Embryology Block I

From the tetis: spermatogonium 46XY, primary spermatocyte 46XY, first meiotic division: secondary spermatocyte 23XY, second meiotic division: 4 spermatid 23XY, sperm 23XY

Reductive division reduces the cells from dipolid 46 chromosomes to haploid 23 chromosomes

produce 4 sperm from 1 primary spermatocyte and only 1 mature oocyte from 1 primary oocyte

from the ovary: primary oocyte 46XX (in the primary follicle), primary oocyte 46XX (in growing follicle), primary oocyte 46XX (in larger follicle), First meotic division: oocyte + 1 polar body 23X (in mature follicle), Ovulation: Fertilized oocyte + 2nd polar body--> second meiotic division complete

Prophase I, Metaphase I (crossover occurs), Anaphase I, Telophase I, 2nd Meiotic division of same process however with haploid cells

• Abnormal chromosomes results from unequal distribution.
• Trisomy 21-Down Syndrome

Double-walled structure at the head of the sperm covering the genetic material, derived from the Golgi region of the spermatid, contains enzymes that are released at the beginning of fertilization to assist the sperm in penetrating the corona radiata and zona pellucida surrounding the secondary oocyte

Mitochondrial sheath which contains ATP for energy and movement to carry the sperm over the long distance to the oocyte.

give nutrition to the embryo before the placenta is developed, produce the vascular system of the placenta

• -GnRH from the hypothalamus triggers LH surge (1st meiotic division of primary oocyte) and FSH release at day 14-ovulation occurs,
• -FSH develops the ovarian follicles and stimulates the production of estrogen by the follicle cells
• -LH triggers ovulation and stimulates follicular cells and corpus luteum to produce progesterone
• -Corpus luteum remains active until no fertilization occurs and it atrophies resulting in menses
• -Ovarian hormones induce growth of endometrium in preparation for implantation

stimulates development of ovarian follicles and production of estrogen by follicular cells

"triggers" ovulation and stimulates follicular cells and corpus luteum to produce progesterone

glandular structure, secretes progesterone and some estrogen, progesterone prepares the walls of the endometrium for embryo implantation, if implantation occurs, the corpus luteum will supply nutrition to the embryo for 20 weeks until the placenta is fully functioning. IF implantation does not occur it will slough off during menses.

This is the series of mitotic cell divisions of the zygote that result in the formation of early embryonic cells, blastomeres

This solid mass of 12 to approximately 32 blastomeres is formed by cleavage of a zygote.

After 2 to 3 days, the morula enters the uterus from the uterine tube (fallopian tube). Soon a fluid-filled cavity, the blastocystic cavity, develops inside it. This change converts the morula into a blastocyst.

Blastocyst become gastrula after completion of the trilaminar embryonic disc (ectoderm, mesoderm, endoderm)

The early embryo during the third and fourth weeks when the neural tube is developing from the neural plate. It is the first appearance of the nervous system and the next stage after the gastrula.

• Embryonic development begins at fertilization and ends at day 56 (end of the 8th week when all body systems are considered to be developed)
• Fetal development begins on day 57 and ends at birth

The beginning or first discernible indication of an organ or structure.

• Infancy: first 12mo after birth
• Childhood: 13mo to puberty
• Puberty: beginning when the secondary sex characteristics develop, age 13-16 for boys, age 12-15 for girls
• Adolescence: at age 11 to 19 characterized as rapid physical and sexual maturation, ending when development of physical structure slows--adulthood begins here

The process during which the blastocyst attaches to the endometrium, the mucous membrane or lining of uterus, and subsequently embeds in it. The preimplantation period of embryonic development is the time between fertilization and the beginning of implantation, a period of approximately 6 days.

The process of formation and development of specialized generative cells (gametes). During gametogenesis, the chromosome number is reduced by half and the shape of the cell is altered. The sperm and oocyte each contain haploid number of chromosomes present in somatic cells.

• Corona Radiata-follicular cells, capable of producing and secreting some estrogen.
• Zona Pellucida-non-cellular structure

• The internal os communicates with the uterine body and is located superior to the cervix.
• The external os communicates with the vagina and is located inferior to the cervix.

• Perimetrium-thin external layer or peritoneal layer
• Myometrium-thick smooth muscle layer, located in the middle of three layers
• Endometrium-thin internal layer, composed of three layers: compact, spongy and basal layer. Basal layer contains its own blood supply and is not sloughed off during menstruation, the compact and spongy layers disintegrate during menstruation and after birth.

The ovaries are almond-shaped reproductive glands located close to the lateral pelvic walls on each side of the uterus that produce oocytes. The ovaries also produce estrogen and progesterone, the hormones responsible for the development of secondary sex characteristics and regulation of pregnancy.

Spiral arteries in the endometrium, become important in early implantation of embryo

• -Fimbriae of the uterian tubes become closely applied to the ovary
• -Follicular fluid escapes from the ovarian follicle
• -Using peristalsis, the ovarian follicle is "swept" into the uterian tube toward the uterus

Epididymis houses the immotile sperm. Peristaltic contractions transport sperm to the urethra for ejaculation into the external os of the vagina. Secretions are added from the seminal glands, prostate and bulborurethral glands, these secretions aid in movement of sperm in the acidic environment of the vagina and also form a 'vaginal plug' to prevent backflow.

approx. 7 hour period of conditioning of sperm, sperm are unable to fertilize oocyte until capacitation

The usual site of fertilization is the ampulla of the uterine tube, its longest and widest part.

Approx. 24 hours

After capacitation, sperm is guided to the oocyte, perforations form in the acrosome of the sperm allowing penetration through the corona radiata and zona pellucida. After penetrating the zona pellucida, the ZP elicits a mechanism (Zona Reaction) to hard the perivitelline space keeping other sperm out. The sperm looses its plasma membrane as it enters the plasma membrane of the oocyte.

Proteolytic enzyme assisting in the sperm's penetration of the zona pellucida.

a change in the properties of the zona pellucida that makes it impermeable to other sperm

• •Stimulates the penetrated oocyte to complete the second meiotic division.
• •Restores the normal diploid number of chromosomes (46) in the zygote.
• •Results in variation of the human species through mingling of maternal and paternal chromosomes.
• •Determines chromosomal sex of the embryo.
• •Causes metabolic activation of the ootid and initiates cleavage (cell division) of the zygote.

Morula

• Trophoblast-encloses the embryoblast and blastocystic cavity and later forms extraembryonic structures and the embryonic part of the placenta, once the zona pellucida is shed, it attaches to the endometrium
• Embryoblast-becomes the embryo
• Blastocystic cavity

• Cytotrophoblast (inner layer)
• Syncytiotrophoblast- (outer layer) invades the endometrial epithelium and underlying connective tissue

• Formation of bilaminar disc: epiblast and hypoblast
• The bilaminar disc gives rise to the germ layer that produce all the tissues and organs.

• -Amniotic cavity
• -Amnion
• -Embryonic disc
• -Umbilical vessel (yolk sac)
• -Connecting stalk
• -Chorionic sac