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Questions and Answers
What is the process of transforming germ cells into mature oocytes called?
What is the process of transforming germ cells into mature oocytes called?
How many chromosomes are in a secondary oocyte after meiosis I?
How many chromosomes are in a secondary oocyte after meiosis I?
What occurs during spermiogenesis?
What occurs during spermiogenesis?
Where does oogenesis primarily take place?
Where does oogenesis primarily take place?
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What is the diploid number of chromosomes in human germ cells?
What is the diploid number of chromosomes in human germ cells?
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When does oogenesis begin in human females?
When does oogenesis begin in human females?
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What is the outcome of the second meiotic division in spermatogenesis?
What is the outcome of the second meiotic division in spermatogenesis?
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Which structure is formed from the Golgi apparatus during spermiogenesis?
Which structure is formed from the Golgi apparatus during spermiogenesis?
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What occurs to primary oocytes before and at birth?
What occurs to primary oocytes before and at birth?
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What is produced when a primary oocyte completes its first meiotic division?
What is produced when a primary oocyte completes its first meiotic division?
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What happens to the secondary oocyte during ovulation?
What happens to the secondary oocyte during ovulation?
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What occurs if a secondary oocyte is fertilized?
What occurs if a secondary oocyte is fertilized?
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Where does fertilization typically occur?
Where does fertilization typically occur?
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What is a characteristic of the first polar body?
What is a characteristic of the first polar body?
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What happens to the secondary oocyte if it is not fertilized?
What happens to the secondary oocyte if it is not fertilized?
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What component surrounds the egg at ovulation?
What component surrounds the egg at ovulation?
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What hormone is primarily responsible for the development of primary follicles during the follicular phase?
What hormone is primarily responsible for the development of primary follicles during the follicular phase?
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Which phase of the ovarian cycle is characterized by the rupture of the mature follicle?
Which phase of the ovarian cycle is characterized by the rupture of the mature follicle?
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What happens to the corpus luteum if the oocyte is not fertilized?
What happens to the corpus luteum if the oocyte is not fertilized?
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Which hormone is secreted by the corpus luteum during the luteal phase?
Which hormone is secreted by the corpus luteum during the luteal phase?
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What is the primary role of Estrogen during the ovarian cycle?
What is the primary role of Estrogen during the ovarian cycle?
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How many primordial follicles does the ovarian cortex typically contain?
How many primordial follicles does the ovarian cortex typically contain?
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Which phase of the menstrual cycle corresponds with the preparation of the endometrium for implantation?
Which phase of the menstrual cycle corresponds with the preparation of the endometrium for implantation?
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What process describes the transformation of spermatogonia into mature sperms?
What process describes the transformation of spermatogonia into mature sperms?
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What is the main goal of cleavage in embryonic development?
What is the main goal of cleavage in embryonic development?
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During which day does the implantation into the uterine wall typically occur?
During which day does the implantation into the uterine wall typically occur?
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What process prevents polyspermy during fertilization?
What process prevents polyspermy during fertilization?
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What do the inner cell mass and trophectoderm give rise to, respectively?
What do the inner cell mass and trophectoderm give rise to, respectively?
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What happens during the acrosome reaction?
What happens during the acrosome reaction?
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After fertilization, what structures are present in the zygote?
After fertilization, what structures are present in the zygote?
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What initiates calcium influx into the egg during fertilization?
What initiates calcium influx into the egg during fertilization?
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What happens to the zona pellucida after the cortical reaction occurs?
What happens to the zona pellucida after the cortical reaction occurs?
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Study Notes
Ovarian Cycle
- The ovarian cycle is controlled by the pituitary gland.
- It is divided into three phases: follicular, ovulatory, and luteal.
- Follicular phase (FSH): ovarian primary follicles develop and become mature, stimulating estrogen production by follicular cells.
- Ovulatory phase (LH): triggers ovulation (rupture of the mature follicle), stimulating the release of estrogen and stimulating corpus luteum to produce progesterone.
- Luteal phase (LH): the ruptured follicle becomes the corpus luteum which secretes progesterone and a small amount of estrogen. These hormones prepare the endometrium for implantation.
- The ovarian cortex contains hundreds of thousands of primordial follicles (400,000 to 500,000). Each primordial follicle consists of a primary oocyte enclosed by a single layer of flat follicular cells.
- Early development of ovarian follicles is induced by FSH. Simple flat follicular cells become cuboidal then columnar and then form many layers around the oocyte; this structure forms the primary follicle.
Gametogenesis
- Gametogenesis is the production of mature male (sperms) and female (ova) gametes.
- Spermatogenesis is the series of changes that transform primitive germ cells (spermatogonia) into mature sperms.
- Oogenesis is the sequence of events that transform primitive germ cells (oogonia) into mature oocytes.
Meiosis
- Meiosis is a cell division that produces male and female gametes.
- It consists of two divisions: meiosis I and meiosis II.
- During meiosis, the diploid number of chromosomes (46) is reduced to the haploid number (23).
Spermatogenesis
- Aim: Formation of sperms with a haploid number of chromosomes.
- Site: Seminiferous tubules of the testis.
- Time: From puberty to old age (about two months).
- Duration: About two months. Sperm are stored and become functionally mature in the epididymis.
- Each daughter spermatogonium grows to give primary spermatocyte (46).
- Primary spermatocyte undergoes meiotic division to give 2 secondary spermatocytes (22 + x) or (22 + y).
- Secondary spermatocytes undergo 2nd meiotic division to form 4 haploid spermatids (half size).
- Spermatids are transformed into 4 mature sperms by a process called spermiogenesis.
Spermiogenesis
- Spermiogenesis is the process of transformation from spermatids to mature sperms.
- It involves changes to shape (metamorphosis).
- It involves the condensing of the nucleus, formation of acrosome by Golgi apparatus, formation of spiral sheath by mitochondria, and elongation of centriole to form the axial filament.
Oogenesis
- Aim: Formation of secondary oocytes with a haploid number of chromosomes.
- Site: Cortex of the ovary.
- Time: Starts during fetal life, completed after puberty, continues until menopause (monthly except during pregnancy).
- Before birth (during early fetal life): primitive ova (Oogonia) proliferate by mitotic division and enlarge to form primary oocytes (46).
- Before and at birth, all primary oocytes complete the prophase of the 1st meiotic division and remain arrested until puberty.
- At puberty, shortly before ovulation, the primary oocyte completes its 1st meiotic division to give secondary oocyte (23) and 1st polar body.
- Secondary oocyte receives almost all the cytoplasm; the 1st polar body receives very little (non-functional, degenerates).
- At ovulation, the secondary oocyte begins meiosis II but progresses only to metaphase where division is arrested. If fertilized, the second meiotic division is completed; otherwise, it degenerates 24 hours after ovulation. Most of the cytoplasm is retained by the mature oocyte (fertilized oocyte). The rest is in the 2nd polar body (which soon degenerates).
Egg (and corona radiata) at ovulation
- Corona radiata, Zona pellucida, Cortical granules
Transport through the oviduct
- At around the midpoint of the menstrual cycle (~day 14), a single egg is ovulated and swept into the oviduct.
- Fertilization usually occurs in the ampulla of the oviduct within 24 hrs. of ovulation.
- Cleavage and differentiation events lead to a blastocyst by day 4 of embryonic development.
- Inner cell mass generates embryonic tissues.
- Outer trophectoderm generates placental tissues.
- Implantation in the uterine wall occurs around day 6 (day 20 of the menstrual cycle).
Fertilization
- It is a multi-step process where multiple sperm bind to the corona radiata. Only a single sperm fertilizes the egg.
- Acrosome reaction (sperm binds to ZP proteins in the zona pellucida; this initiates the release of enzymes from the sperm, allowing it to burrow through the zona pellucida).
- Zona reaction (binding of sperm and egg plasma membranes initiates Ca+ influx into the egg. Release of cortical granules from the egg blocks other sperm from fertilizing the egg).
- Cortical reaction prevents further sperm from fertilizing the egg (polyspermy).
Fertilized egg (zygote)
- Fertilized egg is also called a zygote.
- Fertilized egg (zygote) contains 2 polar bodies and 2 pronuclei.
- Day 1 (0.1 mm).
Cleavage
- Cleavage is cell division.
- Aims to grow the unicellular zygote to a multicellular embryo.
- It begins about 24 hours after pronuclear fusion.
Early Embryonic Stages
- 2-cell stage, 4-cell stage, 8-cell stage
Compaction of the Human Embryo
- Compaction is caused by increased cell-cell adhesion.
- Cells on the outside of the morula become the trophoblast, that will form the placenta.
- The inner cells form the embryo proper (called the inner cell mass, ICM).
Formation of the Blastocyst
- The blastocyst contains the inner cell mass (ICM), blastocoele, and trophoblast.
- ICM forms the embryo.
- Trophoblast forms the placenta.
Developmental Stages
- Zygote → Cleavage → Blastula → Gastrulation
Germ Layers
- Diploblastic (2 germ layers: ectoderm and endoderm)
- Triploblastic (3 germ layers: ectoderm, mesoderm, and endoderm)
Germ Layer Derivatives
- Endoderm (internal layer) derivatives: lung cells, thyroid cells, digestive cells, pancreatic cells.
- Mesoderm (middle layer) derivatives: cardiac muscle cells, skeletal muscle cells, tubule cells of the kidney, red blood cells, smooth muscle cells.
- Ectoderm (external layer) derivatives: skin cells, neurons, pigment cells.
Ectopic Implantation
- Implantation somewhere other than the upper portion of the uterus.
- "Rupture" can lead to life-threatening hemorrhage.
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Description
Explore the intricacies of the ovarian cycle, which is regulated by the pituitary gland and divided into three distinct phases: follicular, ovulatory, and luteal. Understand how hormones like FSH and LH influence the development of ovarian follicles and prepare the endometrium for potential implantation.
