The critical periods in human development. The biological sex determination in a human

An important aspect, concerning mechanisms of reproductive process defects, is that influence of harmful factors is different in different ontogenesis stages. P.G. Svetlov suggested a concept about critical periods of human development based on pathological findings. The most vulnerable periods of embryo structures forming usually coincide with the first stages of some organs formation, occurring mainly in a first 9 weeks of development.

Hence, P.G. Svetlov suggested providing health care for women at that time (elimination of harmful factors, creating less intensive work schedule, providing additionary vacations and so on). However and today these suggestions aren’t used in health care. In his last work, P.G. Svetlov assumed that gametogenesis also has critical periods. It was shown that ovicell especially sensitive to harmful factors influence in a preovulatory period of sex cycle.

During this period alcohol and range of others substances cause more defects. That means that we have to concern not only about early pregnancy stages care but also about preserving of preovulatory period of sex cycle.

The biological sex determination in a human. The division of mankind into two sexes assumes every individual to have full correspondence of anatomical body plan, structure of sexual organs, body’s proportions (growth, shoulder/pelvis width ratio, distribution of adipose tissue and so on), sexual realization ( feeling of self as representative of definite sex), and at least adequate direction of sexual drive and appropriate stereotypes of sexual behavior.

The formation of this system start from genetic sex determination by chromosomes set (pic 5.6). The genetic sex determines gonad (or genuine) sex, identificated by main sign of sex - histological structure of sexual gland. It is genuine because it is allows to determine gamete sex, i.d. ability of sexual gland to produce spermatozoa or ova. Gonads show an individual role in reproduction process.

Pic. 5.6. The human sex formation: 1 - physical deteirainants of sex, 2 - intermediate deteiminant, 3 - social-psycological determinant (by G. S. Vasilchikov, 1990)

Also gonad sex determines hormonal sex - the ability of sexual gland to produce specific sex hormones (during embryonic development only testis are hormone active, whereas in puberty both ovarium and testis are hormone active). Then, the level and dominating directions of hormonal action determine morphological (or somatic) sex (phenotype).

Morphological sex means features of structure and development of internal and external sexual organs, and also secondary sexual characteristics. It is important to note that a term “sex” is composed from many related to each other biological, social and psychological components. Sex — is a union of organism's signs and properties providing participating in reproduction and hereditary information transmission through making gametes.

The biological sex differentiation is programmed by genetic sex chromosome set in zygote after gamete nucleuses fusion.

It was pointed above that embryonic gonad is bisexual. Formation of primary gonads occurs on 5th week of embryonic development. The genetic sex is determined by sex chromosome (X or Y) of sperm. The X chromosome has a gene of testicular feminization (Xtfm), normal allele of which is responsible for receptor synthesis for androgens. Since, male and female organism has at least one X chromosome. That means that both sexes have such receptor. Y chromosome has a gene, which is responsible for synthesis H-Y antigens, which stimulate differentiation of sexual folds’ cells to semeniferous tubules and intersticial cell. If individual has genotype Xtfm Xtfm, the ovarium will be formed from primary gonad cortex. If individual has genotype Xtfm Yh-Y, the testis will be formed from primary gonad medulla.

At 10th week of development the sex of embryo may be determined by two criteria: sex chromosome set and histological structure of sex glands. The sex of mature gonad (gonad’s sex) may be determined by generative elements state: primary follicules with oocyte I in ovariums and semeniferous tubules with spermatozoa in testis.

A hormonal gonad function is producing sex hormone in their intermediate tissues (teca cell in ovarium and Leidig cell in testis). Both ovarium and testis produce main sex hormones: testosterone, estrogen, progesterone, but in different ratio. Ovariums mostly produce estrogens and after ovulation progesterone. Testis mostly produces testosterone. The typical for ovarium and testis features of sex steroid biosynthesis form hormonal sex. It is sexual steroid ratio and their properties, characterizing each sex.

Testosterone, liberating into embryo blood, binds with androgen receptors in a target cell of potential reproductive system. Then complex testosterone-receptor passes to a nucleus, where it changes an activity of genes responsible for tissue growth and development. Testosterone stimulates development of tissues, which give rise only for male reproductive system. That why, male is developed from embryo with sex chromosome set – XY.

Tissues of potential female reproductive system are not activated and they don’t develop. In an embryo with sex chromosome set XX the absence of testosterone allows reproductive system to develop female pattern. On a 10-12th week of embryonic development the internal sex organs are formed. Until differentiation period, both male and female embryo has a rests of pronephros urethra, which are a precursors of sexual organs of both sexes.

So called Muller’s canals are precursors of female sex organs - uterine tubes, uterus, and upper part of vagina. So called Wolf’s ducts are precursors of male reproductive organs - epididymis, vas defferens, seminal vesicles.

After 12* week of development in case of having satisfactory concentration of testosterone, there is musculinisation of external sex organs in a male embryo. It is done in 20* week. There is atrophy of vagina appendix, formation of scrotum suture (scrotum formation), enlargement corpus cavernosa of penis and formation of cavernose part of urethra.

In puberty, the definite level of estrogens provides formation of female sexual characteristics — feminization (female body constitution, mammal glands formation, hymen, vagina and uterus enlargement). Androgens provide male skeleton type, good muscular development, development of larynx cartilages, voice mutation, scrotum and penis enlargement, male type of hair distribution. The synchronization of ovarial cycle (follicule development, ovulation, corpus luteum formation) and pituitary hormone regulation setting also occurs in puberty. Males have stable pituitaiy regulation.