Cell division: mitosis and meiosis
The basis of reproduction and growth consists of two types of cell division. Mitosis ensures tissue growth, regeneration, and somatic cell division. In gametogenesis, it is responsible for the proliferation of precursor cells (spermatogonia and oogonia), maintaining the diploid chromosome set.
Meiosis occurs exclusively in the maturation zone of germ cells and halves the genetic set (to haploid). The process exhibits sex-specific differences:
- In males (spermatogenesis): the first division yields two secondary spermatocytes (differing in sex chromosomes X or Y). The second division produces four equivalent spermatids.
- In females (oogenesis): the first division yields one large secondary oocyte and a small polar (reductional) body. The second division arrests at metaphase and is completed only upon fertilization. The result is one fully functional ovum and polar bodies, which subsequently degenerate.
Note: errors in chromosome segregation during these processes lead to genomic and chromosomal mutations.
Progenesis and preparation for fertilization
The prezygotic period, or progenesis, includes gamete formation, insemination, and fertilization itself—the fusion of male and female cells to form a diploid zygote.
By the time of ovulation, a mature follicle has formed in the female ovary. Inside lies a secondary oocyte surrounded by fluid and follicular cells, which nourish the cell and produce estrogens. Externally, the follicle is covered by a layer of tissue (the theca). The oocyte itself is protected by two layers: the thick zona pellucida (zona pellucida) and the corona radiata (corona radiata).
Embryonic (prenatal) ontogeny
Intrauterine development lasts approximately 270 days. It is traditionally divided into two key stages:
- Embryonic (germinal) — up to 8 weeks. Organogenesis of all systems takes place and basic body features are formed.
- Fetal — from the 9th week until birth. Organs grow and mature.
Embryonic development proceeds through a strict sequence of stages. First, a single-celled zygote is formed, which undergoes cleavage. This results in the formation of the blastula — a single-layered multicellular embryo with a cavity. Next comes gastrulation with the formation of germ layers (gastrula). The early phase concludes with histogenesis and organogenesis, during which the neurula with axial organs is formed.
For example, the neural tube develops rapidly: on the 14th day, the neural plate appears, by the 20th day a neural groove with folds forms, and by the 23rd day the edges fuse into a closed tube.
Postnatal ontogeny
Life after birth is divided into three global stages: prereproductive (growth and development), reproductive (sexual maturity and reproduction), and postreproductive (aging).
In pediatrics and obstetrics, the perinatal period is of particular importance, encompassing the time around birth. It includes the antenatal (prenatal), intranatal (labor and delivery), and postnatal (first 7 days of life) stages. Modern medicine counts the antenatal period from the 22nd–23rd week of pregnancy, when the fetal mass reaches 500 g and it is considered potentially viable.
The development of the reproductive system also has its own periodization. In girls, these periods include neonatal, infancy, the "neutral" phase, prepuberty (prior to the first menstruation — menarche), puberty (up to 15 years), and adolescence.
To remember the stages of early embryogenesis, use the phrase: «Tomorrow Kids Will Go Outside» — Zygote, Cleavage, Blastula, Gastrula, Neurula.
Class and exam questions
What is the main difference between meiosis and mitosis in gametogenesis?
Mitosis ensures the proliferation of precursor cells while preserving the diploid set. Meiosis occurs in the maturation zone, halves the chromosome set, and forms haploid gametes.
Why does oogenesis produce only one functional cell?
During division, the cytoplasm is distributed unevenly. One large ovum storing nutrients is formed, along with small polar bodies that subsequently degenerate.
When does the second meiotic division in oogenesis complete?
It reaches the metaphase stage and arrests. The process resumes and fully completes only if fertilization occurs.
From what gestational age does the antenatal period begin?
Today, the antenatal period is counted from the 22nd–23rd week of gestation, when the fetal mass reaches 500 grams (previously, the threshold was 28 weeks).
What else to study on this topic
- Mechanisms of chromosomal and genomic mutations in cell division disorders
- Structure of the mature follicle and functions of follicular cells
- Critical stages of intrauterine development and their vulnerability
- Detailed chronology of neural tube formation
- Detailed age periodization of human postnatal development
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