Meiosis, Menstrual Cycle, and Histology: Key Concepts
Meiosis
Meiosis: Duplication of DNA, with 2 successive divisions, a mechanism which maintains variability.
Meiosis involves:
- One duplication of DNA, variability through crossing over (metaphase I).
- Two successive divisions: 1st meiotic (prophase I, metaphase I, anaphase I, telophase I, cytokinesis I), 2nd meiotic (II).
Haploid Cells
Haploid cells include sperm and oocytes.
Crossing Over
Crossing over is the exchange of chromatid segments between chromatids.
Prophase I
During prophase I, chromosomes have doubled and are shorter, twisted, and visible under a microscope.
Metaphase I
Centrioles move to opposite poles. Pairs of homologous chromosomes are strongly condensed and aligned on the metaphase plate.
Anaphase I
Chromatids remain attached at their centromeres and move toward the poles.
Telophase I
The nuclear membrane reforms around each set of chromosomes, the spindle disappears, and cytokinesis occurs.
Prophase II
Centrioles are duplicated, the two pairs of centrioles separate into centrosomes, the nuclear membrane disappears, and the spindle forms.
Metaphase II
Each chromosome is aligned at the metaphase equatorial plate.
Anaphase II
Centromeres separate, and the two chromatids of each chromosome move to opposite poles in the spindle.
Telophase II
Telophase II produces 4 haploid daughter cells. Due to crossing over, some chromosomes have recombined segments of the original parental chromosomes.
Menstrual Cycle
The menstrual cycle is a set of events that occurs over approximately 28 days (normal). It involves hormones (gonadotropins (FSH, LH) and sex hormones (estrogen, progesterone)), the ovary, and the endometrium.
Gonadotropins
Gonadotropins control the gonads and are stimulated by the anterior pituitary.
FSH
FSH stimulates the maturation of follicles.
Gametogenesis
Gametogenesis is the process of gamete formation in the gonads through meiosis.
Oocyte I
Oocyte I is found in primary follicles.
Oocyte II
Oocyte II is found in mature or preovulatory antral follicles and is expelled into the uterine tubes during ovulation.
Egg Formation
The egg is formed when sperm comes into contact with oocyte II, restarting meiosis.
- The oocyte contributes mitochondria and mitochondrial DNA.
- Sperm contributes only the nucleus.
Syngamy
Syngamy is the fusion of two sets of chromosomes.
Development of the Zygote
Development of the zygote involves fertilization, gametogenesis, embryo development, segmentation, blastocyst stages, gastrulation, neurulation, and organogenesis.
Histology
Epithelial Tissue
Epithelial tissue consists of one or more layers of cells forming sheets. Functions include protection, absorption, secretion, and excretion.
Connective Tissue
Connective tissue is formed by cells called chondrocytes and elastic and collagen fibers. Types include adipose, fibrous, hematopoietic, and supportive. Functions include connection, support, insulation, production, and hematopoiesis.
Muscle Tissue
Muscle tissue includes striated, smooth, and cardiac muscle. Its primary function is contraction.
Nervous Tissue
Nervous tissue is formed by neurons (nerve endings) and neuroglia. Functions include perceiving sensations and transmitting nerve impulses.
Bone Tissue
Bone tissue consists of osteogenesis, osteoblasts, osteocytes, and osteoclasts.
Haversian Canals
Haversian canals contain blood vessels.
Volkmann’s Canals
Volkmann’s canals are those through which osteocytes communicate.
Lymphocytes
Lymphocytes include T cells (CD4, CD8), B cells (memory, plasma), monocytes (macrophages), eosinophils, basophils, and neutrophils.