to finger-like projections known as villi. The function of villi is to aid in the fixation and absorption of nutrients. When the implantation process takes place, the nutrients are enriched and the cells also grow. This results in the formation of the uterine portion of the placenta and this portion are known as the decidua. The decidua is divided into three layers. They are as follows:

• Decidua Basalis: This layer lies above the myometrium layer and below the chorionic villi.
• Decidua Capsularis: This layer lies between the embryo and the uterine lumen.
• Decidua Parietalis: This layer assists in inlining the uterus.

Ans. The mantle layer of the trophoblast secretes the hormone human chorionic gonadotropin. This hormone can be detected in urine. This is the basis of pregnancy tests. The pregnancy test is also known as the Gravindex test. This HCG hormone helps maintain the corpus luteum and this corpus luteum secretes progesterone. This progesterone hormone inhibits the process of menstruation. Another name for progesterone hormone is pregnancy hormone because this hormone is necessary for the maintenance of pregnancy.

Ans. The procedure of forming three germinal layers from the blastocyst is known as gastrulation.

• During implantation, the zona pellucida disappears and the trophoblast cells become large in size and aggressively invade the uterine tissues and make contact informing the placenta.
• At this time the inner cell mass undergoes differentiation and a layer of very flat cells appears on the inner surface of the inner cell mass which forms the future endoderm. The dorsal inner cell mass is called the epiblast, which is the presumptive material for the ectoderm and mesoderm.
• The formation of three germ layers forms a single-layered blastocyst. The blastocoel becomes undifferentiated and a new cavity called the gastric or archenteron is formed. It is called a gastrula. The process by which blastula is converted into gastrula is called gastrulation.
• The mesodermal cell passes through the primitive streak and differentiates laterally and anteriorly, occupying a position between endoderm and ectoderm.
• The epiblast also contributes some cells to the endoderm. Cells migrate to their future locations by amoeboid movements.

Ans. Pregnancy begins after implantation. The following changes happen during pregnancy:

(i) A finger-like projection appears on the trophoblast called chorionic villi, which are surrounded by uterine tissue and maternal blood.

(ii) Both the uterine tissue and the chorionic villi become interconnected and jointly form a structural and functional unit between the developing embryo and the mother's body known as the placenta.

(iii) The placenta transports oxygen and nutrients to the fetus and removes carbon dioxide and excretory waste products produced by the embryo.

(iv) Placenta also acts as an endocrine tissue and secretes hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogen, progesterone, etc.

(v) A hormone known as relaxin is secreted by the ovaries, in the later stages of pregnancy.

(vi) The umbilical cord connects the placenta to the fetus and helps transport substances to and from the embryo.

(vii) During pregnancy, the level of other hormones like estrogen, progesterone, cortisol, prolactin, thyroxine, etc. in the mother's blood increases many times. These hormones support fetal growth, maintenance of pregnancy, and metabolic changes in the mother.

Ans. Embryonic development begins after pregnancy with the following changes:

(i) The inner cell mass (embryo) differentiates into an outer layer known as ectoderm and an inner layer known as endoderm.

(ii) A middle layer known as mesoderm appears between ectoderm and endoderm.

(iii) The primordial germ layers give rise to all the tissues and organs of the adult. The inner cell mass contains certain cells known as stem cells, which have the power to give rise to all tissues and organs.

(iv) In humans, after one month of pregnancy, the embryo heart forms. A sign of a growing fetus can be confirmed by listening to the heartbeat.

(v) By the end of the second month, limbs and digits develop.

(vi) By the end of the third month (first trimester), most of the major organ systems are formed.

(vii) During the fifth month, the first movements of the fetus and the appearance of hair on the head are observed.

(viii) By the end of the sixth month (second trimester), the body is covered with fine hair, the eyelids separate and the eyelashes form.

(ix) By the end of eight months, the testes descend into the scrotum in the male fetus.

(x) By the end of nine months, the fetus is fully developed and birth-ready.

Ans. Lactation occurs after parturition which requires certain changes in the mother's body. These changes are as follows:

(i) A woman's mammary glands undergo differentiation during pregnancy.

(ii) By the end of pregnancy, they start producing milk through a process known as lactation.

(iii) The milk produced during the first few days of lactation is called colostrum.

It contains several antibodies (IgA, IgG, and IgM), and contains essential nutrients for developing immunity in the newborn.

Ans. In humans, fertilization of the ovum occurs at the ampullary-isthmic junction of the fallopian tube. The events that take place during the procedure of fertilization are:

(i) Sperm reach the junction of the ampulla and the isthmus and come into contact with the zona pellucida layer of the ovum.

(ii) The acrosome of the sperm head releases the enzyme sperm lysine which dissolves the corona radiata and digests the zona pellucida layer and enters the cytoplasm.

(iii) Entry of sperm stimulates the secondary oocyte to complete its stalled second meiotic division, thus producing a haploid egg or seed and a second polar body.

(iv) The sperm and egg nucleus fuse to form a diploid zygote.

Ans. (i) In humans, fertilization of the ovum occurs at the ampullary-isthmic junction of the fallopian tube. The events that happen during the procedure of fertilization are:

(i) Sperm reach the junction of the ampulla and the isthmus and come into contact with the zona pellucida layer of the ovum.

(ii) The acrosome of the sperm head releases the enzyme sperm lysine which dissolves the corona radiata and digests the zona pellucida layer and enters the cytoplasm.

(iii) Entry of sperm stimulates the secondary oocyte to complete its stalled second meiotic division, thus producing a haploid egg or seed and a second polar body.

(iv) The sperm and egg nucleus fuse to form a diploid zygote.

(ii) During fertilization, sperm comes into contact with the zona pellucida layer of the ovary and induces membrane changes (depolarization) that block the entry of additional sperm. This ensures that only one sperm can fertilize the ovum.

Ans. The development of the zygote till implantation takes place in the following order:

• Cleavage division (mitotic) begins in the zygote as it moves through the isthmus of the fallopian tube toward the uterus.
• This division results in 2, 4, 8, and 16 daughter cells known as blastomeres.
• An embryo with 8-16 blastomeres is referred to as a morula.
• The morula continues to divide and convert into a blastocyst as it moves into the uterus.
• The blastomeres in the blastocyst are arranged in an outer layer known as the trophoblast and an inner group of cells attached to the trophoblast known as the inner cell mass.
• The trophoblast layer then fuses with the endometrium and the inner cell mass differentiates as an embryo.
• After attachment, the uterine cells split quickly and cover the blastocyst. It leads to the blastocyst embedding in the endometrium of the uterus. This is known as implantation.

Ans. Post-zygotic events lead to implantation and placenta formation. After implantation of the blastocyst, finger-like projections, called chorionic villi, appear on the trophoblast and are surrounded by uterine tissue and maternal blood that become interdigitated with each other to form the placenta. Events leading to the formation of the placenta. Chorionic villi and uterine tissue, after implantation, interdigitate with each other to form the placenta, which is the structural and functional unit between the fetus and the mother's body.

Functions of Placenta

(i) It facilitates the supply of oxygen and nutrients to the fetus and the removal of C02 and excretory products.

(ii) It acts as an endocrine gland and secretes hormones like hPL, hCG, estrogen, and progesterone.

Ans. (i) Development of the placenta in human females: After implantation of the blastocyst, finger-like projections known as chorionic villi appear on the trophoblast and are surrounded by uterine tissue and maternal blood that become interconnected to form the placenta.

(ii) Placenta is attached to the embryo through an umbilical cord.

(iii) Placenta functions as an endocrine tissue as it secretes hormones such as

(a) hCG (human Chorionic Gonadotropin)

(b) hPL (human Placental Lactogen)

(c) Oestrogens

(d) Progesterone

Ans. The differences between a blastocyst and an embryo are as follows: